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.

Gaseous ethyl cyanate, CH3CH2OCN, has been generated from the gas/solid reaction of O-ethyl thiocarbamate with mercury oxide and characterized in the gas phase by infrared spectroscopy for the first time. Experimental data indicate the presence of two conformers in the gas phase, the gauche (synclinal) and the trans (antiperiplanar) form. The molecular geometries and energetics of the possible conformers are obtained from DFT calculations at the B3LYP level and from ab initio calculations at the MP2, MP3, MP4, QCISD, and CCSD(T) levels of theory. The assignment of the gas-phase infrared spectrum is assisted by normal coordinate calculations based on the scaled computed force field of the two conformers. The kinetic instability of CH3CH2OCN toward isomerization is studied at the B3LYP level, in a vacuum and in solutions. Solvent effects are modeled using the polarized continuum model (PCM). Calculations show that the isomerization is not a unimolecular process at ambient temperatures, and bimolecular processes are responsible for the instability. In polar solvents, the OCN- anion plays a key role in the isomerization, being an effective catalyst for the cyanate-isocyanate rearrangement.

The [3 + 2] and [3 + 3] cyclodimerisation processes of small nitrile oxides, XCNO (X = F, Cl, Br, CN, CH(3)) are investigated by ab initio coupled cluster theory at the CCSD, CCSD(T) and MR-AQCC levels for the first time. The favoured dimerisation process is a multi-step reaction to furoxans (1,2,5-oxadiazole-2-oxides) involving dinitrosoalkene-like intermediates with diradical character. The rate determining step for all but the F-species is the first, corresponding to the C-C bond formation. The kinetic energy barrier depends on the nature of the substituent X, generally increasing with decreasing electronegativity and increasing pi-donor ability of the substituent: F (DeltaG(298) = 0 kJ mol(-1)) < Cl (72) < Br (90) < CH(3) (104) < CN (114) (MR-AQCC(2,2)//UB3LYP/cc-pVTZ). Following initial C-C bond formation, three possible dinitrosoethylene diradical pathways are explored. Two of them are new, and one of them is a low-energy three-step path with implications for cycloreversion, tautomerism and detection of dinitrosoethylene intermediates. Alternative one-step, concerted [3 + 2] and [3 + 3] cyclodimerisation processes leading to 1,2,4-oxadiazole-4-oxides and 1,4,2,5-dioxadiazines have kinetic energy barriers around 100-240 kJ mol(-1) (CCSD//B3LYP), some 1.6 to 2.5 times higher than those leading to furoxans, supporting the experimental observations of furoxan formation as nitrile oxide loss channels during storage, trapping/re-vaporisation and reactions of nitrile oxides. Potential polymerisation initiation processes for NCCNO, involving the 1,2-dipolar NC substituent are also explored.

The equilibrium geometries, stability, and isomerisation of the ethynyl pseudohalides, HCC-X (where X ¼ -NCO, -OCN, -CNO, -ONC), have been investigated by ab initio MP2 and CCSD(T), as well as by B3LYP density functional methods using the 6-311G(2d,2p) basis set. Minimum energy structures and their interconnecting transition states have been calculated, and possible isomerisation pathways are suggested. Calculations have predicted that three isomers, HCC-NCO, HCC-OCN, and HCC-CNO, are kinetically stable toward unimolecular isomerisation or dissociation at room temperature with the lowest kinetic energy barrier of 274.5, 200.3, and 261.6 kJ mol À1 , respectively (CCSD(T)//B3LYP), and other isomers are unstable. As a particular interest, the condensed phase behaviour of HCC-NCO, HCC-OCN, and HCC-CNO, viz. potential cycloaddition reactions, have been also studied at the B3LYP level. Theoretical calculation have indicated that these molecules are stable below room temperature in the condensed phase, but they dimerise and polymerise at elevated temperatures.