Electrostatic interaction energies of independent aspherical atoms in molecules

Matter may be looked upon as consisting of superimposed independent atoms, which are spatially confined around specific positions by the chemical forces and which are weakly deformed thereby. Independent atoms with degenerate ground states are not fixed to be spherically symmetric as often supposed; it is more convenient for the interpretation of molecular and crystal charge distribution to refer to nonspherical atomic ground-state densities, the orientation of which is also determined by the chemical forces. In accordance with the virial theorem, one half of the quasiclassical electrostatic interaction energy, EE, of superimposed independent atoms is an approximation to the total bond energy, BE. The BE≈EE/2 correlation also improves if oriented nonspherical instead of spherically averaged atoms are superimposed. This correlation does not mean that the bond energy has been explained electrostatically, because one must know the atomic positions (and orientations) in advance. Furthermore, density deformations due to the quantum-mechanical interactions contribute 2–3 eV to BE.     

Diatomic interaction energies in the topological theory of atoms in molecules

Summary.  A partitioning of the ab initio total energy into one-center and two-center terms is proposed. The partitioning scheme is developed using the auxiliary function L˜(2, 1; 1, 2)=γ(2, 1)γ(1, 2) and the topological theory of atoms in molecules. It is shown that this scheme can be used at theoretical levels beyond Hartree–Fock. The numerical results indicate that the two-center terms follow the experimental trend of the dissociation energies for a series of related compounds. Received March 5, 1996/Final revision received August 19, 1996/Accepted August 29, 1996     

Diatomic interaction energies in the topological theory of atoms in molecules

Summary A partitioning of theab initio total energy into one-center and two-center terms is proposed. The partitioning scheme is developed using the auxiliary function (2, 1; 1, 2) = γ(2, 1)γ(1, 2) and the topological theory of atoms in molecules. It is shown that this scheme can be used at theoretical levels beyond Hartree-Fock. The numerical results indicate that the two-center terms follow the experimental trend of the dissociation energies for a series of related compounds.     

Remarkable features in the interactions of quadrupolar molecules

Quadrupolar charge fields of molecules and of molecular fragments give rise to unique features in weakly interacting clusters and aggregations. Relative to dipole-dipole interactions, the interactions among quadrupolar molecules tend to allow for greater orientational distortions away from equilibrium. Potential surface regions have been found for several clusters that are attractive and yet very flat for certain directions. There is a notable slipperiness for the interactions in some of these cases. This implies significant vibrational excursions even in the ground state. Furthermore, the coupling of rotations among nearby molecules in pure clusters of quadrupolar molecules is different than for dipolar species, and it can lead to unexpectedly small internal rotation barriers. How these and other features develop and what they might imply for materials and biomolecular simulations are discussed here.     

The effect of the temperature of Na-Y zeolite treatment with ethylenediaminetetraacetic acid on its structure

The variations in the structure of Na-Y zeolite after its dealumination with ethylenediaminetetraacetic acid at 293 and 373 K were studied by IR spectra of lattice vibrations. The resulting deformation of the terminal [O₃SiO](H,Na) tetrahedra is accompanied by the closure of the terminal Si-O(H,Na) groups with the formation of bridge bonds. The saturation of samples dealuminated at 373 K with water decreases the deformation of the terminal tetrahedra which recovers on heating. Long-term exposure to water vapor results in an irreversible decrease in local deformations and in lowering the excessive negative charge of the skeleton, which manifests itself in a general high-frequency shift of the bands by 5–7 cm^–1. These changes are more pronounced when the samples are heated in air at 873 K. The local structure deformations of the samples dealuminated at 293 K are irreversible. Heating them at 873 K causes the reorganization of the structure with the formation of a silica phase.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 655–659, April, 1993.     

Studies on the occurrence of atoms and molecules of aluminum, gallium, indium and their monohalides in an electrothermal carbon furnace

Molecular absorption spectra of monohalides (MX; X = F, Cl and Br) of aluminum, gallium and indium have been observed, where the monohalide species were produced in the electrothermal carbon furnace. In the spectra, some characteristic band structures have been identified for each metal halide. Generally, aluminum halides and/or metal fluorides provided clear and sharp band structures. The spectral bandwidth, background absorptions, and co-existing cations influenced the analytical features of molecular absorption spectrometry utilizing the monohalide bands. The time-dependent signal profiles of atoms and molecules at the stage of atomization in the carbon furnace have been observed for discussing the mechanisms of atomization and molecule formation of the Group III elements. Alkali and alkaline earth ions enhanced the molecular absorptions of monohalides, and transition metal ions reduced the background absorptions due to cutting the oxide. In addition, the application of the molecular absorptions of monohalides to the determination of trace halogens has also been investigated using the electrothermal vaporization technique.     

Second-order atomic Fukui indices from the electron-pair density in the framework of the atoms in molecules theory

Atomic Fukui indices, which are obtained from the electron density, have been previously shown to be useful in predicting which atoms in a molecule are most likely to suffer nucleophilic, electrophilic, or radicalary attacks. Here, we present a second-order generalization of these indices based on the electron pair density. We show how second-order atomic Fukui indices can be used to analyze the effects of electron loss or gain in several molecules from an electron pair point of view. Further, these indices also highlight which atoms or pairs of atoms are more likely to suffer nucleophilic, electrophilic, or radical attacks. In conclusion, second-order indices can complement first-order ones by affording relevant information on molecular reactivity from an electron pair perspective.     

Change in the state of aluminum in heat-and-water vapor dealumination of Y zeolite based on the IR spectrum of framework vibrations

The effect of heat treatment of dealuminated samples of Y zeolite in water vapors and in air on the state of the aluminum in the composition of six-coordinated compounds, Al^VI, O, was investigated by IR Fourier spectroscopy. The existence of at least two crystal chemically different Al^VI, O compounds which act differently to the effect of acids: weakly bound with the framework — out-of-framework, and covalently bound with the framework oxygen — in-framework, and the ratio between them changes with an increase in the temperature of treatment in water vapors toward an increase in the concentration of out-of-framework aluminum compounds, and in heat treatment in air, toward in-framework aluminum compounds.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1299–1303, June, 1991.     

Use of electron scattering data in calculating the exchange interaction of excited atoms with molecules

Theoretical and Experimental Chemistry -     

EPR and DFT study on the stabilization of radiation-generated methyl radicals in dehydrated Na-A zeolite

Electron paramagnetic resonance (EPR) spectroscopy was applied to study paramagnetic species stabilized in Na-A zeolite exposed to gaseous methane and gamma-irradiated at 77 K. Two types of EPR spectra were recorded during thermal annealing of zeolite up to room temperature. Owing to the results for the zeolite exposed to (13)CH(4) the multiplet observed at 110 K was assigned to a (.-)CH(3)...Na(+) complex. After decay of the multiplet, the isotropic quartet of methyl radical was recorded in the temperature range of 170-280 K. On the basis of the EPR parameters it is postulated that (.-)CH(3) radicals in this temperature region are able to freely rotate inside the zeolite cage. The structures of the (.-)CH(3)...Na(+) adsorption complex and respective hyperfine coupling constants were calculated by applying DFT quantum chemical methods. Two different models were applied to represent the zeolite framework: the 6T structure of one six-membered ring and the 3T cluster. The hyperfine coupling constants calculated for the (.-)CH(3)...Na(+) adsorption complex for both applied models show very good agreement with those obtained experimentally.     

Repulsive potentials for the interaction of oxygen atoms with the noble gases and atmospheric molecules

Repulsive potentials between about 0.2 eV and 20 eV are determined for interactions of oxygen atoms with the noble gas atoms and with H₂, N₂, O₂, NO, CO, CO₂ and H₂O molecules from incomplete total scattering cross sections. Good agreement with theory is obtained for the noble gases. The potentials for the interaction with molecules obtained by assuming spherically symmetric potentials are compared with the same potentials computed by averaging the cross sections for different orientations of the molecules. By introducing reduced variables, the problem of finding two isotropic potential parameters is simplified to a search for one, the other being determined by a least-squares constraint.     

Properties of the halogen-hydride interaction: an ab initio and "atoms in molecules" analysis

X-Cl...H-Y interactions are analyzed by applying ab initio methods as well as the Bader theory. All calculations were performed using Pople's basis sets (6-311++G(2df,2pd) and 6-311++G(3df,3pd)) as well as the Dunning-type bases (aug-cc-pVDZ and aug-cc-pVTZ) within the MP2 method. For the complexes analyzed here, X-Cl and H-Y may be treated as a Lewis acid and a Lewis base, respectively. The Cl...H interactions are rather weak or at most moderate since, for the strongest interaction of the F3...HLi complex, the binding energy calculated at the MP2/6-311++G(3df,3pd) level of approximation amounts to -3.4 kcal/mol, and the H...Cl distance is equal to 2.65 A, less than the corresponding sum of van der Waals radii. These interactions may be classified as halogen-hydride interactions. However, some of the complexes analyzed, especially F3SiCl...HBeF and F3SiCl...HBeF, are very weakly bound, probably by typical van der Waals interactions.     

Influence of various aluminum distributions on modification of ZSM-5 zeolite framework with cobalt ions in alkane catalytic cracking

Research on Chemical Intermediates - HZSM-5 (HZ(1) and HZ(2)) zeolites synthesized with various framework aluminum distributions and also cobalt-modified H-form zeolites (Co/HZ(1) and Co/HZ(2))...     

Theoretical study of the reaction of H atoms with vibrationally highly excited HF molecules

Vibrationally highly excited molecules react extremely fast with atoms and probably with radicals. The phenomenon can be utilized for selectively enhancing the rate of reactions of specific bonds. On the basis of quasiclassical trajectory calculations, the paper analyzes mechanistic details of a prototype reaction, H + HF(v). At vibrational quantum numbers v above 2, the reaction exhibits capture-type behavior, that is, the reactive cross section diverges as the relative translational energy of the partners decreases, both for the abstraction and for the exchange channel. The mechanism of the reaction for both channels is different at low and at high translational energy. At low vibrational energy, the reaction is activated, which is switched to capture-type at high excitation. The reason is an attractive potential that acts on the attacking H atom when the HF molecule is stretched. In contrast to the 6-SEC potential surface of Mielke et al., the switch cannot be observed on the Stark-Werner potential surface, due to a small artificial barrier at high H-HF separation, preventing the reactants from obeying the attractive potential and also proving the importance of the latter. The exchange reaction can be observed even when the total energy available for the partners is below the exchange barrier, because at low translational energies the product F atom of a successful abstraction step can re-abstract that H atom from the intermediate product H2 molecule that was originally the attacker.     

EPR study of the mechanism of interaction of NO and NO2 molecules with zeolite surfaces

The adsorption of the paramagnetic molecules of NO and NO₂ by zeolites in the alkali and alkaline earth cationic forms has been studied by EPR and reflectance spectroscopic methods. The change in the EPR spectra of adsorbed nitric oxide with increase in the degree of covering of the surface of the alkali cationic form of the zeolites, and also the nature of the change in the spectra when oxygen is adsorbed on zeolites on which NO has previously been adsorbed, indicate the existence of two types of adsorption center. At low degrees of covering of the surface, on the order of 10¹⁸ g^–1, as can be judged from the EPR spectra, the adsorbed NO molecule is strongly polarized and the unpaired electron is almost completely localized on the oxygen atom. At high degrees of covering, for an appreciable proportion of the NO molecules, the bond with the surface is weaker. In this case, the EPR spectra show a hyperfine structure (HFS) with a constant which changes with change in the cation in the order Li⁺ Na⁺ K⁺. The replacement of the singly charged Na⁺ by the doubly charged Ca²⁺ produces a marked change in the adsorption properties of the zeolite. The adsorption of NO on CaA leads not only to polarization of the adsorbed molecule but also to transfer of the electron from the nitrogen atom to the atoms of the adsorbent; this is recorded in the EPR spectrum in the form of an F-center. On further adsorption, the NO molecules are adsorbed both on the nitrogen atom and on the oxygen atom of the first molecule; thus, NO₂ and N₂O are formed.     

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).