Gas-Phase Generation of the Unstable BrCNO Molecule and Its Stable Dibromofuroxan Dimer. He I Photoelectron, Photoionization Mass Spectroscopy, Mid-Infrared, and ab Initio Studies

Abstract

The haloformonitrile oxide BrCNO has been generated for the first time in the gas phase from the dibromoformaldoxime precursor by pyrolysis or by chemical reaction with HgO(s) or NHsfg). Trapping this gaseous species leads to the ring dimer dibromofuroxan. The BrCNO and (BrCNOh species are characterized in the gas phase by Hel photoelectron spectroscopy, Hel and HLa photoionization mass spectroscopy, and infrared spectroscopy. The geometry of BrCNO obtained from ab initio calculations at the HF, MP" ( = 2-4), QCISD(T), and CCSD(T) levels of theory suggests a quasi-linear structure with a pronounced effect of triple substitutions on the linear-bent question. The dibromofuroxan species, also investigated for the first time by gas-phase methods, is confirmed by ab initio calculations at the HF level as a planar fivemembered ring with a strongly polarized exocyclic nitrile oxide group. This dimer, under thermolysis, does not revert to the BrCNO monomer, but leads to the formation of the unique fulminate, cyanogen bis-N-oxide, ONCCNO. The ab initio calculations lend strong support to the vibrational spectra, but are limited by the application of Koopmans' theorem for the experimental ionization potentials.

Mechanistic aspects of the dehydration and dehydrohalogenation of halo‐hydroxyformaldoxime conformers. A quantum chemical model study

“…Notice that the planar geometries of conformers 9, 12, and 13, correspond to transition states as their Hessian posses one imaginary frequency. In general terms, halogen substitution does not appear to affect the length of the nominally double CϭN bond (deviations of only 0.001 Å were observed), a result comparable to that observed for Cl and Br substitution on formaldoxime 7 and ethylene. 20 However, chloro-or bromo-substitution affects slightly the C-O bond, being shortened by 0.008 Å.…”

“…Thus, ClCNO and BrCNO are predicted to adopt a linear geometry at the B3LYP, MP3, MP4SDQ, QCISD, and CCSD levels and a bent geometry at the MP2, MP4SDTQ, QCISD(T), and CCSD(T) using the 6-31G** basis set. 7 More recent calculations 9 of the molecular parameters of chloro-and bromo-fulminates using large-scale ab initio methods, such as coupled-cluster method, CCSD(T), and basis sets of double-through quintuple-zeta quality revealed that both molecules adopt a bent geometry. Our calculations predicted a trans bent structure for ClCNO and BrCNO in their singlet 1 AЈ ground states.…”

“…[1][2][3] The recent interest of haloformonitrile oxides stems from their application in the synthesis of biologically active compounds such as pesticides and insecticides 4 and dihydromuscinol 5 and the amino acid antibiotic acivicin 6 of use in medicinal chemistry. The haloformonitrile oxides have been recently identified 7 for the first time in the gas phase generated from the dihaloformaldoxime precursor by pyrolysis or by chemical reaction with HgO(s) or NH 3 (g). Trapping these very reactive gaseous species leads through [3ϩ2] cycloaddition reactions to the ring dimer dihalofuroxan.…”

Quantum-chemical study of the structure and stability of ethynyl pseudohalides: HCC–NCO and its isomers

“…The equilibrium structures of HCC-NCO isomers (1,2,3,4,5) and transition states (6,7,8,9,10,11,12) were calculated at the ab initio MP2(full)/6-311G(2d,2p) level and also using the density functional theory in the form of Becke's three-parameter exchange functional in combination with the Lee, Yang and Parr correlation functional, B3LYP/6-311G(2d,2p). Harmonic vibrational frequencies were then calculated at the minimum energy geometries to make sure they were real minima on the potential energy surface (zero imaginary frequency).…”

Dimerisation of nitrile oxides: a quantum-chemical study

“…This concurs with experimental results; the gas-phase thermolyses of dichloro-and dibromofuroxan do not produce nitrile oxide monomers. 26,52,55 This also suggests that gas-phase thermal generation of the unknown FCNO species from the furoxan is unfeasible. On the other hand, our experiments (UPS and/or FTIR) have shown that gas-phase thermal cycloreversion of dimethyl-and dicyanofuroxan, at temperatures for both molecules in the relatively narrow range of 550-600 1C (10 À3 to 10 À1 Torr), rapidly and quantitatively leads to monomeric CH 3 CNO, 27 and NCCNO.…”