Isotope effects on the dynamics of a supercooled van der Waals liquid

Deuteron magnetic resonance was used to study three differently isotope-lab eled species of the fragile glass-forming liquid ortho-terphenyl. The calor imetric glass transition of the isotope deuterated only at the central phen yl ring is significantly lower than that of the perdeuterated one. It is sh own that while the ortho-terphenyl molecule is not as rigid as previously o ften assumed, its overall reorientation geometry is independent of deuterat ion. The characteristic jump angles are found to increase with temperature, thus resolving an apparent discrepancy previously noted when comparing typ ical jump sizes from NMR with other data.