Exploring the potential energy hypersurface of histamine monocation: Tautomerism in gas phase

The potential energy hypersurface of the histamine monocation is determined by ab initio methods at the STO ‐4G level using analytical gradient techniques. Three transition states and two minima have been found for the N τ H tautomer. One of the transition states connects the trans conformational region with a minimum gauche structure, where the proton of the ammonium group is approximately halfway between the N π of the imidazole group and the N of the ammonium group, but nearer to the N π . This minimum connects the potential energy surface of the N τ H tautomer with the imidazolium one. In the latter region, three transition states and two minima have been found. Critical points are discussed in relation with experimental data and histamine H 2 receptor models.