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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 trans-PA had contributions from several empirical modes, indicationg the more complieacy 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~(-1) higher 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.
The results of normal vibrational analyzes 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) the calculated energy was 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 trans-PA had contributions from several empirical modes, indicationg the more complieacy 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 200 cm -1 higher 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.