Crossed-beam investigation of O(3P) + C2H5 → C2H4 + OH

Article abstract of DOI:10.1021/jp910615y

The reaction dynamics of ground-state atomic oxygen [O(³P)] with an ethyl radical (C2H5) in the gas phase was investigated using high-resolution laser spectroscopy in a crossed-beam configuration. An exothermic channel of O(³P) + C₂H₅ → C₂H₄ + OH was identified, and the nascent distributions of OH (X ²π: ν″? 0, 1) showed significant internal excitations with an unusual bimodal feature of low and high rotational N″-components with neither spin-orbit nor λ-doublet propensities. On the basis of the ab initio and statistical calculations, the reaction mechanism can be rationalized by two competing mechanisms: abstraction vs addition. The low N″-components with significant vibrational excitation can be described in terms of the direct abstraction process as a major channel. The extraordinarily hot rotational distribution of high N″-components implies that a portion of the fraction proceeds through the indirect short-lived addition-complex forming process. From the comparative analysis of the reactions of O(³P) + several hydrocarbon molecules and radicals, the reactivity and mechanistic characteristics of the title reaction are discussed. Copyright

Full text of DOI:10.1021/jp910615y

J. Phys. Chem. A 2010, 114, 4891–4895
4891
Crossed-Beam Investigation of O(3P) + C2H5 f C2H4 + OH†
Yong-Pal Park, Kyoo-Weon Kang, Se-Hee Jung, and Jong-Ho Choi*
Department of Chemistry and Center for Electro- and Photo-ResponsiVe Molecules, Korea UniVersity, 1,
Anam-dong, Seoul 136-701, Korea
ReceiVed: NoVember 7, 2009; ReVised Manuscript ReceiVed: February 4, 2010
The reaction dynamics of ground-state atomic oxygen [O(3P)] with an ethyl radical (C2H5) in the gas phase was
investigated using high-resolution laser spectroscopy in a crossed-beam configuration. An exothermic channel of
2
O(3P) + C2H5 f C2H4 + OH was identified, and the nascent distributions of OH (X Π: υ′′ ) 0, 1) showed
significant internal excitations with an unusual bimodal feature of low and high rotational N′′-components
with neither spin-orbit nor Λ-doublet propensities. On the basis of the ab initio and statistical calculations,
the reaction mechanism can be rationalized by two competing mechanisms: abstraction vs addition. The low
N′′-components with significant vibrational excitation can be described in terms of the direct abstraction
process as a major channel. The extraordinarily hot rotational distribution of high N′′-components implies
that a portion of the fraction proceeds through the indirect short-lived addition-complex forming process.
From the comparative analysis of the reactions of O(3P) + several hydrocarbon molecules and radicals, the
reactivity and mechanistic characteristics of the title reaction are discussed.
Introduction
reaction of allyl radicals by means of the crossed-beam
apparatus.7 A supersonic flash pyrolysis source was adopted,
Gas-phase radical-radical reaction dynamics play a signifi-
cant role in understanding the microscopic mechanisms of
elementary reactive processes such as combustion, hydrocarbon
synthesis, interstellar, and atmospheric chemistry. Despite their
mechanistic significance, the studies of radical-radical reactive
scattering at the molecular level, particularly in the field of
reaction dynamics, are quite scarce compared to the more
extensive atom-molecule reactions. Most of the gas-phase
experimental investigations conducted thus far are related to
bulk kinetics under thermally relaxed conditions. This dearth
of a substantial body of investigation is primarily attributed to
the difficulties in generating sufficient levels of clean hydro-
carbon radicals and the realization of reliable and facile
characterization schemes.1
where the labile precursor molecules underwent a rapid thermal
decomposition to produce clean, jet-cooled radical beams
without recombination and/or secondary dissociation.8 With the
aid of the ab initio calculations, the analyses of the nascent-
state distributions of various reaction products have demon-
strated unusual reactivity and mechanistic characteristics not
previously observed in the reactions of O(3P) + closed-shell
hydrocarbon molecules.9
This work aims to provide our gas-phase crossed-beam
investigation of the reaction dynamics of atomic oxygen O(3P)
with an ethyl radical (C2H5). Apart from the fundamental
importance in elementary reactive scattering processes, the
oxidation reaction of ethyl radicals has been known as an
important pathway under lean fuel conditions. The C2H5 radical
was produced by supersonic flash pyrolysis of the synthesized
precursor azoethane (C2H5-N2-C2H5) that underwent a rapid
thermal decomposition inside a high-temperature tube. The
probed channel was the exothermic pathway (1) that produced
a diatomic product OH and a closed-shell molecule ethene:
In previous bimolecular reactive scattering experiments,
Leone and co-workers studied the reactions of O(3P) with ethyl
radicals using time-resolved FT-IR spectroscopy and observed
OH products.2 Direct abstraction mechanism was proposed to
explain the nonstatistical vibrational distributions of the OH
products. Several groups performed kinetic investigations to
obtain rate constants using mass spectrometry and laser spre-
ctroscopy.3 Only a few theoretical studies have been reported
on the title reaction. Harding and co-workers performed quantum
chemical and classical trajectory calculations in an attempt to
obtain high-pressure recombination rate constants.4 Gupta et al.
and Yang et al. carried out independent ab initio calculations
on several reaction channels of O(3P) + C2H5.5
In recent years this lab has performed combined crossed-
beam and theoretical investigations into the reactions of O(3P)
with a series of prototypical hydrocarbon radicals, including
allyl (C3H5), propargyl (C3H3), and tert-butyl (t-C4H9), which
show some intrinsic stability owing to resonance stabilization
or hyperconjugation.1,6 Leonori et al. also studied the oxidation
O(3P) + C2H5 f OH + C2H4 (ethene)
∆H ) -67.3 kcal mol-1 (1)
f H2CO (formaldehyde) + CH3
∆H ) -81.1 kcal mol-1 (2)
f CH3CHO (acetaldehyde) + H
∆H ) -77.3 kcal mol-1 (3)
The reaction enthalpy was determined using the ab initio heat
of formation.10 On the other hand, in the previous mass-
spectrometry experiments for reaction channels 1-3, a second-
order rate constant and relative branching fractions over the
temperature range of 295-600 K were determined to be (2.2
( 0.4) × 10-10 cm3 molecule-1 s-1 and k1:k2:k3 ) 0.23:0.32:
† Part of the special section “30th Free Radical Symposium”.
* Corresponding author. Tel: +82-2-3290-3135. Fax: +82-2-3290-3121.
E-mail: jhc@korea.ac.kr.
10.1021/jp910615y  2010 American Chemical Society
Published on Web 02/19/2010

4892 J. Phys. Chem. A, Vol. 114, No. 14, 2010
Park et al.
2
2
Figure 1. LIF spectrum in the 0-0 and 1-1 band regions of the nascent OH (A Σ+ f X Π) produced from the reaction of O(3P) + C2H5 f
OH + C2H4.
0.40, respectively.3 The high reaction rate suggests that the title
reaction proceeds through fast and irreversible association-
fragmentation mechanisms.
were examined using the LIF scheme of the (0,0) and (1,1)
diagonal bands of the A Σ+ f X Π electronic transition in
the 306-318 nm region. The resulting fluorescence was
collected by a photomultiplier tube interfaced to a Boxcar signal
averager and an IBM-PC for display and analysis. The minor
interfering background signal resulting from the photolysis of
the HONO impurity was subtracted to obtain the OH signal
due solely to the title reaction.
2
2
In the presented crossed-beam scattering experiments, high-
resolution laser-induced fluorescence (LIF) spectroscopy was
employed to interrogate the nascent rovibrational distributions
of the OH products from channel 1. Ab initio calculations using
the density functional method were also performed, along with
a complete basis model to elucidate the reaction mechanisms,
kinetics, and dynamic characteristics of the title oxidation
reaction. To the best of the authors’ knowledge, no gas-phase
crossed-beam study of the O(3P) + C2H5 reaction has been
reported thus far. Therefore, the crossed-beam investigation
presented herein can provide significant mechanistic insights
into the oxidation reaction dynamics of saturated hydrocarbon
radicals as a model system at the molecular level.
Ab initio calculations on the title reaction conducted in this
laboratory have been recently reported in detail elsewhere.10
Here, a brief account necessary to help characterize the
mechanistic pathways and the nascent population analysis of
the observed OH products is presented. All calculations were
performed using the Gaussian 03 system of programs on an
IBM-PC and Compaq-Workstation (XP-1000).12-14 Various
geometries of local minima and transition states along the
reaction coordinates were performed using the density functional
method and the complete basis set models of CBS-Q and CBS-
QB3, employing the 6-311G(d) and 6-311+G(3df,2p) basis sets.
Unless otherwise specified, the single-point energies computed
at the CBS-QB3 levels are presented in the discussion below.
In addition, the vibration frequencies were also computed to
characterize the stationary points and zero-point energy (ZPE)
corrections with a scaling factor of 0.98. Here, it should be
mentioned that Gupta et al. and Yang et al. also carried out
independent ab initio calculations on a few reaction channels
of O(3P) + C2H5 using Gaussian programs.5 However, compared
to those results, our extensive calculations demonstrated that
our estimated heats of formation and reaction enthalpies
correlated strongly with the known experimental values within
Experiment and Ab Initio Calculations
The homemade crossed molecular beam apparatus designed
for the presented investigations of reactive scattering was
described in detail elsewhere.6 Briefly, the apparatus consisted
of two source chambers and a scattering chamber that were
pumped by two 6 in. and one 10 in. baffled diffusion pumps,
respectively. For generation of O(3P), 12% NO2 samples seeded
in ultrahigh purity helium (UHP He: 99.999%) at a 2 atm
stagnation pressure were expanded through a piezoelectrically
actuated pulsed valve and irradiated by ca. 30 mJ pulses of the
355 nm laser beam near the throat of supersonic expansion.
The C2H5 radical was produced by supersonic flash pyrolysis
of the precursor azoethane (C2H5-N2-C2H5). The azoethane
was synthesized using the method of Renaud and Leitch.11 The
precursor seeded in the UHP He at 2 atm was completely
pyrolyzed into C2H5 and N2 inside a hot silicon carbide tube.
At the center of the scattering chamber, both supersonic radical
beams were crossed at right angles under the single collision
condition. Assuming the full expansion of the molecular beams
under the crossed-beam conditions, Ecom was assessed to be 5.2
kcal mol-1. The nascent internal distributions of OH products
an accuracy of <1.5 kcal mol-1 10
.
Experimental Results
Figure 1 displays a typical LIF spectrum of the nascent OH
2
(X Π: υ′′ ) 0, 1) products from the title reaction, together
with the spectral assignment of individual lines. The assignment
was performed by comparison with previously known line
positions and further confirmed by spectral simulations.15 The

Crossed-Beam Investigation of O(3P) + C2H5 f C2H4 + OH
J. Phys. Chem. A, Vol. 114, No. 14, 2010 4893
components. However, for the υ′′ ) 1 level, the rotational
temperatures were found to be 1720 K (F1) and 1660 K (F2)
for the low-N′′ components. The temperatures for high-N′′
components were not estimated due to the limited number of
available populations in the probed distributions. In addition,
all of the rotational populations for the individual branches were
summed up, corrected, and compared to obtain the ratio of
vibrational partitioning (Pυ′′)0:Pυ′′)1). The corrections considered
in the analysis were Franck-Condon factors (0.90 and 0.71 for
the (0,0) and (1,1) bands, respectively) and the lifetime
differences (686.0 ns for υ′ ) 0 and 748.4 ns for υ′ ) 1). The
averaged vibrational partitioning with respect to the low-N′′ )
0 level showed population inversion with a ratio of P0:P1 )
1:1.06 ( 0.44 and were found to be almost the same in the two
spin-orbit states. The relative rotational populations for the two
spin-orbit electronic states and the Λ-doublet propensity were
also examined. No significant spin-orbit preferences were
observed in each vibrational state (1.08 ( 0.31 and 1.34 ( 0.41
for υ′′ ) 0 and 1, respectively). The Λ-doublet propensity was
determined to be 0.97 ( 0.46 for the υ′′ ) 0 level and 1.11 (
0.29 for the υ′′ ) 1 level, thereby implying no noticeable
propensity.
Discussion
The detailed mechanistic pathways that account for the
observed bimodal rotational distributions and the inversion of
vibrational populations of the OH products can be obtained from
ab initio calculations. Figure 3 illustrates the schematic diagram
of two competing reaction pathways on the lowest doublet
potential energy surface: abstraction and addition. It is obvious
that the facile abstraction process is the direct H-atom abstraction
from the methyl group. The entrance barrier along the coordinate
of abstraction reaction was predicted to be negligible. Such
absence of an activation barrier can be attributed to the unusually
small C-H bond dissociation energy (BDE ) 36.2 kcal mol-1)
and the high reaction exothermicity of -67.3 kcal mol-1. Here,
the H-atom abstraction from the CH2 group of C2H5 was not
taken into account due to unfavorable endothermic reaction
pathways to the formation of OH + 1CH3CH and 3CH3CH (∆H
) 8.0 and 5.6 kcal mol-1, respectively). Similar barrierless
systems can be found in the H-atom abstraction reactions of
O(3P) + allyl and tert-butyl radicals, where the absence was
also attributed to the unusually small C-H bond dissociation
energy (BDE ) 57.6 and 35.5 kcal mol-1 for allyl and tert-
butyl, respectively) and the high reaction exothermicities (∆H
) -46.0 and -67.2 kcal mol-1 for allyl and tert-butyl,
respectively).6 The absence is well compared with the substantial
barriers existing in the abstraction reactions of O(3P) with CH3,
C3H3, and closed-shell hydrocarbon molecules, where the large
C-H BDE and unfavorable reaction exothermicities entail
overcoming the high activation barriers along the reaction
coordinates.6,9,10
A second pathway to the formation of C2H4 + OH is
predicted to be the indirect addition reaction, a characteristic
barrierless process of the title radical-radical reaction. As
atomic oxygen attacks C2H5 along the entrance reaction
coordinate, the strong, long-range attractive interaction between
the two open-shell radicals leads to an energized C2H5O
intermediate denoted as eINT1 in Figure 3, with a high
association exothermicity of -92.7 kcal/mol with respect to the
reactant radicals. Due to the plentiful internal energy, eINT1
can undergo further isomerization steps, resulting in the forma-
tion of another two energized isomers, eINT2 and eINT3: the
eINT1 might undergo H-atom migration from either C(1) or
Figure 2. Boltzmann plots of the nascent OH distributions as a function
of rotational energy for the F1 states of the (a) υ′′ ) 0 (9) and (b) υ′′
) 1 (b) vibrational levels. (c) A rotational surprisal plot for the nascent
OH produced in the reaction of O(3P) with the ethyl radical for the F1
state of the υ′′ ) 0 vibrational level.
spectrum clearly demonstrates that the nascent products exhibit
substantial rotational excitation in the observed υ′′ ) 0 and 1
vibrational states. Figure 2 displays the logarithmic plots of the
populations divided by the rotational degeneracy as a function
of rotational energy for the F1 (2Π3/2) spin-orbit state in the
υ′′ ) 0 and 1 levels. The plots show bimodal features composed
of low- and high-N′′ rotational components in each vibrational
state. The feature is more obviously exhibited in the υ′′ ) 0
level, where the fraction of the rotational energy from the total
available energy was higher. When a simple Boltzmann
distribution is assumed for low- and high-N′′ components, the
slopes in Figure 2 correspond to rotational temperatures. For
the υ′′ ) 0 level, the rotational temperatures were estimated to
be 1300 K (F1:2Π3/2) and 1010 K (F2:2Π1/2) for low-N′′
components, and 4750 K (F1) and 4760 K (F2) for high-N′′

4894 J. Phys. Chem. A, Vol. 114, No. 14, 2010
Park et al.
Figure 3. Schematic energy diagram of the potential energy surface with the optimized geometries along the O(3P) + C2H5 f OH + C2H4
reaction coordinate at the CBS-QB3 level of theory. Values in parentheses denote experimental values (kcal mol-1).
C(2) to the oxygen atom to form eINT2 (CH3CHOH) and eINT3
(CH2CH2OH) by overcoming the respective three- and four-
membered transition states (RTS and ꢀTS). The activation
excluded in our analysis. Compared to the observed experi-
mental distributions of the OH products at a collision energy
of 5.2 kcal mol-1, the prior calculations predicted quite different
rotational temperatures [2940 K (F1) and 2920 K (F2) for the
υ′′ ) 0 level and 2650 K (F1) and 2630 K (F2) for the υ′′ ) 1
level] and a smaller fraction (P1/P0 ) 0.20) of population
partitioned into υ′′ ) 1. In addition, statistical surprisal analysis
based upon the prior populations (no) was also performed to
deconvolute the observed bimodal rotational distributions (n)
of the υ′′ ) 0 level. The rotational surprisal analysis reveals
the extent of the deviation from the statistical distribution and
can be used to deduce the relative contribution. The surprisals,
I(fr/fv) ) -ln[n(υ′′,J′′)/no(υ′′,J′′)] were plotted as a function of
the rotational fraction of the available energy, gR ) fr/(1 - fv),
and shown in Figure 2c. Here, fr and fv are the fraction of the
rotational and vibrational energy of the OH product, respectively.
As shown in Figure 2c, it was not possible to deconvolute the
rotational populations due to very poor fits except for the large
negative slope in the low-N′′ regime, suggesting that the low
rotational components are highly populated in comparison to
the expected statistical distribution. When the nascent distribu-
tions were simply deconvoluted, the approximate ratio of
population partitioning for the low- and high-N′′ components
was estimated to be about 1.5: 1.
barriers were calculated to be 27.2 and 28.0 kcal mol-1
,
respectively, and belong in the typical range 20-40 kcal mol-1
for the H-atom migration process. Here, it should be mentioned
that eINT2 and eINT3 were calculated to be mutually convert-
ible by overcoming the three-membered transition state (γTS)
with a barrier height of 45.6 kcal mol-1. Finally, eINT3
undergoes a peculiar isomerization process leading to the title
reaction pathway. After the direct C(1)-OH bond rupture
through the etTS transition state, the H atom of the OH radical
is rotated toward C2H4 to form the so-called van der Waals
complex etC. The complex shows a weakly bound structure with
Cs symmetry, in which the electron-deficient H atom of the OH
radical perpendicularly interacts with the π bond of C2H4. The
OH---C bond length and stabilization energy with respect to
the products (C2H4 + OH) were estimated to be 2.508 Å and
-1.8 kcal mol-1, respectively. The prediction is quite consistent
with the recent theoretical studies on the reaction of ethylene
+ OH, in which as the OH radical approaches ethylene, the
prereaction association adduct was predicted to exist along the
entrance channel.16
The prior statistical theory and surprisal analysis based upon
the aforementioned theoretical calculations were applied to
characterize the observed nascent distributions.17,18 For a
decomposing energized intermediate such as eINT3, the statisti-
cal product-state distributions are assumed to be solely deter-
mined by the available phase space volumes of the two fragment
products C2H4 + OH. In obtaining the rotational temperatures
from the slopes, the very low-N′′ regions were known to be
very susceptible to the possible relaxation even under single-
collision conditions.1,6 Therefore, the lowest three points were
The discrepancy between the observed bimodal internal
distributions and the prior distributions shows that the statistical
picture is not sufficiently appropriate to describe the dynamical
biases manifested in the title atom-radical reactive scattering
processes. On the basis of both crossed-beam results and ab
initio calculations, the reaction mechanism at the molecular level
is believed to proceed through the following two competing
dynamic pathways: abstraction vs addition. The low-N′′ com-
ponents with significant vibrational excitation can be described

Crossed-Beam Investigation of O(3P) + C2H5 f C2H4 + OH
J. Phys. Chem. A, Vol. 114, No. 14, 2010 4895
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in terms of the direct abstraction process as a major channel.
Nevertheless, the minor but extraordinarily hot rotational
distribution of high-N′′ components implies that some fraction
of the reactants proceeds through an indirect short-lived addition-
complex forming process, such as eINT3. A similar abstraction
mechanism was also proposed in the infrared emission study
of OH produced in the reaction of O(3P) + C2H5 performed by
Leone and co-workers.2 Although clear rotational-state distribu-
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to the error (10-50%) associated with the each rotational level
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The competing reaction pathways of the title reaction stand
in sharp contrast with the reaction mechanisms of O(3P) +
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radical.6,9 The former proceeds through the well-known collinear
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stable allene (C3H4) and isobutene (i-C4H8) + OH products.6
As demonstrated above, the work herein represents a step
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molecular level. Extensive crossed-beam investigations com-
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reactions are currently being conducted. It is the hope of the
authors that the gas-phase reaction dynamics studies presented
herein will provide valuable mechanistic insights into the
unexplored elementary oxidation reactions of various hydro-
carbon radicals.
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Acknowledgment. This work was supported by a National
Research Foundation of Korea (NRF) grant funded by the Korea
government (MEST) (No. M10500000023-06J0000). We thank
Professor I. Fischer and Dr. B. Noller for their helpful discussion
in synthesizing the precursor.
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