14N NQR study of molecular reorientation and phase transition in s-triazine

Abstract

14N NQR frequencies, linewidth, and spin–lattice relaxation times were measured in s‐triazine as a function of temperature between 77 and 280 °K. The second order phase transition from hexagonal to monoclinic which takes place at the critical temperature T c =198.8 °K was studied in detail. The splittings of the NQR lines in the monoclinic phase, which are proportional to the order parameter, were found to vanish with the critical exponent β=1/2 on approaching T c . Above 210 °K the temperature dependence of the linewidth and of the spin–lattice relaxation time indicates that the s‐triazine molecule undergoes molecular reorientation around its threefold axis. The activation enthalpy for this motion was found to be ΔH=14.0 kcal/mole. These relaxation phenomena were analyzed using a density matrix formalism. Explicit expressions for the decay exponents and signal function were derived.