| Popis: |
Previous work by Woessner and Gutowsky has shown the behavior of T1 (35Cl) in methylene chloride to be anomalous in the temperature range 150 K to 176 K. Semin and Babushkina report two phase transitions in this compound, and in order to clarify a possible association between these phase transitions and the T1 behavior, we undertook the measurement of deuterium quadrupole coupling and chlorine relaxation in CD2Cl2. Only a single phase was observed in our experiments for both CH2Cl2 and CD2Cl2. The deuteron quadrupole coupling varies qualitatively with temperature as expected from Bayer's model below 150 K, but the temperature coefficient becomes unusually large at higher temperatures. The asymmetry parameter decreases from 0.0308 at 77 K to ca. zero at 150 K. We estimate the static values for coupling constant and asymmetry parameter as 175.67 to 176.33 kHz and 0.0569 to 0.0605, respectively. The ratio of 35Cl to 37Cl T1 values is given by the square of the ratio of the quadrupole moments only up to ca. 150 K. Above this temperature, the ratio decreases steadily toward unity as the melting point is approached. T1 (35Cl) shows typical activated rate process behavior in this short temperature interval, corresponding to the activation energy of 5 kcal/mol quoted by Woessner and Gutowsky. The observed behavior of quadrupole coupling in this material suggests that the “premelting lattice disorder” mentioned by Woessner and Gutowsky is, in fact, manifested in T1 as a slow dynamic process, probably a rotation about the molecular figure axis. |
