Ilya U. Goldschleger

Formation of the HHF complex in the reaction of thermal fluorine atoms with hydrogen molecules in solid Ar

Abstract The reaction of F atoms with H 2 in an Ar matrix was studied with EPR spectroscopy. F 2 photolysis produces two well-known doublets of the H atom trapped in different crystal sites. Heating gives rise to an EPR spectrum assigned to the H 2 F complex ( A H = 50.8, A F = 1.85, A H ⩽ 0.15 mT), the formation of which is attributed to the reaction of diffusing thermal F t atoms with H 2 . The shape and width of the spectrum of the complex changes drastically with temperature due to the transitions between two stable configurations: A ( a F = 3.2 mT, a H ⩽ 3.2 mT) and B ( a F = 0.5 mT, a H = 0.4 mT) with an Arrhenius-like dependence of rate ( E a = 80 cal/mol, ω 0 ∼ 3 × 10 10 s −1 ).

High-resolution electron spin resonance spectroscopy of XeF• in solid argon. The hyperfine structure constants as a probe of relativistic effects in the chemical bonding properties of a heavy noble gas atom

Xenon fluoride radicals were generated by solid-state chemical reactions of mobile fluorine atoms with xenon atoms trapped in Ar matrix. Highly resolved electron spin resonance spectra of XeF* were obtained in the temperature range of 5-25 K and the anisotropic hyperfine parameters were determined for magnetic nuclei 19F, 129Xe, and 131Xe using naturally occurring and isotopically enriched xenon. Signs of parallel and perpendicular hyperfine components were established from analysis of temperature changes in the spectra and from numerical solutions of the spin Hamiltonian for two nonequivalent magnetic nuclei. Thus, the complete set of components of hyperfine- and g-factor tensors of XeF* were obtained: 19F (Aiso=435, Adip=1249 MHz) and 129Xe (Aiso=-1340, Adip=-485 MHz); g(parallel)=1.9822 and g(perpendicular)=2.0570. Comparison of the measured hyperfine parameters with those predicted by density-functional theory (DFT) calculations indicates, that relativistic DFT gives true electron spin distribution in the 2Sigma+ ground-state, whereas nonrelativistic theory underestimates dramatically the electron-nuclear contact Fermi interaction (Aiso) on the Xe atom. Analysis of the obtained magnetic-dipole interaction constants (Adip) shows that fluorine 2p and xenon 5p atomic orbitals make a major contribution to the spin density distribution in XeF*. Both relativistic and nonrelativistic calculations give close magnetic-dipole interaction constants, which are in agreement with the measured values. The other relativistic feature is considerable anisotropy of g-tensor, which results from spin-orbit interaction. The orbital contribution appears due to mixing of the ionic 2Pi states with the 2Sigma+ ground state, and the spin-orbit interaction plays a significant role in the chemical bonding of XeF*.

HFCN open-shell isomers in solid argon. I. Spectroscopy of the ground and excited states of HFC=N radical

Fluoromethylene amidogen radicals, HFC=N•, were generated in solid argon by solid-state chemical reactions of mobile F atoms with hydrogen cyanide. Highly resolved infrared and electroparamagnetic resonance spectra of HFC=N• were obtained in the temperature range 15–30 K. All six vibrational frequencies and the complete set of isotropic hyperfine coupling constants on magnetic nuclei 1H, 19F, and 14N were determined experimentally. Calculated spectroscopic characteristics are in excellent agreement with experiments, showing that HFC=N• radical has a planar structure in the ground state. Two electronic absorption transitions were observed in the near-ultraviolet and visible spectral region. The first excited 2A″ state of HFC=N• radical is calculated to have a planar structure very similar to the ground state, and lies 20 726 cm−1 above the ground state [at the CCSD(T)/cc-pVTZ level of theory], in good agreement with the experimental value, 20 430 cm−1. The observed Franck–Condon envelope in the laser-induc...

HFCN open-shell isomers in solid argon. II. Excited-state tunneling isomerization HFC=N•→FC•=NH

Photochemical reaction of the π-type fluoromethylene amidogen radical HFC=N• was studied by infrared, electroparamagnetic resonance, and laser-induced fluorescence spectroscopies in solid argon matrices. Laser excitation of the 0–0 band of the first excited state of HFC=N• at 488.1 nm leads to complete conversion to another structural isomer, the σ-type fluoroiminomethyl radical FC•=NH. The quantum yield of the photoisomerization reaction is ΦH≈0.5 at temperatures T⩽30 K. Experiments using pulsed laser excitation show that the rate constant for hydrogen atom transfer is kH=5×104 s−1 at T⩽30 K, whereas the rate constant of the analogous process in DFC=N• is only kD=30 s−1 under the same experimental conditions. The large kinetic isotope effect kH/kD≈1400 and absence of temperature dependence of kH show that the mechanism of photoisomerization is intramolecular tunneling transfer of the hydrogen atom in the first excited A 2A″ state. Simple one-dimensional consideration of the tunneling transfer of an H or ...

Reactions of photogenerated fluorine atoms with molecules trapped in solid argon. 4. Spectroscopic characteristics of β-C2H2F· radicals generated in reactions of mobile F atoms with C2H2 molecules trapped in solid argon

Reactions of mobile fluorine atoms with C2H2, C2D2, and C2HD molecules in solid argon were studied by ESR and IR spectroscopic techniques. Highly resolved ESR spectra of the stabilized radicals CHF=·CH, CDF=·CD, CHF=·CD, and CDF=·CH were obtained for the first time. Isotropic hyperfine constants on fluorine and proton nuclei were measured. It was found that the radicals formed in the reaction F + C2H2 correspond to the cis-β-C2H2F· isomer. A comparison of the measured HFC constants with the values calculated by modern quantum-chemical methods allows the identification of the isomeric form of the radical, whereas vibrational analysis of the IR absorption spectra gives unreliable results. The calculation of the energy of the radical isomers predicts that cis-β-C2H2F· is more stable than trans-β-C2H2F· by ∼1.0 kJ mol–1.