Long-Lived Nuclear Spin States in Monodeuterated Methyl Groups

Nuclear magnetic resonance (NMR) experiments are limited by relaxation dynamics. Observing non-equilibrium magnetization is restricted to timescales governed by the longitudinal relaxation time T$_{\mathrm{1}}$, which limits potential applications such as hyperpolarization or transport phenomena. Long-lived states (LLS) have relaxation times much longer than T$_{\mathrm{1}}$, providing a possible approach to overcome relaxation constraints. Often the duration of information capture is extended by an order of magnitude over T$_{\mathrm{1}}$. LLS commonly exist in symmetry-constrained homonuclear pairs termed singlet states, with some multi-spin variants established. Molecular systems exhibiting LLS include; parahydrogen, parawater, gamma-picoline, peptides, fumarates and naphthalenes. A recent addition to the LLS family is the monodeuterated methyl (CH$_{\mathrm{2}}$D) group. Radio frequency pumping of NMR silent spin states is achieved using the spin-lock induced crossing (SLIC) pulse sequence. A LLS decay constant (T$_{\mathrm{S}})$ of 27.0 \textpm 0.6 s was recorded. A number of CH$_{\mathrm{2}}$D-2-x-piperidine derivatives are currently in synthesis to extend singlet lifetimes and to control chemical shift differences, the later of which are to be compared with computational predictions.