Abstract
The vibrational frequency of free OH − ions, which cannot be directly measured, has been claimed to be 3700 cm −1. In solid hydroxides the OH stretching frequency has been found in the range from 3690 to 3100 cm −1. The decrease of the vibrational frequency has been interpreted to be caused by hydrogen bridges or the increase of the metal oxygen bond strength. We suggest an alternative explanation. The vibrational frequency of unperturbed OH − ions is 3570 ± 10 cm −1. In ionic hydroxides this frequency is increased due to repulsion effects of the lattice or decreased if hydrogen bonds are present, to a large extent (up to 400 cm −1) in the case of common OH −….X bridges and only up to 70 cm −1 in the case of XH….OH − bonds.
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