C-13 CHEMICAL-SHIFT TENSOR CORRELATION VIA SPIN-DIFFUSION IN SOLID TROPOLONE USING SWITCHED-ANGLE SPINNING SPECTROSCOPY

In switched-angle spinning spectroscopy (SAS) a sample is spun about d ifferent angles, beta, relative to the magnetic field, during various periods of the experiment. In the present work, SAS is combined with t wo-dimensional exchange spectroscopy in order to correlate carbon-13 c hemical shift tensors of the carbonyl (1) and hydroxyl (2) carbons of tropolone. Experiments were performed on a sample enriched to 25 at. % in each of these sites (at different molecules). At this level of enr ichment the dominant exchange mechanism between the two sites involves spin diffusion, The experiment consists of a preparation period durin g which the sample spins at the magic angle and the magnetization of o ne of the sites is quenched by means of a selective pulse sequence. Du ring the rest of the experiment the sample spins with its axis away fr om the magic angle except for a short period just before the detection where the axis is switched to the magic angle in order to select the magnetization to be detected. Experiments were performed for all four possible combinations of the initial and final magnetizations, thus pr oviding chemical shift correlations between carbons 1,1',2, and 2' in the two magnetically inequivalent (but symmetry related) molecules in the unit cell. Combining these results with the known crystal structur e of tropolone (neglecting a small tilt between the perpendicular to t he molecular plane and the crystallographic c-axis) provides informati on on the orientation and magnitude of the chemical shift tensors of t he two types of carbons, The principal values (in ppm) are sigma(xx)(1 )=65, sigma(yy)(1)=33, sigma(zz)(1)=-98, sigma(xx)(2)=77, sigma(yy)(2) =17, and sigma(zz)(2)=-94. Assuming sigma(zz) to be perpendicular to t he molecular plane, the orientations of the sigma(yy) s' are 12 degree s off the C-1=0 bond (toward the hydroxyl carbon) for carbon 1 and 10 degrees off the C-3=C-2 bond (away from the carbonyl carbon) for carbo n 2. (C) 1995 American Institute of Physics.