Ab initio and DFT computer studies of complexes of quaternary nitrogen cations: trimethylammonium, tetramethylammonium, trimethylethylammonium, choline and acetylcholine with hydroxide, fluoride and chloride anions

Abstract

Density functional theory (DFT by B3LYP) and ab initio (MP2) calculations are reported for complexes of the trimethylammonium, tetramethylammonium, trimethylethylammonium, choline and acetylcholine cations with hydroxide, fluoride and chloride anions. Stable structures are predicted in which the anion is bonded to three hydrogen atoms directed away from the cationic head and on each of three methyl groups of these quaternary nitrogen compounds. These are described as reverse complexes and are compared with normal complexes between F⁻ and Cl⁻ and the N–H group in the first of these cations or alternatively between HF and HCl with (CH₃)₃N. Stable structures are optimised at the B3LYP/6-31+G(d,p) level in all cases and also at the MP2/6-31+G(d,p) level for all complexes studied other than those of choline and acetylcholine. The differences between the DFT and MP2 methods largely follow systematic patterns and are small. Calculated values of structures with reference to Y⋯H distances, binding energies and assignment of harmonic C–H–Y (Y = OH, F and Cl) and other cationic head C–H stretching modes are discussed. The effect of solvents of low (n-hexane) and high (water) relative permittivity on energies are reported and discussed in relation to pharmacological considerations. Some properties of the trans,trans; trans,gauche and gauche,trans conformations of the acetylcholine cation and fluoride are reported. Complexes between quaternary nitrogen cations and the hydroxide, fluoride and chloride anions are described as consequences of hydrogen bonds strengthened by interactions between ions of opposite polarity.