Relaxation of longitudinal and singlet nuclear spin order as a function of solvent viscosity

Abstract

Extending the storage time of nuclear spin order is an important issue in modern NMR spectroscopy since many NMR and MRI applications are limited by the lifetime of spin states. Under normal circumstances the survival of spin magnetization is limited by longitudinal relaxation (T1) which brings the magnetization back to equilibrium within a few seconds. The conversion of longitudinal into singlet order can extend that lifetime to tens of minutes up to an hour.
Most nuclear spin relaxation mechanisms in liquids are a function of the correlation time τ_c of the molecule, which is linked to the molecular tumbling, and inversely proportional to viscosity. Therefore a lower viscosity would reduce the influence of those mechanisms. The increase in the lifetime of longitudinal (T₁) and singlet order (T_S) is shown here as the viscosity of common organic solvents decreases.
The study includes measurements in liquid and supercritical CO₂, that offer an extremely low viscosity regime. The further extension of spin order lifetimes for a singlet bearing molecule in liquid CO₂ is presented. There appears to be an upper limit to the extension of the singlet order lifetime that is caused by relaxation mechanisms acting in different directions.

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ORCID for Giuseppe Pileio: orcid.org/0000-0001-9223-3896

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