MNDO calculations of systems containing hydrogen bonds

A modified MNDO method, which can be used in the studies on structures with hydrogen bonds X-H-X, X, X = N, O, is described. Results for a wide range of molecular complexes are reported. Energies of hydrogen bonds are reproduced with useful accuracy. The modified MNDO seems to give more reliable values of hydrogen bond energies and barrier heights of proton transfers than 4-31G ab initio model.     

Ground states of molecules. 53.MNDO calculations for molecules containing chlorine

MNDO calculations of heats of formation, dipole moments, ionization potentials, and structures are reported for a wide range of compounds containing chlorine in its characteristic valence state (Cl^I) and one or more of the elements H, B, Be, C, N, O, and F. The calculated errors in the heats of formation and the dipole moments are not significantly greater than those previously reported for compounds containing no chlorine. First vertical ionization potentials were on average 0.95 eV too high. The ordering of higher cationic states was found to be correct, even for species such as Cl₂O, Cl₂, and HOCl, where ab initio–Koopmans' theorem calculations predict the incorrect ordering. The calculated energies and geometries of compounds such as CIF₃ are qualitatively incorrect, probably because of the lack of 3d atomic orbitals in the orbital basis set.     

Ground states of molecules. 56. MNDO calculations for molecules containing sulfur

MNDO has been extended to sulfur, but without inclusion of 3d AO s. Calculations are reported for heats of formation, geometries, dipole moments, and ionization energies of a variety of sulfur-containing molecules. The average discrepancy between calculated and observed heats of formation is larger than for compounds of other elements, a difference probably due, at least partly, to the lower accuracy of the thermochemical data for sulfur compounds. The calculated dipole moments agree well with experiment as do the calculated ionization energies, except for those corresponding to ionization from sulfur “lone-pair” orbitals which are too high by ca. 1 eV, probably as a result of the neglect in NDDO of interactions between inner and valence shell orbitals. As in the case of other third-period elements, the calculated heats of formation of compounds of sulfur in its higher valence states (S^IV, S^VI) were too positive by large amounts, due presumably to the neglect of 3d AO s.     

Studies on organophosphorus compounds 52.Structure-reactivity studies of organophosphorus compounds by MNDO calculations

MNDO and MM2(85)methods were used to study the conformation and the structure-reactivity relationship of neutral and acidic phosphorus esters.The calculation results indicate thatfor the most stable conformation,the charge density of phosphoryl oxygen(q_O)is determined notonly by the electronegativity of the substituents,but also by the conformation of the alkoxyl groupson the phosphorus atom.Meanwhile,the conformation of the alkoxyl group provides,as a rule,more important influence on the charge density of the phosphoryl oxygen.However,the energy ofthe highest occupied molecular orbital(EHOMO)is basically dependent on the eletronegativity of thesubstituents,while the donating ability or the withdrawing ability of the neutral phosphorus com-pounds is mainly governed by the EHOMO but not the q_O.This is also true for other kinds of theneutral oxygen-containing compounds.     

MNDO calculations on diazirines

The MNDO molecular orbital method gives good agreement between experimental geometries and, when available, heats of formation of diazirines. MNDO and ab initio calculations have been performed on a novel intermediate, methylenediazirine, proposed in a reaction of 3-chloro-3-methyl-diazirine. The relation of this intermediate to its diazo-isomer, its dimer and the products of the reaction are discussed on the basis of MNDO calculations.     

MNDO method for calculations of magnesium clusters

A new set of parameters for the magnesium atom has been developed within the MNDO method. In contrast to previously published parameters, the new parameters correctly describe molecules with different chemical natures: magnesium halides, organomagnesium compounds and the recently found small magnesium clusters Mg _n (n=2-8). The average errors in the calculated heats of formation and bond lengths of magnesium compounds, including clusters are: 10.7 kcal/mol and 0.167 Å, respectively.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1384–1388, August, 1994.     

Ground states of molecules. 67.MNDO Calculations for compounds containing iodine

MNDO has been parametrized for iodine. Since d atomic orbitals (AOS ) are not included, the calculations are restricted to I^I. Heats of formation, molecular geometries, ionization energies, and dipole moments are reproduced with useful accuracy.     

Ground states of molecules. 64.MNDO Calculations for compounds containing bromine

MNDO has been parametrized for bromine. Since d atomic orbitals (AOS) are not included, the calculations are restricted to Br^I. Heats of formation, molecular geometries, ionization energies, and dioole moments are reproduced with useful accuracy.     

Modification of siloxane coatings with boron and aluminum compounds

Modification of siloxane coatings of protective materials with boron (boric acid, boric anhydride, triethyl borate) and aluminum (aluminum butylate, oxide, hydroxide) compounds was studied with the aim to enhance the fireproofing and physicomechanical properties of the materials.     

Molecular mechanics calculations of 10-vertex boron cage compounds

The model proposed earlier for molecular mechanics calculations of 7- and 12-vertex boranes, carboranes, and metallocarboranes has been extended to the case of 10-vertex borane cage compounds. To use the MM3 program with the standard connectivity file, and to avoid program alterations, the 10-vertex cages of the molecules were presented as a superposition of four formally independent fragments. Interactions between the fragments were described with a Hill-like potential, with the parameters adjusted for valence interactions. Standard values for the bond lengths and bond angles in the 10-vertex boron cage have been found by statistical analysis of X-ray data on borane cage compounds stored in the Cambridge Structural Database. Several substituted neutral molecules and anions have been considered, and good agreement of the calculated and experimental data has been obtained. Using the approach developed, the unknown structure of the [mu-B20H16O(CH2)4O(CH2)2CH(CH3)2]3- ion has been calculated.     

Crown compounds for anions. MNDO calculations of complexes of halide anions with cyclic pentameric difluoromethylidenemercury

Molecular and electronic structures of complexes of halide anions with cyclic pentameric difluoromethylidenemercury [CF₂Hg]₅ have been studied by the MNDO method. Calculations have been performed for [L-X]^– halfsandwich complexes and bipyramidal complexes [X-L-X]^2– having a shape of a spinning top (L is the mercury-containing macrocycle; X = F, Cl, Br, or 1). It has been shown that in complexes of both types, halide anions are bonded to the mercury-containing macrocycle via three generalized chemical bonds: one headlightshaped -bond and two two-lobe n-bonds. The complexes studied have been compared with the analogous complexes of the macrocycles containing three mercury atoms. The similarity and differences in the character of the generalized chemical bonds in relation to the size of the cycle have been considered.V. B. Shur, I. A. Tikhonova, F. M. Dolgushin, A. I. Yanovsky, Yu. T. Struchkov, A. Yu. Volkonsky, and E. V. Solodova, Unpublished results.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 44–48, January, 1996.     

NMR spectra of organogermanium compounds: 7. NMR spectra and MNDO calculations of phenylgermacyclohexanes

¹³C nmr spectra of a variety of 1-phenylgermacyclohexanes indicates that the conformational energy of a phenyl group on germanium is essentially zero (0 – 0.1 kcal/mol), in a sharp contrast with that in phenylcyclohexanes, The results are consistent with the heats of formation of phenylgermacyclohexanes calculated by the MNDO method.     

Ab initio electric field gradient calculations for a series of oxygen-containing boron compounds

Ab initio Hartree-Fock calculations of the electric field gradient tensor at the double-zeta level are reported for orthoboric acid, orthoboric acid dimer, the tetraborate anion and metaboric acid. For orthoboric acid, Cl calculations with an extended basis set have been performed to investigate polarisation and correlation effects. Calculations at the double-zeta level show the best agreement with experiment.     

MNDO calculations of bond strengths in alkanes

MNDO method was used to calculate 22 values of C–H bond strengths in alkanes of various structures. Results of calculation were shown to be in good agreement with the experiment. The average absolute error of calculation (without tert-butyl and cyclobutyl groups) was 1.3 kcal/mol and was comparable with experimental error of C–H bond strength determination.     

Reactions of perchlorofluoro compounds VIII. A structure-reactivity relationship of perchlorofluoroolefins based on MNDO calculations

The electronic structures of 17 perchlorofiuoroolefins and perfluoroolefins were calculated using the MNDO method. Based on these calculations combined with the perturbation theory, a good structure-reactivity relationship of perchlorofluoroolefins has been established. In the case of internal olefins, the direction of nucleophilic attack is governed not only by the perturbation energy of the ground state, but also by the stability of the anionic intermediate and the activation energy of the reaction.     

MNDO calculations of systems with hydrogen bonds S-H

The MNDO method has been modified for the calculation of the properties of complexes with hydrogen bonds S-H-X, X = N, O, F, S. The results obtained are in good agreement with the experimental data.     

MNDO calculations on hydrogen bonds. Modified function for core-core repulsion

The MNDO method has been modified to calculate the properties of the structures involving hydrogen bonds X···H—Y, X, Y = N, O and F. A new method (referred to as MNDO/H) has been tested by calculation of a wide range of molecular systems with weak and strong (ionic) hydrogen bonds. The results obtained are in good agreement with the experimental data. In the cases where direct comparisons are possible, the MNDO/H method seems to give more accurate values of hydrogen bond energy than the ab initio method using STO-4-31G basis set.