Comparative NMR properties of H2 and HD in toluene-d8 and in H2/HD@C60.

Spin-lattice relaxation times, T(1), have been measured from 200-340 K for the protons in H(2) and HD molecules dissolved in toluene-d(8) and incarcerated in C(60). It is found that HD relaxes more slowly than H(2) in both environments and at all temperatures, as expected from the smaller values of the spin-rotation and dipole-dipole coupling in HD compared to H(2). More detailed analysis using models developed to describe relaxation in both condensed media and the gas phase indicates that transitions among the rotational states of H(2) occur at a rate similar to those of HD in both toluene-d(8) solution and in C(60), in contrast to the situation in gas phase collisions between hydrogen and He or Ar, where the lifetimes of rotational states of HD are markedly shorter than those for H(2). Measurements of the relative (1)H chemical shifts of H(2) and HD, the coupling constant J(HD), and the widths of the HD peaks at various temperatures revealed only small effects with insufficient accuracy to warrant more detailed interpretation.