Comparison of large basis set DFT and MP2 calculations in the study of the barrier for internal rotation of 2,3,5,6-tetrafluoroanisole

The barrier for internal rotation around the -OCH3 bond in 2,3,5,6-tetrafluoroanisole was calculated using the density functional theory (DFT) and second-order Moller-Plesset (MP2) methods with Pople's basis sets up to 6-311++G(3df,2p) and Jensen basis sets up to pc-3. The results are converged only if fairly large basis sets are used (at least 6-311++G(3df,2pd) or pc-2). Both the DFT and MP2 potential energy curves show internal structure. Two minima and three maxima are observed on the curves, arising from the interplay between lone-pair delocalization and changes in the hybridization around the oxygen atom, together with the attraction between the positively polarized hydrogens in the methyl group and the negatively polarized fluorine atom at the ortho position. These critical points are somehow ironed out by the addition of zero-point and thermal corrections to the energy curves. At this level, the MP2 method can describe reasonably well the previously determined single-well experimental rotational barrier, 2.7 +/- 2.0 kcal/mol at 298 K, while all DFT methods yield a much smaller result. As observed experimentally, the -OCH3 group is perpendicular to the aryl ring in the equilibrium structure, although two very close minima with an intermediate hump at 90 are still observable. The theoretical free energy barrier of rotation at the MP2(full)/pc-2 level is 2.0 +/- 1.0 kcal/mol, in reasonable agreement with the experimental determination. (c) 2006 Wiley Periodicals, Inc.

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