Relaxation theory of nuclear singlet states in two spin-1/2 systems

摘要

Nuclear magnetic resonance accesses static and dynamic information on molecules that possesses non-zero spin moment nuclei. For a long time, it was believed that this information could be stored for no longer than the longitudinal relaxation time constant, T1, (due to the relaxation of the z-component of the magnetization along the static magnetic field via interaction with the lattice). Till now and in liquid state NMR, this has set a rigid upper limit to NMR procedures and investigations. Recently, this limitation has been overcome [1–4]. It was demonstrated that there are nuclear spin states whose decay constant, TLLS, is much longer than the one of longitudinal magnetization, i.e. TLLS > T1. This is because they are immune to dipolar relaxation. This property is strict for systems of two spin-1/2 nuclei in liquid state NMR where the dipolar relaxation mechanism is the most important one and where there is a spin state, namely the antisymmetric combination of Zeeman spin states for the two nuclei (singlet state), that is completely immune to intramolecular dipolar relaxation and whose decay constant, TS was shown to be up to 40 times longer than T1. The presence of other nuclei in the molecule give rise to other dipolar relaxation sources to which these states are not in general immune. It was found, however, that even in multiple-spin-systems there are long-lived spin states provided that some conditions are met [5–7]. Here, a general overview of the relaxation theory behind this phenomenon is given in the case of molecules containing two spin-1/2 nuclei in liquids.