Long-lived nuclear spin states in monodeuterated methyl groups

Abstract

It is possible to access long-lived nuclear singlet order in monodeuterated methyl groups, in the case that a significant chemical shift difference exists between the CH₂D protons. This occurs when the local environment is chiral, and the CH₂D rotamers have different populations. An experimental demonstration is presented for the case of N-CH₂D-2-methylpiperidine. The ratio of the singlet relaxation time constant T_S to the longitudinal relaxation time constant T₁ is found to be equal to 3.1 ± 0.1, over a wide range of temperatures, solvents, and magnetic fields. The longest observed value of T_S approaches 1 minute. The relaxation mechanisms of the long-lived state are discussed, and a modified model of the CH₂D geometry is proposed to explain the observed ratio of T_S to T₁.