The HKrCCHCO2 complex: an ab initio and matrix-isolation study.

We report an experimental and theoretical study on new noble-gas hydride complex HKrCCHCO2, which is the first known complex of a krypton hydride with carbon dioxide. This species was prepared by the annealing-induced H + Kr + CCHCO2 reaction in a krypton matrix, the CCHCO2 complexes being produced by UV photolysis of propiolic acid (HCCCOOH). The H-Kr stretching mode of the HKrCCHCO2 complex at 1316 cm-1 is blue-shifted by 74 cm-1 from the most intense H-Kr stretching band of HKrCCH monomer. The observed blue shift indicates the stabilization of the H-Kr bond upon complexation, which is characteristic of complexes of noble-gas hydrides. This spectral shift is slightly larger than that of the HKrCCHC2H2 complex (+60 cm-1) and significantly larger than that of the HXeCCHCO2 complex (+32 and +6 cm-1). On the basis of comparison with ab initio computations at the MP2 and CCSD(T) levels of theory, the experimentally observed absorption is assigned to the quasi-parallel configuration of the HKrCCHCO2 complex. The calculated complexation-induced spectral shift of the H-Kr stretching band (60.4 or 72.7 cm-1 from the harmonic calculations at the MP2 and CCSD(T) levels, respectively) agrees well with the experimental value.

Matrix isolation and ab initio study on HCN/CO2 system and its radiation-induced transformations: Spectroscopic evidence for HCN⋯CO2 and trans-HCNH⋯CO2 complexes.

Spectroscopic characteristics and X-ray induced transformations of the HCN⋯CO2 complex in solid Ar and Kr matrices were studied by FTIR spectroscopy and ab initio calculations at the CCSD(T) level. The complex was prepared by deposition of the HCN/CO2/Ng gas mixtures (Ng = Ar or Kr). The comparison of the experiment and calculations prove formation of a linear, H-bonded NCH⋯OCO complex with a substantial red shift of the C-H stretching band and a blue shift of the H-C-N bending band in respect to the monomer. This result is in contrast with the previous gas-phase observations, where only T-shape complex was found. Irradiation of deposited matrices leads to formation of CN radicals and HNC molecules and subsequent annealing results in appearance of H2CN and trans-HCNH in both matrices plus HKrCN in the case of Kr. In the presence of CO2, the strongest absorption of trans-HCNH radical demonstrates an additional blue-shifted (by 6.4 cm-1) feature, which was assigned to the N-coordinated complex of this radical with CO2 on the basis of comparison with calculations. To our knowledge, it is the first experimentally observed complex of this radical. No evidence was found for HKrCN⋯CO2 complex, which was explained tentatively by steric hindrance.

Diketone radical cations: ketonic and enolic forms as revealed by matrix EPR studies and DFT calculations.

Radical cations of 2,3-butanedione, 2,4-pentanedione, 3-methylpentane-2,4-dione, 2,5-hexanedione, and 2,3-pentanedione were investigated by electron paramagnetic resonance (EPR) spectroscopy in a solid Freon matrix and density functional theory (DFT) quantum chemical calculations. All the diketone radical cations in ketonic form show small proton hyperfine couplings (typically unresolved in the EPR spectra). In the cases of 2,4-pentanedione and 3-methylpentane-2,4-dione, enolic forms of the radical cations (pi-type species with main spin population at carbon atom) were characterized. Preferential stabilization of the enolic form of 3-methylpentane-2,4-dione radical cation was explained by trap-to-trap positive hole migration rather than monomolecular relaxation of the ionized ketonic form through H atom transfer.

Structure of radical cations of saturated heterocyclic compounds with two heteroatoms as studied by electron paramagnetic resonance, electron-nuclear double resonance, and density functional theory calculations.

The radical cations of piperazine, morpholine, thiomorpholine, and thioxane were investigated by electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy in a solid Freon matrix. Optimized geometry and magnetic parameters of the radical cations were calculated using a density functional theory (DFT)/Perdew-Burke-Ernzerhof (PBE) method. Both experimental and theoretical results suggest that all the studied species adopt chair (or distorted chair) conformations. No evidence for the boat conformers with intramolecular sigma-bonding between heteroatoms were obtained. In the cases of morpholine and thioxane, the oxygen atoms are characterized by relatively small spin populations, whereas a major part of spin density is located at N and S atoms, respectively. The thiomorpholine radical cation exhibits nearly equal spin population of N and S atoms. In most cases (except for thioxane), the calculated magnetic parameters agree with the experimental data reasonably well.