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 (M_W= 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.