The availability of macroscopic quantities of fullerenes has resulted in a vast number of physical and chemical studies of these new materials. However, the mechanisms that lead to the formation of these spherical carbon allotropes are not well understood. Mass spectral evidence has been obtained for the size-selective growth of fullerenes through the coalescence of cyclo[
n
]carbons, molecular carbon allotropes consisting of monocyclic rings with
n
carbon atoms. Whereas coalescence of cyclo[30]carbon (
cyclo
-C
30
) produces predominantly buckminsterfullerene (C
60
), the smaller rings
cyclo
-C
18
and
cyclo
-C
24
preferentially produce fullerene C
70
through distinct intermediates. The present studies not only provide new insights into fullerene formation mechanisms but also raise the possibility of tailoring the size distributions of fullerenes by variation of the appropriate properties of the precursors.
The deep ultraviolet (λ < ∼250 nm) photochemistry of
chemisorbed organosilane self-assembled films
of the type R(CH2)
n
SiO−surface
where n = 0, 1, 2 and R = phenyl, naphthyl, or
anthracenyl is explored.
Photochemistry is examined using 193 and 248 nm laser irradiation
as well as deep ultraviolet lamp
sources. It is demonstrated for a variety of systems, including
single and multiple rings as well as
heterocycles, that the primary photochemical mechanism is cleavage of
the Si−C bond. Photocleavage
of the organic group generates a polar, wettable silanol surface that
is amenable to subsequent remodification
by organosilane chemisorption, allowing the fabrication of
high-resolution patterns of chemical functional
groups in a single molecular plane. The use of patterned
monolayers as templates of reactivity for subsequent
selective chemical reactions is demonstrated.
The ionization potentials (IPs) of several large carbon clusters Cn (n≥48), including the enhanced abundance (‘‘magic number’’) clusters C50, C60, and C70, have been determined by Fourier transform ion cyclotron resonance (FTICR) mass spectrometric charge transfer bracketing experiments. The IPs of C50, C60, and C70 were bracketed by the same two charge transfer compounds, leading to a common value of 7.61±0.11 eV. The IPs of even numbered clusters adjacent to these magic number clusters were found to be lower by as much as 0.5 eV and all clusters between C50 and C200 were determined to have IPs greater than 6.20 eV. The reaction rates of C+60 and C+70 with metallocenes were anomalously slow in comparison to the other large carbon cluster ions. IP and reactivity results suggest that C50, C60, and C70 may indeed have different or more stable structures than neighboring clusters, which supports the hypothesis of closed-shell, spherical species. The implications of these results for the mechanism of C+n formation by direct laser vaporization are also discussed.
The ion/molecule chemistry of laser-generated carbon cluster ions (C+n; n=3–19) has been studied using Fourier transform mass spectrometry. The ion/molecule reactions of mass-selected carbon cluster ions with D2 and O2 have been studied and reaction rates measured. Reactions of the primary product ions with D2 and O2 were also studied. The change in reactivity observed as a function of cluster size suggests a structural change from linear to monocyclic rings in the cluster ions between n=9 and 10. Evidence for the presence of two structural isomers for the C+7 cluster ion has also been observed and has been attributed to the existence of both a linear and cyclic form of the ion. MNDO calculations have also been used to obtain structural and thermodynamic information on possible reactant and product ions in an attempt to explain the differences observed in the ion/molecule reactions. The results from collision induced dissociation studies of the carbon cluster ions are also discussed.
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