Both experimental and theoretical investigations of solid-state 17O NMR for l-valine and l-isoleucine

We have presented an experimental investigation of the carboxyl oxygen NMR parameters for four distinct sites in l-valine and l-isoleucine. The carboxyl 17O quadrupolar coupling constant, CQ, and isotropic chemical shift, δiso, for these compounds are obtained by analyzing two-dimensional 17O multiple-quantum magic-angle spinning (MQMAS) and/or 1D MAS spectra. The values of CQ and δiso found to be in the range of 7.00-7.85 MHz, and 264-314 ppm, respectively. Extensive quantum chemical calculations at the density functional levels have been performed for a full cluster of l-valine molecules and a few theoretical models. The calculated results indicated that there was a correlation between the 17O NMR parameters and C-O bond lengths, which was helpful for the spectral assignment. They also demonstrated that the torsion angle of l-valine plays an important role in determining the magnitudes of 17O NMR parameters.