Red-shifting and blue-shifting OH groups on metal oxide surfaces – towards a unified picture

Abstract

We analyse the OH vibrational signatures of 56 structurally unique water molecules and 34 structurally unique hydroxide ions in thin water films on MgO(001) and CaO(001), using DFT-generated anharmonic potential energy surfaces. We find that the OH stretching frequencies of intact water molecules on the surface are always downshifted with respect to the gas-phase species while the OH- groups are either upshifted or downshifted. Despite these differences, the main characteristics of the frequency shifts for all three types of surface OH groups (OHw, OsH and OHf) can be accounted for by one unified expression involving the in situ electric field from the surrounding environment, and the gas-phase molecular properties of the vibrating species (H2O or OH-). The origin behind the different red- and blueshift behaviour can be traced back to the fact that the molecular dipole moment of a gas-phase water molecule increases when an OH bond is stretched, but the opposite is true for the hydroxide ion. We propose that familiarity with the relations presented here will help surface scientists in the interpretation of vibrational OH spectra for thin water films on ionic crystal surfaces.

Isolating the Contributions of Specific Network Sites to the Diffuse Vibrational Spectrum of Interfacial Water with Isotopomer-Selective Spectroscopy of Cold Clusters

“…To sample the range available to the A sites, however, we note that these only occur in minimum energy structures at smaller cluster sizes. In general, the free OH frequency has been correlated with the local electric field at the surface, − such that the centroid of the ν sym and ν asym doublet red-shifts as the two bands come closer together. Several examples demonstrating this effect are included in Figure A in which H 2 O is attached to H 2 O, H 3 O + , H + , Mn + and Mn 2+ (Figure A2–A6, respectively) .…”

“…Characterization of the H-bonding strength in the context of partial charge transfer has been discussed at length in the case of the isomers formed by the neutral water hexamer. , What accounts for the large frequency ranges exhibited by the AAD and AADD sites, given that they all share the same local coordination arrangement? One correlation that accounts for long-range interactions is the dependence of the OH b frequencies on the net local electric field at the bond center. , Skinner and Ohno , have developed schemes to classify this dependence according to the local network topology. Those maps were generated by exploiting experimental results on the frequencies of the neutral water hexamer to calibrate the methods, and then using these calculations to follow how they are modified in the various isomeric forms.…”

Selective Transfer Hydrogenation of Cinnamaldehyde with Alcohol on Amorphous TiO2 Catalysts: Competitive Adsorption between H2O and Carbonyl

“…The FT‐IR spectra gave information about the adsorption behavior of CAL on the surface of A‐TiO 2 . In Figure a, the vibration peak of O−H (1627 cm −1 ) belonged to A‐TiO 2 blueshifted, which was attributed to the influence of the C=O group adsorbed on the catalyst . In addition, with the increase of H 2 O content, the degree of blue shift decreased, proving that the introduction of H 2 O suppressed the adsorption of the C=O group.…”

“…Compared with CAL, H 2 O could be adsorbed on the surface of A‐TiO 2 more easily, which would decrease the conversion of CAL. H 2 O molecules could also combine with the surface hydroxyl group of metal oxide catalysts easily via the hydrogen bond . Therefore, the presence of H 2 O in solvent could form a layer of H 2 O molecules.…”

Oxygen-functionalized alkyne precursors in carbon nanotube growth

“…effects, 43 but seems to indicate some incorporation of oxygen groups that escape detection by other methods (XPS and EA, with a detection limit of 2% O, in particular). Overall, in other spectral regions, the standard error is distributed widely about the mean and indicates the triplicates are heterogeneous.…”