FORMIC ACETIC-ANHYDRIDE IN THE GAS-PHASE, STUDIED BY ELECTRON-DIFFRACTION AND INFRARED-SPECTROSCOPY, SUPPLEMENTED WITH AB-INITIO CALCULATIONS OF GEOMETRIES AND FORCE-FIELDS

The structure of formic acetic anhydride was studied by the joint anal ysis of gas-phase electron diffraction and infrared data, supported wi th extensive ab-initio calculations on the 4-21G and 6-31G* levels. A ll data agree with the gas phase at room temperature existing in the p lanar (sp,ap) conformer. Best electron diffraction geometry was:obtain ed using geometrical constraints derived from 4-21G calculations after correction to r(alpha)(o) level. Also, the scaled 4-21G force field p erformed better than its 6-31G* counterpart. The new model of formic, acetic anhydride is self-consistent, reproduces the IR frequencies wi th a root-mean-square deviation of 8.8 cm(-1), and results in an impro ved frequency assignment as well as in a good qualitative agreement be tween observed and calculated IR band intensities. Formic acetic anhyd ride is conformationally and spectroscopically very different from ace tic anhydride,but strongly resembles formic anhydride, although differ ences remain. The similarities with formic anhydride are ascribed to a n attractive nonbonded H (formyl)... O= interaction, while the dissimi larities are ascribed to a larger electronic interaction between two f ormyl moieties rather than between a formyl and an acetyl moiety.