ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
In contrast to the OH stretching frequencies of bound H2O molecules, which are always found at lower wave numbers compared to the free molecule, the experimentally determined frequency of the OH− ion can be either lower or higher than the free-ion value. Optimized geometries and fundamental stretching frequency of OH− have been calculated here by ab initio methods at the Hartree–Fock and second-order Møller–Plesset levels for a number of cation–OH−, HOH⋅⋅⋅OH−, cation–OH−⋅q−, and cation–OH−⋅OH2 complexes for Li+, Mg2+, and Al3+. The importance of electrostatic effects on the OH− frequency has been assessed by comparison with calculations of different point-charge and homogenous-field OH− systems. As long as the interaction is not dominated by electronic overlap, the frequency shift is found to be largely determined by electrostatic forces: with increasing field strength the OH− frequency rises to a maximum and then decreases. The OH− dipole moment and Mulliken charges vary monotonically with the field strength, whereas the equilibrium OH distance goes through a minimum and the bond electron density through a maximum. In strongly polarizing fields, such as in the optimized Al3+⋅OH− and Mg2+⋅OH−⋅⋅⋅OH2 systems, the OH− frequency falls below the free-ion value. Ar experimentally observed frequency downshift for an OH− ion in the condensed phase cannot be used as a criterion for the existence of H bonding. The OH− ion acts as an H-bond donor only when strongly polarized by a neighbor on its oxygen side.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.460808
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