PMR studies of molecular motions, phase transitions and quantum tunnelling in NH4SnCl3 and N(CH3)4SnCl3

Abstract

Molecular reorientation and low temperature relaxation effects of NH+ 4 ion and the effect of CH3 substitution (in place of H) are investigated by proton spin lattice relaxation time (T1) measurements at 10 MHz in NH4SnCl3 and N(CH3)4SnCl3 in the temperature range 4.2 K upto the melting points of the compounds (≊ 440 K). Phase transitions around 360 K in NH4SnCl3 and around 361 and 116K in N(CH3)4SnCl3 have been observed. In NH4SnCl3, the high temperature minimum at 330.5 K is attributed to the translational diffusion of the NH+ 4 ions, while the other T1, minima at 103.5, 60 and 50 K are ascribed to the reorientations of the NH+ 4 ion about the C2 and C3 axes. The low temperature minimum at 13.5 K is attributed to rotational tunnelling of the NH+ 4 ions. In N(CH3)4SnCl3, in addition to the high temperature minima at 212.2 and 182.6 K due to N(CH3)4 tumbling and CH3 reorientation, a temperature independent T1 behaviour between 83 and 31 K is observed, below which T1 decreases and tends to go through a minimum around 5 K. This low temperature minimum is attributed to rotational tunnelling of the CH3 groups. The motional parameters and tunnel frequencies are estimated.