Infrared hydrogen bond, intensity

Color from Vibrations and Rotations. Vibrational excitation states occur in H2O molecules in water. The three fundamental frequencies occur in the infrared at more than 2500 nm, but combinations and overtones of these extend with very weak intensities just into the red end of the visible and cause the blue color of water and of ice when viewed in bulk (any green component present derives from algae, etc). This phenomenon is normally seen only in H2O, where the lightest atom H and very strong hydrogen bonding combine to move the fundamental vibrations closer to the visible than in any other material. 

Infrared studies show that when water is adsorbed on the surface, the background intensity in the hydroxyl region increases new bands may appear but hydrogen-bonding effects make such conclusions uncertain. If such a catalyst is then exposed to hydrogen (or deuterium), no bands due to adsorbed hydrogen (or deuterium) are observed. Thus, adsorption of water apparently occurs on the active sites and blocks out type I chemisorption. 

Figure 7. Librational infrared spectra of methanol clusters [93] (bands B and C due to the tetramer, broad profile due to large clusters, cluster size increases from bottom to top) compared to the absorptions in amorphous and crystalline (zig zag) solid methanol [40]. The large clusters compare well to the amorphous solid, whereas the ring tetramer may be viewed as a small model of the zig zag chains in the crystal. Note that the high frequency band C acquires IR intensity through puckering of the methyl groups above (u) and below (d) the hydrogen bond plane.

T. Di Paolo, C. Bourderon, and C. Sandorfy, Model calculations on the influence of mechanical and electrical anharmonicity on infrared intensities Relation to hydrogen bonding. Can. 

Besides the peaks of the local proton modes typical for hydrogen bond, a sharp peak at 28 meV was observed in KDP [34] and attracted much attention [34,38,39]. This peak exists in DKDP at somewhat higher frequency its intensity decreases in both crystals and its width decreases upon the transition from the FE to the PE phase, without any softening of its frequency [38]. Hence, it is concluded that this mode is connected with the phase transition dynamics, i.e., coupled to the polarization fluctuations. This mode is not the tunneling mode or any local mode of proton or deuteron, but rather some collective optical mode of the lattice that involves substantial proton or deuteron displacement. It has been suggested [38] that this mode corresponds to the mode that has a peak at about 200 cm (25 meV) in Raman scattering and infrared reflectivity spectra, and that it is coupled to the soft mode and usually... 

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