Understanding the molecular dynamics associated with polymorphic transitions of DL-norvaline with solid-state NMR methods.

DL-Norvaline (NVA) is an important pharmaceutical intermediate and undergoes two polymorphic transitions between 140 and 300 K. To understand molecular dynamics of NVA accompanied with these transitions, we conducted a comprehensive study on its molecular motions at multiple time scales (10(-9)-1 s) with various solid-state NMR methods. (13)C CPMAS NMR spectra revealed four sets of clearly resolved signals for NVA carbons corresponding to at least three crystal modifications with two polymorphic transitions. Proton and (13)C relaxation results showed that, apart from the reorientations of methyl and amino groups, NVA had another relaxation process following the second transition with the activation energy of 16-21 kJ/mol corresponding to the side-chain motions. This was further confirmed with the data from dipolar and chemical shift experiments. No motions were detected at CODEX time scale (ms-s). These results provide essential information for understanding the mechanistic aspects of the polymorphic transitions in aliphatic α-amino acids with linear side-chains.