Hydrogen bonding fractionation factor

Although the number of research papers reporting the use of NMR spectroscopy, especially by comparison to and is small the range of applications is extensive (and is described in more detail in the book by Evans et al.). For example, the measurement of specific activity, optical purity, stereochemistry, solution conformation, kinetic acidities, hydrogen isotope exchange, hydrogen bonding, fractionation factors, and radiolysis. 

Figures 7-9 show the fractional conversion of methanol in the pulse as a function of temperature for the three catalysts and the three methanol feeds. Evidently the kinetic isotope effect is present on all three catalysts and over the complete temperature range, indicating that the rate limiting step is the breaking of a carbon-hydrogen bond under all conditions. From these experiments, the effect cannot be determined quantitatively as in the case of the continuous flow experiments, but to obtain the same conversion of CD,0D, the temperature needs to be 50-60° higher. This corresponds to a factor of about three in reaction rate. The difference in activity between PfoCL and Fe.(MoO.), is larger in the pulse experiments compared to tHe steady stateJ results.

It has become clear in the past decade that strong hydrogen bonding has associated with it several characteristic properties. In particular, as hydrogen-bond strength changes, maxima or minima are observed in nmr chemical shifts, the isotope effect on the chemical shift A[5( H) — 8( H)] defined on p. 271, ir Vah/vad band ratios, and in the isotope-fractionation factor, p. 

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. 

A large number of different techniques have been used to measure isotopic fractionation factors, and any method that can be used for measuring the equilibrium position of reaction (3) is appropriate. The discussion here will be limited to the more recent methods and those that have been used for studying hydrogen-bonded species. The discussion is further limited to fractionation factors for species in solution. Recent measurements (Larson and McMahon, 1986, 1987, 1988) of

hydrogen-bonded species in the... 

Similar measurements have given values for the fractionation factor of hydrogen-bonded complexes of the fluoride ion (Emsley et al., 1986c) and the acetate ion (Clark et al., 1988a) in acetic acid solution, [20] and [21]. For the chloride ion in acetic acid, the result (Emsley et al., 1986c) was cp = 1.26, which means that the exchangeable sites in acetic acid molecules in the solvation sphere of the chloride ion are favoured by deuterium compared to the sites in the bulk solvent. 

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. 

Table 6 Fractionation factors ((p) for protons in hydrogen bonds."...

A selection of representative values of the fractionation factors of protons in hydrogen bonds is given in Table 6. The results clearly show that hydrogen bonding reduces the value of fractionation factors and the hydro-... 

Fig. 10 Possible variation of isotopic fractionation factors with bond strength for hydrogen-bonded species. A,AH".

Fractionation factor acid-dissociation constants in H2O and D2O by Kreevoy (Kreevoy et al., 1977 Kresge and Chiang, 1973). This value is not as low as that for the many other systems which are known to have weaker hydrogen bonds. 

When the symmetry factor was introduced by Volmer and Erdey-Gruz in 1930, it was thought to be a simple matter of the fraction of the potential that helps or hinders the transfer of an ion to or from the electrode (Section 7.2). A more molecularly oriented version of the effect of P upon reaction rate was introduced by Butler, who was the first to apply Morse-curve-type thinking to the dependence of theenergy-dis -tance relation in respect to nonsolvent and metal—hydrogen bonds. 

---