Acetic Anhydride in the Gas Phase, Studied by Electron Diffraction and Infrared Spectroscopy, Supplemented With ab Initio Calculations of Geometries and Force Fields

Abstract

Geometry-relaxed ab initio calculations of acetic anhydride at interalia B3LYP/6-31G**, B3LYP/cc-pvtz, and MP2/6-31G** level revealed a mixture of nonplanar (sp,sp) and (sp,ac) energy minima, connected to one another via low-energy rotation barriers, thereby allowing for extensive large-amplitude motions. This model provided the geometrical constraints and force fields necessary to perform the joint analysis of gas-phase electron diffraction and infrared data. The large-amplitude motion is described, using pseudoconformers at 20° intervals around the axes of rotation. It led to a dynamic model consisting of eight pseudoconformers of lowest energy connected to the two local minima ((sp,ac) and nonplanar (sp,sp)) by fixed differences in torsion angles. The main structural parameters were refined using the electron diffraction method and an experimental “conformer” ratio of nonplanar (sp,sp)/(sp,ac) = 37(±15)%:63(±15)% was obtained, in close agreement with the quantum chemical results. The model of acetic anhydride is self-consistent, 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 following experimental spectral peculiarities: (i) the relatively large number of bands with a small intensity and (ii) the changes in band intensities, band shape, and doublet behavior when going from the gas phase to the liquid and to solutions of different polarity.

Gas phase structure and conformational properties of trifluoroacetic anhydride, CF3C(O)OC(O)CF3

“…This result agrees with that for bisfluoroformyl ether (dicarbonic difluoride), FC(O)-O-C(O)F [7], but it is in contrast to the conformational properties of the parent compound, acetyl anhydride, which exists as a 2:1 mixture of [sp, ac] and [sp, sp] forms [6]. The presence of a single conformer in 1 is demonstrated also by matrix IR spectra where two C¼O vibrations are observed at 1883 (vs) and 1816 (s) cm À1 [13].…”

“…For acetyl anhydride, MeC(O)-O-C(O)Me, two GED studies report different conformational properties. Whereas the earlier investigation results in a single conformer with C 2 symmetry and [sc, sc] orientations of the two C¼O bonds [5], the more recent study results in a 2:1 mixture of [sp, ac] and [sp, sp] forms [6]. This latter result is confirmed by quantum chemical calculations.…”

Acetic anhydride at 100 K: the first crystal structure determination

“…Fig. 2 illustrates the various conformations of (I), with definitions of the O-C-O-C torsion angles according to Wu et al (2000). For (I), in the crystal, the torsion angles are ' 1 = ' 2 = 32.13 (7) , corresponding to an [sc,sc] conformation.…”

“…Its synthesis was mentioned in the literature for the first time more than 160 years ago (Gerhardt, 1852). The molecular structure of (I) has been investigated by gasphase electron diffraction, IR spectroscopy and computational methods (Mirone et al, 1970;Vledder et al, 1971a,b;Colthup, 1985;Wu et al, 2000). As far as we are able to ascertain, a crystallographic study of (I), which is liquid under ambient conditions (m.p.…”

Shorter and Easier Syntheses of Di‐tert‐butylketene and Relatedgem‐Di‐tert‐butyl Compounds

“…Di‐ tert ‐butylacetic anhydride ( 40 , Scheme )33,34 was accessible through the acylation of the acid 11 with the ketene 14 when these two reagents were either employed as such or were generated in situ from the potassium or (better) lithium salt ( 39 ) of 11 , which is sufficiently basic to eliminate HCl from the acyl chloride 13 in anhydrous toluene. The initial formation of the ketene 14 from 13 and 39 could be observed ( 1 H NMR in situ) in THF or diglyme solutions at 55 °C.…”