Theoretical study of the infrared absorption intrans-(CH)xandtrans-(CD)x

A calculation of the vibration spectra of trans-${\left(\mathrm{CH}\right)}_x$ and trans-${\left(\mathrm{CD}\right)}_x$ is presented; it includes for the first time the out-of-plane motions and the infrared (ir) absorption intensities. To obtain a reliable set of transferable parameters, extensive simultaneous optimizations of both the force constants and the electro-optical parameters were performed to solve the inverse spectral problem for several short polyenes. The calculations evince a novel major effect in trans-polyacetylene: The $A_u$ in-plane C-H bending mode is expected to give rise to the strongest ir absorption band at ∼ 1300 ${\mathrm{cm}}^{-1\mathrm{}}$ but its intensity is in fact dramatically reduced. The same effect is seen in the deuterated polymer. Lowering of the resonant Raman frequencies and the ir band disappearance are both believed to be caused by coupling of the vibrations to the $\pi$-electron system. For the ir case this is probably caused by the large dipole moments of the vibrating C-H bonds.