Isotope Effects on Chemical Shifts as an Analytical Tool in Structural Studies of Intramolecular Hydrogen Bonded Compounds

Simon      Bolvig; Poul      Erik Hansen

doi:10.2174/1385272003376391

Abstract

Isotope effects on chemical shifts of intramolecularly hydrogen bonded systems are reviewed. The effects are conveniently divided into localized (intrinsic) and equilibrium isotope effects. The review covers both primary and secondary isotope effects on chemical shifts. For the localized one it is very important to distinguish between RAHB and non-RAHB types. For the RAHB systems the OH group is shown to form a stronger hydrogen bond than the OD group, whereas the opposite is true for non-RAHB. Theoretical calculations at the ab initio level (DFT) can be used to provide reliable structures, chemical shifts and isotope effects. Large intrinsic secondary isotope effects can to a good degree be related to the change in the OH(D) bond length upon deuteriation. 2ΔC(XD) isotope effects are shown to be good measures of hydrogen bond strength. So far no evidence for heavy atom movement has been convincingly advanced for RAHB systems. A number of isotope effect types have now been studied in depth, nΔC(XD), X= O, S or N, 1DN(D), 1ΔO(D), 5ΔO(D), nΔH(OD), nΔF(D). It is concluded that the possible over determination of isotope effects in hydrogen bonded systems provide a very powerful tool in studies of structure of hydrogen bonded systems. Isotope effects are studied in detail in sterically hindered systems and parameters are available to distinguish between twist of e.g. RCO groups and steric compression. Furthermore, twist of phenyl rings may also be monitored. Proton transfer reactions such as tautomerism have been studied extensively. Equilibrium isotope effects on chemical shifts have been reported in a large number of cases. The magnitude of the equilibrium isotope effects depends on the equilibrium constant. A series of parameters have been suggested as a good way to establish tautomerism in a number of difficult cases. Both deuterium and tritium primary isotope effects have now been reported in a large number of systems. Large intrinsic primary isotope effects is a good proof of double potential wells. Primary isotope effects are also studied in tautomeric systems and at different temperatures and can under such circumstances be both positive and negative.

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