Hydrogen bonding geometric isotope effects

There are two geometric isotope effects A/ (D — H), which refers to the change in bond length / . .. g of a hydrogen bond AHB on deuteration, and 8/ , which is defined as the change in the distance between the two minima of the potential energy well of the hydrogen bond when it is deuterated. The isotope effect, A/ (D—H), was dealt with previously (see p. 263). 

For resonance-assisted hydrogen bond (RAHB) systems such as enaminones, it was found that the intrinsic one-bond deuterium isotope effects increased with the hydrogen bond strength, being typically larger than 1 ppm [27]. For intermolecular cases, the behavior is very different as, AN(D) decreases as the distance to the hydrogen partner decreases [28, 29]. For a geometric description, see Limbach et al. [20]. 

H. Benedict et al., Solid State 15N NMR and theoretical studies of primary and secondary geometric h/d isotope effects on low-barrier nhn-hydrogen bonds. J. Am. Chem. Soc. 120, 2939-2950 (1998)... 

H/D isotope effects during compression assisted concerted HH-transfer in two cooperatively coupled hydrogen bonds, (c) Geometric H/D isotope effects during compression assisted stepwise HH-transfer in two anticooperatively coupled hydrogen bonds. 

Much evidence has been obtained in support of the El mechanism. For example, El reactions show first-order kinetics, consistent with a rate-limiting, unimolecular dissociation process. Furthermore, El reactions show no deuterium isotope effect. Because rupture of the C-H (or C-D) bond occurs after the rate-limiting step rather than during it, we can t measure a rate difference between a deuterated and nondeuterated substrate. A final piece of evidence is that there is no geometric requirement on the El reaction because the halide and the hydrogen are lost in separate steps, unlike the E2 reaction, where anti periplanar geometry is required. 

Perez—Hern dez, N., Guo, J., Shenderovich, I.G., and Denisov, G.S. (2009) OHO hydrogen bond geometries and NMR chemical shifts from equilibrium structures to geometric H/D isotope effects with apphca-dons for water, protomted rater and compressed i< . Israel J. ChenL,... 

M., Fehlhammer, W.P., Golubev, N.S., and Janoschek, R. (1998) Theoretical studies of primary and secondary geometric H/D isotope effects on low-barrier NHN-hydrogen bonds. 

H. H. (2012) Geometric hydrogen bond H/D isotope effects and isotopic polymorphism of solid... 

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