The dipolar-chemical shift (CS) method has been applied to analyze the carboxyl-methylene carbon isolated spin pair in phenylacetic-13 C 2 acid and potassium hydrogen bisphenylacetate-13 C 2 . The span, Ω, of the CS tensor is decreased significantly for both carbon atoms in the potassium acid salt compared to the acid. The orientations of the carboxyl CS tensor and, more notably, the methylene CS tensor, have a marked dependence on the protonation state of the carboxyl group. Ab initio calculations [RHF/6-311G*, RHF/6-311++G(2d,-2p)] support the experimental findings. In addition to these studies, we demonstrate how rotational resonance (RR) NMR spectroscopy complements the dipolar-CS method in a study of the isolated 13 C spin pairs in phenylacetic-13 C 2 acid. In particular, the higher-order RR experiments provide a stringent check on the CS parameters and the dipolar coupling constant, R, derived from the dipolar-CS analysis. The dipolar-CS method, in combination with a two-dimensional spin-echo experiment, yields R ) 2150 ( 30 Hz for phenylacetic acid, whereas RR indicates that R ) 2100 ( 15 Hz. Although n ) 1 RR can be applied reliably to determine internuclear distances in the absence of CS tensor data, these data are critical for simulations of the n ) 2 RR effects. Specifically, longitudinal magnetization exchange curves are shown to be sensitive to slight rotations of the carboxyl carbon CS tensor about an axis perpendicular to the carboxyl plane, a phenomenon observed upon moving from the acid to the acid salt. Simulations indicate that altering the MAS rate by only a few tens of hertz can drastically alter the higher-order RR line shapes. The ab initio calculations of chemical shielding tensors provide data that are useful in the simulation of rotational resonance effects. We propose phenylacetic-13 C 2 acid as a setup sample for rotational resonance and other homonuclear dipolar recoupling experiments.
IntroductionThe dipolar-chemical shift (CS) method is an effective NMR technique for studying isolated spin-1 / 2 pairs in stationary powdered solids. [1][2][3][4][5][6] Under favorable circumstances, when symmetry dictates the orientation of at least one component of one of the CS tensors, this method will yield the direct dipolar coupling constant (R DD ), the magnitudes of the principal components of the two CS tensors, the relative orientations of the CS tensors, and the orientations of these tensors with respect to the molecular axis system. 6 The fact that this orientational information is derived from a powdered sample makes the method particularly valuable when single crystals are not available. R DD is of interest because of its simple relationship with the distance, r 12 , between nucleus 1 and nucleus 2 (eq 1) where µ 0 is the permeability of free space and γ 1 and γ 2 are the magnetogyric ratios of the nuclei under consideration. It is important to recognize that r 12 is a motionally averaged distance. Complications in measuring very accurate values of r 12 using solid-state NMR will be di...