Heat capacity of one-dimensional chains of methane molecules in the outer grooves of carbon nanotube bundles

The heat capacity at constant pressure C _P of 1D-chains of methane molecules adsorbed in the grooves on the outer surface of the bundles of closed single-walled nanotubes was measured in the temperature range from 2 to 60K for the first time. The behavior of the temperature dependence of C _P below 12K indicates the presence of a Schottky-type anomaly originated from the tunneling between the lowest energy levels of the rotational spectra of the A, T, and E nuclear-spin species of methane molecules. The feature observed in the vicinity of 14K is presumably caused by an orientational phase transition, in which the nature of the rotational motion of the molecules changes from libration to hindered rotation. It was found that the rotational heat capacity in the temperature range of 30-40K is close to that of freely rotating methane molecules. An increase in the derivative dC _P (T)/dT above 40K and the feature in the C _P (T) near 52K are due to the decay of 1D chains of CH ₄ .