On the relative importance of O-H/pi interactions in 7-norbornenols

The geometries of the two H-O-C rotamers of syn- and two of anti-7-norborne nol have been optimized at the ab initio HF/6-31G*" and B3LYP hybrid HF-DFT levels of theory by using a 6-31G** basis set. Contrary to an earlier repo rt, we find that the (nongeometry constrained) anti-trans isomer (1d) is pr edicted to be more stable than the corresponding syn-cis form (1a). The inc reased stability of 1d vis-a-vis la can be accounted for in terms of relati ve H(1)-C(2)-O(3)-H(4) torsion energy effects. The computational results in dicate that the hydroxyl proton in 1a enters into intramolecular hydrogen b onding with the proximate C=C pi bond. Supporting evidence for this conclus ion resides in the fact that the la is predicted to possess the lowest O-H stretching frequency, a result that can be attributed to pi-hydrogen bondin g.