MNDO Approximation Research Articles

Influence of defects in the carbon network on the static polarizability of fullerenes

The static polarizability of the C60, C70, C80, and C186 fullerenes has been calculated within the semiempirical MNDO approximation implemented in the MOPAC quantum-chemical program package. It is demonstrated that the results obtained are comparable with experimental data and the results of the ab initio B3LYP method using the 6–31G(d, p) basis set. The influence of topological defects (five-, seven-, and eight-membered rings), vacancies produced by removing pentagons, and nitrogen and boron atoms on the geometric parameters and the polarizability of the C60, C240, and C540 fullerenes has been investigated by the MNDO method. It is revealed that the polarizability of the fullerene with topological defects is higher than the polarizability of the perfect icosahedral fullerene. The formation of vacancies in the carbon cage leads to a linear decrease in the polarizability of the fullerene and an increase in the specific polarizability. The polarizability of the heterofullerene with nitrogen or boron atoms spaced apart in the carbon cage is higher than that of the fullerene with heteroatoms located adjacent to each other.

Quantum-chemical study of the electronic structure and reactivity of 2-methyl-4H-thieno[3,2-b]pyrrole-5-carboxylic acid methyl ester

We have used the MNDO approximation to carry out a quantum-chemical study showing that the selectivity of acylation of 2-methyl-4H-thieno[3,2-b]pyrrole-5-carboxylic acid methyl ester under Friedel—Crafts reaction conditions in the presence of AlCl3 depends more on the electron density distribution in the complexes than on the structure parameters.

Quantum-chemical study of the structure and thermochemical properties of nitropiperazines and nitrosopiperazines

We have used MNDO and PM3 approximations and the density functional method (6–31G*) for a quantum chemical study of the structure and some physicochemical properties of N-nitro and N-nitroso derivatives of piperazine, furazano[3,4-b]piperazine, and bisfurazano[3,4-b;3′,4′-e]piperazine. We have analyzed the structural, electronic, and thermochemical characteristics and the enthalpies of formation for the compounds in the gas phase and in the solid phase. We have found a correlation between the strength of the N-N bond and the N-N bond length, the pyramidality of the nitrogen atom of the piperazine ring, and the size of the energy gap between the frontier orbitals. Based on calculations by the density functional method, we have carried out a comparative analysis of the thermochemical stability of the compounds in homolytic reactions.

(Fluoroorgano)fluoroboranes and ‐borates. 11 [1] The Electrochemical Oxidation of Polyfluorophenyltrifluoroborate Anions in Acetonitrile

AbstractThe potentials for the electrochemical oxidation of potassium polyfluorophenyltrifluoroborate salts K[C6H5—nFnBF3] (n = 0 — 5) were measured by cyclic voltammetry in CH3CN. In all cases the first oxidation peak corresponded to irreversible processes at the Pt electrode. The good correlation between the observed first oxidation peak potential values, Ep1, and the calculated (RHF, MNDO approximation) vertical ionisation potentials of the polyfluorophenyltrifluoroborate anions, [C6H5—nFnBF3]—, indicates an one‐electron step as first step of the oxidation. The formation of C6F5H and of the adduct BF3·NCCH3 in the potential controlled electrolysis of K[C6F5BF3] in CH3CN is in agreement with this assumption.

Fluorination of carbon nanotubes: Quantum chemical investigation within MNDO approximation

AbstractComputations of (6, 0), (8, 0), (6, 6), (7, 7), and (8, 8) single‐wall carbon nanotubes fluorination processes have been carried out within the framework of molecular cluster model and semiempirical quantum chemical MNDO scheme. The adsorption energy of fluorine atom as a function of distance from the surface carbon has been constructed. Chemical stable and van der Waals metastable stations of an F atom on nanotube surfaces have been found. The adsorption process of two fluorine atoms on adjacent surface carbons has been studied. It has been found that the second fluorine atom is adsorbing without van der Waals metastable station. The probability of the processes has been estimated in quasiclassic approaching. The greatest possible concentration ratio F/C of almost all investigated nanotubes have been obtained equal to 1 besides (8, 8) nanotube, for which the ratio is 0.75. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Thermal decomposition of aminotetrazoles

The thermal decomposition of 5-aminotetrazole (5-AT), 1-methyl-5-aminotetrazole (MAT), 1,5-diaminotetrazole (DAT), poly-1-vinyl-5-aminotetrazole (PVAT) and sodium salt of 5-aminotetrazole (SAT) have been studied by thermogravimetry, thermal volumetric analysis (TVA), DSC, DTA and evolved gas analysis (EGA). The kinetic parameters of the thermal decomposition of aminotetrazoles were calculated either by the Ozawa method or by the method of invariant kinetic parameters (IKP). The gaseous products, volatile condensed products and solid residues were identified by FTIR and gas chromatography–mass-spectrometry (GS/MS). The total energies and the energies of chemical bonds of various isomeric forms of 5-AT and MAT have been calculated ab initio using MP2/6-31G ∗∗ theory level and MNDO approximation. Based on the content of products of thermal decomposition and the kinetic consideration, the mechanism of thermal decomposition of aminotetrazoles has been derived. Two routes of the splitting of tetrazole ring leading either to elimination nitrogen or hydrogen azide are suggested. The contribution of each route is changing upon the advancement of the process. It was assumed, that hydrogen azide splits out from the prothotropic forms of the tetrazole ring, which have hydrogen atoms by nitrogens in the ring. Experimental study as well as literature data on the amino-imino tautomerism are in agreement with the suggested mechanism of the decomposition of the tetrazole ring. It is shown that secondary reactions significantly extend variety of the products of thermal decomposition of aminotetrazoles.

Automatic Quantum-Chemical Procedure for Correction of Optimized Spatial Structures of Free Radical Systems Based on Their Experimental Magnetic Resonance Parameters

The structural parameters of a test set of free radicals optimized by standard ab initio (UHF/6-31G**) and DFT (UB3LYP/6-31G**) procedures and in MNDO approximation are compared with each other and with experimental data. The possibility of quantum-chemical correction of the spatial structures of free radical systems after their optimization using semiempirical schemes is discussed. An automatic correction procedure is proposed and tested based on the requirement that optimal coincidence must be achieved between the magnetic resonance parameters calculated by a quantum-chemical procedure and the experimental parameters. Analysis of the results allowed us to formulate the criterion of applicability for the proposed procedure.

Quantum-chemical estimates of geometric parameters of free radicals

Experimental geometries of the HCO· and H2CO·+ σ-electron radicals were compared with those obtained from calculations with the total energy optimization carried out in the framework of widely used ab initio and semiempirical computational procedures. For each structural form of the radicals, the magnetic resonance parameters calculated in the MNDO approximation were correlated with experimental values determined by ESR spectroscopy. Comparative analysis of the results obtained indicates the possibility of systematic correction of the optimized geometric parameters of free radicals using the results of ESR measurements. A simple computational procedure for automatic geometry correction in the MNDO approximation is developed and evaluated.

Evaluation of MNDO approximation in quantum-chemical calculations ofg-tensors of free radicals

The MNDO approximation was tested for applicability in calculations of theg-tensors of free radicals. The representative test set included 15 free radicals for which the isotropic and anisotropic hyperfine coupling constants were calculated previously in the framework of the MNDO approach.

Evaluation of MNDO approximation in quantum-chemical calculations of anisotropic hyperfine coupling tensors in free radicals

The MNDO approximation was tested for applicability in quantum-chemical calculations of anisotropic hyperfine coupling tensors using the same set of 17 free radicals as that used previously in evaluating a new procedure for quantum-chemical estimates of constants of isotropic hyperfine coupling with protons in the framework of the same approach.

NMR chemical shifts in MNDO approximation: Parameters and results for H, C, N, and O

NMR chemical shifts in MNDO approximation: Parameters and results for H, C, N, and O

NMR chemical shifts in MNDO approximation: Parameters and results for H, C, N, and O

NMR chemical shifts in MNDO approximation: Parameters and results for H, C, N, and O

Semi-empirical calculation and normal coordinate study of the conformation and electronic and vibrational spectra of acrolein

Detailed quantum chemical and normal coordinate calculations have been conducted on acrolein and its deutero derivatives to understand the structural and spectroscopic characteristics of its rotational isomers, in particular the cis isomer. The results of molecular orbital studies in the MNDO (using the modified AM1 Hamiltonian), CS-INDO and CNDO approximations on potential energy curves, rotational barrier heights, conformations and physico-chemical properties of the ground and first excited ( nπ ∗ ) electronic states of acrolein are reported. Using multielectron configuration interaction based on the subroutine CIPSI, the electronic transitions have been explained. Generalized valence force fields have been obtained for the trans and cis conformers of acrolein and their monodeutero derivatives. A complete assignment of the in-plane and out-of-plane vibrational modes has been provided. It is inferred that while in the ground state the trans conformer of acrolein is more stable than the cis conformer. the reverse is the case in the first excited electronic state. In addition, a gauche conformer (Φ = 90°) may also exist in the excited state.

AB Initio and MNDO calculations of atomic interactions in the CH3OCOCl molecule

The CH3OCOCl molecule is calculated by ab initio methods using the split-valence basis sets at RHF/3-21G//RHF/3-21G, RHF/6-31G*//RHF/6-31G*, and RHF/6-311G*//RHF/6-31G* levels of theory and in the MNDO approximation. The optimized geometry of the molecule is consistent with the experimental data. The populations of the p-AOs of this molecule and the MO compositions show that the electron distribution in this molecule should be interpreted without considering the conjugation between the lone electron pairs of the Cl or O atoms and the π-electron system of the carbonyl group. The asymmetry parameters of the electric field gradient on the35Cl nucleus were calculated using the Cl p-AO populations and compared with the corresponding experimental value.

Condensed pyridine bases. Electrophilic substitution in 1,7-dimethyl-3(2H)-benzo[b]furo[2,3-c]pyridone

Nitration, bromination, thiocyanation, and acylation of 1, 7-dimethyl-3(2H)-benzo[b]furo[2, 3-c]pyridone were investigated. It was shown that either the 4-nitro- or the 4,6-dinitro derivative is formed as a function of the nitration conditions. Bromination with bromine and thiocyanation with dithiocyanogen yields a bromo derivative at position 4. The product of substitution at the C(6), atom is obtained in acylation with acetyl chloride, and the product of substitution at atoms C(4) and C(6) is obtained with benzoyl chloride. The results of the calculations were generalized in the MNDO approximation.

Electronic structure and polarizabilities of some heterocycles: I. Hydroxypyrazoles and related systems

The (hyper)polarizabilities of different tautomer forms of hydroxypyrazoles and pyrazolones have been calculated by the finite-field procedure in the MNDO approximation and the sum of states formalism in the PPP approximation, with all singly- and doubly-excited electronic configurations in the CI method. It was shown that while in the ground electronic state the values of the (hyper) polarizabilities are not essentially different, in the first excited singlet Franck-Condon state an increase of the molecular polarizabilities of some tautomers is observed. This increase is attributed to a specific change in the electronic structure of the excited state, demonstrated by the localization of the electronic transition in the different pyrazolone tautomers. The electron-donor capabilities of phenyl-substituted hydroxypyrazoles and pyrazolones are discussed.

Electronic and spatial structures of some derivatives of 5-phosphoranylidenbarbituric and 5-phosphoranylidenthiobarbituric acids

The spatial and electronic structures of some derivatives of 5-phosphoranylidenbarbituric and 5-phosphoranylidenthiobarbituric acids are investigated within the framework of the semiempirical MNDO approximation. The effect of the substituents at the phosphorus and nitrogen atoms on the geometry and distribution of electron density in the studied molecules is considered. Their pseudoaromatic π-system, which is formed by diffuse lone electron pairs of the $$\begin{array}{*{20}c} \backslash \\ / \\ \end{array} C^ - - $$ ion and two N atoms, involves alternating donor and acceptor centers. The pseudoaromatic fragment in these molecules is nearly planar, and the deviations from planarity are dependent on the size of the substituents at the phosphorus and nitrogen atoms.

Electrophilic substitution in benzo[b]furo[2,3-c]pyridines: nitration, acylation

It has been shown that the nitration and acylation of substituted benzo[b]furo[2,3-c]pyridines proceeds exclusively at position 6 of the benzene ring. If position 6 is blocked by a substitutnt, the product of monosubstitution at the C(8) atom is formed. The molecules that were investigated have been calculated in the MNDO approximation.

Cluster calculation of the boron centre in SiC

Calculations using the self-consistent unrestricted Hartree-Fock-Roothan method are carried out in the MNDO approximation for a cluster of 35 host atoms of cubic SiC with a boron impurity. The equilibrium geometry and the hyperfine and quadrupole constants are obtained and compared with experimental values. The computed parameters reveal the main features of boron impurities in SiC.

The electrostatic potential in the semiempirical molecular orbital approximation

A method is described for calculating the molecular electrostatic potential (MEP) within the MNDO, AM1 or PM3 semiempirical approximations. The MEPs are used to obtain atomic charges from which dipole moments are calculated for comparison with the exact semiempirical values. The results for a wide range of neutral molecules and ions show excellent agreement between the dipole moments calculated from MEP-derived atomic charges and the exact semiempirical dipole moments. The neutral molecules and cations showed a mean deviation in the dipole of less than 0.01 D. The maximum errors (≈ 0.04 D) were obtained for some molecules containing third row or heavier elements. By contrast, all previously reported methods based on the semiempirical approximations for generating MEP-charges in general yield significantly larger deviations from the exact dipoles. The maximum error for the anions tested was 0.07 D, with a mean deviation only slightly higher than for neutral species and cations. The correct limiting behaviour is obtained when the MEP surfaces are calculated at large distances, i.e. the exact dipole is approached asymptotically. Quadrupole moments of the MEP-derived charge distributions are reported for some nonpolar and polar molecules.