Bonding in alcohols

Figure 17.1 Hydrogen-bonding in alcohols and phenols. A weak attraction between a positively polarized OH hydrogen and a negatively polarized oxygen holds molecules together. The electrostatic potential map of methanol shows the positively polarized O-H hydrogen (blue) and the negatively polarized oxygen (red).

FIGURE 19.2 The boiling points of ethers are lower than those of isomeric alcohols, because there is hydrogen bonding in alcohols but not in ethers. All the molecules represented here are unbranched. 

There have been many studies on the formation of hydrogen bonds between alcohols and a wide range of bases. If the bases are of similar type (for example, all nitrogen donor atoms in amines), there is also frequently a rather good correlation between the shift of the O-H stretching band and other properties. For example, stretching frequency shifts of the OH bonds in alcohols have been correlated... 

The hydroxyl group of alcohol weakens the a-C—H bond. Therefore, free radicals attack preferentially the a-C—H bonds of the secondary and primary alcohols. The values of bond dissociation energy (BDE) of C—H bonds in alcohols are presented in Table 7.1. The BDE values of C—H bonds of the parent hydrocarbons are also presented. It is seen from comparison that the hydroxyl group weakens BDE of the C—H bond by 23.4 kJ mol 1 for aliphatic alcohols and by 8.0 kJ mol 1 for allyl and benzyl alcohols. 

Bond Dissociation Energies of a-C—H Bonds in Alcohols [1-5]— -continued... 

From a structural point of view the OPLS results for liquids have also shown to be in accord with available experimental data, including vibrational spectroscopy and diffraction data on, for Instance, formamide, dimethylformamide, methanol, ethanol, 1-propanol, 2-methyl-2-propanol, methane, ethane and neopentane. The hydrogen bonding in alcohols, thiols and amides is well represented by the OPLS potential functions. The average root-mean-square deviation from the X-ray structures of the crystals for four cyclic hexapeptides and a cyclic pentapeptide optimized with the OPLS/AMBER model, was only 0.17 A for the atomic positions and 3% for the unit cell volumes. 

Among the wealth of issues relevant to hydrogen bonding in alcohol clusters, this review will focus on aspects related to hydrogen bond patterns and on the dynamical implications over a wide range of time scales. Some key questions connected to these aspects will be formulated. 

R. A. Provencal, R. N. Casaes, K. Roth, J. B. Paul, C. N. Chapo, R. J. Saykally, G. S. Tschumper, and H. F. Schaefer, Hydrogen bonding in alcohol clusters A comparative study by infrared cavity ringdown laser absorption spectroscopy. J. Phys. Chem. A 104, 1423 1429 (2000). 

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