Carbon-13 chemical shift tensor correlation via spin diffusion in solid tropolone using switched-angle spinning spectroscopy

Abstract

In switched-angle spinning spectroscopy (SAS) a sample is spun about different angles, β, relative to the magnetic field, during various periods of the experiment. In the present work, SAS is combined with two-dimensional exchange spectroscopy in order to correlate carbon-13 chemical 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 enrichment the dominant exchange mechanism between the two sites involves spin diffusion. The experiment consists of a preparation period during which the sample spins at the magic angle and the magnetization of one of the sites is quenched by means of a selective pulse sequence. During the rest of the experiment the sample spins with its axis away from 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 providing 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 structure of tropolone (neglecting a small tilt between the perpendicular to the molecular plane and the crystallographic c-axis) provides information on the orientation and magnitude of the chemical shift tensors of the two types of carbons. The principal values (in ppm) are σ1xx=65, σ1yy=33, σ1zz=−98, σ2xx=77, σ2yy=17, and σ2zz=−94. Assuming σzz to be perpendicular to the molecular plane, the orientations of the σyy’s are 12° off the C1=0 bond (toward the hydroxyl carbon) for carbon 1 and 10° off the C3=C2 bond (away from the carbonyl carbon) for carbon 2.