Products of the Propargyl Self-Reaction at High Temperatures Investigated by IR/UV Ion Dip Spectroscopy

Article abstract of DOI:10.1021/acs.jpca.6b08750

The propargyl radical is considered to be of key importance in the formation of the first aromatic ring in combustion processes. Here we study the bimolecular (self-) reactions of propargyl in a high-temperature pyrolysis flow reactor. The aromatic reaction products are identified by IR/UV ion dip spectroscopy, using the free electron laser FELIX as mid-infrared source. This technique combines mass selectivity with structural sensitivity. We identified several aromatic reaction products based on their infrared spectra, among them benzene, naphthalene, phenanthrene, indene, biphenyl, and surprisingly a number of aromatic compounds with acetylenic (ethynyl) side chains. The observation of benzene confirms that propargyl is involved in the formation of the first aromatic ring. The observation of compounds with acetylenic side chains shows that, in addition to a propargyl- and phenyl-based mechanism, the HACA (hydrogen abstraction C₂H₂ addition) mechanism of polycyclic aromatic hydrocarbons formation is present, although no acetylene was used as a reactant. On the basis of the experimental results we suggest a mechanism that connects the two pathways.

Full text of DOI:10.1021/acs.jpca.6b08750

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Article
Products of the Propargyl Self-Reaction at High
Temperatures Investigated by IR/UV Ion Dip Spectroscopy
Philipp Constantinidis, Florian Hirsch, Ingo Fischer, Arghya Dey, and Anouk M. Rijs
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b08750 • Publication Date (Web): 08 Dec 2016
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2nd revised version, Dec. 8th, 2016
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Products of the Propargyl SelfꢀReaction at High
Temperatures Investigated by IR/UV Ion Dip
Spectroscopy
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P. Constantinidis,a) F. Hirsch, a) I. Fischer,* a) A. Dey, b) A. M. Rijsb)
a) Institute of Physical and Theoretical Chemistry, University of Würzburg, Am
Hubland, Dꢀ97074 Würzburg, Germany, phone: +49ꢀ931/31ꢀ86360
b) Radboud University, Institute for Molecules and Materials, FELIX Laboratory,
Toernooiveld 7c, 6525 ED Nijmegen, the Netherlands, phone: +31ꢀ24/3653940
Eꢀmail: ingo.fischer@uniꢀwuerzburg.de
a.rijs@science.ru.nl
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if
required according to the journal that you are submitting your paper to)
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ABSTRACT
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The propargyl radical is considered to be of key importance in the formation of the first
aromatic ring in combustion processes. Here we study the bimolecular (selfꢀ) reactions of
propargyl in a highꢀtemperature pyrolysis flow reactor. The aromatic reaction products are
identified by IR/UV ion dip spectroscopy, using the free electron laser FELIX as midꢀinfrared
source. This technique combines mass selectivity with structural sensitivity. We identified
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several aromatic reaction products based on their infrared spectra, among them benzene,
naphthalene, phenanthrene, indene, biphenyl and surprisingly a number of aromatic
compounds with acetylenic (ethynyl) side chains. The observation of benzene confirms that
propargyl is involved in the formation of the first aromatic ring. The observation of
compounds with acetylenic side chains shows that in addition to a propargylꢀ and phenylꢀ
based mechanism the HACA (hydrogen abstraction C2H2 addition) mechanism of
polycyclic aromatic hydrocarbons formation is present, although no acetylene was used as a
reactant. Based on the experimental results we suggest a mechanism that connects the two
pathways.
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INTRODUCTION
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The objective of the present work is to investigate the role of propargyl radicals in the
formation of PAH (polycyclic aromatic hydrocarbons) beyond the first ring by identifying the
reaction products formed inside a pyrolysis microꢀreactor. Small resonanceꢀstabilized
hydrocarbon radicals1 (RSRs) such as the C3H3 isomer propargyl, HꢀC≡CꢀCH2 play an
important role in PAH growth in combustion processes.2 RSRs can accumulate in flames and
contribute to molecular growth, because their reactivity towards O2 is lower than for their
nonꢀstabilized counterparts.3 In particular the recombination of propargyl is assumed to be the
main route to the formation of the first aromatic ring, i.e. phenyl or benzene, which is
considered to be a rateꢀdetermining step in PAH growth.2 Therefore considerable
computational and experimental studies are dedicated to unravel its chemistry. At high
temperature propargyl can be formed by hydrogen abstraction of allene and propyne as shown
in shock tube studies4ꢀ7 or by the insertion reaction of methylene into acetylene.2,8ꢀ10 It also
appears as a decomposition product in the pyrolysis of biomass.11ꢀ13 Recently, the stepwise
addition of acetylene to propargyl was found to constitute a pathway to indene.14
Unimolecular dissociation reactions of propargyl were investigated in the laboratory using
photoexcitation.15ꢀ19
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PAH growth is mostly explained by the HACA (hydrogen abstraction C2H2 addition)
mechanism, in which acetylene adds stepꢀwise to an aromatic hydrocarbon radical.20 As it was
found that HACA is too slow to account for the formation of large PAHs, alternative
pathways like PAC21 (phenyl addition cyclization) and oddꢀcarbonꢀpathways involving
methyl22 or cyclopentadienyl23 were suggested. Given the importance of propargyl in
combustion, it might also participate in the growth processes to larger PAHs via a C3ꢀ
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pathway. Studies on the recombination of propargyl are numerous, but they focus mostly on
the understanding of benzene (C6H6) formation.24ꢀ28 At low temperatures recombination of
C3H3 primarily leads to linear addition products, while at high temperatures and pressures
isomerization to benzene and fulvene dominates.27,29 In addition the reaction to phenyl, 2
C3H3 → C6H5 + H is relevant, but seems to be less important than benzene formation.26 The
consecutive step in a C3 pathway, the reaction of phenyl/benzene with a further propargyl
radical, could lead to another RSR, phenylpropargyl (C9H7), which was only recently
characterized spectroscopically.30ꢀ32 Previous IR/UV studies showed that it dimerizes to pꢀ
terphenyl and 1ꢀ(phenylethynyl)naphthalene (1ꢀPEN).33 Furthermore phenylacetylene (C8H6)
and phenylpropyne (C9H8) have been found as products of the propargyl selfꢀreaction,29 but
higher masses have not yet been reported. Here we apply IR/UV ion dip spectroscopy34 to
identify the aromatic products of the propargyl selfꢀreaction in a pyrolysis tube.35 The interest
in high temperature reactions36 triggered a number of studies on the chemistry occurring in
such a reactor.11,13,33,37,38 Often photoionization using synchrotron radiation has been applied
and products have been identified by their ionization energy.37,39ꢀ41 In the Discussion section
below we will compare methods applied to identify products in high temperature reactions in
more detail. Here we simply like to point out that combining the massꢀselectivity of UV
photoionization with the structural sensitivity of IR spectroscopy permits an identification of
PAH product isomers33,38 and thus complements photoionization mass spectrometry in the
identification of larger PAH.
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EXPERIMENTAL
Experiments were conducted in a differentially pumped molecular beam apparatus.34,42 High
temperature reactions were carried out by flash pyrolysis in a resistively heated SiC tube
(microꢀreactor), mounted onto a pulsed solenoid valve. Propargyl bromide (stabilized with
MgO, TCI Deutschland GmbH, purity > 97.0 %) kept at room temperature was used as
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precursor for propargyl radical generation and seeded in 1.0 bar of argon. Note that the
absence of toluene (which is often added as a stabilizer) was verified by NMR spectroscopy
(Figure S1).
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Pyrolysis conditions were chosen to maximize bimolecular reactions. The most important
parameters are the radical number density that can be influenced via the temperature of the
sample container, the pyrolysis temperature and the delay between laser and gas pulse. Early
in the gas pulse there are few bimolecular reactions and cold propargyl radicals can be
studied, while in the center of the gas pulse bimolecular reactions become relevant. All
parameters were systematically varied and reaction conditions were optimized with the aid of
photoionization mass spectrometry, using 118 nm radiation (Nd:YAG 9th harmonic) produced
in a xenon gas cell. The pyrolysis tube was heated to around 1000 °C in the experiments
described below as measured by a Type C thermocouple on the outside wall of the reactor, but
there will be a radial temperature gradient inside the reactor. The pressure on the other hand
will slightly decrease along the tube axis and sharply drop at the tube exit. Its variation inside
the tube has been simulated for continuous operation.36 It is not well characterized for pulsed
operation, but estimated to be on the order of several hundred mbar
IR/UV ion dip spectroscopy was carried out at the FELIX free electron laser laboratory43
(Radboud University, Nijmegen, the Netherlands). In this experiment the free jet is crossed by
both FELꢀIR and UV laser radiation. Molecules are ionized by the UV laser in a [1+1]
process at a fixed wavelength. The IR laser is fired about 200 ns before the UV laser. When
an IR active mode of the molecules is excited, a decrease of the UV laser ion signal (dip) is
observed, because the molecular ground state is depopulated. Pulsed UV radiation (≈ 1 mJ)
with a repetition rate of 10 Hz was provided by a frequencyꢀdoubled dye laser, pumped by a
Nd:YAG laser and operating at wavelengths of 263 nm, 270 nm or 275.3 nm. The IR laser
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was triggered at 5 Hz and scanned over the fingerprint region (550ꢀ1750 cmꢀ1) at a step width
of 2.5 cmꢀ1. The signal was obtained by dividing the UV only signal (I0) by the IR+UV signal
(I), taking the decadic logarithm and correcting for IR laser power.The final IR spectra were
obtained after composing and averaging the experimental data at each single UV wavelength.
In certain cases spectra of multiple wavelengths were averaged, which is indicated in the
figures as sum of wavelengths (e.g. “263 + 275.3 nm”). Savitzky–GolayꢀFiltering was applied
to improve the quality of the spectra. Most carriers were identified with the aid of frequency
calculations, conducted at the DFT/B3LYP/6ꢀ311G(p,d) level of theory, employing the
Gaussian09 computational chemistry software.44 The computed wavenumbers were scaled
with a factor of 0.975 and convolved with a Lorentzian function with a full width at halfꢀ
maximum (fwhm) of 10 cmꢀ1. For several molecules gasꢀphase FTꢀIR spectra were recorded
to substantiate the assignment. These measurements were performed in a highꢀtemperature
gas cell specifically designed to investigate molecules with low vapor pressure, incorporated
into a Bruker IFS120HR spectrometer.45
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RESULTS
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a) Mass Spectra
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Figure 1. Single photon ionization (SPI) mass spectrum (upper trace) and [1+1] resonance
enhanced multiphoton ionization (REMPI) mass spectrum (lower trace) of the reaction
products of the propargyl selfꢀreaction. The spectra were recorded under similar conditions in
the pyrolysis reactor.
Figure 1 shows the photoionization mass spectra of the pyrolysis products of propargyl
bromide. They were recorded at a pyrolysis temperature of around 1000 °C, which was
sufficient to fully convert the precursor. The upper trace was recorded by VUV radiation of
118 nm (10.5 eV), which ionizes most molecules in the beam in a single photon process.
While molecular ionization cross sections σion vary strongly around threshold, they become
more similar at higher energies and the value of σion around 1 eV above threshold often
approaches 10 Mb.46,47 The signal size in the 118 nm mass spectrum thus approximates the
relative concentration of species reasonably well. Mass spectra as a function of the pyrolysis
temperature are given in Figure S2 in the supporting information (SI), showing the
development of the relative peak intensities with temperature. Beside the propargyl peak at
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m/z = 39 (C3H3) a group of intense features appears in the spectrum at m/z = 78 (C6H6), 77
(phenyl) 76 (C6H4, possibly orthoꢀbenzyne) and 74 (C6H2) and corresponds to products of
propargyl dimerization followed by Hꢀ or H2 loss. Phenyl and orthoꢀbenzyne are important
reactants for subsequent PAH forming reactions. Peaks at m/z = 115 and 152 indicate that
products with three and four C3ꢀunits are formed in the reactor. Unimolecular decomposition
of propargyl to C3H2 and H leads to minor C3H2 concentrations and is considered less relevant
for the formation of higher masses.26 A particularly interesting reaction product is C4H2 (m/z
= 50), which is attributed to diacetylene. It is most likely produced by decomposition of a
larger species. Dissociative photoionisation (DPI) is unlikely because of the narrow peak
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+
width and the fact that C4H2 has not been observed as a DPI product in previous synchrotron
radiation studies of important larger species, like oꢀbenzyne (here C4H2 is generated at high
temperatures by pyrolysis) or phenylpropargyl. Below we will argue that it originates from
the thermal decomposition of C6H4. Note that species originating from dissociative ionization
will exhibit the IRꢀspectrum of the parent molecule and can thus often be identified.
Acetylene itself cannot be observed by 118 nm photoionization due to its high ionization
energy of 11.40 eV.48 Two equally intense peaks at m/z = 156, 158 might be due to C6H5Br,
possibly formed from a reaction of propargyl with the precursor propargyl bromide (C3H3Br).
Some of the masses have been observed before by Jochnowitz et al.49, who operated a SiC
microreactor under similar conditions. One should note, however that in addition to C3H3 a
bromine atom is formed in the pyrolysis, which can abstract hydrogen atoms from propargyl,
the most abundant reactive molecule in the beam, and form HBr and C3H2. HBr cannot be
detected due to its high IE of 11.68 eV. 50
In the lower trace several of the small mass peaks are absent, because twoꢀphoton ionization
requires an ionization energy (IE) below 9.43 eV. Several other smaller molecules, like
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propargyl possess no excited states around 263 nm. Thus the spectrum does not represent the
relative concentration of the pyrolysis products. On the other hand many higher mass peaks
get more prominent in the 263 nm spectrum because intermediate electronic states resonantly
enhance the ionization probability of almost all polycyclic aromatic molecules. While in the
118 nm mass spectrum growth by adding of C3 units is prominent, in the 263 nm spectrum
spacings of 24/26 or 50 amu between the mass peaks are present, already indicating an
additional growth mechanism by C2 or C4 units.
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b) IR/UV Spectra
For many of the larger molecules that appear in the mass spectra, the chemical structure
cannot be inferred from simple mechanistic pictures, because too many isomers exist. We
therefore apply IR/UV ion dip spectroscopy in combination with quantum chemical
calculations of vibrational spectra to identify aromatic pyrolysis products. When calculations
did not yield satisfactory agreement, FTꢀIR spectra of relevant molecules were recorded in a
highꢀtemperature gas cell. The UV wavelength employed for a given spectrum is indicated in
the figure.
In the next sections the most important IR/UV spectra are shown. Before the assignments are
discussed in detail, we would like to point out that the vast majority of closedꢀshell aromatic
molecules possess electronically excited states in the range between 263 nm and 275 nm, see
for example Ref. 51. Although these resonances are typically broad, they can be resonantly
excited in a [1+1] REMPI (resonance enhanced MPI) process. As previously discussed in
numerous publications, it is not necessary to excite a narrow electronic transition to observe
52ꢀ55
an IRꢀUV ion dip signal.
This is due to the fact that either there are large increases in
absorption strength or large shifts in the REMPI spectrum. Additionally multiple photon
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processes contribute to the signal because of the high IR intensity and the long pulse length of
the FEL pulse allows the absorption of a second photon after vibrational relaxation.55
Nevertheless absorption of the first IR photon constitutes the bottleneck for the process, so IR
spectra of the ground state are observed. It is only possible to record an IR spectrum that
exhibits a resonant intermediate electronic excited state at the selected UV range using IRꢀUV
ion dip spectroscopy, therefore no spectrum was recorded for propargyl itself. However, its
formation in the pyrolysis has been verified in recent experiments using synchrotron
radiation.56 In addition the signal also depends on the IR cross section and will be weak for
modes with a small change in dipole moment. Thus for m/z = 115 we also did not succeed in
recording IR/UV spectra of sufficient signal/noise (S/N) ratio. In a previous study, using a
phenylpropargyl precursor as the starting material, IR spectra of moderate S/N ratio were
recorded,33 but in the present experiments the concentration of phenylpropargyl was most
likely too small given the low IR cross section.
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Benzene and Simple Polycyclic Hydrocarbons
Figure 2. The IR/UV spectrum of m/z = 78 (upper trace) appears as a gain spectrum (upside
down) and was assigned to benzene by comparison with a laboratory spectrum (lower trace).
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Figure 2 shows the IR/UV spectrum of m/z = 78. Since only a weak ion signal is visible in the
twoꢀphoton mass spectrum at m/z = 78, a subsequently low signal/noise (S/N) ratio is
observed in the IR/UV spectrum (see upper trace of Figure 2). Benzene is the most likely
carrier of this peak, by comparison with a reference gasꢀphase IR spectrum. The IR/UV
spectrum appears here inverted, as the ion signal increases (gain) when IR radiation is present.
We therefore encounter a “hole filling” rather than a “hole burning” process.57 Vibrational
excitation in the ground state seems to increase the photoionization cross section, probably
because the S1 ← S0 transition is formally symmetryꢀforbidden and only allowed by vibronic
coupling. A comparison of the IR/UV spectrum with computations for benzene and other
important C6H6 isomers is given in Figure S3. The observation of benzene agrees with theory,
which suggests that the recombination of two propargyl radicals leads via several linear C6H6
intermediates to the thermodynamically stable cyclic isomer benzene.24,25,28,58 However, many
relevant (in particular open chain) isomers, like the primary dimerization product 1,5ꢀ
hexadiyne cannot be detected in our experiment, because they do not absorb between 260 nm
and 275 nm. Thus no information on the relative importance of the various isomers can be
derived.
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Figure 3. Naphthalene is the sole product with m/z = 128.
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The first polycyclic molecule, naphthalene (C10H8) was readily identified by its characteristic
band around 780 cmꢀ1 in the IR/UV spectrum shown in Figure 3. Several routes have been
proposed for the formation of naphthalene, for example stepꢀwise addition of two acetyleneꢀ
units to phenyl, followed by Hꢀatom loss.39 Alternatively a pathway (1) starting from two
phenyl units has been computed,59 according to the reaction
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2 C6H5
→
C10H8 + C2H2
ꢀHR = ꢀ289 kJ
⋅
molꢀ1
(1)
Note that acetylene is formed as the second product and can be consumed in further reactions.
Figure 4. Biphenyl was identified by its computational spectrum. The band at 900 cmꢀ1
indicates the possible presence of a second isomer.
Mass m/z = 154 was identified as biphenyl (Figure 4), a molecule that contains four C3 units.
It has already been observed as the main reaction product in the selfꢀreaction of phenyl
radicals in a previous study.38 Therefore its appearance in the present experiment is not
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surprising and regarded as a consequence of phenyl/benzene formation from propargyl.
Several possible pathways for the reaction have been computed.59 Beside the phenyl
recombination, the reaction of phenyl + benzene (2) and the disproportionation of benzene
and triplet orthoꢀbenzyne (3) are relevant:
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C6H6 + C6H5
→
C12H10 + H
C12H10
ꢀ
RH= ꢀ42 kJ
⋅
molꢀ1
molꢀ1
(2)
(3)
C6H6 + oꢀC6H4 (triplet)
→
ꢀRH= ꢀ503 kJ
⋅
In the spectrum given in Figure 4 a prominent band is present at ≈ 900 cmꢀ1 that is neither
described by the computations nor was observed in an earlier experiment.38 A likely carrier is
2ꢀethenylnaphthalene (Figure S4).
Figure 5. The IR/UV spectrum of m/z = 178 is assigned to phenanthrene.
The peak at m/z = 178 was assigned to phenanthrene (Figure 5). Clearly, the computed
spectrum of this isomer matches the experimental spectrum much better than other relevant
isomers (Figure S5). Phenanthrene appears in flames in significantly higher amounts than
anthracene.60,61 The reason for the selectivity of its formation is not easily understood within
the HACA model. It was explained by the contribution of a reaction path proceeding via
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acenaphthylene.62 In a recent study, Parker et al. however asserted that phenanthrene is not
formed via such a twoꢀstep HACA reaction, originating from naphthalene.63 Formally the
formation of phenanthrene could also follow from the reaction of acetylene with biphenyl.
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Figure 6. Benzo[a]anthracene was assigned by its computational spectrum.
Mass m/z = 228 has been prominently observed in previous experiments on phenyl and
phenylpropargyl. Since the mass corresponds to C18 hydrocarbons, its appearance can be
expected within a mechanism based on C3 or C6 addition. Phenylpropargyl dimerization leads
to 1ꢀ(phenylethynyl)naphthalene (1ꢀPEN),33 while the selfꢀreaction of phenyl yielded most
likely triphenylene.38 Mass 228 was also observed in a fuelꢀrich premixed toluene flame.64 In
our present study we again observe a peak at m/z = 228 (Figure 1, lower trace), which we
assign to benzo[a]anthracene (Figure 6). A comparison with computed IR spectra of other
isomers is given in Figure S6. Thus although each of the radicals investigated so far produces
selectively a hydrocarbon of mass 228, it is always a different isomer, indicating that the
reaction is kinetically controlled and not thermodynamically. Triphenylene as the suggested
product of the PAC mechanism21 can be ruled out as well as 1ꢀPEN. This indicates that the
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m/z=228 product is not formed by condensation of phenyl units, but rather by sequential
addition of C3 units.
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Figure 7. The spectrum at m/z = 116 is probably best represented by a mixture of indene and
1ꢀphenylpropyne.
The 600ꢀ850 cmꢀ1 region of the IR/UV spectrum of m/z = 116 in the top trace of Figure 7
shows typical features of indene, a common product in highꢀtemperature reactions.40
However, the bands between 1400 cmꢀ1 and 1600 cmꢀ1 are neither well represented by the
computed spectrum (Figure S7) nor by the FTꢀIR reference spectrum given in the center trace
of Figure 7. The IR/UV spectrum seems to contain the contribution of a further isomer, most
likely 1ꢀphenylpropyne. Further isomers do not contribute significantly, as can be concluded
from Figure S7. Indene constitutes the most stable isomer on the C9H8 potential energy
surface and can be formed by several routes. Under our experimental conditions the reaction
of a C6ꢀunit (benzene, phenyl or oꢀbenzyne) with propargyl will dominate.65,66 In particular
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the phenyl + propargyl radical/radical reaction, which proceeds via a phenylpropargyl
intermediate is highly exothermic.50
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9
C6H5 + C3H3 → C9H8 (indene)
ꢁHR = ꢀ517 kJ/mol
(4)
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Starting from 1ꢀ and 3ꢀphenylpropargyl radicals in a previous IR/UV study led to indene as
the sole isomer of m/z = 116.33 A new pathway to indene was recently explored and involves
consecutive addition of acetylene to propargyl.14 Matsugi et al. found that the reaction of oꢀ
benzyne with propargyl leads to indenyl as the dominant product.66 Finally openꢀchain C9H8
isomers have been observed in a VUV photoionization study on the reaction of phenyl with
propyne.40 1ꢀPhenylpropyne was also suggested to be the main C9H8 product isomer in a
GC/MS analysis of the propargyl radical selfꢀreaction.29
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Reaction products with ethynyl side chains
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Figure 8. IR/UV spectra of reaction products with ethynyl side chains. Ethynyl compounds
are identifiable by a pair of bands in the 600ꢀ680 cmꢀ1 range (CH bending fundamentals) and
an intense band around 1220 cmꢀ1 (overtone/combination band).
In addition to the polycyclic molecules discussed above, a number of further reaction products
are visible in the MPI mass spectrum, for example m/z = 102, 126, 150, 152, 176, 200, 202,
226, 250, and higher masses. A spacing of 24 amu between peaks is striking and suggests a
growth by addition of C2 (ethyne) units. Note that within the HACA mechanism the first step
is hydrogen abstraction from an aromatic molecule and formation of a radical. This step is
followed by addition of acetylene and finally loss of another Hꢀatom. All the IR/UV spectra in
Figure 8 exhibit a prominent band around 1220 cmꢀ1 and a pair of equally intense bands in the
600ꢀ680 cmꢀ1 range.
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Figure 9. 1,4ꢀdiethynylbenzene was identified by its gasꢀphase FTIR spectrum. An
anharmonic frequency computation illustrates the contribution of fundamental, overtone and
combination bands.
In the case of the m/z=126 product, simple computations for possible carriers do not yield a
match for the 1220 cmꢀ1 band, while the peak is observed in the gasꢀphase FTꢀIR spectrum of
1,4ꢀdiethynylbenzene (center trace of Figure 9). This band is typical for terminal alkynes and
is caused by overtones and/or combination bands of the acetylenic ≡CH bending mode.67,68
Figure 9 depicts the comparison between experiment and calculation for 1,4ꢀ
diethynylbenzene. To account for the overtone/combination band, we conducted anharmonic
calculations, applying generalized 2nd order vibrational perturbation theory (GVPT2), as
implemented in the Gaussian09 package.44 A good fit of the experimental spectrum was
achieved using a hybrid scheme69, where anharmonic frequency computations on the
(B3LYP/6ꢀ311g++(g,d)) level were combined with high level harmonic computations
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(B2PLYP/augꢀccꢀpVTZ). The various isomers of diethynylbenzene can be distinguished by
the 830 cmꢀ1 band, which is redꢀshifted in the 1,2ꢀ and 1.3ꢀisomers and in phenyldiacetylene
(Figure S8). Note that this comparison was based on harmonic calculations of the isomers.
Although small contributions cannot be ruled out, the 1,4ꢀisomer dominates the spectrum. The
absence of the 1,2ꢀisomer can either be explained by steric hindrance or by a Bergmann
cyclization, which consumes this isomer by ultimately forming naphthalene.70
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Proceeding in a similar fashion several of the other massꢀselected spectra assembled in Figure
8 can be assigned. The spectrum of m/z = 102 is assigned to phenylacetylene (Figure S9). In
general phenylacetylene is a typical product of the HACA mechanism and formed via
C6H5 + C2H2 → C8H6 + H
(5)
Alternatively it can emerge from the reaction of oꢀbenzyne (C6H4) with acetylene.71
One of the most interesting masses is m/z = 152, which has been observed before in a number
of high temperature experiments. The reaction of 1ꢀnaphthyl and 2ꢀnaphthyl with acetylene
was assumed to yield primarily acenaphthylene, beside 1ꢀ or 2ꢀethynylnaphthalene.63,72 It also
appears as product of oꢀbenzyne dimerization, where it was assumed to correspond to
biphenylene.73 In contrast our IR/UV spectra provide compelling evidence for the presence of
an alkynyl side chain in the compound. The best match is given by the 2ꢀethynylnaphthalene
isomer, as demonstrated from a gasꢀphase IRꢀspectrum of the pure compound (Figure 10).
Figure S10 shows the 1ꢀethynylnaphthalene isomer provides an inferior match, while
acenaphthylene and biphenylene can be excluded as carriers of the spectrum. In our prior
study on the selfꢀreaction of phenyl radicals m/z = 152 could also not be attributed to either
biphenylene and acenaphthylene.38 With the present information, we can now assign m/z=152
to an ethynyl compound. Interestingly recent computations suggested ethynylnaphtahelene to
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play an important role in PAH growth above 1000 K.62 The small naphthylethylene
contribution visible in the IR spectrum of m/z = 154 (vide supra) might also be connected to
2ꢀethynylnaphthalene via a H2 addition or loss process.
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Figure 10. M/z = 152 is assigned to 2ꢀethynylnaphthalene based on comparison with a gasꢀ
phase IR spectrum (center trace) and computations (bottom trace).
Assignment of the higher masses becomes successively more difficult. For m/z = 150 the
possible carriers are either 1,2,4ꢀtriethynylbenzene or (4ꢀethynylphenyl)diacetylene based on
the absorption bands at 1480 cmꢀ1 and 830 cmꢀ1 (Figure S11). Mass 176 might originate from
naphthalene substituted with one diacetylene group or two ethynyl groups, while m/z = 200
corresponds to addition of a third ethynyl group to a naphthyl radical (see Figure S12 for the
IR/UV spectra) and m/z = 202 to a phenanthrenyl with one acetylene unit added (Figure S13).
Due to the large numbers of possible isomers, we cannot assign the carriers unambiguously.
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DISCUSSION
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Before we start to discuss the results we would like to compare IR/UV spectroscopy to other
methods that have been applied to detect products of highꢀtemperature reactions. Each method
has its unique strengths and weaknesses: Haefliger and Zenobi51 applied [1+1] REMPI to the
identification of PAH molecules and compared it to GC/MS. The disadvantages of the latter
are the comparably large necessary sample amounts and the laborious sample preparation. Its
advantage over the highly sensitive REMPI on the other hand is the availability of
quantitative information. Furthermore REMPI relies on the existence of a UV chromophore.
As an alternative VUV oneꢀphoton ionization is increasingly applied to detect products in
pyrolysis tubes,39,40,63 chemical reactors74 or flames.75 It is not as sensitive as REMPI, but a
fully universal detection scheme, because every molecule can be ionized. Sufficiently above
the ionization threshold the ionization cross sections are similar enough to allow extracting
relative concentrations. While isomer sensitivity has been accomplished for small molecules,
it is on the other hand difficult to distinguish isomers of larger molecules, like PAH. Here
photoelectronꢀphotoion coincidence spectroscopy offers distinct advantages. It has been
employed to monitor the chemistry inside a pyrolysis source,76,77 and recently been applied to
flames78, but whether isomers of PAH can be distinguished remains to be seen. As an
alternative IR/UV spectroscopy comes in. While not applicable to small molecules that cannot
be electronically excited with common dye lasers, isomerꢀselective identification of aromatic
molecules and PAH is straightforward. It thus offers unique benefits and supplements the
methods mentioned above. In particular when the results are combined with those from oneꢀ
photon ionization it constitutes a powerful and versatile approach. An example is the
molecule with m/z=152 (see Figure 10), which is identified as 2ꢀethynylnaphthalene.
Acenaphtylene and biphenylene can be ruled out, as can be seen in Figure S10.
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The HACA mechanism, i.e. addition of acetylene to hydrocarbon radicals followed by Hꢀ
atom loss is still considered to be the main path to PAHs, although computations suggest that
it can only explain part of the PAH formation.62,79 Alternative models were developed, relying
for example on reactions of phenyl radicals,21 which lead to polycyclic molecules in a few
steps. Considering that C3H3 plays an important role in the formation of the first aromatic
ring, one can assume C3ꢀhydrocarbons to be also effective in PAH growth. While several
computational studies investigated the reaction of C3H3 with oꢀbenzyne66, phenyl, benzene65,
benzyl66 and naphthyl or naphthalene80, which all lead to PAH molecules, there is a lack of
experimental data that provide benchmarks for these computations. The aim of this study was
to identify the most important product isomers and provide such a guideline. Computations
will then help to extract mechanistic information on these reactions.
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Scheme 1. The propargyl radical and the reaction products of its selfꢀreaction identified by
IR/UV ion dip spectroscopy (solid boxes). Small aliphatic hydrocarbons, phenyl and benzyne
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have not been detected by IR/UV, but are supposed to participate in the growth process based
on mass spectrometric detection (dashed box).
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Scheme 1 summarizes the identified reaction products. While the oneꢀphoton mass spectrum
is dominated by groups of peaks separated by C3 units (m/z = 76/77/78, 115, 152/154, 190),
the [1+1] twoꢀphoton mass spectrum shows series of peaks separated by C2ꢀunits (m/z = 102,
126, 128, 150, 152, 176, 178, 200, 202, 226, 228). The carriers were identified by IR/UV ion
dip spectroscopy to be (1) oꢀfused 6ꢀringꢀPAHs like naphthalene and phenanthrene and (2)
compounds with ethynyl (ꢀC2H) side chains, which supports the presence of an acetylene
addition pathway. Interestingly no cyclopentaꢀfused aromatic molecules were observed,
which were considered to be markers for the HACA mechanism.70.
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Benzene is the sole identifiable C6H6 isomer in our experiment, but as noted above isomers
that do not absorb in the range between 260 nm and 275 nm like 1,5ꢀhexadiyne will not be
detected in the experiment. Phenyl (C6H5) and oꢀbenzyne (C6H4) are formed by successive H
or H2 loss and constitute important reactants in PAH growth, due to their high reactivity. The
peaks at m/z = 74 (C6H2) and m/z = 98 (C8H2) indicate in addition the formation of polyynes.
Biphenyl (m/z = 154) and benzo[a]anthracene (m/z = 228) originate from further
condensation. Since the m/z=228 product observed here is different from the one found in the
previous study on phenyl condensation (triphenylene),38 it is probably formed by successive
addition of C3 species rather than a condensation of phenyl radicals (PACꢀmechanism). Other
products of the PAC mechanism such as terphenyls are not observed. To form even larger
PAH during the 10ꢀ100 ꢁs residence time in the reactor81 the concentration of propargyl and
phenyl are not high enough.
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The first species beyond benzene in a possible C3 growth pathway is m/z = 115 (C9H7), which
shows an intense ion signal in the oneꢀphoton mass spectrum (Figure 1). While in a former
study the IR/UV spectra of 1ꢀ and 3ꢀphenylpropargyl (m/z = 115) were recorded and assigned
after selective generation from a bromide precursor,33 m/z = 115 could not be unambiguously
identified here. The high reactivity and presumably short excitedꢀstate lifetimes make
resonant ionization difficult. Yet the observation of indene (m/z = 116) is an indicator for the
presence of C9H7 isomers in the pyrolysis. Dimerization products of phenylpropargyl have not
been observed.
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The large number of peaks separated by 24 amu in the [1+1] mass spectrum indicates the
presence of an additional C2 growth process. The formation of several products with ethynyl
side chains suggests reactions with acetylene in a HACAꢀtype reaction. We want to
emphasize that signal strength in [1+1] ionization depends strongly on the resonance
enhancement, thus the ionization probability significantly increases for larger aromatic
molecules, which all resonantly absorb at around 263 nm.51 The smaller molecules that
dominate the 118 nm spectrum, like propargyl itself, possess no intermediate states in this
wavelength that can be resonantly excited.
Since in contrast to earlier studies based on photoionization detection no acetylene was added
to the reactant,39,63 the occurrence of these products is nevertheless surprising. Therefore, we
considered alternative explanations for their appearance. For example, formation of
phenylacetylene by a reaction of benzene (6) or phenyl (7) with propargyl, according to
C6H6 + C3H3 → C8H6 + CH3
C6H5 + C3H3 → C8H6 + CH2
ꢁHR = +30 kJ/mol
ꢁHR = +15 kJ/mol
(6)
(7)
is endothermic and thus thermochemically unlikely (ꢁHR was calculated from ꢁHf values,
obtained from Ref 50). In addition no CH3 is observed in the 118 nm mass spectrum. For
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comparison, the formation of indene according to (4) is highly exothermic. Shafir et al.
observed phenylacetylene in their propargyl study in a heated quartz reactor and explained it
by a reaction of phenyl with propyne.29 However, in their experiments propyne was used as a
propargyl precursor and thus present in considerable amounts. Other investigations of the
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C9H9, C9H8 65, C9H7 PES did not provide evidence for a fragmentation pathway leading to
molecules with ethynyl side chains, nor did other computational studies of propargyl
reactions.66,80 The challenge for the interpretation of the experimental results is therefore to
identify a possible acetylene source that is responsible for the formation of HACAꢀtype
reaction products.
We suggest m/z = 76 (benzyne) as a possible acetylene source, because its high intensity in
the mass spectrum is remarkable. Jochnowitz et al. observed C6H4 along with C6H6 in a
similar reactor as soon as the reaction conditions were suitable for propargyl dimerization.49
In our experiments this mass seems to be associated with the formation of larger products,
m/z = 115, 152 (76 + 39, 2 × 76). An explanation for its appearance could be Hꢀloss from
phenyl or H2ꢀloss from benzene yielding oꢀbenzyne. Both processes would carry away the
excess energy available after propargyl dimerization. The reaction channel leading from
propargyl directly to phenyl + H was considered less relevant in the temperature range of
1100 – 2100 K,26 but our experiments were conducted at the lower end of this range. In fact
Hꢀatom loss is the dominant decomposition pathway of phenyl radicals.59 A further route to
C6H4 might proceed via propargylene, a possible propargyl dissociation product,82 because the
dimerization of propargylene has been observed experimentally, the dimer probably being an
openꢀchain compound.83 Linear C6H4 could also be formed directly by
abstraction/elimination of linear C6H6, the initial reaction product of propargyl dimerization.
H
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Computational studies found two high temperature decomposition pathways for oꢀ
benzyne:84,85
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C6H4 → C4H2 + C2H2
C6H4 → C6H2 + H2
(9)
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(10)
Reaction enthalpies, rate constants and branching ratios for (9) and (10) have been
computed.85 Thus C6H4, which appears with significant intensity in the mass spectrum in
Figure 1, constitutes an acetylene source via reaction (9). The second product, diacetylene,
C4H2 could be involved in the addition of C4 units, an alternative pathway to some of the
products observed. Furthermore the signal at m/z = 74 can be explained by reaction (10).
Fragmentation of oꢀbenzyne (and/or other C6 species) could thus provide the reactants for
several of the observed reaction products. It would also link a pathway based on C3 or phenyl
addition with the HACA mechanism. The hypothesis that C6H4 plays a central role will be
tested in future experiments, exploring the bimolecular chemistry of oꢀbenzyne in a chemical
reactor.
SUMMARY AND CONCLUSION
Products of the selfꢀreaction of propargyl in a pyrolytic flow reactor were investigated in situ
by IR/UV ion dip spectroscopy. This method combines the massꢀselectivity of resonant
photoionization with the structural sensitivity of IR spectroscopy and thus complements
VUVꢀphotoionization for establishing the chemical structure of aromatic reaction products.
Benzene was observed, confirming the role of propargyl dimerization in the formation of the
first aromatic ring. Beside PAH molecules like naphthalene and phenanthrene that are
expected to form in a high temperature environment, two further classes of products were
detected, (a) molecules whose formation can be described by the addition of C3 or C6 units
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and (b) molecules with ethynyl side chains which are expected to be formed in a HACAꢀlike
process. Indene, 1ꢀphenylpropyne and biphenyl indicate the further participation of propargyl
in the formation of hydrocarbons beyond benzene in a C3 growth pathway. Aromatic
molecules with ethynyl side chains were identified by characteristic vibrational bands at
around 1220 cmꢀ1 and 600ꢀ680 cmꢀ1 that are due to the acetylenic CH bending mode. The
appearance of molecules with acetylenic side chains is surprising, because the experiments
were performed using a clean propargyl bromide sample without any coꢀreagent. Their
formation has not been detected before in the propargyl selfꢀreaction. The 118 nm
photoionization mass spectrum displays an intense peak at m/z = 76 that might be assigned to
oꢀbenzyne, thought to be a further product of propargyl dimerization. Decomposition of C6H4
to C4H2 + C2H2 is known to be a major decomposition pathway at elevated temperatures.
Acetylene produced in this reaction can participate in PAH growth in the pyrolysis reactor.
Our results thus suggest a close connection between a propargylꢀ and phenylꢀbased pathway
and a HACA pathway to polycyclic aromatic hydrocarbons. Orthoꢀbenzyne is a possible
candidate for this connection. The hypothesis will be tested in an upcoming study on the selfꢀ
reaction of oꢀbenzyne.
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ACKNOWLEDGEMNTS:
This work was supported by the German Science
Foundation, DFG through contract FI 575/8ꢀ2. Furthermore the research leading to
these results has received funding from the European Community's Seventh
Framework Programme (FP7/2007ꢀ2013) under grant agreement no. 312284. We
gratefully thank the FELIX staff for their experimental support and we acknowledge
the Stichting voor Fundamenteel Onderzoek der Materie (FOM) for the support of the
FELIX laboratory (project no. N2300N).
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SUPPORTING INFORMATION PARAGRAPH
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A comparison of IR/UV spectra with additional computations, discussing alternative
assignments, and full ref. 12 and 44 are given in the supporting information. This material is
available free of charge via the Internet at http://pubs.acs.org.
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