Conformational effects on NMR chemical shifts of half-cage alcohols calculated by GIAO-DFT

Half-cage compounds have played an important role in the investigation of the way steric compression affects physical and chemical properties of organic molecules. Recent theoretical studies of half-cage alcohols have also shown that rotation around the carbon-oxygen bond of the hydroxyl group leads to low-energy conformers in which hyperconjugation affects bond lengths, bond angles, and charge distribution on carbon and hydrogen atoms in its vicinity while charge distribution is also affected by electrostatic effects. Chemical shifts are also sensitive to such variations, but we found that in smaller model systems steric effects may strongly attenuate those due to hyperconjugation so we optimized geometries for low energy rotamers of 'outside' and 'inside' half-cage alcohols, where these effects can be separated, and calculated their respective hydrogen and carbon-13 chemical shifts by gauge-independent atomic orbital (GIAO) methods at the 133LYP/6-31G(d) level. Results are compared to those obtained for the corresponding norbornyl alcohols as well as for the half-cage hydrocarbon. Carbon-13 chemical shifts respond more strongly to effects owing to hyperconjugation while hydrogen chemical shifts are more sensitive to electrostatic effects due to the proximity of the hydroxyl group. (C) 2002 Elsevier Science B.V. All rights reserved.