An investigation of the relative stabilities of the isomers of CF2N2: Comparison of ab initio and MNDO calculations

Ab initio SCF calculations at the HF/3-21G level and semi-empirical MNDO calculations have been used to locate the stationary points on the CF₂N₂ energy surface. Perfluorodiazomethane is predicted to be most stable isomer, but perfluorodiazirine is predicted to lie only ca 41 kJ higher in energy at the SCF level. There are significant differences between the ab initio and MNDO results for the ordering of some of the isomers. Frequency calculations give results in good agreement with the limited experimental data on these molecules.     

Electron donor-acceptor complexes: Evaluation of MNDO as a computational tool to probe intermolecular interactions

Donor-acceptor pairs form EDA complexes that exist as conformational isomers exhibiting different ground-state and photochemical properties. We have sought a rapid, general, and accurate quantum mechanical computational method to generate potential energy surfaces that are representative of the donor-acceptor intermolecular interactions at the self-consistent field (SCF) level. The semiempirical molecular orbital (MO) method MNDO has been compared to ab initio methods to assess its behavior with respect to energy, dipole moment and ionization potential shifts. MNDO correctly distinguishes between repulsive and bound EDA complex states at the SCF level and produces potential curves that are smooth and free of spurious minima or cusps. MNDO curves are systematically more repulsive than those for ab initio STO-3G calculations; calculated interaction energies exhibit a mean absolute deviation of 2.90 kcal/mol. MNDO appears to provide a reliable qualitative estimate of the nondispersion portion of the interaction energy. Limitations and errors arising from minimal basis sets, single determinants, and neglect of dispersion are discussed.     

Calculation of energies of excited states using MNDO

The energies of the lowest singlet (S₁) and triplet (T₁) states of 28 molecules have been calculated by the “half-electron” (MNDO -HE ) and spin-unrestricted (UMNDO ) versions of MNDO . While most of the calculated values are too negative, because of overestimation of the correlation energy in MNDO -HE and UMNDO , the errors are systematic and depend in an understandable way on the nature of the molecular orbitals (MO S) involved. When appropriate corrections are applied, the calculated energies agree with experiment almost as well as they do for ground states. This justifies the use of MNDO -HE or UMNDO for studies of excited state processes.     

Semiempirical mo studies on the proton transfer equilibria of amides

Intramolecular proton transfer equilibria of acetamide and methyl carbamate have been studied by AMI MO calculations, and the results are compared with those of the MNDO method. It was found that the two semi- empirical methods predict essentially the same proton transfer mechanism, but MNDO tends to overestimate the activation barriers. Participation of one solvate vater in the proton transfer led to a considerable lovering of the activation barrier, by nearly the same amount, in both methods. One notable conflict between the two methods was that the methoxy-O-protonated form of methyl carbamate, in the gas phase, can be a local energy minimum with MNDO, whereas it leads to dissociation into two species Instead of an optimized structure with AM1. It was concluded that the MNDO method can be useful for this type of process, especially when one is interested in the relative activation barriers only.     

Electronic structure and properties of the reaction centers of phospha-alkenes in the MNDO approximation

The electronic structure of several phospha-alkenes has been studied by the MNDO method. The frontier orbitals (occupied and unoccupied) of these compounds are of the type. The conjugation effects have a significant influence on their energy and structure, as well as on the intramolecular distribution of the electron density. The results of the calculations are consistent with the available experimental data. The MNDO method makes it possible to predict the possible paths for the chemical conversion of phospha-alkenes in reactions with charge and orbital control.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 2, pp. 143–149, March–April, 1989.     

Quantum-chemical investigation of the mechanism of nucleophilic addition in the HCNO molecule

Calculations by the MNDO, AM1, and MNDO/M methods were undertaken for the reaction of HCNO and CH₃OH. It was shown that the MNDO and AM1 methods overestimate the activation energy of the reaction involving proton transfer, and this leads to an incorrect conclusion about the mechanism of the process. It was concluded that the reaction of the HCNO molecule with nucleophiles takes place in a single stage by a concerted asynchronous mechanism. The product in the Z configuration is formed as a result of the stereospecific reaction.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 6, pp. 719–722, November–December, 1989.     

A computational study of the reaction of methane with methyl radical

The activation energy and optimized transition-state geometry for the abstraction of a hydrogen atom from methane by methyl radical have been calculated by the semiempirical methods MINDO /3 and MNDO . These results are compared with other semiempirical and ab initio results. The MINDO /3 method, based upon accuracy of the computed energy of activation, appears to be the computational method of greatest reliability. A method of locating the transition state on semiempirical surfaces is demonstrated.     

Limitations on mndo and mndo/ci computations of activation barriers

The ability of MNDO and MNDO/CI for predicting activation barriers of unimolecular reactions is tested for decompositions of methane, ethane, nitromethane, and methyl nitrite. MNDO is not useful for this purpose; MNDO/CI gives energy barriers in agreement with experiment to about ±10 $\text{kcal}\text{mole}$ for the types of reactions considered.     

Theoretical study of the thermal unimolecular rearrangement of fluoroethylidenes

The thermal unimolecular isomerization of fluoroethylidenes to the corresponding fluoroethylenes has been studied by the MNDO method. It has been shown that fluorine substitution on the carbene carbon increases the activation energy in comparison with the ethylidene rearrangement. To understand the reason for this increase in the activation energy, the charge-transfer effects have been analyzed. Fluorine substitution at other positions does not significantly affect the activation energies. The thermodynamic parameters for the reaction have been evaluated, using vibrational and rotational spectral data calculated in this work. RRKM calculations have been performed and high-pressure Arrhenius parameters calculated. Hydrogen–deuterium kinetic isotope effects indicate that the reaction rates are altered considerably on isotopic substitution, and the change in reaction rates depends upon the position of deuterium substitution, as well as on the number of hydrogens replaced by deuterium atoms. © 1992 John Wiley & Sons, Inc.     

An MNDO treatment of sigma values

The Hammett σ and Taft σ⁰ constants have been examined using the MNDO computational procedure. They are both best explained by using computed properties based upon the benzoate ion, especially the reciprocal of the energy of the highest occupied molecular orbital.     

Conformation of BEDT-TTF molecules in the crystal

Based on comparison of low-temperature x-ray diffraction data for BEDT-TTF salts and quantum chemical MNDO calculations, we have shown that the most energetically favorable conformation of the BEDT-TTF molecule is not realized in the crystal. A local energy minimum 0.035 eV above the ground state corresponds to the conformation detected in the crystal. The x-ray diffraction data and the MNDO results suggest weak participation of the heteroatoms of the six-membered rings in the conjugation with the -system of the TTF-fragment in the BEDT-TTF and BEDO-TTF molecules.Institute of Chemical Physics at Chernogolovka, Russian Academy of Sciences, Chernogolovka 142432. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 4, pp. 890–894, April, 1992.     

Theoretical studies of [n]paracyclophanes and their valence isomers. I. Geometries,strain energies,and enthalpies of the inter-conversions of [n]paracyclophanes and their Dewar benzee isomers

Four semiempirical methods (AM1, MNDO, PM3, and MINDO/3) are used to calculate the deformation angles of [n]paracyclophanes and their Dewar benzene isomers for n = 3… 10. The results obtained by all these methods are in good agreement with data from X-ray studies. We have determined the strain energies that, in both series of compounds, are due to two components: (1) the strain energy of deformation of the cycle (aromatic or Dewar Benzene skeletons) and (2) the strain energy of the oligomethylene chain. In [6]paracyclophane, the strain energy [SE_ring(MNDO) ≈? 32.9 kcal/mol] almost compensates the resonance energy (E_resonance ≈ 36 kcal/mol) so that its chemical properties are closer to alkenes than to benzenic compounds. To better reproduce the enthalpy of the valence isomerization [n]Dewar bezene → [n]paracyclophane, which is poorly calculated with these methods, a correction is proposed and the reaction enthalpy of [6]paracyclophane is estimated to be about ΔH_r ≈ 15 ± 15 kcal/mol. It is found that MNDO and MINDO/3 need the smallest corrections, but MNDO leads to better geometries than MINDO/3. In conclusion, MNDO seems to be the best technique for further studies of these compounds. © 1992 by John Wiley & Sons, Inc.     

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.     

甘油快原子轰击质谱的量子化学研究

用标准MNDO方法优化了甘油快原子轰击正离子谱中的主要离子构型,计算了生成热、质子化能以及亚稳断裂的活化势垒,研究了质子化甘油二步脱水反应的机理,并指出脱水产物C_3H_5O~+离子存在2种不同的构型。     

Theoretical calculations of β-lactam antibiotics. III. AM1, MNDO,and MINDO/3 calculations of hydrolysis of β-lactam compound (azetidin-2-one ring)

Semiempirical AM1, MINDO/3, and MNDO methods have been used in the study of the alkaline hydrolysis of β-lactam antibiotics through a base-catalyzed, acyl-cleavage, bimolecular mechanism. In this work, the hydroxyl ion has been chosen as nucleophilic agent and the azetidin-2-one ring like a model of β-lactam antibiotic. The MINDO/3 method does not predict correctly the energies of small rings. This, together with the fact that, like MNDO, it cannot detect the occurrence of hydrogen bonds, gives rise to uncertain estimates of energy barriers. The AM1 method can be considered the most suitable for studying the hydrolysis of β-lactam compounds.     

2-(2′-羟基苯基)间氮杂硫茚激发态质子转移反应的理论研究

人们早就认识到质子转移互变异构反应的存在。近年来,发现的激发态分子内质子转移反应更引入注目,因为它有重大的应用前景,如     

Semiempirical study of compounds with O-H?O intramolecular hydrogen bond

Application of MNDO, AM1, PM3, MNDO/H, and MNDO/M methods to a set of compounds with intramolecular hydrogen bonds suggested that none of these methods accurately modeled the characteristics of the hydrogen bonds. Since the MNDO/H and MNDO/M methods work well for intermolecular hydrogen bonds, we followed their example and modified MNDO for intramolecular hydrogen bonds by altering the empirical core–core repulsion energy function for all pairs of atoms involved in intramolecular O-H? O bonds. The resulting modified method models the behavior of these bonds quite well, especially as regards their geometry and the barrier to proton transfer. © 1992 by John Wiley & Sons, Inc.     

Structures and energies of D-galactose and galabiose conformers as calculated by ab initio and semiempirical methods

Optimized geometries and total energies of some conformers of alpha- and beta-D-galactose have been calculated using the RHF/6-31G* ab initio method. Vibrational frequencies were computed at the 6-31G* level for the conformers that favor internal hydrogen bonding, in order to evaluate their enthalpies, entropies, Gibbs free energies, and then their structural stabilities. The semiempirical AM1, PM3, MNDO methods have also been performed on the conformers GG, GT, and TG of alpha- and beta-D-galactose. In order to test the reliability of each semiempirical method, the obtained structures and energies from the AM1, PM3, and MNDO methods have been compared to those achieved using the RHF/6-31G* ab initio method. The MNDO method has not been investigated further, because of the large deviation in the structural parameters compared with those obtained by the ab initio method for the galactose. The semiempirical method that has yielded the best results is AM1, and it has been chosen to perform structural and energy calculations on the galabiose molecule (the disaccharides constituted by two galactose units alpha 1,4 linked). The goal of such calculations is to draw the energy surface maps for this disaccharide. To realize each map, 144 different possible conformations resulting from the rotations of the two torsional angles psi and phi of the glycosidic linkage are considered. In each calculation, at each increment of psi and phi, using a step of 30 degrees from 0 to 330 degrees, the energy optimization is employed. In this article, we report also calculations concerning the galabiose molecule using different ab initio levels such as RHF/6-31G*, RHF/6-31G**, and B3Lyp/6-31G*.     

Asorption Behavior of CIO4-and ItsInfluence on Electronic Properties of Polypyridinopyridine

The adsorption behavior of CIO₄^- on the surface of polypyridino-pyridine (PPyPy)and its influence on the energy band structure of the poly mer have been studied using duantum chemistry MNDO, CNDO/2 and EHMO-CO methods.