An experimental and theoretical study of the structure of Lamotrigine in its neutral and protonated forms: evidence of Lamotrigine enantiomers

The energies, geometries, and NMR chemical shifts have been calculated at the B3LYP/6-311++G(d,p) level for 17 structures of the anticonvulsant drug Lamotrigine and 29 structures of protonated Lamotrigine, including tautomers and E/Z isomers of the imino groups. The calculations were compared with solid state (X-ray and CPMAS NMR) and solution experimental results both reported in the literature and determined in this work. The conclusion is that Lamotrigine exists as the diamino tautomer and that its protonation takes place on the N2 atom. Using ABTE and/or deuterated ABTE as chiral solvating agent, it has been demonstrated for the first time by NMR in solution that Lamotrigine is a racemate of rapidly interconverting enantiomers. The crystal structure of two new solvated salts of Lamotrigine, both saccharinates, has been determined. Both salts present the same arrangement in chains of Lamotrigine and saccharinate joined by hydrogen bonds and stacking interactions. No isostructurality is present because of the different arrangement of the chains in both crystal structures.