The results of normal vibrational analyses of trans

trans-2

4-hexadiene and the backbone of trans

trans-1

4-diphenyl-butadiene were used in the normal mode computation of trans-polyacetylene (PA). Only 8 non-zero calculated frequencies were obtained which obeys the rule of 3N-4. The potential energy distribution (PED) data were in good agreement with the empirical assignment of Shirakawa et al. and Kozmany

but most of the vibrational frequencies of ttrans-PA had contributions from several empirical modes

indicationg the more conplicacy in trans-PA molecular vibration than in the model molecule case.The calculated C=C and C-C stretching frequencies of trans-PA were over 200cm-1higher and lower than the observed ones

respectively

due to the longer effective conjugate length in the trans-PA.This was shown by the dependence of the PED-weighted average frequencies of C=C and C-C stretchings on the force constants f(C=C)2 and f(C-C)2.