Isotope effects on chemical shifts as an analytical tool in structural studies of intramolecular hydrogen bonded compounds

Isotope effects on chemical shifts of intramolecularly hydrogen bonded syst ems are reviewed. The effects are conveniently divided into localized (intr insic) and equilibrium isotope effects. The review covers both primary and secondary isotope effects on chemical shifts. For the localized one ii is v ery 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, ch emical shifts and isotope effects. Large intrinsic secondary isotope effect s can to a good degree be related to the change in the OH(D) bond length up on deuteriation. (2)Delta C(XD) isotope effects are shown to be good measur es of hydrogen bond strength. So far no evidence for heavy atom movement ha s been convincingly advanced for RAHB systems. A number of isotope effect t ypes have now been studied in depth, (n)Delta C(XD), X=O, S or N, (1)Delta N(D), (1)Delta O(D), (5)Delta O(D), (n)Delta H(OD), (n)Delta F(D). It is co ncluded that the possible over determination of isotope effects in hydrogen bonded systems provide a very powerful tool in studies of structure of hyd rogen bonded systems. Isotope effects are studied in detail in sterically h indered systems and parameters are available to distinguish between twist o f e.g. RCO groups and steric compression. Furthermore, twist of phenyl ring s may also be monitored. Proton transfer reactions such as tautomerism have been studied extensively. Equilibrium isotope effects on chemical shifts h ave been reported in a large number of cases. The magnitude of the equilibr ium isotope effects depends on the equilibrium constant. A series of parame ters have been suggested as a good way to establish tautomerism in a number of difficult cases. Both deuterium and tritium primary isotope effects hav e now been reported in a large number of systems. Large intrinsic primary i sotope effects is a good proof of double potential wells. Primary isotope e ffects are also studied in tautomeric systems and at different temperatures and can under such circumstances be both positive and negative.