Transition from hydrogen bonding to ionization in (HCl)(n)(NH3)(n) and (HCl)n(H2O)(n) clusters: Consequences or anharmonic vibrational spectroscopy

Anharmonic vibrational frequencies and intensities are calculated for 1:1 a nd 2:2 (HCl)(n)(NH3)(n) and (HCl)(n-)(H2O)(n) complexes, employing the corr elation-corrected vibrational self-consistent field method with ab initio p otential surfaces at the MP2/TZP computational level. In this method, the a nharmonic coupling between all vibrational modes is included, which is foun d to be important for the systems studied. For the 4:4 (HCL)(n)- (H2O)(n) c omplex, the vibrational spectra are calculated at the harmonic level, and a nharmonic effects are estimated. Just as the (HCl)(n)(NH3)(n) Structure swi tches from hydrogen-bonded to ionic for n = 2, the (HCl)(n)-(H2O)(n) switch es to ionic structure for n = 4. For (HCl)(2)(H2O)2, the lowest energy stru cture corresponds to the hydrogen-bonded form. However, configurations of t he ionic form are separated from this minimum by a barrier of less than an O-H stretching quantum. This suggests the possibility of experiments on ion ization dynamics using infrared excitation of the hydrogen-bonded form. The strong cooperative effects on the hydrogen bonding, and concomitant transi tion to ionic bonding, makes an accurate estimate of the large anharmonicit y crucial for understanding the infrared spectra of these systems. The anha rmonicity is typically of the order of several hundred wavenumbers for the proton stretching motions involved in hydrogen or ionic bonding, and can al so be quite large for the intramolecular modes. In addition, the large coop erative effects in the 2:2 and higher order (HCl)(n)(H2O)(n) Complexes may have interesting implications for solvation of hydrogen halides at ice surf aces.