Carbon-13 chemical-shift tensors in single-crystal methoxybenzenes
Using a two-dimensional NMR orientational correlation technique, carbon-13 chemical-shift tensors have been measured in single crystals of 1,4-dimethoxybenzene, 1,3,5-trimethoxybenzene and 1,2,3-trimethoxybenzene. The two-dimensional technique greatly extends the single-crystal method to materials with a much larger number of different carbon-13 resonance lines. The manner of dealing with chemically identical, but magnetically inequivalent carbons in the unit cell is discussed. The X-ray structure of 1,2,3-trimethoxybenzene is reported for the first time, and a redetermination of the X-ray structure for 1,4-dimethoxybenzene is given. The principal values of all carbon-13 chemical-shift tensors in the three molecules and the orientation of their principal axes have been obtained. Using multiple regressional analysis the principal values of the tensors have been discussed in terms of additive substituent effects. Ab initio calculations of the shielding tensors in anisole (methoxybenzene) and benzene were used to calculate substituent effects which agree closely with the parameters obtained from the regressional analysis.
