Acetic anhydride in the gas phase, studied by electron diffraction and infrared spectroscopy, supplemented with ab initio calculations of geometries and force fields

Geometry-relaxed ab initio calculations of acetic anhydride at interalia B3 LYP/6-31G**, B3LYP/cc-pvtz, and MP2/6-31G** level revealed a mixture of non planar (sp,sp) and (sp,ac) energy minima, connected to one another via low- energy rotation barriers, thereby allowing for extensive large-amplitude mo tions. This model provided the geometrical constraints and force fields nec essary to perform the joint analysis of gasphase electron diffraction and i nfrared data. The large-amplitude motion is described, using pseudoconforme rs at 20 degrees intervals around the axes of rotation. It led to a dynamic model consisting of eight pseudoconformers of lowest energy connected to t he two local minima ((sp,ac) and nonplanar (sp,sp)) by fixed differences in torsion angles. The main structural parameters were refined using the elec tron diffraction method and an experimental "conformer" ratio of nonplanar (sp,sp)/(sp,ac) = 37(+/-15)%:63(+/-15)% was obtained, in close agreement wi th the quantum chemical results. The model of acetic anhydride is self-cons istent, reproduces the IR frequencies, with a root-mean-square deviation of about 10 cm(-1). and results in an improved frequency assignment. Assisted by MP2/6-31G**-based IR band intensities, the model also explains the foll owing experimental spectral peculiarities: (i) the relatively large number of bands with a small intensity and (ii) the changes in band intensities, b and shape and doubler behavior when going from the gas phase to the liquid and to solutions of different polarity.