Slow-Neutron Scattering and Rotational Freedom of Methyl Groups in Several Organic Compounds

The total cross-sections of CH3NO2, (CH3)2CO2, (CH3)2SO, (CH3CO)2O, CH3CH, (CH3)2SiCl2 and dimethyl polysiloxane (silicone oil) have been measured for neutrons with wavelength λn in the range 5 - 10 Å. The scattering cross-sections per H atom, os/H, may be approximated by straight lines os/H = as + bsλn, for λn > Å. An empirical correlation is observed to exist between the slopes bs and the barrier heights for internal rotation of CH3 groups in the molecules, and a calibration curve is plotted using some of the experimentally determined slopes and published values of barrier heights determined by other methods. From the slopes 12.3 ± 0.5 b/Å-H for (CH3)2SiCl2, and 12.3 ± 0.3 b/Å-H for dimethyl polysiloxane, it is concluded that the internal rotation of CH3 groups in these compounds is practically free. An average barrier height of ∼ 1 kcal/mole is estimated for (CH3CO)2O, , from the experimentally determined slope 10.8 ± 0.3 b/Å-H. (author)