While the strength of an intermolecular hydrogen bond can be measured by the binding energy of the appropriate bimolecular complex, this technique is not available for the strength of intramolecular hydrogen bonds. The strength of the latter has been estimated by ab initio calculations and by looking at red shifts in the IR (Badger-Bauer). Both methods have their drawbacks. In this study, the intramolecular bond strength is measured by measuring the binding energy of the complex with a proton acceptor that is capable of disrupting the intramolecular bond. The difference between this complex and that of a suitable control gives the intramolecular bond energy. This is illustrated by complexing 4-fluoro- and 2-fluorophenol with DMSO and measuring the binding constant using 1H and 19F NMR. Both methods give similar results and these results are in line with estimates from other methods. The method is then extended to other 2-X-phenols that are capable of forming hydrogen bonds. The results show a strength ordering of OCH3 &gt; Br ≈ about equal Cl &gt; F.

Using 19F and 1H NMR to determine the strength of intramolecular hydrogen bonds

253rd National Meeting (2017)

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