Calculation of molecular energy indices by the bond interaction method

Conclusions Parameters were determined for an interacting bond model for hydrocarbons and oxygen- and sulfur-containing organic molecules. The parameters obtained were used to calculate the homolytic bond dissociation energies and molecular formation energies.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1680–1683, July, 1988.     

The structure and stability of some charged fragments derived from methanol: an indo mo study

INDO geometry searches on nine fragment ions derived from methanol are reported. The correct energy ordering is found, but only if full geometry optimization is performed. Optimum geometries are similar, though not identical, to those of ab initio calculations, where these are available for comparison. The effects of molecular relaxation on the energy curves for CH and OH bond stretching in CH₃OH⁺ is also discussed.     

取代乙烯顺反异构体键能和稳定性的密度泛函理论研究

对十甲种取代乙烯（XX’CCYY’）顺反异构体进行了DFT-B3LYP/6-31G（d)水 平的几何全优化的总能量、总键能的计算。结果表明，对无共轭大π键的二取代乙 烯，其顺式异构体的总能量比反式异构体的低，前沿轨道能隙、总键能和总π键能 均比反式异构体的大，而对有共轭大π键的体系，其情况则刚好相反。表明对于前 者其顺式异构体比反式异构体稳定，而对于后者则反式异构体比顺式异构体稳定。     

Calculations of electronic structure of molecules and complexes by the modified horn-murrel method

We describe the characteristic features of the VAI and VSAI calculation schemes, designed for nonempirical valence-basis calculations of the electronic structure of molecules and complexes and carried out using the program KOMPLEKS-2. The calculation results presented demonstrate good agreement with all-electron calculations. We show that the approximate VSAI method reproduces sufficiently satisfactorily the Hartree-Fock-Roothaan calculations.Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 25, No. 4, pp. 427–432, July–August, 1989.     

Estimation of hydrogen bond energy in n-alkanols from the gas chromatography data

The retention factors of C1—C9 n-alkanols on a capillary column with the SE-30 nonpolar phase were determined for temperatures between 320 and 400 K. From the temperature dependence of the free energy change, variations in the enthalpy of sorption of n-alkanols on the SE-30 polymethylsiloxane phase were calculated. The difference between the enthalpies of condensation and sorption was used to estimate the contribution of hydrogen bonding to self-association of pure liquid alcohols. The calculated energies of hydrogen bonds in liquid n-alkanols are in good agreement with the published data.     

Investigation of the electronic structure and the structural stability of selected penicillins by density functional calculations of 14N nuclear quadrupole resonance parameters

A computational study was carried out by density functional theory (DFT) to investigate the relative stability and reactivity in three selected penicillins: penicillin-G, penicillin-V and carbenicillin. The geometry of the investigated molecules was optimized at the B3LYP/6-31G(d) level of theory. Then, the nuclear quadrupole resonance (NQR) parameters of ¹⁴N and ²H nuclei and natural bond orbital (NBO) analysis in these molecules were calculated on the geometrically optimized models at the B3LYP level using 6-311++G(d,p) basis sets in the gas phase. The NBO analysis shows that the occupancy of the LP(N) decreases with increasing p character of the lone pair of nitrogen. A comparison between the results obtained for these penicillins and related 6-APA structures of them indicates that the presence of a bulky side group in the acyl side chain can lead to more stability of the β-lactam ring. On the other hand, NBO analysis was applied to rationalize the ¹⁴N NQR parameters in the charge distribution around nitrogen atoms. Inspection of the present results illustrates that the largest component of EFG tensor (q _zz ), the nuclear quadrupole coupling constant, C _Q, and the NQR frequency values of nitrogens decrease with decreasing occupancy values of LP(N). We suggest that the reason for this trend can be found in increasing contribution of delocalized electrons of nitrogen in the intramolecular interactions and hence stability of these structures increases in the order: PG < PV < CA. Finally, a good relationship is found between most of the calculated ²H NQR parameters and the related intramolecular hydrogen bonds.     

Effect of the intramolecular hydrogen bond on the electronic structure of organic molecules with a planar quasicycle

The electronic structure of the following organic molecules is studied using the HF/6-311G(d,p) method: malonic dialdehyde, acetylacetone, thiomalonic aldehyde, 2-XC₆H₄NH₂ aniline derivatives, 2-XC₆H₄OH phenol derivatives, 2-XC₆H₄SH thiophenol derivatives (X = CHO, COOH, COO^?, NO, NO₂, OH, OCH₃, SH, SCH₃, F, Cl, Br), 8-hydroxyquinoline, 8-mercaptoquinoline, tropolone. It is found that the intramolecular hydrogen bond (IHB) leads to a local electronic redistribution in the quasi-cycle, and above all to the electron density transfer among the immediate participants of IHB — from the hydrogen atom to the proton-acceptor atom. When the IHB of the S-H?O type forms, the electron density mainly decreases on sulfhydryl hydrogen atom and increases on sulfur atom.     

Molecular and electronic structure,and stability of organic cations,dications and donor-acceptor complexes

The MNDO method with optimization of geometric parameters has been used to study the fragments of the potential energy surface for the acylium cation MeC⁺, radical dication MeCO^2+., and the (1 n) complexes of general formula MeCOCl ·nAlCl₃ (wheren=1, 2, 3). The stability of these complexes with respect to certain dissociation mechanisms has been studied. The calculations show that the the hitherto unknown RCOCl · 2AlCl₃ and RCOCl · 3AlCl₃ can actually exist. Moreover, it has been shown that there can be complexes where the charge on the strongly electrophilic carbon atom is greater than that of the carbon in the acylium cation. The result of the study provided an explanation for the superacidity of the RCOX · 2AlX₃ systems in reactions with saturated hydrocarbons.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 854–858, May, 1993.     

A density functional study of the electronic structure and spin Hamiltonian parameters of mononuclear thiomolybdenyl complexes

The electron paramagnetic resonance spin Hamiltonian parameters of mononuclear thiomolybdenyl complexes based upon the tris(pyrazolyl)borate ligand, together with their molybdenyl analogues, are calculated using density functional theory. The electronic g matrix and 95Mo hyperfine matrix are calculated as second-order response properties from the coupled-perturbed Kohn-Sham equations. The scalar relativistic zero-order regular approximation (ZORA) is used with an all-electron basis and an accurate mean-field spin-orbit operator which includes all one- and two-electron terms. The principal values and relative orientations of the g and A interaction matrices obtained from the experimental spectra in a previous EPR study are compared with those obtained from unrestricted Kohn-Sham calculations at the BP86 and B3LYP level, and the latter are found to be in good quantitative agreement. A quasi-restricted approach is used to analyze the influence of the various molecular orbitals on g and A. In all complexes the ground state magnetic orbital is dX2-Y2-based and the orientation of the A matrix is directly related to the orientation of this orbital. The largest single contribution to the orientation of the g matrix arises from the spin-orbit coupling of the dYZ-based lowest-unoccupied molecular orbital into the ground state. A number of smaller, cumulative charge-transfer contributions augment the d-d contributions. A comparison of the theoretical EPR parameters obtained using both crystallographic and gas-phase geometry-optimized structures of Tp*MoO(bdt) (Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate, bdt = 1,2-benzenedithiolate) suggests a correspondence between the metal-dithiolate fold angle and the angle of noncoincidence between g and A.     

Electronic structure of free and doped actinides: N and Z dependences of energy levels and electronic structure parameters

Theoretical study of electronic structure of antinide ions and its dependence on N and Z are presented in this paper. The main 5f^N and excited 5f^Nn′l′^N′ configurations of actinides have been studied using Hartree-Fock-Pauli approximation. Results of calculations of radial integrals and the energy of X-ray lines for all 5f ions with electronic state AC⁺¹−AC⁺⁴ show approximate dependence on N and Z. A square of N and cubic of Z are ewalized for the primary electronic parameters of the actinides. Theoretical values of radial integrals for free actinides and for ions in a cluster AC⁺ⁿ:[L]_k are compared, too.     

Theoretical investigation on electronic structure and stability of some inverted cucurbiturils by density functional theory

Structure and stability of diastereoisomers of cucurbit[n]urils (CB[n = 5–10]), the inverted CB[n]s, were investigated by density functional theory (DFT) computations. All the inverted CB[n]s were less stable than their normal CB[n]s and the mono-inverted ones with one inverted glycoluril unit in their structures were more stable than their doubly-inverted isomers. Relative change in dipole moments and molecular electrostatic potentials (MEP) were discussed with the deformation in geometric structure and charge distribution of the normal and inverted CB[n]s.     

A method to calculate the energy levels of nanoobjects with a periodic framework structure

A method to calculate electronic and vibrational energy levels in nanoobjects with a periodic internal structure is proposed. The approach allows us to reduce the general problem to a set of order problems corresponding to the repeating atomic arrangement. Further refinement is performed using perturbation theory.     

The magnetic and electronic structure of vanadyl pyrophosphate from density functional theory

We have studied the magnetic structure of the high symmetry vanadyl pyrophosphate ((VO)(2)P(2)O(7), VOPO), focusing on the spin exchange couplings, using density functional theory (B3LYP) with the full three-dimensional periodicity. VOPO involves four distinct spin couplings: two larger couplings exist along the chain direction (a-axis), which we predict to be antiferromagnetic, J(OPO) = -156.8 K and J(O) = -68.6 K, and two weaker couplings appear along the c (between two layers) and b directions (between two chains in the same layer), which we calculate to be ferromagnetic, J(layer) = 19.2 K and J(chain) = 2.8 K. Based on the local density of states and the response of spin couplings to varying the cell parameter a, we found that J(OPO) originates from a super-exchange interaction through the bridging -O-P-O- unit. In contrast, J(O) results from a direct overlap of 3d(x(2)-y(2)) orbitals on two vanadium atoms in the same V(2)O(8) motif, making it very sensitive to structural fluctuations. Based on the variations in V-O bond length as a function of strain along a, we found that the V-O bonds of V-(OPO)(2)-V are covalent and rigid, whereas the bonds of V-(O)(2)-V are fragile and dative. These distinctions suggest that compression along the a-axis would have a dramatic impact on J(O), changing the magnetic structure and spin gap of VOPO. This result also suggests that assuming J(O) to be a constant over the range of 2-300 K whilst fitting couplings to the experimental magnetic susceptibility is an invalid method. Regarding its role as a catalyst, the bonding pattern suggests that O(2) can penetrate beyond the top layers of the VOPO surface, converting multiple V atoms from the +4 to +5 oxidation state, which seems crucial to explain the deep oxidation of n-butane to maleic anhydride.     

Harmonic force fields from molecular energy calculations : Tests of a method using indo and MINDO/2 semiempirical molecular energies

A numerically stable method for simultaneous calculation of force constants and coupling constants from energy data has been designed and tested. It has been used to obtain harmonic force fields for a set of small molecules from INDO and MINDO/2 calculations.