An ab initio study of the OH stretching frequencies in ice II, ice VIII, and ice IX

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Abstract
Ab initio studies of the uncoupled, anharmonic OH and OD stretching frequency shifts in the three proton‐ordered ice phases known, ice II, ice VIII, and ice IX, are presented. The ice structures are simulated by (H2O)5 supermolecules surrounded by point charges representing the correct crystal potentials. The calculations include electron correlation at the MP2 (DZP) level. For the eight different OH (OD) vibrators studied, the crystal environment leads to a downshift of the anharmonic OD frequency in the range 195–265 cm−1, in good agreement with experimental values (222–281 cm−1) when corrections are made for the limited supermolecular size (∼−45 cm−1), and, for ice VIII, also for the effects of the nonhydrogen bonded network (∼+75 cm−1). Also the agreement between absolute experimental and theoretical OD frequencies is good when errors due to basis set limitation (∼−75 cm−1) are taken into account. The calculations suggest a reassignment of two of the experimental OD bands in ice II and all three experimental OD bands in ice IX. Calculations for charge‐embedded (H2O)9 and (H2O)13 ice clusters show that at least a nonamer is needed to avoid boundary effects from the size of the supermolecule. Theoretical correlation curves between H‐bond parameters–R(O...O), ν(OH), re(OH), and infrared absorption intensity—are presented for the three ice phases and are compared to liquid water computations.

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