TY - JOUR
T1 - Transition from hydrogen bonding to ionization in (HCl)n(NH3)n and (HCl)n(H2O)n clusters
T2 - Consequences for anharmonic vibrational spectroscopy
AU - Chaban, Galina M.
AU - Gerber, R. Benny
AU - Janda, Kenneth C.
PY - 2001/9/13
Y1 - 2001/9/13
N2 - Anharmonic vibrational frequencies and intensities are calculated for 1:1 and 2:2 (HCl)n(NH3)n and (HCl)n-(H2O)n complexes, employing the correlation-corrected vibrational self-consistent field method with ab initio potential surfaces at the MP2/TZP computational level. In this method, the anharmonic coupling between all vibrational modes is included, which is found to be important for the systems studied. For the 4:4 (HCl)n-(H2O)n complex, the vibrational spectra are calculated at the harmonic level, and anharmonic effects are estimated. Just as the (HCl)n(NH3)n structure switches from hydrogen-bonded to ionic for n = 2, the (HCl)n-(H2O)n switches to ionic structure for n = 4. For (HCl)2(H2O)2, the lowest energy structure corresponds to the hydrogen-bonded form. However, configurations of the 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 ionization dynamics using infrared excitation of the hydrogen-bonded form. The strong cooperative effects on the hydrogen bonding, and concomitant transition to ionic bonding, makes an accurate estimate of the large anharmonicity crucial for understanding the infrared spectra of these systems. The anharmonicity is typically of the order of several hundred wavenumbers for the proton stretching motions involved in hydrogen or ionic bonding, and can also be quite large for the intramolecular modes. In addition, the large cooperative effects in the 2:2 and higher order (HCl)n(H2O)n complexes may have interesting implications for solvation of hydrogen halides at ice surfaces.
AB - Anharmonic vibrational frequencies and intensities are calculated for 1:1 and 2:2 (HCl)n(NH3)n and (HCl)n-(H2O)n complexes, employing the correlation-corrected vibrational self-consistent field method with ab initio potential surfaces at the MP2/TZP computational level. In this method, the anharmonic coupling between all vibrational modes is included, which is found to be important for the systems studied. For the 4:4 (HCl)n-(H2O)n complex, the vibrational spectra are calculated at the harmonic level, and anharmonic effects are estimated. Just as the (HCl)n(NH3)n structure switches from hydrogen-bonded to ionic for n = 2, the (HCl)n-(H2O)n switches to ionic structure for n = 4. For (HCl)2(H2O)2, the lowest energy structure corresponds to the hydrogen-bonded form. However, configurations of the 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 ionization dynamics using infrared excitation of the hydrogen-bonded form. The strong cooperative effects on the hydrogen bonding, and concomitant transition to ionic bonding, makes an accurate estimate of the large anharmonicity crucial for understanding the infrared spectra of these systems. The anharmonicity is typically of the order of several hundred wavenumbers for the proton stretching motions involved in hydrogen or ionic bonding, and can also be quite large for the intramolecular modes. In addition, the large cooperative effects in the 2:2 and higher order (HCl)n(H2O)n complexes may have interesting implications for solvation of hydrogen halides at ice surfaces.
UR - http://www.scopus.com/inward/record.url?scp=0035856085&partnerID=8YFLogxK
U2 - 10.1021/jp011567k
DO - 10.1021/jp011567k
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AN - SCOPUS:0035856085
SN - 1089-5639
VL - 105
SP - 8323
EP - 8332
JO - Journal of Physical Chemistry A
JF - Journal of Physical Chemistry A
IS - 36
ER -