14N nuclear quadrupole resonance and relaxation in tetramethylpyrazine

Resonance frequencies and spin–lattice relaxation times (T1) of nitrogen-14 nuclei in solid tetramethylpyrazine from liquid-helium temperature to room temperature have been measured by the pulse method and the temperature dependence of the quadrupole resonance frequencies is compared with those in the low-temperature phase of pyrazine. Below 150 K, the relaxation times are governed by the reorientation of the methyl groups. The activation energy and the pre-exponential factor of the correlation time are found to be Ea= 6.0 kJ mol–1 and τ0= 2 × 10–13 s, respectively. Above 150 K the relaxation times depend on torsional lattice vibrations and are determined by the indirect two-phonon processes so that T–11∝T2.