Long-Lived Heteronuclear Spin-Singlet States in Liquids at Zero Magnetic Field
Transferring 2-spin system from high to zero magnetic field generates population difference between triplet and singlet states (top). By applying DC magnetic field pulses in different directions, it is possible to use zero-field NMR to differentially probe triplet and singlet relaxation (bottom).
The NMR Program has demonstrated the formation of a long-lived spin-singlet (zero spin) state in a spin pair comprised of unique nuclear constituents, which is possible due to symmetries of NMR spectra acquired in an environment screened from magnetic fields (zero-field NMR).
Significance and Impact
Pines and colleagues have overturned the assumption that singlet states can only be formed between identical constituents. This may be used to increase lifetimes of nonequilibrium nuclear spin polarization (hyperpolarization) and greatly enhance MRI contrast.
When two or more nuclear spins are coupled together, the resulting state can have several different values for the total angular momentum. For the case of an even number of spin-1/2 particles, it is possible to form a singlet state, which has zero spin. Combining into a state with zero spin eliminates any angular/magnetic momenta, producing a stable diamagnetic combination (a diamagnetic material creates a magnetic field that opposes any externally applied magnetic field). Because the state has no net magnetic moment by which a fluctuating environment can cause dephasing of the coherent state, singlet states are long-lived. This property has important practical implications for nuclear magnetic resonance spectroscopy (NMR) and imaging (MRII): the long lifetimes can be used a) to extend the range of dynamic phenomena that can be probed by NMR, or b) to store hyperpolarization and enhance the sensitivity of NMR.
Previously, it had been assumed that singlet states could only be formed by combining identical constituents. This work, however, has demonstrated the formation of a long-lived spin-singlet state made of two different spin-1/2 particles: a hydrogen nucleus and a carbon-13 nucleus covalently bound in formic acid. By using our newly developed zero-field NMR techniques, it is possible to separately query states of different symmetries, i.e. singlet (total spin 0) vs. triplet (total spin 1) states, in order to measure their lifetimes. We have measured a lifetime for the heteronuclear spin-singlet state that is three times longer than that of the triplet state, confirming the formation of a long-lived singlet-like state. This demonstration overturns theoretical preconceptions and substantially extends the range of systems accessible to singlet-state-enhanced NMR and MRI techniques.