Hydrogen bonding effects phenols

From a more general perspective, the example at hand highlights a situation where PCM alone is unable to account fully for solvent effects on spectroscopic properties (e.g. the aN values in solution computed with PCM are 15.70, 15.75 and 15.80 G, versus experimental values of 16.58, 16.15 and 16.91 G for phenol, methanol and water respectively) this is typically related to the presence of strong, specific H-bond interactions. As shown in Figure 2.6, inclusion of specific hydrogen bond effects results in a further increase of the computed aN values, with final results close to their experimental counterparts (16.35, 16.15 and 16.51 G). 

Hydrogen-bonding effects of ethanol, phenol, and water on the e.s.r. spectra of the anion radicals of pyrazine mono- and di-A -oxides have been studied quantitatively (751). 

For phenolic oxygens the picture is as seen for 15 (see above) and 16. This shows a low frequency shift caused by hydrogen bonding. A similar picture is seen for intramolecularly hydrogen bonded nitro compounds. However, these data consist of both intra- and intermolecular hydrogen bonding effects as well as proximity effects. 

O chemical shifts of phenols hydrogen bonded to heteroaromatic nitrogens in systems like o-hydroxypyridines or similar compounds with one or more nitrogens or hydroxy groups show OH chemical shifts that are very similar (94-97 ppm), with the exception of a para-substituted methoxy derivative (90 ppm) , but this can be ascribed to a simple substituent effect (see above). 

In the following sections, some recent work in this field is reviewed. In a number of cases, references are given to recent publications with discussions of earlier work. The main focus is on IR investigations of key phenols that serve as reference compounds, particularly in relation to the study of hydrogen bonding effects. IR spectroscopy of biological systems is considered to fall outside the scope of this survey. For an example... 

A comparison of phenol acidity in DMSO versus the gas phase also shows an attenuation of substituent effects, but not nearly as much as in water. Whereas the effect of ubstituents on AG for deprotonation in aqueous solution is about one-sixth that in the gas phase, the ratio for DMSO is about one-third. This result points to hydrogen bonding of the phenolate anion by water as the major difference in the solvating properties of water and DMSO. ... 

The urea usually is added to the finished PF-resin and causes a distinct decrease of the viscosity due to disruption of hydrogen bonds [95] and due to dilution effects. There is obviously no co-condensation of this post-added urea with the phenolic resin. Urea only reacts with the free formaldehyde of the resin forming methylols, which, however, do not react further due to the high pH [19]. Only at the higher temperatures of the hot-press does some phenol-urea co-condensation occur [93,94,96]. 

The reaction of 2,4,6-tribromopyridine with phenoxide ion illustrates, in our opinion, the effect of hydrogen bonding as discussed in Section II, B, 3. Reaction (150°, 24 hr) in water gave approximately equal amounts (18% yields) of 2- and 4-monosubstitution, but in phenol under the same conditions only the 2-phenoxy derivative (in high yield plus a small amount of the 2,6-diphenoxy compound) was formed. In water, reaction at the adjacent 2- and 6-position is hindered by the hydrogen bonding (cf. 61) of the solvent to the azine-nitrogen, compared to reaction at the 4-position. On the other hand, in... 

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