Theoretical study of (XYO· · ·H· · ·OXY)þ (X, Y ¼ H, F, Cl) systems. From the asymmetrical to the symmetrical (O· · ·H· · ·O)þ hydrogen bonds

The hydrogen bonds in (XYO· · ·H· · ·OXY)þ systems (X, Y ¼ H, Fl, Cl) are investigated theoretically by using DFT-type B3LYP functional with the 6-311þþG(2d, 2p) basis set. The optimized geometries, binding energies, harmonic vibrational frequencies and intensities, and the natural charges on the different atoms are calculated. The proton affinities of the monomers are calculated as well. When the proton is bonded between H2O, HFO, Cl2O and F2O, the hydrogen bond is symmetrical and the O· · ·O distances are short (2.40–2.48 A° ). In the other systems, the proton is not centered and the O· · ·O distances range between 2.44 and 2.68 A ° . The binding energies vary within a broad range, from 22.2 to 143.5 kJ mol21. The largest binding energies are predicted for the symmetrical (O· · ·H· · ·O)þ bonds. The harmonic frequencies are calculated between 750 and 2400 cm21, the lowest frequencies being predicted for the symmetrical systems. The charge transfer occurring from the XYO molecule to the XHOHþ cation comprises between 0.08 and 0.22e. The correlations between the OH and O· · ·O distances and the correlations between the elongations of the OH bond and the binding energies or the variations of the electronic charges on the O and H atoms illustrate the large differences between the symmetrical and asymmetrical hydrogen bonds. q 2004 Elsevier B.V. All rights reserved.