MP2/6-31G** ab-initio molecular orbital calculations have been performed to obtain geometries, H-bond energies and vibrational properties of the C 3H 6HX, C 2H 4HX and C 2H 2HX H-bonded complexes with XF or Cl. The more pronounced effects on the structural parameters of the isolated molecules due to complexation are verified to the CC and HX bond lengths, which are directly involved in the H-bond formation. They are increased after complexation. The calculated H-bond lengths for the hydrogen complexes for XF are shorter than those for xCl by about 0.55 Å, whereas the corresponding experimental value is 0.58 Å. The H-bond energies are essentially determined by the nature of the proton donor molecule. For XF, the AE mean value is 20 kJ/mol, whereas it is approximately 14.5 kJ/mol for XCl. The H-bond energies including zero-point corrections show a good correlation with the H-bond lengths. The more pronounced effect on the normal modes of the isolated molecules after complexation occurs to the HX stretching mode. The HX stretching frequency is shifted downward, whereas its IR intensity is much enhanced upon H-bond formation. The new vibrational modes arising from complexation show several interesting features.