CONNECTION
COMMUNICATION, -and, about communication, in connection and in connection, w.
1. (due). A relationship of mutual dependence, conditionality, commonality between something. C. theory and practice. Causal p.
2. (due). Close communication between someone or something. Friendly village Strengthen international relations.
3. (in connection and in connection). Love relationship, cohabitation. Lyubovnaya s. To be in touch with someone.
4. pl. h. Close acquaintance with someone, providing support, patronage, benefit. Have connections in influential circles. Great connections.
5. (due). Communication with someone, as well as means that make it possible to communicate, to communicate. Kosmicheskaya village Live s.(via contacts). Air village Intercity telephone s.
6. (due). Industry National economy, relating to the means of such communication (mail, telegraph, telephone, radio), as well as the totality of such means concentrated in the relevant institutions. Communication service. Communication workers.
7. (in connection), usually plural. h. Part of a building structure connecting its main elements (special).
Due to how, preposition with TV. n. as a result of something, because of something, being conditioned by something. Late due to skidding.
Due to, union for the reason that, on the basis of the fact that. I inquired because accurate information was needed.
CONNECTION What is it CONNECTION, meaning of the word CONNECTION, synonyms for CONNECTION, origin (etymology) CONNECTION, CONNECTION stress, word forms in other dictionaries
+ CONNECTION- T.F. Efremova New dictionary Russian language. Explanatory and word-formative
COMMUNICATION is
connection
and.
a) Mutual relations between someone or something.
b) Community, mutual understanding, internal unity.
a) Communication with someone.
b) Love relationships, cohabitation.
3) Relationships between someone that create mutual dependence, conditionality.
4) Consistency, coherence, harmony (in thoughts, presentation, etc.).
5) The ability to communicate with someone or something. on distance.
6) Means by which communication is carried out at a distance.
7) A set of institutions that provide means of communication at a distance (telegraph, mail, telephone, radio).
a) Connection, fastening of something.
b) Cohesion, mutual attraction (molecules, atoms, electrons, etc.).
+ CONNECTION- Modern Dictionary ed. "Big Soviet Encyclopedia»
COMMUNICATION is
CONNECTION
1) transmission and reception of information using various technical means. In accordance with the nature of the means of communication used, it is divided into postal (see Mail) and electric (see Telecommunications). 2) The branch of the national economy that ensures the transmission and reception of postal, telephone, telegraph, radio and other messages. In the USSR in 1986 there were 92 thousand communications enterprises; 8.5 billion letters, 50.3 billion newspapers and magazines, 248 million parcels, 449 million telegrams were sent; the number of telephone sets on the general telephone network amounted to 33.0 million. 60s in the USSR, the Unified Automated Communications Network (EASC) is being introduced. 3) Military communications are provided by the Signal Corps. --- in philosophy - the interdependence of the existence of phenomena separated in space and time. Connections are classified according to objects of cognition, according to forms of determinism (unambiguous, probabilistic and correlational), according to their strength (rigid and corpuscular), according to the nature of the result that the connection gives (connection of generation, connection of transformation), according to the direction of action (direct and reverse), by the type of processes that defines this connection(functioning connection, development connection, control connection), according to the content that is the subject of the connection (connection that ensures the transfer of matter, energy or information).
+ CONNECTION- Small academic dictionary Russian language
COMMUNICATION is
connection
AND, sentence about communication, in connection and in connection, and.
Mutual relationship between someone or something.
Link between industry and agriculture. Connection between science and production. Trade connections. Economic connections between regions. Family ties.
Mutual dependence, conditionality.
Causality.
We only want to say that all sciences are closely connected with each other and that the lasting acquisitions of one science should not remain fruitless for others. Chernyshevsky, Grammar notes. V. Klassovsky.
The connection between Petrov-Vodkin’s work and the traditions of ancient Russian painting is obvious.
L. Mochalov, The uniqueness of talent.
Coherence, harmony, consistency (in connecting thoughts, in presentation, in speech).
Thoughts were confused in his head, and words had no connection. Pushkin, Dubrovsky.
There is not enough consistency in my thoughts, and when I put them on paper, it always seems to me that I have lost my sense of their organic connection. Chekhov, A boring story.
Closeness with someone, inner unity.
That invisible connection grew between them, which was not expressed in words, but only felt. Mamin-Sibiryak, Privalovsky millions.
When a writer deeply feels his blood connection with the people, it gives him beauty and strength. M. Gorky, Letter to D.N. Mamin-Sibiryak, October 18. 1912.
Communication (friendly or business), relations with someone or something.
Keep in touch with smb. Make connections in the literary world.
(Ivan Ivanovich and Ivan Nikiforovich) broke off all ties, while previously they were known as the most inseparable friends! Gogol, The story of how Ivan Ivanovich quarreled with Ivan Nikiforovich.
Drozdov's connections with one of the revolutionary organizations were established, and arrests were made. M. Gorky, Story about a hero.
Love relationship; cohabitation.
(Matvey) entered into a relationship with a bourgeois woman and had a child with her. Chekhov, Murder.
(Sophia:) What right do you have to talk about my infidelity?.. You had dozens of relationships. M. Gorky, The Last.
|| pl. h.(connections, -ey).
Close acquaintance with influential persons who can provide support and patronage.
Good B. decided to find a home for his stepfather. He already had great connections and immediately began to ask and recommend his poor comrade. Dostoevsky, Netochka Nezvanova.
Thanks to the connections of my late engineer father, I was enrolled in the Mikhailovsky School. Pertsov, From autobiography.
Communication, communication with someone or something. using various means.
In the cabin, using a speaking tube, the commander could communicate with the bridge, and by telephone with any department of the ship. Novikov-Priboy, Captain 1st Rank.
Morozka was among the cavalry assigned to communicate with the platoons during the battle. Fadeev, Defeat.
Now there was only one way of communication left - through the Volga. Simonov, Days and Nights.
|| Those.
Transmitting and receiving information using special means.
5. usually with a definition.
The means by which communication and transmission of information are carried out.
Radiotelephone communication. Telegraph communication. Dispatcher communication.
Signalmen at night artillery regiment managed to establish a telephone connection to the tank. V. Kozhevnikov, Seven days.
The set of institutions serving technical means communication at a distance (telegraph, mail, telephone, radio).
Communication workers.
|| Military
A service that allows communication between military units(using telephone, radio, messengers, etc.).
Arkhip Khromkov became the head of intelligence and communications. Markov, Strogovs.
A liaison officer arrived from army headquarters with an urgent package. Popovkin, Rubanyuk Family.
Connection, fastening of something.
Bonding stones and bricks with clay.
In the Trinity Cathedral, he introduces iron into the masonry of the building to connect the corners. Pilyavsky, Works of V.P. Stasov in Leningrad.
Cohesion, mutual attraction (molecules, atoms, electrons, etc.).
The connection of electrons with the nucleus.
A device that binds or fastens parts of something. buildings or structures; clamp.
It was a huge decorative workshop - a dome intertwined at the top with iron rafters and braces. A. N. Tolstoy, Egor Abozov. logic, coherence, continuity, foldability, sequence, harmony, interaction, connection, articulation, concatenation, cohesion, communication, means of communication, intercourse, communication, contact, association, relation, relationship, dependence, binding, ties, romance, connecting link, union, causation, public relations, tomba, intimate relationships, intrigue, correlation, duplex, umbilical cord, intercourse, bonding, religion, cohabitation, parataxis, connecting thread, continuity, adhesion, interconnectedness, correlation, conditionality, connection, kinship, putty, bond, cupids, affair, synapse, context, love, thread, mail, message, quadruplex. Ant. fragmentation
Chemical bond
All interactions leading to the combination of chemical particles (atoms, molecules, ions, etc.) into substances are divided into chemical bonds and intermolecular bonds (intermolecular interactions).
Chemical bonds- bonds directly between atoms. There are ionic, covalent and metallic bonds.
Intermolecular bonds- connections between molecules. These are hydrogen bonds, ion-dipole bonds (due to the formation of this bond, for example, the formation of a hydration shell of ions occurs), dipole-dipole (due to the formation of this bond, molecules of polar substances are combined, for example, in liquid acetone), etc.
Ionic bond- a chemical bond formed due to the electrostatic attraction of oppositely charged ions. In binary compounds (compounds of two elements), it is formed when the sizes of the bonded atoms are very different from each other: some atoms are large, others are small - that is, some atoms easily give up electrons, while others tend to accept them (usually these are atoms of the elements that form typical metals and atoms of elements forming typical nonmetals); the electronegativity of such atoms is also very different.
Ionic bonding is non-directional and non-saturable.
Covalent bond- a chemical bond that occurs due to the formation of a common pair of electrons. A covalent bond is formed between small atoms with the same or similar radii. Prerequisite- the presence of unpaired electrons in both bonded atoms (exchange mechanism) or a lone pair in one atom and a free orbital in the other (donor-acceptor mechanism):
| A) | H· + ·H H:H | H-H | H 2 | (one shared pair of electrons; H is monovalent); |
| b) | NN | N 2 | (three shared pairs of electrons; N is trivalent); | |
| V) | H-F | HF | (one shared pair of electrons; H and F are monovalent); | |
| G) | NH4+ | (four shared pairs of electrons; N is tetravalent) |
- Based on the number of shared electron pairs, covalent bonds are divided into
- simple (single)- one pair of electrons,
- double- two pairs of electrons,
- triples- three pairs of electrons.
Double and triple bonds are called multiple bonds.
By distribution electron density between the bonded atoms, the covalent bond is divided into non-polar And polar. A non-polar bond is formed between identical atoms, a polar one - between different ones.
Electronegativity- a measure of the ability of an atom in a substance to attract common electron pairs.
The electron pairs of polar bonds are shifted towards more electronegative elements. The displacement of electron pairs itself is called bond polarization. The partial (excess) charges formed during polarization are designated + and -, for example: .
Based on the nature of the overlap of electron clouds ("orbitals"), a covalent bond is divided into -bond and -bond.
-A bond is formed due to the direct overlap of electron clouds (along the straight line connecting the atomic nuclei), -a bond is formed due to lateral overlap (on both sides of the plane in which the atomic nuclei lie).
A covalent bond is directional and saturable, as well as polarizable.
To explain and predict the mutual direction of valence bonds use the hybridization model.
Hybridization atomic orbitals and electronic clouds- the supposed alignment of atomic orbitals in energy, and electron clouds in shape when an atom forms covalent bonds.
The three most common types of hybridization are: sp-, sp 2 and sp 3 -hybridization. For example:
sp-hybridization - in molecules C 2 H 2, BeH 2, CO 2 (linear structure);
sp 2-hybridization - in molecules C 2 H 4, C 6 H 6, BF 3 (flat triangular shape);
sp 3-hybridization - in molecules CCl 4, SiH 4, CH 4 (tetrahedral form); NH 3 (pyramidal shape); H 2 O (angular shape).
Metal connection- a chemical bond formed by sharing the valence electrons of all bonded atoms of a metal crystal. As a result, a single electron cloud of the crystal is formed, which easily moves under the influence of electrical voltage- hence the high electrical conductivity of metals.
A metallic bond is formed when the atoms being bonded are large and therefore tend to give up electrons. Simple substances with a metallic bond are metals (Na, Ba, Al, Cu, Au, etc.), complex substances are intermetallic compounds (AlCr 2, Ca 2 Cu, Cu 5 Zn 8, etc.).
The metal bond does not have directionality or saturation. It is also preserved in metal melts.
Hydrogen bond - intermolecular bond, formed due to the partial acceptance of a pair of electrons from a highly electronegative atom by a hydrogen atom with a large positive partial charge. It is formed in cases where one molecule contains an atom with a lone pair of electrons and high electronegativity (F, O, N), and the other contains a hydrogen atom bound by a highly polar bond to one of such atoms. Examples of intermolecular hydrogen bonds:
H—O—H OH 2 , H—O—H NH 3 , H—O—H F—H, H—F H—F.
Intramolecular hydrogen bonds exist in polypeptide molecules, nucleic acids, proteins, etc.
A measure of the strength of any bond is the bond energy.
Communication energy- the energy required to break a given chemical bond in 1 mole of a substance. The unit of measurement is 1 kJ/mol.
The energies of ionic and covalent bonds are of the same order, energy hydrogen bond- an order of magnitude less.
The energy of a covalent bond depends on the size of the bonded atoms (bond length) and on the multiplicity of the bond. The smaller the atoms and the greater the bond multiplicity, the greater its energy.
The ionic bond energy depends on the size of the ions and their charges. The smaller the ions and the greater their charge, the greater the binding energy.
Structure of matter
According to the type of structure, all substances are divided into molecular And non-molecular. Among organic matter molecular substances predominate; among inorganic substances, non-molecular substances predominate.
Based on the type of chemical bond, substances are divided into substances with covalent bonds, substances with ionic bonds (ionic substances) and substances with metallic bonds (metals).
Substances with covalent bonds can be molecular or non-molecular. This significantly affects their physical properties.
Molecular substances consist of molecules connected to each other by weak intermolecular bonds, these include: H 2, O 2, N 2, Cl 2, Br 2, S 8, P 4 and others simple substances; CO 2, SO 2, N 2 O 5, H 2 O, HCl, HF, NH 3, CH 4, C 2 H 5 OH, organic polymers and many other substances. These substances do not have high strength, they have low temperatures melting and boiling, do not carry out electricity, some of them are soluble in water or other solvents.
Non-molecular substances with covalent bonds or atomic substances(diamond, graphite, Si, SiO 2, SiC and others) form very strong crystals (layered graphite is an exception), they are insoluble in water and other solvents, have high melting and boiling points, most of them do not conduct electric current (except graphite , which has electrical conductivity, and semiconductors - silicon, germanium, etc.)
All ionic substances are naturally non-molecular. These are solid, refractory substances, solutions and melts of which conduct electric current. Many of them are soluble in water. It should be noted that in ionic substances ah, the crystals of which consist of complex ions, there are also covalent bonds, for example: (Na +) 2 (SO 4 2-), (K +) 3 (PO 4 3-), (NH 4 +)(NO 3-) etc. The atoms that make up complex ions are connected by covalent bonds.
Metals (substances with metallic bonds) very diverse in their physical properties. Among them there are liquid (Hg), very soft (Na, K) and very hard metals(W, Nb).
The characteristic physical properties of metals are their high electrical conductivity (unlike semiconductors, it decreases with increasing temperature), high heat capacity and ductility (for pure metals).
In the solid state, almost all substances are composed of crystals. Based on the type of structure and type of chemical bond, crystals (“crystal lattices”) are divided into atomic(crystals are not molecular substances with a covalent bond), ionic(crystals of ionic substances), molecular(crystals of molecular substances with covalent bonds) and metal(crystals of substances with a metallic bond).
Tasks and tests on the topic "Topic 10. "Chemical bonding. Structure of matter."
- Types of chemical bond - Structure of matter grade 8–9
Lessons: 2 Assignments: 9 Tests: 1
- Assignments: 9 Tests: 1
Having worked through this topic, you should understand the following concepts: chemical bond, intermolecular bond, ionic bond, covalent bond, metal connection, hydrogen bond, simple connection, double bond, triple bond, multiple bonds, non-polar bond, polar bond, electronegativity, bond polarization, - and - bond, hybridization of atomic orbitals, bond energy.
You must know the classification of substances by type of structure, by type of chemical bond, the dependence of the properties of simple and complex substances on the type of chemical bond and the type of “crystal lattice”.
You must be able to: determine the type of chemical bond in a substance, the type of hybridization, draw up diagrams of bond formation, use the concept of electronegativity, a number of electronegativity; know how electronegativity changes chemical elements one period, and one group to determine the polarity of a covalent bond.
After making sure that everything you need has been learned, proceed to completing the tasks. We wish you success.
Recommended reading:
- O. S. Gabrielyan, G. G. Lysova. Chemistry 11th grade. M., Bustard, 2002.
- G. E. Rudzitis, F. G. Feldman. Chemistry 11th grade. M., Education, 2001.
Electronegativity is the ability of atoms to displace electrons in their direction when forming a chemical bond. This concept was introduced by the American chemist L. Pauling (1932). Electronegativity characterizes the ability of an atom of this element attract the common electron pair in a molecule. Electronegativity values determined different ways, differ from each other. IN educational practice most often they use not absolute ones, but relative values electronegativity. The most common is a scale in which the electronegativity of all elements is compared with the electronegativity of lithium, taken as one.
Among the elements of groups IA - VIIA:
The patterns of changes in electronegativity among d-block elements are much more complex.electronegativity with increasing atomic number, as a rule, increases in periods (“from left to right”), and in groups it decreases (“from top to bottom”).
These include: hydrogen, carbon, nitrogen, phosphorus, oxygen, sulfur, selenium, fluorine, chlorine, bromine and iodine. According to a number of characteristics, they are also classified as non-metals. standing group noble gases (helium-radon).Elements with high electronegativity, whose atoms have a high affinity for electrons and high energy ionization, i.e., prone to the addition of an electron or the displacement of a pair of bonding electrons in its direction, are called non-metals.
Most elements are metals Periodic table.
This set of physical properties that distinguish metals from non-metals is explained by the special type of bond that exists in metals. All metals have a clearly defined crystal lattice. Along with atoms, its nodes contain metal cations, i.e. atoms that have lost their electrons. These electrons form a socialized electron cloud, the so-called electron gas. These electrons are in the force field of many nuclei. This bond is called metallic. The free migration of electrons throughout the volume of the crystal determines the special physical properties of metals.Metals are characterized by low electronegativity, i.e., low ionization energy and electron affinity. Metal atoms either donate electrons to nonmetal atoms or mix pairs of bonding electrons from themselves. Metals have a characteristic luster, high electrical conductivity and good thermal conductivity. They are mostly durable and malleable.
Metals include all d and f elements. If from the Periodic Table you mentally select only blocks of s- and p-elements, i.e., elements of group A and draw a diagonal from the left top corner to the lower right corner, it turns out that non-metallic elements are located in right side from this diagonal, and metal ones - on the left. Adjacent to the diagonal are elements that cannot be unambiguously classified as either metals or non-metals. These intermediate elements include: boron, silicon, germanium, arsenic, antimony, selenium, polonium and astatine.
The concepts of covalent and ionic bonds played important role in the development of ideas about the structure of matter, but the creation of new physical and chemical methods research fine structure substances and their use have shown that the phenomenon of chemical bonding is much more complex. It is currently believed that any heteroatomic bond is both covalent and ionic, but in different proportions. Thus, the concept of covalent and ionic components of a heteroatomic bond is introduced. How more difference in the electronegativity of the bonding atoms, the greater the polarity of the bond. When the difference is more than two units, the ionic component is almost always predominant. Let's compare two oxides: sodium oxide Na 2 O and chlorine oxide (VII) Cl 2 O 7. In sodium oxide, the partial charge on the oxygen atom is -0.81, and in chlorine oxide -0.02. This effectively means that the Na-O bond is 81% ionic and 19% covalent. The ionic component of the Cl-O bond is only 2%.
List of used literature
- Popkov V. A., Puzakov S. A. general chemistry: textbook. - M.: GEOTAR-Media, 2010. - 976 pp.: ISBN 978-5-9704-1570-2. [With. 35-37]
- Volkov, A.I., Zharsky, I.M. Big chemical reference book/ A.I. Volkov, I.M. Zharsky. - Mn.: Modern school, 2005. - 608 with ISBN 985-6751-04-7.
Lecture for teachers
A chemical bond (hereinafter referred to as a bond) can be defined as the interaction of two or more atoms, as a result of which a chemically stable polyatomic microsystem (molecule, crystal, complex, etc.) is formed.
The doctrine of bonding occupies a central place in modern chemistry, since chemistry as such begins where the isolated atom ends and the molecule begins. In essence, all properties of substances are determined by the characteristics of the bonds in them. The main difference between a chemical bond and other types of interactions between atoms is that its formation is determined by a change in the state of the electrons in the molecule compared to the original atoms.
Communication theory should provide answers to a number of questions. Why are molecules formed? Why do some atoms interact while others do not? Why do atoms combine in certain ratios? Why are atoms arranged in a certain way in space? And finally, it is necessary to calculate the bond energy, its length and other quantitative characteristics. The correspondence of theoretical concepts to experimental data should be considered as a criterion for the truth of the theory.
There are two main methods for describing communication that allow you to answer the questions posed. These are the methods of valence bonds (BC) and molecular orbitals (MO). The first one is more visual and simple. The second is more strict and universal. Due to greater clarity, the focus here will be on the BC method.
Quantum mechanics allows us to describe the connection based on the most general laws. Although there are five types of bonds (covalent, ionic, metallic, hydrogen and intermolecular interaction bonds), the bond is uniform in nature, and the differences between its types are relative. The essence of communication is in Coulomb interaction, in the unity of opposites - attraction and repulsion. The division of communication into types and the difference in methods of describing it indicate not the diversity of communication, but rather the lack of knowledge about it at the present stage of development of science.
This lecture will cover topics such as chemical bond energy, the quantum mechanical model of covalent bonds, exchange and donor-acceptor mechanisms of covalent bond formation, atomic excitation, bond multiplicity, hybridization of atomic orbitals, electronegativity of elements and covalent bond polarity , concept of the molecular orbital method, chemical bonding in crystals.
Chemical bond energy
According to the principle of least energy, internal energy of a molecule compared to the sum of the internal energies of the atoms that form it should decrease. The internal energy of a molecule includes the sum of the interaction energies of each electron with each nucleus, each electron with each other electron, and each nucleus with each other nucleus. Attraction must prevail over repulsion.
The most important characteristic of a bond is energy, which determines its strength. A measure of the strength of a bond can be both the amount of energy spent on breaking it (bond dissociation energy) and the value that, when summed over all bonds, gives the energy of formation of a molecule from elementary atoms. The energy of breaking a bond is always positive. The energy of bond formation is the same in magnitude, but has a negative sign.
For a diatomic molecule, the binding energy is numerically equal to the energy of dissociation of the molecule into atoms and the energy of formation of the molecule from atoms. For example, the binding energy in a HBr molecule is equal to the amount of energy released in the process H + Br = HBr. It is obvious that the binding energy of HBr is greater than the amount of energy released during the formation of HBr from gaseous molecular hydrogen and liquid bromine:
1/2Н 2 (g.) + 1/2Вr 2 (l.) = НBr (g.),
on the energy value of evaporation of 1/2 mol Br 2 and on the energy value of decomposition of 1/2 mol H 2 and 1/2 mol Br 2 into free atoms.
Quantum mechanical model of covalent bonding using the valence bond method using the example of a hydrogen molecule
In 1927, the Schrödinger equation was solved for the hydrogen molecule by German physicists W. Heitler and F. London. This was the first successful attempt to apply quantum mechanics to solve communication problems. Their work laid the foundations for the method of valence bonds, or valence schemes (VS).
The calculation results can be presented graphically in the form of dependences of the interaction forces between atoms (Fig. 1, a) and the energy of the system (Fig. 1, b) on the distance between the nuclei of hydrogen atoms. We will place the nucleus of one of the hydrogen atoms at the origin of coordinates, and the nucleus of the second will be brought closer to the nucleus of the first hydrogen atom along the abscissa axis. If the electron spins are antiparallel, the attractive forces (see Fig. 1, a, curve I) and repulsive forces (curve II) will increase. The resultant of these forces is represented by curve III. At first, the forces of attraction predominate, then the forces of repulsion. When the distance between the nuclei becomes equal to r 0 = 0.074 nm, the attractive force is balanced by the repulsive force. The balance of forces corresponds to the minimum energy of the system (see Fig. 1, b, curve IV) and, therefore, the most stable state. The depth of the “potential well” represents the bond energy E 0 H–H in the H 2 molecule at absolute zero. It is 458 kJ/mol. However, at real temperatures, bond breaking requires slightly less energy E H–H, which at 298 K (25 ° C) is equal to 435 kJ/mol. The difference between these energies in the H2 molecule is the energy of vibrations of hydrogen atoms (E coll = E 0 H–H – E H–H = 458 – 435 = 23 kJ/mol).
Rice. 1. Dependence of the forces of interaction between atoms (a) and the energy of the system (b)
on the distance between the nuclei of atoms in the H 2 molecule
When two hydrogen atoms containing electrons with parallel spins approach each other, the energy of the system constantly increases (see Fig. 1, b, curve V) and a bond is not formed.
Thus, the quantum mechanical calculation provided a quantitative explanation of the connection. If a pair of electrons has opposite spins, the electrons move in the field of both nuclei. Between the nuclei there appears an area with high density electron cloud – redundant negative charge, which attracts positively charged nuclei. From the quantum mechanical calculation follow the provisions that are the basis of the VS method:
1. The reason for the connection is the electrostatic interaction of nuclei and electrons.
2. The bond is formed by an electron pair with antiparallel spins.
3. Bond saturation is due to the formation of electron pairs.
4. The strength of the connection is proportional to the degree of overlap of the electron clouds.
5. The directionality of the connection is due to the overlap of electron clouds in the region of maximum electron density.
Exchange mechanism of covalent bond formation using the BC method. Directionality and saturation of covalent bonds
One of the most important concepts of the BC method is valence. The numerical value of valence in the BC method is determined by the number of covalent bonds that an atom forms with other atoms.
The mechanism considered for the H2 molecule for the formation of a bond by a pair of electrons with antiparallel spins, which belonged to different atoms before the formation of the bond, is called exchange. If only the exchange mechanism is taken into account, the valence of an atom is determined by the number of its unpaired electrons.
For molecules more complex than H2, the principles of calculation remain unchanged. The formation of a bond is caused by the interaction of a pair of electrons with opposite spins, but with wave functions of the same sign, which are summed. The result of this is an increase in electron density in the region of overlapping electron clouds and contraction of nuclei. Let's look at examples.
In the fluorine molecule, the F2 bond is formed by 2p orbitals of fluorine atoms:
The highest density of the electron cloud is near the 2p orbital in the direction of the symmetry axis. If the unpaired electrons of fluorine atoms are in 2p x orbitals, the bond occurs in the direction of the x axis (Fig. 2). The 2p y and 2p z orbitals contain lone pairs of electrons that are not involved in the formation of bonds (shaded in Fig. 2). In what follows we will not depict such orbitals.
Rice. 2. Formation of the F 2 molecule
In the hydrogen fluoride molecule HF, the bond is formed by the 1s orbital of the hydrogen atom and the 2p x orbital of the fluorine atom:
The direction of the bond in this molecule is determined by the orientation of the 2px orbital of the fluorine atom (Fig. 3). The overlap occurs in the direction of the x axis of symmetry. Any other overlap option is energetically less favorable.
Rice. 3. Formation of the HF molecule
More complex d- and f-orbitals are also characterized by the directions of maximum electron density along their symmetry axes.
Thus, directionality is one of the main properties of a covalent bond.
The direction of the bond is well illustrated by the example of the hydrogen sulfide molecule H 2 S:
Since the symmetry axes of the valence 3p orbitals of the sulfur atom are mutually perpendicular, it should be expected that the H 2 S molecule should have a corner structure with an angle between the S–H bonds of 90° (Fig. 4). Indeed, the angle is close to the calculated one and is equal to 92°.
Rice. 4. Formation of the H 2 S molecule
Obviously, the number of covalent bonds cannot exceed the number of electron pairs forming the bonds. However, saturation as a property of a covalent bond also means that if an atom has a certain number of unpaired electrons, then all of them must participate in the formation of covalent bonds.
This property is explained by the principle of least energy. With each additional bond formed, additional energy is released. Therefore, all valence possibilities are fully realized.
Indeed, the stable molecule is H 2 S, not HS, where there is an unrealized bond (the unpaired electron is designated by a dot). Particles containing unpaired electrons are called free radicals. They are extremely reactive and react to form compounds containing saturated bonds.
Excitation of atoms
Let's consider the valence possibilities according to metabolic mechanism some elements of the 2nd and 3rd periods of the periodic table.
The beryllium atom at the outer quantum level contains two paired 2s electrons. There are no unpaired electrons, so beryllium must have zero valence. However, in compounds it is divalent. This can be explained by the excitation of the atom, which consists in the transition of one of the two 2s electrons to the 2p sublevel:
In this case, excitation energy E* is expended, corresponding to the difference between the energies of the 2p and 2s sublevels.
When a boron atom is excited, its valence increases from 1 to 3:
and the carbon atom has from 2 to 4:
At first glance, it may seem that excitation contradicts the principle of least energy. However, as a result of excitation, new, additional connections arise, due to which energy is released. If this additional energy released is greater than that expended on excitation, the principle of least energy is ultimately satisfied. For example, in a CH4 methane molecule, the average C–H bond energy is 413 kJ/mol. The energy expended for excitation is E* = 402 kJ/mol. The energy gain due to the formation of two additional bonds will be:
D E = E additional light – E* = 2,413 – 402 = 424 kJ/mol.
If the principle of least energy is not respected, i.e. E add.st.< Е*, то возбуждение не происходит. Так, энергетически невыгодным оказывается возбуждение атомов элементов 2-го периода за счет перехода электронов со второго на третий квантовый уровень.
For example, oxygen is only divalent for this reason. However, the electronic analogue of oxygen - sulfur - has greater valence capabilities, since the third quantum level has a 3d sublevel, and the energy difference between the 3s, 3p and 3d sublevels is incomparably smaller than between the second and third quantum levels of the oxygen atom:
For the same reason, the elements of the 3rd period - phosphorus and chlorine - exhibit variable valence, in contrast to their electronic analogues in the 2nd period - nitrogen and fluorine. Excitation to the appropriate sublevel can explain the formation chemical compounds elements of group VIIIa of the 3rd and subsequent periods. No chemical compounds were found in helium and neon (1st and 2nd periods), which have a completed external quantum level, and they are the only truly inert gases.
Donor-acceptor mechanism of covalent bond formation
A pair of electrons with antiparallel spins forming a bond can be obtained not only by the exchange mechanism, which involves the participation of electrons from both atoms, but also by another mechanism, called donor-acceptor: one atom (donor) provides a lone pair of electrons for the formation of the bond, and the other (acceptor) – vacant quantum cell:
The result for both mechanisms is the same. Often bond formation can be explained by both mechanisms. For example, an HF molecule can be obtained not only in the gas phase from atoms according to the exchange mechanism, as shown above (see Fig. 3), but also in an aqueous solution from H + and F – ions according to the donor-acceptor mechanism:
There is no doubt that molecules produced by different mechanisms are indistinguishable; connections are completely equivalent. Therefore, it is more correct not to distinguish the donor-acceptor interaction in special kind bond, but consider it only a special mechanism for the formation of a covalent bond.
When they want to emphasize the mechanism of bond formation precisely according to the donor-acceptor mechanism, it is denoted in structural formulas by an arrow from the donor to the acceptor (D® A). In other cases, such a connection is not isolated and is indicated by a dash, as in the exchange mechanism: D–A.
Bonds in the ammonium ion formed by the reaction: NH 3 + H + = NH 4 +,
are expressed by the following scheme:
The structural formula of NH 4 + can be represented as
.
The second form of notation is preferable, since it reflects the experimentally established equivalence of all four connections.
The formation of a chemical bond by the donor-acceptor mechanism expands the valence capabilities of atoms: valence is determined not only by the number of unpaired electrons, but also by the number of lone electron pairs and vacant quantum cells involved in the formation of bonds. So, in the example given, the valence of nitrogen is four.
The donor-acceptor mechanism is successfully used to describe the bonding in complex compounds using the BC method.
Multiplicity of communication. s- and p -Connections
The connection between two atoms can be carried out not only by one, but also by several electron pairs. It is the number of these electron pairs that determines the multiplicity in the BC method - one of the properties of a covalent bond. For example, in the ethane molecule C 2 H 6 the bond between the carbon atoms is single (single), in the ethylene molecule C 2 H 4 it is double, and in the acetylene molecule C 2 H 2 it is triple. Some characteristics of these molecules are given in table. 1.
Table 1
Changes in bond parameters between C atoms depending on its multiplicity
As the bond multiplicity increases, as one would expect, its length decreases. The bond multiplicity increases discretely, that is, by an integer number of times, therefore, if all bonds were the same, the energy would also increase by a corresponding number of times. However, as can be seen from table. 1, the binding energy increases less rapidly than the multiplicity. Consequently, the connections are unequal. This can be explained by differences in the geometric ways in which the orbitals overlap. Let's look at these differences.
A bond formed by overlapping electron clouds along an axis passing through the nuclei of atoms is called s-bond.
If the s-orbital is involved in the bond, then only s - connection (Fig. 5, a, b, c). This is where it got its name, since the Greek letter s is synonymous with the Latin s.
When the p-orbital (Fig. 5, b, d, e) and d-orbital (Fig. 5, c, e, f) participate in the formation of a bond, the s-type overlap occurs in the direction highest density electron clouds, which is the most energetically favorable. Therefore, when forming a connection, this method is always implemented first. Therefore, if the connection is single, then this is mandatory s - connection, if multiple, then one of the connections is certainly s-connection.
Rice. 5. Examples of s-bonds
However, from geometric considerations it is clear that between two atoms there can be only one s -connection. In multiple bonds, the second and third bonds must be formed by a different geometric method of overlapping electron clouds.
The bond formed by the overlap of electron clouds on either side of an axis passing through the nuclei of atoms is called p-bond. Examples p - connections are shown in Fig. 6. Such overlap is energetically less favorable than s -type. It is carried out by the peripheral parts of electron clouds with lower electron density. Increasing the multiplicity of the connection means the formation p -bonds that have lower energy compared to s - communication. This is the reason for the nonlinear increase in binding energy in comparison with the increase in multiplicity.
Rice. 6. Examples of p-bonds
Let's consider the formation of bonds in the N 2 molecule. As is known, molecular nitrogen is chemically very inert. The reason for this is the formation of a very strong NєN triple bond:
A diagram of the overlap of electron clouds is shown in Fig. 7. One of the bonds (2рх–2рх) is formed according to the s-type. The other two (2рz–2рz, 2рy–2рy) are p-type. In order not to clutter the figure, the image of the overlap of 2py clouds is shown separately (Fig. 7, b). To get the general picture, Fig. 7, a and 7, b should be combined.
At first glance it may seem that s -bond, limiting the approach of atoms, does not allow the orbitals to overlap p -type. However, the image of the orbital includes only a certain fraction (90%) of the electron cloud. The overlap occurs with a peripheral region located outside such an image. If we imagine orbitals that include a large fraction of the electron cloud (for example, 95%), then their overlap becomes obvious (see dashed lines in Fig. 7, a).
Rice. 7. Formation of the N 2 molecule
To be continued
V.I. Elfimov,
professor of Moscow
State Open University
Exclusively great importance in biological systems has a special type of intermolecular interaction, a hydrogen bond, which occurs between hydrogen atoms chemically combined in one molecule and the electronegative atoms F, O, N, Cl, S belonging to another molecule. The concept of "hydrogen bond" was first introduced in 1920 by Latimer and Rodebush to explain the properties of water and other associated substances. Let's look at some examples of such a connection.
In paragraph 5.2 we talked about the pyridine molecule and it was noted that the nitrogen atom in it has two outer electrons with antiparallel spins that do not participate in the formation of a chemical bond. This "free" or "lone" pair of electrons will attract the proton and form with it chemical bond. In this case, the pyridine molecule will go into an ionic state. If there are two pyridine molecules, they will compete to capture a proton, resulting in a compound
in which three dots indicate new type intermolecular interaction called hydrogen bonding. In this compound, the proton is closer to the left-handed nitrogen atom. With the same success, the proton may be closer to the right nitrogen atom. Therefore, the potential energy of a proton as a function of the distance to the right or left nitrogen atom at a fixed distance between them (approximately ) should be depicted by a curve with two minima. A quantum mechanical calculation of such a curve, carried out by Rhine and Harris, is shown in Fig. 4.
The quantum mechanical theory of the A-H...B hydrogen bond based on donor-acceptor interactions was one of the first to be developed by N. D. Sokolov. The reason for the bond is the redistribution of electron density between atoms A and B caused by the proton. Briefly, they say that a “lone pair” of electrons is shared. In fact, in
Rice. 4. Potential curve of proton energy as a function of the distance between the nitrogen atoms of two pyridine molecules.
Other electrons of molecules also participate in the formation of potential hydrogen bond curves, although to a lesser extent (see below).
Typical hydrogen bond energies range from 0.13 to 0.31 eV. It is an order of magnitude less than the energy of chemical covalent bonds, but an order of magnitude greater than the energy of van der Waals interactions.
The simplest intermolecular complex formed by hydrogen bonding is the complex. This complex has linear structure. The distance between fluorine atoms is 2.79 A. The distance between atoms in a polar molecule is 0.92 A. When a complex is formed, an energy of about 0.26 eV is released.
With the help of hydrogen bonding, a water dimer is formed with a binding energy of about 0.2 eV. This energy is approximately one-twentieth the energy of the OH covalent bond. The distance between two oxygen atoms in the complex is approximately 2.76 A. It is less than the sum of the van der Waals radii of oxygen atoms, equal to 3.06 A. In Fig. Figure 5 shows the change in the electron density of water atoms calculated in the work during the formation of the complex. These calculations confirm that when a complex is formed, the distribution of electron density around all atoms of the reacting molecules changes.
The role of all atoms in the establishment of hydrogen bonds in the complex can also be judged by the mutual influence of two hydrogen bonds between the nitrogenous bases, thymine and adenine, that are part of the double helix of the DNA molecule. The location of the minima of the proton potential curves in two bonds reflects their mutual correlation (Fig. 6).
Along with the usual or weak hydrogen bond formed by hydrogen with an energy release of less than 1 eV, and characterized by a potential energy with two minima, hydrogen forms some complexes with a large energy release. For example, when creating a complex, an energy of 2.17 eV is released. This type of interaction is called strong
Rice. 5. Change in electron density around atoms in a complex formed by hydrogen bonds from two water molecules.
Electron charge accepted equal to one. In a free water molecule, the charge of 10 electrons is distributed so that near the oxygen atom there is a charge of 8.64, and at the hydrogen atoms
Rice. 6. Hydrogen bonds between nitrogenous bases: a - thymine (T) and adenip (A), which are part of the DNN molecules (arrows indicate the places of attachment of the bases to the chains of sugar and phosphoric acid molecules); - potential hydrogen bond curves; O - oxygen; - hydrogen; - carbon; - nitrogen.
hydrogen bond. When complexes with strong hydrogen bonds are formed, the configuration of the molecules changes significantly. The proton's potential energy has one relatively flat minimum located approximately at the center of the bond. Therefore, the proton is easily displaced. Easy displacement of the proton under the influence external field determines the high polarizability of the complex.
Strong hydrogen bonding does not occur in biological systems. As for the weak hydrogen bond, it has crucial in all living organisms.
Exclusively big role hydrogen bonding in biological systems is primarily due to the fact that it determines secondary structure proteins, which are of fundamental importance for all life processes; with the help of hydrogen bonds, base pairs are held in DNA molecules and their stable structure as double helices, and finally, hydrogen bonding is responsible for the very unusual properties of water, which are important for the existence of living systems.
Water is one of the main components of all living things. Animal bodies are almost two-thirds water. The human embryo contains about 93% water during the first month. There would be no running water. Water serves as the main medium in which biochemical reactions occur in the cell. It forms the liquid part of blood and lymph. Water is necessary for digestion, since the breakdown of carbohydrates, proteins and fats occurs with the addition of water molecules. Water is released in the cell when proteins are built from amino acids. Physiological
Rice. 7. Ice structure. Each water molecule is connected by hydrogen bonds (three points) to four water molecules located at the vertices of the tetrahedron.
Rice. 8. Hydrogen bond in a dimer and “linear” hydrogen bond
properties of biopolymers and many supramolecular structures (in particular, cell membranes) depend very significantly on their interaction with water.
Let's look at some properties of water. Each water molecule has a large electrical torque. Due to the high electronegativity of oxygen atoms, a water molecule can form hydrogen bonds with one, two, three, or four other water molecules. The result is relatively stable dimers and other polymer complexes. On average, each molecule in liquid water has four neighbors. The composition and structure of intermolecular complexes depend on water temperature.
Crystalline water (ice) has the most ordered structure at normal pressure and temperature below zero degrees Celsius. Its crystals have a hexagonal structure. The unit cell contains four water molecules. The cell structure is shown in Fig. 7. Around the central oxygen atom there are vertices regular tetrahedron at distances of 2.76 A there are four other oxygen atoms. Each water molecule is connected to its neighbors by four hydrogen bonds. In this case, the angle between OH bonds in the molecule approaches the “tetrahedral” value of 109.1°. In a free molecule it is approximately 105°.
The structure of ice resembles that of diamond. However, in diamond, between the carbon atoms there are chemical forces. Diamond crystal is large molecule. Ice crystals are classified as molecular crystals. The molecules in a crystal retain essentially their individuality and hold each other together through hydrogen bonds.
Rice. 9. Experimental value shifts in the infrared vibration frequency in water during the formation of a hydrogen bond at an angle.
The ice lattice is very loose and contains many “voids”, since the number of nearest water molecules for each molecule (coordination number) is only four. When melting, the ice lattice is partially destroyed, at the same time some voids are filled and the density of water becomes more density ice. This is one of the main water anomalies. With further heating to 4° C, the compaction process continues. When heated above 4° C, the amplitude of anharmonic vibrations increases, the number of associated molecules in complexes (swarms) and the density of water decreases. According to rough estimates, the composition of swarms at room temperature includes about 240 molecules, at 37 ° C - about 150, at 45 and 100 ° C, respectively, 120 and 40.
The contribution of hydrogen bonding to the total energy of intermolecular interactions (11.6 kcal/mol) is about 69%. Due to hydrogen bonds, the melting points (0 ° C) and boiling points (100 ° C) of water differ significantly from the melting and boiling points of other molecular liquids, between the molecules of which only van der Waals forces act. For example, for methane these values are respectively -186 and -161° C.
In liquid water, along with the remnants of the tetrahedral structure of ice, there are linear and cyclic dimers and other complexes containing 3, 4, 5, 6 or more molecules. It is important that the angle P formed between the OH bond and the hydrogen bond changes depending on the number of molecules in the cycle (Fig. 8). In a dimer this angle is 110°, in a five-membered ring it is 10°, and in a six-membered ring and hexagonal ice structure it is close to a bullet (“linear” hydrogen bond).
It turns out that highest energy one hydrogen bond corresponds to the angle The energy of a hydrogen bond is proportional (Badger-Bauer rule) to the shift in the frequency of stretching infrared vibrations of the OH group in a water molecule but compared to the vibration frequency of a free molecule. The maximum displacement is observed in the case of a “linear” hydrogen bond. In a water molecule in this case, the frequency decreases by , and the frequency decreases by . In Fig. Figure 9 shows a graph of the displacement ratio
frequency to maximum offset from angle . Consequently, this graph also characterizes the dependence of the hydrogen bond energy on the angle . This dependence is a manifestation of the cooperative nature of the hydrogen bond.
Multiple attempts have been made to theoretically calculate the structure and properties of water, taking into account hydrogen bonds and other intermolecular interactions. According to statistical physics thermodynamic properties systems of interacting molecules located in volume V at constant pressure P in statistical equilibrium with a thermostat are determined through statistical sum states
Here V is the volume of the system; To - Boltzmann constant; T - absolute temperature; means that we need to take the trace of the statistical operator in curly brackets, where H is the quantum operator of the energy of the entire system. This operator equal to the sum operators of kinetic energy of translational and rotational movements molecules and operator potential energy interactions of all molecules.
If all the eigenfunctions and the full energy spectrum E of the operator H are known, then (6.2) takes the form
Then the Gibbs free energy G of the system at pressure P and temperature T is determined by the simple expression
Knowing the Gibbs free energy, we find the total energy entropy volume.
Unfortunately, due to complex nature interactions between molecules in water (anisotropic dipole molecules, hydrogen bonds leading to complexes of variable composition, in which the energy of hydrogen bonds itself depends on the composition and structure of the complex, etc.), we cannot write the operator H in explicit form. Therefore, we have to resort to very large simplifications. Thus, Nameti and Scheraga calculated the partition function based on the fact that only five energy states of molecules in complexes can be taken into account, according to
with the number of hydrogen bonds they form (0, 1, 2, 3, 4) with neighboring molecules. Using this model, they even managed to show that the density of water is maximum at 4° C. However, later the authors themselves criticized the theory they developed, since it did not describe many experimental facts. Other attempts at theoretical calculations of the structure of water can be found in the review by Ben-Naim and Stillinger.
Due to the dipole nature of water molecules and big role Hydrogen bonds also play an extremely important role in the interactions of water molecules with ions and neutral molecules in living organisms. Interactions leading to hydration of ions and special type interactions, called hydrophobic and hydrophilic, will be discussed in the following sections of this chapter."
Speaking about the role of water in biological phenomena, it should be noted that all living organisms have very successfully adapted to a certain amount of hydrogen bonding between molecules. This is evidenced by the fact that the replacement of heavy water molecules has a very significant effect on biological systems. The solubility of polar molecules decreases, the rate of passage decreases nerve impulse, the work of enzymes is disrupted, the growth of bacteria and fungi slows down, etc. Perhaps all these phenomena are associated with the fact that the hydrogen interaction between molecules is stronger than the interaction between Na molecules higher value hydrogen bond between heavy water molecules indicates the heat its melting point (3.8° C) and high heat of fusion (1.51 kcal/mol). For ordinary water, the heat of fusion is 1.43 kcal/mol.