Dipole effects on cation-π interactions:: Absolute bond dissociation energies of complexes of alkali metal cations to N-methylaniline and N,N-dimethylaniline

Threshold collision-induced dissociation of M+(nMA)(x) with Xe is studied using guided ion beam mass spectrometry, where nMA = N-methylaniline and N,N-dimethylaniline and x = 1 and 2. M+ includes the following alkali metal cations: Li+, Na+, K+, Rb+, and Cs+. In all cases, the primary dissociation pathway corresponds to the endothermic loss of an intact nMA ligand. The primary cross section thresholds are interpreted to yield 0 and 298 K bond dissociation energies (BDEs) for (nMA)(x-1)M+-(nMA) after accounting for the effects of multiple ion-neutral collisions, the internal and kinetic energy distributions of the reactants, and the dissociation lifetimes. Density functional theory calculations at the B3LYP/6-31G* level of theory are used to determine the structures of these complexes, which are also used in single-point calculations at the MP2(full)/6-311+G(2d,2p) level to determine theoretical BDEs. The results of these studies are compared to previous studies of the analogous M+(aniline)(x) complexes to examine the effects of methylation of the amino group on the binding interactions. Comparisons are also made to a wide variety of cation-pi complexes previously studied to elucidate the contributions that ion-dipole, ion-induced -dipole, and ion-quadrupole interactions make to the overall binding.