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

Deuteron magnetic resonance was used to study three differently isotope-labeled species of the fragile glass-forming liquid ortho-terphenyl. The calorimetric glass transition of the isotope deuterated only at the central phenyl ring is significantly lower than that of the perdeuterated one. It is shown that while the ortho-terphenyl molecule is not as rigid as previously often assumed, its overall reorientation geometry is independent of deuteration. The characteristic jump angles are found to increase with temperature, thus resolving an apparent discrepancy previously noted when comparing typical jump sizes from NMR with other data. In order/disorder phase transitions of crystalline solids isotope effects are known to play an important role [1]. But in studies of glass-forming liquids it is often tacitly assumed that such effects are negligible. This is surprising in view of the fact that isotope labeling is a much used tool in studies dealing with soft condensed matter when employing nuclear magnetic resonance (NMR) as well as other experimental techniques [2]. There are only a few reports where either anomalous [3] or else effectively missing [4] isotope effects have provided important insights into the dynamics of the amorphous state. On the other hand, isotope shifts of melting points, Tm, particularly those emerging from proton vs. deuteron substitution are on the order of 0.5-5 K as documented for a large number of substances [1]. The calorimetric glass transition temperature, Tg, for many families of glass formers is related to Tm via the empirical relationship $T_{m}/T_{g} approx 1.5$ [5]. This rule thus rationalizes the observation of corresponding Tg shifts detected in adiabatic calorimetry studies [6]. However, reports concerning the impact of deuteration on the dynamics of supercooled small molecule liquids do not seem to be available. We have observed significant isotope effects on differently deuterated samples of ortho-terphenyl (OTP). This substance has played a key role in previous theoretical and experimental investigations dealing with the vitrification of fragile van der Waals glass-forming liquids [7]