Formation of fulvene in the reaction of C2H with 1,3-butadiene

Article abstract of DOI:10.1016/j.ijms.2014.08.025

Abstract Products formed in the reaction of C₂H radicals with 1,3-butadiene at 4 Torr and 298 K are probed using photoionization time-of-flight mass spectrometry. The reaction takes place in a slow-flow reactor, and products are ionized by tunable vacuum-ultraviolet light from the Advanced Light Source. The principal reaction channel involves addition of the radical to one of the unsaturated sites of 1,3-butadiene, followed by H-loss to give isomers of C₆H₆. The photoionization spectrum of the C₆H₆ product indicates that fulvene is formed with a branching fraction of (57 ± 30)%. At least one more isomer is formed, which is likely to be one or more of 3,4-dimethylenecyclobut-1-ene, 3-methylene-1-penten-4-yne or 3-methyl-1,2-pentadien-4-yne. An experimental photoionization spectrum of 3,4-dimethylenecyclobut-1-ene and simulated photoionization spectra of 3-methylene-1-penten-4-yne and 3-methyl-1,2-pentadien-4-yne are used to fit the measured data and obtain maximum branching fractions of 74%, 24% and 31%, respectively, for these isomers. An upper limit of 45% is placed on the branching fraction for the sum of benzene and 1,3-hexadien-5-yne. The reactive potential energy surface is also investigated computationally. Minima and first-order saddle-points on several possible reaction pathways to fulvene + H and 3,4-dimethylenecyclobut-1-ene + H products are calculated.

Full text of DOI:10.1016/j.ijms.2014.08.025

International Journal of Mass Spectrometry 378 (2015) 232–245
Contents lists available at ScienceDirect
International Journal of Mass Spectrometry
journal homepage: www.elsevier.com/locate/ijms
Formation of fulvene in the reaction of C2H with 1,3-butadiene
Jessica F. Lockyear a,1, Martin Fournier b,c, Ian R. Sims b, Jean-Claude Guillemin c,
Craig A. Taatjes d, David L. Osborn d, Stephen R. Leone a,
*
a
Departments of Chemistry and Physics, University of California at Berkeley, and Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
Institut de Physique de Rennes, UMR CNRS-UR1 6251, Université de Rennes 1, 263 Avenue du Général Leclerc, 35042 Rennes CEDEX,France
Ecole Nationale Supérieure de Chimie de Rennes, CNRS UMR 6226, 11 Allée de Beaulieu, CS 50837, 35708 Rennes CEDEX 7, France
Combustion Research Facility, Mailstop 9055, Sandia National Laboratories, Livermore, CA 94551-0969, USA
b
c
d
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 30 May 2014
Received in revised form 21 July 2014
Accepted 11 August 2014
Available online 20 August 2014
Products formed in the reaction of C2H radicals with 1,3-butadiene at 4 Torr and 298 K are probed using
photoionization time-of-flight mass spectrometry. The reaction takes place in a slow-flow reactor, and
products are ionized by tunable vacuum-ultraviolet light from the Advanced Light Source. The principal
reaction channel involves addition of the radical to one of the unsaturated sites of 1,3-butadiene, followed
by H-loss to give isomers of C6H6. The photoionization spectrum of the C6H6 product indicates that
fulvene is formed with a branching fraction of (57 Æ30)%. At least one more isomer is formed, which is
likely to be one or more of 3,4-dimethylenecyclobut-1-ene, 3-methylene-1-penten-4-yne or 3-methyl-
1,2-pentadien-4-yne. An experimental photoionization spectrum of 3,4-dimethylenecyclobut-1-ene and
simulated photoionization spectra of 3-methylene-1-penten-4-yne and 3-methyl-1,2-pentadien-4-yne
are used to fit the measured data and obtain maximum branching fractions of 74%, 24% and 31%,
respectively, for these isomers. An upper limit of 45% is placed on the branching fraction for the sum of
benzene and 1,3-hexadien-5-yne. The reactive potential energy surface is also investigated
computationally. Minima and first-order saddle-points on several possible reaction pathways to
fulvene + H and 3,4-dimethylenecyclobut-1-ene + H products are calculated.
Keywords:
Combustion chemistry
Astrochemistry
Fulvene
Benzene
Polycyclic aromatic Hydrocarbons
Photoionization
Published by Elsevier B.V.
1. Introduction
Titan [2,8] and in numerous combustion and flame studies [9,10];
thus, many reactions have been postulated to explain its formation.
There are many parallels between combustion chemistry and
the chemistry of planetary atmospheres and interstellar space. In
these environments, reactions of radicals with hydrocarbons lead
to molecular-weight growth and formation of polycyclic aromatic
hydrocarbons (PAHs). Further, it is often the same radicals that are
thought to be responsible for these important processes in such
varied environments. PAHs are significant in astrochemistry
because they are complex molecules whose formation may give
us clues to the chemistry of a prebiotic earth [1,2]. In combustion,
the mechanism of PAH formation is investigated in order to curtail
it, because PAHs and soot emitted from combustion engines [3–5]
are significant pollutants.
In addition, it is vital to know the pre-cyclization chemistry as well
as the critical ring-forming reactions to understand the generation
of PAHs [4]. In combustion environments, propargyl (C3H3)
recombination is generally thought to be one of the main benzene
formation pathways [4,11–13]. However, several studies have
shown that benzene is a minor, or non-existent, product of
propargyl recombination at low temperatures and pressures
[12,14]. The C3H3 + C3H3 route to C6H6 can only proceed by
relatively inefficient radiative association in the very low pressures
of astrochemical environments and so alternative mechanisms
must be invoked. Another popular candidate for benzene forma-
tion is the reaction between the 1,3-butadien-1-yl radical (n-C4H5)
and acetylene (C2H2), which undergoes an addition–elimination
reaction to yield C6H6 + H [3,4,15]. However, there are conflicting
accounts of the importance of C4H5 in benzene formation, not the
least of which is that 1,3-butadien-2-yl (i-C4H5) is more stable than
n-C4H5, but it is preferable to have the latter for facile benzene
formation [4]. In 1989 Westmoreland et al. outlined 18 suggested
pathways to benzene formation in combustion [9]. Despite all
current and past efforts, even the most rigorous combustion
Benzene (C6H6), the simplest closed-shell aromatic ring, is
likely to be a key primary reactant in PAH chemistry and it has been
detected in the circumstellar envelopes [6,7], the atmosphere of
* Corresponding author. Tel.: +1 510 643 5467.
E-mail addresses: jess.lockyear@gmail.com (J.F. Lockyear), srl@berkeley.edu
(S.R. Leone).
1
Tel.: +1 5103295313.
http://dx.doi.org/10.1016/j.ijms.2014.08.025
1387-3806/Published by Elsevier B.V.

J.F. Lockyear et al. / International Journal of Mass Spectrometry 378 (2015) 232–245
233
models available are unable to accurately predict concentrations of
species such as C6H6, C5H6, and C8H6[16]. For example, the
concentrations of 1-butyne (CH3CH2CCH), 2-butyne (CH3CCCH3),
1,3-butadiene (CH2CHCHCH2), C4H5, C4H4, C4H3 and 1,3-butadiyne
(C4H2) from a butanol flame experiment are not well replicated by
the associated model and the authors of this study state that this
discrepancy arises because the base C4 chemistry has not been
thoroughly validated for recombination reactions that lead to
highly unsaturated C4 and larger hydrocarbons [16].
A promising species in the search for cyclization reactions is
the ethynyl radical (C2H), which is abundant in many astronomi-
cal environments [17–19]. The C2H radical readily undergoes
barrierless addition–elimination reactions with unsaturated
hydrocarbons [20]. The fact that these reactions are barrierless
is important in low temperature astrochemical environments,
where entrance barriers are difficult or impossible to overcome.
Reactions of C2H have been implicated in the generation of long
chain polyynes that have been detected in the interstellar
medium (ISM) [6,21,22]. Moreover, the eliminated moiety in
the addition–elimination reactions of this radical is often a
hydrogen atom, meaning that C2H reacting with C4H6 yields
isomers of C6H6 + H. Thus, C2H has been suggested by Kaiser and
co-workers to be a critical species in benzene formation in the
ISM [23]. Although in combustion environments C2H is present at
might expect a small branching fraction for benzene, with the
remainder of the products being 1,3-hexadien-5-yne.
Despite the likely importance of C2H reactions with C4H6
isomers, very few other studies examining them have been
published. The reasons for this are manifold, including the
technique chosen to produce the C2H radical. Generally, in flow
tube experiments, C2H is formed via 193 nm photolysis of C2H2, or
occasionally 193 nm photolysis of trifluoromethylacetylene
(CF3CCH). Recent work in our group has employed 193 nm
photolysis of C2H2 and CF3CCH to determine the products and
rates of reactions of C2H with acetylene [32], ethene [33], propene
[33], butenes [34], allene [35] and propyne [35], among others.
However, polyunsaturated and acetylenic C4 (and higher) hydro-
carbons tend to absorb strongly at 193 nm, so there are concurrent
photodissociation yields of radicals that can interfere with the
reaction of interest or give rise to secondary reactions that
complicate the analysis. Our group has reported one experimental
study of C2H with an isomer of C4H6, specifically 1-butyne [36],
which employed 193 nm photolysis of C2H2 to generate the radicals
and in which the contribution of products due to 1-butyne
photolysis was carefully examined and accounted for. Clearly a
method that avoids photodissociation of the molecular co-reactant
is preferable. At 300 K, 1,3-butadiene absorbs strongly at 193 nm,
but only weakly at 248 nm. Consequently, for the experimental
work here, a different C2H precursor was synthesized, bromoa-
cetylene (BrC2H), which photolyses efficiently at 248 nm to give
Br + C2H [37]. Details of the synthetic method are given in
Section 2.
Determining the ratio between straight chain and cyclic isomer
products of the title reaction is valuable for understanding
astrochemical and combustion chemistry. By synthesizing bro-
moacetylene, an excellent C2H precursor when photolysed at
248 nm, problems with interfering dissociative products of the 1,3-
butadiene are minimized. Obtaining product masses is achieved by
tunable synchrotron ionization, time-of-flight mass spectrometry.
This method is a powerful multiplexed technique that allows the
identification of the product isomers formed. In the following
sections, the experimental techniques are described and the
recorded reaction spectra are presented. These results are followed
by a discussion of the extracted branching fractions and possible
reaction pathways, calculated at the CBS-QB3 level. Finally, the
implications of current results for our understanding of astro-
chemical and combustion environments are discussed.
concentrations that are too small to make
a substantial
contribution to soot formation, C2H is formed via several
reactions: between the hydroxyl radical (OH) and C2H2 to form
water (H2O) and C2H [4], between C3H3 and oxygen atoms (O) to
form formaldehyde (CH2O) + C2H, between C3H3 and methyl
radicals (CH3) to form ethyl radicals (C2H5) + C2H [24], or between
C3H3 and methylene radicals (CH2) to form ethene (C2H4) + C2H
[11]. In addition, C2H can be formed by the reaction of hydrogen
atoms (H) with C2H2 at higher temperatures [5,25].
Recently, Mebel and co-workers have carried out an extensive
ab initio investigation into the reactions of C2H with unsaturated
hydrocarbons, including all isomers of C4H6 (1,3-butadiene [23],
1,2-butadiene [20], 1-butyne and 2-butyne [26]). In a combined
crossed-molecular beams and computational study, Jones et al.
suggested that benzene is formed in the reaction between C2H and
1,3-butadiene [23], at kinetic energies higher than thermal. In the
crossed-molecular beams experiment products were ionized by
electron impact at 80 eV, and isomer identification was achieved
by examination of product translational energy distributions. The
experimental part of the study indicated a branching fraction for
benzene of 30%, with the other 70% of products being 1,3-
hexadien-5-yne. The theoretical study found a benzene branching
fraction between 40% and 20% at collision energies between
0 kJ molÀ1 and 45 kJ molÀ1. To our knowledge, this is the only
experimental report of the products of the C2H + 1,3-butadiene
reaction, and therefore it is valuable to investigate this reaction via
additional methods.
The reaction of the cyano radical (CN), which is isoelectronic
with C2H, with 1,3-butadiene has also been studied [27]. Previous
studies of the reactions of the isoelectronic C2H and CN radicals
with unsaturated hydrocarbons have shown that they react
similarly, both in terms of reaction rate and major products
[28–31]. Thus, one might expect the outcome of the CN + 1,3-
butadiene reaction to provide clues to the C2H + 1,3-butadiene
reaction mechanism. CN + 1,3-butadiene proceeds via addition and
H-elimination to yield isomers of C5H5N. Crossed-molecular beams
experiments suggest a maximum of 3–6% pyridine and 94–97% 1-
cyano-1,3-butadiene is formed under single collision conditions.
Rice–Ramsperger–Kassel–Marcus (RRKM) calculations predict
only 0.02% pyridine formation at 0 eV collision energy, with a
maximum of 6% at the limits of the errors of the calculations [27]. If
C2H + 1,3-butadiene were analogous to CN + 1,3-butadiene, one
2. Experimental details
The experimental technique comprises several components.
Synthesized BrC2H gas is mixed with 1,3-butadiene and flowed
through a quartz tube housed in a vacuum chamber. Photodissoci-
ation of BrC2H forms Br + C2H, which react with the closed-shell
1,3-butadiene. Gases exit the reactor through a pinhole and are
ionized with tunable vacuum ultra-violet light in a differentially
pumped chamber. The mass-analysis of the products is carried out
using a time-of-flight mass-spectrometer. Details of the spectrom-
eter and the synthesis procedure will be given in the following
sections. In addition, the computational methodologies employed
for supporting interpretation of the data are also described.
2.1. Multiplexed photoionization mass spectrometer
The C2H + 1,3-butadiene reaction takes place in
a laser
photolysis, slow flow reactor coupled to a multiplexed photoioni-
zation mass spectrometer at the Advanced Light Source of
Lawrence Berkeley National Laboratory. Comprehensive details
of the experimental apparatus are given elsewhere [38–40] and
only an overview is presented here. The reactions occur inside a

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J.F. Lockyear et al. / International Journal of Mass Spectrometry 378 (2015) 232–245
quartz tube with 1.05 cm inner diameter at 4 Torr (533 Pa) and
298 K. The gas flow consists of small amounts of the C2H radical
precursor, BrC2H, and 1,3-butadiene in a large excess of helium
buffer gas. Dilutions of BrC2H and 1,3-butadiene in helium are
prepared in stainless steel cylinders with a total pressure of
2000 Torr. The gases are mixed in situ at the entrance of the reactor
using calibrated mass flow controllers. The total flow is 100 sccm
(standard cubic centimeters per minute), sufficient to completely
replenish the gas in the reactor tube between consecutive laser
pulses (4 Hz repetition rate). Reactor pressure is maintained with a
butterfly valve after the reactor under active feedback control.
A uniform initial concentration of C2H radicals is produced in the
axialandradialdirectionsofthereactortubeby248 nmphotolysisof
BrC2H using the unfocused beam of an excimer laser. The typical
photolysis fluence inside the reaction flow tube is ꢀ70 mJ cmÀ2 in a
20 ns pulse. There is effectively no attenuation of the photon flux
along the length of the flowtube due to absorption, asverified by the
stable concentrations of products of photolytically-initiated reac-
tions. The initial BrC2H and 1,3-butadiene number densities are both
6.5 Â1013 cmÀ3 in a total gas density of 1.3 Â1017 cmÀ3. Depletion of
the BrC2H signal at t = 0 when the laser fires is not apparent in the
time profile for this species above the signal-to-noise ratio and
therefore the percentage depletion of the radical precursor is
estimated to be <1%. Fangtong et al. employed 193 nm BrC2H
photodissociation in a crossed-molecular beams experiment and
indicated that approximately 6% of the BrC2H is decomposed into
Br + C2Hwithalaserfluenceofꢀ400 mJ cmÀ2 [41].Theabsorptionof
248 nm light by BrC2H is reported to be around 30 times lower than
that of 200 nm light [42]. Due to the absence of absorption data at
193 nm, the assumption is made that the absorption at 193 nm is
similar to 200 nm. Correcting for the laser fluence and different
cross-section at 200 nm and 248 nm, an estimate of 0.035% for
percentage of BrC2H that photodissociates at 248 nm is calculated.
Thus, an estimate for the number density of the C2H radicals of
ꢀ2 Â1010 cmÀ3 is derived.
each photon energy. Full time- and photon-energy-dependent
spectra are collected three times to check for consistency, with
200 laser pulses at each VUV photon energy.
2.2. Bromoacetylene synthesis
The C2H precursor in our experiment was bromoethyne
(bromoacetylene, BrC2H), which was synthesized according to
the methodology of Bashford et al [43]. Synthesis was carried out
in
a triple-necked round-bottomed flask in a temperature-
controlled water bath, connected to a water-cooled condensing
column and a phosphorus pentoxide drying column. 10 g (50% w/
w) sodium hydroxide solution was placed in the flask. The
synthesis apparatus was purged of air using a helium flow and
covered in aluminum foil to prevent any photolytic decomposition
of the BrC2H product. 15 g of 1,2-dibromoethylene and 18 ml of
ethanol were injected into the flask. The flask was then heated to
85 ꢁC for one hour, whilst maintaining a slow helium flow. Reaction
products were collected in a liquid nitrogen cooled cold trap.
Purification of BrC2H was carried out using three cold traps,
covered in aluminum foil, attached in series and connected to a
rotary pump, which attained a pressure of 100 m Torr. The temper-
atures of the three successive cold fingers were À65ꢁC, À115 ꢁC and
À196 ꢁC.Theimpureproductwaspumpedthroughthetrapsandwas
isolated in pure form at À115 ꢁC and then allowed to sublime into a
stainless steel cylinder with internal teflon coating. Helium was
addedtothecylindertomakea5%mixtureofBrC2H;adilutemixture
is used to reduce the rate of degradation and polymerization.
The vapor pressure of BrC2H was measured to be greater than
100 Torr at 298 K. A very high degradation rate is observed when
exposed to light in solid state, and there is a high degradation rate
at elevated temperatures, even in gaseous phase. Oxygen reacts
extremely violently with BrC2H and induces explosive polymeri-
zation [43]. The purity of the sample was checked with time-of-
flight mass spectrometry using the multiplexed photoionization
mass spectrometer (see Section 2.2) and the mass spectrum
showed very little impurity.
A portion of the gas escapes from the flow tube through a
650 mm diameter pinhole in the side of the tube into a chamber at a
pressure of 10À5 Torr. The nearly effusive beam from this pinhole is
skimmed by a 1.5 mm diameter skimmer before entering a
differentially pumped ionization region. The gas beam is crossed
by tunable synchrotron undulator vacuum ultraviolet (VUV)
radiation that is dispersed by a 3 m monochromator (Chemical
Dynamics Beamline at the Advanced Light Source of Lawrence
Berkeley National Laboratory). The monochromatic VUV radiation
energy is calibrated using known atomic resonances of Xe; at the
conditions employed in these experiments the VUV light has a
bandwidth of 20 meV. The VUV photon flux at each photon energy
is measured using a calibrated photodiode (SXUV-100 Interna-
tional Radiation Detectors, Inc.). The masses of all cations formed
in the ionization region are monitored with an orthogonal
acceleration time-of-flight mass spectrometer equipped with a
micro-channel plate detector. Mass spectra are taken at intervals of
2.3. Computational methodology
Identification of the isomers produced in the reaction relies on
fitting the measured product photoionization spectra to known
spectraofthepurecompounds.Incaseswherethesespectrahavenot
been measured, due to the instability of the molecules, the spectra
are simulated using computational methods. The photoionization
spectrum of a molecule is computed by integrating the Franck–
Condon factors between the vibrational modes of the ground and
ionic states [44]. Employing the Gaussian 09 suite of programs [45],
adiabatic ionization energies (AIEs) are calculated using the CBS-
APNO or CBS-QB3 composite methods [46]. The geometries,
harmonic vibrational modes and force constants for the molecules
of interest are calculated at the B3LYP/cc-pVTZ level, and photoelec-
tronspectraaresimulatedmakinguseofthebuilt-inFranck–Condon
overlapfeatureofGaussian09[47,48]. Note thatthis methodis based
on the assumption of direct ionization to the lowest-lying cationic
state and does not describe autoionizing and shape resonances,
dissociative ionization, and ionization to excited cationic states.
Where applicable, the semiempirical method of Bobeldijk et al [49].
is used to estimate the absolute photoionization cross sections.
Photoionization spectra used in the following analysis, simulated or
experimental, are given in the Supporting Information.
20
m
s, from À20 ms to 130 ms relative to photolysis. By additionally
scanning the photon energy, time- and photon energy-resolved
mass spectra are recorded, leading to a three-dimensional matrix
of data consisting of ion intensity as a function of m/z (mass-to-
charge ratio), reaction time, and photon energy.
The data can be examined in three ways: m/z vs. kinetic time, m/
z
vs. photon energy and photon energy vs. kinetic time.
Photoionization spectra are constructed by integrating the data
first over the desired m/z range and then over a suitable time
window that corresponds to production of the species of interest in
the reaction. Background contributions are removed by subtrac-
tion of the average pre-photolysis signal taken in the 20 ms before
the photodissociation laser pulse. Finally, these background-
subtracted signals are normalized for the VUV photon flux at
Minima and first-order saddle points on the C2H + 1,3-butadi-
ene potential energy surface (PES) are explored using the M06-2X
method [50] with a 6-31G* basis set, with energies calculated using
the CBS-QB3 composite method [51,52]. The quantum-chemical
calculations are performed using Gaussian 09 [45].

J.F. Lockyear et al. / International Journal of Mass Spectrometry 378 (2015) 232–245
235
[(Fig._1)TD$FIG]
Fig. 1. Top panel is the background subtracted time-dependent mass spectrum integrated over the 8.2–11 eV photon energy range with the radical precursor BrC2H and the
closed-shell reactant 1,3-butadiene present in the flow tube. The products that are dependent on the photolysis laser pulse (t = 0) appear in the spectrum. Bottom panel shows
a one-dimensional mass spectrum integrated over the 0–100 ms time range.
3. Results
that there are more species in the flow that can react with the
radical. The peak at m/z = 50 is predominantly due to photolysis of
3.1. Experimental results
BrC2H, with a very small contribution from 1,3-butadiene
photolysis, Fig. 2(b). The component of the m/z = 50 signal that
is due to 1,3-butadiene photolysis has a slower rise time than that
for the total signal when both reactants are present and the signal
is due to BrC2H photolysis. This slower rise time suggests that the
source of this species is a secondary reaction rather than primary
photolysis of 1,3-butadiene. Fig. 2(c) shows that the peak at m/
z = 52 is primarily due to photolysis of 1,3-butadiene, yet the
concentration of this species increases when both reactants are
entrained in the flow. The relative areas of the peaks at m/
z = 52 with only 1,3-butadiene in the flow and with both reactants
in the flow reveal that approximately 73% of the products at this
mass arise from photolysis of 1,3-butadiene. Possible sources of the
remaining 27%, which arises only when both reactants are present,
will be discussed later. The time profiles for m/z = 78 shown in
Fig. 2(d) reveal that the majority of the signal, approximately 90%,
only appears when both reactants are present in the flow. When
only 1,3-butadiene is present, a small signal appears at m/
z = 78 that has a very slow rise time that accounts for no more
than 10% of the total when both reactants are present. This signal is
likely due to the recombination of C3H3 radicals (m/z = 39) that are
formed by 1,3-butadiene photolysis, a process that has been
observed before [53], or a reaction of C3H3 with 1,3-butadiene.
Nevertheless, it is clear that formation of C6H6 (m/z = 78) is the
predominant reaction channel in the C2H + 1,3-butadiene reaction.
The peak at m/z = 50 has been identified by its photoionization
spectrum as 1,3-butadiyne [54] (C4H2), Fig. 3(a). Upon photolysis of
BrC2H, 1,3-butadiyne forms by reaction of the photolytically
produced C2H radical with the radical precursor via the mechanism
outlined in Scheme 1 in Fig. 4 [55]. The peaks at m/z = 39 and
52 have been identified as propargyl radicals [56] (C3H3), Fig. 3(b),
and vinyl acetylene [54] (C4H4), Fig. 3(c), respectively. Few studies
have dealt with the fate of 1,3-butadiene following excitation at
248 nm. However, numerous studies explored the excitation and
photolysis of 1,3-butadiene in the 193–220 nm range, fromwhich it
is found that the primary dissociative channel leads to formation of
C3H3 + CH3 [14,57–60]. In addition to C3H3 + CH3, the following
minor photolysis channels have also been reported in the 193–
220 nm energy range: C4H5 + H, C4H4 + H2, C2H3 + C2H3 and C2H2 +
C2H4 with varying relative ratios [14,59,60]. The 1,3-butadiene
cross-section for absorption at 248 nm is nearly two orders of
C2H + 1,3-butadiene reaction data were recorded over the 8.2–
11.5 eV photon energy range, the À20–130 ms kinetic time range
and up to 160 atomic mass units. Fig. 1 shows the background-
subtracted product mass spectrum integrated over the 8.2–11 eV
photon energy range as a function of kinetic time, along with the
one-dimensional mass spectrum resulting from integration of the
time-dependent spectrum over the 0–100 ms range. Four main
peaks are present in the background subtracted mass spectrum, at
m/z = 39, 50, 52 and 78, corresponding to molecular formulae C3H3,
C4H2, C4H4 and C6H6, respectively. The peak at m/z = 79 is
approximately 8% of the magnitude of the peak at m/z = 78, 6.1%
of which is due to the presence of 12C513CH6 at the natural
abundance. The remaining 1.9% is likely due to detection of C6H7,
the reaction adduct, which may undergo radiative or collisional
stabilization.
Mass spectra were also recorded at a single photon energy of
10.6 eV with only BrC2H or only 1,3-butadiene in the flow. These
data inform us of any direct photolysis products of the reactants, or
secondary reactions involving these photolysis products that do
not require both reactants entrained in the flow. One-dimensional
mass spectra recorded with only BrC2H or 1,3-butadiene entrained
in the flow are given in the Supplementary Information and
compared to a spectrum recorded when both reactants were
entrained in the flow; these mass spectra are not background
subtracted. When only BrC2H is present in the flow, a peak at m/
z = 50, due to detection of a species with molecular formula C4H2, is
observed. When only 1,3-butadiene is present in the flow, four
peaks appear in the spectrum: at m/z = 39, m/z = 50, m/z = 52 and m/
z = 78 due to detection of species with the molecular formulae
C3H3, C4H2, C4H4 and C6H6, respectively. The background
subtracted temporal evolutions of these four signals, at m/z = 39,
50, 52 and 78, are examined and shown in Fig. 2. In Fig. 2, the solid
black and dotted blue lines show the evolution of the signal when
only BrC2H and only 1,3-butadiene, respectively, are present in the
flow and the dashed red line is the time profile of the signal when
both reactants are present.
Fig. 2(a) shows that m/z = 39 is solely due to 1,3-butadiene
photolysis. The depletion rate of the m/z = 39 concentration when
both reactants are present (dashed red line) is faster due to the fact

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J.F. Lockyear et al. / International Journal of Mass Spectrometry 378 (2015) 232–245
[
[(Fig._3)TD$FIG]
Fig. 3. Photoionization spectra of the species detected at (a) m/z = 50, (b) m/
z = 39 and (c) m/z = 52. The red solid lines show the photoionization spectra of (a)
1,3-butadiyne, (b) propargyl and (c) vinyl acetylene. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
mass-spectrometer means detection of species with m/z = 39 is
nearly twice as likely as species with m/z = 15 [56]. The majority of
the signal at m/z = 52, vinyl acetylene, is due to 1,3-butadiene
photolysis, Scheme 3 in Fig. 4, or secondary reactions of photolysis
products, but a small additional amount is due to the presence of
both BrC2H and 1,3-butadiene in the flow. Vinyl acetylene could
potentially be a product of C2H + 1,3-butadiene reaction via a
Fig. 2. Background subtracted time profiles for m/z = 39, 50, 52 and 78 with only
BrC2H and He in the flow (solid black line), with only 1,3-butadiene and He in the
flow (dotted blue line) and with both BrC2H and 1,3-butadiene (and He) in the flow
(dashed red line). (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
magnitude smaller than at 193 nm and certain channels are
energetically inhibited, such as formation of two vinyl radicals
(C2H3) by cleavage of the central CÀÀC bond. It has been shown that
in the 220–260 nm range, photolysis of 1,3-butadiene leads to
formation of CH3 radicals (in conjunction with C3H3) with a
branching of 75% [61]. Propargyl radicals are formed in conjunction
with CH3 (Scheme 2 in Fig. 4), although methyl is not detected in
our experiment. The absence of CH3 signal is likely due to the fact
that the photoionization cross-section of CH3 is much smaller than
that of propargyl radicals; at 10.413 eV, for example, it is around 4
times lower [56]. In addition, the mass discrimination factor of the
mechanism of addition and
b
-CÀÀC bond scission to yield vinyl
acetylene + C2H3, Scheme 4 in Fig. 4. However, vinyl radicals (m/
z = 27) are not observed above the background level in the mass
spectra, perhaps because they rapidly dissociate or react.
The current experiment shows the predominant channel in the
C2H + 1,3-butadiene reaction to be addition of the radical to one of
the unsaturated carbon sites of 1,3-butadiene followed by H-loss to
give C6H6. The fact that the amount of vinyl acetylene (m/z = 52)
detected increases by 27% when both BrC2H and 1,3-butadiene are
present in the flow suggests that a pathway to vinyl acetylene

J.F. Lockyear et al. / International Journal of Mass Spectrometry 378 (2015) 232–245
237
[(Fig._4)TD$FIG]
Fig. 4. Scheme 1: Reaction mechanism for formation of 1,3-butadiyne via photolysis of BrC2H at 248 nm to give C2H + Br, followed by reaction of the C2H with BrC2H and Br
elimination. Photolysis of 1,3-butadiene by 248 nm light to give C3H3 + CH3 (Scheme 2) and vinyl acetylene + H2 (Scheme 3). Scheme 4: Possible reaction mechanism for
production of vinyl acetylene + vinyl radicals from the reaction of C2H with 1,3-butadiene.
(C4H4) + vinyl radicals (C2H3) may also represent a small fraction of
the reactivity. In addition, the peak at m/z = 79 has an area that is
31% larger than expected from 12C513CH6 alone, suggesting that
some reactive intermediates are stabilized radiatively or by
collisions. Based solely on the peak areas of these respective
signals, and not accounting for the absolute photoionization cross-
sections of the constituent species, the process that results in m/
z = 78 accounts for 96% of reactivity, that resulting in vinyl
acetylene (m/z = 52) accounts for 4% and the proportion of
intermediates that are stabilized is 0.2%. To identify isomers
formed, the measured photoionization spectrum of the products at
m/z = 78 is examined. As shown earlier, there is a contribution to
the peak at m/z = 78 when only 1,3-butadiene is present in the flow,
arising from C3H3 recombination. This C3H3 recombination
accounts for less than 10% of the total signal at m/z = 78 when
both reactants are present in the flow. The photoionization
spectrum of the C6H6 products from C3H3 recombination [62]
has a significantly different shape than that of the m/z = 78

238
J.F. Lockyear et al. / International Journal of Mass Spectrometry 378 (2015) 232–245
[(Fig._5)TD$FIG]
Fig. 5. Black open circles show the measured photoionization spectrum for the products of the C2H + 1,3-butadiene reaction at m/z = 78. The dark blue dotted line shows the
photoionization spectrum for fulvene, which fits the measured spectrum up to a photon energy of 8.85 eV, as shown in (a). In (b), the dashed green line shows the
experimental photoionization spectrum of 3,4-dimethylenecyclobut-1-ene and the solid red line shows the fit to the data, constructed by adding weighted contributions of
the fulvene and 3,4-dimethylenecyclobut-1-ene spectra. The fit in (c), shown by the solid red line, is constructed by adding a contribution of 3-methylene-1-penten-4-yne, the
photoionization spectrum shown by the pale blue line, to the fulvene component. The fit in (d), shown by the solid red line, is constructed by adding a contribution of 3-
methyl-1,2-pentadien-4-yne, the photoionization spectrum shown by the orange dot-dashed line, to the fulvene component. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)
products observed in this experiment. Correcting the spectrum for
the products of the C2H + 1,3-butadiene reaction for the 10%
contribution from C3H3 recombination has a negligible effect on
the shape of the measured spectrum. Nevertheless, this corrected
spectrum is used in the following analysis. The onset of the
measured photoionization spectrum of the products of C2H + 1,3-
butadiene with m/z = 78 is at 8.36 eV, which matches exactly with
the ionization onset of fulvene [63]. Indeed, the measured
photoionization spectrum can be reproduced by the experimen-
tally measured photoionization spectrum of fulvene [36] up to a
photon energy of ꢂ8.85 eV, Fig. 5(a). From the deviation of the
measured signal from the fulvene photoionization spectrum above
8.85 eV, it is evident that additional isomers must contribute to the
overall signal. There are three isomers of C6H6 that have ionization
onsets in the 8.85 eV region; these are 3,4-dimethylenecyclobut-1-
ene (8.8 eV [63]), 3-methylene-1-penten-4-yne (no experimental
value available, calculated in this work using the CBS-APNO
methodology to be 8.89 eV [64]) and 3-methyl-1,2-pentadien-4-
yne (no experimental value available, calculated in this work using
the CBS-APNO methodology to be 8.90 eV [64]). It seems likely that
one or more of these isomers is formed.
fulvene, as shown in Fig. 5(c), a good fit of the measured spectrum
can be obtained up to a photon energy of around 9.5 eV. The
photoionization spectrum of 3-methylene-1-penten-4-yne has not
been measured experimentally and so a simulated spectrum
reported by Soorkia et al. [36] is used in the current analysis. These
simulations often reproduce the experimental spectra for the
transition from the ground electronic state of the neutral to the
ground electronic state of the cation. However, excited electronic
states of the cation and autoionization are not considered in the
calculation and so the simulated spectrum may not have the same
shape as the real spectrum at higher photon energies. For this
reason it is difficult to determine whether the deviation of the fit
from the measured spectrum in Fig. 5(c) at 9.5 eV is due to
formation of other isomers with higher ionization energies or due
to photo-excitation to excited electronic states of 3-methylene-1-
penten-4-yne. Finally, a good fit to the measured spectrum can also
be obtained if a contribution of 3-methyl-1,2-pentadien-4-yne is
included, in addition to the fulvene component, Fig. 5(d). An
experimental photoionization spectrum of 3-methyl-1,2-penta-
dien-4-yne is not available and so was simulated using the
methodology outlined in the Experimental section. Using fulvene
and 3-methyl-1,2-pentadien-4-yne, the fit begins to deviate from
the data at around 9.75 eV, which could be due to excited electronic
states of the 3-methyl-1,2-pentadien-4-yne cation or additional
isomers being formed.
A good fit to the measured photoionization spectrum can be
obtained with a weighted contribution of the experimentally
measured photoionization spectra of fulvene [36] and 3,4-
dimethylenecyclobut-1-ene [36], as shown in Fig. 5(b). Adjusting
the scaling factors for the absolute photoionization cross-sections
of these two isomers yields branching ratios of 51% fulvene and
49% dimethylenecyclobut-1-ene. Alternatively, if a contribution
from 3-methylene-1-penten-4-yne is included in addition to
Benzene has vertical ionization energy of 9.244 eV [63]. There is
not a distinct rise in the measured photoionization spectrum at
9.244 eV, thus, it seems that benzene is not a major product of the
title reaction. 1,3-hexadien-5-yne has an ionization energy of

J.F. Lockyear et al. / International Journal of Mass Spectrometry 378 (2015) 232–245
239
[
[(Fig._7)TD$FIG]
Fig. 6. Black open circles show the measured photoionization spectrum of the C6H6
products of the C2H + 1,3-butadiene reaction at 4 Torr and 298 K. Dark blue dashed
line shows the experimental photoionization spectrum of fulvene. Pale blue dotted
line shows the simulated photoionization spectrum of 3-methylene-1-penten-4-
yne. Green dot-dashed line shows the experimental photoionization spectrum of
Fig. 7. Large red circles show the time profile of the C6H6 product integrated over
the 8.2–10.5 eV range. Small black circles show the time profile of the fulvene
product, which is obtained by integrating over the 8.2–8.6 eV range. Although the
signal-to-noise of the fulvene-only signal is low, the risetime is consistent with that
of the sum of all C6H6 isomers and is inconsistent with contributions from H-atom
assisted isomerization. The laser fires at t = 0. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)
benzene. The solid red line shows
a fit to the data constructed using an
appropriately weighted sum of each of these calibration photoionization spectra
(dark blue, pale blue and green lines). (For interpretation of the references to colour
in this figure legend, the reader is referred to the web version of this article.)
sum of the PI spectra of each isomer multiplied by a scaling factor.
The scaling factors need to be adjusted for the absolute
photoionization cross-section of each isomer. An uncertainty of
50% is estimated for the cross-sections calculated using the semi-
empirical method of Bobeldjik et al. [49], and an uncertainty of 20%
is estimated for the absolute cross-section of benzene, which has
been experimentally measured. Adjusting the scaling factors for
the absolute photoionization cross-section of each isomer, a
branching fraction for the fulvene product with an associated error
and maximum possible branching fractions for 3,4-dimethylene-
cyclobut-1-ene, 3-methylene-1-penten-4-yne and 3-methyl-1,2-
pentadien-4-yne are calculated and given in Table 1. Also shown in
Table 1 is the maximum possible branching fraction for the sum of
benzene and 1,3-hexadien-5-yne, given the significance of the
benzene molecule in a variety of environments. The ground state
reaction exothermicities for formation of each proposed isomer + H
from C2H + 1,3-butadiene, calculated at the CBS-QB3 level of
theory, are also given in Table 1.
9.2 eV [63], very close to that of benzene. Reasonable fits to the
measured photoionization spectrum can be made with weighted
contributions of both of these isomers, but only benzene will be
discussed further, given its significance in combustion and
astrochemical environments. However, since two possible combi-
nations of isomers can reproduce the data, the maximum
branching fraction extracted is for a sum of benzene and 1,3-
hexadien-5-yne. As shown in Fig. 6, a fit to the measured spectrum
can be obtained with a combination of the experimental PI
spectrum of fulvene [36], the simulated spectrum of 3-methylene-
1-penten-4-yne and the experimental PI spectrum of benzene [54].
The scaling factors for each PI curve that give the best fit to the data
are adjusted for the absolute photoionization cross-section of each
species, yielding a maximum possible branching for the sum of
benzene and 1,3-hexadien-5-yne.
It is clear from the product photoionization spectrum that the
dominant product of the C2H + 1,3-butadiene reaction is fulvene.
The identity of the other isomers formed includes one or a
combination of 3,4-dimethylenecyclobut-1-ene, 3-methylene-1-
penten-4-yne and 3-methyl-1,2-pentadien-4-yne. Using the ex-
perimental or simulated photoionization spectra available at this
time, the measured product PI spectrum can be reproduced by the
The results strongly indicate that under the conditions of this
experiment (4 Torr and 298 K), fulvene has a branching fraction of
(57 Æ30)%. The maximum branching fractions for the other
[F(ig._8)TD$FIG]
Table 1
Branching fraction for fulvene and upper limits for the branching fractions for 3,4-
dimethylenecyclobut-1-ene, 3-methylene-1-penten-4-yne, 3-methyl-1,2-penta-
dien-4-yne and the sum of benzene and 1,3-hexadien-5-yne derived from fitting
the measured photoionization spectrum of m/z = 78 in the reaction of C2H + 1,3-
butadiene. Also given are the CBS-QB3 ground state reaction exothermicities for
forming the specific product isomer + H from C2H + 1,3-butadiene.
Product isomer
Branching
fraction/%
Reaction exothermicity (including H atom
co-product)a/kJ molÀ1
Fulvene
3,4-
57 Æ 30
À247
Dimethylenecyclobut-1-ene
<74
À123
<24
3-Methylene-1-
penten-4-yne
3-Methyl-1,2-
pentadien-4-yne
Benzene
À101
À59
<31
<45
À384
À116
1,3-Hexadien-5-yne
Fig. 8. Schematic of the potential energy profile showing the enthalpy vs reaction
coordinate, illustrating four possible reaction pathways to fulvene formation in the
C2H + 1,3-butadiene reaction.
a
All reaction exothermicities are ground state and calculated at the CBS-QB3
level of theory.

240
J.F. Lockyear et al. / International Journal of Mass Spectrometry 378 (2015) 232–245
[(Fig._9)TD$FIG]
isomers. The reaction of C2H + 1,3-butadiene may be a significant
source of cyclic molecules in combustion and astrochemical
environments.
Since the reactions take place in a collisional environment, it is
necessary to verify that collisions with species other than He in the
flow do not alter the product distribution. To verify that H-assisted
isomerization, or any other secondary process, does not change the
isomer distribution, the signal rise time of the fulvene product is
compared to the signal rise time for the m/z = 78 peak at all photon
energies. By integrating the data over the 8.2–8.6 eV photon energy
range the temporal behavior of the fulvene component can be
examined without contribution from other product isomers. It is
found that the rise time of the fulvene product after the laser pulse
is the same as the rise time for the peak as a whole, Fig. 7,
confirming that fulvene formation is not a result of H-assisted
isomerization from another product, at least not on the millisecond
time scale. The initial C2H concentration (and hence the H-atom
and C6H6 concentrations) is estimated to be 2 Â1010 cmÀ3, so the
characteristic time for H-atom assisted isomerization even for an
unrealistically large rate coefficient of 10À9 cm3 sÀ1 would be in
excess of 50 ms. In addition, the photoionization spectrum was
examined in 5 ms intervals over the 0–100 ms kinetic time range;
the shape of the spectrum, and therefore the isomer distribution,
remained the same over the entire range. Another point to address
is the possibility of unimolecular isomerization of the C6H6
product. Isomerization of the closed-shell products formed by the
H-loss involves prohibitively high energetic barriers and does not
occur in the current experiment at 298 K.
Fig. 9. Schematic of the potential energy profile showing the enthalpy vs reaction
coordinate, illustrating four possible reaction pathways to 3,4-dimethylenecyclo-
but-1-ene formation in the C2H + 1,3-butadiene reaction.
possible cyclic isomers, 3,4-dimethylenecyclobut-1-ene and ben-
zene, are estimated to be 74% and 45%, respectively. What is
intriguing about these values is that the branching fraction for
fulvene and the maxima for 3,4-dimethylenecyclobut-1-ene and
benzene are significant, and could potentially account for nearly all
products formed, suggesting that cyclization reactions can
compete favorably with direct H-loss to give straight-chain
[(Fig._10)TD$FIG]
Fig. 10. Schematic of the addition reactions of C2H on the unsaturated sites of 1,3-butadiene, 1,2-butadiene, 1-butyne and 2-butyne. The most facile inter-conversions
between the initial adducts are shown. Transition state energies relative to the infinitely separated C2H and 1,3-butadiene reactants are in kJ molÀ1. Also given are the energies
of the infinitely separated C2H and 1,2-butadiene, 1-butyne and 2-butyne relative to the infinitely separated C2H and 1,3-butadiene in kJ molÀ1
.

J.F. Lockyear et al. / International Journal of Mass Spectrometry 378 (2015) 232–245
241
3.2. Computational results
cyclization isomerization and a secondary H-shift, followed by
H-loss.
A quantum chemical investigation of possible reaction mecha-
nisms resulting in fulvene and 3,4-dimethylenecyclobut-1-ene
was carried out. The principal aim of the quantum chemical
calculations is to uncover reaction pathways to fulvene and 3,4-
dimethylenecyclobut-1-ene formation that may be able to
compete with the pathways to 1,3-hexadien-5-yne and benzene
formation outlined by Jones et al. Structures of stationary points
and transition states were determined at the M06-2X/6-31G* level
and energies computed using the CBS-QB3 composite method.
These calculations do not attempt to predict the products of the
title reaction quantitatively, but they do provide auxiliary
information to the experimental results obtained. Four energeti-
cally possible pathways to fulvene formation and four pathways to
3,4-dimethylenecyclobut-1-ene formation in the C2H + 1,3-butadi-
ene reaction were found. Schematics of the energy profile of these
pathways as a function of the reaction coordinate are given in
Figs. 8 and 9.
C2H can add to either the terminal or central unsaturated
carbon atoms of 1,3-butadiene, leading to two structurally
different encounter complexes, termed E1 and E2, respectively.
E1 and E2 have energies of À286 kJ molÀ1 and À213 kJ molÀ1
relative to the asymptote for the infinitely separated reactants.
Transformation of E1 to E2 and vice versa can occur via a 3-
membered-ring isomer, as shown schematically in Fig. 10. In
addition, E1 could directly lose an H atom to yield 1,3-hexadien-5-
yne or 1,2-hexadien-5-yne and E2 could lose an H atom to yield 3-
methylene-1-penten-4-yne. Alternatively, isomerization of E1 or
E2 via H-shifts or cyclization could occur. The simplest pathway to
fulvene formation that was found involves cyclization of E1 via a
low barrier that is 90 kJ molÀ1 above the energy of E1, a single H-
shift and H-loss to yield fulvene + H, denoted by the thick black line
in Fig. 8. Three other possible fulvene-formation pathways were
also determined that involve isomerization of E1 via H-shift as
their first step; these pathways are denoted by the green, blue and
red curves in Fig. 8 and correspond to 2,1-, 2,3- and 1,3-H-shifts,
respectively. These three alternate pathways involve an additional
H-shift step via transition states that lie high in energy compared
to E1 and also to the barriers involved in the first pathway
described. Consequently, it seems very likely that the simplest
pathway, involving cyclization prior to H-shift, would be more
likely to account for the fulvene formation in the C2H + 1,3-
butadiene reaction than the pathways involving H-shift prior to
cyclization.
Four energetically possible reaction pathways to 3,4-dimethy-
lenecyclobutene-formation were also found, two from addition of
the C2H radical to the terminal carbon atom and two from addition
of the radical to the central carbon atom of 1,3-butadiene, Fig. 9. All
routes to 3,4-dimethylenecyclobutene that have been calculated
involve a primary H-shift isomerization of E1 or E2 followed by
cyclization, a secondary H-shift and finally H-loss. E1 can undergo
4,1- or 2,1-H-shift to yield straight-chain radicals that can undergo
cyclization reactions to give the 4-membered ring backbone of 3,4-
dimethylenecyclobut-1-ene. The cyclic intermediate then under-
goes a secondary H-shift, followed by H-loss. The 2,1-H-shift has a
very high barrier, lying only 12 kJ molÀ1 below the energy of the
separated reactants and so it is postulated that this route is
unlikely to play an important role in 3,4-dimethylenecyclobut-1-
ene formation compared to the pathway that involves 4,1-H-shift.
E2 could isomerize via a shift of the H that was originally bound to
the central carbon atom on which the radical addition took place to
the neighboring terminal position. Alternatively, one of the H
atoms bound to the terminal carbon furthest from the site of
radical addition could shift to the opposite terminal position. The
radicals resulting from these H-shifts could then undergo a
4. Discussion
To understand the experimental results obtained in this study
of the C2H + 1,3-butadiene reaction, it is helpful to put them in the
context of the PES considering multiple C4H6 isomers. Despite the
fact that the isomeric form of the reactant C4H6 dictates where the
C6H7 surface is accessed, which may influence the product
distributions, these starting points are inherently connected, as
illustrated in Fig. 10. Only the simplest inter-conversions (fewest
steps) between the primary adducts are given in Fig. 10, although
others exist. CBS-QB3 energies for each transition state involved in
the transformations shown in Fig. 10 have been calculated relative
to the C2H + 1,3-butadiene ground state asymptote. Conversion of
the initial adduct of terminal addition of C2H on 1,3-butadiene (E1)
to the initial adduct of terminal addition on 1,2-butadiene (E3)
occurs via a single H-shift with a transition state that lies
23.6 kJ molÀ1 below the asymptote for the separated C2H + 1,3-
butadiene reactants. The dominant products of the C2H + 1,2-
butadiene reaction are predicted by Jamal et al. to be 3-methylene-
1-penten-4-yne + H [20]. However, CH3-loss is also predicted by
Jamal et al. to occur in the C2H + 1,2-butadiene reaction, giving
isomers of C5H4 with a branching fraction between 9 and 16% of the
reactivity [20]. The fact that C5H4 is not detected above the signal-
to-noise ratio in the current experimental study of the C2H + 1,3-
butadiene reaction implies that the inter-conversion via a H-shift
from the initial adduct of the C2H + 1,3-butadiene terminal addition
to the initial adduct of the C2H + 1,2-butadiene terminal addition is
unfavorable. Conversion of E1 and E2 to the initial adducts of the
C2H + 1-butyne or 2-butyne reactions involves at least two H-shifts
and so one might expect these conversions to be less favorable than
to the initial adducts of the C2H + 1,2-butadiene reaction. Indeed,
the C2H + 2-butyne and C2H + 1-butyne reactions are predicted by
Jamal et al. to yield high branching fractions of C5H4 isomers [26],
and the experimental study of the C2H + 1-butyne reaction by
Soorkia et al. found a branching of 56% for C5H4 [36], whereas no
C5H4 isomers are detected in this study. In summary, it seems that
E1 and E2 do not isomerize to explore the areas of the C6H7 PES
accessed in the C2H + 1,2-butadiene, 1-butyne and 2-butyne
reactions. These insights indicate that isomerizations of the
adducts E1 and E2 via H-shift are not favorable under the current
conditions.
Mebel and co-workers have published ab initio investigations of
the C2H reaction with all non-cyclic isomers of C4H6 (1,3-
butadiene, 1,2-butadiene, 1-butyne and 2-butyne) [20,23,26]. All
of the branching ratios reported in each of those papers are given in
Table 2. The experimentally obtained branching fractions in the
C2H + 1-butyne reaction reported by Soorkia et al. [36], are also
given in Table 2. The results of Soorkia et al. were obtained by
employing the same experimental apparatus as in the present
study. The branching fractions for the C2H + 1-butyne reaction
reported by Soorkia et al [36]. are significantly different from the ab
initio results of Jamal et al [26]. In another computational study by
Mandal et al. [65], the experimental findings of Soorkia et al. are
corroborated and reaction pathways to each detected isomer are
found. Unfortunately, overall branching fractions are not given in
the publication of Mandal et al.
The experimentally derived branching ratios in the C2H + 1,3-
butadiene reaction presented here, particularly the high branching
fraction for fulvene, are not in agreement with the experimental
and computational results published by Jones et al [23]. Some
possible reasons for these disagreements are discussed here, and,
clearly, there are still unanswered questions in C2H + C4H6 reaction
systems that merit further investigation. Firstly, the branching

242
J.F. Lockyear et al. / International Journal of Mass Spectrometry 378 (2015) 232–245
Table 2
Previously published experimental and theoretical branching fractions for the products of C2H reactions with 1,3-butadiene, 1,2-butadiene, 1-butyne and 2-butyne.
Product channel
C2H + 1,3-butadiene [23]
(experiment and theory, Jones
et al.)
C2H + 1,2-butadiene [20] C2H + 2-butyne [26]
C2H + 1-butyne [26]
(theory, Jamal et al.)
C2H + 1-butyne [36]f
(experiment, Soorkia et al.)
(theory, Jamal et al.)
(theory, Jamal et al.)
Benzene + H
40a-20b,30b,c
0
0
0e
0
0e
<10
–
1,3-Hexadien-5-yne + H 60a-80b, 70b,c
3-Methylene-1-penten- 0e
4-yne + H
3.6a-2.4d
57a-36d
90.8a-83.68d
0.4a-0.5d
2.4
Penta-1,4-diyne + CH3
1,2-Pentadien-4-
yne + CH3
1,4-Hexadiyne + H
3-Methyl-1,4-
0e
0e
6.98a-11.86d
1.78a-3.38d
98.6a-97.7d
0
0e
–
32.9a-42.6d
56
0e
0e
0.10a-0.26d
0.06a-0.32d
0e
0e
0e
0e
–
–
pentadiyne + H
Diacetylene + C2H5
0e
0.12a-0.18d
0.04a-0.1d
0.02a-0.08d
0
0
4.2a-15.5d
1.3a-1.2d
0
–
2,3-Hexadien-5-yne + H 0e
8.4
–
3-Methyl-1,2-
0e
0e
1.0a-1.7d
pentadien-4-yne + H
1,3-Hexadiyne + H
0
0
0.9a-1.8d
4.2
14
5.4
9.6
Methyldiacetylene + CH3 0e
0e
0e
0e
0e
0
0.03a-0.2d
Fulvene + H
3,4-
0e
0e
0
0
0
Dimethylenecyclobut-
1-ene + H
1,2,3,4-Hexatetraene + H 0e
1,2-Hexadien-5-yne + H 0e
1,2-Hexadien-4-yne + H 0e
0
0
0
0e
0e
0e
0e
0e
0e
–
–
–
a
At zero collision energy.
b
At 45 kJ molÀ1 collision energy.
c
Experimental value.
d
At 29 kJ molÀ1 collision energy.
e
Not specified so assumed to be zero.
f
Calculated from the relative m/z = 64:m/z = 78 peak areas in Ref. [36] and corrected for the isomeric composition of each peak and the estimated photoionization cross-
section of each isomer that are reported in the paper.
fractions of (70 Æ10)% and (30 Æ10)% for 1,3-hexadien-5-yne and
benzene, respectively, in the C2H + 1,3-butadiene reaction reported
by Jones et al [23]. were obtained employing a crossed-molecular-
beams technique. In this technique the reaction between the
radical and reactant takes place in a single-collision environment
at elevated collision energy. Although their collision energy of
45 kJ molÀ1, translates to a thermal energy of 5400 K, the excess
energy is likely rapidly randomized within the initial C6H7 reactive
complex. In contrast, in the current study the reactions take place
in a collisional environment and at 298 K. Excess translational or
internal energy of the C2H reactant is collisionally relaxed prior to
reaction with 1,3-butadiene, and collisional redistribution of
energy in the C6H7 complex is also possible. The distribution of
angular momentum states in the C6H7 may also be substantially
different in the two experiments. In the crossed-molecular beams
investigation of Jones et al., products are ionized via electron
impact at 80 eV, the angularly resolved TOF spectra are recorded
and a product translational energy distribution in the center-of-
mass frame is calculated. The identification of the isomers is
carried out by fitting the experimental data to modeled
distributions that are obtained by forward convolution. The
molecular beam experiments are particularly selective for
benzene, the most exothermic channel, because it can appear at
energies that are closed for other reaction products. The
identification of isomers in the present study is spectroscopic,
and the identification of fulvene by its photoionization spectrum is
independent of the energy disposal in the reaction. The present
experiments can detect fulvene with particular selectivity because
it has the lowest photoionization energy of the C6H6 isomers and
appears at photon energies where other reaction products are
absent. It is not specified in the publication of Jones et al. whether
inferior fits resulted from using isomers other than benzene and
1,3-hexadien-5-yne. As the reported maximum benzene yields are
consistent with the benzene yields observed in the molecular
beam experiments, the major discrepancy with the Jones et al.
report is the fraction of fulvene. A composite photoionization
spectrum has been simulated that is composed of 70% 1,3-
hexadien-5-yne and 30% benzene (Fig. 11), the branching fractions
reported by Jones et al. [23]. It is clear from the comparison of this
simulated composite spectrum and the actual spectrum measured
in this experiment, also shown in Fig. 11, that the product
branching fractions resulting in the onset and shape of the
measured spectrum are unequivocally different to the values
observed by Jones et al.
Jones et al. also report a computational investigation into the
C2H + 1,3-butadiene reaction that corroborates their experimental
components [23]. The computational branching fractions are
reported from 0 to 45 kJ molÀ1 collision energy. At lower collision
energies, which would be more comparable to the current flow
tube conditions, they predict that the benzene product increases in
relative yield compared to the 1,3-hexadien-5-yne, (branching
given in Table 2). Jones et al. outline two possible pathways to
benzene formation, which are shown in Fig. 12, cyclization of the
initial adduct followed by H-shift or H-shift followed by cycliza-
tion, both followed by H-loss to yield benzene; all values in Fig. 12
are reproduced from that paper. Jones et al. state that 99% of the
benzene molecules are formed by the first pathway. 1,3-hexadien-
5-yne can be formed via direct H-loss from the adduct formed by
terminal addition of C2H to 1,3-butadiene via a transition state that
lies 13 kJ molÀ1 higher in energy than the separated 1,3-hexadien-
5-yne + H products. Jones et al. do not mention the possibility of
any products other than benzene or 1,3-hexadien-5-yne and so it is
difficult to know if formation of any other isomers was considered.
From the computational studies carried out and presented here,
the transition state with the highest energy in the proposed
fulvene-formation pathway lies at À116 kJ molÀ1 relative to the
asymptote for the reactants, whereas the highest energy transition
state on the 1,3-hexadien-5-yne -formation pathway of Jones et al.

J.F. Lockyear et al. / International Journal of Mass Spectrometry 378 (2015) 232–245
243
[(Fig._11)TD$FIG]
branching fractions measured (30% at 45 kJ molÀ1 collision energy)
and predicted (40% at 0 kJ molÀ1 collision energy) by Jones et al.
[23]. Given the significance of benzene in PAH chemistry in both
astrochemical and combustion environments, it is important to
identify reactions that are likely or unlikely to form benzene. Jones
et al. propose that the C2H + 1,3-butadiene reaction is a likely
candidate for benzene production in the interstellar medium.
However, the experimental evidence presented here suggests that
the C2H + 1,3-butadiene reaction may also contribute significantly
to yields of the 5-membered ring species fulvene in such
environments. It would be very interesting to search for the major
product of the C2H + 1,3-butadiene reaction, fulvene, in the
interstellar medium as this molecule possesses a substantial
dipole moment [66] and may be present as a result of the title
reaction where C2H radicals are abundant, for example in
circumstellar shells.
One factor that would assist greatly in the interpretation of the
photoionization spectrum would be accurate absolute photoioni-
zation spectra for 3,4-dimethylenecyclobut-1-ene, 3-methylene-
1-penten-4-yne and 3-methyl-1,2-pentadien-4-yne. Alternatively,
the system could be studied using different experimental
methodologies, or current methodology could be augmented with
a secondary component for identification. A full time-dependent,
multi-well master equation investigation of this system, which
includes the new pathways presented here, would be very
valuable, particularly given the significance of the title reaction
in formation of cyclic molecules in astrochemical and combustion
environments.
Fig.11. Black circles show the measured photoionization spectrum for the products
with m/z = 78 in the current experiment. Red triangles show a simulated spectrum
with a composition of 70% 1,3-hexadien-5-yne and 30% benzene, the branching
fractions reported by Jones et al. [23]. Both spectra are normalized to the same
maximum value.
lies at À103 kJ molÀ1, a difference of 13 kJ molÀ1. It may be that this
fulvene formation path may be able to effectively compete with the
proposed 1,3-hexadien-5-yne formation path by Jones et al., and
with the lower-energy benzene formation pathway shown in
Fig. 12, depending on the entropy of each transition state and the
effect of the collisional stabilization in our experiment.
All of the proposed 3,4-dimethylenecyclobut-1-ene pathways
involve preliminary H-shifts, which, due to the lack of C5H4
products detected in the current experiment, have been postulated
to be unfavorable, as discussed earlier. Furthermore, the assign-
ment of 3,4-dimethylenecyclobutene is less definitive than the
observation of fulvene formation (see above). Consequently, in the
case of the 3,4-dimethylenecyclobut-1-ene product, further
experimental or theoretical work would be particularly valuable
to corroborate its formation. It is possible that lower energy
pathways to both fulvene and 3,4-dimethylenecyclobut-1-ene
exist that have not been found in the present calculations.
Although we observe no spectroscopic evidence for benzene,
the signal-to-noise ratio in the photoionization spectrum indicates
that benzene could account for a maximum of 45% of products of
the C2H + 1,3-butadiene reaction under the current conditions. The
maximum branching fraction of 45% is in fair agreement with the
5. Conclusion
The reaction of C2H with 1,3-butadiene was investigated using a
slow flow reactor at 4 Torr and 298 K coupled with synchrotron
photoionization time-of-flight mass spectrometry. The primary
channel in the reaction is H-loss from the adduct to yield C6H6. No
signal is observed from the C5H4 + CH3 channel, which is prominent
when C2H reacts with other C4H6 isomers, implying that the title
reaction does not access the entrance channels involved in reaction
with the other C4H6 isomers. The measured photoionization
spectrum of this C6H6 product reveals that fulvene is formed with
branching fraction of (57 Æ30)%. From the deviation of the
measured spectrum from that of fulvene, it seems very likely that
additional isomers of C6H6 are formed. Using the currently
available calibration photoionization spectra for each isomer,
maximum branching fractions for 3,4-dimethylenecyclobut-1-ene,
3-methylene-1-penten-4-yne and 3-methyl-1,2-pentadien-4-yne
are calculated to be 74%, 24% and 31%, respectively. The upper limit
of the branching fraction for the sum of benzene and 1,3-hexadien-
5-yne under the current conditions is 45%. The observation of
fulvene as the major product of the title reaction is not in good
agreement with the interpretation of Jones et al. [23], suggesting
that further investigation of the C2H + 1,3-butadiene reaction
would be valuable to better understand formation of cyclic species
in combustion and astrochemical environments.
[(Fig._12)TD$FIG]
Several energetically feasible reaction pathways to fulvene and
3,4-dimethylenecyclobut-1-ene from the C2H + 1,3-butadiene
reactants have been determined using quantum chemical meth-
ods. For fulvene formation a route involving relatively low energy
transition states has been calculated, which may be able to
effectively compete with the benzene-formation pathway calcu-
lated by Jones et al. [23] under the current conditions. Given the
potentially significant reaction channels and the disagreement
encountered between studies thus far, a full time-dependent
multi-well master equation calculation and further experimental
investigation of the C2H + 1,3-butadiene system would be extreme-
ly valuable.
Fig. 12. Minima and first-order saddle points on two possible reaction pathways to
benzene formation in the C2H + 1,3-butadiene reaction. Values and structures are
from the publication by Jones et al. [23].The thick black line shows the pathway that
accounts for 99% of benzene products, according to Jones et al., and the thin red line
shows the pathway that accounts for the remaining 1% of benzene products.

244
J.F. Lockyear et al. / International Journal of Mass Spectrometry 378 (2015) 232–245
[19] E.H. Wilson, S.K. Atreya, Current state of modeling the photochemistry of
Acknowledgements
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S.R.L. and J.F.L. are supported by the Director, Office of Science,
Office of Basic Energy Sciences of the U.S. Department of Energy
under Contract DE-AC03-76SF0098 at Lawrence Berkeley National
Laboratory. M.F. thanks the I.R.S. acknowledges the France Berkeley
Fund and the Université de Rennes 1 and the France-Berkeley Fund
for financial support to enable his stay at the Advanced Light
Source. I.R.S. acknowledges the Université de Rennes 1. J.-C.G.
thanks the Centre National d'Etudes Spatiales (CNES) for financial
support. Sandia authors, and the development and maintenance of
the multiplexed photoionization mass spectrometry kinetics
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Geosciences, and Biosciences, the Office of Basic Energy Sciences
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