IR POLARIZATION DIRECTIONS IN S-TRANS-1,3-BUTADIENE AND THE AVERAGE TOPOCHEMISTRY OF THE S-CIS TO S-TRANS PHOTOISOMERIZATION IN MATRIX-ISOLATION

Vibrational transition moment directions of s-trans-1,3-butadiene were deduced from measurements of its IR linear dichroism on samples partially oriented (i) by photoselective photoisomerization of s-cis-1,3-butadiene isolated in inert-gas matrices with linearly polarized light, (ii) in stretched polyethylene, and (iii) in stretched poly(ethylene-d4). The moment directions are in quite good agreement with those predicted by theory at the MP2/6-311G** level of approximation. With semiempirical methods and at very low levels of ab initio theory, the agreement is just as good for a few vibrations (C = C stretch, as CH2 stretch) but poor for others. The average relative orientation of the C1-C4 axis of the s-cis starting material and the C1-C4 axis of the s-trans photoproduct in the matrix has been determined; the two directions nearly coincide. A general computational procedure for relating the orientation of reactant and product molecules for an arbitrary reaction path in the absence of net torque on the environment has been formulated. For two plausible mechanisms of s-cis to s-trans isomerization, it predicts the observed near coincidence of the C1-C4 directions. For another reaction path, it predicts a 30-degrees angle between them. However, at present, it is unknown whether matrix photoisomerizations indeed proceed without exertion of a net torque on the environment.