Carbon-13 chemical shift tensors and molecular conformation of anisole

The first direct measurement of the ortho steric effect of the methoxy group on the C-13 chemical shifts in anisole is reported. The ortho steric effect on the isotropic C-13 chemical shifts was obtained from a low-temperature MAS spectrum, and the effect on both the isotropic and the censor principal components was determined from a low-temperature 2D magic angle turning (MAT) experiment. From the low-temperature MAS spectrum, the C-13 chemical shift of the ortho carbon cis to the methoxy carbon is found to be 7.0 ppm lower that of the ortho carbon trans to the methoxy carbon, in good agreement with previous estimates. From the low-temperature MAT experiment, a 6.8 ppm decrease in the chemical shift is observed in the isotropic chemical shift, while the effects on the difference (cis minus traits) between the individual tensor components are measured to be -9 ppm in delta(11), 1 ppm in delta(22), and -14 ppm in delta(33), in reasonable agreement with the results of a previous linear regression substituent analysis on several di- and trimethoxybenzenes. Comparison of the experimental results with calculations, including thermal averaging considerations, further demonstrates that at room temperature the methoxy group in anisole undergoes stochastic jumps between the two equivalent planar configurations. This work demonstrates the feasibility of using the low-temperature MAT experiment at low temperature to measure the principal values of the C-13 chemical shift tensors in molecules that are liquids at room temperature.