Ab initio and atoms in molecules (AIM) theoretical studies have been used to show that in a 1: 1 complex formed between CH4 and H2O, CH4 acting as a hydrogen bond acceptor leads to the global minimum structure. There is a bond path connecting C to the H of H2O revealing a penta-coordinate carbon that could be the precursor to the CH5+. It appears to be a general feature in CH4 center dot center dot center dot HX complexes (X = F, Cl, and SH). It is in agreement with the experimental structure for analogous complexes and also the C2H6 center dot center dot center dot HF/HCN complexes, suggesting that it is also a general feature for hydrocarbon-HX interaction. (C) 2008 Elsevier B. V. All rights reserved.