O---H Stretching Vibrations

FIGURE 3.14. Benzyl alcohol. A. O—H stretch intermolecular hydrogen bonded, 

Strong intramolecular hydrogen bonding occurs in o-hydroxyacetophenone. The resulting absorption at 

FIGURE 3.16. Phenol. A. Broad intermolecular hydrogen bonded, O—H stretch, 3373 cm-1. B. Aromatic C—H stretch, 3045 cm-1. C. Overtone or combination bands, 2000-1667 cm-1. D. The C—C ring stretch, 1595,1499, 1470 cm-1. E. In-plane O—H bend, 1360 cm-1. F. The C—O stretch, 1224 cm-1. G. Out-of-plane C—H bend, 810, 752 cm1. H. Out-of-plane ring C—C bend, 690 cm-1. I. (Broad) hydrogen-bonded, out-of-plane O—H bend, about 650 cm-1. 

In structures such as 2,6-di-r-butylphenol, in which steric hindrance prevents hydrogen bonding, no bonded hydroxyl band is observed, not even in spectra of neat samples. 

FIGURE 3.17. Infrared spectrum of the O—H stretching region of cyclohexylcarbinol in CC14. Peak A at 0.03 M (0.406-mm cell) Peak B at 1.00 M (0.014-mm cell). 

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The broadband at 3270 cm-1 is due to the O-H stretching vibration of the hydroxyl group. Moreover, the N-H stretching vibration absorption for open-chain amides occurs near 3270 cm-1 in the niclosamide solid state. 

O—H Stretching Vibrations In the liquid or solid state, and in CC14 solution at concentrations much over 0.01 M, carboxylic acids exist as dimers due to strong hydrogen bonding. 

Table 1. The central frequency Wo, width Aw, vibrational lifetime T and spectral diffusion time tc of the O-H stretch vibration of different hydrogen-bonded O-H groups.

The values of rc of the solvation shells are surprisingly long in comparison to the value of rc of 500 100 fs of the O-H- -O hydrogen bond in bulk liquid water, but are quite comparable to the recently calculated residence time of 18 ps of water in the solvation shell of Br- [10]. However, one should be very careful with this comparison since the characteristic time of the fluctuations of the hydrogen bond is not the same as the residence time in the solvation shell because the breaking of the hydrogen bond does not automatically mean that the water molecule really leaves the shell. The narrow width and long rc of the O-H- Y absorption component imply that the first solvation shell forms a stable and well-defined structure. The solvation shells of F and of the cations likely show similar dynamics, but unfortunately these dynamics could not be measured because the O-H stretch vibrational lifetime of the water molecules in these solvation shells is comparable to that of bulk HDO D20. 

In addition to frequency shifts, new vibrational bands occasionally appear in the liquid state as compared to the gas phase. An example is formic acid, HCOOH. At high temperatures, the vapor shows a characteristic O—H stretching vibrational band at 3750 cm-1 as the temperature of the gas is lowered, this band disappears and a new band at 3080 cm -1 appears. In the liquid state, only the 3080 cm"1 band is evident. The new band is due to O—H vibration in the hydrogen-bonded dimer. Hydrogen bonding also affects the IR spectra of liquid CH3OH and H20. 

Besides the O—H stretching vibrations of alcohols, there is a bending O—H vibration normally observed in the region 1410-1260 cm-1. There also is a strong C—O stretching vibration between 1210 cm-1 and 1050 cm-1. Both these bands are sensitive to structure as indicated below ... 

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