A comparative density functional study of the torsional potential of 4-fluoro (trifluoromethoxy)benzene and related species

B3LYP density functional methods with conventional and especially derived basis sets are applied to determine the torsional potential around the aryl-O bond in 4-fluoro (trifluoromethoxy)benzene (1), trifluoromethoxybenzene (2), 4-fluoro anisole (3) and anisole (4). It is found that both 1 and 2 exhibit a minimum at the perpendicular-planes [o]-form conformation, opposite to 3 and 4, which present minima at the eclipsed [e]-form conformation. A very flat secondary minimum for the [e]-form is found for both 1 and 2 with the larger basis sets, but it is unable to sustain any vibrational level. The precision and accuracy of the B3LYP calculations are determined through the study of methyl vinylether and trifluoromethyl vinylether at the MP2 and QCISD levels.