On the basis of extensive quantum chemical calculations for 1,2 elimination of H X from CzHsX (X = F, C1, Br, and I) we have obtained a strongly asymmetric transition state that differs greatly from previous models for this reaction. The effect of a-chloro/fluoro substitution was also investigated, specifically for HF and H C l elimination. W e confirm the electron donor/acceptor property of these substituents but find the conventional explanation of their effect on the activation energy to be unsatisfactory.

Ab initio restricted Hartree–Fock 6-31G calculations are reported for the static longitudinal hyperpolarizability of the linear polyenes C4H6 through C44H46. Using a new extrapolation technique the infinite chain value in polyacetylene is determined with an accuracy similar to that achieved for small molecules. This is the first in a series of articles leading to a comprehensive ab initio treatment for the nonlinear optical properties of conjugated polymers.

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The evolution in the static longitudinal polarizability and second hyperpolarizability of the C 4 H 2 through C 54 H 2 oligomers of polyyne is studied at the Hartree-Fock level of theory. We find that the calculated values are highly sensitive to the adopted geometry. Using improved geometries, new extrapolation procedures and careful finite field determinations we extend earlier work by others so as to provide reliable estimates for the above properties in the infinite polymer limit.

Nonlinear optical properties of the linear polyenes C4H6 through C30H32 are calculated at frequencies below the first resonance using ab initio time-dependent Hartree–Fock theory. The results are used to determine accurate values for the infinite polymer, polyacetylene, by means of a new extrapolation method. Dispersion coefficients describing the low frequency behavior are obtained for the largest oligomer.