A H-2 NMR-STUDY OF THE MOLECULAR-DYNAMICS OF SOLID CYCLOPENTANE

We report models for the molecular motions of cyclopentane in its three solid phases, based upon analyses of equilibrium and partially relaxed H-2 NMR line shapes. At temperatures below -150-degrees-C, phase III, the spectrum of [H-2(10)]cyclopentane is a Pake pattern having a quadrupole coupling constant of 171 kHz. This observation together with the T1 value, 9.0 s, observed at -178-degrees-C, shows that molecular reorientation of the C-H-2 bonds is limited to small-amplitude picosecond motions in phase III. In contrast, in the -150 to -172-degrees-C range, in supercooled phase II, line shapes are axially asymmetric, order parameters are small (S2 ca. 0.01-0.05), and T1 values are 50-80 ms. The data indicate that fast-limit motion predominates in this phase, which is simulated by using jump models involving four sites, which we interpret as being due to a combination of pseudorotation and a rocking motion about the 5-fold molecular axis. In the temperature range -135 to -150-degrees-C, normal phase II, spectra lack fine structure and the line shape has a Lorentzian appearance. In the temperature range -97 to -135-degrees-C, phase I, the spectra are usually composed of a number of doublets, whose splittings depend upon the orientation of the sample relative to the external field, because in phase I the sample typically consists of a nonrandom distribution of crystallites. Occasionally a nearly random distribution of crystallites is obtained in phase I, based upon the observation of a nearly ideal Pake pattern. Throughout phase I the observed quadrupole splitting is less than 3 kHz, showing that cyclopentane reorientation in this phase is, in effect, nearly isotropic.