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Halogen−Halogen Interactions in Halomethanes

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Abstract

We report the study of the electronic structure of CHFI(2), CHBrI(2), CHBr(2)I, CHClI(2), and CHCl(2)I using VUV photoelectron spectroscopy (UPS). The lone pair orbital interactions are discussed on the basis of experimental results and comparisons with other halomethanes.

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Die Einwirkung lithiumorganischer Reagenzien auf ein Gemisch von Fluordibrommethan und einem Olefin bietet einen direkten Zugang zu Fluorcyclopropanen. — Die Addition des Fluorcarbens vollzieht sich stereospezifisch unter Erhalt der vom Olefin vorgegebenen Konfiguration. Sie verläuft außerdem stereoselektiv unter Bevorzugung des syn-isomeren Fluorcyclopropans. Londonsche Dispersionskräfte scheinen demnach für die syn-Stereoselektivität der Carben-Addition nicht verantwortlich zu sein. Fluoro Organic Syntheses, III. Monofluorocarbene Fluorocyclopropanes are obtained by treating a mixture of dibromofluoromethane and an olefin with organolithium compounds. — The addition of the monofluorocarbene occurs stereospecifically with retention of the olefin configuration and stereoselectively with preference of the syn-isomeric fluorocyclopropane. Therefore, London dispersion forces seem not to be the origin of the syn-stereoselectivity observed with carbene addition reactions.
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The valence shell photoelectron spectra of 2-bromothiophene and 3-bromothiophene have been studied, both experimentally and theoretically, in order to characterise the main bands due to single-hole states and to assess the importance of electron correlation in the formation of satellite states. Synchrotron radiation has been employed to measure photoelectron angular distributions and branching ratios in the photon energy range 13–115 eV, and the results indicate that the photoionisation dynamics of the , and ( states are affected by Cooper minima. The experimental data demonstrate that the 12a′ and 2a″ orbitals, corresponding to the Br 4p lone pairs, retain their atomic properties to a substantial degree, and that the same is true, although to a lesser extent, for the 11a′ orbital which is related to the S 3p subshell. The results are compared with similar measurements on chlorothiophene and bromobenzene. The vertical ionisation energies and spectral intensities of the entire valence shell photoelectron spectrum have been computed using the third-order algebraic–diagrammatic construction approximation scheme for the one-particle Green's function. These theoretical predictions have allowed assignments to be proposed for all the prominent structure observed in the experimental spectra. The outer valence Green's function method has also been used and the calculated ionisation energies show good agreement with the experimental values. Mulliken atomic populations have been computed for some of the outer valence orbitals, and the calculated Br 4p and S 3p content possessed by the 12a′, 2a″ and 11a′ molecular orbitals is in accordance with the strength of the Cooper minimum observed in the photoelectron asymmetry parameters associated with these orbitals.
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Photoelectron spectra for the dihalomethanes CH2BrCl, CH2ClI and CH2Br2 have been recorded with photons in the energy range 19–115 eV using Daresbury Laboratory Synchroton Radiation Source. Ionization energies have been measured for all valence falling within this energy including Br 3d and I 4d orbitals. Asymmetry parameters have been measured for all intense ionization processes and characteristic Cooper minima observed for halogen lone pair orbitals. For CH2ClI lone pair orbital β spectra show strong evidence of mixed halogen character although a similar situation is not observed for CH2BrCl. Partial photoionization cross sections are tabulated for the observed ionization processes. Detailed spectra are presented for the molecule CH2ClI since this appears to be the first photoelectron study of this molecule.
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