Inter- and intramolecular O-H center dot center dot center dot pi hydrogenbonding in the methanol-ethene complex and syn-7-norbornenol, probed by IR, H-1 NMR and quantum chemistry

The geometries and energies of syn-7-norbornenol (1) have been investigated with theoretical and experimental methods, while its epimer anti-7-norborn enol (2) has been investigated by theoretical methods only. It was found th at 1 is intramolecularly hydrogen bonded and exists almost exclusively in i ts hydrogen bonded form, the Anti conformer. Compound 2, for which intramol ecular hydrogen bonding is impossible, was found to exist as the Gauche con former. A theoretical investigation of the model complex between methanol a nd ethene showed that the interaction energy is -3.1 +/- 0.1 kcal mol(-1) ( estimated Hartree-Fock basis set limit plus correlation contribution, not i ncluding vibrational zero point energy). Electron correlation is essential in the evaluation of the interaction energy and also for the geometry of th e complex. It was found that the potential energy surface around the minimu m energy structure is flat, with an almost freely rotating methanol part. ( C) 2001 Elsevier Science B.V. All rights reserved.