Abstract
The populations of long-lived spin states, in particular, populations of singlet states that are comprised of antisymmetric combinations of product states, |alpha(I)beta(S)> - |beta(I)alpha(S)>, are characterized by very long lifetimes because the dipole-dipole interaction between the two "active" spins I and S that are involved in such states is inoperative as a relaxation mechanism. The relaxation rate constants of long-lived (singlet) states are therefore determined by the chemical shift anisotropy (CSA) of the active spins and by dipole-dipole interactions with passive spins. For a pair of coupled spins, the singlet-state relaxation rate constants strongly depend on the magnitudes and orientations of the CSA tensors. The relaxation properties of long-lived states therefore reveal new information about molecular symmetry and structure and about spectral density functions that characterize the dynamic behavior.
Highlights
CSA relaxation is obviously detrimental for the lifetimes TLLS of the long-livedsingletstatesand limits the time scales of the slow processes that can be studied, this effect can be put to good use
It will be shown in this paper that the CSA relaxation of long-livedsingletstates critically depends on molecular symmetry properties
The relaxation rate constant RLLS of a long-lived state involving two active spins I and S is affected by DD interactions I-Ri and S-Ri with passive spins Riwith i = 1, 2, . . . , N.͒ The resulting DD contributions RLDLDS to the relaxation rate constants of long-lived states depend on the molecular symmetry
Summary
CSA relaxation is obviously detrimental for the lifetimes TLLS of the long-livedsingletstatesand limits the time scales of the slow processes that can be studied, this effect can be put to good use. Eters of proton CSA tensors are often difficult to determine with reasonable accuracy, and it is often necessary to exploit the field dependence of CSA autorelaxation rate constants and/or CSA/DD cross-correlation effects It will be shown in this paper that the CSA relaxation of long-livedsingletstates critically depends on molecular symmetry properties. The CSA contribution to relaxation vanishes if the two tensors have identical principal components and parallel principal axes This relaxation mechanism is similar to that due to random field fluctuations in the absence of a static magnetic field.[11] The relaxation rate constant RLLS of a long-lived state involving two active spins I and S is affected by DD interactions I-Ri and S-Ri with passive spins Riwith i = 1 , 2 , . This paper focuses on the interpretation of the experimentally determined relaxation rate constants RLLS of long-lived states in terms of CSA ten-
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