Investigation of the methyl torsion in isotactic polypropylene — comparison between neutron inelastic scattering spectra and normal coordinate calculations

Incoherent neutron scattering spectra for polypropylene and its partially deuterated analogue allow identification of the methyl torsion at 230 cm^?1. Comparison of the experimental spectra with the calculated densities of states shows discrepancies with the expected intensity of the methyl torsional band. Results from a stretch-oriented sample arranged so that the wave vector $\text{Q}$ is first parallel ($\text{Q}{}^{\mathrm{\parallel }}$) and then perpendicular ($\text{Q}{}^{\mathrm{\perp }}$) to the helical chain axis indicate that the torsion is more intense and probably has a small frequency dispersion around 240 cm^?1 for $\text{Q}{}^{\mathrm{\parallel }}$ and is weaker with a broad dispersion centred at 220 cm^?1 for $\text{Q}{}^{\mathrm{\perp }}$.