Studies of an isolated N-15-N-15 spin pair. Off-angle past-sample-spinning NMR and self-consistent-field calculations for diazo systems

An off-magic-angle spinning study of the nonassociated molecular solid, doubly N-15-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, C6H5N2+, 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 N-15-labeled isotopomers in which information concerning the electric field gradient at the alpha and beta sites was deduced. (C) 1997 Academic Press.