A theoretical study of some hydrogen-bonded complexes using the theory of atoms in molecules

π-Type hydrogen-bonded complexes consisting of hydrogen halide HX (X = Cl, F) and the carbon–carbon triple or double bond of vinyl acetylene (1-buten-3-yne, HC≡C-CH=CH2) have been studied. The vinyl acetylene molecule contains two possible π-bonding sites (C≡C and C=C). It offers three possible structures of [Formula: see text] that comprise two T-type bonds to C≡C (endo and exo approaches) and one T-type bond to C=C (perpendicular approach). The optimized geometries and the hydrogen-bond stabilization energies, based on MP2(FULL)/6-311 ++ G(d,p)//6-31G(d,p) calculations, indicate that the π-type hydrogen bond to a C≡C triple bond leads to a more stable complex than for an analogous bond to C=C. The calculated global minima for the complexes with HF and HCl correspond to the H—X moiety lying along a bisector of the C≡C triple bond in the endo approach, predictions that are in good agreement with the reported FTMS results. The topological properties of the electron density distributions of these two systems have been analyzed in terms of the theory of atoms in molecules. The nature of π-type hydrogen bonds has also been discussed using the Laplacian of the electron density, [Formula: see text] The complexes [Formula: see text] and [Formula: see text] as well as the hydrogen-bonded complex consisting of 2-butyne (CH3-C≡C-CH3) and HCl were also studied for comparison. Key words: ab initio calculation, hydrogen bonding, topological analysis of electron density, vinyl acetylene, 2-butyne.