¹⁴N Nuclear Quadrupole Resonance and Relaxation in 2,5-Dimethylpyrazine—Evidence of Level Crossing between NQR and Tunnel Systems—

Abstract

NQR frequencies and spin-lattice relaxation times (T₁) have been measured from 4.2 to 262 K by the pulse method. The quadrupole coupling constant (4.795 MHz) and the asymmetry parameter (0.4964) at 4.2 K are compared with those in tetramethylpyrazine. The temperature dependence of T₁ shows peculiar behavior: Above 200 K, the fluctuation of the electric field gradient dominates the relaxation, and the activation energy (E_a) of the molecular motion has a value of 55±5 kJ mol⁻¹. Below 100 K, T₁ is governed by dipolar interactions between ¹⁴N and ¹H. T₁ minima are observed at 69 K and at 52 K. The methyl-group reorientation (E_a=6.5 kJ mol⁻¹) contributes to T₁(min) at 69 K, giving the pre-exponential factor of the correlation time, τ₀=5×10⁻¹³ s. T₁(min) at 52 K is explained with the use of a tentative model of the level crossing between NQR and tunnel systems.