Hydrogen-bonded protons

Figures 2.a-c show the pyridine adsorption results. Bronsted acidity is manifested by the bands at 1440-1445,1630-1640 and 1530-1550 cm . Bands at 1600-1630 cm are assigned to pyridine bonded to Lewis acid sites. Certain bands such as the 1440-1460 and 1480-1490 cm can be due to hydrogen-bonded, protonated or Lewis-coordinated pyridine species. Under continuous nitrogen purging, spectra labeled as "A" in Figures 2a-c represent saturation of the surface at room temperature (90 25 unol pyridine/g found in all three tungsta catalysts) and "F" show the baseline due to the dry catalyst. We cannot entirely rule out the possibility of some extent of weakly bound pyridine at room temperature. Nevertheless, the pyridine DRIFTS experiments show the presence of Brpnsted acidity, which is expected to be the result of water of reduction that did not desorb upon purging at the reduction temperature. It is noted that, regardless of the presence of Pt, the intensity of the DRIFTS signals due to pyridine are...

Over the past 20 years, with the availability of fast reaction techniques (Eigen and de Maeyer, 1963 Hammes, 1974 Bemasconi, 1976), numerous kinetic studies have been made of the reactivity of hydrogen-bonded protons towards an external base (52). The majority of such studies have been made with hydroxide ion as the external base. Some examples of proton transfer to... 

The conclusions reached about proton transfer from phenylazoresorcinol monoanions are quite different from the behaviour which has been described for other hydrogen-bonded acids. For phenylazoresorcinol monoanions, it appears that direct attack by base on the hydrogen-bonded proton is an important process and can compete with two-step proton removal. For two-step proton transfer through an open form of the phenylazoresorcinol monoanion it is found that the rate of proton transfer from the open form is... 

The hydrogen-bonded protons wander all over the lot. Where you find them, and how sharp their signals are, depends at least on the solvent, the concentration, and the temperature. 

Table 5 Isotope effects on the chemical shift of hydrogen-bonded protons.

The value of the fractionation factor for any site will be determined by the shape of the potential well. If it is assumed that the potential well for the hydrogen-bonded proton in (2) is broader, with a lower force constant, than that for the proton in the monocarboxylic acid (Fig. 8), the value of the fractionation factor will be lower for the hydrogen-bonded proton than for the proton in the monocarboxylic acid. It follows that the equilibrium isotope effect on (2) will be less than unity. As a consequence, the isotope-exchange equilibrium will lie towards the left, and the heavier isotope (deuterium in this case) will fractionate into the monocarboxylic acid, where the bond has the larger force constant. 

The equilibrium constant for the isotope-exchange equilibrium can be expressed (6) in terms of the solvent isotope effects on the acid-dissociation constants and of the monocarboxylic acid and dicarboxylic acid monoanion, respectively. It follows that a lower value for the fractionation factor of the hydrogen-bonded proton means that the solvent isotope effect on the acid-dissociation constant will be lower for the dicarboxylic acid monoanion than for the monocarboxylic acid. 

The values of the fractionation factors in structures [15]-[21] are not strictly comparable since they are defined relative to the fractionation in different solvent standards. However, in aqueous solution, fractionation factors for alcohols and carboxylic acids relative to water are similar and close to unity (Schowen, 1972 Albery, 1975 More O Ferrall, 1975), and it seems clear that the species [15]-[21] involving intermolecular hydrogen bonds with solvent have values of cp consistently below unity. These observations mean that fractionation of deuterium into the solvent rather than the hydrogen-bonded site is preferred, and this is compatible with a broader potential well for the hydrogen-bonded proton than for the protons of the solvents water, alcohol and acetic acid. 

The major conclusion which was reached from the work of Jarret and Saunders was that intramolecularly hydrogen-bonded protons generally give values of (p below the value for a similar proton which is not involved in an intramolecular hydrogen bond. For example, a value of (p of 0.77 was obtained for the hydrogen-bonded site in the maleate ion [12] compared to 0.94 for acetic acid. The value for the phthalate ion [10] was higher, (p = 0.95. The value of (p = 0.95 was obtained for the salicylate ion [22] in comparison to the result for phenol, tp = 1.13. 

There is no clear correlation between the chemical shift or the ir absorption frequency and the length and linearity of the hydrogen bonds in the protonated diamines. The most reliable test for the nature of the potential function appears to be the sign of the isotope effect on the chemical shift of the hydrogen-bonded proton. 

Eyring (Haslam et ai, 1965a Eyring and Haslam, 1966 Jensen et ai, 1966) preferred to explain the reduced rate in terms of a one-step mechanism in which the base directly attacked the hydrogen-bonded proton as in (24). 

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