High-field chlorine NMR spectroscopy of solid organic hydrochloride salts: A sensitive probe of hydrogen bonding environment

A series of organic hydrochloride salts has been investigated using solid-state Cl-35 and Cl-37 NMR spectroscopy at a lied magnetic field strengths of 9.4 and 18.8 T. Magic-angle spinning, static Hahn-echo, and quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) echo experiments have been applied to investigate the chlorine electric field gradient (EFG) and chemical shift (CS) tensors for L-tyrosine hydrochloride, L-cysteine methyl ester hydrochloride, L-cysteine ethyl ester hydrochloride, quinuclidine hydrochloride, and tris sarcosine calcium chloride. Chlorine-35 nuclear quadrupolar coupling constants for these compounds range from 2.23 to 5.25 MHz, and isotropic chemical shifts range from approximately 9 to 53 ppm relative to the chloride ion in aqueous solution. The results demonstrate the feasibility and benefits of high-field Cl-35/37 NMR studies of organic chloride salts. A discussion of the data in the context of the known X-ray or neutron diffraction structures for these compounds suggests that the chlorine EFG tensor is a valuable probe of hydrogen bonding to the chloride ion. Because the anisotropies of the CS tensors are rather small, precise determination of the chlorine CS tensors proved to be challenging and was only feasible for L-cysteine ethyl ester hydrochloride, where the span, Omega, was found to be 47 +/- 4 ppm. This represents the first determination of Omega (Cl) from a powder sample. Results of ab initio calculations of the chlorine EFG and CS tensors in L-tyrosine hydrochloride are presented and compared with the experimental data.