Abstract
A series of MP2 and CCSD(T) computations have been carried out with correlation consistent basis sets as large as aug-cc-pV5Z to determine the intrinsic equatorial-axial conformational preference of CH 3-, F-, OCH 3-, and OH-substituted cyclohexane and tetrahydropyran rings. The high-accuracy relative electronic energies reported here shed new light on the intrinsic energetics of these cyclic prototypes for the anomeric effect. At the CCSD(T) complete basis set (CBS) limit, the energy of the equatorial conformation relative to the axial position (ΔE CCSD(T)CBC) is -0.75, -0.20, -0.21, and -0.56 kcal mol -1 in methyl-, fluoro-, methoxy-, and hydroxy cyclohexane, respectively, while ΔE CCSD(T)CBC is -2.83, +2.45, +1.27, and +0.86 kcal mol -1 for 2-methyl-, 2-fluoro-, 2-methoxy-, and 2- hydroxy tetrahydropyran, respectively. Note that the equatorial and axial conformers are nearly electronically isoenergetic in both fluoro- and methoxy cyclohexane. For all eight cyclic species, a zero-point vibrational energy correction decreases ΔE CCSD(T)CBC by a few tenths of a kilocalorie per mole. Relative energies obtained with popular methods and basis sets are unreliable, including Hartree-Fock theory, the B3LYP density functional, and the 6-31G and 6-311G families of split-valence basis sets. Even with the massive pentuple-ξ basis sets, the HF and B3LYP methods substantially overestimate the stability of the equatorial conformers (by as much as 0.99 and 0.73 kcal mol-1, respectively, for 2-methoxytetrahydropyran). Only because of a consistent cancellation of errors do these popular approaches sometimes provide reasonable estimates of the anomeric effect. © 2005 American Chemical Society.
Recommended Citation
A. J. Weldon et al., "Intrinsic Conformational Preferences of Substituted Cyclohexanes and Tetrahydropyrans Evaluated at the CCSD(T) Complete Basis Set Limit: Implications for the Anomeric Effect," Journal of Physical Chemistry A, vol. 109, no. 48, pp. 11073 - 11079, American Chemical Society, Dec 2005.
The definitive version is available at https://doi.org/10.1021/jp0550311
Department(s)
Chemistry
International Standard Serial Number (ISSN)
1089-5639
Document Type
Article - Journal
Document Version
Citation
File Type
text
Language(s)
English
Rights
© 2024 American Chemical Society, All rights reserved.
Publication Date
08 Dec 2005
