Nitrogen Electric Quadrupole and Proton Magnetic Resonances in Thiourea

Abstract

The nitrogen nuclear electric quadrupole coupling constants for the two nonequivalent thiourea molecules in the low‐temperature phase of solid thiourea, SC(NH₂)₂, have been determined as a function of temperature between 4° and 169°K. At 77°K, the coupling constants and asymmetry parameters are, for Molecule (1) e²q₁Q/h=3121.6 kc/sec and η₁=0.3954, and for Molecule (2) e²q₂Q/h=3099.6 kc/sec and η₂=0.3930. The two highest‐frequency transitions in each spectrum exhibit temperature dependences that follow Bayer's theory of electric‐field gradient averaging by local oscillators. Zeeman studies place the x principal axis of the electric field gradient out of alignment with the C,N bond, the axis of symmetry in the NH₂ group. Magnetic dipole perturbation coupling between nitrogen and hydrogen nuclei is zero in first order, and the second‐order dipole coupling shifts the quadrupole resonance line near values of the Zeeman field, which make the proton Larmor frequency equal to a nitrogen quadrupole transition frequency. The energy‐conserving ¹⁴N‐proton mutual spin flips, which occur at these applied magnetic fields, make several cross‐relaxation experiments possible. The temperature dependences of the proton spin—lattice relaxation times is characterized by that of thermally activated molecular reorientation at temperatures above the ferroelectric transition temperature, 169°K. Measured values of the activation energy and pre‐exponential frequency factor are 10 kcal/mole and 7.5×10¹³ sec^—1. This molecular reorientation inhibits measurement of the quadrupole coupling above 169°K. Nitrogen quadrupole‐lattice relaxation times appear to be the same for all level transitions and to follow a T^—2 temperature dependence between 8° and 165°K.