Charles Allan Brown

Vibrational Raman and infrared spectra of chromatographically separated C60 and C70 fullerene clusters

Abstract We report vibrational Raman and infrared spectra for chemically separated C 60 and C 70 fullerenes. Thin film samples were prepared by subliming the chromatographically separated species onto appropriate substrates. The C 60 Raman spectrum shows eight clear lines and two weaker ones. If C 60 in fact that has the proposed buckminsterfullerene structure (as is strongly indicated by recent experiments), the present Raman measurements together with the four observed IR frequencies give a complete set of Raman and infrared active fundamental frequencies for this molecule. A comparison of this set with the calculated spectrum for buckminsterfullerene shows satisfactory agreement.

Bond lengths in free molecules of buckminsterfullerene, c60, from gas-phase electron diffraction.

Electron diffraction patterns of the fullerene C60 in the gaseous state have been obtained by volatilizing it from a newly designed oven-nozzle at 730�C. The many peaks of the experimental radial distribution curve calculated from the scattered intensity are completely consistent with icosahedral symmetry for the free molecule. On the basis of this symmetry assumption, least-squares refinement of a model incorporating all possible interatomic distances led to the values rg(C1-C2) = 1.458(6) angstroms (�) for the thermal average bond length within the five-member ring (that is, for the bond fusing five- and six-member rings) and rg(C1-C6) = 1.401(10) � for that connecting five-member rings (the bond fusing six-member rings). The weighted average of the two bond lengths and the difference between them are the values 1.439(2) � and 0.057(6) �, respectively. The diameter of the icosahedral sphere is 7.113(10) �. The uncertainties in parentheses are estimated 2σ values.

Electron spin relaxation times of C−60 in solution

Abstract Potassium and calcium salts of C − 60 have been studied in solid and liquid solutions by ESR and optical spectroscopies. Over the temperature interval from 8 to 45 K the electron spin relaxation times T 1 and T M (the phase memory time) were measured by saturation recovery and electron spin echo spectroscopies. For both salts T 1 decreased from a few milliseconds at 8 K to less than 1 μs at 40 K. T M was 1 to 3 μm between 8 and 25 K and decreased rapidly with increasing temperature to become equal to T 1 near 40 or 50 K. Below 40 K the linewidth of the anion signal was about 5.5 G, which is much larger than the linewidth calculated from T M (0.02 G at 10 K). Above about 50 K the linewidth was relaxation time determined and increased rapidly with increasing temperature until reaching a plateau near the melting point of the solution. Analysis of the relaxation rates for C − 60 between 35 K and the melting (softening) point of the solutions gives an Arrhenius activation energy between 2.2 and 2.9 kJ/mol (180–240 cm −1 ) depending on the solvent. This measured range of activation energies for relaxation is close to and somewhat less than the lowest vibrational mode (≈ 270 cm −1 of C 60 ; in C − 60 this vibration may couple to the triply degenerate electronic ground state to produce a dynamic Jahn—Teller distortion.

Induced conjugate addition of simple 2-lithio-1,3-dithians to cyclic αβ-unsaturated ketones

In sharp contrast to prior observations that 2-lithio-1,3-dithians undergo only carbonyl (1,2–) additions to enones, the successful conjugate (1,4–) addition of the parent 2-lithio-1,3-dithian and its simple derivatives has been achieved by the addition of 1·0–2·0 equiv. of hexamethylphosphoramide to the reaction mixture.

Characterization of fullerenes and doped fullerenes

Abstract The results of a variety of experiments used to characterize fullerenes, metallofullerenes and alkali-metal intercalated C 60 are reported. It is shown that differential scanning calorimetry and NMR characterization of the orientational phase transition in C 60 provide sensitive means to assess the purity and crystallinity of fullerene samples. The results of a mass-spectrometric investigation of metallofullerene samples produced by co-vaporization of carbon and metals are described. An account is given of some electron paramagnetic resonance results obtained for LaC 82 and solid-state NMR results obtained on an alkali-intercalated fullerite, Rb 3 C 60 , which show for the first time the presence of magnetically inequivalent carbons in underivatized C 60 . Together these experiments yield a great deal of information about the phase purity, molecular dynamics and structure of a variety of fullerene materials.

Facile reaction of potassium hydride with trialkylboranes, a convenient general synthesis of simple and hindered potassium trialkylborohydrides under exceptionally mild conditions

The reaction of potassium hydride in tetrahydrofuran with simple and hindered trialkylboranes proceeds readily at 25°C (<0.5 h) with the formation of the corresponding trialkylborohydrides. Even some highly hindered trialkylboranes react smoothly under these conditions. This reaction provides a convenient synthesis in high yields of such hindered and highly hindered potassium trialkylborohydride under very mild reaction conditions.

Generation of 2-lithio-2-(methylthio)-1,3-benzodithioles, new carboxyl carbanion equivalents, and their application to the syntheses of unsymmetrical hexathioorthooxalates

2-Lithio-2-(methylthio)-1,3-benzodithioles are readily generated either by thiophilic addition of methyllithium to o-phenylenetrithiocarbonates or by deprotonation of 2-(methylthio)- 1,3-benzodithioles with n-butyllithium in THF; these anions are protonated (aqueous NH4Cl) or methylated (MeI) as expected, but undergo an unexpected, overall carbophilic addition reaction with cyclic trithiocarbonates to yield, after alkylation, unsymmetrical hexathioorthooxalates, compounds which have been difficult to prepare by traditional methods.

The isomerization of optically-active propargyl alcohols to terminal acetylenes.

Abstract The isomerization of optically-active secondary propargyl alcohols, RCHOHCC(CH2)nCH3, to terminal acetylenic alcohols, RCHOH(CH2)n+1 C=CH, by potassium 3-aminopropylamide (KAPA) proceeds without loss of configuration at the hydroxy center.

Facile synthesis of unsymmetrical benzotetrathiafulvalenes via cleavage of the corresponding hexathioorthooxalates

Abstract Unsymmetrical hexathioorthooxalates of types ( 1 ) and ( 2 ) undergo elimination of dialkyl disulfide on heating in an organic solvent; the reaction, which is catalyzed by acid, proceeds without fission of the central C:C bond and provides the first general, high yield synthesis of unsymmetrical benzotetrathiafulvalenes of types ( 3 ) and ( 4 ).

Electronic structure of single crystal C60

Abstract We report angle-resolved photoemission data from single crystals of C60 cleaved in UHV. Unlike the other forms of pure carbon, the valence band spectrum of C60 consists of many sharp features that can be essentially accounted for by the quantum chemical calculations describing individual molecules. This suggests that the electronic structure of solid C60 is mainly determined by the bonding interactions within the individual molecules. We also observe remarkable intensity modulations of the photoemission features as a function of photon energy, suggesting strong final state effects. Finally, we address the issue of the band width of the HOMO state of C60. We assert that the width of the photoemission peak of C60 does not reflect the intrinsic band width because it is broadened by the non 0-0 transitions via the Franck-Condon principle. Our view point provides a possible reconciliation between these photoemission data and those measured by other techniques.