MATRIX-ISOLATION INFRARED AND ABINITIO STUDIES ON CONFORMERS OF FORMIC-ACID ANHYDRIDE

High-energy conformers of formic acid anhydride were produced in low-t emperature Ar matrices with the aid of UV irradiation. One previously unknown conformer, EE, was detected from the FTIR spectra of the irrad iated matrices. Connected with the assignment of this new species, the stabilities and vibrational spectra of three previously assumed confo rmers, obtained with respect to internal rotation about the C-O bonds, were studied by ab initio methods. At all levels used, the AE conform er was the lowest energy species. Considerable basis set dependence in the conformer energetics was found: MP2/4-31G and MP4/4-31G//MP2/4-31 G levels of theory resulted in the same energetic order of the three c onformers as did the calculations at HF level, but calculations at hig her levels (e.g., MP2/6-31G* and MP4/6-31G**//MP2/6-31G**) turned the AA shallow minimum into an unstable saddle point. In this paper are g iven optimized geometries, experimental and calculated spectra, and a vibrational analysis of formic acid anhydride based on the ab initio c alculated force field.