Conformational Analysis. 24. Structure and Composition of Gaseous Oxalyl Fluoride, C₂F₂O₂:  Electron-Diffraction Investigation Augmented by Data from Microwave Spectroscopy and Molecular Orbital Calculations

Abstract

The molecular structure and composition of gaseous oxalyl fluoride (OXF) has been investigated by electron diffraction (GED) at nozzle-tip temperatures of -10, 149, and 219 degrees C. The GED data were augmented by molecular orbital calculations, and the analysis was aided by use of rotational constants from microwave (MW) spectroscopy. As in the other oxalyl halides, there are two stable species, of which the more stable is periplanar anti (i.e., trans). However, unlike these other halides in which the second form is gauche, the second form of oxalyl fluoride was known from MW work to be periplanar syn (i.e., cis). Our results are consistent with a mixture of trans and cis forms, and yield values for the structural parameters, the composition of the system at the three temperatures cited, and the thermodynamic quantities deltaG(o), deltaH(o), and deltaS(o) for the reaction trans --> cis. Some trans/cis distances (r(g)/Angstrom) and angles (<(alpha)/deg) at -10 degrees C are r(C=O) = 1.178(2)/1.176(2), r(C-F) = 1.323(2)/1.328(2); r(C-C) = 1.533(3)/1.535(3), <(C-C=O) = 126.4(2)/124.2(2), <(C-C-F) = 109.8(2)/112.2(2), and <(O-C-F) = 123.8(2)/123.6(2). The mixture compositions (percent trans) at -10 degrees C/149 degrees C/219 degrees C are 75(3)/58(7)/52(8), from which deltaH(o) and deltaSO) are found to be 1.14 kcal/mol and 2.12 cal/(mol x deg). The system properties are discussed.

Vibronic spectra, ab initio calculations, and structures of conformationally non-rigid molecules of oxalyl halides in the ground and lowest excited electronic states. Part III: Theoretical investigation of oxalyl fluoride

“…Nine excited states of the torsional fundamental mode were assigned and the torsional fundamental was determined to be 26(5) cm À1 . The results of the recent electron diffraction investigation by Friesen et al [15] confirmed the geometry of (COF) 2 to be a mixture of trans and cis conformers. The electron diffraction [15], microwave (absolute intensities measurements) [14], and vibrational spectroscopy [12,13,16] investigations of (COF) 2 all determined the conformer energy (enthalpy) difference in the gas phase to be 230-400 cm À1 ; the fraction of (COF) 2 molecules in the cis conformer at room temperature is hence significant.…”

“…The electron diffraction [15], microwave (absolute intensities measurements) [14], and vibrational spectroscopy [12,13,16] investigations of (COF) 2 all determined the conformer energy (enthalpy) difference in the gas phase to be 230-400 cm À1 ; the fraction of (COF) 2 molecules in the cis conformer at room temperature is hence significant. This conclusion was supported by theoretical calculations [12][13][14][15][17][18][19][20][21].…”

Vibronic spectra, ab initio calculations, and structures of conformationally non-rigid molecules of oxalyl halides in the ground and lowest excited electronic states. Part I: Reanalysis of the 3680Å and 4100Å absorption systems of oxalyl chloride

“…However, no data on the structure of the second conformers of oxalyl halides in excited electronic states are available. Earlier, only transitions of the trans-conformers were observed in the vibronic absorption spectra of oxalyl halides [14,16,[18][19][20], although according to vibrational spectroscopy and electron diffraction studies, the values of conformer energy (enthalpy) differences of oxalyl halides in the ground electronic state in the gas phase were relatively small, and the content of the second conformers was rather high [2,[4][5][6][9][10][11]22].…”

“…It was proven experimentally that all molecules investigated in the vapor phase have two conformers in the ground electronic state: the planar trans-conformer (dOCCO = 180°, C 2h symmetry), which is lower in energy [1][2][3][4][5][6], and the second conformer-planar cis (dOCCO = 0°, C 2v symmetry) for glyoxal (COH) 2 [7] and oxalyl fluoride (COF) 2 [8,9] or non-planar gauche-conformer (dOCCO > 0°, C 2 symmetry) for oxalyl chloride (COCl) 2 [10] and oxalyl bromide (COBr) 2 [11].…”

Vibronic spectra, ab initio calculations, and structures of conformationally non-rigid molecules of oxalyl halides in the ground and lowest excited electronic states. Part V: Oxalyl chloridefluoride (COCl–COF)

“…For oxalyl chloridefluoride, there are no experimental data on geometries of conformers in the ground electronic state. The comparison of calculated values could be made only with geometric parameters of oxalyl fluoride (COF) 2 and oxalyl chloride (COCl) 2 obtained experimentally [29,30] or theoretically [2,3]. In general, if the CCSD(T)/ cc-pVTZ or cc-pVQZ values are compared, the respective geometric parameters of CFO, CClO fragments of COCl-COF, (COF) 2 , and (COCl) 2 molecules are very similar.…”