59346-70-2Relevant academic research and scientific papers
Halogenation of cubane under phase-transfer conditions: Single and double C - H-bond substitution with conservation of the cage structure
Fokin,Lauenstein,Gunchenko,Schreiner
, p. 1842 - 1847 (2007/10/03)
The first highly selective C - H chlorination, bromination, and iodination of cubane (1) utilizing polyhalomethanes as halogen sources under phase-transfer (PT) conditions is described. Isomeric dihalocubanes with all possible combinations of chlorine, bromine, and iodine in ortho, meta, and para positions were also prepared by this method; m-dihalo products form preferentially. Ab initio and density functional theory (DFT) computations were used to rationalize the pronounced differences in the reactions of 1 with halogen (Hal·) vs carbon-centered trihalomethyl (Hal3C·) radicals (Hal = Cl, Br), For Hal3C radicals the C - H abstraction pathway is less unfavorable (ΔG?298 = 21.6 kcal/mol for C3C· and 19.4 kcal/mol for Br3C· at B3LYP/6-311+G**//B3LYP/6-31G**) than the fragmentation of the cubane skeleton via SH2-attack on one of the carbon atoms of 1 (ΔG?298 = 33.8 and 35.1 kcal/mol, respectively). In stark contrast, the reaction of 1 with halogen atoms preferentially follows the fragmentation pathway (ΔG?298 = 2.1 and 7.5 kcal/mol) and C - H abstraction is more unfavorable (ΔG?298 = 4.6 and 12.0 kcal/mol). Our computational results nicely agree with the behavior of 1 under PT halogenation conditions (where Hal3C· is involved in the activation step) and under free-radical photohalogenation with Hal2 (Della E. W., et al. J. Am. Chem. Soc. 1992, 114, 10730). The incorporation of a second halogen atom preferentially in the meta position of halocubanes demonstrates the control of the regioselectivity by molecular orbital symmetry.
Molecular Reorientation and Self-Diffusion in Solid Cubane by Deuterium and Proton NMR
Detken, A.,Zimmermann, H.,Haeberlen, U.,Poupko, R.,Luz, Z.
, p. 9598 - 9604 (2007/10/03)
Deuterium and proton NMR line shape and relaxation measurements are reported for the sym-cubane-d2 isotopomer (D3d symmetry) in the temperature range from 200 K to the melting point of the solid.The results confirm the polymorphic phase sequence: solid II solid I liquid, reported earlier by White et al.The deuterium spectrum below 215 K exhibits a Pake doublet corresponding to an axially symmetric quadrupole coupling tensor with a coupling comstant of Qc = 178 kHz.Above this temperature the spectrum develops features characteristic of the onset of dynamic processes leading to a single line at about room temperature.Longitudinal relaxation time measurements show a T1 minimum at 344 K.On transition to solid I there is a discontinuous narrowing of the line and a concomitant increase in T1.These results are quantitatively interpreted in terms of reorientational jumps of the cubane molecules between their various equivalent orientations.The jump rate k in solid II follows an Arrhenius behavior over the range of almost seven decades with an activation energy E = 62 kJ mol-1.Transition to solid I results in a discontinuous increase of k.Proton line width measurements in solid II show two motional narrowing steps.The first, at around 240 K, is due to the cubic jumps while the second, at around 375 K, corresponds to molecular self-diffusion between the lattice sites.The activation energy for this process is Ed = 83 kJ mol-1.On transition to solid I there is also a discontinuous increase in the rate of this process.The proton T1 values are predominantly affected by the reorientation process and are consistent with the deuteron data.At temperatures above 340 K, where the deuterium NMR spectrum is expected to be a single Lorentzian with a width of less than 400 Hz, it actually exhibits, in both phases II and I, a Pake doublet corresponding to an axially symmetric quadrupole coupling tensor with a very small coupling constant Q'c = 0.48 kHz.This indicates that the reaorientation process is not perfectly cubic.Possible reasons for this surprising effect are discussed.
Crystal and molecular structures of 1,4-dibromo- and 1,4-diiodocubane
Kawai, N. T.,Gilson, D. F. R.,Britten, J. F.,Butler, I. S.,Farrell, P. G.
, p. 910 - 914 (2007/10/02)
The crystal structure of 1,4-dibromocubane has been determined by X-ray diffraction.The crystal is monoclinic, P21/n, a = 6.878(1), b = 6.960(7), c = 8.793(2) Angstroem, β = 113.08(2) deg, Z = 2, R = 0.055.The C-Br bond length, at 1.926 Angstroem, is shorter than usual and the carbon framework is probably distorted by longer bonds along the edges.The direct dipole coupling constants between protons, obtained from the partially oriented NMR spectrum, show that a distortion of the proton positions from the ideal cube occurs in the bromo compound but not in diiodocubane, presumably because of the higher strain energy in the former. Key words: halocubanes, crystal structures, NMR spectroscopy, molecular modelling calculations.
