Conformational energies for 2‐substituted butanes

The conformational free energies for some 2‐substituted butanes where X = F, Cl, CN, and CCH were calculated using G3‐B3, CBS‐QB3, and CCSD(T)/6‐311++G(2d,p) as well as other theoretical levels. The above methods gave consistent results with free energies relative to the trans conformers as follows: X = CCH, g+ = 0.77 ± 0.05 kcal/mol. g− = 0.88 ± 0.05 kcal/mol; X = CN, g+ = 0.85 ± 0.05 kcal/mol, g− = 0.75 ± 0.05 kcal/mol; X = Cl, g+ = 0.70 ± 0.05 kcal/ml, g− = 0.80 ± 0.05 kcal/mol; and X = F, g+ = 0.53 ± 0.05 kcal/mol, g− = 0.83 ± 0.05 kcal/mol. The conformational free energies also were estimated using the observed liquid phase IR spectra and intensities calculated using B3LYP/6‐311++G** and MP2/6‐311++G**. The rotational free energy profiles for all of the compounds were estimated at the G3‐B3 level. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 1127–1132, 2004