A high-field solid-state 35/37Cl NMR and quantum chemical investigation of the chlorine quadrupolar and chemical shift tensors in amino acid hydrochlorides
Physical Chemistry, Chemical Physics - PCCP
A series of six L-amino acid hydrochloride salts has been studied by 35/37 Cl solid-state NMR spectroscopy (at 11.75 and 21.1 T) and complementary quantum chemical calculations. Analyses of NMR spectra acquired under static and magic-angle-spinning conditions for the six hydrochloride salts, those of aspartic acid, alanine, cysteine, histidine, methionine and threonine, allowed the extraction of information regarding the chlorine electric field gradient (EFG) and chemical shift tensors,
... ft tensors, including their relative orientation. Both tensors are found to be highly dependent on the local environment, with chlorine-35 quadrupolar coupling constants (C Q ) ranging from À7.1 to 4.41 MHz and chemical shift tensor spans ranging from 60 to 100 ppm; the value of C Q for aspartic acid hydrochloride is the largest in magnitude observed to date for an organic hydrochloride salt. Quantum chemical calculations performed on cluster models of the chloride ion environment demonstrated agreement between experiment and theory, reproducing C Q to within 18%. In addition, the accuracy of the calculated values of the NMR parameters as a function of the quality of the input structure was explored. Selected X-ray structures were determined (L-Asp HCl; L-Thr HCl) or re-determined (L-Cys HCl Á H 2 O) to demonstrate the benefits of having accurate crystal structures for calculations. The self-consistent charge field perturbation model was also employed and was found to improve the accuracy of calculated quadrupolar coupling constants, demonstrating the impact of the neighbouring ions on the EFG tensor of the central chloride ion. Taken together, the present work contributes to an improved understanding of the factors influencing 35/37 Cl NMR interaction tensors in organic hydrochlorides. Electronic Supplementary Information (ESI) available: Crystallographic information files (CIFs) for aspartic acid hydrochloride, cysteine hydrochloride monohydrate, and threonine hydrochloride. Difference NMR spectra for stationary samples. See This journal is c the Owner Societies 2007 Phys. Chem. Chem. Phys., 2007, 9, 6219-6230 | 6219 PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics Chart 1 Structures of the amino acids whose hydrochloride salts were examined in this study.