Long-Lived Heteronuclear Spin-Singlet States in Liquids at a Zero Magnetic field

We report an observation of long-lived spin-singlet states in a C-13-H-1 spin pair in a zero magnetic field. In C-13-labeled formic acid, we observe spin-singlet lifetimes as long as 37 s, about a factor of 3 longer than the T-1 lifetime of dipole polarization in the triplet state. In contrast to common high-field experiments, the observed coherence is a singlet-triplet coherence with a lifetime T-2 longer than the T-1 lifetime of dipole polarization in the triplet manifold. Moreover, we demonstrate that heteronuclear singlet states formed between a H-1 and a C-13 nucleus can exhibit longer lifetimes than the respective triplet states even in the presence of additional spins that couple to the spin pair of interest. Although long-lived homonuclear spin-singlet states have been extensively studied, this is the first experimental observation of analogous singlet states in heteronuclear spin pairs.