Non-empirical SCF studies of the ring and open forms of ozone

Non-empirical SCF calculations have been used to investigate the relative stabilities of the open and closed forms of ozone. In these studies several different sized basis sets have been employed, the best basis set being at about the level of a double-zeta set plus d-polarization functions. In contrast to the predictions of the CNDO/2, INDO and MINDO methods, the non-empirical SCF studies correctly predict that the open form of ozone is more stable. Thus these results show that the area of deficiency in the CNDO/2, INDO and MINDO results for this system is not in the SCF approximation.     

MINDO/3 calculations of ESCA chemical shifts

Calculations of ESCA chemical shifts, using Jolly's equivalent core approximation and the MINDO/3 semi-empirical SCF MO method, have given results in reasonable agreement with experiment.     

A theoretical study of the anomalous substituent effect of the hydroxyl group

A theoretical study by MINDO/3 and STO-3G SCF calculations has been used to examine the effect of a H-bonded water molecule on the ability of the hydroxyl group to stabilise a carbocation centre. Alkoxy groups are better stabilising groups in the gas phase, hydroxyl is more effective in protic solvents.     

Notiz über die Photoelektronen-Spektren des Nortricyclens und des Triasterans

The photocelectron spectra of nortricyclene (=tricyclo[2.2.1.0^2,6]heptane) and of triasterane (=tetracyclo[3.3.1.0^2,8.0^4,6]nonane) have been recorded and a tentative assignment of the bands has been put forward on the basis of MINDO/2 SCF calculations.     

A nonelectrocyclic path from the cyclobutene cation radical to the 1,3-butadiene cation radical

The conversion of the cyclobutene cation radical to the 1,3-butadiene cation radical has been studied using MINDO /3 and ab initio SCF MO methods. Not only smooth electrocyclic but also stepwise, non-electrocyclic routes were considered. Both calculational methods agree that the preferred reaction path is a novel nonelectrocyclic one proceeding through an intermediate “cyclopropylcarbinyl cation radical.” The quantitative agreement in the activation parameters calculated by the two methods is excellent. The proposed intermediate also provides an attractive explanation for the mass spectrometric fragmentation patterns of the cyclobutene and butadiene cation radicals.     

Development and status of MINDO/3 and MNDO

The historical development of MINDO/3 and MNDO is outlined in relation to the parallel developments of the ab initio SCF approach. It is pointed out that both treatments are purely empirical so far as chemistry is concerned and complement one another, MINDO/3 and MNDO allowing calculations to be carried out when ab initio methods of comparable performance are inapplicable because they need one-thousand times more computing time.     

Faster finite-difference calculations of molecular properties

A very simple yet effective device for reducing the number of self-consistent-field (SCF) cycles in central finite-difference calculations of molecular properties is presented. The efficiency of this is demonstrated by MINDO/2 calculations of the force constants of a variety of molecules. The device consists of using information about the wavefunction at two values of a perturbation parameter to provide an estimate of the wavefunction at a third. This estimate is then used as an initial guess in an SCF procedure. The underlying theory is discussed and applications to other problems are suggested.     

Calculated photoelectron spectra of singlet and triplet methylene

Vertical ionization potentials for singlet and triplet methylene are calculated by a CI perturbation method based on ab initio SCF molecular orbitals (6–31 G^** basis). The shape and vibrational fine structure of the first photoelectron band are investigated using the MINDO/3 method. The computed singlet-triplet splitting for methylene is 16.4 kcal/mol, in reasonable agreement with the experimental value of 19.5 kcal/mol.     

Ab initio calculations including electron correlation,and mindo/3 calculations on the system C2H+7

Ab initio SCF and CEPA PNO calculations have been performed together with MINDO/3 calculations on the system C₂H⁺₇. In agreement with experimental assignment, but in contradiction to MINDO/3 results, the ab initio methods show the CC protonated structure to be more stable than the CH protonated structure. The energy difference is 8.5 kcal/mol at the SCF level and 6.3 kcal/mol with inclusion of electron correlation. Additionally, ΔH⁰₃₀₀ for the reaction C₂H⁺_s + H₂ = C₂H⁺₇ and the proton affinity of ethane are computed.     

Anodic behavior of ephedrine at solid electrodes

The electrooxidation of the alkaloid ephedrine at solid electrodes in a wide range of concentrations and pH values of solutions has been studied by the methods of potentiodynamic voltammetric curves, preparative electrolysis, and quantum-chemical calculations by the SCF MO LCAO method in the MINDO/3 approximation. A quantitative basis has been given for the sequence of stages in the electrooxidation of ephedrine proposed previously by the authors of one of the cited papers. Good agreement has been established between the values found theoretically and experimentally. Institute of Organic Synthesis and Colloid Chemistry, Kazakh SSR Academy of Sciences, Karaganda. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 523–526, July–August, 1989.     

Combined immobilization of lytic and proteolytic enzymes

The electrooxidation of the alkaloid ephedrine at solid electrodes in a wide range of concentrations and pH values of solutions has been studied by the methods of potentiodynamic voltammetric curves, preparative electrolysis, and quantum-chemical calculations by the SCF MO LCAO method in the MINDO/3 approximation. A quantitative basis has been given for the sequence of stages in the electrooxidation of ephedrine proposed previously by the authors of one of the cited papers. Good agreement has been established between the values found theoretically and experimentally.Institute of Organic Synthesis and Colloid Chemistry, Kazakh SSR Academy of Sciences, Karaganda. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 523–526, July–August, 1989.     

N-取代乙基吡唑热消除反应的量子化学研究

用MINDO/3方法研究了N-取代乙基吡唑热消除反应H₃C₃N₂CH₂CH₂X→H₄C₃N₂+H₂C=CHX(反应Ⅰ～Ⅲ,对应X=H,CH₃,F)的机理,结果表明,甲基的超共轭效应以及F的强吸电子作用和邻基参与使得反应Ⅱ和反应Ⅲ的活化势垒降低,比反应Ⅰ更易进行。     

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.     

Bond lengthening and through-bond interactions in p,p'-dibenzene and related molecules

The structure of p,p'-dibenzene (PDB) has been investigated by full geometry optimizations using the empirical force field (EFF) and MINDO/3 methods. While other structural parameters are in good agreement, the central bond length calculated by MINDO/3 (1.595 Å), as confirmed by an ab initio (STO-3G basis set) optimization (1.596 Å), is in striking contrast to the corresponding length calculated by EFF (1.543 Å). A detailed analysis of the electronic structure of PDB based on a quantitative perturbational molecular orbital treatment reveals that through-bond coupling of the four π systems is responsible for an elongation of the σ bond which mediates this interaction. Further studies using the EFF and MINDO/3 approaches demonstrate that extended C-C single bonds can arise even in structures with fewer than four π systems. The effect of substituents on the central bond length in PDB has been briefly investigated. (MINDO/3). A variety of other structures have been identified in which bond lengthening may result from through-bond coupling.     

Electronic structure and mass spectra of substituted hexahydro-1,3,5-triazine-2-thiones

The electron density in the molecules of substituted hexahydro-1,3,5-triazine-2-thiones was calculated by the LCAO-MO SCF method in the MINDO/3 approximation. The mass spectra of substituted 5-methylhexahydro-1,3,5-triazine-2-thiones were studied, and a fragmentation scheme for the compounds is proposed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 976–980, July, 1986.     

Quantum-chemical cluster models for hypervalency coordination environments in aluminum oxides

The MINDO/3 semiempirical approximation in SCF MO LCAO has been used to examine whether a cluster scheme can be used to calculate the electronic structures for aluminum oxides. Conditions are defined under which the hypervalent environment of the aluminum atom is stable under geometry optimization. A univalent pseudoatom model is compared with an uncharged ionic one to estimate the geometrical optimization. A univalent pseudoatom model is compared with an uncharged ionic one to estimate the geometrical and energy characteristics for clusters representing three-coordinated, four-coordinated, and six-coordinated Al.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 5, pp. 581–586, September–October, 1989.     

Optimum geometries and relative energies for cytosine,thymine, uracil,the imino tautomer of cytosine,the enol tautomer of thymine,and the enol tautomer of uracil by the MINDO/2 SCF MO method

A MINDO /2 SCF MO geometry optimization of cytosine (C), thymine (T), uracil (U), the imino tautomer of cytosine (C*), the enol tautomer of thymine (T*), and the enol tautomer of uracil (U*)was made. The optimized geometries for cytosine, thymine, and uracil agree well with crystallographic data. The optimized geometries for the tautomers show the correct trends in bond lengthening and bond angle except for the C4—O4 length and C4—O4—H angle of T* and U*. The energies of tautomerization were found to be 10.3, ?9.0, and ?14.2 kcal/mol for C?C*, T?T*, and U?U*, respectively, when optimized geometries are used. The overestimation of the C4—O4—H angle is speculated to arise because of an inadequacy in the parametrization of the one-center integrals in MINDO /2.     

Proton affinity correlations for alkyl chlorides

Ab initio SCF calculations with minimal STO-3G and extended 44-31G basis sets have been performed on the simple alkyl chlorides, HCl to t-BuCl, and their protonated analogs. MINDO/3 calculations are also reported for these species and a variety of cyclic and bicyclic chlorides. The much closer agreement with experiment for STO-3G proton affinities than for 44-31G values is in sharp contrast to the results for first-row bases. An excellent correlation is found between both the STO-3G and MINDO/3 proton affinities and the charge on the CIH fragment in RCIH⁺. For the acyclic chlorides, correlations of PA's with the polar substituent constant, σ*, and IP's are also reasonable. In addition, the calculated carbonium ion-HCl interaction energy for t-BuClH⁺ indicates that protonated tertiary chlorides are no more than marginally stable in the gas phase.     

Semi-empirical molecular orbital calculations of the properties of 2:1 hydrogen bonded complexes of water with some ketones and ethers

The properties (molecular geometries and hydrogen bond energies) of a number of 2:1 complexes formed between water and some ketones and ethers have been calculated using both the CNDO/2 and the MINDO/3 semi-empirical molecular orbital methods. The CNDO/2 results have been compared with those obtained using the MINDO/3 method, and have been correlated with the i.r. spectroscopic properties of the complexes. The results for the 2:1 complexes have also been compared with those of 1:1 complexes formed with the same solvents.     

Quantum-chemical investigation of the 1,2-proton shift in protonated five-membered aromatic heterocycles

Calculations of the energetics of the 1,2-proton shift in protonated five-membered aromatic heterocycles — pyrrole, furan, and thiophene — have been carried out by the SCF MO LCAO method in the MINDO/3 approximation and nonempirically on the OST-3GF (OST-3GF*) basis. The general features of this process, and also the influence of solvation and of taking into account the vacant d-AOs of the sulfur atom in the protonated form of thiophene on the results of the calculation are considered. The results obtained have been used for a discussion of the activity and selectivity of the heterocycles considered in aromatic electrophilic substitution reactions.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1186–1190, September, 1984.