Information obtainable from N.M.R. longitudinal relaxation as measured by a non-selective pulse sequence in an ABC system

Abstract

From proton and carbon-13 longitudinal relaxation and ¹³C Overhauser enhancement measurements, the diffusion constants for the molecule acrylonitrile dissolved in carbon disulphide are found to be: D_xx =(0·11±0·06) × 10¹² s^-1; D_yy = (0·38 ± 0·12) × 10¹² s^-1; D_zz = (0·25 ± 0·02) × 10¹² s^-1 (the z axis is perpendicular to the molecular plane; the y axis is almost parallel to the C≡N bond). The diffusion and inertial principal axis systems are shown to be nearly coincident. From these diffusion constants, the nitrogen-14 quadrupolar coupling constant was calculated to be (4·9±0·8) MHz. Notwithstanding the fact that we can study more relaxation rates due to the presence of more lines in the proton ABC spectrum, as compared to an AB ₂ or AX ₂ system, we obtain less information. This is a result of the fact that the unknown spectral densities occur in linear combinations in the expression for the recovery rates. A procedure to obtain the maximum available information is described.