D.W. Davies

Valency electron molecular orbital calculations: II. Self-consistent perturbation theory and applications to electric polarizabilities

A self-consistent molecular orbital perturbation method suitable for use with the CNDO/2 approximations is formulated. The first-order perturbed equation is solved with an electric field perturbation for several molecules, and the polarizabilities are obtained. The results are too low, particularly for polarizabilities along short molecular axes, with a valency orbital basis set. The inclusion of 2p orbitals on hydrogen atoms gives improved results; but it is concluded that additional basis orbitals on other atoms are required for a valid interpretation of polarizabilities.

Valency electron molecular orbital calculations

The approximate self-consistent molecular orbital method CNDO/2 is applied to calculations on all the valency electrons of some fluorobenzenes and fluoronitrobenzenes. The charge distribution, which shows σ-electron acceptance and π-electron donation by the fluorine atoms, is in accordance with general chemical ideas. It leads to satisfactory values for the dipole moments of the fluorobenzenes and slightly less good values for the fluoronitrobenzenes. The spin densities on the carbon, nitrogen and oxygen atoms obtained for the anions of the fluoronitrobenzenes by the unrestricted form of the method give a satisfactory representation of the experimental hyperfine splitting constants. The elements of the bond order matrix are used with the average energy approximation and theoretical values for r -3 integrals to obtain relative shielding constants for the fluorine nuclei. The assumption made by other workers that the σ-electron terms are effectively constant for the fluorobenzenes is found to be justified. ...

Close-coupling calculations of polarized rotational cross sections for (Ar, LiF)

Close-coupling calculations with 121 channels at an energy of 155 cm-1 have been made for the rotational scattering of LiF by Ar, using a Lennard-Jones type potential with long range anisotropic terms. Integral cross sections for transitions (jmj )→(j′mj′ ) are given for j = 0, 1, 2, 3, 4 and j′ = 0, 1, 2, 3, 4, 5, and also for the j = 0→8 and j = 0 → 10 transitions. Differential cross sections were obtained for Δj = 1 and j = 0, 1, 2, 3, and for j = 1 →1, 1 →3, 0 →2 and 3 →3 transitions. Rotated scattering amplitudes were used to obtain integral cross sections by numerical integration, but these show little evidence that for Δj ≠ 0 the Δm = 0 transition has a relatively large cross section along an axis perpendicular to the bisector of the angle between the initial and final relative velocity vectors. Certain rules for Δm are suggested, and a comparison with experimental results for (Ar, LiF) and (Ar, CsF) is given.

An ab initio configuration interaction calculation for the polarizability of CsF

Extended gaussian basis set ab initio molecular orbital calculations, both with and without configuration interaction, have been made for the polarizability components of CsF, using the distant charge finite field method. Preliminary calculations to obtain a suitable basis were made for the polarizability of LiF and for the dipole and quadrupole moments of LiF, RbF and CsF. A revised value is suggested for the quadrupole moment of RbF. The polarizability anisotropy of CsF is compared with previous empirical values.

Proton and fluorine nuclear shielding anisotropies

Experimental values for 1H and 19F nuclear shielding anisotropies obtained from solid state, nematic phase, and molecular beam measurements are reviewed. Certain discrepancies are pointed out. An anisotropy operator is defined as a sum of two operators corresponding to the Lamb and high frequency terms. Approximations to the high frequency operator are given. Scales of absolute shielding constants are obtained, so that the magnitudes of the anisotropies can be discussed on the basis of non-empirical and semi-empirical calculations, and of the experimental results. The hypothesis due to Ramsey that the potential is constant for a given nucleus in different molecules is used to obtain approximate values for the Lamb term in the nuclear shielding constant. The distinction between shielding in large and small molecules is stressed. For the fluoromethanes and fluorobenzene the results of CNDO/2 molecular orbital calculations are used for a qualitative discussion of the anisotropies.

Interaction potentials and rotational scattering for H2 and MuH with He

Elastic and inelastic differential cross-sections for the rotational scattering of p-H2 and MuH by He at a single collision energy are calculated with potential surfaces obtained by varying the parameters in a model potential. The importance of the attractive and repulsive terms in the potential, and of the available kinetic energy in the different channels is demonstrated, and it is shown that the whole potential surface determines the scattering cross-sections. Inelastic and elastic scattering are compared, and the differences between the rotational scattering of homonuclear and heteronuclear homopolar molecules are ascribed to the molecular constants rather than to the numbers of open channels.