Influence of rotational diffusion on the magnetic relaxation of α-13C nuclei of poly(γ-benzyl-L-glutamate) in the helical conformation

Abstract

Spin–lattice relaxation times, T1, measured at two resonance frequencies, 25.1 and 75.5 MHz, for α-13C nuclei of poly(γ-benzyl-L-glutamate) fractions in deuterated dimethyl formamide were found to increase with increase in molecular weight (text-decoration:overlineMW= 31000–81000 g mol–1). The spin–spin relaxation times, T2, for the α-13C nuclei were found to decrease with increase in molecular weight. Within experimental error T1 was found to be independent of concentration over the dilute solution range covered (12.5–100 mg cm–3), whilst T2 was found to increase significantly with decrease in concentration. It is suggested that T2 for the helical molecules is influenced by both the axial and end-over-end diffusion coefficients, while T1 is governed mainly by the axial diffusion coefficient. The effect of internal librational motion on the α-C—H bond must be taken into account in order to explain the relaxation results quantitatively.