Anthony Khong | Yale University (original) (raw)
Papers by Anthony Khong
Journal of Organic Chemistry, 2006
The [2 + 2] photoadditions of 3-methyl-2-cyclohexenone to C70 and 3He@C70 have been studied by a ... more The [2 + 2] photoadditions of 3-methyl-2-cyclohexenone to C70 and 3He@C70 have been studied by a combination of HPLC chromatography and FAB-MS, as well as IR and 1H and 3He NMR spectroscopies. The total yield of the mixture of monoadducts was 55% (67% on the basis of the recovered C70). The use of 3He NMR was especially powerful in determining the regioselectivity of the photoaddition reaction of enone to C70. Results of the 3He NMR experiments conducted on the product mixture implicate the two [6,6] bonds closest to the poles of the fullerene (C1-C2 and C5-C6) in the photoaddition process. This reaction mode is analogous to that of most thermal addition reactions to C70. Separation and characterization of the product mixture shows that eight distinct monoadducts are formed in the photoaddition, namely, the four diastereomeric adducts to the C1-C2 and C5-C6 bonds of the C70 cage, each consisting of cis- and trans-fused isomers in a ratio of 2:3. The major mode of photoaddition, accounting for 65% of the product mixture, involves addition to the C1-C2 bond of the ovoid fullerene. Mechanistic implications of these findings are discussed.
Journal of The Acoustical Society of America, 2002
As was reported previously, stably levitated bubbles were observed in LiCl and LiBr solutions und... more As was reported previously, stably levitated bubbles were observed in LiCl and LiBr solutions under SBSL conditions. Stable bubbles were recorded for LiCl concentrations ranging from 0.47 to 1.4 M. Beyond 1.4 M, no SL was detected. In contrast, stable SBSL can be detected over a larger range of LiBr concentrations, from 0.56 to 2.5 M. At 3.0 M, unstable short-lived transient bubbles were noticed. A striking feature common to both salt solutions is the pronounced decrease in SL light intensity, measured with a PMT with peak detection sensitivity at 400 nm, as the salt concentration increases. Light intensities were close to one order of magnitude less than in pure water under similar conditions. The focus of the current study is geared toward resolving the observed reduction in light intensity with respect to chemical processes occurring in the bubble. UV spectroscopy will be key to procuring vital information relating to these processes based on the absorption patterns and the spectral ranges of peak absorptions. Similar salt solutions in D2O, aimed at revealing differences in the chemical processes in the case of heavy water, will also be investigated. [Work supported by a generous grant from University of Washington.]
Physical Review E, 2003
Calculations based on a consideration of gas diffusion of gas dynamics in a sonoluminescing bubbl... more Calculations based on a consideration of gas diffusion of gas dynamics in a sonoluminescing bubble filled with a noble gas and water vapor are carried out. Xenon-, argon-, and helium-filled bubbles are studied. In the absence of shock waves, bubble temperatures are found to be decreased, a decrease attributable to the large heat capacity of water vapor. Peak bubble temperature reductions are seen in bubbles containing Xe or Ar but not in those containing He. Further extrapolations provide evidence for the occurrence of shock waves in bubbles with Xe and water vapor. No shock waves are observed in bubbles with Ar or He.
The photochemistry and photophysics of two sets of stilbene-derivatized fullerene isomers, in whi... more The photochemistry and photophysics of two sets of stilbene-derivatized fullerene isomers, in which stilbene is covalently linked to C60, are described. Synthesis and characterization of cis- and trans-stilbene substituted methanofullerenes 1 and 2, and cis- and trans-stilbene substituted fulleropyrrolidines 7 and 8 are described. While UV irradiation of the stilbene-substituted ketal precursors to 1 and 2 lacking the fullerene moiety afforded a photostationary state with 90:10 cis:trans ratio, similar to that of other model stilbene systems, direct and fluorenone-sensitized irradiation of 1 and 2 led to complete conversion to the trans isomer 2, as determined by HPLC analysis. The same results were obtained using cis-trans isomers 7 and 8, namely, the photostationary state on excitation below 350 nm is essentially 100% trans. No isomerisation in either system was obtained on excitation above 400 nm, where all the light is absorbed by the fullerene moiety. By analogy to previous studies of quenching of stilbene excited states, these results suggest that both singlet and triplet excited states of the trans-stilbene moiety in 2 and 8 are being quenched by intramolecular energy transfer to the attached C60, while the much shorter lived cis-stilbene excited states are not similarly quenched. Fluorescence studies on compound 8 support this hypothesis, since the characteristic fluorescence emission of trans-stilbene and trans-stilbene derivatives is not observed in the case of adduct 8. Because of the well established fact that trans-stilbene S1 states are longer lived than the S1 states of the corresponding cis isomers, rapid intramolecular singlet-singlet energy transfer to the appended C60 moiety, ket approximately 10(12) s(-1), is able to compete effectively with radiative and radiationless deactivation of the trans-stilbene S1 states in 2 and 8, but not in the corresponding cis isomers 1 and 7.
Chemical Physics Letters, 1998
C70 was heated with 22Ne at high pressure. Analysis by mass spectrometry unexpectedly showed the ... more C70 was heated with 22Ne at high pressure. Analysis by mass spectrometry unexpectedly showed the formation of Ne2@ C70 in addition to Ne@ C70. The ratio of empty C70 to mono-occupied to di-occupied was 1000: 1: 0.02. A mechanism where a small fraction of the ...
International Journal of Mass Spectrometry, 2001
Several new diatomic molecule-containing endohedral fullerenes were prepared by heating C 60 or C... more Several new diatomic molecule-containing endohedral fullerenes were prepared by heating C 60 or C 70 under high pressures of the corresponding gases. The species prepared are N 2 @C 60 , N 2 @C 70 , 13 CO@C 60 , and 3 He 22 Ne@C 70 . Their existence was demonstrated through high sensitivity, wide dynamic range mass spectrometry. Out of two thousand C 60 molecules about one is observed to incorporate N 2 . The nitrogen molecule containing endohedral molecules are stable and the mass spectrometric signal is not lost even after several hours heating at 500 K. The corresponding endohedral ions undergo the Rice shrink-wrap mechanism; a mass-analyzed ion kinetic energy spectrum demonstrates the loss of a C 2 unit from the cage. The observation of 13 CO@C 60 by mass spectrometry opens up the possibility for future NMR studies of this molecule. The observation of 3 He 22 Ne@C 70 is in accordance with the "promoter" mechanism of Thiel and co-worker [J. Am. Chem. Soc. 118 (1996) 7164; Helv. Chim. Acta 80 (1997) 495], whereby the singly and doubly occupied fullerenes are in equilibrium but the empty and filled fullerenes are not. (Int J Mass Spectrom 210/211 (2001) 241-247)
Journal of Organic Chemistry, 2000
Chemical Physics Letters, 1998
Tetrahedron, 1996
Noble gas atoms can be introduced into the interior of flfllerene molecules. A procedure, using a... more Noble gas atoms can be introduced into the interior of flfllerene molecules. A procedure, using a mass spectrometer system, is described for measuring the rates and amounts of noble gas released on heating these compounds. We find that the half life for Ne~C66 reacting to Ne + C60 is at least many weeks at 630°C. Additionally, the rate of thermal decomposition of fullerenes can be very substantially faster if traces of trapped solvent are not removed.
Tetrahedron Letters, 1997
... Cited By in Scopus (33). Permissions & Reprin... more ... Cited By in Scopus (33). Permissions & Reprints. 3 He NMR spectra of highly reduced C 60. WE Billups * , Alexander Gonzalez, Christoph Gesenberg, Weimei Luo, Terry Marriott and Lawrence B. Alemany. Martin Saunders * , Hugo A. Jiménez-Vázquez and Anthony Khong. ...
The Journal of Physical Chemistry, 1996
Ar@C 60 , synthesized by heating primarily C 60 (with trace amounts of C 70 ) under a high pressu... more Ar@C 60 , synthesized by heating primarily C 60 (with trace amounts of C 70 ) under a high pressure of argon gas, was purified by high-performance liquid chromatography (HPLC) and examined by Fourier transform mass spectrometry (FTMS). The sample, which consisted of 0.1% Ar@C 60 , was dissolved in toluene and separated with HPLC using a COSMOSIL 5PYE column as the stationary phase and toluene as the eluent (UV detection). Ar@C 60 was enriched by a factor of about 400 relative to the starting material. Ar@C 60 was also examined by X-ray photoelectron spectroscopy (XPS) but was found to be sensitive to radiation damage and subsequent depletion. Low-energy collisional dissociation of Ar@C 60 + in the FTMS yields primarily C 58 + , generated by loss of the argon atom and C 2 . The ionization potential of Ar@C 60 was bracketed by charge-exchange reactions and was found to be between 7.53 and 7.8 eV, which is indistinguishable from that of C 60 (7.65 eV).
Journal of Physical Chemistry A, 2000
... Thiel, W. Private communication. (23) Smith, R.; Beardmore, K.; Belbruno, J. J. Chem. Phys. 1... more ... Thiel, W. Private communication. (23) Smith, R.; Beardmore, K.; Belbruno, J. J. Chem. Phys. 1999, 111, 9227−9232. ... (24) Saunders, M.; Khong, A.; Shimshi, R.; Jiménez-Vázquez, HA; Cross, RJ Chem. Phys. Lett. 1996, 248, 127. ...
Journal of The American Chemical Society, 2000
Cheminform, 2010
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
Journal of The American Chemical Society, 2001
Isomers of C 60 H 36 and He@C 60 H 36 have been synthesized by the Birch or dihydroanthracene red... more Isomers of C 60 H 36 and He@C 60 H 36 have been synthesized by the Birch or dihydroanthracene reduction of C 60 and isolated by preparative high-pressure liquid chromatography. 3 He, 13 C, and 1 H NMR spectroscopic properties were then determined. A comparison of experimental chemical shifts against those computed using density functional theory (B3LYP) with polarized triple-and double-basis sets for He and C,H, respectively, allowed provisional assignment of structure for several isomers to be made. Theoretical calculations have also been carried out to identify low-energy structures. The transfer hydrogenation method using dihydroanthracene gives a major C 60 H 36 isomer and a minor C 60 H 36 isomer with C 3 symmetry as determined by the 13 C NMR spectrum of C 60 H 36 and the 3 He NMR spectrum of the corresponding sample of 3 He@C 60 H 36 . In view of the HPLC retention times and the 3 He chemical shifts observed for the Birch and dihydroanthracene reduction products, the two isomers generated by the latter procedure can be only minor isomers of the Birch reduction. A significant energy barrier apparently exists in the dihydroanthracene reduction of C 60 for the conversion of the C 3 and C 1 symmetry isomers of C 60 H 36 to the T symmetry isomer previously predicted by many calculations to be among the most stable C 60 H 36 isomers. Many of the 1 H NMR signals exhibited by C 60 H 36 (and C 60 H 18 , previously reported) are unusually deshielded compared to "ordinary" organic compounds, presumably because the unusual structures of C 60 H 36 and C 60 H 18 result in chemical shift tensors with one or more unusual principal values. Calculations clearly show a relationship between exceptionally deshielded protons beta to a benzene ring in C 60 H 18 and C 60 H 36 and relatively long CsC bonds associated with these protons. The additional information obtained from 1D and 2D 1 H NMR spectra obtained at ultrahigh field strengths (up to 900 MHz) will serve as a critical test of chemical shifts to be obtained from future calculations on different C 60 H 36 isomers.
Chemistry-a European Journal, 1997
Bis- to hexakisadducts of C60 (1–6) and mono- to tetrakisadducts of C70 (7–13) containing 3He ato... more Bis- to hexakisadducts of C60 (1–6) and mono- to tetrakisadducts of C70 (7–13) containing 3He atoms (endo-hedral helium complexes) were prepared and studied by 3He NMR spectroscopy to determine the influence of degree of functionalization and addition pattern on the chemical shift of the 3He atom. In the series of C60 derivatives, which included the previously measured 6–6 monoadduct 3He@C61H2, the 3He resonance was shifted considerably upfield relative to that of 3He@C60 (δ = −6.36) up to the bisadduct 1 (δ = −11.45). The resonances of the higher adducts 3–6, however, were shifted only slightly further up-field and all appeared in a rather narrow spectral range between δ = −11.84 and −12.26. The absence of further substantial upfield shifts was rationalized in terms of the compensation of deshielding due to the functionalization-induced decrease in the diamagnetic π-electron ring currents that extend around the fullerene sphere by the shielding that results from the weaker pentagonal-ring paramagnetic currents and the increased number of localized benzenoid substructures in 3–6. In contrast, all 3He resonances of the C70 adducts are shifted downfield relative to the signal of 3He@C70 (δ = −28.81). A monotonic relationship exists between the chemical shift and the degree of functionalization, whereby the 3He chemical shifts steadily decrease from monoadduct 7 (δ = −27.53) to the isomeric tetrakisadducts 12 (δ = −21.09) and 13 (δ = −20.68). This deshielding was explained by the reduction of the strong diamagnetic ring currents extending along the polar corannulene perimeters as a result of the functionalization at α- and β-type polar bonds in 7–13. In both series, the addition pattern was also found to have a distinct influence on the 3He chemical shift.
Journal of The Chemical Society-perkin Transactions 2, 1999
Both i 3 HeC 60 F 18 and i 3 HeC 60 F 36 have been prepared by fluorinating i 3 HeC 60. The 3 He ... more Both i 3 HeC 60 F 18 and i 3 HeC 60 F 36 have been prepared by fluorinating i 3 HeC 60. The 3 He NMR spectrum of i 3 HeC 60 F 18 shows a single line at–16.66 ppm, very close to the value of–16.45 ppm, recorded previously for the isostructural i 3 HeC 60 H 18. Density ...
Journal of The American Chemical Society, 1998
The reduction of 3 He@C 60 and 3 He@C 70 by lithium metal to give solutions of the corresponding ... more The reduction of 3 He@C 60 and 3 He@C 70 by lithium metal to give solutions of the corresponding hexaanions in THF-d 8 has been achieved under gentle conditions, at subambient temperatures, without sonication, by capitalizing on the ability of corannulene (1) to serve as an efficient electron carrier between the lithium metal and the solid fullerenes, which are virtually insoluble in the reaction medium. The 3 He inside the C 60 hexaanion is found to be more strongly shielded (by nearly 20 ppm!) than any previously reported 3 He in a fullerene [δ( 3 He@C 60 6-) ) -48.7 ppm, relative to dissolved 3 He gas in the solution], whereas the 3 He inside the C 70 hexaanion is actually deshielded [δ( 3 He@C 70 6-) ) +8.3 ppm], resonating at nearly 15 ppm lower field than any previously reported 3 He in a fullerene. These results stand in complete accord with earlier predictions that the magnetic properties of C 60 and C 70 would be altered dramatically, and in opposite directions, by reduction of the fullerenes to their hexaanions. The phenomenal ability of C 60 6to shield an endohedral 3 He from a powerful external magnetic field provides the most compelling evidence to date for the ability of electrons to move freely about the surface of a spheroidal π-system. S0002-7863(98)00583-6 CCC: $15.00
Science, 1996
Heating fullerenes at 650°C under 3000 atmospheres of the noble gases helium, neon, argon, krypto... more Heating fullerenes at 650°C under 3000 atmospheres of the noble gases helium, neon, argon, krypton, and xenon introduces these atoms into the fullerene cages in about one in 1000 molecules. A "window" mechanism in which one or more of the carbon-carbon bonds of the cage is broken has been proposed to explain the process. The amount of gas inside the fullerenes can be measured by heating to 1000°C to expel the gases, which can then be measured by mass spectroscopy. Information obtained from the nuclear magnetic resonance spectra of helium-3-labeled fullerenes indicates that the magnetic field inside the cage is altered by aromatic ring current effects. Each higher fullerene isomer and each chemical derivative of a fullerene that has been studied so far has given a distinct helium nuclear magnetic resonance peak.
Photochemical & Photobiological Sciences, 2003
The photochemistry and photophysics of two sets of stilbene-derivatized fullerene isomers, in whi... more The photochemistry and photophysics of two sets of stilbene-derivatized fullerene isomers, in which stilbene is covalently linked to C60, are described. Synthesis and characterization of cis- and trans-stilbene substituted methanofullerenes 1 and 2, and cis- and trans-stilbene substituted fulleropyrrolidines 7 and 8 are described. While UV irradiation of the stilbene-substituted ketal precursors to 1 and 2 lacking the fullerene moiety afforded a photostationary state with 90:10 cis:trans ratio, similar to that of other model stilbene systems, direct and fluorenone-sensitized irradiation of 1 and 2 led to complete conversion to the trans isomer 2, as determined by HPLC analysis. The same results were obtained using cis-trans isomers 7 and 8, namely, the photostationary state on excitation below 350 nm is essentially 100% trans. No isomerisation in either system was obtained on excitation above 400 nm, where all the light is absorbed by the fullerene moiety. By analogy to previous studies of quenching of stilbene excited states, these results suggest that both singlet and triplet excited states of the trans-stilbene moiety in 2 and 8 are being quenched by intramolecular energy transfer to the attached C60, while the much shorter lived cis-stilbene excited states are not similarly quenched. Fluorescence studies on compound 8 support this hypothesis, since the characteristic fluorescence emission of trans-stilbene and trans-stilbene derivatives is not observed in the case of adduct 8. Because of the well established fact that trans-stilbene S1 states are longer lived than the S1 states of the corresponding cis isomers, rapid intramolecular singlet-singlet energy transfer to the appended C60 moiety, ket approximately 10(12) s(-1), is able to compete effectively with radiative and radiationless deactivation of the trans-stilbene S1 states in 2 and 8, but not in the corresponding cis isomers 1 and 7.
Journal of Organic Chemistry, 2006
The [2 + 2] photoadditions of 3-methyl-2-cyclohexenone to C70 and 3He@C70 have been studied by a ... more The [2 + 2] photoadditions of 3-methyl-2-cyclohexenone to C70 and 3He@C70 have been studied by a combination of HPLC chromatography and FAB-MS, as well as IR and 1H and 3He NMR spectroscopies. The total yield of the mixture of monoadducts was 55% (67% on the basis of the recovered C70). The use of 3He NMR was especially powerful in determining the regioselectivity of the photoaddition reaction of enone to C70. Results of the 3He NMR experiments conducted on the product mixture implicate the two [6,6] bonds closest to the poles of the fullerene (C1-C2 and C5-C6) in the photoaddition process. This reaction mode is analogous to that of most thermal addition reactions to C70. Separation and characterization of the product mixture shows that eight distinct monoadducts are formed in the photoaddition, namely, the four diastereomeric adducts to the C1-C2 and C5-C6 bonds of the C70 cage, each consisting of cis- and trans-fused isomers in a ratio of 2:3. The major mode of photoaddition, accounting for 65% of the product mixture, involves addition to the C1-C2 bond of the ovoid fullerene. Mechanistic implications of these findings are discussed.
Journal of The Acoustical Society of America, 2002
As was reported previously, stably levitated bubbles were observed in LiCl and LiBr solutions und... more As was reported previously, stably levitated bubbles were observed in LiCl and LiBr solutions under SBSL conditions. Stable bubbles were recorded for LiCl concentrations ranging from 0.47 to 1.4 M. Beyond 1.4 M, no SL was detected. In contrast, stable SBSL can be detected over a larger range of LiBr concentrations, from 0.56 to 2.5 M. At 3.0 M, unstable short-lived transient bubbles were noticed. A striking feature common to both salt solutions is the pronounced decrease in SL light intensity, measured with a PMT with peak detection sensitivity at 400 nm, as the salt concentration increases. Light intensities were close to one order of magnitude less than in pure water under similar conditions. The focus of the current study is geared toward resolving the observed reduction in light intensity with respect to chemical processes occurring in the bubble. UV spectroscopy will be key to procuring vital information relating to these processes based on the absorption patterns and the spectral ranges of peak absorptions. Similar salt solutions in D2O, aimed at revealing differences in the chemical processes in the case of heavy water, will also be investigated. [Work supported by a generous grant from University of Washington.]
Physical Review E, 2003
Calculations based on a consideration of gas diffusion of gas dynamics in a sonoluminescing bubbl... more Calculations based on a consideration of gas diffusion of gas dynamics in a sonoluminescing bubble filled with a noble gas and water vapor are carried out. Xenon-, argon-, and helium-filled bubbles are studied. In the absence of shock waves, bubble temperatures are found to be decreased, a decrease attributable to the large heat capacity of water vapor. Peak bubble temperature reductions are seen in bubbles containing Xe or Ar but not in those containing He. Further extrapolations provide evidence for the occurrence of shock waves in bubbles with Xe and water vapor. No shock waves are observed in bubbles with Ar or He.
The photochemistry and photophysics of two sets of stilbene-derivatized fullerene isomers, in whi... more The photochemistry and photophysics of two sets of stilbene-derivatized fullerene isomers, in which stilbene is covalently linked to C60, are described. Synthesis and characterization of cis- and trans-stilbene substituted methanofullerenes 1 and 2, and cis- and trans-stilbene substituted fulleropyrrolidines 7 and 8 are described. While UV irradiation of the stilbene-substituted ketal precursors to 1 and 2 lacking the fullerene moiety afforded a photostationary state with 90:10 cis:trans ratio, similar to that of other model stilbene systems, direct and fluorenone-sensitized irradiation of 1 and 2 led to complete conversion to the trans isomer 2, as determined by HPLC analysis. The same results were obtained using cis-trans isomers 7 and 8, namely, the photostationary state on excitation below 350 nm is essentially 100% trans. No isomerisation in either system was obtained on excitation above 400 nm, where all the light is absorbed by the fullerene moiety. By analogy to previous studies of quenching of stilbene excited states, these results suggest that both singlet and triplet excited states of the trans-stilbene moiety in 2 and 8 are being quenched by intramolecular energy transfer to the attached C60, while the much shorter lived cis-stilbene excited states are not similarly quenched. Fluorescence studies on compound 8 support this hypothesis, since the characteristic fluorescence emission of trans-stilbene and trans-stilbene derivatives is not observed in the case of adduct 8. Because of the well established fact that trans-stilbene S1 states are longer lived than the S1 states of the corresponding cis isomers, rapid intramolecular singlet-singlet energy transfer to the appended C60 moiety, ket approximately 10(12) s(-1), is able to compete effectively with radiative and radiationless deactivation of the trans-stilbene S1 states in 2 and 8, but not in the corresponding cis isomers 1 and 7.
Chemical Physics Letters, 1998
C70 was heated with 22Ne at high pressure. Analysis by mass spectrometry unexpectedly showed the ... more C70 was heated with 22Ne at high pressure. Analysis by mass spectrometry unexpectedly showed the formation of Ne2@ C70 in addition to Ne@ C70. The ratio of empty C70 to mono-occupied to di-occupied was 1000: 1: 0.02. A mechanism where a small fraction of the ...
International Journal of Mass Spectrometry, 2001
Several new diatomic molecule-containing endohedral fullerenes were prepared by heating C 60 or C... more Several new diatomic molecule-containing endohedral fullerenes were prepared by heating C 60 or C 70 under high pressures of the corresponding gases. The species prepared are N 2 @C 60 , N 2 @C 70 , 13 CO@C 60 , and 3 He 22 Ne@C 70 . Their existence was demonstrated through high sensitivity, wide dynamic range mass spectrometry. Out of two thousand C 60 molecules about one is observed to incorporate N 2 . The nitrogen molecule containing endohedral molecules are stable and the mass spectrometric signal is not lost even after several hours heating at 500 K. The corresponding endohedral ions undergo the Rice shrink-wrap mechanism; a mass-analyzed ion kinetic energy spectrum demonstrates the loss of a C 2 unit from the cage. The observation of 13 CO@C 60 by mass spectrometry opens up the possibility for future NMR studies of this molecule. The observation of 3 He 22 Ne@C 70 is in accordance with the "promoter" mechanism of Thiel and co-worker [J. Am. Chem. Soc. 118 (1996) 7164; Helv. Chim. Acta 80 (1997) 495], whereby the singly and doubly occupied fullerenes are in equilibrium but the empty and filled fullerenes are not. (Int J Mass Spectrom 210/211 (2001) 241-247)
Journal of Organic Chemistry, 2000
Chemical Physics Letters, 1998
Tetrahedron, 1996
Noble gas atoms can be introduced into the interior of flfllerene molecules. A procedure, using a... more Noble gas atoms can be introduced into the interior of flfllerene molecules. A procedure, using a mass spectrometer system, is described for measuring the rates and amounts of noble gas released on heating these compounds. We find that the half life for Ne~C66 reacting to Ne + C60 is at least many weeks at 630°C. Additionally, the rate of thermal decomposition of fullerenes can be very substantially faster if traces of trapped solvent are not removed.
Tetrahedron Letters, 1997
... Cited By in Scopus (33). Permissions & Reprin... more ... Cited By in Scopus (33). Permissions & Reprints. 3 He NMR spectra of highly reduced C 60. WE Billups * , Alexander Gonzalez, Christoph Gesenberg, Weimei Luo, Terry Marriott and Lawrence B. Alemany. Martin Saunders * , Hugo A. Jiménez-Vázquez and Anthony Khong. ...
The Journal of Physical Chemistry, 1996
Ar@C 60 , synthesized by heating primarily C 60 (with trace amounts of C 70 ) under a high pressu... more Ar@C 60 , synthesized by heating primarily C 60 (with trace amounts of C 70 ) under a high pressure of argon gas, was purified by high-performance liquid chromatography (HPLC) and examined by Fourier transform mass spectrometry (FTMS). The sample, which consisted of 0.1% Ar@C 60 , was dissolved in toluene and separated with HPLC using a COSMOSIL 5PYE column as the stationary phase and toluene as the eluent (UV detection). Ar@C 60 was enriched by a factor of about 400 relative to the starting material. Ar@C 60 was also examined by X-ray photoelectron spectroscopy (XPS) but was found to be sensitive to radiation damage and subsequent depletion. Low-energy collisional dissociation of Ar@C 60 + in the FTMS yields primarily C 58 + , generated by loss of the argon atom and C 2 . The ionization potential of Ar@C 60 was bracketed by charge-exchange reactions and was found to be between 7.53 and 7.8 eV, which is indistinguishable from that of C 60 (7.65 eV).
Journal of Physical Chemistry A, 2000
... Thiel, W. Private communication. (23) Smith, R.; Beardmore, K.; Belbruno, J. J. Chem. Phys. 1... more ... Thiel, W. Private communication. (23) Smith, R.; Beardmore, K.; Belbruno, J. J. Chem. Phys. 1999, 111, 9227−9232. ... (24) Saunders, M.; Khong, A.; Shimshi, R.; Jiménez-Vázquez, HA; Cross, RJ Chem. Phys. Lett. 1996, 248, 127. ...
Journal of The American Chemical Society, 2000
Cheminform, 2010
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
Journal of The American Chemical Society, 2001
Isomers of C 60 H 36 and He@C 60 H 36 have been synthesized by the Birch or dihydroanthracene red... more Isomers of C 60 H 36 and He@C 60 H 36 have been synthesized by the Birch or dihydroanthracene reduction of C 60 and isolated by preparative high-pressure liquid chromatography. 3 He, 13 C, and 1 H NMR spectroscopic properties were then determined. A comparison of experimental chemical shifts against those computed using density functional theory (B3LYP) with polarized triple-and double-basis sets for He and C,H, respectively, allowed provisional assignment of structure for several isomers to be made. Theoretical calculations have also been carried out to identify low-energy structures. The transfer hydrogenation method using dihydroanthracene gives a major C 60 H 36 isomer and a minor C 60 H 36 isomer with C 3 symmetry as determined by the 13 C NMR spectrum of C 60 H 36 and the 3 He NMR spectrum of the corresponding sample of 3 He@C 60 H 36 . In view of the HPLC retention times and the 3 He chemical shifts observed for the Birch and dihydroanthracene reduction products, the two isomers generated by the latter procedure can be only minor isomers of the Birch reduction. A significant energy barrier apparently exists in the dihydroanthracene reduction of C 60 for the conversion of the C 3 and C 1 symmetry isomers of C 60 H 36 to the T symmetry isomer previously predicted by many calculations to be among the most stable C 60 H 36 isomers. Many of the 1 H NMR signals exhibited by C 60 H 36 (and C 60 H 18 , previously reported) are unusually deshielded compared to "ordinary" organic compounds, presumably because the unusual structures of C 60 H 36 and C 60 H 18 result in chemical shift tensors with one or more unusual principal values. Calculations clearly show a relationship between exceptionally deshielded protons beta to a benzene ring in C 60 H 18 and C 60 H 36 and relatively long CsC bonds associated with these protons. The additional information obtained from 1D and 2D 1 H NMR spectra obtained at ultrahigh field strengths (up to 900 MHz) will serve as a critical test of chemical shifts to be obtained from future calculations on different C 60 H 36 isomers.
Chemistry-a European Journal, 1997
Bis- to hexakisadducts of C60 (1–6) and mono- to tetrakisadducts of C70 (7–13) containing 3He ato... more Bis- to hexakisadducts of C60 (1–6) and mono- to tetrakisadducts of C70 (7–13) containing 3He atoms (endo-hedral helium complexes) were prepared and studied by 3He NMR spectroscopy to determine the influence of degree of functionalization and addition pattern on the chemical shift of the 3He atom. In the series of C60 derivatives, which included the previously measured 6–6 monoadduct 3He@C61H2, the 3He resonance was shifted considerably upfield relative to that of 3He@C60 (δ = −6.36) up to the bisadduct 1 (δ = −11.45). The resonances of the higher adducts 3–6, however, were shifted only slightly further up-field and all appeared in a rather narrow spectral range between δ = −11.84 and −12.26. The absence of further substantial upfield shifts was rationalized in terms of the compensation of deshielding due to the functionalization-induced decrease in the diamagnetic π-electron ring currents that extend around the fullerene sphere by the shielding that results from the weaker pentagonal-ring paramagnetic currents and the increased number of localized benzenoid substructures in 3–6. In contrast, all 3He resonances of the C70 adducts are shifted downfield relative to the signal of 3He@C70 (δ = −28.81). A monotonic relationship exists between the chemical shift and the degree of functionalization, whereby the 3He chemical shifts steadily decrease from monoadduct 7 (δ = −27.53) to the isomeric tetrakisadducts 12 (δ = −21.09) and 13 (δ = −20.68). This deshielding was explained by the reduction of the strong diamagnetic ring currents extending along the polar corannulene perimeters as a result of the functionalization at α- and β-type polar bonds in 7–13. In both series, the addition pattern was also found to have a distinct influence on the 3He chemical shift.
Journal of The Chemical Society-perkin Transactions 2, 1999
Both i 3 HeC 60 F 18 and i 3 HeC 60 F 36 have been prepared by fluorinating i 3 HeC 60. The 3 He ... more Both i 3 HeC 60 F 18 and i 3 HeC 60 F 36 have been prepared by fluorinating i 3 HeC 60. The 3 He NMR spectrum of i 3 HeC 60 F 18 shows a single line at–16.66 ppm, very close to the value of–16.45 ppm, recorded previously for the isostructural i 3 HeC 60 H 18. Density ...
Journal of The American Chemical Society, 1998
The reduction of 3 He@C 60 and 3 He@C 70 by lithium metal to give solutions of the corresponding ... more The reduction of 3 He@C 60 and 3 He@C 70 by lithium metal to give solutions of the corresponding hexaanions in THF-d 8 has been achieved under gentle conditions, at subambient temperatures, without sonication, by capitalizing on the ability of corannulene (1) to serve as an efficient electron carrier between the lithium metal and the solid fullerenes, which are virtually insoluble in the reaction medium. The 3 He inside the C 60 hexaanion is found to be more strongly shielded (by nearly 20 ppm!) than any previously reported 3 He in a fullerene [δ( 3 He@C 60 6-) ) -48.7 ppm, relative to dissolved 3 He gas in the solution], whereas the 3 He inside the C 70 hexaanion is actually deshielded [δ( 3 He@C 70 6-) ) +8.3 ppm], resonating at nearly 15 ppm lower field than any previously reported 3 He in a fullerene. These results stand in complete accord with earlier predictions that the magnetic properties of C 60 and C 70 would be altered dramatically, and in opposite directions, by reduction of the fullerenes to their hexaanions. The phenomenal ability of C 60 6to shield an endohedral 3 He from a powerful external magnetic field provides the most compelling evidence to date for the ability of electrons to move freely about the surface of a spheroidal π-system. S0002-7863(98)00583-6 CCC: $15.00
Science, 1996
Heating fullerenes at 650°C under 3000 atmospheres of the noble gases helium, neon, argon, krypto... more Heating fullerenes at 650°C under 3000 atmospheres of the noble gases helium, neon, argon, krypton, and xenon introduces these atoms into the fullerene cages in about one in 1000 molecules. A "window" mechanism in which one or more of the carbon-carbon bonds of the cage is broken has been proposed to explain the process. The amount of gas inside the fullerenes can be measured by heating to 1000°C to expel the gases, which can then be measured by mass spectroscopy. Information obtained from the nuclear magnetic resonance spectra of helium-3-labeled fullerenes indicates that the magnetic field inside the cage is altered by aromatic ring current effects. Each higher fullerene isomer and each chemical derivative of a fullerene that has been studied so far has given a distinct helium nuclear magnetic resonance peak.
Photochemical & Photobiological Sciences, 2003
The photochemistry and photophysics of two sets of stilbene-derivatized fullerene isomers, in whi... more The photochemistry and photophysics of two sets of stilbene-derivatized fullerene isomers, in which stilbene is covalently linked to C60, are described. Synthesis and characterization of cis- and trans-stilbene substituted methanofullerenes 1 and 2, and cis- and trans-stilbene substituted fulleropyrrolidines 7 and 8 are described. While UV irradiation of the stilbene-substituted ketal precursors to 1 and 2 lacking the fullerene moiety afforded a photostationary state with 90:10 cis:trans ratio, similar to that of other model stilbene systems, direct and fluorenone-sensitized irradiation of 1 and 2 led to complete conversion to the trans isomer 2, as determined by HPLC analysis. The same results were obtained using cis-trans isomers 7 and 8, namely, the photostationary state on excitation below 350 nm is essentially 100% trans. No isomerisation in either system was obtained on excitation above 400 nm, where all the light is absorbed by the fullerene moiety. By analogy to previous studies of quenching of stilbene excited states, these results suggest that both singlet and triplet excited states of the trans-stilbene moiety in 2 and 8 are being quenched by intramolecular energy transfer to the attached C60, while the much shorter lived cis-stilbene excited states are not similarly quenched. Fluorescence studies on compound 8 support this hypothesis, since the characteristic fluorescence emission of trans-stilbene and trans-stilbene derivatives is not observed in the case of adduct 8. Because of the well established fact that trans-stilbene S1 states are longer lived than the S1 states of the corresponding cis isomers, rapid intramolecular singlet-singlet energy transfer to the appended C60 moiety, ket approximately 10(12) s(-1), is able to compete effectively with radiative and radiationless deactivation of the trans-stilbene S1 states in 2 and 8, but not in the corresponding cis isomers 1 and 7.