A THEORETICAL-STUDY OF SOME X-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-PI HYDROGEN-BONDED COMPLEXES USING THE THEORY OF ATOMS IN MOLECULES

pi-Type hydrogen-bonded complexes consisting of hydrogen halide HX (X = Cl, F) and the carbon-carbon triple or double bond of vinyl acetylen e (1-buten-3-yne, HC=C-CH=CH2) have been studied. The vinyl acetylene molecule contains two possible pi-bonding sites (C=C and C=C). It offe rs three possible structures of C4H4 ... HX that comprise two T-type b onds to C=C (endo and exo approaches) and one T-type bond to C=C (perp endicular approach). The optimized geometries and the hydrogen-bond st abilization energies, based on MP2(FULL)/6-311++G(d,p)//6-31G(d,p) cal culations, indicate that the pi-type hydrogen bond to a C=C triple bon d leads to a more stable complex than for an analogous bond to C=C. Th e calculated global minima for the complexes with HF and HCl correspon d to the H-X moiety lying along a bisector of the C=C triple bond in t he endo approach, predictions that are in good agreement with the repo rted 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 pi-type hydrogen bonds has also been discussed using the Laplacian of the electron density, del( 2) rho. The complexes C2H2 ... HX and C2H4 ... HX as well as the hydro gen-bonded complex consisting of 2-butyne (CH3-C=C-CH3) and HCl were a lso studied for comparison.