Nuclear relaxation and molecular motion of pyridine-N-oxide in carbon tetrachloride

Abstract

Proton spin lattice relaxation of pyridine-N-oxide-2,6-d2 in dilute solution (0·2 M) with carbon tetrachloride has been measured at -12·6, 6·5, and 26·6°C. Using 2H and 14N decoupling, eight transitions were resolved and recovery was monitored after non-selective inversion. In absence of 14N decoupling, scalar relaxation produced clearly visible effects on the proton recovery curves. Combined with measurements of deuterium relaxation of the 2,6-d2 and 4-d derivatives, proton relaxation data (including effects of dipolar cross correlation) were used to determine all three rotational diffusion constants at each temperature. The in-plane spinning motion was found to be ∼four-fold more strongly dependent on viscosity than predicted by the slip rotational diffusion model, while motion about the other two principal axes proceeds at rates within ∼40 per cent of slip predictions. 14N spin lattice relaxation data were combined with the anisotropic reorientation rates in efforts to determine liquid phase values of the 14N quadrupole coupling parameters. A very small decrease from solid phase values provides the best overall fit to the relaxation data.