The reorientations between unequal wells in Cl3P=NCCl(CF3)2 crystal as studied by the 35Cl nuclear quadrupole resonance method

Abstract

The crystal Cl3P=NCCl(CF3)2, which discovered autonomous reorientations of the asymmetric fragment CCl(CF3)2 between unequal potential wells, is a unique object that allowed to observe, using the NQR method, the effect of unequal-wells reorientations on quadrupole nuclei, both participating in the hindered motion and "fixed" quadrupole nuclei of neighboring atoms, which are connected with the moving fragment by nonvalent interactions. This study repeats the quantitative analysis of the temperature dependences of the spin-lattice relaxation time T1 measured in this compound earlier by the NQR method for both types of quadrupole nuclei. An experimental proof is obtained that the dependence T1(T) of quadrupole nuclei directly involved in the unequal-wells reorientations provides information about the potential barrier of the transition from the ground state to the metastable, which is the opposite of the previously accepted point of view. This fact eliminates contradictions in the earlier interpretation of the NQR data for the "fixed" quadrupole nuclei. New analysis gives a self-consistent adequate description of the experimental results obtained by the NQR method and clarifies the real parameters of all molecular movements affecting the relaxation behavior of quadrupole nuclei in different molecular fragments. On the other hand, the dependence T1(T) of quadrupole nuclei involved in unequal-wells reorientation directly allowed to establish the principal feature of such reorientation, which consists in reducing the efficiency of the reorientation mechanism and manifests itself in shifting the dependence T1(T) towards higher temperatures. Quantitatively, we can observe such a shift as a decrease in the pre-exponential factor in the Arrhenius equation, which describes the reorientation contribution to the dependence T1(T) of moving nuclei. This decrease can reach several orders in comparison with the usual value of the factor (1012–1013 s–1) and makes it possible to evaluate the difference D between the energy levels of the main and metastable orientations. On the basis of the obtained data, it is assumed that the general form of the expression for the quadrupole spin-lattice relaxation rate of moving nuclei should contain a factor depending on the energy difference D of the orientational positions, and a factor depending on the angle between the direction of the electric field gradient on the quadrupole nucleus and the axis of the reorientational motion.