Relaxation effects in the nuclear magnetic double resonance spectra of a symmetrical three spin system (AB₂)

Abstract

Relaxation effects are studied in the steady-state proton-proton double resonance spectra of a symmetrical three spin system of the type AB₂, formed by the ring protons of 2,6-dibromoaniline. The experiments were performed on a Varian HR-100 high resolution N. M. R. spectrometer suitably modified for field-frequency locking and added by the usual equipment for the purpose of frequency-sweep double resonance. The sample used is a 20 per cent solution in CCl₄, purified and sealed in vacuum. The parameters describing the single resonance spectrum of the molecule were obtained to be |v_A - v_B|=89.95±0.1Hz and J _AB=7.95±0.1 hz. Double resonance spectra obtained by irradiating each of the eight transitions in the single resonance spectrum, show significant relaxation effects for amplitudes of irradiation varying from 0.1 hz to 9.0 hz. These features were analysed using the density matrix theory of double resonance. The inhomogeneity of the applied magnetic field is found to be an important factor in the interpretation of the relaxation effects, and has been introduced into the density matrix calculations. Two relaxation mechanisms, (i) by isotropic random fields and (ii) by internal dipole-dipole interactions, along with possible correlations between the different terms in the relaxation hamiltonian, were considered for the analysis. It has been shown that it is particularly useful to study the spectra obtained with high as well as low values of the strength of irradiation, since these two categories possess some distinctive features both from experimental and theoretical points of view, thus permitting more accurate conclusions to be reached about the relaxation processes. The present analysis showed that the relaxation of the ring protons in 2,6-dibromoaniline is primarily due to random fields arising from intermolecular dipolar interactions, with the mean square fields at the 'A' (position 4) and 'B' (positions 3 and 5) nuclei having a ratio of about 2 : 1. Further it was found that the fields at the two B nuclei are highly correlated while those at nucleus A and any one of the B nuclei are practically uncorrelated.