Revisiting the vibrational spectrum of formic acid anhydride

The potential energy surface of formic acid anhydride has been investigated at highly accurate quantum chemical methods. The rotation of CHO group in global minimum conformer of formic acid anhydride can lead two different local minimum conformers. Both local minimum conformers are around three kcal/mol higher in energy than the global minimum conformer. One conformer is planar and another has two rotamers which rapidly interconvert to each others via planar transition state. In some earlier studies, this planar transition state has incorrectly assigned to be a local minimum structure. We calculated anharmonic vibrational frequencies for the global and local minimum energy conformers and compared the theoretical wavenumbers with experimental gas phase and argon matrix measurements. Our results suggest that some of the experimentally detected peaks are overtone and combination bands. In previous studies with harmonic calculations, those peaks have assigned to be fundamental bands with zero intensities. We confirmed that the higher energy conformer produced in argon matrix by ultra violet induced rotamerization of global minimum conformer belongs to the planar conformer.