Structure, spectroscopy, and dynamics of the phenol-(water)₂ cluster at low and high temperatures

Aqueous solutions are complex due to hydrogen bonding (HBing). While gas-phase clusters could provide clues on the solution behavior, most neutral clusters were studied at cryogenic temperatures. Recent results of Shimamori and Fujii provide the first IR spectrum of warm phenol-(H₂O)₂ clusters. To understand the temperature (T) effect, we have revisited the structure and spectroscopy of phenol-(H₂O)₂ at all T. While older quantum chemistry work concluded that the cyclic isomers are the most stable, the inclusion of dispersion interactions reveals that they are nearly isoenergetic with isomers forming π-HBs with the phenyl ring. Whereas the OH-stretch bands were previously assigned to purely local modes, we show that at low T they involve a concerted component. We have calculated the (static) anharmonic IR spectra for all low-lying isomers, showing that at the MP2 level, one can single out one isomer (udu) as accounting for the low-T spectrum to 3 cm^-1 accuracy. Yet no isomer can explain the substantial blueshift of the phenyl-OH band at elevated temperatures. We describe the temperature effect using ab initio molecular dynamics with a density functional and basis-set (B3LYP-D3/aug-cc-pVTZ) that provide a realistic description of OH⋯O vs. OH⋯π HBing. From the dipole moment autocorrelation function, we obtain good description for both low- and high-T spectra. Trajectory visualization suggests that the ring structure remains mostly intact even at high T, with intermittent switching between OH⋯O and OH⋯π HBing and lengthening of all 3 HBs. The phenyl-OH blueshift is thus attributed to strengthening of its OH bond. A model for three beads on a ring suggests that this shift is partly offset by the elimination of coupling to the other OH bonds in the ring, whereas for the two water molecules these two effects nearly cancel.