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Abstract

The dynamic disorder in solid acenaphthylene has been studied by deuterium NMR spectroscopy of deuteriated powder samples. The spectra exhibit lineshape and relaxation discontinuities in the transitions from Phase III, through Phase II, to Phase I (the latter being stable above 127 K). A detailed analysis of the lineshape and T1 relaxation in Phase I is presented. The results show the occurrence of two distinct equally populated, dynamic sites. The molecules in both sites undergo planar reorientation, but with different Arrhenius kinetic parameters: ko1 = 1.5 × 1012 s-1, ΔE1 = 10.2 kJ mol-1; ko2 = 4.8 × 1012 s-1 and ωE2 = 18.3 kJ mol-1. The results are consistent with earlier proton second moment and relaxation measurements and provide additional information on the nature of the motion. It is argued that the subspectra with the slower and faster motions are associated, respectively, with the (A,B) and C layers in the crystal.