CORRELATED VIBRATIONAL FREQUENCIES OF POLYMERS - MBPT(2) FOR ALL-TRANS POLYMETHINEIMINE

Polymethineimine is a conjugated polymer, isovalent with polyacetylene , which was first synthesized in 1971. It has been found to be a semic onductor, though prior theory did not support that. Except for its vib rational frequencies which have been measured, little about the polyme r is known. MBPT(2) is applied to calculate its equilibrium structure, band gap, and vibrational frequencies. The latter represents the firs t time an ab initio correlated method has been applied to this problem for an infinite system. The calculations for three basis sets, STO-3G , 6-31G, and 6-31G* demonstrate that both basis and electron correlat ion have a strong influence on its optimized geometry. The MBPT(2) ban d gap with a polarized basis set (6-31G*) is 4.7826 eV, compared to 8 .54 for Hartree-Fock. According to our MBPT(2) calculations, the band gap of all-trails polymethineimine would be estimated to be about 1 eV larger than that of polyacetylene, i.e. similar to 2.75+/-0.5 eV. Ele ctron correlation is demonstrated to have an important effect on the c omputed vibrational frequencies, which we find to be in good agreement with experiment. (C) 1998 American Institute of Physics.