Gas-Phase Synthesis of Triphenylene (C18H12)

Article abstract of DOI:10.1002/cphc.201801154

For the last decades, the hydrogen-abstraction?acetylene-addition (HACA) mechanism has been widely invoked to rationalize the high-temperature synthesis of PAHs as detected in carbonaceous meteorites (CM) and proposed to exist in the interstellar medium (ISM). By unravelling the chemistry of the 9-phenanthrenyl radical ([C₁₄H₉]^.) with vinylacetylene (C₄H₄), we present the first compelling evidence of a barrier-less pathway leading to a prototype tetracyclic PAH – triphenylene (C₁₈H₁₂) – via an unconventional hydrogen abstraction–vinylacetylene addition (HAVA) mechanism operational at temperatures as low as 10 K. The barrier-less, exoergic nature of the reaction reveals HAVA as a versatile reaction mechanism that may drive molecular mass growth processes to PAHs and even two-dimensional, graphene-type nanostructures in cold environments in deep space thus leading to a better understanding of the carbon chemistry in our universe through the untangling of elementary reactions on the most fundamental level.

Full text of DOI:10.1002/cphc.201801154

Accepted Article
Title: Gas Phase Synthesis of Triphenylene (C₁₈H₁₂)
Authors: Long Zhao, Bo Xu, Utuq Ablikim, Wenchao Lu, Musahid
Ahmed, Mikhail M. Evseev, Eugene K. Bashkirov, Valeriy N.
Azyazov, A. Hasan Howlader, Stanislaw F. Wnuk, Alexander
M. Mebel, and Ralf I. Kaiser
This manuscript has been accepted after peer review and appears as an
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the VoR from the journal website shown below when it is published
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To be cited as: ChemPhysChem 10.1002/cphc.201801154
Link to VoR: http://dx.doi.org/10.1002/cphc.201801154
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10.1002/cphc.201801154
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Gas Phase Synthesis of Triphenylene (C18H12)
Long Zhao,[a] Bo Xu,[b] Utuq Ablikim,[b] Wenchao Lu,[b] Musahid Ahmed,[b],* Mikhail M. Evseev,[c] Eugene
K. Bashkirov,[c] Valeriy N. Azyazov,[c,d] A. Hasan Howlader,[e] Stanislaw F. Wnuk,[e] Alexander M.
Mebel,[c,e],* Ralf I. Kaiser[a],*
Abstract: For the last decades, the Hydrogen-Abstraction/
aCetylene-Addition (HACA) mechanism has been widely invoked to
rationalize the high-temperature synthesis of PAHs as detected in
carbonaceous meteorites (CM) and proposed to exist in the
interstellar medium (ISM). By unravelling the chemistry of the 9-
phenanthrenyl radical ([C14H9]•) with vinylacetylene (C4H4), we
present the first compelling evidence of a barrier-less pathway
leading to a prototype tetracyclic PAH - triphenylene (C18H12) - via
an unconventional Hydrogen Abstraction – Vinylacetylene Addition
(HAVA) mechanism operational at temperatures as low as 10 K. The
barrier-less, exoergic nature of the reaction reveals HAVA as a
versatile reaction mechanism that may drive molecular mass growth
processes to PAHs and even two dimensional, graphene-type
nanostructures in cold environments in deep space thus leading to a
better understanding of the carbon chemistry in our universe through
the untangling of elementary reactions on the most fundamental
level.
alkylated counterparts in carbonaceous chondrites such as in
Allende and Murchison suggests that PAHs may account for up
to 20% of the cosmic carbon budget,[1-2] although most of them
still remains unidentified.[3] Results from laser desorption – laser
multiphoton ionization mass spectrometry (L2MS) along with D/H
and 13C/12C isotopic analyses of meteoritic PAHs reveal that
meteoritic PAHs are not terrestrial contaminants, but rather have
4]
an extraterrestrial origin.[1,
Contemporary astrochemical
models tackling the molecular growth processes to PAHs have
been proposed to involve iron-based organometallic catalysis[5]
or have been extrapolated from combustion chemistry reaction
networks.[6]
The 18-π aromatic triphenylene molecule (C18H12) has been at
the center of attention in unraveling the underlying molecular
mass growth processes leading to complex polycyclic aromatic
hydrocarbons (PAHs) - organic molecules comprising fused
benzene rings - on the most fundamental, microscopic level
(Figure 1).[1] The ubiquitous presence of PAHs along with their
[a]
[b]
[c]
Dr. L. Zhao, Prof. Dr. R. I. Kaiser
Department of Chemistry
University of Hawaii at Manoa
Honolulu, Hawaii, 96822 (USA)
E-mail: ralfk@hawaii.edu
Dr. B. Xu, Dr. U. Ablikim, Dr. Wenchao Lu, Dr. M. Ahmed
Chemical Sciences Division
Lawrence Berkeley National Laboratory
Berkeley, CA 94720 (USA)
E-mail: mahmed@lbl.gov
Dr. M. M. Evseev, Prof. Dr. E. K. Bashkirov, Prof. Dr. V. N. Azyazov,
Prof. Dr. A. M. Mebel
Samara National Research University
Samara 443086 (Russia)
E-mail: mebela@fiu.edu
[d]
[e]
Prof. Dr. V. N. Azyazov
Department of Chemical & Electric Discharge Lasers
Lebedev Physical Institute of RAS
Samara 443011 (Russia)
Dr. A. H. Howlader, Prof. Dr. S. F. Wnuk, Prof. Dr. A. M. Mebel
Department of Chemistry and Biochemistry
Florida International University
Figure 1. Molecular structures of triphenylene (1), coronene (2), and
circumcoronene (3) highlighting the potential role of triphenylene as a key
building block in the formation of graphene-type two dimensional
nanostructures. The carbon atoms of the triphenylene building block are
highlighted in black.
Miami, FL 33199 (USA)
Supporting information for this article is given via a link at the end of
the document.
The popular Hydrogen-Abstraction/aCetylene-Addition (HACA)
mechanism[7] has been remarkably prominent in making an
effort to untangle the formation of PAHs in high temperature
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environments such as in combustion flames[6b-d] and in outflows
of carbon-rich asymptotic giant branch (AGB) stars such as
IRC+10216.[8] HACA involves a recurring sequence of atomic
hydrogen abstractions from an aromatic hydrocarbon such as
benzene followed by consecutive addition of one or two
acetylene molecule(s) prior to cyclization and aromatization[8a, 8b,
attributed to phenanthrene and 13C-phenanthrene formed via
hydrogen abstraction by or hydrogen addition to the 9-phe-
nanthrenyl radical; finally, ion counts at m/z = 176 and 177 are
connected to phenanthryne isomers (m/z = 176) together with
the 9-phenanthrenyl radical ([C14H9]•+; m/z = 177) (Supporting
Information, Fig. S1).
9]
with Parker et al. revealing that the three simplest PAHs
carrying two, three, and four fused benzene rings - naphthalene
(C10H8), phenanthrene (C14H10), and pyrene (C16H10) - can be
synthesized at elevated temperatures via HACA-type
mechanisms.[6b, 6d, 10] However, very recently, the ubiquity of the
HACA concept was challenged when it was revealed that
naphthalene can be synthesized via a barrier-less reaction at
temperatures as low as 10 K involving the reaction of the phenyl
radical ([C6H5]•) with vinylacetylene (C4H4), the latter of which is
supposed to be an important component in the atmosphere of
large planets and interstellar medium,[11] through the hydrogen
12]
abstraction – vinylacetylene addition (HAVA) pathway.[6a,
However, despite its potential to synthesize PAHs in the cold
interstellar medium, the legitimacy of HAVA to form PAHs
beyond naphthalene has not been demonstrated experimentally,
via molecular mass growth processes starting with aryl radicals
and vinylacetylene.
In this Communication, by untangling the chemistry of the 9-
phenanthrenyl radical ([C14H9]•; 177 amu) with vinylacetylene
(C4H4; 52 amu), we reveal the hitherto unknown chemistry
synthesizing triphenylene (C18H12; 228 amu) - the prototype of a
tetracyclic, benzenoid PAH composed only of benzene rings -
along with atomic hydrogen (1 amu) (reaction (1)) via a de facto
barrier-less HAVA reaction sequence. Briefly, a chemical reactor
was utilized, products being detected isomer-specifically via
fragment-free photoionization of the products in a molecular
beam by tunable vacuum ultraviolet (VUV) time of flight mass
spectrometry (Supporting Information).
Figure 2. Comparison of photoionization mass spectra recorded at
a
photoionization energy of 9.50 eV for the (a) 9-phenanthrenyl ([C14H9]•) -
vinylacetylene (C4H4) and (b) 9-phenanthrenyl ([C14H9]•) - helium (He) systems.
The mass peaks of the newly formed C16H10 (m/z = 202) and C18H12 (m/z =
228) species along with the 13C-substituted species (m/z = 203 and 229) are
highlighted in red.
Our objective is to assign the structural isomer(s) of C18H12
synthesized in the elementary reaction of 9-phenanthrenyl with
(1)
[C14H9]• + C4H4 → C18H12 + H•
vinylacetylene, This requires
a detailed analysis of the
corresponding photoionization efficiency (PIE) curve, which
illustrates the intensity of the ion at m/z of 228 (C18H12+) as a
function of the photon energy from 7.30 eV to 10.00 eV (Figure
3a). These data are fit with established reference PIE curves for
An illustrative mass spectrum collected at a photoionization
energy of 9.50 eV for the reaction of the 9-phenanthrenyl radical
with vinylacetylene is portrayed in Figure 2a; reference spectra
were also recorded by replacing the vinylacetylene reactant with
non-reactive helium carrier gas (Figure 2b). These data deliver
clear evidence on the synthesis of molecules with the molecular
formulae C16H10 (202 amu) and C18H12 (228 amu) in the 9-
phenanthrenyl – vinylacetylene system (Figure 2a), which are
not present in the control experiment (Figure 2b). The C18H12
isomer(s) and atomic hydrogen is formed via reaction (1) of 9-
phenanthrenyl with vinylacetylene while signal connected with
C16H10 might be linked to the reaction of 9-phenanthrenyl with
acetylene (C2H2; 26 amu) (Supporting Information, Fig. S1).
Finally, ion counts at mass-to-charge ratios (m/z) of 259
(C1313CH981Br+), 258 (C14H981Br+), 257 (C1313CH979Br+), 256
distinct
C18H12
isomers
(triphenylene,
chrysene,
benz(a)anthracene, benzo(c)phenanthrene, 9-(but-3-en-1-yn-1-
yl)-phenanthrene and (E)-9-(but-1-en-3-yn-1-yl)-phenanthrene
(Supporting Information, Fig. S2). Here, the experimentally
derived PIE curve at m/z of 228 (black) can be effectively
replicated by the reference PIE curves (C18H12+) of three isomers
including triphenylene, 9-(but-3-en-1-yn-1-yl)-phenanthrene and
(E)-9-(but-1-en-3-yn-1-yl)-phenanthrene. The experimental PIE
curve reveals an onset of the ion signal at 7.60 ± 0.05 eV; this
onset compares favorably with the adiabatic ionization energy of
9-(but-3-en-1-yn-1-yl)-phenanthrene at 7.60 ± 0.05 eV measured
in this work. It should be noted that the PIE curve for m/z = 229
(Figure 3b) is, after scaling, superimposable on the PIE curve of
m/z = 228. Therefore, the PIE function of m/z = 229 can be
associated with the 13C substituted isomers (C1713CH12) of
triphenylene, 9-(but-3-en-1-yn-1-yl)-phenanthrene and (E)-9-
(but-1-en-3-yn-1-yl)-phenanthrene (C18H12). It is crucial to
emphasize that the PIE curves of C18H12 isomers of triphenylene
+
(C14H979Br+), 179 (C1313CH10+), 178 (C14H10+), 177(C14H9 /
+
+
C1413CH8 ), and 176 (C14H8 ) are detectable in both the 9-
phenanthrenyl - vinylacetylene and the 9-phenanthrenyl - helium
systems. Therefore, these species are not associated with the
reaction between 9-phenanthrenyl and vinylacetylene. Signal at
m/z = 259 to 256 can be linked with the non-pyrolyzed 9-
bromophenanthrene precursor; signal at m/z = 178 and 179 is
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10.1002/cphc.201801154
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are characteristically associated to each molecule highlighting
that the co-existence of other isomers in the molecular beam
would change the shape of the PIE significantly and hence can
be excluded. Therefore, we determine that within our error limits,
triphenylene, 9-(but-3-en-1-yn-1-yl)-phenanthrene and (E)-9-
(but-1-en-3-yn-1-yl)-phenanthrene signify the contribution to
signal at m/z of 228 and 229. Two sources add to the errors: ±
10% based on the accuracy of the photodiode and a 1  error of
the PIE curve averaged over the individual scans.
bars consist of two parts: ±10% based on the accuracy of the photodiode and
a 1  error of the PIE curve averaged over the individual scans.
The complex [1] can isomerize via addition of the radical center
to the H2C moiety of the vinylacetylene molecule through a
barrier of only 5 kJ mol–1 leading to a resonantly stabilized free
radical intermediate [2], which is stabilized by 192 kJ mol–1 with
respect to the separated reactants. The corresponding transition
state lies 3 kJ mol-1 lower in energy than the separated reactants.
In this sense, a barrier to addition does exist, but this barrier is
located below the energy of the separated reactants and hence
is called a submerged barrier. For the overall reaction from 9-
phenanthrenyl plus vinylacetylene to intermediate [2], the
reaction is de facto barrier-less. The C18H13 intermediate [2]
isomerizes via a hydrogen migration from the C10 carbon atom
of the phenanthrenyl segment to the vinylacetylene moiety
yielding [3], thus effectively shifting the radical center from the
aliphatic side chain to the aromatic ring. The latter undergoes a
facile ring closure via a barrier of only 42 kJ mol–1 yielding
intermediate [4] with the latter revealing the triphenylene carbon
backbone. A second hydrogen migration – this time from the
methylene moiety to the carbene carbon atom – is required to
form intermediate [5], which then undergoes hydrogen atom loss
and aromatization to triphenylene (C18H12, R1) via a tight exit
transition state located 23 kJ mol–1 above the separated
products. The existence of a tight exit transition state, in which
the hydrogen atom is emitted at an angle of 89º almost
perpendicularly to the molecular plane, is sensible since the
reversed reaction involves the addition of a hydrogen atom to a
closed shell aromatic molecule, and a barrier of addition of a
similar order of magnitude of 37 kJ mol–1 was computed for
addition of atomic hydrogen to benzene – the prototype aromatic
system.[13] Branched from intermediate [2] one isomer of
triphenylene (E)-9-(but-1-en-3-yn-1-yl)-phenanthrene (R2) is
produced from hydrogen loss of [2] via a barrier of 177 kJ mol-1.
Moreover, the van-der-Waals complex [6] can isomerize via
addition of the radical center to the ethynyl moiety of the
vinylacetylene molecule through a barrier of 13 kJ mol–1 leading
to intermediate [7], stabilized by 146 kJ mol–1 compared with the
reactants, and further lose one hydrogen atom to yield the third
isomer 9-(but-3-en-1-yn-1-yl)-phenanthrene (R3) via a tight exit
transition state located 20 kJ mol-1 above the products.
Consequently, the computational prediction of the formation of
three structural isomers of C18H12 is well matched by our
experimental studies and directly reflects two distinct entrance
channels via two van-der-Waals complexes leading to three
discrete structural isomers, defining a benchmark of a molecular
mass growth process to PAHs.
The present work reveals that the prototype of a benzenoid PAH
composed of four benzene rings – triphenylene – can be
synthesized via the elementary reaction of the 9-phenanthrenyl
radical with vinylacetylene. To untangle the underlying mechani-
sm of formation, we performed electronic structure calculations
on the relevant C18H12 and C18H13 potential energy surfaces
(PESs) (Figure 4, Supporting Information, Fig. S3). Our
computations reveal an overall barrier-less pathway leading to
triphenylene at temperatures as low as 10 K found in cold
molecular clouds in interstellar space. Here, the formation of
triphenylene is initiated by the barrier-less formation of weakly
bound van-der-Waals complexes of 9-phenanthrenyl and
vinylacetylene [1] and [6], in which the radical center of 9-
phenanthrenyl points to the vinyl and ethynyl moieties of the
vinylacetylene reactant, respectively.
An alternative possibility for the formation of triphenylene (R1)
might be the reaction of phenanthryne with vinylacetyelene. Our
calculations (Figure S4) reveal that R1 can be plausibly
produced in this reaction via vinylacetylene addition to the triple
bond of phenanthryne, six-member ring closure and two
hydrogen migrations after overcoming entrance barriers of 29-36
kJ mol-1. However, according to the earlier calculations,[14] a rate
constant for the hydrogen atom loss from an aryl radical to
produce a triple bond in an aromatic ring (like in phenanthryne)
is typically on the range of 103 s-1 under the conditions of the
Figure 3. Photoionization efficiency (PIE) curves for m/z = 228 and 229.
Black: experimentally derived PIE curves; Colored lines: reference PIE curves
for triphenylene (green), (E)-9-(but-1-en-3-yn-1-yl)-phenanthrene (blue) and 9-
(but-3-en-1-yn-1-yl)-phenanthrene (cyan), red: overall fit. The overall error
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10.1002/cphc.201801154
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present experiment making the lifetime of the aryl radical with
respect to its decomposition to be ~1 ms, that is longer than the
typical residence time in the reactor of a fes 10 µs. When
phenanthryne is produced that late in the reactor, it may not
have enough time to react with vinylacetylene and hence can be
detected as in the present study. In the meantime, the
phenanthryne plus vinylacetylene reaction cannot account for
the formation of R2 and R3, as no feasible pathways to these
products could be found in our calculations of the pertinent
potential energy surface (Figure S4).
hydrogen bond cleavage leading to 9-phenanthrenyl and phenyl,
respectively, followed by formation of a weakly bound van-der-
Waals complex, addition of the radical center to the methylene
moiety of the vinylacetylene reactant via a submerged barrier,
hydrogen shift from the aromatic moiety to the former
vinylacetylene reactant establishing a second methylene group,
ring closure, hydrogen shift, and eventually hydrogen atom loss
followed by aromatization and formation of triphenylene and
naphthalene, respectively. Consequently, the aforementioned
features reveal that at ultralow temperatures in molecular clouds,
Figure 4. Potential energy surface (PES) for the 9-phenanthrenyl [C14H9]• reaction with vinylacetylene (C4H4) calculated at the G3(MP2,CC)//B3LYP/6-
311G(d,p) level of theory leading to triphenylene (R1), (E)-9-(but-1-en-3-yn-1-yl)-phenanthrene (R2) and 9-(but-3-en-1-yn-1-yl)-phenanthrene (R3). The
relative energies are given in kJ mol–1
.
This synthesis of triphenylene via ring expansion from 9-
phenanthrenyl defines a standard de-facto barrier-less molecular
mass growth process involving vinylacetylene addition followed
by isomerization via hydrogen shifts and ring closure along with
aromatization via atomic hydrogen loss. In cold molecular clouds
such as Taurus Molecular Cloud -1 (TMC-1), the 9-
phenanthrenyl-vinylacetylene route to triphenylene can be
initiated by photolysis of phenanthrene (C14H10) by the internal
ultraviolet field present even deep inside molecular clouds[15]
thus leading, e.g., via atomic hydrogen loss from the C9-position
to the 9-phenanthrenyl radical ([C14H9]•). Even at molecular
cloud temperatures as low as 10 K, 9-phenanthrenyl reacts with
HAVA might represent the key mechanism driving the molecular
mass growth of PAHs via de facto barrier-less, successive ring
expansions involving bimolecular reactions of aryl radical with
vinylacetylene as a building block with the aryl radical generated
from the corresponding aromatic precursor via photolysis by the
internal ultraviolet field. However, at elevated temperatures such
as in circumstellar envelopes of carbon stars of up to a few
1,000 K and in combustion flames, the reaction could be also
initiated by hydrogen abstraction from phenanthrene followed by
formation of triphenylene upon reaction with vinylacetylene. The
critical role of the hydrogen abstraction – vinylacetylene addition
(HAVA) mechanism in the 9-phenanthrenyl-vinylacetylene
system to form triphenylene is based on the fact that the
classical HACA route with the acetylene reactant cannot
synthesize triphenylene, but rather produces acephenanthrylene
(C20H12) in strong resemblance to the 1-naphthyl ([C10H7]•) –
acetylene system leading to acenaphthylene thus blocking
cyclization to a fourth six-membered ring (Figure 5).[6c] Likewise,
vinylacetylene via
a submerged barrier yielding eventually
triphenylene via an overall exoergic bimolecular gas phase
reaction. In these interstellar environments, the reaction
mechanism of the phenanthrene – triphenylene ring expansion
mirrors the benzene – naphthalene[6a] mass growth process
starting with the photolysis of the aromatic precursor via carbon-
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with just the HACA mechanism, the coronenyl radical ([C24H11]•)
cannot generate higher PAHs containing solely six-membered
rings, but leads to a PAH carrying a five-membered ring:
cyclopenta[bc]coronene (C26H12) (Figure 6).
respectively. Overall, three repetitive HACA sequences could
form ultimately coronene from triphenylene. The subsequent
molecular growth processes of coronene are rather tricky. HACA
– initiated via abstraction of any of the chemically equivalent
hydrogen atoms of coronene – does not lead to the growth of a
six-membered ring (Figure S4); the abstraction of a hydrogen
atom leads essentially to a 1-naphthyl moiety, which has been
shown to react with acetylene to form a five-membered ring such
as in acenaphthylene synthesized in the 1-naphthyl (C10H7) –
acetylene system.[6c] On the other hand, a transfer of the
aforementioned HAVA pathway to coronene suggests that three
repetitive HAVA sequences can form three six-membered rings
yielding
eventually
tribenzo[fgh,pqr,za1b1]trinaphthylene
(C36H18). From here, additional HACA pathways can take over
forming circumcoronene via diphenanthro[3,4,5,6-efghi:3',4',5',6'-
uvabc]ovalene (C48H18). This reaction sequence also backs up
Naraoka et al.’s results of a detailed 13C/12C isotopic analysis of
PAHs in meteorites such as in Asuka-881458 identifying
triphenylene as a key PAH on the path to more complex
structures consisting of fused benzene rings possibly leading to
the build-up of two-dimensional graphene-type nanostructures.[1]
Here, starting essentially from triphenylene, HAVA pathways
result in acene-type growth patterns in three sectors of the
molecular plane separated by 120°, whereas the classical HACA
mechanisms close the bays.
Figure 5. a) HACA mechanism involving the reaction of 9-phenanthrenyl
([C14H9]•) with acetylene (C2H2) to acephenanthrylene (C16H10) plus atomic
hydrogen (H). b) HACA mechanism involving the reaction of 1-naphthyl
([C10H7]•) with acetylene (C2H2) to acenaphthylene (C12H8) plus atomic
hydrogen (H).[5]
As reported in Parker et al.’s study on the formation of
acenaphthylene (C12H8) from the reaction of 1-naphthyl ([C10H7]•)
radical with acetylene (C2H2),[6c] which has an analogous bay
structure as the 1-naphthyl radical, the 9-phenanthrenyl radical
([C14H9]•) is expected to react via a similar pathway with
acetylene to produce acephenanthrylene (C16H10) along with
atomic hydrogen, but not with two acetylene molecules to form
triphenylene. It is noticeable that though no acephenanthrylene
was observed under our experiment conditions, it is still a
potential formation pathway as shown in Fig. 5a.
Figure 6. HACA reaction from coronenyl radical ([C24H11]•) to
cyclopenta[bc]coronene (C26H12). Starting from the coronenyl radical, after
addition of one acetylene molecule followed by atomic hydrogen, a five-
member ring is generated,[6c] but no expansion occurs with an extra six-
membered ring formation.
Figure 7. Potential schematic mass growth processes starting with
triphenylene (C18H12) to circumcoronene (C54H18) via tandem HACA and HAVA
reactions. These mass growth processes have not been studied
experimentally yet.
The pathways involve complementary HACA (red) and HAVA
(blue) sequences with HACA leading to bay closure and HAVA
supporting six-membered ring expansion eventually forming
graphene-type nanostructures (Figure 7). The red and blue
numbers within the benzene rings define the step in the growth
sequence. Considering the molecular structures D3h symmetric
triphenylene and D6h symmetric coronene molecule, a hydrogen
abstraction from any of the six bay carbon atoms in triphenylene
followed by acetylene addition could lead to bay closure via
HACA similarly as verified in the reaction of the biphenylyl
radical (C6H5-C6H4) and 4-phenanthrenyl radical with acetylene
leading to phenanthrene (C14H10)[6d] and pyrene (C16H10),[10]
To conclude, the facile route to triphenylene (C18H12), as
identified in carbonaceous chondrites, via the bimolecular
reaction of the 9-phenanthrenyl radical with vinylacetylene
following the hydrogen abstraction – vinylacetylene addition
(HAVA) mechanism represents the prototype of a barrier-less
reaction leading through submerged barriers and resonantly
stabilized free radical (RSFR) intermediates to facile molecular
growth processes in PAHs via ring expansion through
a
bimolecular collision at temperatures as low as 10 K. Therefore,
HAVA represents the key mechanism driving the molecular
mass growth processes of PAHs via de facto barrier-less,
successive ring expansions involving bimolecular reactions of
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10.1002/cphc.201801154
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ionization energies were also computed at the G3(MP2,CC)//B3LYP/6-
311G(d,p) level of theory. The GAUSSIAN 09[21] and MOLPRO 2010
program packages[22] were utilized for the ab initio calculations.
aryl radical with vinylacetylene as a molecular building block.
Starting from triphenylene - HAVA and HACA may even operate
in
tandem
to
eventually
synthesize
graphene-type
nanostructures and after condensation of multiple layers
graphitized carbon as identified in carbonaceous chondrites like
Allende[16] ultimately changing our paradigm on the interstellar
carbon chemistry and the progression of carbonaceous matter in
the universe on the most fundamental, microscopic level.
Acknowledgements
This work was supported by the US Department of Energy,
Basic Energy Sciences DE-FG02-03ER15411 (experimental
studies) and DE-FG02-04ER15570 (computational studies and
organic synthesis of 9-(but-3-en-1-yn-1-yl)-phenanthrene and
(E)-9-(but-1-en-3-yn-1-yl)-phenanthrene) to the University of
Hawaii and Florida International University, respectively. Ab
initio calculations of the C18H13 PES relevant to the reaction of 9-
phenanthrenyl radical with vinylacetylene at Samara University
were supported by the Ministry of Education and Science of the
Russian Federation under Grant No. 14.Y26.31.0020. MA, UA,
BX, and the experiments at the chemical dynamics beamline at
the ALS are supported by the Director, Office of Science, Office
of Basic Energy Sciences, of the U.S. Department of Energy
under Contract No. DE-AC02-05CH11231, through the Gas
Phase Chemical Physics Program, Chemical Sciences Division.
Method - Experimental
The experiments were conducted at the Advanced Light Source (ALS) at
the Chemical Dynamics Beamline (9.0.2.) exploiting a high-temperature
chemical reactor consisting of a resistively-heated silicon carbide (SiC)
tube of 20 mm length and 1 mm inner diameter.[6c, 6d, 17] This reactor is
incorporated into a molecular beam apparatus operated with a Wiley-
McLaren reflectron time-of-flight mass spectrometer (Re-TOF-MS). This
setup investigates discrete chemical reactions to simulate PAH growth in
situ through the reaction of radicals. Here, 9-phenanthrenyl radicals
[C14H9]• were prepared at the concentration of less than 0.1% in situ via
pyrolysis of the 9-bromophenanthrene precursor (C14H9Br; TCI-America,
> 98%) seeded in vinylacetylene/helium (5% C4H4; 95% He; Airgas)
carrier gas at a pressure of 300 Torr. The temperature of the SiC tube
was monitored using a Type-C thermocouple and was maintained at
1450 ± 10 K. At this temperature, 9-bromophenanthrene dissociates to
the 9-phenanthrenyl radical plus atomic bromine in situ and reacts with
vinylacetylene, and the target signal of m/z = 228 was the strongest
compared with those at other temperatures. The reaction products
synthesized in the reactor were expanded supersonically and passed
through a 2-mm diameter skimmer located 10 mm downstream the pyro-
lytic reactor and enter the main chamber, which houses the Re-TOF-MS.
The products within the supersonic beam were then photoionized in the
extraction region of the spectrometer by exploiting quasi-continuous
tunable synchrotron vacuum ultraviolet (VUV) light and detected with a
microchannel plate (MCP). It is important to highlight that VUV single
Keywords: Hydrogen Abstraction – Vinylacetylene Addition
(HAVA)
• polycyclic aromatic hydrocarbons • gas-phase
chemistry • mass spectrometry • interstellar medium
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
The triphenylene molecule – a
potential precursor to two
Long Zhao, Bo Xu, Utuq Ablikim,
Wenchao Lu, Musahid Ahmed,* Mikhail
M. Evseev, Eugene K. Bashkirov,
Valeriy N. Azyazov, A. Hasan Howlader,
Stanislaw F. Wnuk, Alexander M.
Mebel,* Ralf I. Kaiser*
dimensional graphite nano sheets in
the interstellar medium – can be
formed without entrance barrier at
temperatures as low as 10 K in
molecular clouds.
Page No. – Page No.
Gas Phase Synthesis of Triphenylene
(C18H12)
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