Dipolar-Chemical Shift and Rotational Resonance13C NMR Studies of the Carboxyl−Methylene Carbon Spin Pair in Solid Phenylacetic Acid and Potassium Hydrogen Bisphenylacetate

Abstract

The dipolar-chemical shift (CS) method has been applied to analyze the carboxyl−methylene carbon isolated spin pair in phenylacetic-13C2 acid and potassium hydrogen bisphenylacetate-13C2. 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 13C spin pairs in phenylacetic-13C2 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-13C2 acid as a setup sample for rotational resonance and other homonuclear dipolar recoupling experiments.