| Abstract
| - 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 ClH as proton acceptors have been determined. Geometries were optimized with both the MP2 and theB3-LYP methods in conjuction with the 6-311+G(3df,2p) basis set. In general, we note good agreementbetween MP2 and B3-LYP hydrogen-bonded structures. However, for some very weakly bound complexeslarger differences exist, particularly in the r(H···X) distance, and in these instances the MP2 results aredetermined (from comparative CCSD(T) calculations) to be more reliable. The CCSD(T)/6-311+G(3df,2p)binding energies (De), which include corrections for basis set superposition errors, are very similar for theMP2 and B3-LYP geometries, reflecting the relative insensitivity of De to geometry for weakly boundcomplexes. The C−H···X hydrogen-bond strength (D0) shows a considerable dependence on the acidity ofthe C−H donor group and on the nature of the proton-accepting group. The strongest hydrogen bonds areformed between acetylene and either NH3 (9.2 kJ mol-1) or OH2(7.7 kJ mol-1). These values decreasesignificantly 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 betweenethylene and PH3, SH2 or ClH, as well as the complexes between ethane and NH3 or H2O, are more weaklybound again and have binding energies less than 1.0 kJ mol-1.
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