Treatment of hydrogen bonding within CNDO/2 and MINDO/3: CNDO/2H and MINDO/3H

A formalism has been developed to treat hydrogen-bonded A—H…?B systems within the CNDO /2 and the MINDO /3 methodologies. In this formalism the interactions are divided into three distinct classes; those between (a) two hydrogen-bonded atoms, (b) one hydrogen-bonded and non-hydrogen-bonded atom, and (c) two non-hydrogen-bonded atoms. The last class of interactions is treated solely by the existing CNDO /2 or MINDO /3 method. For A –H…?B systems, the core resonance integrals are individually parametrized depending upon the class of the interaction. Three types of A—H…?B systems have been thus far parametrized. Nine hydrogen-bonded dimers have been studied using the new formalism and the current CNDO /2 and the MINDO /3 methods. MINDO /3 predicts very large interatomic (A –B) distances for the equilibrium geometry, and relatively small stabilization values for the hydrogen-bond energies. CNDO/2 predicts the reverse. The new formalism for both CNDO /2 and MINDO /3 predicts accurate geometries as well as energies for all nine dimers. The new formalisms are called CNDO /2H and MINDO /3H. A general discussion of the nature of hydrogen bonding as exhibited by CNDO /2H and MINDO /3H is presented.     

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.     

CNDO/2 and MINDO/2 energy band structures of some homopolynucleotides

The all-valence electron band structures of the periodic DNA models polyC, polyT, and polyA have been calculated with the aid of the CNDO /2 and MINDO /2 crystal orbital (CO ) method. According to the obtained results the valence and conduction bands are always of π type. The widths of the bands are usually larger in the CNDO case, than those obtained for the same systems in the PPP CO approximation. On the other hand the MINDO results show similar widths as the PPP ones. The forbidden bandwidths are by 3–4 eV larger in the CNDO /2 case, than in the PPP one, while the MINDO /2 CO method has resulted in similar gaps like the PPP calculation.     

Calculation of optimum geometries and force fields by the CNDO/force method

CNDO/Force calculations have been performed on a series of molecules, H₂CO, F₂CO, CF₄, CHF₃, CH₂F₂ and CH₃F. The optimum geometries and force fields are reported. It is found that the method can successfully predict the geometries of polyatomic molecules. The bending force constants and interaction force constants are, in general, comparable with experimental values both with respect to sign and magnitude. The stretching force constants have higher values than the experimental force constants. However, the trend in stretching force constants of a series of molecules is comparable with that of the corresponding experimental values.     

多原子分子力场的模型势函法与MNDO,MINDO／3方法的研究

用量子力学方法获得分子的模型力场现已占有很重要的位置.然而由于理论上很难获得体系高精度解析势能面,使力场的理论计算遇到了困难.直到Pulay提出梯度法后,量子化学方法才被广泛用于多原子分子的振动分析.但由于该方法采用数值微分通过能量的第二阶导数所获得的力场矩阵对角元误差很大,需将其伸缩、变角对角力常数分别扣除10％、20％,才能获得与观测值相近的结果;且ab initio计算花机时太多,应用于大分子计算存在一定的困难.为此本文把前文提出的确定力常数的模型势函法与MNDO、MINDO/3等半经验方法结合起来,构造多原子分子的力场,并用于振动分析,获得了满意的结果.     

Predicted vibrational frequencies and assignment of the spectra of uracil and its n,n-dideutero derivative by quantum chemical methods

A recent theoretical study on the vibrational spectrum and force field of maleimide (ref.1) lead to scale factors transferable to molecules of similar structure (ref.2). The theoretical vibrational spectrum of uracil (refs.2–3) calculated from scaled CNDO/2 force constants using the scale factors of ref.1 agrees better with the experimental results (ref.4) than former calculations by $\text{ab initio}$ STO-3G (ref.5) or MINDO/3 (ref.6) methods. The scaled CNDO/2 force field of uracil has also been used to predict the normal frequencies of a set of its deuterated derivatives.     

Determination of force fields for formaldehyde,acetaldehyde and acetone by the CNDO/force method

CNDO/Force calculations have been done for formaldehyde, acetaldehyde and acetone, and the theoretical force fields evaluated. Experimental force fields are obtained from vibrational frequencies using the least-squares refinement method. The initial force fields considered are based on the bending and interaction force constants obtained from the CNDO/Force calculations and the stretching force constants transferred from chemically related molecules. Vibrational frequencies of H₂CO, D₂CO, HDCO, H₂¹³CO and D₂¹³CO for formaldehyde, CH₃CHO, CH₃CDO, CD₃CHO, CD₃CDO and CH₂DCHO for acetaldehyde, and CH₃COCH₃ CD₃COCH₃ and CD₃COCD₃ for acetone are employed in the force field refinements. The final force fields obtained are found to be reasonable with respect to the diagonal and interaction force constants.     

A comparative study of the structure and bonding of HOO,HOS, HSO,and HSS radicals by CNDO/2 and INDO methods

Equilibrium geometries, force constants, barriers to linearity, charge distributions, dipole moments, and electron spin density of HOO, HOS, HSO, and HSS radicals are calculated by CNDO/2 and INDO methods using respectively the original and some recently introduced scheme of parametrization. Three sets of calculations, namely, CNDO/2(sp), CNDO/2(spd), and INDO, are performed, and the results are compared with the ab initio and experimental values, wherever available. A good agreement is obtained for geometry in the case of CNDO/2 (sp) and INDO calculations. The performance of CNDO/2 (spd) calculations in this regard is quite unreliable. The stretching force constants are considerably overestimated by all the methods, while the bending force constants are in reasonable agreement with the ab initio values. With respect to dipole moments, the CNDO/2 values are in better agreement with the ab initio results than the INDO values. In all the cases, the dipole moment vector directions are in complete disagreement with the ab initio predictions.     

Study of the electronic energy band structure of polyethylene using MINDO/3

The electronic structure of polyethylene has been calculated using crystal orbital theory and the latest version (MINDO/3) of the MINDO semi-empirical SCF MO method. The results are in better agreement with the observed photoelectron spectrum than those given by an analogous calculation using MINDO/2, which in turn has been shown superior to treatments based on EHT, CNDO, INDO, or ab initio SCF procedures.     

Force constants in hydrogen-bonded crystals

Hydrogen bond stretching force constants in crystals of imidazole, urea and cyanuric acid are calculated using both a modified CNDO/2 method and a constrained least squares fitting of interatomic pair potentials to the lattice vibrations. Results show that the modified CNDO/2 method gives closer agreement with the experimentally derived force constants than the normal CNDO/2 parameterization.     

Prediction of vtbrational spectra by the CNDO/2 force method: I. Out-of-plane vibrations of benzene and fluorobenzenes

The out-of-plane force constants of benzene and 12 fluorobenzenes have been calculated by the CNDO/2 force method. Two empirical scaling factors had to be applied to obtain good agreement. The mean deviation between the 88 calculated and experimental frequencies is 16 cm^?1.     

Electronic states in clusters of H forms of zeolites with variation of the Si/Al ratio

Fragments of H forms of zeolites of the faujasite type including up to 12 silicon- and aluminum-oxygen tetrahedrons and having different Si/Al ratios have been calculated in the cluster approximation by the MINDO/3 and CNDO/2 methods. The dependence of the integral and orbital densities of electronic states in the clusters on the aluminum content has been investigated. It has been shown that the profiles of the s- and p-orbital density of states of Al remain practically unchanged as the Si/Al ratio is lowered and that the maxima of the orbital density of states of Si broaden, and new maxima appear at the bottom and top of the valence band. When the acidity of the structural OH groups is lowered, the maxima of the orbital density of states of the H atoms are displaced appreciably only in the deep valence band, while in the upper valence band the positions of the peaks of the s-orbital density of states of the H atoms remain constant. Satisfactory agreement of the calculated orbital densities of states of Si, Al, and O with the corresponding x-ray photoelectron spectra has been obtained. In the deep valence band the data from the MINDO/3 method are better than those from the CNDO/2 method and reproduce the positions of the maxima in the x-ray photoelectron spectra.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 1, pp. 95–102, January–February, 1987.     

CH acidity comparative CNDO/2 and NDDO calculations on the reactivity of azabenzenes

It is shown that in contradiction to results obtained with the CNDO/2 method the NDDO method is successful in accounting for the effect of nitrogen lone pairs on the stability of π-delocalized and σ-localized carbanions derived from various azabenzenes.     

Block copolymers : Edited by D.C. Allport and W.H. Janes,applied science,Barking, Essex, 1973, pp. xxiii+ 620, price £12.00

A CNDO/2 parametric adaptation including Fischer-Kollmar and Sadlej modifications is made. This method leads to much better agreement between theory and experiment for bonding force constants of diatomic molecules and ethylenic and saturated hydrocarbons. However, these force constants are very sensitive to first and second derivatives of molecular integrals, so it is not possible to obtain a unique parametric formulation for various types of bond.     

Electronegativity and inductive and mesomeric parameters of atomic groups: A semiempirical quantum chemical estimation

The electronegativity, the inductive parameter, and the mesomeric dipole moments of substituents depend on the electronic structure characteristics of organic molecules calculated by the CNDO/2, INDO, MINDO, MINDO/3, MNDO, and AMI methods. It is shown that the electronegativity as well as the inductive and mesomeric characteristics of atomic groups may be estimated by semiempirical quantum chemical methods.     

Semi-empirical studies of molecular vibrations:: Part 2 A study of the vibrations of the benzene and perdeutero benzene molecules

The MINDO/2 prime method is applied to the study of the force field of the benzene molecule. Force constants are recorded and compared with the data of Duinker and Mills. The wavenumbers of vibration are calculated for both perhydro and perdeuterobenzene, and the values compared with experiment.     

A comparison of the HAM/3 semiempirical method with the MINDO/3 and CNDO/2 semiempirical methods and the GAUSSIAN 70 ab initio method; a comparison of ionization potential calculations and experimental photoelectron spectra of some imines and diimines

Molecular orbital calculations have been performed on eight molecules (containing 16–80 electrons) using the GAUSSIAN 70, MINDO/3, CNDO/2 and HAM/3 programs. The molecules contain nitrogen—nitrogen (diimines) or carbon—nitrogen (imines) double bonds. A comparison of the results of each method with experimental photoelectron data and an analysis of the cost effectiveness indicates that the HAM/3 method will prove a useful tool for photoelectron spectroscopists.     

A semiempirical and ab initio MO study of the tautomers ofN-unsubstituted pyrazolones [hydroxy pyrazoles]

Semiempirical (MNDOC, MINDO/3, AM1, and MNDO) and ab initio (STO-3G and 4-31G basis sets) calculations on the relative stabilities, structures, and dipole moments of the 8 theoretically possible tautomeric forms of pyrazolone are reported. It is shown that MNDO + CI and MINDO/3 predict that 5-hydroxy pyrazole, 3-hydroxy pyrazole, and 2-pyrazolin-5-on are the most stable. These results correspond to the known experimental data. Of all used quantum chemical methods, the MINDO/3 results for the dipole moments of the investigated tautomers are in best agreement with the known experimental data. The electronic excitation energies were calculated using the CNDO/S-CI method. The results are in good agreement with the experimental UV spectra.     

Conformational Effect on CNDO Force Field of Furan-2-Aldehyde

The complete sets of force constants for both O, O-cis and O,O-trans furand-2-aldehyde conformers were calculated using the CNDO force method. The force constants were corrected by ten empirical scaling factors. The calculated and observed frequencies agree well in most cases. Comparing the force constants of the two conformers, the largest deviations can be found in the interaction terms belonging to the aldehyde group.     

Quantenchemische modellierung der potentialhyperfläche des äthylkations im bereich der klassischen und nichtklassischen strukturen

Properties of the potential energy surface of protonated ethylene are calculated by semiempirical methods (CNDO/2-FK, MINDO/2, MINDO/3). These methods were chosen in respect of the agreement with new strong energy data concerning the relative stability of the nonclassical and a classical structure of the ethyl cations. They give no stationary point in that region of the hypersurfaces where until now was assumed to exist the most stable of the classical ethyl cations. The H-eclipsed classical cation is a saddle point on the CNDO/2-FK- and the MINDO/2-hypersurface, which represents the barrier of potential energy for the H-exchange in the ethyl cation; on the MINDO/3-surface a flat sea modified the character of the saddle point. Finally the results of the different quantum chemical methods (including NDDO and ab initio) are analysed concerning their agreement in calculating geometry and the distribution of electron density in the ethyl cations.