Long-lived States: Progress and Prospects

Long-lived states are configurations of nuclear spins that display long relaxation times, relative to ordinary nuclear magnetization. A primary example is the state of singlet order in a pair of spins-1/2, defined as the mean population difference between the spin-0 singlet state and the spin-1 triplet state. In favourable circumstances the lifetime of nuclear singlet order may exceed that of longitudinal magnetization by a factor of 50 or more. Singlet order does not itself provide an NMR signal but it is possible to convert magnetization into singlet spin order, and back again, providing that a small symmetry-breaking mechanism is provided (rather like the keyhole in a safe).

We have used molecular dynamics, quantum chemistry and spin dynamical theory to guide the design of molecular systems providing exceptionally long-lived singlet order. Target systems could then be synthesised through a collaboration with synthetic organic chemists. An example will be given of a molecular system that exhibits a nuclear singlet lifetime of more than 1 hour in a room-temperature liquid.

Systems of this kind hold promise as transport agents for nuclear hyperpolarization and for molecular sensing and imaging applications.

Long-lived states are not restricted to two-spin systems. If time permits, some experiments on long-lived states in the three-proton systems of methyl groups will also be presented.