Semiempirical study of compounds with intramolecular O?H…︁O hydrogen bonds. II. Further verification of a modified MNDO method

In the forerunner of this article, we described a MNDO modification designed for studies of compounds with intramolecular O? H…?O hydrogen bonds. Here, we report the further verification of the modification by means of its application to 14 compounds not considered in its development. Comparison of the calculated structural parameters and proton transfer characteristics with available experimental or ab initio results, and with those obtained using MNDO, AM1, MNDO/H, MNDO/M, and PM3, supports the validity of the new modification for prediction of hydrogen bond characteristics. © 1994 by John Wiley & Sons, Inc.     

A density functional treatment of organolithium compounds: Comparison to ab initio,semiempirical, and experimental results

Density functional calculations on several classes of organolithium compounds are described. The compounds studied include lithium bonds to carbon, oxygen, and nitrogen and are representative of most types of organolithium compounds that have appeared in the recent literature. The computational results are compared to those using MNDO, which has been shown to have some serious deficiencies in compounds involving carbon–lithium bonds, and to PM3 results, which offer some improvement over MNDO for many organolithium compounds. Most of the density functional calculations with a large basis set are in good agreement with available ab initio and experimental data. Calculated carbon–lithium bond lengths were slightly shorter than those calculated by other ab initio methods and were substantially longer than those calculated by MNDO, which is known to underestimate carbon–lithium bond lengths severely. Dimerization energies of methyllithium, calculated by DMol, were also in good agreement with those of other ab initio calculations. Lithium–nitrogen bonds in lithium amides were calculated to be slightly shorter by DMol than by MNDO, although the two methods were in qualitative agreement for this type of compound. © 1995 by John Wiley & Sons, Inc.     

偶氮染料分子的电子结构与生物降解活性(Ⅱ)-取代基非定域活性对偶氮键还原裂解的影响

通过ＭＮＤＯ计算及相关实验结果,用取代基非定域活性概念分析了羟基、氨基及硝基在—ＮＮ—上的电子效应,阐述了取代基影响偶氮键生物降解活性的电子结构机理,并提出将ＭＮＤＯ统计集居分析中的活性电荷ＱＡ（其值可反映原子在分子中的化学活性）作为关联偶氮键生物降解活性的电子结构参数     

Vibrational modes and frequencies of 2-pyrrolidinones and their deutero-isotopomers

MNDO-based calculations have been used to predict the fundamental vibrational frequencies of 2-pyrrolidinone, N-methyl 2-pyrrolidinone, and one deutero-isotopomer of each of the two compounds. A good over-all fit between observed and calculated frequencies was obtained. Comparisons of the MNDO results were made with previously published normal coordinate analyses of the same compounds. A critical and important result is that MNDO correctly calculates the in-plane NH deformational mode of 2-pyrrolidinone not to mix with either the amide bond or carbonyl bond stretches.     

Population analysis of pair densities: A link between quantum chemical and classical picture of chemical structure

The pairon population analysis based on the geminal expansion of pair densities is introduced and applied. As demonstrated by numerical data calculated for a series of simple molecules by the semiempirical MNDO method, the resulting populations provide a new simple means of visualizing the molecular structure. In addition to the reproduction of classical structural formula including the multiplicity of individual bonds, the resulting populations confirm the transferability of bond energies and also provide a simple interpretation of the concept of quantum chemical valence. © 1994 John Wiley & Sons, Inc.     

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 proton transfers in hydrogen bonding interactions with semi-empirical MNDO/H

A method for calculating proton transfer enthalpies by a proper modification of the recent H-bonding version of MNDO is presented. This method perceives the proton as being both “bonded” and “hydrogen bonded” to the two electronegative atoms involved in the hydrogen bond: as it moves from one potential minimum at X-H---Y to the other at X---H-Y, a hydrogen bonding function is attached to the proportion of the distance that is to be traversed. The method is applied to two proton transfers within anionic oxygen H-bonded complexes and is shown to reduce the previously calculated barriers which were too high. Gas phase results for the single step of proton transfer over a barrier are required to evaluate the results obtained by this method.     

Formation constants for complexes of transition-metal cations with O- and N-donor ligands in aqueous solutions

A method has been developed for the theoretical calculation of stability constants for transitionmetal complexes proceeding from molecular properties. An analytical form of the cation-ligand interaction potential including the partial bond covalence has been developed. The potential is expressed through molecular parameters and a covalence parameter; the latter may be determined from experimentally measured or theoretically calculated gas-phase data. The stability constants have been calculated for 1: 1 complexes of divalent metal cations with hydroxide anion, acetic, glycolic, and lactic acid anions, and ammonia in aqueous solutions. The covalence parameters have been calculated for the bonds of metal cations with O-and N-donor ligands.     

Conformational analysis of 2-methyl-1,3,2-dioxaborinane in the presence of an ammonia molecule

Nonempirical quantum chemical methods RHF//STO-3G, 3-21G and 6-31G(d) were used for the investigation of the conformational properties of molecular complexes of 2-methyl-1,3,2-dioxaborinane with ammonia (1:1). Two types of possible associates are revealed: with donor-acceptor N→B bond and with intermolecular NH···O hydrogen bond. Their calculated relative stability and conformational behavior are determined not only by the spatial orientation of the donor and the acceptor but also by the concepts of the used calculation approach.     

Polycyclic aromatic hydrocarbons with five-membered rings: Modeling of experimental X-ray and neutron-diffraction structures

Several of the readily available theoretical programs are evaluated as tools for modeling the structures of polycyclic aromatic hydrocarbons with five-membered rings (CPAHs). The experimentally determined bond lengths and angles are compared to calculated values. Experimental bond lengths are also compared to Pauling and Huckel molecular orbital (HMO) bond orders. Previously published experimental X-ray and neutron-diffraction structures of acenaphthene, acenaphthylene, fluoranthene, cyclopent[o,p,q,r]benz[c]phenanthrene, and corannulene are modeled by the programs MMX, AM1, MNDO, and PM3, and previously reported STO-3G and 6-31G * data are also evaluated. In general, the error differences between the experimental and calculated results for all of the semiempirical programs were small. However, PM3 performed slightly better than AM1 and MMX, while MNDO generated structures which exhibited the largest deviation from experiment. Although the standard deviations for all programs are shown to be of comparable magnitude, a particular bond length or bond angle in any given theoretical calculation can exhibit significant error from the experimental data. The scatter in the bond order data computed from Huckel molecular orbital theory and valence bond theory is contrary to results obtained with alternant systems. It appears that these approaches are less successful at modeling accurately the nonalternant hydrocarbon systems described in this paper.     

An Explanation on the Electron Ionization Mass Spectra of Crown Ethers by MNDO Computational Method

ＡｎＥｘｐｌａｎａｔｉｏｎｏｎｔｈｅＥｌｅｃｔｒｏｎＩｏｎｉｚａｔｉｏｎＭａｓｓＳｐｅｃｔｒａｏｆＣｒｏｗｎＥｔｈｅｒｓｂｙＭＮＤＯＣｏｍｐｕｔａｔｉｏｎａｌＭｅｔｈｏｄＹＵＡＮＨｕ－ｓｈａｎ，ＴＡＮＧＭｉｎｇ－ｓｈｅｎｇａｎｄＷＵＹａｎｇ－ｊｉｅ（...     

多硝基甲烷Mannich碱稳定性的理论研究

用MNDO SCF-MO方法全优化了系列多硝基甲烷Mannich碱的几何构型, 计算了它们的电子结构。在胺、醛组分相同时, 标题物>N-CH2-Y的特征键CH2-Y的键级随酸组分亲核性的增强而增大。该CH2-Y键级较小是造成Mannich碱在溶液中不稳定的重要原因。C-NO2键的键级在分子中较小, 可能是热或光解等受激分解的引发键, 从电子结构特征上阐明了以硝仿为酸组分的Mannich碱稳定性较差的原因。     

Extension of MNDO to d orbitals: Parameters and results for the halogens

A recently proposed extension of the MNDO formalism to d orbitals has been parameterized for the halogens CI, Br, and I. Extensive test calculations indicate slight consistent improvements for normalvalent molecules and dramatic improvements for hypervalent molecules, in comparison with established MNDO -type methods without d orbitals. The mean absolute errors in calculated heats of formation are 3.9 kcal/mol for 155 normalvalent compounds and 2.8 kcal/mol for 23 hypervalent compounds. The predicted structures of the hypervalent molecules are qualitatively correct, with a mean absolute error of 2° in 19 bond angles.     

Computational chemistry of the silicon nitride surface. 1. Water,ammonia, and water-ammonia complex

This paper deals with computational modeling of structure and properties of the silicon nitride surface zone using combined computational and real experiments. The computational experiment implies quantum chemical calculations of structure and vibrational spectra of polyatomic clusters. The real experiment suggests measurement and analysis of vibrational spectra. For quantum chemical calculations, semiempirical methods (MNDO and AM1) were chosen. In most calculations, the MNDO/H method was preferred because of the presence of many H-bonds in the surface zone. For verification of calculations, we calculated the structures and vibrational spectra of water and ammonia molecules and the water-ammonia complex and compared the results with experimental and ab initio (extended basis) data; MNDO/H proved to be an optimal method giving reliable results. Russian Peoples' Friendship University. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 1, pp. 58–69, January–February, 1995. Translated by L. Smolina     

Quantum-mechanical simulation of the interaction of aluminum- and boron-containing zeolite clusters with molecules of water and ammonia

Quantum-mechanical computations of zeolite clusters with molecules of water and ammonia have been carried out. The clusters consisted of ten atoms of silicon and aluminum, where one atom of aluminum was also replaced with an atom of boron. Values of the bond length and bond angles have been obtained; the geometry of adsorption complexes and the bond energy for molecules of water and ammonia with atoms of aluminum and boron of a zeolite fragment have been determined. The computed values of bond energy for molecule probes yield the quantitative strength characteristic of zeolite aprotic acid centers.     

Hyperpolarizabilities of hydrogen-bonded complexes of phenol derivatives with ammonia: PM3 and ab initio studies

Polarizability and first hyperpolarizability values of the hydrogen-bonded complexes formed by nitrosubstituted phenols with ammonia have been calculated using PM3 and ab initio (STO-3G) methods. It has been shown that enhancement of the polarizability (Δα) as well as the first hyperpolarizability (Δβ) of the complex arises from the hydrogen bond interaction between the phenol derivative and ammonia.     

Molecular orbital treatments of hydrogen bonded systems. 2. Dimers of water and HCN

Calculations were performed to investigate the reliabilities of the CNDO/2, PRDDO, and MNDO approximate molecular orbital methods. Systems selected for study included the linear, cyclic, and bifurcated dimers of water as well as the linear and cyclic dimers of HCN. The PRDDO method was found to provide the most consistently accurate reproduction of ab initio and experimental data. CNDO/2 performed fairly well in a number of cases but yielded extremely poor results for the cyclic dimers of both H₂O and HCN. Hydrogen bond strengths were consistently underestimated by MNDO which also furnished erroneously large intermolecular separations. In addition, MNDO calculations indicate the bifurcated water dimer to be most stable in contrast to other quantum mechanical and experimental information.See Ref. [1] for Paper 1 of this series.     

Quantum-chemical interpretation of regioselective coordination in the series of 2-aminoazole—metal complexes

Relative thermodynamic stabilities and competitive bonding (regioselective coordination) of Pearson acids MCl₂ (hard BeCl₂, intermediate ZnCl₂, and soft HgCl₂) with aminohetero-cycles L (2-aminooxazole, 2-aminothiazole, and 2-aminoimidazole) have been calculated by the quantum-chemical MNDO method. It follows from the results of the calculation that in tetrahedral MCl₂·2L complexes the coordination bond is formed by the endocyclic pyridine-type nitrogen atom independent of the type of ligands.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2378–2380, December, 1995.This work was financially supported by the Russian Foundation for Basic Research (Project Nos. 94-03-09731 and 93-03-18692).     

Optimization of parameters for semiempirical methods II. Applications

MNDO/AM1-type parameters for twelve elements have been optimized using a newly developed method for optimizing parameters for semiempirical methods. With the new method, MNDO-PM3, the average difference between the predicted heats of formation and experimental values for 657 compounds is 7.8 kcal/mol, and for 106 hypervalent compounds, 13.6 kcal/mol. For MNDO the equivalent differences are 13.9 and 75.8 kcal/mol, while those for AM1, in which MNDO parameters are used for aluminum, phosphorus, and sulfur, are 12.7 and 83.1 kcal/mol, respectively. Average errors for ionization potentials, bond angles, and dipole moments are intermediate between those for MNDO and AM1, while errors in bond lengths are slightly reduced.