Longitudinal acoustic mode in polymers. VIII. Nature of mass perturbations in terminally brominated paraffins

Raman spectra have been obtained of the longitudinal acoustic mode (LAM) in α‐BrC20H41 and α, ω‐BrC20H40Br, and compared with such spectra for n‐C20H42. Normal coordinate analyses of these three molecules clearly account for (i) the frequency decreases on Br substitution; (ii) the presence of doublet LAM modes in both brominated molecules; and (iii) the decrease in the total intensity of the LAM mode resulting from the added masses, as distinct from the increase predicted by the elastic rod model. These results thus provide experimental support for predictions made from normal coordinate and intensity analyses regarding the influence on LAM bands of mass perturbations at the ends of paraffinic chains.