Spin-lattice relaxation times of 13C satellites in proton nuclear magnetic resonance spectra and their use in determining molecular rotational diffusion constants

Abstract

The nuclear magnetic spin-lattice relaxation times of the naturally abundant ¹³C satellites in the proton n.m.r. spectra of CH₂Cl₂, CH₃I, CH₃CN, CH₃CCl₃, (CH₃)₃CCl, (CH₃)₄Si and mesitylene have been measured and compared with the relaxation times of the predominant ¹²C-containing molecules. Although each satellite forms a coupled relaxation system with the other satellite and the ¹³C spin, it is shown that the relaxation is closely exponential with an effective relaxation time T^*₁ which depends on the ¹³C relaxation probabilities as well as on the proton relaxation mechanisms. T^*₁ also depends on the method of measurement. Computer calculation of the coupled relaxation system has been used to determine intermolecular, spin-rotation, and intramolecular contributions to the proton relaxation. The intramolecular part has been used to evaluate molecular rotational diffusion constants. There is generally good agreement with those parameters that have been determined by other methods.