Forbidden nuclear magnetic resonance transitions between singlet and triplet states in spin-12 pair systems in rotating solids

An ensemble of paired spin(-1/2) nuclei in a rotating solid: Polarization evolution and NMR spectrum in a wobbling frame

The evolution of the magnetic polarization of an ensemble of paired spin(-1/2) nuclei in an MAS NMR (nuclear magnetic resonance) experiment and the induced spectrum are described theoretically by means of a Liouville-von Neumann equation representation in a wobbling rotating frame in combination with the averaged Hamiltonian theory. In this method, the effect of a high-intensity external static magnetic field and the effects of the leftover interaction components of the Hamiltonian that commute with the approximate Hamiltonian are taken into account simultaneously and equivalently. This method reproduces details that really exist in the recorded spectra, caused by secular terms in the Hamiltonian, which might otherwise be smoothed out owing to the approximate treatment of the effects of the secular terms. Complete analytical expressions, which describe the whole NMR spectrum including the rotational sideband sets, and which consider all the relevant intermolecular interactions, are obtained.

1D and 2D MAS NMR spectra of a dipolar-coupled homonuclear spin-(1/2) pair

An analytical approach based on the average Hamiltonian theory is proposed for efficient calculation of the MAS NMR spectra of a dipolar-coupled homonuclear spin-(1/2) pair. For this purpose a superoperator formalism is developed which allows one to describe the spectra over a broad span of sample spinning rates, including the exact rotational resonances. This formalism can also be applied to the description of 2D polarization exchange spectra, which in many cases turns out to be useful for measuring the coupling strength. The experimental MAS NMR and the 2D spectra of doubly 13C-labeled zinc acetate were found to be in good agreement with the calculated spectra. Copyright 1997 Academic Press. Copyright 1997Academic Press

Studies of an Isolated 15 N-15 N Spin Pair. Off-Angle Fast-Sample-Spinning NMR and Self-Consistent-Field Calculations for Diazo Systems

An off-magic-angle spinning study of the nonassociated molecular solid, doubly 15 N-labeled 5-methyl-2-diazobenzenesulphonic acid hydrochloride (I ) is reported. The validity of the off-magic-angle spinning approach under fast-spinning conditions is verified by average Hamiltonian theory. Ab initio SCF calculations were performed on the simpler molecule, C6 H5 N2 + , to provide the shielding parameters, the dipolar coupling between the two nitrogen nuclei, and the electric field gradient existing at both the alpha-nitrogen and beta-nitrogen sites. The calculated values are in good agreement with the shielding and effective dipolar coupling data elucidated in the present investigation, and with a previous study of the two singly 15 N-labeled isotopomers in which information concerning the electric field gradient at the alpha and beta sites was deduced.

Density operator formalism for the description of coherence evolution of a homonuclear two-spin system in rotating solids

Based on a simple disentangling technique of the propagator, a density operator formalism is developed to describe the behavior of a homonuclear two-spin system in rotating solids. It is shown that the coherence evolutions can be characterized in terms of quaternions or a rotation matrix in a three-dimensional space; the coherence transfers between two different spins can thus be analysed well.