RELATIVE ORIENTATION OF CHEMICAL SHIELDING AND DIPOLAR COUPLING TENSORS - MIXED SINGLE-QUANTUM AND DOUBLE-QUANTUM HOMONUCLEAR ROTARY RESONANCE NUCLEAR-MAGNETIC-RESONANCE OF ROTATING SOLIDS

A novel two-dimensional magic-angle spinning nuclear magnetic resonanc e (NMR) method for determination of relative orientation of dipolar an d chemical shielding tensors for dipolar-coupled homonuclear spin-1/2- pairs of powder samples is described. Simultaneous recoupling of aniso tropic chemical shielding and dipolar coupling interactions is accompl ished using a homonuclear rotary resonance pulse sequence with the amp litude omega(rf) of a rf irradiation field matched to the spinning fre quency omega(r) according to omega(rf)=omega(r). Employing this techni que in the first dimension of a two-dimensional experiment leads to po wder spectra exhibiting strong dependence on the magnitudes and the re lative orientation of the two shielding tensors and the dipolar coupli ng tensor correlated to a high-resolution spectrum in the sampling dim ension. Various aspects of the recoupling experiment are described the oretically and the applicability of the method for determination of re lative orientation of these three anisotropic tensors through numerica l simulation is demonstrated on basis of experiments for a doubly C-13 -labeled powder of L-alanine. With reference to this sample (and amino acids in general), minor effects from simultaneous recoupling of the h eteronuclear dipolar coupling between C-alpha and the amide N-14 nucle us are evaluated. In the present case, the C-13-N-14 dipolar interacti on is used in numerical simulations to refine our structural analysis and to obtain information about the absolute orientation of the C-13 c hemical shielding tensors relative to the molecular coordinate system. (C) 1997 American Institute of Physics.