Complexation of anions to gas-phase amino acids: Conformation is critical in determining if the global minimum is canonical or zwitterionic

We report the first MP2 computational study of the Cl−·Arginine and Br−·Arginine complexes, with the clusters being systematically classified into conformational families prior to optimization. The canonical tautomers in particular adopt numerous conformeric structures with close relative energies. Conformer energy differences are of similar magnitude to the canonical–zwitterionic tautomer differences, indicating that it is essential to obtain reliable conformer energies to determine whether the global minimum structure is zwitterionic or canonical for these prototype anionic systems. The zwitterionic and canonical minima are found to be very close in energy for Br−·Arginine, whereas the canonical form is significantly lower for Cl−·Arginine.