| Abstract
| - The energetics of intramolecular hydrogen bonds (H-bonds) is a subject of fundamental importance in chemistryand biochemistry. In contrast with intermolecular H-bonds, whose enthalpy can be determined by experimentor accurately evaluated through a supermolecular approach, there is no general accepted procedure to determinethe enthalpy of an intramolecular H-bond. In this work, different ways for assessing the energetics ofintramolecular H-bonds of selected aromatic systems were applied and compared. They include the widelyused conformational analysis approach (cis−trans method), a recently proposed isodesmic reaction method,and a new procedure that we designate as the ortho−para method. Energy calculations were carried out atseveral theory levels, including a modified complete basis set extrapolation method (CBS-QMPW1), in whichthe geometries are based on MPW1PW91/aug-cc-pVDZ density functional theory optimizations. The obtainedresults, together with a simple dipole−dipole interaction model, help to explain why the enthalpies ofintramolecular H-bonds are often overestimated by the cis−trans method. The results also show thatintramolecular H-bond enthalpies based on the isodesmic reaction method may be unreliable. The ortho−para method, which can be applied when accurate theoretical or experimental standard enthalpies of formationare available, is probably the best way of estimating the enthalpies of intramolecular hydrogen bonds. Finally,our results illustrate the important role played by intramolecular H-bonds in the energetics of homolyticdissociation reactions involving di-substituted benzenes.
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