Gas-Phase Infrared and ab Initio Study of the Unstable CF₃CNO Molecule and Its Stable Furoxan Ring Dimer

Abstract

The unstable trifluoroacetonitrile N-oxide molecule, CF3CNO, has been generated in high yield in the gas phase from CF3BrC=NOH and studied for the first time by gas-phase mid-infrared spectroscopy. Cold trapping of this molecule followed by slow warming forms the stable ring dimer, bis(trifluoromethyl)furoxan, also investigated by gas-phase infrared spectroscopy. The spectroscopy provides an investigation into the vibrational character of the two molecules, the assignments supported by calculations of the harmonic vibrational frequencies using in the case of CF3CNO both ab initio (CCSD(T)) and density functional theory (B3LYP) and B3LYP for the ring dimer. The ground-state structures of both molecules were investigated at the B3LYP level of theory, with CF3CNO further investigated using coupled-cluster. The CCSD(T) method suggests a slightly bent (C(s)) structure for CF3CNO, while the B3LYP method (with basis sets ranging from 6-311G(d) to cc-pVTZ) suggests a close-to-linear or linear CCNO chain. The CCN bending potential in CF3CNO was explored at the CCSD(T)(fc)/cc-pVTZ level, with the results suggesting that CF3CNO exhibits strong quasi-symmetric top behavior with a barrier to linearity of 174 cm(-1). Since both isomerization and dimerization are feasible loss processes for this unstable molecule, the relative stability of CF3CNO with respect to the known cyanate (CF3OCN), isocyanate (CF3NCO), and fulminate (CF3ONC) isomers and the mechanism of the dimerization process to the ring furoxan and other isomers were studied with density functional theory.

A one-pot synthesis of 3-trifluoromethyl-2-isoxazolines from trifluoromethyl aldoxime

“…Upon energy minimization, a structure with a geometry close to linearity was found for trifluoroacetonitrile oxide (Fig. 4), which is consistent with earlier findings [44]. …”

An exploration of the mechanisms for the formation of 3,4-bis(4-aminofurazan-3-yl)-furoxan by density functional theory

“…The initial step corresponds to the formation of intermediate 9 via a singlet diradical TS (TS8), in which the forming C1AC4 bond is 1.949 Å. This length is close to those obtained in the dimerization of acetonitrile oxide (1.957 Å) [20] and trifluoroacetonitrile oxide (1.963 Å) [21]. Two features of the structure of TS8 are worthy of mention.…”

“…3, the dimerization occurs with a very large exothermicity (À26.0 kcal/mol) and the C1AC4 bond forming process (TS8) is the rate-determining step, which is only 14.6 kcal/mol higher in energy than the reactants, suggesting, as observed, that dimerization can occur at room and higher temperature. Aliphatic nitrile oxides can easily dimerize to furoxans via this diradical intermediate mechanism, and the overall pathway is similar to the diradical dimerization routes previously determined for acetonitrile oxide [20] and trifluoroacetonitrile oxide [21].…”

“…Dimerization reactions of nitrile oxides, including acetonitrile oxide [20], trifluoroacetonitrile oxide [21] and benzonitrile oxide [23], have been theoretically studied recently. These reactions are found to be stepwise processes involving diradical intermediates.…”

“…DFT method has also been applied to study the vibrational spectra of furoxan and dichlorofuroxan [19], dimerization of nitrile oxides for the formation of furoxans 2210-271X/$ -see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.comptc.2012.07.042 via stepwise process, where intermediates and TSs are found to have diradical character [20][21][22][23].…”