C-H•••X hydrogen bonds of acetylene, ethylene, and ethane with first- and second-row hydrides

The structures and binding energies of a series of C-H X hydrogen-bonded complexes involving acetylene, ethylene, and ethane as proton donors and the first- and second-row hydrides CH4, NH3, OH2, FH, PH3, SH2, and C1H as proton accepters have been determined. Geometries were optimized with both the MP2 and the B3-LYP methods in conjunction with the 6-311+G(3df,2p) basis set. In general, we note good agreement between MP2 and B3-LYP hydrogen-bonded structures. However, for some very weakly bound complexes larger differences exist, particularly in the r(H . . .X) distance, and in these instances the MP2 results are determined (from comparative CCSD(T) calculations) to be more reliable. The CCSD(T)/6-311+G(3df,2p) binding energies (D-e), which include corrections for basis set superposition errors, are very similar for the MP2 and B3-LYP geometries, reflecting the relative insensitivity of D-e to geometry for weakly bound complexes. The C-H X hydrogen-bond strength (D-0) shows a considerable dependence on the acidity of the C-H donor group and on the nature of the proton-accepting group. The strongest hydrogen bonds are formed between acetylene and either NH3 (9.2 kJ mol(-1)) or OH2 (7.7 kJ mol(-1)). These values decrease significantly for the corresponding complexes between acetylene and FH, CH4 or the second-row hydrides. The binding energies for the complexes between ethylene and either NH3 or OH2 (2.1 and 1.5 kJ mol(-1), respectively) are much smaller than those of the corresponding acetylene complexes. The complexes between ethylene and PH3, SH2 or C1H, as well as the complexes between ethane and NH3 or H2O, are more weakly bound again and have binding energies less than 1.0 kJ mol(-1).