DYNAMIC DISORDER IN ACENAPHTHYLENE - A SOLID-STATE DEUTERIUM NMR-STUDY

The dynamic disorder in solid acenaphthylene has been studied by deute rium NMR spectroscopy of deuteriated powder samples, The spectra exhib it lineshape and relaxation discontinuities in the transitions from Ph ase III, through Phase II, to Phase I(the latter being stable above 12 7 K), A detailed analysis of the lineshape and T-1 relaxation in Phase I is presented, The results show the occurrence of two distinct equal ly populated, dynamic sites, The molecules in both sites undergo plana r reorientation, but with different Arrhenius kinetic parameters: k(1) degrees = 1.5 x 10(12) s(-1), Delta E-1 = 10.2 kJ mol(-1); k(2) degre es = 4.8 x 10(12) s(-1) and Delta E-2 = 18.3 kJ mol(-1). The results a re consistent with earlier proton second moment and relaxation measure ments and provide additional information on the nature of the motion, It is argued that the subspectra with the slower and faster motions ar e associated, respectively, with the (A,B) and C layers in the crystal .