Pk. Rajan et al., PROTON NMR-STUDIES OF MOLECULAR-DYNAMICS IN [(CH3)(4)N](3)BI(2)X(9) (X=CL AND BR), Solid state communications, 100(7), 1996, pp. 535-540
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