Ab initio molecular orbital calculations of electron distribution in tetramethylammonium ion

Ab initio molecular orbital calculations of electron distribution in tetramethylammonium ion and its uncharged isoelectronic analogue, neopentane, have been carried out. Comparison of the two compounds permits a detailed description of the delocalization of the positive charge of tetramethylammonium ion. The van der Waals surface of this ion is found to be characterized by "patches" of positive charge associated with the methyl groups, interspersed with essentially neutral regions. The consequences of this nonspherical charge distribution for interaction with anions have been explored by calculations of the interaction energy of fluoride ion with tetramethylammonium ion and neopentane in several mutual orientations. The lowest-energy orientation is found to be one in which the anion approaches a "face" of the tetrahedral cation (opposite to a C--N bond direction). The origins of this preference and the electron redistribution produced by the interaction with fluoride are discussed. The tetramethylammonium ion is clearly not a featureless positively charged sphere but will have appreciable geometrical specificity in its interaction with a presumed anionic group on the acetylcholine receptor.