The physical origins of the interactions in the acetophenone cation adducts [M+Na](+), [M+NH4](+), and [M+H](+) were explored by localized molecular orbital-energy decomposition analysis and density functional theory. The analyses highlighted the differences in the interactions in the three adduct ions. Electrostatic energy was important in [M+Na](+) and there was little change in the acetophenone orbital shape. Both electrostatic and polarization energy were important in [M+NH4](+), and a considerable change in the orbital shape occurred to maximize the strength of the hydrogen bond. Polarization energy was the major attractive force in [M+H](+). (C) 2017 Elsevier B.V. All rights reserved.