Unusual proton Zeeman spin relaxation in an organic solid: several crystal polymorphs or different glass structures?

Solid state proton Zeeman relaxation rate R1z measurements in two isomers of an organic solid (1- and 2-ethylnaphthalene) are reported. The samples are liquids at room temperature and the temperature T and Larmor frequency omega dependence of R1z depends strongly on how the sample is solidified. Methyl group (CH3) rotation is responsible for the proton spin relaxation and the methyl groups serve as probes of the local environment. The R1z measurements clearly distinguish between different solid states due to the differences in local structure at the several-molecule level. The experiments cannot be used to determine the states of these Van der Waals solids although interpreting the relaxation rate data suggests the states are unusual. We propose that these systems might exist in two (2-ethylnaphthalene) or more (1-ethylnaphthalene) polycrystalline polymorphs or that we are observing distinguishable glassy states, or, both. A phase transition is observed in 1-ethylnaphthalene. Variable temperature X-ray studies of organic systems that solidify well below room temperature are difficult, or at least not routine, and proton spin relaxation measurements serve as a convenient starting point for investigating such systems.