Hydrogen bonds, infrared function

In dimers composed of equal molecules the dimer components can replace each other through tunneling. This effect has been discovered by Dyke et al. [1972] as interconversion splitting of rotational levels of (HF)2 in molecular beam electric resonance spectra. This dimer has been studied in many papers by microwave and far infrared tunable difference-frequency laser spectroscopy (see review papers by Truhlar [1990] and by Quack and Suhm [1991]). The dimer consists of two inequivalent HE molecules, the H atom of one of them participating in the hydrogen bond between the fluorine atoms (fig. 60). PES is a function of six variables indicated in this figure. 

Dkhissi, A., Alikhani, M. E., Bouteiller, Y., 1997, Methodological Study of Becke3-LYP Density Functional Adapted to the Determination of Accurate Infrared Signature for Hydrogen-Bonded Complexes , 7. Mol. Struct. 

The infrared spectrum of erythromycin is commonly used for its identification. Figure 2 shows the spectrum of a 75 mg./ml. chloroform solution. The bands at 1685 and 1730 cm- are due to the ketone carbonyl and the lactone carbonyl, respectively. The absorption peaks between 1000 and 1200 cm-1 are due to the ethers and amine functions. The CH2 bending is evidenced by peaks between 1340 and 1460 cm-, and alkane stretching peaks appear between 2780 and 3020 cm-. Hydrogen bonded OH and water appear as bands between 3400 and 3700 cm-1. 

Figure 14.4. Infrared spectra showing common absorption bands. The —OH of acids and alcohols are hydrogen-bonded, leading to broadening.The —OH of hexanoic acid, which is unusually sharp, absorbs in the 3500-2500 cm-1 region. Bottom spectrum shows absorptions due to functional groups in the compounds making up the mixture.

Since infrared spectroscopy also provides information about physical structure, infrared imaging can be used to determine spatial distribution of physical properties as well. Some of the properties include intermolecular and intramolecular order, hydrogen bonding, protein secondary structure, complexation and functional group orientation. 

A.J. Bienko et al., Low barrier hydrogen bond in protonated proton sponge. X-ray diffraction, infrared, and theoretical ab initio density functional theory studies. J. Chem. Phys. 119, 4313 4319 (2003)... 

Various authors5 17,35 37,38 39,40 41 42 43 have tried to model the dehydroxylation of bridged hydroxyls as a function of hydrogen bonding strength. However, the obtained results are often inconclusive, if not contradictory. The results can probably best be summarized by Hoffmann s7 statement that the complexity of some spectral intervals in the mid infrared is still beyond the scope of a detailed and unambiguous structural interpretation . 

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