Hydrogen Bonding and Phase Transitions of RbOH · H2O and CsOH · H2O Studied by IR and Raman Spectroscopy

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Abstract
The infrared and Raman spectra of RbOH · H2O and of trimorphic CsOH · H2O, LTM, RTM, and HTM, in the range of 95–401 K are presented and discussed in terms of hydrogen bonding, disorder of the hydrogen atoms, and phase transitions. The different behaviour of OH− ions and H2O molecules as hydrogen bond donor and acceptor groups results in strong hydrogen bonding for HOH···OH− linkages and only weak H‐bonds for OH−···OH2 groupings. This is, for example, shown from the very different OH stretching modes of OH− and H2O in CsOH · H2O RTM, viz., 3496 and 2800 cm−1, observed for the same O···O distance of 264 pm. The temperature shifting of the OH− stretching modes in the range 340–395 K of CsOH · H2O can be interpreted by a Landau type order‐disorder phase transition. The halfwidths of the OH− bands exhibit an Arrhenius type behaviour. The activation energies derived reflect the band energies of lattice and librational modes (vibrational dephasing) as well as the energy barriers of proton reorientation processes, i.e., 17 kJ/mol for CsOH · H2O HTM.

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