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

The structure of formic anhydride was studied by the joint analysis of gas-phase electron diffraction, microwave, and infrared data. The exp erimental data are supported with geometrical constraints and force fi elds from geometry-relaxed ab-initio calculations on the 4-21G and 6-3 1G* levels. All data agree with the gas phase at room temperature con sisting of the planar [sp,ap] conformer, in which the two O=CH-O moiet ies differ significantly in geometrical as well as vibrational paramet ers. Geometrical least-squares constraints taken from the 4-21G calcul ations performed slightly better than those from 6-31G* calculations. In contrast, the 6-31G*-derived scaled force field performed better in the IR analysis, reproducing the frequencies with a root-mean-squar e deviation of 6.7 cm(-1) and a largest discrepancy of 14.1 cm(-1). As sisted by 6-31G*-based IR band intensities, a significantly improved assignment of IR frequencies was made. The reported self-consistent mo lecular model of formic anhydride is in agreement with all diffraction and spectroscopic data available to date, contains a complete force f ield of which diagonal and off-diagonal constants allow a physically c onsistent interpretation, and also points to a rationalization why for mic anhydride is thermolabile and acetic anhydride is not.