Ab initio and DFT computer studies of equilibria between quaternary nitrogen hydroxides, including those of choline and acetylcholine, with the water complexes of their ylidic forms

Theoretical calculations (DFT and ab initio) are reported for the hydroxides of trimethylammonium, tetramethylammonium, trimethylethylammonium, choline and acetylcholine and the water complexes of the ylidic forms of these quaternary nitrogen compounds. These are carried out at the B3LYP/6-31+G(d,p) and MP2/6-31+G(d,p) levels of theory in all cases. The DFT methods are found to be more sensitive than the ab initio methods for identifying and characterizing vibrational modes associated with hydrogen bonding or ion pair type interactions. Potential energy scans at both these levels correspond to equilibria involving proton transfer between the hydroxides and their ylides and reveal double minima corresponding to stable optimized structures. The calculated information is compared with the limited experimental data available for related systems. Comparisons are made with previous studies of corresponding phosphorus systems to show an inversion of relative stabilities of the hydroxide and the ylidic water complexes. The equilibria are discussed in terms of selected geometrical values and harmonic modes of vibration of the component complexes. Binding energies in each complex are reported in the free state with BSSE corrections and in solvents of low and high relative permittivity. These calculations may have importance in understanding mechanism of neurotransmission, including anesthetics, at the molecular level.