An ab initio study of some structural features and the rotational barriers in performic acid,formic acid and hydrogen peroxide

Calculations on performic acid at the 4-31G level, with and without bond functions with complete geometry optimization, and at the (9, 5) level, with and without polarization functions and rigid rotation, all give no sign of a well in the potential energy curve for rotation about the O/O bond axis in the region of 50° – 90° ; and all but the unaugmented 4-31G basis set find the cis-cis planar conformer to be the most stable form. Calculations at the (9,5) level with rigid rotation find the energies of the other planar conformers, relative to the cis-cis conformer, to be 0.94, 1.50 and 14.80 kcal mol^?1 for the trans-trans, cis-trans and trans-cis structures respectively. These energies and also that for the barrier separating the cis-cis and cis-trans conformers, 1–2 kcal mol^?1, are discussed in relation to corresponding data for formic acid, hydrogen peroxide and several planar four heavy-atom molecules. Dipole moment calculations using the same basis sets would seem to favor a skew conformation as the most stable for performic acid, but comparisons between calculated and experimental values for formic acid and for hydrogen peroxide cast doubt on the validity of such results.