Gas phase formation of cyclopentanaphthalene (benzindene) isomers: Via reactions of 5- And 6-indenyl radicals with vinylacetylene

Article abstract of DOI:10.1039/d0cp03846f

The tricyclic polycyclic aromatic hydrocarbons (PAHs) 3H-cyclopenta[a]naphthalene (C13H10), 1H-cyclopenta[b]naphthalene (C13H10) and 1H-cyclopenta[a]naphthalene (C13H10) along with their indene-based bicyclic isomers (E)-5-(but-1-en-3-yn-1-yl)-1H-indene, (E)-6-(but-1-en-3-yn-1-yl)-1H-indene, 5-(but-3-ene-1-yn-1-yl)-1H-in-dene, and 6-(but-3-ene-1-yn-1-yl)-1H-indene were formed via a directed synthesis in a high-temperature chemical micro reactor at the temperature of 1300 ± 10 K through the reactions of the 5- and 6-indenyl radicals (C9H7) with vinylacetylene (C4H4). The isomer distributions were probed utilizing tunable vacuum ultraviolet light by recording the photoionization efficiency curves at mass-to-charge of m/z = 166 (C13H10) and 167 (13CC12H10) of the products in a supersonic molecular beam. The underlying reaction mechanisms involve the initial formation of van-der-Waals complexes followed by addition of the 5- and 6-indenyl radicals to vinylacetylene via submerged barriers, followed by isomerization (hydrogen shifts, ring closures), and termination via atomic hydrogen elimination accompanied by aromatization. All the barriers involved in the formation of 3H-cyclopenta[a]naphthalene, 1H-cyclopenta[b]naphthalene and 1H-cyclopenta[a]naphthalene are submerged with respect to the reactants indicating that the mechanisms are in fact barrierless, potentially forming PAHs via the hydrogen abstraction-vinylacetylene addition (HAVA) pathway in the cold molecular clouds such as Taurus Molecular Cloud-1 (TMC-1) at temperatures as low as 10 K. This journal is

Full text of DOI:10.1039/d0cp03846f

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A. H. Howlader and S. F. Wnuk, Phys. Chem. Chem. Phys., 2020, DOI: 10.1039/D0CP03846F.
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Determination of the thermal, oxidative and photochemical
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Page 1 of 21
Physical Chemistry Chemical Physics
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DOI: 10.1039/D0CP03846F
Gas Phase Formation of Cyclopentanaphthalene (Benzindene)
Isomers via Reactions of 5- and 6-Indenyl Radicals with
Vinylacetylene
1
1*
Long Zhao , Ralf I. Kaiser
Department of Chemistry, University of Hawaii at Manoa, Honolulu, Hawaii, 96822
USA)
1
(
2
2
2*
Wenchao Lu , Oleg Kostko , Musahid Ahmed
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720
2
(USA)
3
3
Mikhail M. Evseev, Eugene K. Bashkirov
3
Samara National Research University, Samara 443086, Russian Federation
3
,4
3,4
Artem D. Oleinikov , Valeriy N. Azyazov
3
Samara National Research University, Samara 443086, Russian Federation
4
Lebedev Physical Institute, Samara 443011
3
,5
5
5
Alexander M. Mebel * A. Hasan Howlader, Stanislaw F. Wnuk
3
Samara National Research University, Samara 443086
(
Russian Federation)
5
Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199
(USA)
*
Corresponding author:
Ralf I. Kaiser <ralfk@hawaii.edu>
Musahid Ahmed <mahmed@lbl.gov>
Alexander M. Mebel <mebela@fiu.edu>
1

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Abstract:
The tricyclic polycyclic aromatic hydrocarbons (PAHs) 3H-cyclopenta[a]naphthalene (C H ),
1
3
10
1
H-cyclopenta[b]naphthalene (C H ) and 1H-cyclopenta[a]naphthalene (C H ) along with
13 10 13 10
their indene-based bicyclic isomers (E)-5-(but-1-en-3-yn-1-yl)-1H-indene, (E)-6-(but-1-en-3-yn-
-yl)-1H-indene, 5-(but-3-ene-1-yn-1-yl)-1H-in-dene, and 6-(but-3-ene-1-yn-1-yl)-1H-indene
1
were formed via a “directed synthesis” in a high-temperature chemical micro reactor at the
•
temperature of 1300 ± 10 K through the reactions of the 5- and 6-indenyl radicals (C H ) with
9
7
vinylacetylene (C H ). The isomer distributions were probed utilizing tunable vacuum ultraviolet
4
4
light by recording the photoionization efficiency curves at mass-to-charge of m/z = 166 (C H )
13
10
1
3
and 167 ( CC H ) of the products in a supersonic molecular beam. The underlying reaction
1
2
10
mechanisms involve the initial formation of van-der-Waals complexes followed by addition of
the 5- and 6-indenyl radicals to vinylacetylene via submerged barriers, followed by isomerization
(
hydrogen shifts, ring closures), and termination via atomic hydrogen elimination accompanied
by aromatization. All the barriers involved in the formation of 3H-cyclopenta[a]naphthalene,
H-cyclopenta[b]naphthalene and 1H-cyclopenta[a]naphthalene are submerged with respect to
1
the reactants indicating that the mechanisms are in fact barrierless, potentially forming PAHs via
the hydrogen abstraction – vinylacetylene addition (HAVA) pathway in the cold molecular
clouds such as Taurus Molecular Cloud-1 (TMC-1) at temperatures as low as 10 K.
2

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1
. INTRODUCTION
Polycyclic aromatic hydrocarbons (PAHs) carrying five membered rings such as fluorene
C H ) and cyclopentanaphthalenes (C H ) represent fundamental molecular building blocks
(
1
3
10
13 10
1
-5
of non-planar PAHs like corannulene (C H ) along with fullerenes (C , C ) (Scheme 1).
2
0
10
60
70
These species necessitate five-membered rings in the carbon backbone of the PAH to ‘curve’
planar PAHs out of the plane. An intimate knowledge of the elementary mechanisms forming
PAHs carrying five-membered ring(s) is therefore critical in aiding our understanding of the
early stage chemistry in combustion systems as to how precursor PAHs lead to three dimensional
(bowl-shaped) carbonaceous structures and ultimately soot particles. However, the elementary
steps, chemical dynamics, and reaction mechanisms to form PAHs carrying five-membered
ring(s) on the molecular level are largely elusive as detailed synthetic routes have not been
investigated comprehensively to date. The presence of five-membered rings in PAHs is also
important from the viewpoint of molecular mass growth processes and ring expansion
6
pathways. In combustion flames, ubiquitous open shell reactants such as atomic hydrogen can
easily abstract a hydrogen atom from the CH moiety of, e.g. the five-membered ring of indene
2
resulting in the 1-indenyl radical. The radical-radical reaction with a methyl radical has been
revealed to lead via ring expansion from a five- to a six-membered ring thus effectively
converting molecular building blocks relevant to the formation of three-dimensional
carbonaceous nanostructures to those of critical importance to graphene-type two-dimensional
nano-sheets at elevated temperatures.6
Previously, the simplest prototype of a PAH carrying a single six and five membered ring –
indene (C H ) (Scheme 1) has been synthesized under single collision conditions exploiting a
9
8
•
crossed molecular beams machine via the reactions of phenyl radicals (C H ) with allene
6
5
(H CCCH ) and methylacetylene (CH CCH) along with their (partially) deuterated counterparts
2 2 3
(
Scheme 2).7-11 After overcoming barriers to addition of the phenyl radical to the π-electron
-1
density of methylacetylene or allene of 1 to 26 kJ mol , the initial collision complexes isomerize
predominantly via hydrogen shifts and ring closures followed ultimately by atomic hydrogen loss
accompanied by aromatization and indene formation (Scheme 2).12 These findings were
confirmed in recent studies exploiting a micro reactor coupled with vacuum ultraviolet (VUV)
photoionization of the neutral reaction products leading to the isomer-selective identification of
indene along with its phenylallene, 1-phenyl-1-propyne, and 3-phenyl-1-propyne isomers.10
3

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Alternatively, chemical micro reactor studies revealed that indene can also be synthesized at
elevated temperatures of 600 ± 100 K exclusively through the reaction of the resonantly
•
stabilized, aromatic benzyl radical (C H ) with acetylene (C H ) after overcoming a significant
7
7
2
2
-1
13
barrier to addition of 51 kJ mol followed by ring closure and hydrogen atom elimination, i.e.
a derivative of the Hydrogen-Abstraction-aCetylene-Addition (HACA) mechanism. These C6-
C3 and C7-C2 pathways effectively lead to ring annulation to an existing C6-benzene-type
ring.12
•
HACA is also implicated in the reaction of the 2-naphthyl radical (C H ) with allene
1
0
7
(
H CCCH ) and methylacetylene (CH CCH) leading to 3H-cyclopenta[a]naphthalene (C H )
2 2 3 13 10
and 1H-cyclopenta[b]naphthalene (C H ) (Scheme 2).14 However, 1H-cyclopenta
1
3
10
[a]naphthalene was not reported in the study due to low yields. Previous experimental and
theoretical investigations also suggested that 3H-cyclopenta[a]naphthalene can be synthesized
from 1-methylnaphthalene (C H ) via hydrogen abstraction from the methyl group followed by
1
1
10
acetylene addition reaction, isomerization, and hydrogen elimination (HACA).2, 15-25 However,
these molecular mass growth processes operate only at elevated temperatures as found in
combustion systems and in circumstellar envelopes of Asymptotic Giant Branch (AGB) carbon
-1
stars close to the photosphere; entrance barriers to addition of 8 to 11 kJ mol would effectively
block these reactions at low temperatures such as in molecular clouds, e.g., Taurus Molecular
Cloud -1 (TMC-1) (10 K), and in hydrocarbon rich atmospheres of planets and their moons like
2
6
Saturn’s satellite Titan (70 – 200 K). Therefore, as of now, ring annulation sequences resulting
in an efficient addition of a five-membered ring to an existing benzene moiety have only been
exposed to operate at high temperatures; pathways leading to molecular mass growth processes
involving five-membered rings at ultralow temperatures are still elusive.
The elucidation of the hydrogen abstraction – vinylacetylene addition (HAVA) reaction
mechanisms has opened up barrier-less low temperature pathways to form PAHs even at
2
7
•
temperatures as low at 10 K. In the prototype phenyl (C H ) – vinylacetylene (C H ) system,
6
5
4
4
Parker et al. and Zhao et al. provided compelling evidence via crossed molecular beams and
chemical micro reactor studies coupled with electronic structure calculations on the formation of
2
8, 29
naphthalene (C H ) plus atomic hydrogen.
The reaction is initiated by the formation of a
1
0
8
van-der-Waals complex followed by addition of the phenyl radical to the CH moiety of the
2
4

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vinylacetylene reactant leading to a resonantly stabilized radical intermediate (C H ). Although
1
4
11
-1
a barrier to addition of 9 kJ mol does exist, the transition state is located below the energy of
the separated reactants, and hence the barrier to addition is ‘submerged’. The existence of
submerged barriers is also important in the formation of methyl-substituted naphthalenes such as
1
-methylnaphthalene (C H ) and 2-methylnaphthalene (C H ) as demonstrated through the
11 10
11 10
•
30
reactions of m- and p-tolyl radicals (C H ) with vinylacetylene.
7
7
Considering that the HAVA mechanism operates at ultralow temperatures and the fact that
substituted phenyl-type radicals such as tolyl also react with vinylacetylene barrierlessly leading
to ring annulation of a benzene ring, we explore to what extent the reactions of the 5- and 6-
•
indenyl radicals (C H ) with vinylacetylene lead to effective (barrierless) ring annulation of a
9
7
benzene ring ultimately leading to prototype tricyclic polycyclic aromatic hydrocarbons (PAHs)
carrying two six membered and one five membered ring (C H ) in the gas phase (Scheme 2).
1
3
10
•
The 5- and 6-indenyl radicals (C H ) can be rationalized as ‘disubstituted’ phenyl radicals, in
9
7
which the five-membered ring acts as a spectator moiety leading to PAH growth in low
temperature environments from aromatic radical reactions with key hydrocarbon molecules
(
vinylacetylene).31
2
. EXPERIMENTAL
The experiments were carried out at the Advanced Light Source (ALS), Lawrence Berkeley
National Laboratory. A detailed description of the experimental set-up was provided
previously.6, 10, 13, 27, 32-36 Briefly, by studying the reactions of the 5- and 6-indenyl radicals
•
(
C H ) with vinylacetylene (C H ; Applied Gas; 5% C H seeded in 95 % He) under simulated
9
7
4
4
4
4
combustion conditions, we deliver experimental and computational evidence of the molecular
growth processes to benzindene (C H ) isomers. In separate experiments, a continuous beam of
1
3
10
•
5
5
- and 6-indenyl radicals (C H ) was prepared in situ through the pyrolysis of the corresponding
9 7
- and 6-iodoindene (C H I) precursors. The in-house synthesis of both iodoindene precursors is
9
7
described in the Supplementary Information.37 In separate experiments, the precursors were
seeded in the vinylacetylene/helium carrier gas at an inlet pressure of 300 ± 5 Torr. Control
experiments were also performed by just replacing the vinylacetylene/helium gases with pure
helium while all the other settings (temperature, pressure) remained identical. The gas mixture
was expanded through a 0.1 mm diameter orifice into the high-temperature pyrolytic reactor, i.e.
5

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a resistively heated silicon carbide (SiC) tube with 1.0 mm inner diameter and 20 mm heating
length. The SiC tube was held at 1300 ± 10 K as monitored by a Type-C thermocouple attached
to the center of the heating area. The residence time in the reactor tube under our experimental
condition are up to a few tens of µs,38, 39 with the number of collisions up to a few hundreds.39
The products formed in the reactor were expanded supersonically, passed through a 2 mm
diameter skimmer located 10 mm downstream of the pyrolytic reactor, and entered into the
photoionization chamber housing the Wiley–McLaren reflectron time-of-flight mass
-6
spectrometer (ReTOF-MS), which was kept at 10 Torr during the experiment. The quasi-
continuous tunable synchrotron VUV light from ALS intersected the neutral molecular beam
perpendicularly in the extraction region of the ReTOF-MS. VUV single photon ionization is
essentially a fragment-free ionization technique, which would preserve the original information
4
0
of the target molecules. The ions of the photoionized molecules were accelerated to the field-
free TOF region and then collected by a microchannel plate (MCP) detector in the ReTOF mode.
The time-dependent ion signal was amplified by a fast preamplifier and convoluted by a
multichannel digitizer card.
Photoionization efficiency (PIE) curves, which report the ion count as a function of photon
energy with a step interval of 0.05 eV at a well-defined mass-to-charge ratio (m/z), were
produced by integrating the signal recorded at the specific m/z for the species of interest from
7
.30 eV to 10.00 eV and normalized by photon fluxes. Experimentally measured PIE curves are
usually used for species identification by fitting the calibration PIE curve(s) of the species of
interest linearly. To fit the PIE curve and identify the target products lied at m/z = 166 (C H ),
1
3
10
the calibration curves of the individual isomers were newly obtained at the photoionization
energy from 7.3 to 10.0 eV with the 0.05 eV step interval, as presented in the Supporting
Information (Figure S1). Among these isomers, 1H-cyclopenta[a]naphthalene (P3) and the
branched molecules [(E)-5-(but-1-en-3-yn-1-yl)-1H-indene (P4), 5-(but-3-ene-1-yn-1-yl)-1H-
indene (P5), (E)-6-(but-1-en-3-yn-1-yl)-1H-indene (P6) and 6-(but-3-ene-1-yn-1-yl)-1H-indene
(
P7)] were synthesized for the current work,37 while the PIE calibration curves for 3H-
cyclopenta[a]naphthalene (P1), and 1H-cyclopenta[b]naphthalene (P2) were already measured
and taken from our previous study.14
3
. COMPUTATIONAL
6

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The calculations of the energies and molecular parameters of various intermediates and
transition states for the reactions of 5- and 6-indenyl with vinylacetylene occurring on the C H
1
3
11
potential energy surface (PES), as well as of the reactants and possible products were carried out
at the G3(MP2,CC)//B3LYP/6-311G(d,p) level of theory. Within this theoretical scheme,
geometries were optimized and vibrational frequencies were calculated using the density
functional B3LYP method with the 6-311G(d,p) basis set. Then, single-point total energies were
improved using a series of coupled clusters CCSD(T) and second-order Møller-Plesset
perturbation theory MP2 calculations, and the final energy was computed as
E[G3(MP2,CC)] = E[CCSD(T)/6-311G(d,p)] + E[MP2/G3Large)] – E[MP2/6-311G(d,p)] + ZPE
[
B3LYP/6-311G(d,p)]41-43
The G3(MP2,CC) model chemistry approach normally provides chemical accuracy of 0.01–0.02
–
1
Å for bond lengths, 1–2º for bond angles, and 3–6 kJ mol for relative energies of hydrocarbons,
their radicals, reaction energies, and barrier heights in terms of average absolute deviations.42
4
4
45
The GAUSSIAN 09 and MOLPRO 2010 program packages were employed for the ab initio
calculations.
4
. RESULTS & DISCUSSION
Illustrative mass spectra collected at a photoionization energy of 9.50 eV are presented in
Figure 1 for the reactions of 5-indenyl and 6-indenyl with vinylacetylene (Figs. 1b and d) to
extract the molecular formulae of the reaction products connected to the formation of benzindene
isomers. An analysis of these data discloses the formation of C H molecules (166 amu) along
1
3
10
1
3
with their C isotopologues at m/z = 167 in both systems. Considering the molecular weight of
the reactants and the products, the C H isomer(s) are products of the reaction of the 5-/6-
1
3
10
indenyl radical with vinylacetylene followed by hydrogen atom elimination. The ion counts at
+
+
13
+
+
mass-to-charge ratios (m/z) of 242 (C H I ), 115 (C H ), and 116 (C CH , C H ) are
9
7
9
7
8
7
9
8
detectable in the control experiments as well and hence cannot be linked to the reaction of 5- or
-indenyl radicals with vinylacetylene. These species are associated with the 5- and 6-iodoindene
6
precursors (242 amu), 5-/6-indenyl radicals (115 amu), and indene (116 amu) with the indene
likely formed through recombination of 5- or 6-indenyl with a hydrogen atom in the reactor.
7

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The investigation of the mass spectra delivered convincing evidence on the formation of
C H (165 amu) and C H (166 amu) isomer(s) along with their 13C substituted species
1
3
9
13 10
C1213CH (167 amu) via the reaction of 5-/6-indenyl radicals with vinylacetylene. It is our goal
10
not only to identify the molecular formulae of the reaction products, but also to unravel which
isomers are contributing to these mass-to-charge ratios. Thus, an in-depth analysis of the
underlying PIE curves of the m/z ratios of interest (Fig. 2) is necessary to reveal the isomers
produced. PIE curves for other m/z ratios are provided in the Supporting Information (Figures S2
and S3). Each PIE curve reports the ion counts at a well-defined m/z ratio as a function of the
photon energy ranging from 7.30 eV to 10.00 eV (Fig. 2). The shapes of the PIE curves of
different C H isomers are distinct and therefore unique; these calibration curves were recorded
1
3
10
1
4
in separate experiments reported in previous works and here. A linear combination of these
base functions is used to fit the experimental PIE curves shown for m/z = 165, 166, and 167 (Fig.
2
1
). Even after scaling, these calibrated PIE curves are not superimposable suggesting that m/z =
65 does not represent a photoionization fragment of m/z = 166, but rather distinct isomer(s).
Due to the lack of reference PIE curves of any C H isomer, it is not feasible to elucidate its
1
3
9
1
3
structure(s). Considering a 1.1% natural abundance of C, 14.3 % of the ion signal of m/z = 165
+
13
+
(
C H ) could contribute to ion counts of m/z = 166 (C CH ) (Fig. 2). Fitting of the PIE
13 9 12 9
curves at m/z = 166 (C1213CH /C H ) reveals the formation of seven C H isomers: 3H-
+
+
9
13 10
13 10
cyclopenta[a]naphthalene (P1), 1H-cyclopenta[b]naphthalene (P2), 1H-cyclopenta[a]naphtha-
lene (P3), (E)-5-(but-1-en-3-yn-1-yl)-1H-indene (P4), 5-(but-3-ene-1-yn-1-yl)-1H-indene (P5),
(E)-6-(but-1-en-3-yn-1-yl)-1H-indene (P6), and 6-(but-3-ene-1-yn-1-yl)-1H-indene (P7). The
corresponding PIE curves of m/z = 167 (C1213CH ) match the linear fit of the aforementioned
+
10
1
3
seven PAH isomers and reveal that ion signal at m/z = 167 originate from their C-isotoplogue
PAHs (C H ). It should be noticed that for the PIE calibration experiments of the branched
1
3
10
indene isomers P4, P5, P6, and P7, only calibration mixtures of (E)-5-(but-1-en-3-yn-1-yl)-1H-
indene (P4) and (E)-6-(but-1-en-3-yn-1-yl)-1H-indene (P6) (40:60) as well as 5-(but-3-ene-1-yn-
1
-yl)-1H-indene (P5) and 6-(but-3-ene-1-yn-1-yl)-1H-indene (P7) (40:60) has been synthesized
and hence exploited for the calibration curve (Supporting Information). Note that actual product
branching ratios could not be derived due to unknown photoionization cross sections for any of
the C H isomers formed and hence only mechanistic information for five membered PAH ring
1
3
10
formation is reported here. For the calibration PIE curves of the tricyclic isomers
8

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cyclopentanaphthalenes (Fig. S1a-S1c), their ionization energies (IEs) lie in the range of 7.5-7.6
eV, which match the onsets of the experimental PIE measurement (7.55 ± 0.05 eV, Fig. 2).
While for the branched indene isomers (Fig. S1d-S1e), their IEs are around 7.70 eV. As the
photoionization thresholds are specific for individual species,46 the cyclopentanaphthalene
isomers are critical for the fit in the lower photon energy range, especially at the onsets of PIE
curves. Moreover, the experimental PIE curves cannot be fit without the calibration curves of the
branched isomers at photon energies higher than 7.70 eV. With the fit presented in Fig. 2, it is
clear that all the isomers are necessary for the overall fit. However, due to the two component
cis/trans mixtures of the branched isomers, it might not be concluded that all branched isomers
were detected. As a conclusion, it is tentative to claim that all the four branched indene isomers
are produced in this work. But for the tricyclic isomers, they are critical to replicate the
experimentally determined PIE curve.
To extract the underlying reaction pathways, the experimental data are corroborated with
electronic structure calculations (Figs. 3 and 4). Figure 3 reveals the potential energy surface
(PES) for the reaction of 5-indenyl with vinylacetylene. The approach of 5-indenyl to the C4 and
C1 carbon atoms of vinylacetylene is dictated by an attractive long range potential and leads to
-1
the barrierless formation of van der-Waals-complexes [i0a] and [i0b] located 10 kJ mol and 7
-1
-1
kJ mol below the energy of the separated reactants. By overcoming a barrier of 6 kJ mol to
addition, [i0a] isomerizes to intermediate [i1]; this complex represents the key intermediate
leading to the formation of the benzindene molecules. It should be stressed that a barrier to
addition does exist, but this barrier lies below the energy of the separated reactants and hence is
called ‘submerged barrier’. Therefore, the formation of [i1] from the reactants represents
essentially a barrierless entrance process. With a hydrogen shift from C4 and C6 of the indenyl
moiety to the β position of the branched C4 chain, [i1] can isomerize to [i3] and [i6],
respectively, followed by cyclization processes to intermediates [i4] and [i7]. Further [1,2]-type
hydrogen shifts from CH to the bare carbon atom in the newly formed six-membered ring lead
2
to intermediates [i5] and [i8], which contain the carbon backbones of the benzindene isomers.
Distinct hydrogen atom losses accompanied by aromatization forms 3H-cyclopenta
[a]naphthalene (P1) (from [i5]) and 1H-cyclopenta[b]naphthalene (P2) (from [i8]). Besides the
reaction sequences leading to P1 and P2, intermediate [i1] can also undergo an immediate
hydrogen atom loss to produce (E)-5-(but-1-en-3-yn-1-yl)-1H-indene (P4) by overcoming a
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-1
barrier of 168 kJ mol . All the barriers in the routes initiated by the van der-Waals complex [i0a]
lie below the separate reactants. On the other hand, [i0b] isomerizes to intermediate [i2],
followed by hydrogen atom loss yielding 5-(but-3-ene-1-yn-1-yl)-1H-indene (P5). The barrier in
-1
the process from [i0b] to [i2] is 3 kJ mol above the separated reactants indicating that this is not
a barrierless route thus requiring somewhat elevated temperatures to proceed.
Figure 4 exhibits the PES for the 6-indenyl - vinylacetylene system. The reaction mechanisms
are similar to those described in the aforementioned system of 5-indenyl reacting with
-1
-1
vinylacetylene. First, two van der-Waals complexes [i’0a] (-11 kJ mol ) and [i’0b] (-7 kJ mol )
are produced via the approach of 5-indenyl to the C4 and C1 atoms of vinylacetylene,
respectively. [i’0a] isomerizes to intermediate [i’1] by overcoming a submerged barrier of 7 kJ
-1
mol . Followed by isomerization (hydrogen shifts, cyclization) and hydrogen atom elimination,
H-cyclo-penta[a]naphthalene (P3) and 1H-cyclopenta[b]naphthalene (P2) can be produced via
1
the reaction sequences [i’1] → [i’3] → [i’4] → [i’5] → P5 and [i’1] → [i’6] → [i’7] → [i’8] →
P2, respectively. Alternatively, a hydrogen atom elimination leads [i’1] to (E)-6-(but-1-en-3-yn-
1
-yl)-1H-indene (P6). In addition, the van der-Waals complex [i’0b] may isomerize to [i’2]
-1
through a barrier of 11 kJ mol , followed by hydrogen atom loss leading to 6-(but-3-ene-1-yn-1-
yl)-1H-indene (P7). This pathway does not represent a barrierless entrance process since the
-1
barrier from [i’0b] to [i’2] lies 4 kJ mol above the energy of the separated reactants.
As the experiments were conducted under combustion-like condition at 1300 ± 10 K, the
-1
barriers of 3 and 4 kJ mol can be easily overcome. Thus, even 5-(but-3-ene-1-yn-1-yl)-1H-
indene (P5) and 6-(but-3-ene-1-yn-1-yl)-1H-indene (P7), can be produced. Recall that due to the
isomer mixture of (E)-5-(but-1-en-3-yn-1-yl)-1H-indene (P4) and (E)-6-(but-1-en-3-yn-1-yl)-
1
H-indene (P6) as well as 5-(but-3-ene-1-yn-1-yl)-1H-indene (P5) and 6-(but-3-ene-1-yn-1-yl)-
H-indene (P7), four branched indene molecules are involved in the fits of the PIE curves of the
1
C H isomers in both systems. However, based on the theoretical calculations, we may
1
3
10
conclude that P4 and P5 are only accessible in the 5-indenyl – vinylacetylene system, while P6
and P7 may only be formed in the reaction of 6-indenyl with vinylacetylene.
5
. CONCLUSION
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Our combined experimental and computational studies revealed critical mass growth processes
from aromatic aryl radicals (5-indenyl and 6-indenyl) leading to distinct tricyclic PAHs carrying
two six- and one five-membered rings: 3H-cyclopenta[a]naphthalene, 1H-cyclopenta[b]
naphthalene, and 1H-cyclopenta[a]naphthalene. The underlying reaction mechanisms involve the
formation of van-der-Waals complexes and the de-facto barrier-less addition of the carbon-
centered radicals of the 5- and 6-indenyl species to the π-electron density of vinylacetylene
reactant at the terminal carbon atoms. The resonantly stabilized free radical intermediates formed
further isomerize through hydrogen shifts and ring closure; these processes are terminated by
atomic hydrogen elimination accompanied by aromatization leading to the tricyclic PAHs in an
overall exoergic reaction. The reaction mechanisms essentially mirror the formation of the
1
2, 28, 29
naphthalene molecule (C H ) in the phenyl-vinylacetylene system
and suggest that the
1
0
8
five-membered ring in the indenyl radicals acts as a spectator. These overall routes to these
PAHs are essentially barrierless since all the transition states involved are located below the
energy of the separated reactants. The submerged barriers represent a crucial prerequisite for a
bimolecular reaction to proceed at low temperatures, since any transition state located above the
energy of the separated reactants cannot be overcome at low temperatures in cold molecular
clouds such as TMC-1 at temperatures as low as 10 K. Here, the hydrogen abstraction –
vinylacetylene addition (HAVA) pathway signifies a versatile reaction mechanism to generate
benzindene molecules through barrierless, stepwise ring expansion via elementary gas-phase
reactions of an aryl radical with vinylacetylene. The aryl radical can be formed inside molecular
clouds from the corresponding aromatic precursor via photolysis by an internal ultraviolet field.
In circumstellar envelopes of carbon stars and even under combustion conditions with
temperatures of up to a few 1,000 K, molecular mass growth processes could also be triggered by
HAVA as a facile key mechanism propelling molecular mass growth processes of PAHs. This
study potentially suggests that the five-member-ring contained PAHs can lead, via a low-
temperature propagation mechanism, to more complex 3D-structured PAHs such as corannulene
and even fullerenes observed in the galaxy.
Acknowledgments
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1

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DOI: 10.1039/D0CP03846F
This work was supported by the US Department of Energy, Basic Energy Sciences DE-FG02-
3ER15411 (experimental studies) and DE-FG02-04ER15570 (computational studies; synthesis
of the C H isomers) to the University of Hawaii (UH) and Florida International University
0
1
3
10
(FIU), respectively. W.L., O.K., and M.A. 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 of the Chemical Sciences
Division. The ALS is supported under the same contract. Ab initio calculations at Samara
University were supported by the Ministry of Science and Higher Education of the Russian
Federation under Grant No. 14.Y26.31.0020.
Author Contributions
R.I.K. designed the experiment; L.Z., W.L. and O.K. carried out the experimental measurements;
M.A. supervised the experiment; L.Z. performed the data analysis; M.M.E., E.K.B., A.D.O.,
V.N.A. and A.M.M. carried out the theoretical analysis; A.H.H. and S.F.W. synthesized the
compounds, A.M.M., and M.A. discussed the data; R.I.K., A.M.M. and L.Z. wrote the
manuscript.
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Scheme 1. Molecular building blocks of the 5-member-ring carbon skeleton (black) in indene
1), fluorene (2), 1H-cyclopenta[a]naphthalene (3), 3H-cyclopenta[a]naphthalene (4), 1H-
cyclopenta[b]naphthalene (5), corannulene (6), C -fullerene (7) and C -fullerene (8).
(
6
0
70
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•
Scheme 2. Reaction schemes: [a] phenyl (C H ) reacts with allene/methylacetylene (C H ) and
6
5
3
4
1
1
•
yields indene; [b] 2-naphthyl (C H ) reacts with allene/methylacetylene (C H ) leading to the
1
0
7
3
4
1
4
formation of 3H-cyclopenta[a]naphthalene/1H-cyclopenta[b]naphthalene (C H ); [c] 5-/6-
1
3
10
•
indenyl radicals (C H ) is proposed to react with vinylacetylene (C H ) to produce 3H-
9
7
4
4
cyclopenta[a]naphthalene, 1H-cyclopenta[b]naphthalene, and 1H-cyclopenta[a]naphthalene
C H ).
(
1
3
10
1
6

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Figure 1. Comparison of photoionization mass spectra recorded at a photon energy of 9.50 eV
with the reaction temperature at 1300 ± 10 K. (a) 5-iodoindene (C H I) – helium (He) system;
9
7
(b) 5-iodoindene (C H I) – vinylacetylene (C H ) system; (c) 6-iodoindene (C H I) – helium
9 7 4 4 9 7
(
He) system; and (d) 6-iodoindene (C H I) – vinylacetylene (C H ) system. The mass peaks of
9
7
4
4
1
3
the newly formed species along with the C-counterparts are highlighted in red.
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Figure 2. Photoionization efficiency (PIE) curves for signal at m/z = 166 and 167 in the reactions
•
of indenyl radicals with vinylacetylene. Upper panels (a and b): 5-indenyl (C H ) +
9
7
•
vinylacetylene (C H ); Lower panels (c and d): 6-indenyl (C H ) + vinylacetylene (C H ). Black
4
4
9
7
4
4
lines present experimentally derived PIE curves along with the error bars indicated in gray;
colored lines (green, blue, olive and orange) present the reference PIE curves; and red lines are
the overall fit. The overall error bars consist of two parts: ±10% based on the accuracy of the
photodiode and a 1 σ error of the PIE curve averaged over the individual scans.
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8

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Physical Chemistry Chemical Physics
•
Figure 3. Potential energy surface (PES) for the 5-indenyl (C H ) reaction with vinylacetylene (C H ) leading to the formation of 3H-
9
7
4
4
cyclopenta[a]naphthalene (P1), 1H-cyclopenta[b]naphthalene (P2), (E)-5-(but-1-en-3-yn-1-yl)-1H-indene (P4), and 5-(but-3-ene-1-
–
1
yn-1-yl)-1H-indene (P5). The relative energies are given in kJ mol .
1
9

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•
Figure 4. Potential energy surface (PES) for the 6-indenyl (C H ) reaction with vinylacetylene (C H ) leading to the formation of 1H-
9
7
4
4
cyclopenta[b]naphthalene (P2), 1H-cyclopenta[a]naphthalene (P3), (E)-6-(but-1-en-3-yn-1-yl)-1H-indene (P6), and 6-(but-3-ene-1-
–
1
yn-1-yl)-1H-indene (P7). The relative energies are given in kJ mol .
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The reaction of indenyl radicals with vinylacetylene leads to cyclopentanaphthalene at low
temperature.