On the absolute photoionization cross section and dissociative photoionization of cyclopropenylidene

Article abstract of DOI:10.1039/c6cp01068g

We report the determination of the absolute photoionization cross section of cyclopropenylidene, c-C₃H₂, and the heat of formation of the C₃H radical and ion derived by the dissociative ionization of the carbene. Vacuum ultraviolet (VUV) synchrotron radiation as provided by the Swiss Light Source and imaging photoelectron photoion coincidence (iPEPICO) were employed. Cyclopropenylidene was generated by pyrolysis of a quadricyclane precursor in a 1 : 1 ratio with benzene, which enabled us to derive the carbene's near threshold absolute photoionization cross section from the photoionization yield of the two pyrolysis products and the known cross section of benzene. The cross section at 9.5 eV, for example, was determined to be 4.5 ± 1.4 Mb. Upon dissociative ionization the carbene decomposes by hydrogen atom loss to the linear isomer of C₃H⁺. The appearance energy for this process was determined to be AE_0K(c-C₃H₂; l-C₃H⁺) = 13.67 ± 0.10 eV. The heat of formation of neutral and cationic C₃H was derived from this value via a thermochemical cycle as Δ_fH_0K(C₃H) = 725 ± 25 kJ mol^-1 and Δ_fH_0K(C₃H⁺) = 1604 ± 19 kJ mol^-1, using a previously reported ionization energy of C₃H.

Full text of DOI:10.1039/c6cp01068g

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On the Absolute Photoionization Cross Section and Dissociative
Photoionization of Cyclopropenylidene
Fabian Holzmeier,a Ingo Fischer,*,a Benjamin Kiendl,b Anke Krueger,b Andras Bodi,c and Patrick
Hemberger*,c
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
We report the determination of the absolute photoionization cross section of cyclopropenylidene, c-C3H2, and the heat of
formation of the C3H radical and ion derived by the dissociative ionization of the carbene. Vacuum ultraviolet (VUV)
synchrotron radiation as provided by the Swiss Light Source and imaging photoelectron photoion coincidence (iPEPICO)
were employed. Cyclopropenylidene was generated by pyrolysis of a quadricyclane precursor in a 1:1 ratio with benzene,
which enabled us to derive the carbene’s near threshold absolute photoionization cross section from the photoionization
yield of the two pyrolysis products and the known cross section of benzene. The cross section at 9.5 eV, for example, was
determined to be 4.5±1.4 Mb. Upon dissociative ionization the carbene decomposes by hydrogen atom loss to the linear
isomer of C3H+. The appearance energy for this process was determined to be AE0K(c-C3H2; l-C3H+)=13.67±0.10 eV. The heat
of formation of neutral and cationic C3H was derived from this value via a thermochemical cycle as ꢀfH0K(C3H) = 725 ±
www.rsc.org/
25 kJ mol-1 and ꢀfH0K(C3H+)
= 1604 ± a previously reported ionization energy of C3H.
19 kJ mol-1, using
concentration. Due to their importance in kinetics and
analytical chemistry, the cross sections of numerous species
relevant in hydrocarbon combustion processes have already
been determined.9, 10 Absolute photoionization cross sections
are usually extracted by comparing the photoion yield of the
1. Introduction
In this paper we report the absolute photoionization cross
section of cyclopropenylidene, c-C3H2, near its ionization
threshold. In addition, we investigated the dissociative
photoionization (DPI) of the carbene employing imaging
photoelectron photoion coincidence spectroscopy (iPEPICO)
with VUV synchrotron radiation.
two components of
a binary mixture with a defined
stoichiometry, from which the cross section of one component
is exactly known. While such mixtures can be easily prepared
for stable molecules, the determination of photoionization
cross sections for unstable molecules, e.g. radicals and
carbenes, is challenging. Subsequently, σi is known for only
few reactive open-shell species, vinyl,11 propargyl,11-13 allyl,14
2-propenyl,14 phenyl,15 methyl,16, 17 and ethyl18 among them.
Here we employ the strategy of producing a reactive molecule
together with a well-studied stable molecule from a joint
precursor. Cyclopropenylidene can be generated by pyrolysis
of a quadricyclane derivative (see Fig. 1),19, 20 and benzene is
generated as a second pyrolysis product. The latter’s absolute
photoionization cross section has been determined
previously10, 21 and it has an IE similar to cyclopropenylidene.22
As both pyrolysis products are generated in a 1:1 ratio, it is
possible to determine the absolute photoionization cross
section of cyclopropenylidene by comparing the
photoionization efficiency (PIE) curves of c-C3H2 and benzene.
In a second experiment, the dissociative photoionization of
cyclopropenylidene was investigated and an H-loss appearance
energy determined. Recording and analysing the breakdown
diagram of unstable intermediates can potentially yield
valuable thermochemical data, but has proved to be rather
Along with its isomer propargylene, cyclopropenylidene was
identified in a rich cyclopentene flame by photoionization
mass spectrometry (PIMS),1 based on its ionization energy (IE)
of 9.17 eV as determined by conventional2-4 and threshold
photoelectron spectroscopy (TPES).5 Furthermore, it is
assumed to dominate the dissociation dynamics of the
resonantly stabilized propargyl radical at low combustion
temperatures or low pressures.6 For a deeper insight into
flame chemistry, reaction rates are of great interest. These can
be extracted from PIMS experiments if the absolute
concentrations of all reactants in the flame can be
8
determined.7, Therefore, it is necessary that the absolute
photoionization cross section σi of each of the involved species
is known to relate the observed ion signal to the
24
challenging. Selected data were obtained for the allyl,23,
propargyl,25 and ethyl radicals,25 but no full breakdown
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Phys. Chem. Chem. Phys.
diagram of a polyatomic reactive intermediate has been
reported yet to the best of our knowledge. The generally low
signal levels, overlapping peaks from the dissociative
photoionization of the precursor and the radical intermediate,
and broad internal energy distributions because of the
increased temperature complicated measurements. A further
difficulty is the spin state of the parent and daughter ions.
Open-shell radicals yield stable, closed-shell parent ions, which
typically dissociate into two open-shell fragments or two
2. Experimental
The experiments were performed at the VUV X04DB beamline
at the Swiss Light Source (SLS) storage ring utilizing the
iPEPICO (imaging photoelectron photoion coincidence)
endstation. Details on the beamline optics46 and the iPEPICO
48
setup47, can be found in the literature and are thus only
briefly described here. Vacuum ultraviolet (VUV) synchrotron
radiation is provided by a bending magnet and collimated onto
a
grazing incidence monochromator equipped with
150 lines/mm plane grating. The photon energy resolution is
approximately E/
E = 103. The monochromatic radiation is
focused into a rare gas filter operated with a mixture of
Ne/Ar/Kr (7 eV < h < 14 eV) or pure Ne (11 eV < h < 21 eV) to
a
25
closed-shell ones over a non-negligible reverse barrier.24,
Since cyclopropenylidene is a singlet carbene, it is a prime
candidate to circumvent this issue, as the cation should
fragment into an open-shell and a stabilized closed-shell
species. A second observation is that even in the few cases
with experimental data on the breakdown diagram for a
pyrolysis product, the deduced internal energy distribution
ꢀ
ν
ν
suppress higher harmonic radiation and is then passed to the
endstation, where it crosses the sample molecular beam.
The precursor molecule, a quadricyclane depicted in Fig. 1, was
synthesized according to the route described in Ref.20. The
liquid was held at 15°C to prevent decomposition, seeded in
1.8 bar of argon and expanded into the experimental chamber
through a 100 µm orifice passing a SiC tube electrically heated
to 740 K as measured by a Type C thermocouple. Details on
the pyrolysis source can be found elsewhere.49 The pyrolysis
products were then ionized by VUV radiation and detected in
the iPEPICO spectrometer. Electrons and ions are extracted by
a 120 V cm-1 electric field and the electrons are velocity map
imaged onto a position sensitive delay line anode (Roentdek
DLD40), while the ions are detected in coincidence in a Wiley-
McLaren time-of-flight (TOF) mass spectrometer. The photon
energy was calibrated on the 11s´, 12s´, and 13s´
autoionization resonances of Ar in first and second order.
Mass-selected threshold photoelectron (ms-TPE) spectra were
extracted by selecting the central part of the photoelectron
image corresponding to electrons with a kinetic energy of less
than 5 meV which were followed by an ion event in the
corresponding time-of-flight range. Hot electron contribution
was subtracted as proposed by Sztáray and Baer.50 The
photoion yield (PIY) was obtained by summing all electron
signal detected in coincidence with the selected mass range
and the signal was normalized to the photon flux as measured
by a SXUV-100 photodiode from IRD, taking into account the
energy dependent response. Due to the limited size of the
electron image, only electrons with a kinetic energy below
about 1 eV are completely imaged onto the detector. The
electron signal was accumulated for 6 min in steps of 10 meV
for the low energy spectra presented in this paper, while the
step size and the acquisition time were increased to 50 meV
and 8.5 min, respectively, for the high energy spectra.
corresponded to
experimental one.24,
a much higher temperature than the
26
This is at odds with Franck–Condon
simulations of the hot bands in the more readily available
threshold photoelectron spectra of the species, which typically
shows a lower and more realistic temperature. As will be
shown below, the cyclopropenylidene breakdown diagram is
also affected, and we identify three possible underlying
reasons and discuss their likelihood. Dissociative
photoionization yields access to validated thermochemical
information. In addition, as dissociative photoionization
processes take place on the same potential energy surface as
ion–molecule reactions, and each dissociation corresponds to
the inverse of an association reaction,27 breakdown diagrams
may also be relevant from an astrochemical point of view, as
cations are abundantly formed and found in space.28
Cyclopropenylidene is indeed one of the most abundant
hydrocarbons in interstellar space and appears with significant
concentration in diffuse interstellar clouds.29-34 The high
intensity of high-energy electromagnetic radiation in the
interstellar medium leads to molecular ions and therefore, ion-
molecule reactions dominate interstellar chemistry due to
their long range potential and often negligible barriers.35-39 The
+
cationic potential energy surface of C3H2 has been explored
for this reason.36, 40 Investigating the DPI of cyclopropenylidene
+
sheds light onto bonding properties of the c-C3H2 cation and
will provide access to the thermochemistry of daughter ions,
thus leading to
a better understanding of interstellar
chemistry. Since c-C3H2 is the most abundant cyclic
hydrocarbon in the interstellar space, the presence of
energetic radiation may lead to other fragments, e.g., l-C3H+,
which has been observed in the Horsehead photodissociation
region41 and toward Sgr B2(N).42, 43
Unless stated otherwise, energy computations were
52
These two experiments complete a series of investigations on
the cyclopropenylidene carbene in our group, in which we
analysed the near-threshold photoionization by threshold
photoelectron spectroscopy (TPES),5 investigated the
photophysics and photochemistry of the 1B1 excited state by
multiphoton ionization, Doppler spectroscopy,44 femtosecond
performed on CBS-QB3 level of theory,51,
utilizing the
Gaussian09 suite of programs.53 Ionization energies were
obtained by subtracting the neutrals’ energy from the cations’
one and appearance energies are derived from the difference
of the product energies (daughter ion + neutral fragment) and
the parent’s energy, if there is no reverse barrier. Scans of the
reaction coordinates for the localization of transition states
were performed on B3LYP/6-31G(d) and CASSCF-
time-resolved
photoionization
and
photoelectron
spectroscopy,45 and studied the photodissociation dynamics by
photofragment velocity map imaging.20
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MP2(7,10)/cc-pVTZ level of theory. Franck-Condon simulations
including hot band contribution were conducted with the
program ezSpectrum54 and the program MinimalPEPICO55 was
used to fit the breakdown diagram of cyclopropenylidene.
3. Results and Discussion
3.1 Photoionization Cross Section
As visible in the mass spectrum given in Fig. 1 and shown
previously,19, 20 the quadricyclane derivative (m/z=116) yields
cyclopropenylidene, c-C3H2, and benzene upon pyrolysis with a
high conversion efficiency. The only observed dissociative
photoionization channel of the precursor is associated with H-
atom loss and is responsible for the peak at m/z=115. A further
peak at m/z=109 is due to 3-methoxypyridine, which was
studied previously and does not interfere with the present
experiment.63 Benzene has a similar ionization energy as
cyclopropenylidene and its absolute photoionization cross
section is available from previous works.10, 21 As m/z = 38 and
m/z = 78 are the only relevant fragments generated upon
pyrolysis of the quadricyclane precursor, c-C3H2 and benzene
must be generated in a stoichiometric quantity. Other species
(m/z = 39, 40 and 66) are only present in minuscule amounts,
originate from a previous experiment, and are therefore
neglected. The mass-selected threshold photoelectron (ms-
TPE) spectra corresponding to these two mass peaks were
hence recorded to check if the observed signals can indeed be
assigned to cyclopropenylidene and benzene or if other
isomers also contribute to the mass peaks. As expected, the
ms-TPE spectrum corresponding to m/z = 78 (not shown in this
paper) represents a perfect match with previously reported
spectra of benzene confirming an IE of 9.24 eV.22 The ms-TPE
spectrum corresponding to m/z=38 is shown in Fig. 2. It
matches the earlier TPE spectrum of cyclopropenylidene
obtained utilizing the 1-chlorocycloprop-2-ene precursor5 and
confirms the previously reported ionization energy of 9.17 eV
for c-C3H2. The signal/noise ratio is significantly improved in
the new spectrum and the observed vibrational structure
confirms the previous assignment of vibrational bands. A
pronounced band at 9.02 eV was observed in the TPE
spectrum of Hemberger et al., which was discussed to
originate either from an unexpectedly intense hot band or
Fig. 2 Mass-selected threshold photoelectron spectrum of cyclopropenylidene. The
Franck-Condon simulation (blue sticks; red line: convolution with Gaussian
fwhm=30 meV) assuming a vibrational temperature of 740 K confirms that the cooling
of the generated pyrolysis products in the continuous molecular beam is negligible.
more likely, the contribution of another isomer, propargylene
HCCCH.5 The new TPE spectrum confirms the latter
explanation, as the band at 9.02 eV is almost completely
suppressed with the quadricyclane precursor (Fig. 2). The data
enable us to determine the absolute photoionization cross
section of cyclopropenylidene by the following relation:
ꢇꢈꢉꢊꢋꢌ ꢎꢁ ꢃ
ꢇꢈꢍꢊꢍꢌ
ꢂ
ꢄ
( 1 )
ꢀ
ꢐ
ꢐ
ꢁꢂꢃꢄ
ꢁꢂꢃꢄ
ꢁꢅꢃꢅ
ꢏ
ꢆ
∙
∙
ꢅ
ꢎꢏꢁ ꢃ
ꢅ
ꢀ
ꢁꢅꢃꢅ
ꢀ
stands for the detected ion signals of the respective species,
ꢇꢈꢉꢊꢋꢌ and ꢇꢈꢍꢊꢍꢌ correspond to the concentrations of
cyclopropenylidene and benzene, ꢎꢏ is the absolute
photoionization cross section, and
ꢐ represents the apparatus
functions. As the two pyrolysis products are generated in a 1:1
ratio, the fraction of the species concentrations equals 1. The
apparatus function comprises a mass-dependent element, the
so-called mass discrimination factor.56 As shown in the
Electronic Supplementary Information (ESI), the mass
discrimination factor can be approximated to be comparable
for m/z = 38 and m/z = 78 and the ratio of apparatus function
terms in equation ( 1 ) is close to unity. Since the employed
continuous molecular beam has an effusive character, the
sampling efficiency is mass independent and the mass
dependence of the channel plate gain is small over the
investigated mass range. The Franck-Condon simulation of the
carbene’s TPE spectrum at T=740 K, the pyrolysis temperature,
agrees well with the experimental spectrum, as illustrated in
Fig. 2 and supports the assumption of an effusive beam.57, 58
Thus, equation ( 1 ) reduces to:
ꢀ
( 2 )
ꢁꢂꢃꢄ
ꢎꢏꢁ ꢃ
ꢆ
∙ ꢎꢏꢁꢅꢃꢅ
ꢂ
ꢄ
ꢀ
ꢁꢅꢃꢅ
The absolute photoionization cross section of c-C3H2 was then
determined normalizing the PIE curve of benzene on the
absolute photoionization cross section reported in the
literature by Cool et al.10 These literature values of Cool et al.
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Phys. Chem. Chem. Phys.
were obtained by measuring the benzene cross section in a
binary mixture relative to propene and were afterwards 3.2 Dissociative Photoionization
normalized to the previously determined curve of Rennie et
A
previous investigation of the DPI of c-C3H2 using 1-
al.,21 who used nitrogen oxide as a reference. To evaluate the
influence of the beam temperature on the cross sections, we
recorded PIE curves of benzene at various temperatures (see
Fig. S2). The obtained photoionization cross section for
cyclopropenylidene as a function of the photon energy is
depicted in Fig. 3 (see ESI for the tabulated values). Steps in
the respective PIE curves are observed at the ionization
energies for cyclopropenylidene and benzene. The non-zero
ion signal below the IE indicate that pyrolysis products are
generated vibrationally hot, which is expected for this
experimental setup. Note that, as discussed in the
experimental section, it is only possible to determine the cross
section for ions generated with an excess energy less than
about 1 eV due to the limited size of the electron imaging
detector.
chlorocycloprop-2-ene as the precursor was challenged by the
presence of HCl, which appears as a direct product after α-
elimination.5,
44, 45
The H37Cl isotopologue has the same m/z
ratio as cyclopropenylidene and therefore contributes to the
parent ms-TPE signal. disentanglement of the two
A
contributions to the m/z = 38 channel therefore requires a
subtraction of the HCl signal weighted by an empirical factor
that includes the Cl isotopic ratio, which introduces
a
significant source of error. Employing the quadricyclane
precursor, it is hence possible to investigate the dissociative
photoionization of cyclopropenylidene by analysing the mass-
selected
TPE
spectra
without
this
perturbation.
Cyclopropenylidene undergoes dissociative photoionization at
higher photon energies to form a daughter ion of m/z = 37, i.e.
a hydrogen atom loss to C3H+ takes place. A breakdown
diagram for the dissociative photoionization of c-C3H2 was
recorded between 12.5 and 14.2 eV. In Fig. 4, the fractional
abundances of the parent and daughter ion’s TPE signal are
plotted as a function of the photon energy. The simultaneously
recorded TOF distribution shows no asymmetry for the
daughter ion peak (see ESI), which indicates that the
dissociation is fast and there is no kinetic shift. The 0 K
appearance energy is the photon energy at which even the
We now analyse the various sources of errors for the absolute
photoionization cross section of cyclopropenylidene given in
Fig. 3. The first source is the assumption of an effusive beam
and mass-independent apparatus function. As discussed
further in the ESI, the associated error seems to be small.
Further error sources are the integration limits for the studied
mass peaks (including the 13C isotopologues), the subtraction
of false coincidences, and the normalization of benzene’s
recorded photoion yield curve above the IE to the absolute
photoionization cross section of Cool et al.10 The value of
benzene’s cross section itself accounts by far for the greatest
source of error, because its uncertainty is stated to be 20 %.10
The overall error for the absolute photoionization cross section
reported in this paper is therefore evaluated to be around ±30
initially
0 K neutrals gain enough internal energy to
dissociatively photoionize. The parent ion’s disappearance
energy corresponds to the 0 K appearance energy of the
lowest lying dissociative photoionization channel in small
molecules and in the absence of a kinetic shift.59, 60
The signal/noise ratio is very low due to unfavourable Franck-
Condon factors in the investigated energy range presumably
+
because no excited cationic state of c-C3H2 is close in energy.
% of
σi, indicated in grey in Fig. 3. In comparison to other
open-shell hydrocarbons like methyl16 or ethyl,18 the cross
section of cyclopropenylidene shows a faster increase above
the ionization threshold.
A decrease of the parent signal (m/z = 38) is observed with a
simultaneous rise of the daughter ion signal (m/z = 37), setting
in around 12.9 eV. The parent ion signal drops to zero at
Fig. 3 Absolute photoionization cross section of cyclopropenylidene (black curve),
which was generated in stoichiometric quantities from the quadricyclane precursor.
The second fragment is benzene (red curve), which acts as reference cross section. The
margin of error for the cross section is indicated by the shaded grey area and
evaluated to be ±30%.
Fig. 4 Breakdown diagram of the dissociative photoionization of cyclopropenylidene to
HCCC+ (black squares: parent ion m/z = 38; red circles: daughter ion m/z = 37). Two fits
of the breakdown curves are shown for temperatures of T=740 K (dotted lines,
daughter blue) and T=1200 K (solid lines, daughter red) (see discussion for details).
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around 13.6 eV. Due to the absence of a kinetic shift the
parent ion’s energy of disappearance should therefore
correspond the appearance energy AE0K(C3H2; C3H+).59
Quantum chemistry was employed to analyse further the
dissociative photoionization of cyclopropenylidene to C3H+. In
agreement with previous theoretical studies,36 the direct
product of H atom loss, cyclic C3H+ was found to be a transition
state on the way to the linear isomers HCCC+ on the B3LYP/6-
311G(d,p) level of theory. The activation barrier for the
dissociation via the cyclic C3H+ cationic transition state is
predicted to lie at +14.29 eV relative to neutral c-C3H2 and
Fig. 5 Computed mechanism for the dissociative photoionization of cyclopropenylidene
to HCCC+.
Here, ꢙꢔꢚꢗ is the thermal energy distribution of the
dissociating ion, which is assumed to be
a Boltzmann
distribution that can be calculated based on the density of
states (DOS) of neutral cyclopropenylidene.55 In RRKM (Rice-
Ramsperger-Kassel-Markus) calculations, the isomerization
rates for both pathways to H2CCC+ and HCCCH+ were found to
be several orders of magnitude higher at the energy of the
dissociation threshold than the rates for the respective
hydrogen loss step. Consequently, the rearrangement of the
cation does not influence the modelling of the breakdown
diagram. Using phase space theory, the dissociation rate at
threshold energy was computed to be 5·109 s-1 for H2CCC+ and
5·106 s-1 for HCCCH+, respectively. Therefore, the dissociation is
fast on the timescale of the experiment, regardless whether
H2CCC+ or HCCCH+ is the dissociating ion, with calculations
suggesting that the former pathway is favoured. The proposed
dissociation mechanism is in good agreement with the
experimentally observed fast dissociation rate, as visible from
the symmetric TOF distribution.
+
5.12 eV above c-C3H2 (CBS-QB3). This value is significantly
higher than the observed energy of disappearance for the
parent ion at around 13.6 eV. A rearrangement of the
dissociating ion to another isomer must therefore be
considered. HCCCH+ and H2CCC+ were found to represent
+
further stable isomers of the composition C3H2 (see Fig. 5), in
agreement with the literature.36 The former one is computed
+
to be only 0.25 eV (24 kJ mol-1) less stable than c-C3H2 . Wong
and Radom reported an activation barrier of 1.75 eV for the
isomerization of c-C3H2 to HCCCH+ as computed on the MP2/6-
31G(d) level. This transition state could not be localized in the
present study, but CASSCF calculations yielded a potential
energy curve suggesting a conical intersection in this energy
range. Subsequent hydrogen atom loss was found to proceed
without a reverse barrier, but necessitating a significantly
higher activation energy of 4.18 eV relative to the HCCCH+
structure. An isomerization barrier of 3.29 eV is predicted for
The breakdown diagram was then fitted using equation ( 3 )
and ( 4 ) by setting the neutral temperature to T=740 K, as
obtained from the Franck-Condon simulation of the TPE
spectrum (Fig. 2), calculating the internal energy distribution of
the neutral, and shifting it by the photon energy to obtain the
fractional abundances of the parent and daughter ions, i.e. the
fraction of the distribution below and above the dissociation
threshold, at each photon energy. Assuming a vibrational
temperature of 740 K yielded the best fit to the high-energy
part of the breakdown diagram and an AE0K of 13.67 eV, as
depicted as a dashed line in Fig. 4. However, the measured
daughter ion abundances between 13.0 and 13.5 eV are
significantly underestimated by the model. In fact, the best
agreement between experiment and fit at the low energy side
was obtained for a temperature of T = 1200 K, which is shown
as a full line in Fig. 4. We want to emphasize that such a high
temperature is not only in contrast with the absence of hot
bands in the TPE spectrum (see ESI), it is also unrealistically
high for a 740 K pyrolysis experiment, as confirmed by the TPE
spectrum modelling. Anharmonicity leads to a higher density
of states at high internal energies, and may increase the
abundance of high-energy neutrals considerably already at
temperatures of 700–800 K. We therefore expect
anharmonicity to be the main reason of the temperature
discrepancy. Second, in modelling the breakdown diagram, we
assume that the internal energy distribution of the neutral is
shifted by the photon energy onto the ionic manifold without
being distorted. In other words, we assume that the threshold
photoionization cross section is independent of the internal
+
the rearrangement of c-C3H2 to H2CCC+, which lies 1.96 eV
(189 kJ mol-1) higher in energy than the cyclic cation. The H
atom loss again proceeds without a reverse barrier leading to
the same products as the pathway involving the HCCCH+
isomer. Yielding the same fragments without an overall
reverse barrier (see Fig. 5), both pathways are calculated to
have the same appearance energy of 13.60 eV at the CBS-QB3
level of theory. This value could be confirmed by other
composite methods, namely CBS-APNO (13.64 eV), W1U
(13.70 eV), and G3 (13.58 eV), employing different geometry
optimization and electron correlation approaches. As
discussed above, the TOF distributions indicate
a fast
dissociation on the timescale of the experiment. The parent
(BDP) and daughter (BDD) ion signal in the breakdown diagram
can then be modelled using the following expressions:55
ꢛꢜꢝꢞꢟ
( 3 )
ꢔ
ꢗ
ꢔ ꢗ
ꢙ ꢚ dꢚ
ꢑꢒꢓ ꢕꢖ
ꢆ
ꢆ
ꢘ
ꢠ
ꢣ
ꢔ
ꢗ
ꢔ ꢗ
ꢙ ꢚ ꢢꢚ for ꢕꢖ ꢤ ꢐꢚ
ꢑꢒꢡ ꢕꢖ
ꢘ
ꢛꢜꢝꢞꢟ
ꢔ
ꢗ
ꢒꢓ ꢕꢖ ꢆ 0
( 4 )
ꢔ
ꢗ
ꢑꢒꢡ ꢕꢖ ꢆ 1 for ꢕꢖ ꢥ ꢐꢚ
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ARTICLE
Phys. Chem. Chem. Phys.
energy. While this has proven to be a generally reliable thermal enthalpy), together with the W1U (and W1Usc)
assumption in room temperature or colder systems,55 small calculated 0 K ½ H2 and H2 abstraction energies of 230 (224)
deviations have been observed.61 If the threshold and 369 (367) kJ mol-1, yield, on average,
ꢀfH0K(C3H) = 719 ±
photoionization cross-section for hot neutrals at high photon 11 kJ mol-1, in good agreement with the newly determined
energies were up to an order of magnitude larger than for experimental value. For the C3H+ cation, an experimental heat
rovibrationally cold ones, the increased width of the of formation of
breakdown diagram could be explained. While this reason
cannot be ruled out, the fact that up to a factor ten
enhancement would be needed for the hot neutrals, we think
it is unlikely that it plays an important role in the broadening of
ꢀ
fH0K(C3H+) = 1604 ± 19 kJ mol-1 is obtained.
4. Conclusions
the breakdown diagram. Third, depending on the reaction
coordinate, the pyrolysis products may be formed with
significant vibrational excitation energy in certain modes. If re-
thermalization in the pyrolysis source were inefficient and
vibrational cooling in the molecular beam expansion negligible,
this vibrational excess energy could remain trapped in the
pyrolysis products. However, it has been observed several
The
absolute
photoionization
cross
section
of
cyclopropenylidene was determined and its dissociative
photoionization was investigated using imaging photoelectron
photoion coincidence spectroscopy. A quadricyclane precursor
was employed, which yields benzene and the desired carbene
in stoichiometric quantities. Absolute photoionization cross
sections of cyclopropenylidene up to 9.8 eV were derived from
the photoionization yield curves and the known cross section
of benzene. The absolute cyclopropenylidene photoionization
cross section at 9.5 eV, for example, was found to be 4.5 ±
1.4 Mb.
Dissociative photoionization of cyclopropenylidene to C3H+ was
observed in the mass-selected threshold photoelectron
spectra. Computations reveal that the cyclopropenylidene
cation first isomerizes to HCCCH+ or H2CCC+, before an H atom
loss occurs, leading in both cases to linear C3H+. Based on the
symmetric and narrow daughter ion TOF peak shapes, the
dissociation is fast on the experimental time scale. The
breakdown diagram is markedly broader than expected based
on the cyclopropenylidene internal energy distribution
calculated within the harmonic approximation at the
estimated sample temperature of 740 K. Significant
anharmonicity is suggested to be the main reason for the
broadening of the internal energy distribution, and increasing
threshold photoionization cross sections with increasing
internal energy of the neutral as well as a non-Boltzmann
internal energy distribution due to vibrational excitation along
the pyrolysis reaction coordinate are also discussed. The
breakdown diagram fit, using a realistic temperature, yields an
appearance energy of AE0K(c-C3H2; C3H+)=13.67±0.10 eV, as
confirmed by several computations with independent
methods. The appearance energy of C3H+ enables to derive the
times that re-thermalization by collisional heating is prompt
under our pyrolysis conditions,62,
63
which makes it highly
unlikely that collisional cooling is slow and that this effect plays
any role in the temperature discrepancy. Furthermore, the
breakdown diagram obtained after manual subtraction of the
HCl signal in experiments using the 1-chlorocycloprop-2-ene
precursor also required an unrealistically high temperature for
the fit,26 and it is also unlikely that the two different reaction
coordinates should lead to the deposition of similar amounts
of excess internal energy in the product carbene. A last, rather
remote possibility is that the dissociative photoionization of
the unconverted precursor affects m/z = 37 smoothly, which in
the end yields a breakdown diagram of apparently higher
temperature. The chances for this affecting several high-
temperature breakdown diagrams are minuscule and
computations on the precursor’s DPI do not support this
hypothesis. None of these explanations challenge the validity
of the computationally confirmed experimental appearance
energy of C3H+ from cyclopropenylidene. We therefore accept
the
low-temperature
fit
value
of
AE0K(c-C3H2;
C3H+)=13.67±0.10 eV because it describes best the region
where the parent ion signal drops to zero and is in agreement
with our own calculations as well as a recent theoretical paper.
40
Utilizing this appearance energy, the heat of formation of
cyclopropenylidene ꢀ
fH0K(c-C3H2) = 501 ± 9 kJ mol-1 (converted
from the 298 K value of Chyall and Squires64 using the W1U
thermal enthalpy and in very good agreement with high-level
computations by Lau and Ng65) and the ionization energy for
heat of formation for neutral and ionic l-C3H via
thermochemical cycle and we obtained values of fH0K(C3H) =
725 ± 25 kJ mol-1 and fH0K(C3H+) = 1604 ± 19 kJ mol-1.
a
ꢀ
ꢀ
linear C3H determined by Kaiser et al. as IE(l-C3H)
=
9.15±0.05 eV,66 the heat of formation of l-C3H can be
A
cknowledgement
determined in a thermochemical cycle:
The experiments were conducted at the VUV beamline of the
Swiss Light Source storage ring, Paul Scherrer Institute. We
acknowledge the assistance of Isabella Wagner and Engelbert
Reusch during the experiments. The work was supported by
the Deutsche Forschungsgemeinschaft, contract FI575/7-3, the
GRK 1221, and the Swiss Federal Office for Energy (BFE
Contract Number 101969/152433 & SI/501269-01). Travel
subsidies were provided by the European Commission
ꢔ
ꢗ
ꢔ
ꢗ
ꢔ
ꢨꢗ
Δꢦꢊꢠꢧ CꢉH ꢆ Δꢦꢊꢠꢧ CꢉHꢋ + ꢐꢚ CꢉH
ꢔ ꢗ
( 5 )
− Δꢦꢊꢠꢧ H − ꢩꢚꢔCꢉHꢗ
The obtained value of 725 ± 24 kJ mol-1 lies above the first
estimation of Saturno from the C-H bonding dissociation
energy of C3H3 and C3H2 (583 kJ mol-1).67 However, the heats of
formation of c-C3H2 and propargyl68
(ꢀfH0K(C3H3) = 342 ±
4 kJ mol-1 converted from room temperature using the W1U
6 | J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
T. Schüßler, H.-J. Deyerl, S. Dümmler, I. Fischer, C. Alcaraz
and M. Elhanine, J. Chem. Phys., 2003, 118, 9077–9080.
I. Fischer, T. Schüßler, H.-J. Deyerl, M. Elhanine and C.
Alcaraz, Int. J. Mass Spectrom., 2007, 261, 227–233.
T. Schüßler, W. Roth, T. Gerber, I. Fischer and C. Alcaraz,
Phys. Chem. Chem. Phys., 2005, 7, 819–825.
P. Hemberger, Dissertation, Julius-Maximilians Universität
Würzburg, 2011.
T. G. Rowland, S. Borkar, A. Bodi and B. Sztáray, Int. J.
Mass Spectrom., 2015, 378, 134-142.
J. Bouwman, B. Sztáray, J. Oomens, P. Hemberger and A.
Bodi, J. Phys. Chem. A, 2015, 119, 1127–1136.
J. M. Vrtilek, C. A. Gottlieb and P. Thaddeus, ApJ, 1987,
314, 716–725.
P. Thaddeus, J. M. Vrtilek and C. A. Gottlieb, ApJ, 1985,
299, L63-L66.
P. Cox, R. Guesten and C. Henkel, Astron. Astrophys.,
1987, 181, L19-L22.
P. Cox, R. Guesten and C. Henkel, Astron. Astrophys.,
1988, 206, 108-116.
P. Cox, C. M. Walmsley and R. Guesten, Astron.
Astrophys., 1989, 209, 382-390.
program “CALIPSO Transnational Access”. Computations were
performed at the Linux-cluster of the Leibniz-Rechenzentrum
der Bayerischen Akademie der Wissenschaften (LRZ) and the
Merlin 4 HPC cluster at PSI.
References
1.
C. A. Taatjes, S. J. Klippenstein, N. Hansen, J. A. Miller, T.
A. Cool, J. Wang, M. E. Law and P. R. Westmoreland, Phys.
Chem. Chem. Phys., 2005, 7, 806–813.
2.
3.
4.
5.
H. Clauberg, D. W. Minsek and P. Chen, J. Am. Chem. Soc.,
1992, 114, 99–107.
H. Clauberg and P. Chen, J. Phys. Chem., 1992, 96, 5676–
5678.
H. Clauberg and P. Chen, J. Am. Chem. Soc., 1991, 113,
1445-1446.
P. Hemberger, B. Noller, M. Steinbauer, I. Fischer, C.
Alcaraz, B. K. Cunha de Miranda, G. A. Garcia and H. Soldi-
Lose, J. Phys. Chem. A, 2010, 114, 11269–11276.
S. J. Klippenstein, J. A. Miller and A. W. Jasper, J. Phys.
Chem. A, 2015, 119, 7780-7791.
T. A. Cool, J. Wang, N. Hansen, P. R. Westmoreland, F. L.
Dryer, Z. Zhao, A. Kazakov, T. Kasper and K. Kohse-
Höinghaus, Proc. Combust. Inst., 2007, 31, 285-293.
C. A. Taatjes, N. Hansen, D. L. Osborn, K. Kohse-
Höinghaus, T. A. Cool and P. R. Westmoreland, Phys.
Chem. Chem. Phys., 2008, 10, 20–34.
6.
7.
S. C. Madden, W. M. Irvine, D. A. Swade, H. E. Matthews
and P. Friberg, AJ, 1989, 97, 1403-1422.
N. G. Adams and D. Smith, ApJ, 1987, 317, L25-L27.
M. W. Wong and L. Radom, J. Am. Chem. Soc., 1993, 115,
1507–1514.
35.
36.
8.
9.
37.
38.
39.
D. Gerlich and S. Horning, Chem. Rev., 1992, 92, 1509–
1539.
J. Berkowitz, Photoabsorption, Photoionization, and
Photoelectron Spectroscopy, Academic Press, New York,
1979.
S. D. Prodnuk, S. Grocert, V. M. Bierbaum and C. H.
DePuy, Org. Mass Spectrom., 1992, 27, 416–422.
G. B. I. Scott, D. A. Fairley, C. G. Freeman, M. J. McEwan,
N. G. Adams and L. M. Babcock, J. Phys. Chem. A, 1997,
101, 4973–4978.
J. P. Sánchez, N. F. Aguirre, S. Díaz-Tendero, F. Martín and
M. Alcamí, J. Phys. Chem. A, 2016, 120, 588–605.
J. Pety, P. Gratier, V. Guzmán, E. Roueff, M. Gerin, J. R.
Goicoechea, S. Bardeau, A. Sievers, F. Le Petit, J. Le
Bourlot, A. Belloche and D. Talbi, Astron. Astrophys.,
2012, 548, A68.
A. M. Brett, P. B. Carroll, A. L. Ryan, A. B. Geoffrey, M. H.
Jan, J. L. Frank, R. J. Philip and J. R. Anthony, ApJ, 2013,
774, 56.
P. Botschwina, C. Stein, P. Sebald, B. Schröder and R.
Oswald, ApJ, 2014, 787, 72.
P. Hemberger, B. Noller, M. Steinbauer, K. Fischer and I.
Fischer, J. Phys. Chem. Lett., 2010, 1, 228–231.
P. Hemberger, J. Kohler, I. Fischer, G. Piani, L. Poisson and
J.-M. Mestdagh, Phys. Chem. Chem. Phys., 2012, 14,
6173–6178.
M. Johnson, A. Bodi, L. Schulz and T. Gerber, Nucl. Instr.
Meth. Phys. Res. A, 2009, 610, 597–603.
A. Bodi, M. Johnson, T. Gerber, Z. Gengeliczki, B. Sztáray
and T. Baer, Rev. Sci. Instrum., 2009, 80, 034101.
A. Bodi, P. Hemberger, T. Gerber and B. Sztáray, Rev. Sci.
Instrum., 2012, 83, 083105.
10.
11.
12.
13.
14.
15.
16.
T. A. Cool, J. Wang, K. Nakajima, C. A. Taatjes and A.
Mcllroy, Int. J. Mass Spectrom., 2005, 247, 18–27.
J. C. Robinson, N. E. Sveum and D. M. Neumark, J. Chem.
Phys., 2003, 119, 5311–5314.
J. D. Savee, S. Soorkia, O. Welz, T. M. Selby, C. A. Taatjes
and D. L. Osborn, J. Chem. Phys., 2012, 136, 134307.
H. Xu and S. T. Pratt, J. Phys. Chem. A, 2013, 117, 9331–
9342.
J. C. Robinson, N. E. Sveum and D. M. Neumark, Chem.
Phys. Lett., 2004, 383, 601–605.
N. E. Sveum, S. J. Goncher and D. M. Neumark, Phys.
Chem. Chem. Phys., 2006, 8, 592–598.
40.
41.
42.
B. Gans, L. A. V. Mendes, S. Boyé-Péronne, S. Douin, G.
Garcia, H. Soldi-Lose, B. K. Cunha de Miranda, C. Alcaraz,
N. Carrasco, P. Pernot and D. Gauyacq, J. Phys. Chem. A,
2010, 114, 3237–3246.
43.
44.
45.
17.
18.
J.-C. Loison, J. Phys. Chem. A, 2010, 114, 6515-6520.
B. Gans, G. A. Garcia, S. Boyé-Péronne, J.-C. Loison, S.
Douin, F. Gaie-Levrel and D. Gauyacq, J. Phys. Chem. A,
2011, 115, 5387–5396.
46.
47.
48.
49.
50.
51.
19.
20.
H. P. Reisenauer, G. Maier, A. Riemann and R. W.
Hoffmann, Angew. Chem. Int. Ed., 1984, 23, 641.
M. S. Schuurman, J. Giegerich, K. Pachner, D. Lang, B.
Kiendl, R. J. MacDonell, A. Krueger and I. Fischer, Chem.
Eur. J., 2015, 21, 14486–14495.
21.
22.
E. E. Rennie, C. A. F. Johnson, J. E. Parker, D. M. P.
Holland, D. A. Shaw and M. A. Hayes, Chem. Phys., 1998,
229, 107–123.
L. A. Chewter, M. Sander, K. Müller-Dethlefs and E. W.
Schlag, J. Chem. Phys., 1987, 86, 4737–4744.
D. W. Kohn, H. Clauberg and P. Chen, Rev. Sci. Instrum.,
1992, 63, 4003–4005.
B. Sztáray and T. Baer, Rev. Sci. Instrum., 2003, 74, 3763–
3768.
J. A. Montgomery Jr., M. J. Frisch, J. W. Ochterski and G.
A. Petersson, J. Chem. Phys., 1999, 110, 2822–2827.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Physical Chemistry Chemical Physics
Page 8 of 8
View Article Online
DOI: 10.1039/C6CP01068G
ARTICLE
52.
Phys. Chem. Chem. Phys.
J. A. Montgomery Jr., M. J. Frisch, J. W. Ochterski and G.
A. Petersson, J. Chem. Phys., 2000, 112, 6532–6542.
Gaussian 09, Revision D.01, M. J. Frisch, G. W. Trucks, H.
B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G.
Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H.
Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F.
Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada,
M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A.
Montgomery, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J.
Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R.
Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C.
Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M.
Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C.
Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O.
Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski,
R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P.
Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö.
Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J.
Fox, Gaussian, Inc., Wallingford CT, 2009.
53.
54.
55.
56.
57.
V. A. Mozhayskiy and A. I. Krylov, ezSpectrum,
http://iopenshell.usc.edu/downloads.
B. Sztáray, A. Bodi and T. Baer, J. Mass Spectrom., 2010,
45, 1233–1245.
J. D. Bittner, Dissertation, Massachusetts Institute of
Technology, 1981.
B. K. Cunha de Miranda, C. Alcaraz, M. Elhanine, B. Noller,
P. Hemberger, I. Fischer, G. A. Garcia, H. Soldi-Lose, B.
Gans, L. A. V. Mendes, S. Boyé-Péronne, S. Douin, J. Zabka
and P. Botschwina, J. Phys. Chem. A, 2010, 114, 4818–
4830.
58.
J. D. Savee, J. Zádor, P. Hemberger, B. Sztáray, A. Bodi and
D. L. Osborn, Mol. Phys., 2015, 113, 2217-2227.
T. Baer, Int. J. Mass Spectrom., 2000, 200, 443–457.
T. Baer, A. Guerrero, J. Z. Davalos and A. Bodi, Phys.
Chem. Chem. Phys., 2011, 13, 17791-17801.
A. Bodi, N. S. Shuman and T. Baer, Phys. Chem. Chem.
Phys., 2009, 11, 11013–11021.
S. Liang, P. Hemberger, N. M. Neisius, A. Bodi, H.
Grützmacher, J. Levalois-Grützmacher and S. Gaan, Chem.
Eur. J., 2015, 21, 1073–1080.
59.
60.
61.
62.
63.
F. Holzmeier, I. Wagner, I. Fischer, A. Bodi and P.
Hemberger, J. Phys. Chem. A, 2015, DOI:
10.1021/acs.jpca.5b10743.
64.
65.
66.
L. J. Chyall and R. R. Squires, Int. J. Mass Spectrom. Ion
Proc., 1995, 149/150, 257.
K. C. Lau and C. Y. Ng, Chin. J. Chem. Phys., 2006, 19, 29-
38.
R. I. Kaiser, L. Belau, S. R. Leone, M. Ahmed, Y. Wang, B. J.
Braams and J. M. Bowman, ChemPhysChem, 2007, 8,
1236-1239.
67.
68.
A. F. Saturno, NASA Tech. Memo., 1982, 84537.
W. Tsang, in Energetics of Organic Free Radicals, eds. J. A.
Martinho Simoes, A. Greenberg and F. F. Liebman, Blackie
Academic and Professional, London, 1996, pp. 22-58.
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