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Doppler broadening studies of the quasi-elastic peak of cold neutrons scattered from poly-(dimethylsiloxanes) are reported. The Doppler broadening is observed in the low-molecular-weight liquids and persists in high-molecular-weight gums and even in the cross-linked rubbers. The selfdiffusion constants determined for the low-molecular-weight liquids are larger than the corresponding value obtained from n.m.r. spin echo measurements. Since the latter technique undoubtedly measures the translational contribution to self-diffusion, it is concluded that the neutroiiscattering result includes a major contribution from internal rotational motions of the polymer chain.The activation energy for the neutron diffusional process is 2.0 kcal mol-' compared with 4.0 kcal mol-' for the translational diffusion process. A similar Doppler broadening phenomenon has been observed in poly(propy1ene)oxide at elevated temperatures.In a neutron incoherent scattering experiment 1 *

Torsional frequencies have been assigned in the far infra-red and Raman spectra of C2H5C1, CH3. CHC12, CHzCl . CHzCI, CHCl2CH2C1, CH2Cl. Cc13, C H C l z . CHC12 and CHc12. CCl3. Barrier heights have been calculated for the symmetric top molecules ; V3 ranges from 3.3 to 14-2 kcalmole-1. The problem of estimating barriers in asymmetric top molecules is more difficult. Nevertheless, for these molecules, barrier heights ranging from 5.1 to 12-0 kcal mole-1 have been calculated, which are comparable in magnitude to those obtained for the symmetric top molecules. * We thank I.C.I. Fibres Ltd., Harrogate, for the use of this spectrometer.