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

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: JAB=3.55±0.05Hz, |νA-νB|=18.8±0.1Hz. (b) Scalar coupling with 14N: JAN≈9.7 Hz and JBN≈2.9 Hz. JAN and JBN are of same sign. τq (relaxation time (T1 or T2) of (14N)=0.61 ms. (c) The other important mechanism of proton relaxation is internal dipole-dipole interaction, with τc =7.0×10-12 s. Sample II (40 per cent solution of 2,3,4-trichloronitrobenzene in CS2). (a) Single resonance: JAB=8.85±0.1 Hz, |νA-νB|=11.3±0.1Hz. (b) Scalar coupling with 14N: JAN≈3.0 Hz, JBN≈0.4 hz. Unambiguous determination of relative signs of JAN and JBN was not possible since JBN is small. τq(14N)=1.88 ms. (c) The other important mechanism of relaxation is internal dipole-dipole interaction with τe=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