Double resonance study of spin relaxation in two spin systems (AB) couple to a quadrupolar nucleus

Abstract

A frequency sweep proton-proton double-resonance study of spin relaxation is made in strongly coupled two spin systems (AB) formed by the protons in 2-bromothiazole (sample I) and in 2,3,4-trichloronitrobenzene (sample II). The single-resonance transitions of both these molecules exhibit broadening due to scalar coupling with nearby ¹⁴N nuclei modulated by the rapid quadrupolar relaxation of the latter. A density-matrix analysis of double-resonance spectra obtained over a wide range of irradiation strengths coupled with a determination of ¹⁴N resonance linewidths and approximate measurements of proton relaxation times leads to a determination of the spin-spin coupling constants of ¹⁴N with the two protons, and the parameters describing the other mechanisms of proton relaxation in these molecules. It has been shown that a study of relaxation by double resonance is, in general, capable of yielding the relative signs of the coupling constants involving the ¹⁴N nucleus. The results obtained may be summarized as follows: Sample I (2-bromothiazole, neat liquid). (a) Single resonance: J_AB=3.55±0.05Hz, |ν_A-ν_B|=18.8±0.1Hz. (b) Scalar coupling with ¹⁴N: J_AN≈9.7 Hz and J_BN≈2.9 Hz. J_AN and J_BN are of same sign. τ_q (relaxation time (T₁ or T₂) of (¹⁴N)=0.61 ms. (c) The other important mechanism of proton relaxation is internal dipole-dipole interaction, with τ_c =7.0×10^-12 s. Sample II (40 per cent solution of 2,3,4-trichloronitrobenzene in CS₂). (a) Single resonance: J_AB=8.85±0.1 Hz, |ν_A-ν_B|=11.3±0.1Hz. (b) Scalar coupling with ¹⁴N: J_AN≈3.0 Hz, J_BN≈0.4 hz. Unambiguous determination of relative signs of J_AN and J_BN was not possible since J_BN is small. τ_q(¹⁴N)=1.88 ms. (c) The other important mechanism of relaxation is internal dipole-dipole interaction with τ_e=1.0×10^-10 s. Dipolar interaction with ¹⁴N or with any of the chlorine nuclei, scalar coupling with the chlorine nuclei and intermolecular dipolar interactions, do not make significant contributions.