A CNDO/2 calculation on stilbene

The equilibrium conformations of trans- and cis-stilbene and the rotational barrier between the two isomers were calculated using the CNDO/2 method. The results for the equilibrium angles and distances are in good agreement with experimental data. However, according to the calculations the phenyl rings would have a nearly free rotation which is only limited by steric hindrance. This result obviously deviates from the current view. The results are discussed in relation to previous theoretical and experimental evidence.

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Zusammenfassung Die Gleichgewichtskonformation von trans- und cis-Stilben und die Rotationsbarriere zwischen zwei Isomeren wurde nach der CNDO/2-Methode berechnet. Die Ergebnisse in bezug auf die Geometrie stimmen mit experimentellen Daten gut überein, die Rotationsbarriere für die Drehung der Phenylringe dagegen ist praktisch Null.

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Résumé La méthode CNDO/2 a été employée pour calculer les conformations d'équilibre du trans- et du cis-stilbène ainsi que la barrière de rotation entre ces deux isomères. Angles d'équilibre et distances calculés sont en bon accord avec les données expérimentales. Cependant les calculs indiquent que les cycles phényles devraient posséder une rotation libre soumise seulement à l'empêchement stérique. Ce résultat s'écarte de l'opinion courante. Les résultats sont discutés en relation avec les données théoriques et expérimentales antérieures.

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This work was supported by grant No 2741-4 from The Swedish Natural Science Research Council and grant No 69-927/U 697 from The Swedish Board for Technical Development.     

A CNDO/2 calculation of a rearrangement in cyclopentadienylsilane

The ground state geometry of cyclopentadienylsilane and the transition state geometry for SiH₃ group migration have been calculated by the CNDO technique, a semi-empirical version of the LCAO MO SCF approach.     

CNDO calculation of polarizabilities including polarization functions in the basis set

Electric polarizabilities are calculated by solving the first- and second-order perturbation equations through a variational procedure. Satisfactory numerical results are obtained for a number of molecules using the CNDO approximation with an extended basis set.     

On the use of CNDO/2 calculations of molecular charge and/or spin distributions for chemical predictions

Caution is recommended in the uncritical use of charge or spin densities, calculated by the CNDO/2 method, for chemical predictions. In particular, it is shown that serious errors may arise either through a poor choice of geometry or through ineffective convergence of the relevant density matrices.     

A CNDO/2 calculation on a complex between ethylene and chlorine

CNDO/2 calculations have been carried out for an ethylene chlorine complex, with the halogen orthogonal or parallel to the C-C bond. For comparison similar calculations were also carried out for ethylene and fluorine. The results are compared to estimates from experiments on related complexes and from Mulliken's resonance theory. It is concluded that the CNDO/2 calculations grossly overestimate the ethylene-chlorine interaction in particular if d-orbitals on chlorine are included in the AO-basis. The explanation for this may be the too high d-orbital exponent used here and the wrong asymptotic behaviour of the Slater orbitals.

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Zusammenfassung Es werden CNDO/2-Rechnungen für zwei Konformationen eines Äthylen-Chlor-Komplexes und eines Fluor-Analogens berichtet und die Resultate mit der Resonanztheorie von Mulliken und mit experimentellen Daten verglichen. Es ergibt sich, daß bei CNDO/2-Rechnungen die ChlorÄthylen-Wechselwirkung stark überschätzt wird, insbesondere, wenn d-Orbitale eingeschlossen werden. Letzteres könnte auch an einem großen Exponenten bei diesen Funktionen oder am falschen asymptotischen Verlauf von Slater-Funktionen liegen.

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Résumé Calculs CNDO/2 pour un complexe ethylène-chlore avec l'halogène orthogonal ou parallèle à la liaison C-C. A titre de comparaison des calculs analogues ont été aussi effectués pour l'éthylène et le fluor. Les résultats sont comparés à des estimations expérimentales sur des complexes voisins et à des estimations provenant de la théorie de la résonance de Mulliken. La conclusion est que les calculs CNDO/2 surestiment beaucoup l'interaction éthylène-chlore, en particulier lorsque l'on inclut des orbitales d dans la base des orbitales atomiques. La raison peut s'en trouver dans les exposants trop élevés des orbitales d et dans le mauvais comportement asymptotique des orbitales de Slater.

     

CNDO/2 calculation of prototropic rearrangement in cyclopentadiene

A self-consistent procedure has been worked out for the optimisation of geometrical parameters of ground and transition states of molecular systems and chemical reactions. For cyclopentadiene molecule, the CNDO/2 method is used to calculate a number of the geometrical configurations which lie at the reaction co-ordinates corresponding to possible channels of the proton migration.     

2-取代吡啶衍生物的CNDO/2计算和分子光谱研究

本文用CNDO/2方法计算了吡啶及其2-取代衍生物,测了其紫外光谱.通过红外,紫外光谱和量子化学计算的综合分析,确定2-吡啶酮与2-羟基吡啶是互变异构体,并推断2-氨基吡啶的互变异构体存在,从而很好地得到了此系列紫外光谱的线性规律.     

A pair-excitation MCSCF calculation based on CNDO/INDO approximation I

Second-order multiconfigurational self-consistent field (MCSCF ) calculation has been programmed on the basis of CNDO /INDO molecular orbital bases, in which the configuration space employed is restricted within pair-excitations. Test calculations have been carried out for 17 small molecules. All the MCSCF ground states of these molecules have been successfully converged to their respective optimal states by employing a simple weighting scheme. This procedure provides a great savings in computer time. The MCSCF calculation on azetidine required only 27 min on a HITAC M-680H. The MCSCF energies of HF, F₂, and BH show improved behaviors up to large atomic distances (～7au).     

胺丶醇丶醚类化合物气相碱性的CNDO/2计算

The gas-phase basicities of compounds can be measured by their proton affinities. In this paper we he calculated the gas-phase basicities of about seventy compounds containing N or O by means of the method CNDO/2. For the alkylamines, alcohols, ethers and carbonyl compounds, computational results agree qualitatively with the experimental values. The sequences of gas-phase basicities for the series of these compounds are as follows: Et2NH>Me3N>t-BuNH2>Me2NH>i-PrNH2>n-BuNH2>n-PrNH2>EtNH2>MeNH2>NH3; Et2O>EtOMe>t-BuOH>Me2O>i-PrOH>n-BuOH>n-PrOH>EtOH>MeOH>H2O; n-PrCHO>EtCHO>MeCHO>HCHO; n-BuCO2H>n-PrCO2H>EtCO2H>MeCO2H>HCO2H; HCO2Bu-n>HCO2Pr-N>HCO2Et>HCO2Me>HCO2H Obviously, alkyl substitution plays a role to increase the gas-phase basicities. The squence of increasing effectiveness is t-Bu>i-Pr>n-Bu>n-Pr>Et>Me For the amines containing heteroatoms investigated here, the gas-phase basicities have the following order repectively: CH3NH2>NH2NH2>NH2OH>NH2F>NHF2>NF3 The gas-phase basicities of these compounds change regularly with various substitutents. For the aliphatic compounds, the gas-phase basicity increases with thosizo and the degree of branching of the alkyl groups. For the amines containing heteroatoms, the gas-phase basicity decreases with increasing of the electro-negativity of the substitutent. For the relationship between the gas-phase basksity and the charge distribution and the ionization potentials, the conclusions are as follows: (1) The gas-phase basicities of the homologous compounds are proportional to the electron density of the atom N or O, but those of Rn NH3-n and Rn OH2-n are inversely proportional to the electron denisty of atom N or O. This shows that the base strength of the molecule cannot be determined solely by the electron density of the individual atom. (2) In the protonation reaction the alkyl groups spread the charges from the charged center. This effect enables protonated cations to become more stable because of the charge distribution av     

CNDO/2 calculation on gas-phase basicity of amines,alcohols and ethers

The gas-phase basicity of about seventy compounds containing N or O have been calculated by means of the CNDO/2 method. For alkylamines, alcohols, ethers and carbonyl compounds, computational results agree qualitatively with the experimental values. We have also discussed the relationships between gas-phase basicities and charge distributions and ionization potentials of these molecules.     

CNDO/2 (complete neglect of differential overlap)-method for third-row molecules

An extension of the CNDO/2 method to compounds containing third-row elements (Germanium, Arsenic, Selenium and Bromine) is presented. Bond lengths, bond angles, dipole moments, and ionization potentials are considered.     

A CNDO/2 study of the ethylene-chlorine reaction

CNDO/2 calculations have been carried out of the energies of reaction and changes in charge distribution for some reactions involving the ethylene chlorine complex. The addition of chlorine to ethylene may involve the attack of a chloride ion on the ethylene chlorine complex.     

A further study on the CNDO/2 structures of water dimer

The structure of water dimer has been further studied by the CNDO/2 method. Some results have been found to be different from those in Thiel's paper.     

An application of the CNDO/2 and INDO theories to the calculation of molecular quadrupole moments

Values of molecular quadrupole moments are calculated using the CNDO/2 and INDO methods and compared with experiment and other calculations. The agreement is good, except for π systems both linear and planar. Consideration of this disagreement suggests that use of π AO's which are less compressed perpendicular to the molecular plane or the internuclear axis may produce better results. A preliminary method of this type is adopted to obtain satisfactory theoretical results for π systems, too.     

CNDO/2 calculations of the interactions of formamide and imidazole: Another failure of CNDO/2

The interaction energy curve of the formamide-imidazole system resulting from the CNDO/2 method exhibits a superfluous minimum at a much smaller separation than the sum of the van der Waals radii of the closest atoms, and a low maximum at this latter distance. The relationship of the interaction energies using CNDO/2 and those using the atomic charges on the two molecules is investigated for this system.     

Vibrational corrections to the charge and momentum densities of H2

Vibrational corrections to the charge and momentum densities of H₂ are calculated using the Wang wavefunction and various expressions for the dependence of the effective nuclear charge on the interatomic distance. Both harmonic oscillator and Morse potentials are employed for the vibrational wavefunctions, and excited as well as ground vibrational states are considered. The corrections to the momentum density, though considerably smaller than the charge density corrections, appear to be somewhat more sensitive qualitatively to the choice of both nuclear and electronic wavefunctions.     

A CNDO/2 study of the polymerization of beryllium chloride

The LCGO-CNDO/2 method has been applied in a study of polymer molecules of the type (BeCl₂) _n for values of n between 2 and 6. The calculations predict chain stretching and increasing. covalent character of the Be-Cl ring bonds as polymerization proceeds.