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PubMed2001

Pre-reactive complexes in mixtures of water vapour with halogens: characterisation of H2O...ClF and H2O...F2 by a combination of rotational spectroscopy and ab initio calculations.

Complexes H2O...ClF and H2O...F2 were detected by means of their ground-state rotational spectra in mixtures of water vapour with chlorine monofluoride and difluorine, respectively. A fast-mixing nozzle was used in conjunction with a pulsed-jet, Fourier-transform microwave spectrometer to preclude the vigorous chemical reaction that these dihalogen species undergo with water. The ground-state spectra of seven isotopomers (H2 16O...35ClF, H2 16O...ClF, H2 18O...35ClF, D2 16O... 35ClF, D2 16O...37ClF, HDO...35ClF and HDO...37ClF) of the ClF complex and five isotopomers (H2O...F2, H2 18O...F2, D2O...F2, D2 18O...Fi and HDO...F2) of the F2 complex were analysed to yield rotational constants, quartic centrifugal distortion constants and nuclear hyperfine coupling constants. These spectroscopic constants were interpreted with the aid of simple models of the complexes to give effective geometries and intermolecular stretching force constants. Isotopic substitution showed that in each complex the H2O molecule acts as the electron donor and either CIF or F2 acts as the electron acceptor, with nuclei in the order H2O...ClF or H2O...F2. For H2O...ClF, the angle phi between the bisector of the HOH angle and the O...Cl internuclear line has the value 58.9(16)degrees, while the distance r(O...Cl)= 2.6081(23) A. The corresponding quantities for H2O...F2 are phi = 48.5(21)degrees and r(O...Fi) = 2.7480(27) A, where Fi indicates the inner F atom. The potential energy V(phi) as a function of the angle phi was obtained from ab initio calculations at the aug-cc-pVDZ/MP2 level of theory for each complex by carrying out geometry optimisations at fixed values of phi in the range +/-80degrees. The global minimum corresponded to a complex of Cs symmetry with a pyramidal configuration at O in each. The function V(phi) was of the double-minimum type in each case with equilibrium values phie = +/-55.8degrees and +/-40.5degrees for H2O...ClF and H2O...F2, respectively. The barrier at the planar C2v conformation was V0= 174cm(-1) for H2O...ClF and 7cm(-1) for H2O...F2. For the latter complex, the zero-point energy level lies above the top of the barrier.

Journal Article↗
PubMed2001

Activation of C--H... Halogen (Cl, Br, and I) hydrogen bonds at the organic/inorganic interface in fluorinated tetrathiafulvalenes salts.

The electrocrystallization of fluorinated bis(2,2'-difluoropropylenedithio)tetrathiafulvalene (1) in the presence of linear (ICl2-, IBr2-, I2Br-) or cluster ([Mo6Cl14]2-) anions affords 1:1 and 2:1 cation radical salts such as [1][ICl2] and [1]2[Mo6Cl14].(CH3CN)2. In both salts, the 1*+ radical ion adopts a boat conformation and envelops the anion through C-H...Hal(anion) (Hal(anion) = Cl, Br, I) hydrogen bonds. This demonstrates the activating role of the neighboring electron-withdrawing CF2 moieties in the stabilization of bi- or trimolecular neutral entities. With smaller linear anions, fluorine segregation controls the solid-state associations of the bimolecular [1]*+[X] entities, and gives rise to layered materials with a limited overlap interaction between the open-shell organic cations and magnetic spin chain behavior. With the larger [Mo6Cl14]2 ions, a strong overlap interaction between radical cations gives rise to diamagnetic [1]2(2+) dimers, which alternate with the cluster anions to form hybrid organic/inorganic ...[1]2(2+)[Mo6Cl14]2... chains. This behavior is also observed in [2]2(2+)[Mo6Cl14]2-.(CH2Cl2)2, in which compound 2 is the unsymmetrically substituted (ethylenedithio)(2,2'-difluoropropylenedithio)tetrathiafulvalene. On the other hand, the unsymmetrically substituted 2,2'-difluoropropylenedithiotetrathiafulvalene (3) affords a mixed-valence 4:1 salt with [Mo6Cl14]2, which is formulated as [3]4[Mo6Cl14].(CH3CN)2. This semiconducting salt is characterized by the coexistence of both the fluorine/fluorine segregation (with solvent inclusion) and the organic/inorganic segregation (with delocalized overlap interactions). Both Csp2-H...Cl and Csp3-H...Cl hydrogen bonds facilitate the stabilization of the organic/inorganic interface and the presence of conducting organic slabs.

Journal Article↗
PubMed2001

Selective radical reactions in multiphase systems: phase-transfer halogenations of alkanes.

The present paper shows that selective radical reactions can be initiated and carried out in multiphase systems. This concept is applied to the selective functionalization of unactivated aliphatic hydrocarbons, which may be linear, branched, and (poly)cyclic, strained as well as unstrained. The phase-transfer system avoids overfunctionalization of the products and simplifies the workup; the selectivities are excellent and the yields are good. This is the only method for direct preparative iodination of alkanes applicable to large scale as well. We demonstrate that the reaction systems are indeed phase-transfer catalyzed through a systematic study of variations of the reactants, solvents, catalysts, and by measuring as well as computing the H/D kinetic isotope effects for the rate-limiting C-H abstraction step by *CHal3 radicals which are held responsible for the observed radical reactions. In the case of *CBr3, this key intermediate could also be trapped under otherwise very similar reaction conditions. To stimulate further work, the tolerance of some functional groups was tested as well.

Journal Article↗