An analysis of the through-bond interaction using the localized molecular orbitals—IV: Long-range proton hyperfine coupling in exo- and endo-hydrogens in bicyclo[2.1.1]hex-5-yl radical

The INDO calculations were performed on bicyclo[2.1.1]hex-5-yl radical. From these calculations, it was confirmed that the hyperfine coupling constants depend largely on the geometry of the α hydrogen. The localized MO's were obtained from the canonical MO's calculated by using the INDO method. With the use of the localized MO's thus obtained, the variation in the hyperfine coupling constants at the 6exo- and 6endo-protons in this radical was explained in terms of the through-bond and/or the through-space interactions according to the procedure which we proposed previously. That is by the procedure we can selectively pick up a particular interaction between the specified localized MO's. The hyperfine coupling constant in this radical can be expressed by the summation of several interaction terms. The difference in the hyperfine spin coupling constants of the H_6exo and H_6endo in the radical now concerned has been attempted to explain using MO coefficients of the occupied orbitals.     

An attempt to approach the solutions of the N-electron CNDO hamiltonian for conformational studies of conjugated systems

While dealing with the same N-electron hamiltonian, the SCF approximation and the third order PCILO results completely disagree about the conformation of conjugated systems. An attempt is made, using the CIPSI algorithm, to approach the exact solution of the N-electron CNDO hamiltonian for the butadiene molecule starting from either a priori localized, SCF localized or SCF delocalized MO's. The partial CI's performed from SCF delocalized MO's give arbitrary results when increasing the number of doubly excited determinants and Pancir?'s recent results is fortuitous; on the contrary the results obtained from a priori and SCF localized MO's have better convergence and consistence; from our best calculations the final solution seems to be a rather flat potential curve, the stable minimum being the trans planar minimum, with a second gauche minimum for θ = 120-150° (ΔH ≈ 1 kcal/mole), separated by a weak barrier (ΔH ≈ 2 kcal/mole). The third order PCILO solution is in much better agreement with this estimate of the exact solution than the SCF one.     

A straightforward improvement of AB initio SCF wavefunctions

A way of estimating SCF MO's in terms of a large uncontracted basis set from known SCF MO's in terms of a small uncontracted basis set is suggested by using the principle of maximum overlap of both kinds of molecular orbitals. A significant improvement of contours and one-electron properties is observed from the test calculations for H₂O.     

Multiconfiguration self-consistent wavefunctions for CH4, C2H4 and C2H6

The results of several MC SCF calculations on CH₄, C₂H₄ and C₂H₆ with minimun bases of Slater type AO's are reported. The computing method is a quadratically convergent process. Better final energies are obtained if localized MO's are used.     

Synthese und EPR-spektroskopie dicyclopentadienylanaloger der schlenkschen und tschitschibabinschen kohlenwasserstoffe

The synthesis of several hydrocarbon biradicals consisting of two tetraphenylcyclopentadienyl moieties with different bridge fragments is described. The ESR powder spectra show that there is an intramolecular coupling of two unpaired electron spins to a triplet spin state. The magnitude of the zero field splitting (zfs) is used to discriminate between different molecular conformations. More detailed structural informations are obtained from the observed zfs by quantummechanical calculations of the zfs-parameter D as a function of various torsional angles and cis- or trans-orientations of molecular fragments. The behaviour of the four highest occupied MO's with respect to structural changes in the biradicals is discussed.     

Topological effects displayed in absorption and photoelectron spectra

For topologically related pairs of isomers, S and T, a theorem we term TEMO (Topological Effects on Molecular Orbitals) has been derived; it organizes the MO pattern of the isomers so that in the range of two subsequent MO's of S there are alternately two and no MO's of T. It follows that depending on the number of electrons, the HOMO-LUMO separation in S and T differs in a topologically determined manner. The wave-lengths of the absorption maxima of the longest wave-length transitions in polycyclic aromatic hydrocarbons, polymethines, and heterocyclic compounds are in accord with TEMO. The photoelectron spectra of these compounds exhibit the TEMO pattern of their MO's even in cases where Koopmans' theorem fails.     

Etude structurale de composes organosilicies par diffusion rayleigh depolarisee : V. Proprietes acceptrices d'electrons du silicium dans les systemes SiOC et SiOSi

Among the characteristic properties of the SiOC and SiOSi systems relative to those of the corresponding COC systems, the optical anisotropy is a new example which shows the electron-acceptor ability of silicon by (p → d)π interaction in compounds of the type R₃SiO∑ (R = Me, Et, Pr; ∑ = H, Me, SiMe₃, CH₂-t-Bu, CHMe₂, CMe₃). Effectively, the existence of a linear relationship between the optical anisotropy or the principal optical polarisabilities of the OSiR₃ groups and the Taft's parameter σ^★ of the ∑ groups in this series of compounds, shows that the electron-accepting character of silicon increases with increasing electron-donating ability of the group ∑; this relationship simultaneously translates an increase of the electronic density and mobility perpendicular to the OSi axis to the detriment of the parallel direction. In application, we calculated the σ^★ values for the groups ∑ = SiEt₃, SiPr₃ and the OSiMe₃ optical anisotropy value in PhOSiMe₃.     

Vibration-to-rotation and vibration-to-vibration energy transfer between diatomic molecules

A classical model for VR and VV energy transfer between two dissimilar diatomic molecules is proposed. For the exponential repulsive potential, in the limit as the rotation of the molecules goes to zero, the present model reduces to the previous results for VT and VV energy transfer. For the hydrogen halides, whose relaxation is generally accepted as being governed by VR energy transfer, the Pr's predicted by the present theory agree reasonably well with the experimental data. In particular, the positive and negative temperature dependence of the VV Pr's for N₂HI and N₂DI are predicted.     

Configuration interaction study of H2 using a modified slater-type orbital

A non-standard 1s basis function, χ = (1 + ar²) exp(?ζr), is used to approximate the ground state of the hydrogen molecule in a configuration interaction framework and using numerical integration over MO's. Results are compared to the traditional 1s STO.     

Electron diffraction and vibrational spectroscopic investigation of the molecular structure of (chloromethyl)trichlorosilane

An electron diffraction analysis of the molecular structure of the title compound has been carried out, and related vibrational spectroscopic measurements and calculations have been made. The main bond lengths (r_g and bond angles r_α) are as follows: SiCl, 202.8(2); SiC, 185.1(10); CCl, 179.4(11); CH, 111.2(18) pm; SiCCl, 111.7(4);l ClSiC, 109.95(21)°. The conformation of the molecule is staggered. The barrier to internal rotation is estimated to be around 10 kJ mol^?1.     

Structure-resonance theory for homoaromatic bridged annulenes

The π systems of bridged annulenes can ben represented by π molecular graphs with homoconjugative interactions at the bridge positions. Nonbonding MO's of odd molecular graphs derived from the parent bridged annulene graphs can be used to carry out structure-resonance theory calculations. A general outline of possible applications is given. Specific comparisons are made between calculated bond orders and bond lengths, and between calculated and experimental ionization potentials. The concept of neutral homoaromaticity is supported by good agreement between calculated and experimental properties.     

Photoelektronenspektroskopische untersuchung von tris(trimethylelement(IV)aminen, -phosphinen, -arsinen und -stibinen

The PE spectra of [(CH₃)₃Sn]₃N, [(CH₃)₃E]₃P (E = C, Si, Sn), [(CH₃)₃Si]₃As and [(CH₃)₃E]₃Sb (E = C, S, Ge, Sn) are analyzed. The constant level of the ionization potentials of the n-hybrid orbitals of N, P, As, and Sb, together with changes in the hybridization and interactions with ligand MO's is discussed by qualitative MO investigations.     

Infrared Spectroscopic and dielectrometric investigation of the molecular geometry of isocyanato- and isothiocyanato-silane derivatives

The molecular geometry of compounds of the series (CH₃)_nSi(NCX)_4-n, (where n  0, 1, 2, 3 X  S, O) were investigated with the objective of determining the magnitudes of the SiNC angles. Measurement of the infrared spectra and dipole moments were carried out. The infrared spectra unambiguously suggest linearity, and this is supported by dipole moment calculations. The deviation from linearity observed by electron diffraction is a consequence of shrinkage effects δ(SiNCX) resulting from low frequency, large amplitude vibrations. From the dipole moment values it is concluded that the N—Si—N angle in the di- and trifunctional isothiocyanates is larger than the tetrahedral angle. The structural differences between the silicon and carbon compounds can be attributed to (dp)π bonding in the former.     

Cleavage of silicon- and germanium-transition metal bonds. Dependences of the stereochemistry on the nature of the ligands and the geometry of complexes

The cleavages of some new optically active complexes containing CoSi (orGe), MnSi (orGe), ReGe and WGe bonds are described. Electrophiles cleave the CoSi bond with good retention of configuration at silicon, while the MnSi bond is not cleaved under the same conditions. The M′Si and M′Ge bonds (where M'  transition metal) are cleaved by nucleophiles with retention or inversion of configuration. In the case of triginal bipyramidal geometry (cobalt complexes) the stereochemical outcome of the reaction is strongly dependent upon electronic effects, the size of the ligand trans to the CoSi (orGe) bond, and the nature of the nucleophilic reagant, in accord with the general rules for nucleophilic substitution at silicon. In contrast the transition metalsilicon orgermanium bonds in the octahedral complexes of manganese, rhenium and tungsten are always cleaved with poor retention of configuration regardless of the nature of the ligands or the nucleophilic reagent. The results provide the first cases in which the stereochemistry of nucleophilic displacement at silicon is independent of the electronic features of both the leaving group and the nucleophile.     

Non-empirical calculations of chlorine quadrupole coupling parameters in chloromethanes and chlorosilanes

Non-empirical calculations of the field gradient tensor at the chlorine nucleus are resorted for chloromethane, trichloromethane, chlorosilane and trichlorosilane. The calculations agree quite well with the experimental quadrupole coupling constants. The calculated asymmetry parameters in trichloromethane and trichlorosilane are low and the principal z axis of the field gradient tensor is found nearly to coincide with the CCl and SiCl bond directions.     

Electron diffraction study on the molecular structure of methyltrimethoxysilane

The electron diffraction data for methyltrimethoxysilane are consistent with a C₃ symmetry model, the predominant forms of which have rotational angle(s) between 100 and 155° around the SiO bond (the anti conformation of the CSiOC chain would respond to 0°). There is probably large amplitude motion around the SiO bonds. The following bond lengths and bond angles were determined: r_a(CH) 1.093 ± 0.005, r_a(SiC) 1.842 ± 0.013, r_a(SiO) 1.632 ± 0.004, r_a(OC) 1.425 ± 0.004 », ∠CSiO 109.6 ± 0.5°. and ∠SiOC 123.6 ± 0.5°.     

Photoelectron spectroscopy study of the triphenyl derivatives of the group IV elements

The photoelectron He(I) spectra of several phenyl derivatives of the Group IV elements are reported, and the first few bands are assigned to the corresponding MO's. A sizable interaction among the π orbitals of the rings has been found for the carbon derivatives HCPh₃, HC(mesityl)₃ and H₂CPh₂, but not for the Si, Ge and Sn triphenyl derivatives. The exceptional behaviour of the carbon compounds has been attributed to the short central atom—ring distance, by analogy with findings for the Group V triphenyl derivatives.The charge transfer from the rings towards the Si atom is substantially reduced compared with that in H₃SiPh.     

Calculation of valence electron vertical ionization potentials using a limited Cl approach

A method is outlined for the calculation of valence electron vertical ionization potentials based on CNDO/2 and CNDO/S SCF molecular ground states improved by including a limited number of doubly excited configurations and ionic states obtained from a single Cl calculation considering a limited set of singly and doubly excited configurations based on the closed shell ground state MO's of the parent molecule. From numerical results on carbon dioxide, sulphur dioxide, ethylene, and trans-butadiene it is shown that the Cl procedure leads to the right sequence of ionic states and their reasonable relative location in contrast to respective Koopmans' ionic states predictions.     

Calculations of vibrational energy transfer using semi-classicals matrix theory

We utilize an extension of Miller's semi-classical S matrix theory to calculate resonant and non-resonant vibrational energy transfer probabilities. The collisions under study are collinear D₂D₂ interactions at energies below and above the classical dynamic transition threshold and collinear H₂D₂ interactions above the dynamic threshold. Below threshold we employ an initial angle representation. We find that the computed probabilities are in substantial agreement with exact quantum mechanical computations and represent a major improvement over quasi-classical results. At energies above threshold we apply the first order and the classical semi-classical versions of the theory. The results indicate fair agreement with quantum mechanical calculations, but no significant improvement over quasi-classical results.     

N,N'-diarylformamidinato complexes of the platinum group metals

Some platinum group metal hydrides react with N,N'-diarylcarbodiimides in boiling toluene to yield products containing novel bidentate N,N'-diarylformamidinate (ArNCHNAr) ligands.