The Hydrogen-Bonding Equilibrium

Possible methods of detecting hydrogen bonding were surveyed by Pimentel and McClellan (1960a). Here only two particular methods will be considered briefly to represent essential implications in usual equilibrium constant determinations. 

The second method considered here is superior to n.m.r. in respect of the more or less distinct appearance of absorption peaks assigned to the stretching vibration of free and bonded species. The direot evidence of 

In equilibrium constant determinations, we need extinction coefficients to evaluate equilibrium concentrations for free and bonded species. The absorption peaks have to be resolved. This is in general attainable by using an inert diluent as third component besides proton-donor and proton-acceptor molecules. Let us now consider difficulties emerging from infrared studies. 

Owing to the aforementioned difficulties, equilibrium constants of hydrogen bonding determined by different authors are often contradictory. Nevertheless, some general trends are unambiguously established. 

The influence of substitution of either phenol, as proton donor, or aromatic proton acceptors has been indicated to be almost of the same magnitude on hydrogen bonding (Ryabokobylko and Chekunov, 1970). However, both electron-donor and electron-withdrawing substituents have been found to decrease the degree of selfassociation of phenol (Vanderborgh et al., 1970). 

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Sceats, M. G. Rice, S. A. (1980) The Water-water Pair Potential Near the Hydrogen Bonded Equilibrium Configuration, Journal of Chemical Physics 72, 3236-3247... 

It is seen that the three values for the equilibrium constant (k) range from 0.00443 to 0.00565 with an average value of 0.00504. The two values for the densities of the methanol/water associate are in reasonable agreement and have a magnitude that would be expected for the hydrogen bonded associate. 

First, the hydrogen bond is a bond by hydrogen between two atoms the coordination number of hydrogen does not exceed two.7 The positive hydrogen ion is a bare proton, with no electron shell about it. This vanishingly small cation would attract one anion (which we idealize here as a rigid sphere of finite radius—see Chap. 13) to the equilibrium intemuclear distance equal to the anion radius, and could then similarly attract a second anion, as shown in Figure 12-1, to form... 

Fig. 2 The experimentally determined potential energy V(), expressed as a wavenumber for convenience, as a function of the angle in the hydrogen-bonded complex H20- HF. The definition of

Philippova and Starodubtzev have also extensively studied the complex-ation behavior of polyacids and PEG, especially, the system of crosslinked of poly(methacrylic acid) and linear poly(ethylene glycol) (Philippova and Starodubtzev, 1995 Philippova et al., 1994). They observed that decreasing the molecular weight of PEG from 6000 to 1500 resulted in its slower diffusion into the swollen network of PMAA, and a drastic decrease in both the stability and equilibrium composition of the intermacromolecular complex. Analysis of dried polymer networks of PMAA with absorbed PEG chains by FT-IR spectroscopy revealed the presence of two types of hydrogen bonded structures (1) dimers of methacrylic acid at absorption frequency of 1700 cm-1 and (2) interpolymer complexes of PMAA and PEG at 1733 cm-1. In addition, they also suggested as a result of their studies, that the hydrogen bonded dimer of PMAA forms preferentially to the intermacromolecular complex between the PMAA network and PEG chains. 

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