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Lawrence T Scott

Publications and source records attributed to Lawrence T Scott.

At least 19 recordsLinked to original sources
PubMed2006

Oligoindenopyrenes: a new class of polycyclic aromatics.

A new class of polycyclic aromatic hydrocarbons--oligoindenopyrenes--has been synthesized featuring a Pd-catalyzed Suzuki-Heck coupling cascade. The oligoindenopyrenes are robust, highly colored substructures of C(70) and have properties that might prove useful in new organic materials or devices. After excitation, the tetraindenopyrene derivative 3d undergoes efficient deactivation (99%) by internal conversion to the ground state. The small fluorescence quantum yield (0.004) is in accordance with the short (0.6 ns) fluorescence decay time.

Fluorescence↗
PubMed2006

X-ray and density functional theory structural study of 1,3,5,7,9-penta-tert-butylcorannulene, C40H50.

The first X-ray structural characterization of an alkyl-substituted corannulene, namely, 1,3,5,7,9-penta-tert-butylcorannulene (C40H50), has been accomplished. The addition of bulky tert-butyl groups to the corannulene core flattens the bowl and affects the solid-state packing. The presence of two enantiomers, in addition to positional disorder of the C40H50 bowls in the solid-state structure, has prevented the acquisition of accurate geometric parameters of this open geodesic polyarene. Therefore, DFT calculations have been used to describe its molecular geometry and to access bond lengths, bond angles, and a bowl depth.

Journal Article↗
PubMed2006

C20 carbon clusters: fullerene-boat-sheet generation, mass selection, photoelectron characterization.

Electron-impact ionization in a time-of-flight mass spectrometer of C(20)H(0-3)Br(14-12) probes-secured from C(20)H(20) dodecahedrane by a "brute-force" bromination protocol-provided bromine-free C(20)H(0-2(3)) anions in amounts that allowed the clean mass-separation of the hydrogen-free C(20) (-) ions and the photoelectron (PE) spectroscopic characterization as C(20) fullerene (electron affinity (EA)=2.25+/-0.03 eV, vibrational progressions of 730+/-70). The extremely strained C(20) fullerene ions surfaced as kinetically rather stable entities (lifetime of at least the total flight time of 0.4 ms); they only very sluggishly expel a C(2) unit. The HOMO and LUMO are suggested to be almost degenerate (DeltaE=0.27 eV). The assignment as a fullerene was corroborated by the PE characterization of the C(20) bowl (EA=2.17+/-0.03 eV, vibrational progression of 2060+/-50 cm(-1)) analogously generated from C(20)H(10) corannulene (C(20)H(1-3)Br(9-8) samples) and comparably stable. Highly resolved low-temperature PE spectra of the known C(20) ring (EA=2.49+/-0.03 eV, vibrational progressions 2022+/-45 and 455+/-30 cm(-1)), obtained from graphite, display an admixture of, most probably, a bicyclic isomer (EA=3.40+/-0.03 eV, vibrational progression 455+/-30 cm(-1)). The C(20) (+(-)) and C(20)H(2) (+(-)) cluster ions generated from polybrominated perylene (C(20)H(0-2)Br(12-10)) have (most probably) retained the planar perylene-type skeleton (sheet, EA=2.47+/-0.03 eV, vibrational progressions of 2089+/-30 and 492+/-30 cm(-1) and EA=2.18+/-0.03 eV, vibrational progressions of 2105+/-30 and 468+/-30 cm(-1)).

Alkynes↗
PubMed2006

Reductive Bergman-type cyclizations of cross-conjugated enediynes to fulvene and fulvalene anions: the role of the substituent.

Various cross-conjugated enediynes undergo "Bergman-type" cycloaromatizations upon reduction with potassium metal, generating anions of fulvenes and fulvalene derivatives. This new anionic cyclization is considerably more facile than the classic Bergman cyclization with linear enediynes, creating highly reactive diradicals at -78 degrees C. Not all cross-conjugated enediynes yield cyclized dianions upon reduction; some give uncyclized, Y-shaped, cross-conjugated dianions, while others apparently yield radical-anions that either dimerize or persist as monomers. One system yields both a cyclized and an uncyclized dianion. The substituents are thus shown to be a critical factor in determining the outcome of the reduction. Cyclization occurs within a specific "window of opportunity" that is governed by the substituents.

Journal Article↗
PubMed2006

Reduction of bowl-shaped hydrocarbons: dianions and tetraanions of annelated corannulenes.

[structure: see text] The reduction of several annelated corannulene derivatives was undertaken using lithium and potassium metals. It was found that annelation affects the annulenic character of corannulene by changing its charge distribution; the dianions of derivatives that are annelated with six-membered rings have less annulenic character and are less paratropic than corannulene dianion. This effect is even more pronounced in corannulenes that are peri-annelated with five-membered rings. The alkali metal used in the reduction process has a great influence on the outcome, especially on the degree of reduction. Most derivatives get reduced to tetraanions only with potassium, and not with lithium, the exception being systems that can stabilize the tetraanion with lithium by special means, such as aggregation or dimerization. One such system is cyclopenta[bc]corannulene (acecorannulylene), which gives a coordinative dimer that consists of two cyclopentacorannulene tetraanions, bound together in a convex-convex fashion by lithium cations. The points of contact in this dimer are two rehybridized carbons from each cyclopentacorannulene unit, which are bridged together by two lithium cations.

Journal Article↗
PubMed2005

Interconversions of aryl radicals by 1,4-shifts of hydrogen atoms. A synthesis of benzo[a]corannulene.

Aryl radicals generated ortho to aryl substituents by flash vacuum pyrolysis (FVP) of the corresponding aryl chlorides are shown to be capable of transferring hydrogen atoms between the ortho and ortho' positions (1,4-shifts of hydrogen atoms). In the examples described here, the rearranged aryl radicals are trapped by subsequent radical cyclization reactions. For example, FVP of 2-(o-chlorophenyl)benzo[c]phenanthrene gives 1-phenylbenzo[ghi]fluoranthene as the major product by homolysis of the C-Cl bond, 1,4-shift of a hydrogen atom out of the sterically congested cove region to the radical center, cyclization of the rearranged radical, and rearomatization of the molecule by loss of the other cove region hydrogen. In a control experiment run under the same conditions, FVP of 2-phenylbenzo[c]phenanthrene, which lacks a radical precursor, gave primarily recovered starting material. When the FVP was repeated using 2-(2,6-dichlorophenyl)benzo[c]phenanthrene as the starting material, benzo[a]corannulene was obtained as the major product, presumably by the same cascade of events to produce 1-(o-chlorophenyl)benzo[ghi]fluoranthene, which then suffers a second radical cyclization spontaneously under the high-temperature conditions to give the geodesic polyarene.

Journal Article↗
PubMed2005

Concave polyarenes with sulfide-linked flaps and tentacles: new electron-rich hosts for fullerenes.

Pentakis(1,4-benzodithiino)corannulene (6) in CS2 formed the strongest 1:1 complexes with C60 and C70 of any corannulene derivative yet reported. The 1,3,5,7,9-pentakis(propylthio)corannulene (4b) formed weaker 1:1 complexes with C60 and C70 in CS2, whereas the decakis(propylthio)corannulene (5b) and unsubstituted corannulene (1) showed no evidence for complexation with either C60 or C70 in CS2.

Journal Article↗
PubMed2005

X-ray quality geometries of geodesic polyarenes from theoretical calculations: what levels of theory are reliable?

[structure: see text] Computational methods for calculating molecular geometries have not been well calibrated heretofore against X-ray data for bowl-shaped polycyclic aromatic hydrocarbons (PAHs). The analysis presented here capitalizes on a rare opportunity provided by corannulene to account explicitly for molecular distortions from crystal packing forces. Within the error limits of an extensive X-ray data set, B3LYP/6-31G* calculations were found to correctly reproduce all of the experimental bond distances and bond angles. The reliability and shortcomings of geometry calculations at other levels of theory are enumerated.

Journal Article↗
PubMed2005

Coordination chemistry of buckybowls: from corannulene to a hemifullerene.

This work highlights the progress made in coordination chemistry of transition-metal centers to open geodesic polyaromatic hydrocarbons that map onto the surface of C60, the family of compounds known as buckybowls or 'fullerene fragments'. In particular, an overview of our recent gas-phase coordination studies of several bowl-shaped polyarenes toward the dinuclear metal complex, [Rh2(O2CCF3)4], is given. Selected buckybowls include corannulene (C20H10) and two of its derivatives, namely dibenzo[a,g]corannulene (C28H14) and 1,3,5,7,9-penta-tert-butylcorannulene (C40H50), as well as a hemifullerene (C30H12). This study has resulted in the first X-ray structural characterization of buckybowl coordination complexes and has revealed eta2-rim coordination preferences of open geodesic polyarenes in rhodium(ii) binding reactions.

Journal Article↗
PubMed2005

Ball-and-socket stacking of supercharged geodesic polyarenes: bonding by interstitial lithium ions.

Unprecedented supramolecular stacks of highly reduced geodesic pi-systems were prepared by the reduction of the derivatized fullerenes Me(5)C(60)H and Ph(5)C(60)H and corannulene with lithium metal (R(5)C(60)(5)(-)/Cor(4)(-)/9Li(+)). The host--guest assemblies form because of the enhanced electrostatic interactions between the lithium cations and the anionic moieties, in addition to the structural compatibility between the curved hydrocarbons. The high stability of these new supramolecular assemblies (heterodimers) enables the introduction of another organization motif to the system. This is achieved by using tethered corannulenes as host molecules, which leads to the formation of tethered bis-heterodimers ((Me(5)C(60)(5)(-)/Cor(4)(-))(2)(CH(2))(8)/18Li(+)).

Journal Article↗
PubMed2005

Interstellar chemistry: a strategy for detecting polycyclic aromatic hydrocarbons in space.

Polycyclic aromatic hydrocarbons (PAHs) have long been postulated as constituents of the interstellar gas and circumstellar disks. Observational infrared emission spectra have been plausibly interpreted in support of this hypothesis, but the small (or zero) dipole moments of planar, unsubstituted PAHs preclude their definitive radio astronomical identification. Polar PAHs, such as corannulene, thus represent important targets for radio astronomy because they offer the possibilities of confirming the existence of PAHs in space and revealing new insight into the chemistry of the interstellar medium. Toward this objective, the high-resolution rotational spectrum of corannulene has been obtained by Fourier transform microwave spectroscopy, and the dipole moment (2.07 D) of this exceptionally polar PAH has been measured by exploiting the Stark effect.

Journal Article↗
PubMed2004

Dicyclobuta[de,ij]naphthalene and dicyclopenta[cd,gh]pentalene: a theoretical study.

The structures, energetics, and aromatic character of dicyclobuta[de,ij]naphthalene, 1, dicyclopenta[cd,gh]pentalene, 2, dihydrodicyclobuta[de,ij]naphthalene, 3, and dihydrocyclopenta[cd,gh]pentalene, 4, have been examined at the B3LYP/6-311++G//B3LYP/6-31G level of theory. All molecules are bowl-shaped, and the pentalene isomers, 2 and 4, are most stable. A comparison with other C(12)H(6) and C(12)H(8) isomers indicates that 2 is approximately 25 kcal/mol less stable than 1,5,9-tridehydro[12]annulene and 4 is approximately 100 kcal/mol higher in energy than acenaphthylene, both of which are synthetically accessible. The transition state structure for bowl-to-bowl inversion of 1 is planar (D(2)(h)()) and lies 30.9 kcal/mol higher in energy than the ground state; the transition state for inversion of 2 is C(2)(h)() and lies 46.6 kcal/mol higher in energy. Symmetry considerations, bond length alternations, and NICS values (a magnetic criterion) all indicate that the ground states of 1, 3, and 4 are very aromatic; however, HOMA values (a measure of bond delocalization) indicate that 3S and 4S are aromatic but that 1S is less so. NICS values for the ground state of 2 strongly indicate aromaticity; however, bond localization, symmetry, and HOMA values argue otherwise.

Journal Article↗