Study of Methylidyne Radical (CH and CD) Reaction with 2,5-Dimethylfuran Using Multiplexed Synchrotron Photoionization Mass Spectrometry

Article abstract of DOI:10.1021/acs.jpca.8b04140

At 298 K the reactions of 2,5-dimethlyfuran + CH(X²Θ) and + CD radicals were investigated using synchrotron radiation coupled with multiplexed photoionization mass spectrometry at the Lawrence Berkeley National Laboratory. Reaction products wer

Full text of DOI:10.1021/acs.jpca.8b04140

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A: Environmental, Combustion, and Atmospheric Chemistry; Aerosol Processes,
Geochemistry, and Astrochemistry
Study of Methylidyne Radical (CH and CD) Reaction with 2,5-Dimethylfuran
Using Multiplexed Synchrotron Photoionization Mass Spectrometry
Erica Carrasco, and Giovanni Meloni
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b04140 • Publication Date (Web): 29 Jun 2018
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Study of Methylidyne Radical (CH and CD) Reaction with 2,5-Dimethylfuran Using
Multiplexed Synchrotron Photoionization Mass Spectrometry
Erica Carrasco1 and Giovanni Meloni2,*
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1Department of Chemistry, University of San Francisco, San Francisco, CA United States
2Department of Physical and Chemical Sciences, Università degli Studi de L’Aquila, L’Aquila,
Italy
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Abstract
At 298 K the reactions of 2,5-dimethlyfuran + CH (X2Π) and + CD radicals were
investigated using synchrotron radiation coupled with multiplexed photoionization mass
spectrometer at the Lawrence Berkeley National Laboratory. Reaction products were
characterized based on their photoionization spectra and kinetic time traces. CBS-QB3 level of
theory was used for all energy calculations and potential energy surface scans were used to
determine thermodynamically favorable reaction mechanisms. The two entrance pathways
observed in the reactions are CH insertion within the C-O bond and CH addition to the л-bond
system. Both yield initial 6-membered ring radical intermediates. Primary products from the CH
addition pathway were observed at m/z = 108, 66, and 42. The two C7H8O isomers at m/z = 108
formed are 1,2,4-heptatrien-6-one and 3-hepten-5-yne-2-one. At m/z = 66, the three C5H6
isomers observed are 1,3-cyclopentadiene, 3-penten-1-yne (E), and 1-pent-4-yne. Ketene (m/z =
42) is also observed. From CH insertion entrance channel, the three C6H8 isomers produced are
1,2,4-hexatriene (Z), 2-hexen-4-yne (E), and 1,3,4-hexatriene. Patterns of H-loss, CHO-loss, and
CO-loss observed were also in agreement with trends observed in other similar studies. H-
assisted isomerization pathways have been considered as well for the formation of m/z = 66, 80,
and 108 isomers.
*giovanni.meloni@univaq.it
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Introduction
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As fossil fuel sources continue to diminish, the need for a sustainable source of energy
and fuel is rapidly increasing.1 Carbon based fuels are promising as immediate alternatives, since
they would allow for combustion engines in place to still be utilized.2-3 More specifically, second
generation biofuels are currently the most attractive option.4 Unlike first generation biofuels,
production of second generation biofuels would use sugars that cannot be consumed by humans
as feed stock.4-5 Therefore, fuels would not compete with food production.4-5 In several studies,
2,5-dimethylfuran (DMF), along with other furanic compounds, has been proven to have a high
yield, low cost synthesis process using several unconsumable sugars.6-8 Roman-Leshkoy et al.7
were able to selectively remove five oxygens from fructose to produce DMF with 5-
hydroxymethylfuran (HMF) as an intermediate. Shortly after, Zhao et al.8 produced a high yield
of the HMF intermediate without the use of acid catalysts, which reduced production costs and
allowed for glucose to serve as a feedstock as well. In 2008, cellulose was also successfully
converted in to several furanic compounds, including DMF.3
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Due to several chemical properties, many consider DMF a better contender for a future
biofuel than ethanol.1 When comparing mass production potential, DMF is advantageous over
ethanol because it can avoid the fermentation step seen in ethanol synthesis, which emits carbon
dioxide.3 DMF is nearly 40% higher in energy density, meaning it has better fuel mileage for the
same size fuel tank.1, 6 Additionally, DMF is insoluble in water, which not only makes water
contamination risk negligible, but it also allows for easier storage.1, 9 The heat of vaporization for
DMF is very similar to gasoline and is therefore able to overcome the cold start engine issues
that arise when using ethanol.6, 10 With a high boiling point and low volatility, DMF would be
easily transported and stored as a liquid.1, 9 Currently, DMF and other furan derivatives are used
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as antiknock and octane improvers for gasoline.1, 11 Ignition studies show that DMF has similar
spray patterns and flame propagation as gasoline, which make it compatible with modern spark-
ignition engines without any major modifications.1, 6, 9-10 The U.S. Environmental Protection
Agency and Clean Air Act require new fuels to submit data on the possible health impacts, as
well as demonstrate compatibility with current vehicle and distribution systems.2 DMF
combustion studies have already proven fulfilment of the compatibility requirements, but further
investigation of its combustion reactivity will be essential for its viability as a biofuel.1 The
known disadvantages associated with DMF combustion are toxic emissions of NOx, CO,
harmful intermediates, and particulate matter.1, 6 However, these values are very comparable, if
not less, than those measured from gasoline and ethanol combustion.1, 6 Therefore, DMF is still
perceived as a very promising fuel alternative and the advantages are enough to drive further
research into its reactivity by many scientists.1
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Studying methylidene radical (CH) initiated combustion reactions is an essential starting
point to understanding and predicting the reactivity of more complex hydrocarbons for biofuel
combustion modeling in the future.12-14 CH radical reactions are generally very fast and barrier-
less due to the carbon atom having one singly filled and one empty nonbonding molecular
orbital.13, 15 Present in combustion flames, CH radical significantly affects the energetics of gas-
phase environments.12, 16 Several studies have come to agreeing conclusions about the primary
entrance channel of the CH radical and pathway patterns.12-13, 15-16 Reactions with simple alkenes
have all determined CH radical addition to the π-bond as the dominant entrance channel with the
formation of an initial cyclic intermediate.12-13, 15-16 In 2009, Goulay et al.13 studied the reaction
of CH with several small alkenes and found a pattern of linear and cyclic isomers formed via H-
loss pathways. Trevitt et al.16 investigated reactions of CH and CD radicals and several small
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hydrocarbons with similar experimental methods and was able to establish the pattern: CH+
CxHy à Cx+1Hy + H. Furthermore, studies focused on CH radical reactions with acetones and
acetaldehyde have also followed these patterns.12-13, 15
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Our previous investigation studied the reactions of furan and 2-methylfuran with CH
radicals and also found CH addition entrance channel to be dominant along with patterns of H-
loss and linear and cyclic isomer products.17 The research presented in this work seeks to
continue to provide valuable information about the reactivity of furanic biofuels and hydrocarbon
radicals. Specifically, the reactions of 2,5-dimethylfuran with ground state CH and CD radicals
at 298 K and 4 Torr are presented and discussed. Multiplexed photoionization mass spectrometry
using synchrotron radiation is utilized to detect reaction species, which are identified and
characterized by photoionization (PI) spectra and kinetic time plots. By studying the same
reaction with CD radicals, further details about the formation mechanism for primary products
can be confirmed based on the observed isotopologues. Primary product mechanisms are
proposed using electronic structure calculations and potential energy surface (PES) scans.
Experimental Methods
Experiments were carried out using the Chemical Dynamics beamline at the Advanced
Light Source (ALS) of Lawrence Berkeley National Laboratory. Reaction species were
characterized using multiplexed time- and energy-resolved mass spectrometry coupled with
tunable synchrotron radiation. Details of the instrumentation has been described in depth
elsewhere.18-21
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After purification through freeze-pump-thaw technique, resulting 2,5-dimethylfuran
(purity ≥99%, Sigma-Aldrich) vapor was diluted to 1% with helium gas.19 To produce
bromoform vapors, helium gas was flowed into a glass vessel containing liquid bromoform at
8 ̊
C and 725 torr and bubbled through the liquid. From the bromoform vapors, CH (X2∏)
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radicals were generated by photolysis using an unfocused 4 Hz-pulsed 248 nm XeF excimer laser
of a 1% bromoform/helium gas mixture. The same technique was used to generate CD radicals
with a bromoform-d1. Due to bromoform’s multiple photodissociation products and pathways,
quantifying CH radical production and calculating branching ratios is not feasible.12, 17, 22-23 Our
previous investigation has explained the problematic nature of bromoform dissociation in more
detail.17
DMF and CH/CD (X2∏) radicals were flowed into a 62 cm long heated slow-flow quartz
reactor with an inner diameter of 1.05 cm by calibrated mass flow controllers. A Roots pump
maintains a desired pressure and is connected by a feedback controlled throttle valve to the
reaction cell. Excess helium gas was also added to the reaction cell. A concentration of 3.9 x 1013
molecules cm-3 of DMF was reacted with a concentration of 2.3 × 1013 molecules cm-3 of
bromoform. The same experimental methods were used to react DMF (3.9 x 1013 molecules
cm-3) with CD radicals using bromoform-d1 at a concentration of 6.8 × 1013 molecules cm-3.
Nitrogen with a concentration of 9.1 x 1015 was also added to the reaction with CD radicals.
Nitrogen is present to quench any vibrationally excited CH/CD (X2∏) radicals that were formed
when bromoform was photolyzed and may have been faster than the reaction.13 However, a study
done by Trevitt et al.16 found no experimental difference between PI spectra for reactions with
and without N2 addition. The purpose of the additional reaction with CD radicals is to provide
further clarity on the reaction mechanisms. The deuterium from the CD radicals can be used to
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trace the movement of the original hydrogen from the CH radical reactant in the reaction with
CH radicals. If the reactions have matching experimental PI spectra at the same m/z ratios, then
it can be concluded that the initial hydrogen added with the CH radical reactant is lost. However,
if the spectra are not in agreement, then the original hydrogen is still present in the final product
and a hydrogen from the 2,5-dimethylfuran reactant was lost instead. After each reaction
occurred, the reaction species flowed from the reaction cell to a differentially vacuumed
ionization region through a 650 µm wide pinhole. Reaction species were then photoionized by
tunable synchrotron radiation and the resulting cations were accelerated through a 50 kHz-pulsed
orthogonal acceleration time-of-flight mass spectrometer for detection. A mass resolution of
approximately 1,600 was used in this study. A calibrated photodiode was also employed to
measure the photocurrent of the ALS to be used for background subtraction and normalization of
the ion signal.
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Reaction time, ion intensity, and mass-to-charge ratio were recorded simultaneously over
the photon range of 8 to 11 eV at 0.025 eV increments. The resulting three-dimensional data
block was “sliced” and reduced to two two-dimensional data images to represent m/z ratio vs.
photon energy and m/z ratio vs. reaction time.24 At specific m/z ratios, the ion intensity from the
m/z ratio vs. reaction time image is integrated over the entire photon range and results in kinetic
time plots. Varyingly, integration of ion intensity over a specific time range for the m/z ratio vs.
photon energy image yields a PI spectrum. To reduce any possible signal from secondary
reactions, a time range of 0-30 ms was used in this study. Using time plots and PI spectra,
reaction species at specific mass-to-charge ratios can be characterized and identified. Kinetic
time plots show the progress of the reaction species depleting or forming over the selected
reaction time. To characterize a reaction species as a primary product the kinetic time plot of the
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reactant is inversed and overlaid onto the kinetic time plot of the proposed species. If curves
have initial onsets that are in good agreement, the reaction species is considered a primary
product, since it will be forming at the rate that the reactant initially depletes. PI spectra curves
are unique to individual compounds due to differing Frank-Condon (FC) factors, which allows
spectra to be distinguishable between multiple isomers. Reactions species can therefore be
confidently identified by comparing experimental PI spectra to literature or calculated
photoionization plots. Adiabatic ionization energy (AIE) derived from PI spectra through linear
extrapolation can also aid in product identification.25 For experimental AIE, an estimated
uncertainty of 0.05 eV is present for reaction species identified with an onset due to photon
energy step size, energy resolution, and possible hot bands.
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Computational Methods
The goal of this study is to identify all primary products and propose thermodynamically
feasible mechanisms for their formation. Electronic structure calculations are used to compute
AIE, simulate PI spectra, and perform potential energy scans to investigate the reaction
mechanisms. Using Gaussian 09 software program with CBS-QB3 composite model, all
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electronic structure calculations were performed.18,
The composite model has highly
accurate energetics with an absolute deviation of 4-5 kJ/mol.26-27 It also provides reliable
optimized molecular structural parameters such as bond angles, bond lengths, and vibrational
frequencies.20 If the AIE of a species is not available in the literature, it can be calculated using
the formula below:
AIE = E0, cation – E0, neutral
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where E0 represents the zero-point energy corrected total electronic energy. Heat of reactions are
also determined using the corrected total electronic energies of the neutral states of the proposed
products and starting reactants to confirm that product formation is thermodynamically feasible.
Reference spectra were always used if available for product identification. However, if no
reference spectra were found, photoelectron (PE) spectra were simulated using Gaussian 09 for
the proposed reaction species. Franck-Condon (FC) and Franck-Condon-Herzberg-Teller
methods are used to approximate FC factors based on the vibronic transition from the neutral to
the cationic state.29-31 FC overlap integrals are also calculated using a recursive formula
developed by Ruhoff.32 Following integration, PE spectra are expressed as calculated PI spectra
for comparison to experimental spectra.
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After all products were identified based on agreeing PI spectra and AIE, reaction
mechanisms for each primary product were determined. Proposed mechanism pathways were
confirmed with relaxed PES scans using B3LYP/6-31G(d) level of theory.33 The energies of all
transition states and minima determined in the PES scans were recalculated using the CBS-QB3
composite method and verified for thermodynamic feasibility. Intrinsic reaction coordinate (IRC)
calculations were also completed to further confirm the forward and reverse pathways, local
minima, and saddle points based on the proposed transition state.
Results
The two entrance channels for the CH (X2Π) radical found here were CH addition to the π
bond system and CH insertion into the C-O bond within the ring in the 2,5-dimethylfuran
compound. As seen in previous studies, the dominant entrance channel is the prior and yields a
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majority of the identified primary products.12-13, 16-17 All reaction enthalpies calculated and
reported in this work are at 0 K.
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The dissociation of bromoform produced a significant number of brominated species,
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which were identified and found to not interfere with product analysis. Based on the Br/81Br
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isotopic ratio, suspected brominated species were characterized by overlaying their
photoionization spectra and kinetic time traces. If the relative ion signals of the plots are in good
agreement at a ~1 to 1 ratio, then the species were considered brominated. In both reactions,
species observed at m/z = 144/146, 130/132, 116/118, 104/106, and 79/81 were identified as
brominated and, therefore, no longer investigated in this study.
Product Identification
The primary products for the reaction of 2,5-dimethylfuran + CH and + CD at 298 K
were observed at m/z = 108, 80, 66, and 42. The formation of a product must match the depletion
of the reactant over time to be considered as primary, meaning the product’s kinetic time trace is
in good agreement with the negative plot of the reactant. Figure 1 and 2 show the kinetic plot
comparisons for DMF + CH and + CD, respectively, for the four primary products. A species at
m/z = 52 is also observed and forming as fast as DMF is depleting but based on our computation
of the potential energy surface this species cannot be a primary product because the energetics
are not favorable (see Mechanism Pathways section). Products found at m/z = 108, 66, and 42
are believed to result from a CH addition entrance channel, while the products identified at m/z =
80 are proposed to result from a CH insertion pathway.
Experimental PI spectrum observed at m/z = 108 was found in very good agreement with
a summation of two acyclic C7H8O isomers (Figure 3). Due to the absence of literature values,
FC simulated curves for the products were calculated. The onset of the experimental curve was
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observed at 8.78 ± 0.05 eV and matches with the FC simulated spectra of 3,5,6-heptatrien-2-one.
The CBS-QB3 calculated AIE for this isomer is 8.81 eV, which provides further product
confirmation. The latter section of the experimental curve was found in a very good agreement
with the simulated curve for 3-hepten-5-yne-2-one, which has a calculated AIE of 9.34 eV. The
formation of C7H8O isomers indicates a H-loss mechanism and follows the CH + CxHy à Cx+1Hy
+ H pattern first discussed by Trevitt et al.16
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At m/z = 66, the signal observed was determined to be a summation of three C5H6
isomers. The products yielded were cyclic and acyclic isomers, which was a trend also seen in
similar studies with CH radicals and unsaturated hydrocarbons.15-16 The experimental PI curve
shown in Figure 4 has an onset of 8.55 ± 0.05 eV and is in good agreement with the literature PI
curve for 1,3-cyclopentadiene measured by Taatjes et al.34 Additionally, the literature AIE of
8.57 ± 0.05 eV is within the experimental uncertainty and supported by the CBS-QB3 calculated
AIE of 8.56 eV.34 The middle section of the experimental spectra matches a literature PI
spectrum of 3-penten-1-yne (E), which has an AIE of 9.05 eV.35 The calculated AIE for 3-
penten-1-yne is 9.10 eV. The third C5H6 isomer, which matches the final section of the
experimental curve, was identified as 1-pent-4-yne using a literature curve taken by Hansen et al.
and the reported AIE was 9.92 eV ± 0.05 eV.35 To provide further confirmation, the AIE was
calculated and found to be 9.93 eV. The proposed mechanism to form these products will be
discussed later in this work and is believed to form congruently with the primary product
observed m/z = 42.
A literature spectrum of ketene taken by Yang and coworkers36 was overlaid onto the
experimental curves at m/z = 42 and determined to be in very good agreement (Figure 5). The
literature AIE of 9.62 eV, with an overall uncertainty of 20%, is within the uncertainty of the
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experimental AIE measured at 9.59 ± 0.05 eV.36 Furthermore, the CBS-QB3 calculated AIE is
9.62 eV and matches very well both values. Ketene is an acyclic alkene product that is believed
to form along the same mechanism pathway as the three C5H6 isomers describe above.
Signal observed at m/z = 52 is in very good agreement with the literature PI curve for
vinylacetylene.37 The AIE of the experimental spectrum shown in Figure 6 is 9.53 ± 0.05 eV
matching the literature AIE of 9.58 eV ± 0.05 eV. Providing further validation, the calculated
AIE is 9.58 eV as well. Vinylacetylene is formed from CH addition to the π bond system and
would yield a C3H4O fragment that optimizes to cyclopropanone as well. However, the cation of
cyclopropanone is unbound and, therefore, no signal is observed at m/z = 56. Due to the high
transition barrier (see Mechanism Pathways section) necessary for explaining its formation, this
species is regarded as a secondary product.
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The primary products identified at m/z = 80 are proposed to result from the insertion of
CH radical into the C-O bond within the 2,5-dimethlyfuran ring followed by loss of a CHO
fragment. Unlike the previously described experimental spectra, the spectra at the same m/z ratio
shown in Figure 7 and 8 for the two reactions do not match. For the reaction of DMF + CH, the
experimental spectrum shown in Figure 7 is in good agreement with a summation of three C6H8
isomers and has an onset of 8.35 ± 0.05 eV. Due to the absence of literature PI or PE spectra, FC
simulation calculations were completed for the three proposed products. The onset of the
experimental curve is in good agreement with the simulated spectra for 1,2,4-hexatriene (Z). The
calculated AIE is 8.37 eV and within the experimental uncertainties. The middle of the curve is
in good agreement with the simulated spectra for 1,3,4-hexatriene, which has a calculated AIE of
8.54 eV. The latter part of the curve match with the simulated spectra of 2-hexen-4-yne (E),
which has a calculated AIE of 8.55 eV. Opposingly, the experimental PIE at m/z = 80 for the
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reaction of DMF + CD agrees with the summation of only 2-hexen-4-yne (E) and 1,2,4-
hexatriene (Z) (Figure 8). The onset of the experimental curve is 8.40 ± 0.05 eV and matches the
FC simulation for 1,2,4-hexatriene (Z). The latter part of the curves is in good agreement with
the FC simulation of 2-hexen-4-yne (E). In Figure 9, the experimental curve at m/z = 81 for the
reaction of DMF + CD has been modified by subtracting out the signal at m/z = 79 with respect
to the 79Br/81Br isotopic ratio and the signal at m/z 80 with respect to the 12C/13C ratio. The
remaining signal is found in good agreement with the FC simulation of 1,3,4-hexatriene meaning
the product is d5-1,3,4-hexatriene, which contains deuterium and, therefore, appears at m/z = 81
instead of m/z = 80. Despite all three products resulting from a loss of CHO, there are two
separate pathways described in the next section, which are distinguished by the specific
hydrogen that is lost.
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Intermediates, if detected, would be observed at m/z = 109, 67, and 53. All intermediates
to yield products at m/z = 108 would have signals shown at m/z = 109. Similarly, the
intermediates leading to products detected at m/z = 66 and 52 would be observed at m/z = 67 and
53. The mechanisms will be described in detail in the next section. To verify the identity of the
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signals observed at m/z = 109 and 67, the isotopic portion for C from the detected m/z = 108
and 66 signals were subtracted, respectively. The remaining signal at m/z = 109 is attributed to
the summation of intermediates C1 and C2 from Figure 12 and B2 and B3 from Figure 11.
Similarly, the summation of intermediates F4 from Figure 14 and C3, C4, and E1 from Figure 13
is determined to match the resulting signal at m/z = 67. These intermediates are the only
compounds with bound cation and ionization energies within the relative experimental photon
energy ranges. However, the experimental PI curves of both remaining signals cannot be
confidently identified due to the large number of possible intermediates and low signal-to-noise
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ratios. Intermediate D1 in Figure 12, which leads to vinylacetylene, would be observed at m/z =
53. However, no signal is observed at m/z = 53 and the cation of the intermediate was found to
be unbound.
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Mechanism Pathways
By comparing the reaction of DMF with CH (X2Π) with the reaction using CD radicals, a
mechanism can be confidently proposed by tracing the movement of the deuterium. There are
two entrance channels observed in this study, CH addition and CH insertion. As previously
mentioned, 4 of the 5 primary signals result from CH addition to the л-bond system in the DMF
ring, while only 1 primary signal results from CH insertion within the C-O bond. Both pathways
have relatively low initial energy barriers and are overall exothermic. In the energy diagrams, a
red line at 0 kJ/mol represents the zero reference point used for all energetic calculations of the
products. The zero reference point is based on the energy calculated from the sum of the 2,5-
dimethylfuran and CH (X2Π) radical optimized structures. Any reaction species or activation
barriers found to be above the zero reference point are considered thermodynamically and
kinetically unfavorable.
The two initial entrance channels are shown below in Scheme 1. Both pathways were
verified by three-dimensional PES scans and in line with pathways seen in previous studies of
CH radicals with small unsaturated alkenes.12-13, 15-17 Intermediate radical A results from CH
(X2Π) addition to the л-bond to form a bicyclic ring intermediate, which optimized to the 6-
membered ring compound. The calculated activation barrier is 18 kJ/mol and the addition
releases 480 kJ/mol of energy. Overcoming a barrier of 185 kJ/mol, intermediates B and C result
from the cleavage of one of the specific C-O bonds and have an enthalpy change of -298 kJ/mol.
From acyclic intermediates B and C, mechanism pathways diverge to produce primary products
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at m/z = 108, 66, 52, and 42. The second entrance channel observed in this investigation is CH
(X2Π) insertion within the C-O bond. Possible interactions of CH radical with only the oxygen
atom were also explored. However, no intermediates that could lead to the identified primary
products were produced. Intermediate radical D is formed with a reaction enthalpy of -469
kJ/mol and an activation barrier of 24 kJ/mol. The bond between the oxygen and the carbon with
the methyl group in the 6-membered ring (D) is then cleaved yielding the acyclic radical
intermediate E. The calculated energy of intermediate E falls 278 kJ/mol below the reference
point and forms after overcoming a barrier of 209 kJ/mol. Primary products at m/z = 80 form
from diverging mechanism pathways starting with the shared intermediate E.
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Scheme 1. Initial 6-membered radical ring intermediate (A and D) for CH addition and CH
insertion in 2,5-dimethylfuran reactions. Shared intermediates (B, C, and E) are also shown and
lead to all final products through diverging pathways.
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CH Addition
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The two C7H8O isomers, 3-hepten-5-yn-2-one and 3,5,6-heptatrien-2-one, are formed
from intermediate B through H-loss pathways. After two dihedral rotations of intermediate B, a
hydrogen is lost yielding 3-hepten-5-yn-2-one. In the energy diagram shown in Figure 10, the
two rotated intermediates are represented by A1 and A2 with calculated energies falling below
the zero-point reference by 317 and 321 kJ/mol, respectively. The corresponding activation
barriers to form A1 and A2 are relatively very small at 10 and 4 kJ/mol. Overcoming a barrier of
164 kJ/mol, a hydrogen is lost from intermediate A2 forming the final product 3-hepten-5-yn-2-
one. The calculated reaction enthalpy for the H-loss is -174 kJ/mol and overall exothermic for
product formation. A second isomer is observed at m/z = 108, 3,5,6-heptatrien-2-one, and
produced after two hydrogen transfers and a dihedral rotation detailed in Figure 11. A hydrogen
from the terminal carbon is transferred to the 6th carbon in the chain, where the radical is located,
and releases 415 kJ/mol of energy to form intermediate B1. The transition state barrier for the
transfer is 72 kJ/mol. Intermediate B1 then undergoes a dihedral rotation with a barrier of 295
kJ/mol and an enthalpy change of -267 kJ/mol forming a rotated intermediate represented by B2.
Overcoming a barrier of 120 kJ/mol, a second hydrogen transfer occurs moving the hydrogen
bonded to the 6th carbon to the 5th carbon in the chain producing intermediate B3. The heat of
reaction for the transfer is calculated to be -295 kJ/mol. Intermediate B3 then loses a hydrogen
and undergoes a final dihedral rotation producing 1,2,4-heptatrien-6-one as the final primary
product with a calculated energy falling 168 kJ/mol below the reference point. The activation
barriers for the H-loss and rotation are 148 kJ/mol and 18 kJ/mol, respectively. The formation of
the final intermediate B4 has a calculated reaction enthalpy of -159 kJ/mol. The pathways
forming both C7H8O isomers are overall exothermic and thermodynamically favorable. Figure 3
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shows matching experimental curves at m/z = 108 for both reactions, which indicates that
deuterium is not present in the final products and H in the entering CH radical is the specific
hydrogen that is lost in the H-loss pathways described here.
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From intermediate C, four primary products are produced from two main diverging
pathways. The energy diagrams represented in Figure 12 and 13 detail the pathways yielding two
C5H6 isomers, and ketene. Vinylacetylene formation pathway shows that this species cannot be
regarded as primary product. The formation of 1-penten-4-yne and 1,3-cyclopentadiene begin
with two hydrogen transfers followed by the loss of ketene and a resulting radical fragment at
m/z = 67. A hydrogen from the terminal carbon is transferred to the 6th carbon and overcomes an
activation barrier of 72 kJ/mol to form intermediate C1 (Figure 12). The corresponding enthalpy
change is -347 kJ/mol. Then, the α-hydrogen is transferred to the γ-carbon with a barrier of 240
kJ/mol and causes a slight rotation. The resulting intermediate is represented by C2 and has a
calculated energy that falls 347 kJ/mol below the reference point. From intermediate C2, the
bond between the β-carbon and γ-carbon is cleaved yielding ketene and a radical C5H7 fragment
(C3). The cleavage has an energy barrier of 98 kJ/mol and an overall enthalpy change of -309
kJ/mol. The resulting C5H7 fragment then loses a hydrogen forming two C5H6 isomer products
via distinct pathways detailed in the energy diagram shown in Figure 13. The β-hydrogen is
transferred to the γ-carbon after overcoming a barrier of 143 kJ/mol forming intermediate E1.
The calculated enthalpy change for the transfer is -178 kJ/mol. The final product 1-pent-4-yne is
formed from the loss of hydrogen from intermediate E1 and has a calculated energy that falls 27
kJ/mol below the reference point. The corresponding energy barrier for the H-loss is 151 kJ/mol.
Alternatively, intermediate C3 is also observed to undergo a dihedral rotation causing a ring
formation, which ultimately yields 1,3-cyclopentadiene after a hydrogen loss. The dihedral
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rotation has a relatively small barrier of 38 kJ/mol of energy and a calculated reaction enthalpy
of -296 kJ/mol. The rotated intermediate represented by C4 then forms a ring intermediate (C5)
with a small energy barrier of 24 kJ/mol and an enthalpy change of -346 kJ/mol. Intermediate C5
then overcomes an activation barrier of 181 kJ/mol and loses a hydrogen forming the final
product 1,3-cyclopentadiene. The calculated reaction enthalpy is -165 kJ/mol for the H-loss and
overall exothermic for product formation.
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Another pathway yielding the third C5H6 isomer observed and ketene begins with
intermediate C followed by a dihedral rotation, two hydrogen transfers, and a final H-loss. The
mechanism is detailed in the energy diagram shown in Figure 14. The dihedral rotation of
intermediate C has an activation barrier of 6 kJ/mol and releases 293 kJ/mol of energy to yield an
intermediate represented by F1. Then, the α-hydrogen moves to the 6th carbon in the chain
producing intermediate F2. The transfer has an energy barrier of 26 kJ/mol and a reaction
enthalpy of -352 kJ/mol. The bond between the β-carbon and γ-carbon in F2 is cleaved after
overcoming a barrier of 192 kJ/mol of energy forming ketene and a C5H7 radical fragment (F3).
The calculated enthalpy change for the cleavage falls 180 kJ/mol below the reference point. In
the C5H7 fragment, the β-hydrogen is transferred to the alpha carbon resulting in intermediate F4.
The activation barrier for the transfer is 189 kJ/mol and the calculated heat of reaction is -237
kJ/mol. A hydrogen is lost from intermediate F4 forming the final product 2-pent-4-yne after
overcoming an activation barrier of 187 kJ/mol. The enthalpy change is calculated to be -49
kJ/mol and overall production formation is exothermic. In Figure 4 and 5, experimental curves of
m/z = 66 and 42 for the two investigated reactions are shown to be matching and, therefore, the
specific hydrogen lost from the C5H7 fragment must be the deuterium. This provides additional
support for the proposed mechanisms.
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Vinylacetylene is produced from an alternate pathway of intermediate C2 shown in the
energy diagram detailed in Figure 12. Instead of the cleavage of the β-carbon and γ-carbon bond,
the bond between the δ-carbon and ε-carbon is cleaved forming two fragments: C3H4O and C4H5.
The corresponding activation barrier and heat of reaction were calculated to be 259 kJ/mol and -
88 kJ/mol, respectively. The C4H5 fragment would be detected at m/z = 56, however, no signal is
observed in this investigation. The C3H4O fragment loses a hydrogen from the β-carbon to form
vinylacetylene after overcoming a barrier of 73 kJ/mol. The calculated energy change for
vinylacetylene formation lies above the reference point by 52 kJ/mol and for this reason it is
regarded as a secondary product, because the computed pathway cannot explain its formation
both kinetically and thermodynamically. The experimental PI curves for m/z = 52 shown in
Figure 6 are in very good agreement.
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For all products formed via CH addition, both reactions share the same primary products
and have matching experimental PI spectra. By comparing the two reactions, it is deduced that
deuterium must have been lost in the reaction with CD radicals. Therefore, the it is also the
hydrogen that is introduced with the initial CH radical reactant that is lost in the reaction with
CH radicals.
CH Insertion
The three primary products observed at m/z = 80 are formed from CH insertion pathways
with a shared intermediate E (Scheme 1) and detailed in the energy diagram shown in Figure 15,
16, and 17. All products are C6H8 isomers, exothermic, and thermodynamically favorable. As
previously mentioned, the two experimental PI curves for m/z = 80 do not match, meaning the
initial H added with the CH radical is present in one of the three final products. The two shared
isomers are 2-hexen-4-yne and 1,2,4-hexatriene. From intermediate E, 2-hexen-4-yne is
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produced from a dihedral rotation, hydrogen transfer, and a loss of the aldehyde group. The
rotated intermediate is represented by intermediate A1 and forms after overcoming a relatively
small barrier of 8 kJ/mol (Figure 15). The calculated enthalpy change of the rotation is -297
kJ/mol. A hydrogen transfer occurs in intermediate A1 moving the γ-hydrogen to the ε-carbon
releasing 301 kJ/mol of energy forming intermediate A2. The corresponding activation barrier
for the transfer is 179 kJ/mol. Finally, 2-hexen-4-yne is formed with the loss of the aldehyde
group after overcoming a barrier of 99 kJ/mol. The heat of reaction is -214 kJ/mol and product
formation is overall exothermic. Opposingly, intermediate E also undergoes a series of four
hydrogen transfers and the loss of the aldehyde group to form 1,2,4-hexatriene as shown in
Figure 16. The first hydrogen transfer has an activation barrier of 17 kJ/mol and transfers the
hydrogen on the terminal carbon in the chain to the ε-carbon. The resulting intermediate is
represented by B1 and has a calculated energy that falls 399 kJ/mol below the reference point.
The second and third hydrogen transfer moves the δ-hydrogen to the carbon with the aldehyde
group and the γ-hydrogen to the δ-carbon forming intermediate B2 then B3. The calculated
reaction enthalpies for the transfers are -398 and -309 kJ/mol with activation barriers of 1 and 92
kJ/mol, respectively. The final hydrogen results in intermediate B4 after the β-hydrogen moves
to the γ-carbon. The calculated activation barrier is 7 kJ/mol and the heat of reaction is -314
kJ/mol for the transfer. With a calculated energy that falls 196 kJ/mol below the reference point,
1,2,4-hexatriene is formed by the loss of the aldehyde group from intermediate B4. The
corresponding activation barrier is 122 kJ/mol.
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The third C6H8 isomer observed is 1,3,4-hexatriene. In the reaction with CD radicals, the
product is observed as d5-1,3,4-hexatriene at m/z = 81. From intermediate E, 1,3,4-hexatriene is
formed after a dihedral rotation, two hydrogen transfers, and the loss of a carbonyl group and
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hydrogen detailed in energy diagram shown in Figure 17. C1 represents the rotated intermediate
and has a calculated energy that falls 295 kJ/mol below the reference point. The corresponding
activation energy is relatively small at 5 kJ/mol. Overcoming a barrier of 16 kJ/mol, a hydrogen
from the aldehyde group is transferred to the ε-carbon with a calculated enthalpy change of -371
kJ/mol. The resulting intermediate (C2) loses a carbonyl group to form intermediate C3. The
activation barrier and heat of reaction for the loss is 94 and -288 kJ/mol, respectively. Next, the
α-hydrogen is transferred to the β-carbon with an energy barrier of 174 kJ/mol yielding
intermediate C4. The calculated reaction enthalpy for the transfer is -405 kJ/mol. Finally, the δ-
hydrogen from intermediate C4 is lost forming the primary product 1,3,4-hexatriene that has an
energy that falls 42 kJ/mol below the reference point. The final activation barrier is 284 kJ/mol
and product formation is overall exothermic. Mechanism pathways based on the presence or
absence of deuterium further support product identification.
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Unlike all the other primary products, 1,3,4-hexatriene is observed at different m/z ratios
for the two reactions. This indicates that the hydrogen from the CH radical reactant is present in
the final compound and the mechanism is an H-loss pathway rather than a D-loss pathway. The
CO-loss followed by H-loss pathway is the only mechanism that loses the initial carbon and does
not lose the initial hydrogen from the CH radical reactant.
H-Assisted Isomerization
From the number densities of the reactants and assuming an average molecular diameter
of 0.25 nm for the reacting species, a few thousand collisions are expected at room temperature.
Under these conditions the H-assisted isomerization should also be considered as possible
reaction pathways to yield the observed products at m/z = 66, 80, and 108.
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Two C7H8O isomers are formed at m/z = 108 and Figure 18 details the proposed H-
assisted isomerization of 3,5,6-heptatriene-2-one into 3-hepten-5-yn-2-one. An H atom adds to
the terminal carbon, with an exothermicity of 151 kJ/mol of energy. The addition results in the
loss of the ε-hydrogen to form 3-heptene-5-yn-2-one. The respective barriers for each step are
relatively very small at 9 and 10 kJ/mol.
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Of the three C5H6 isomers observed at m/z = 66, the only possible H-assisted
isomerization is from 3-penten-1-yne to 1-penten-4-yne (Figure 19). An H atom can be added to
the γ-carbon of 3-penten-1-yne after overcoming a barrier of 13 kJ/mol and releasing -123
kJ/mol. Then, 1-penten-4-yne forms from the loss of the δ-hydrogen with an activation energy of
26 kJ/mol.
At m/z = 80, three C6H8 isomers are also observed and two H-assisted isomerizations are
possible. Figure 20 shows the proposed mechanism for 1,3,4-hexatriene isomerization to form 2-
hexen-4-yne. The addition of an H atom to the α-carbon on the 1,3,4-hexatriene is barrier-less
with an exothermicity of 186 kJ/mol of energy. The resulting intermediate then loses the
hydrogen on the 5th carbon in the chain, which is also the deuterium in the reaction with CD
radicals, to form 2-hexen-4-yne. The corresponding activation barrier for the H/D-loss is 180
kJ/mol. Alternatively, 2-hexen-4-yne can also be formed from 1,2,4-hexatriene as shown in
Figure 21. On 1,2,4-hexatriene, a hydrogen atom can add to the α-carbon, which will then cause
the loss of hydrogen from γ-carbon to yield 2-hexen-4-yne. The respective energy barrier for the
hydrogen loss is 149 kJ/mol, while the hydrogen addition is barrier-less with an exothermicity of
152 kJ/mol of energy.
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The reactions of 2,5-dimethlyfuran + CH (X2Π) and + CD radicals at 298 K were studied
using synchrotron radiation coupled with multiplexed photoionization mass spectrometry at the
Lawrence Berkeley National Laboratory. Based on kinetic time traces and photoionization
spectra, primary products were identified. The comparison of the experimental curves at the
same m/z ratios for the two reactions allowed for further support of product identification and
mechanism pathways based on the presence or absence of deuterium. CBS-QB3 level of theory
was used for all energy calculations and to determine reaction mechanisms. The two entrance
channels are observed in this work are CH addition to the л-bond system and CH insertion within
the C-O bond, yielding initial 6-membered ring radical intermediates. As seen in previous
studies, CH addition pathway was more favored and lead to the formation of nearly all primary
products observed. Patterns of H-loss, CHO-loss, and CO-loss observed were also in agreement
with trends observed in other similar studies. From CH addition pathway, products at m/z = 108,
66, and 42 were observed. At m/z = 108, the two C7H8O isomers formed were 1,2,4-heptatrien-6-
one and 3-hepten-5-yne-2-one through H-loss mechanisms. Diverging from the shared 6-
membered ring intermediate, ketene and three C5H6 isomers were formed congruently. The
isomers were identified as 1,3-cyclopentadiene, 3-penten-1-yne (E), and 1-pent-4-yne.
Vinylacetylene is also observed but based on the computation of the potential energy scan it is
not regarded as a primary product. In the CH insertion entrance pathways, CH radical is inserted
within the C-O bond to also yield an initial 6-membered ring radical intermediate. Three C6H8
isomers are produced via CH insertion. The two isomers shared by both reactions at m/z = 80 are
1,2,4-hexatriene (Z) and 2-hexen-4-yne (E) and result from the loss of the aldehyde group.
Through the loss of a carbonyl group then hydrogen, 1,3,4-hexatriene is formed and observed in
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the reaction with CH radicals at m/z = 80. In the reaction with CD radicals, the product is
observed as d5-1,3,4-hexatriene at m/z = 81, meaning that deuterium in present. This CO-loss
pathway is the only mechanism that keeps the initial hydrogen added with the radical reactant.
Finally, H-assisted isomerization has also been considered and the results presented for the m/z =
66, 80, and 108 isomers.
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Acknowledgements
This work is supported by the American Chemical Society – Petroleum Research Fund Grant #
56067-UR6 and the University of San Francisco via the Faculty Development Fund. The authors
would also like to acknowledge Drs. Taatjes and Osborn from Sandia National Laboratories for
the use of the experimental apparatus. The Advanced Light Source is supported by the Director,
Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under
Contract No. DE-AC02-05CH11231.
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Figure 1. Kinetic time traces of primary products of 2,5-DMF + CH radical at 298 K. Time
traces of m/z = 108 (green line), m/z = 80 (blue line), m/z = 66 (teal line), m/z = 52 (red line) and
m/z = 42 (orange line) overlaid onto the kinetic time trace of the reactant 2,5-dimethylfuran
(black line). The reactant time trace has been multiplied by −1 for comparison purposes.
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Figure 2. Kinetic time traces of primary products of 2,5-DMF + CD radical at 298 K. Time
traces of m/z = 108 (green line), m/z = 80 (blue line), m/z = 66 (teal line), m/z = 52 (red line) and
m/z = 42 (orange line) overlaid onto the kinetic time trace of the reactant 2,5-dimethylfuran
(black line). The reactant time trace has been multiplied by −1 for comparison purposes.
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Figure 3. Summation (black line) of FC simulated spectra of 3,5,6-heptatrien-2-one (green line)
and 3-hepten-5-yne-2-one (pink line) overlaid onto the experimental data for DMF + CH radical
(red line) and DMF + CD radical (blue line) at m/z = 108 taken at 298 K.
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Figure 4. Summation (black line) of literature PI spectra for 1,3-cyclopentadiene (pink line), 3-
penten-1-yne (green line), and 1-pent-4-yne (orange line) overlaid onto the experimental data for
DMF + CH radical (red line) and DMF + CD radical (blue line) at m/z = 66. Experimental
spectra were measured at 298 K.
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