PROTON NMR-STUDIES OF MOLECULAR-DYNAMICS IN [(CH3)(4)N](3)BI(2)X(9) (X=CL AND BR)

Proton spin lattice relaxation time (T-1) and second moment (M(2)) mea surements were made on TEMACB and TEMABB, and T-1 data show signatures of two phase transitions at 151K and 90K in TEMACB and 183K and 120K in TEMABB. The transition at 151K in TEMACB and at 183K in TEMABB are associated with a discontinuous change in the correlation time corresp onding to the overall motion of TEMA cation (tau(c1)), and there is co nsiderable dynamic inequivalence among methyl groups in both the compo unds. Activation energies for the dynamics of the TEMA cation and of d ifferent methyl groups are derived from the frequency dispersion and t emperature dependence of T-1 data. Comparison with similar studies on two other related TEMA compounds shows that the higher temperature tra nsition in all these compounds occurs whenever tau(c1) reaches a typic al value (similar to 3 x 10(-10) s), indicating that the high temperat ure phase becomes unstable whenever the overall motion of TEMA slows d own sufficiently, resulting in a phase transition. Copyright (C) 1996 Published by Elsevier Science Ltd