The influence of double bonds on the Raman active longitudinal acoustical model (LAM) in trans-alkenes

Raman measurements on a series of trans-alkenes having the double bond at various locations along the backbone have been obtained. Compared to the n-alkanes, a dramatic decrease in the intensity of LAM-1 has been observed, while that of LAM-3 and LAM-5 remains relatively unchanged. Normal coordinate calculations indicate that in the trans alkenes there is a significant long-range coupling between longitudinal and transverse motion in LAM-1 due to the presence of the double bond which effectively diminishes its intensity. In LAM-3 and LAM-5 the longitudinal/transverse coupling is less, and much more localized about the double bond, preserving both the longitudinal character and intensity of these modes. Raman intensity variations in the -CH2- bending region have been utilized to study the lateral packing of methylene groups in the crystalline subcell, and the trans-alkenes have been found to crystallize in structures analogous to their polyalkenamer counterparts. © 1982 American Institute of Physics.