A deuteron NMR study of the molecular dynamics of solid cyclopentane
| Autor: | Dennis A. Torchia, James W. Mack |
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| Rok vydání: | 1991 |
| Předmět: | |
| Zdroj: | The Journal of Physical Chemistry. 95:4207-4213 |
| ISSN: | 1541-5740 0022-3654 |
| DOI: | 10.1021/j100164a009 |
| Popis: | The authors report models for the molecular motions of cyclopentane in its three solid phases, based upon analyses of equilibrium and partially relaxed {sup 2}H NMR line shapes. At temperatures below {minus}150C, phase 3, the spectrum of ({sup 2}H{sub 10})cyclopentane is a Pake pattern having a quadrupole coupling constant of 171 kHz. This observation together with the T{sub 1} value, 9.0 s, observed at {minus}178C, shows that molecular reorientation of the C-{sup 2}H bonds is limited to small-amplitude picosecond motions in phase 3. In contrast, in the {minus}150 to {minus}172C range, in supercooled phase 2, line shapes are axially asymmetric, order parameters are small (S{sup 2} ca. 0.01-0.05), and T{sub 1} 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 they interpret as being due to a combination of pseudorotation and a rocking motion about the 5-fold molecular axis. In the temperature range {minus}135 to {minus}150C, normal phase 2, spectra lack fine structure and the line shape has a Lorentzian appearance. In the temperature range {minus}97 to {minus}135C, phase 1, the spectra are usually composed of a number of doublets, whose splittings depend uponmore » the orientation of the sample relative to the external field, because in phase 1 the sample typically consists of a nonrandom distribution of crystallites. Occasionally a nearly random distribution of crystallites is obtained in phase 1, based upon the observation of a nearly ideal Pake pattern. Throughout phase 1 the observed quadrupole splitting is less than 3 kHz, showing that cyclopentane reorientation in this phase is, in effect, nearly isotropic.« less |
| Databáze: | OpenAIRE |
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