Toward a Dynamic Lewis Notation

Because the presently used Lewis diagrams have their roots in static electron theories of the early twentieth century, this paper proposes substituting a double-arrow symbolism for the customary double dots. This means that students should be taught that a bond line (or stick as some call it) means a double arrow, not a double dot. Lone pairs are to be represented by double-curved arrows and later abbreviated to the old familiar double dot. There are obvious advantages to this new notation when atoms have different electronegativities. The double arrows can suggest the shifting of the bond pair equilibrium toward the most electronegative atom. The new notation is more compatible with VSEPR theory, and it seems to suggest the loci of largely imaginary MOs and AOs by the configuration of the arrows. With the help of some imagination, the new symbolism can clarify resonance in aromatic molecules and conjugated chains. The most radical suggestion is that resonance forms be abandoned in favor of conjugated blur bonds. The concept of blur bonds has the merit of being easily extendable to the explanation of metal bonding and electron conduction.     

The formation of three-center bonds in chemisorption

Summary It is proposed that molecular adsorption may be brought about by the formation of three-center bonds. In this two donor-acceptor bonds are formed between the molecule to be adsorbed and an atom in the adsorbent surface. The molecule to be adsorbed, for instance, ethylene, acts as a donor because of its -electron pair and the adsorbent atom because of its p-orbital. At the same time the adsorbent atom is a donor on account of its electron pair in a d_XZ-orbital and the molecule an acceptor on account of its antibonding ¹-²- orbital; by this a dative bond is formed. In the case of a metal without d-electrons (for instance, Al) a dative bond is impossible, but the metal can give an acceptor bond because of its empty p-orbital.Such a three-center chemisorption must proceed with a small activation energy, that is, be reversible, unactivated and rapid. Most often chemisorption at low temperature has these properties. The heat of such a chemisorption may vary between a few times ten or a few kcal.Three-center chemisorption may be of considerable importance in catalytic reactions and also in exchange processes on the surface.Only a considerable activation energy and a very high heat of adsorption is a reliable proof that the adsorbed substance occurs on the surface as atoms or in a state in which one adsorbent atom forms a stable chemical bond with only one adsorbate atom.Translated from Zhurnal Strukturnoi Khimii, Vol. 1. No. 2, pp. 189–199, July–August, 1960     

Chemical Bond Effects on Line Intensities in Arsenic X-Ray Emission Spectrum

This paper offers a critical analysis of chemical bond effects using X-ray fluorescent spectroscopy based on currently available equipment. Most chemical bond effects cannot be investigated with standard X-ray fluorescent spectrometers, primarily because of low spectral resolution. Therefore, we suggest an approach based on recording the intensity ratio of the last emission line (IR LEL) of the given X-ray series to the line of an electron transition between the core levels of the same atom. Using various chemical compounds of arsenic as an example, we investigated the dependence of the intensity ratio of the As K ₂ and As K ₁ lines (IR LEL of the arsenic K-series) on various structural parameters. An experimental study points to the existence of linear dependences of IR LEL on the radius of the first coordination sphere, the covalent radius of the nearest atom, and the concentration ratio between the elements of the corresponding phase, defining the deviation from stoichiometry. Using such dependences, one can determine, using X-ray data, the bond length of arsenic in various compounds on a low-resolution spectrometer.     

Experimental and theoretical study of the transannular intramolecular interaction and cage effect in the atrane framework of boratrane and 1-methylsilatrane

On the basis of the high-resolution single-crystal X-ray diffraction data and theoretical B3LYP/6-311+G(d) and B3LYP/cc-pVDZ calculations, the electron density distribution in boratrane and 1-methylsilatrane has been investigated. The B-N bond in the boratrane molecule was found to correspond to the "shared" (covalent) interatomic interaction in terms of Bader's "atoms in molecules" (AIM) theory. On the contrary, the Si...N bond in 1-methylsilatrane and B-N bond in a series of some acyclic donor-acceptor boron complexes and other boratrane derivatives correspond to interaction of an "intermediate" type. Such character of the B-N bond is caused by an influence of the atrane "cage effect". The influence of this effect results in deformation of the boron atom and displacement of the bond critical point (3,-1) to the region of the electron lone pair of the nitrogen atom. This changes the type of the interatomic interaction in comparison to other compounds investigated and explains high hydrolytic stability of boratrane as well as silatranes and germatranes.     

Conformational Analysis of Tris(3-methylphenyl)phosphine and Its Chalcogenides

Conformational analysis of tris(3-methylphenyl)phosphine and its chalcogenides has been performed by methods of dipole moments, IR spectroscopy, and quantum chemistry [DFT B3PW91/6-31G(df,p)]. In solution, these compounds exist as an equilibrium of conformers with gauche- and cis-arrangement of the substituents at the phosphorus atom relative to the P=X bond (X = lone electron pair, O, S, Se).     

Quantum-chemical investigation of nature of neutral intermolecular hydrogen bond in complex H3PO...HF

Results are presented from modeling the complex H₃PO...HF by the ab initio Hartree-Fock-Roothaan method. A generalized method is proposed for the quantitative fragmentary analysis of molecular orbitals (MOs). It is shown that when the complex is formed, the chemical bond between the phosphine oxide and the HF molecule is formed by means of a shift of the electron pair of the -bond of the HF to a 2p orbital of the O atom and the formation of a bonding three-center MO localized on the O, H, and F atoms, and also through a shift of an electron pair from the O atom to a 2p orbital of the F atom and the formation of a nonbonding MO localized on the O and F atoms.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 1, pp. 38–41, January–February, 1992.     

1,3-Thiazepines. 6. UV Spectroscopic and Theoretical Study of the Electronic Structures of 2-Aminotetrahydro- and 2-Iminotetrahydro-1,3-thiazepines

On the basis of studies of the electronic absorption spectra and quantum-chemical calculations of the energies and bond orders of a series of N-acetyl-N-aryl substituted 2-amino-4,5,6,7-tetrahydro-1,3-thiazepines and N-aryl substituted 2-iminohexahydro-1,3-thiazepines it has been concluded that the reason for the weak basicity of the ring nitrogen atom of the former is the acceptor properties of the amide carbonyl which obstructs the interaction of the unshared pair (USP) of this atom with the phenyl ring; in the case of the derivatives of hexahydroazepine the strengthening of the basic properties of the exocyclic nitrogen atom is associated with the conjugation of its USP with the -electrons of the benzene ring, which is strengthening by delocalisation of the USP of the sp³-hybridized ring nitrogen atom into the azomethine bond.     

Hydrogen complexes of <Emphasis Type="Italic">O</Emphasis>-vinylacetoxime with trifluoroacetic acid

According to B3LYP/6-31G** calculations, the stable antiperi-, antiperiperiplanar and antiperi-, synperiperiplanar conformers of O-vinylacetoxime (1) form with trifluoroacetic acid strong H complexes of two types: with N···H-O and O···H-O hydrogen bonds. The former are more stable. Complexation changes N-O and C-O bond lengths in molecule 1, as well as mutual orientation of its vinyl and azomethine groups. The structural effect depends on the orientation of the H bond, which, in its turn, is determined by the nature of the electron-donor center. When the nitrogen atom of oxime 1 is involved in complex formation, the H bond lies in the molecular plane, whereas the H bond involving the oxygen atom is directed in parallel with its lone electron pair.     

Electronic interaction between the nitrogen,oxygen, and silicon atoms in the molecules of α-aziridin-1-ylalkoxy(triethyl)silanes

By the PMR method we have established the existence of a high frequency of the inversion of the nitrogen atom in the molecules of -aziridin-1-ylalkoxysilanes. This is due to the fact that the high (because of p-d bonding with the silicon atom) electronegativity of the oxygen atom in the Si-O-C-N system makes possible an interaction between the unshared electron pair of the nitrogen atom and the antibonding orbital of the C-O bond. The latter, in its turn, increases the degree of p-d bonding between the oxygen and silicon atoms (in these compounds the order of the Si-O bond is greater than in the alkoxysilanes).     

Mechanism of the polarographic reduction of 6-pyridazones

The mechanism of the polarographic reduction of substituted 6-pyridazones in acidic media depends substantially on the nature of the substituent in the 3 position. One reduction wave due to the reduction of the >C=C< bond is observed in the case of electron-donor substituents, whereas, further electrical reduction involving the system of >C=N-N< bonds with cleavage of the >N-N< bond is also observed in the case of electron acceptors. Only one two-electron wave, which reflects reduction at the >C=C< bond, is observed in alkaline media. The half-wave potentials of 3-substituted 6-pyridazones in both acidic and alkaline media correlate linearly with the Hammett a constants. It is assumed that the transfer of the first electron in acidic media is realized at the quaternary nitrogen atom in the 2 position, whereas in alkaline media it is realized at the carbonyl group.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 668–673, May, 1977.     

Nature of weak inter- and intramolecular interactions in crystals. 1. The F...O and F...H contacts in the crystal of 2-trifluoroacetyl-5-trifluoromethylpyrrole

The topological analysis of the electron density distribution in the crystal of 2-trifluoroacetyl-5-trifluoromethylpyrrole revealed that the F...H and F...O intermolecular contacts correspond to attractive interactions. The energies of these interactions were estimated from the experimental data and it was shown that these contacts are similar to the C—H...O contacts. Analysis of the deformation electron density revealed that the F...O contacts correspond to transfer of the lone electron pair of a fluorine atom to the antibonding -orbital of the C=O bond.     

Moment characterization of localized orbitals

The parallel between orbital first and second electric moments and statistical first and second central moments is noted. Three measures of orbital spatial distribution in terms of their moments are proposed, and applied to the LMO's in a series of ten-electron hydrides. Consistent differences between bond and lone pair distributions are found. Using the statistical interpretation, for each LMO an effective solid angle around the central atom is postulated.     

取代基对N—H…O＝C氢键三聚体中氢键强度的影响

使用MP2方法研究了氢键三聚体中N-H…O=C氢键强度,探讨了氢键受体分子中不同取代基对N-H…O=C氢键强度的影响.研究表明,不同取代基对氢键三聚体中N-H…O=C氢键强度的影响是不同的:取代基为供电子基团,氢键键长r(H…O)缩短,氢键强度增强;取代基为吸电子基团,氢键键长r(H…O)伸长,氢键强度减弱.自然键轨道(NBO)分析表明,N-H…O=C氢键强度越强,氢键中氢原子的正电荷越多,氧原子的负电荷越多,质子供体和受体分子间的电荷转移越多.供电子基团使N-H…O=C氢键中氧原子的孤对电子n(O)对N-H的反键轨道σ~*(N-H)的二阶相互作用稳定化能增加,吸电子基团使这种二阶相互作用稳定化能减小.取代基对与其相近的N-H…O=C氢键影响更大.     

Intramolecular hydrogen bonding with the participation of the nitrogen atom of five- and six-membered heterocycles

A comparison of the frequencies of the valence vibrations of the OH group and of the chemical shifts of the protons of the hydroxyl groups in -naphthol derivatives containing the nitrogen atom of the condensed ring of pyridine, pyrazine, 1,2,5-selenadiazole, 1,2,5-thiadiazole, 1,2,5-oxadiazole, and imidazole in the peri position to the hydroxyl group is indicative of the decisive effect of the molecular geometry on intramolecular hydrogen bonding in systems with rigidly fixed configurations. All conditions being equal, the intramolecular hydrogen bond is considerably weaker when the nitrogen atom is part of a five-membered rather than a six-membered heterocycle. This is explained not only by an increase in the distance between the proton donor and acceptor (which may be the same in some cases), but also by the greater deviation of the orbital of the unshared electron pair of the nitrogen of the five-membered heterocycle from the O...N line and, thus, by its greater distance from the hydrogen atom. For the same favorable molecular geometry, the OH...N bond is stronger than the OH...O bond because of the high basicity of the nitrogen atom.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 238–244, February, 1971.     

Electron interaction in molecules

An idea of electron interaction in molecule has been applied to the SCF MO calculations of theπ-electronic structure of some complex aromatic hydrocarbons and their derivatives. The theoretical results for singlet and triplet transition energies, first ionization potentials and bond lengths agree fairly well with the experimental data. A correlation equation between the valence state ionization potential and the one center electron repulsion integral has been proposed. It has been shown that the electron repulsion in molecule is considerably smaller than in free atom. The present calculation shows that we can treat sulphur as a normal heteroatom analogous to oxygen and nitrogen.     

Gilbert N. Lewis and the chemical bond: the electron pair and the octet rule from 1916 to the present day

We describe the development of Lewis's ideas concerning the chemical bond and in particular the concept of the electron pair bond and the octet rule. We show that the concept of the electron pair bond has endured to the present day and is now understood to be a consequence of the Pauli principle. In contrast the octet rule is now regarded as much less important than was originally generally believed, although Lewis himself knew several exceptions and regarded it as less important than what he called the rule of two (the electron pair). The octet rule was more strongly promoted by Langmuir who is also responsible for the term covalent bond. However, many more exceptions to the octet rules than were known to Lewis are now known and the terms hypervalent and hypovalent used to describe such molecules are no longer particularly useful. Today it is realized that bonding electron pairs in many molecules are not as well localized as Lewis believed, nevertheless resonance structures, i.e., plausible alternative Lewis structures, are still often used to describe such molecules. Moreover electrons are not always found in pairs, as for example in linear molecules, which can, however, be satisfactorily described by Linnett's double quartet theory. The electron density distribution in a molecule can now be analyzed using the ELF and other functions of the electron density to show where electron pairs are most probably to be found in a molecule.     

Epoxidation of Alkenes by Peracids: From Textbook Mechanisms to a Quantum Mechanically Derived Curly-Arrow Depiction

Using the intrinsic bond orbital (IBO) analysis based on accurate quantum mechanical calculations of the reaction path for the epoxidation of propene using peroxyacetic acid, we find that the four commonly used curly arrows for representing this reaction mechanism are insufficient and that seven curly arrows are required as a result of changes to σ and π bonding interactions, which are usually neglected in all textbook curly arrow representations. The IBO method provides a convenient quantitative method for deriving curly arrows in a rational manner rather than the normal ad hoc representations used ubiquitously in teaching organic chemistry.     

An ab initio theoretical prediction: an antiaromatic ring pi-dihydrogen bond accompanied by two secondary interactions in a "wheel with a pair of pedals" shaped complex FH . . . C4H4 . . . HF

By the counterpoise-correlated potential energy surface method (interaction energy optimization), the structure of the pi H-bond complex FH cdots, three dots, centered FH . . . C4H4 . . . HF has been obtained at the second-order M?ller-Plesset perturbation theory (MP2/aug-cc-pVDZ) level. Intermolecular interaction energy of the complex is calculated to be -7.8 kcal/mol at the coupled-cluster theory with single, double substitutions and perturbatively linked triple excitations CCSD (T)/aug-cc-pVDZ level. The optimized structure is a "wheel with a pair of pedals" shaped (1mid R:1) structure in which both HF molecules almost lie on either vertical line passing through the middle-point of the C[Double Bond]C bond on either side of the horizontal plane of the C4 ring for cyclobutadiene. In the structure, an antiaromatic ring pi-dihydrogen bond is found, in which the proton acceptor is antiaromatic 4 electron and 4 center pi bond and the donors are both acidic H atoms of HF molecules. In accompanying with the pi-dihydrogen bond, two secondary interactions are exposed. The first is a repulsive interaction between an H atom of HF and a near pair of H atoms of C4H4 ring. The second is the double pi-type H bond between two lone pairs on a F atom and a far pair of H atoms.     

A General Procedure to Obtain Quantum Mechanical Charge and Bond Order Molecular Parameters

In an approach alternative to that of Mayer, a Hermitian operator is defined within the LCAO MO framework, which allows to obtain molecular charges and bond orders as expectation values of the first and second-order densities respectively. Such expectation values result to be nothing else than Mullikens atom and bond populations. Thus, Mulliken populations appear to be non arbitrary condensed electron density partitions, obtained according to quantum mechanical usual procedures for molecular one and two electron observables. The theoretical simplicity of the outlined procedure can be easily extended in order to obtain the expectation values for higher-order electronic chemical bonds.