Estimating the binding ability of onium ions with CO2 and π systems: a computational investigationElectronic supplementary information (ESI) available: Tables and figures of QTAIM analysis, NPA charge transfer and binding energies at the M06-2X/6-311++G(d,p)//M06-2X/6-31G(d) level of theory. LMO-EDA analysis at the M06-2X/6-31G(d) level of theory. See DOI: 10.1039/c4cp03434a

Density functional theory (DFT) calculations have been employed on 165 complexes of onium ions (NH 4 + , PH 4 + , OH 3 + , SH 3 + ) and methylated onium ions with CO 2 , aromatic (C 6 H 6 ) and heteroaromatic (C 5 H 5 X, X = N, P; C 4 H 5 Y, Y = N, P; C 4 H 4 Z, Z = O, S) systems. The stability of CO 2 onium, CO 2 π and onium π complexes was shown to be mediated through various noncovalent interactions such as hydrogen bonding, NH-π, PH-π, OH-π, SH-π, CH-π and π-π. We have discussed 17 complexes wherein the proton transfer occurs between the onium ion and the heteroaromatic system. The binding energy is found to decrease with increasing methyl substitution of the complexes containing onium ions. Binding energy components of all the noncovalent complexes were explored using localized molecular orbital energy decomposition analysis (LMO-EDA). The CO 2 π complexes were primarily stabilized by the dispersion term followed by contributions from electrostatic and polarization components. In general, for onium ion complexes with CO 2 or π systems, the electrostatic and polarization terms primarily contribute to stabilize the complex. As the number of methyl groups increases on the onium ion, the dispersion term is seen to have a key role in the stabilization of the complex. Quantum theory of atoms in molecules (QTAIM) analysis and charges based on natural population analysis (NPA) in various complexes have also been reported in order to determine the nature of noncovalent interactions in different complexes. The impact of increasing methyl substitution on onium ions in their complexes with CO 2 and aromatic systems has been analyzed using DFT calculations.