Product study of the reaction of CH3 with OH radicals at low pressures and temperatures of 300 and 612 K

Article abstract of DOI:10.1021/jp045792o

The product distribution for the title reaction was studied using our time-of-flight mass spectrometer (TOFMS) connected to a tubular flow reactor. The methyl and hydroxyl radicals were produced by an excimer laser pulse (?? = 193 nm) photolyzing acetone and nitrous oxide in the presence of excess water or hydrogen. Helium was used as the bath gas; the total density was held constant at 1.2 ?? 10¹⁷ cm^-3. At 300 K the observations were consistent with singlet methylene (¹CH₂) and water as the main product channel with a small contribution of methanol. In contrast, at about 610 K three channels - formaldehyde isomers and methanol in addition to ¹CH₂ + H₂O - are formed with similar yields. When acetone-d₆ was used, the production of both CHDO and CD₂O was observed, indicating that two different formaldehyde-producing channels are operating simultaneously. These experimental results are compared with RRKM and master equation calculations on the basis of the properties of the methanol potential energy surface from a recent ab initio study. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp045792o

J. Phys. Chem. B 2005, 109, 8415-8427
8415
Product Study of the Reaction of CH3 with OH Radicals at Low Pressures and
Temperatures of 300 and 612 K†
Christopher Fockenberg, Ralph E. Weston, Jr.,* and James T. Muckerman
Chemistry Department 555A, BrookhaVen National Laboratory, P. O. Box 5000, Upton, New York 11973-5000
ReceiVed: September 16, 2004; In Final Form: NoVember 1, 2004
The product distribution for the title reaction was studied using our time-of-flight mass spectrometer (TOFMS)
connected to a tubular flow reactor. The methyl and hydroxyl radicals were produced by an excimer laser
pulse (λ ) 193 nm) photolyzing acetone and nitrous oxide in the presence of excess water or hydrogen.
Helium was used as the bath gas; the total density was held constant at 1.2 × 1017 cm-3. At 300 K the
observations were consistent with singlet methylene (1CH2) and water as the main product channel with a
small contribution of methanol. In contrast, at about 610 K three channelssformaldehyde isomers and methanol
1
in addition to CH2 + H2Osare formed with similar yields. When acetone-d6 was used, the production of
both CHDO and CD2O was observed, indicating that two different formaldehyde-producing channels are
operating simultaneously. These experimental results are compared with RRKM and master equation
calculations on the basis of the properties of the methanol potential energy surface from a recent ab initio
study.
TABLE 1: Enthalpies of Formation at 298 K (kcal/mol)
1. Introduction
Sandia11
NIST12
The reaction of hydroxyl radicals with methyl radicals is an
important process in the combustion of methane and larger
hydrocarbons.1,2 The overall rate constant for this reaction has
been measured over wide temperature and pressure ranges by
a number of investigators.3-10 The reaction involves direct
addition to form a vibrationally excited methanol molecule,
which can then either be stabilized by collision, dissociate back
to reactants, or form one of several possible products, shown
here as reactions R1-R7. Enthalpies of formation at 298 K from
CH3OH
CH3
-49.0 ( 1.0
34.9 ( 1.2
9.2 ( 1.0
102.7 ( 1.8
52.1 ( 1.0
-57.8 ( 1.0
-25.9 ( 1.0
27.8 ( 1.8
23.3 ( 1.7
-48.0
34.82
9.32
101.34
52.10
-57.80
-27.7
OH
1CH2
H
H2O
H2CO
cis-HCOH
trans-HCOH
CH3O
H2COH
4.1
-2.0
-4.9 ( 1.1
∆H° (298 K,
kcal/mol
TABLE 2: Enthalpies of Reaction (kcal/mol)
CH3OH f CH3 + OH
f 1CH2 + H2O
89.43
90.13
77.10
72.92
21.16
107.57
99.51
(R1)
(R2)
(R3)
(R4)
(R5)
(R6)
(R7)
Yu and
Muckerman14
Sandia11 NIST12
f cis-HCOH + H2
f trans-HCOH + H2
f H2CO + H2
reaction
(298 K) (298 K) (0 K)a (298 K)
CH3OH f CH3 + OH
1CH2 + H2O
93.1
93.9
76.8
72.3
23.1
92.14 87.57 89.43
91.54 88.07 90.13
75.32 77.10
71.14 72.92
19.39 21.16
104.2 103.20 107.57
95.09 99.51
f CH2OH + H
f CH3O + H
cis-HCOH + H2
trans-HCOH + H2
H2CO + H2
two different databases11,12 (Table 1) can be used to calculate
enthalpies of reaction for the reactions shown above. (Although
a slightly lower enthalpy of formation for hydroxyl, 8.83 kcal/
mol at 298 K, has been proposed,13 for the sake of consistency
of relative values we have used the values in Table 1 labeled
“Sandia”.) The 0 K values obtained by Yu and Muckerman14
have been extended to 298 K by including the translational,
rotational, and vibrational contributions to ∆H°. These are the
values shown in R1-R7. They are compared with the experi-
mental values in Table 2.
20.3
CH3O + H
H2COH + H
106.0
a Values at 0 K from ref 14 were converted to 298 K by correcting
for rotational and vibrational contributions using data from this source.
reaction of OH with methane. Anastasi et al. again used
absorption at 216.4 nm to follow the course of the reaction.4
Pulse radiolysis of SF6 produced F atoms, which reacted with
CH4 and H2O to produce the required radicals. Fagerstrøm et
al. used the same method to determine the pressure dependence
of the rate constant.3
Hochanadel et al. studied this reaction by following the
concentration of methyl radicals with time-resolved absorption
spectroscopy at 216.4 nm.5 Hydroxyl was generated by the
photolysis of water vapor, and methyl was produced by the
Laszlo et al. used flash photolysis of acetone to produce
methyl radicals and to produce O(1D) from nitrous oxide.6 The
† Part of the special issue “George W. Flynn Festschrift”.
10.1021/jp045792o CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/02/2005

8416 J. Phys. Chem. B, Vol. 109, No. 17, 2005
Fockenberg et al.
oxygen atoms reacted with H2 to form the required hydroxyl
radical. No pressure dependence was found over a range of 34-
350 Torr.
Bott and Cohen measured rate constants at high temperatures
in a shock tube.7 Hydroxyl radicals were produced by the
dissociation of tert-butylhydroperoxide, and tert-butylperoxide
provided methyl radicals. UV absorption at 309 nm was used
to follow the decay of OH. These researchers suggested that
stabilization of methanol accounted for about 75% of the
reaction at a pressure of 1 atm.
Grotheer and co-workers have carried out a number of flow-
tube experiments on this reaction system.8,9 When a microwave
discharge was used to generate H and F atoms, the required
radicals were produced by the reactions
F + CH4 f CH3 + HF
H + NO2 f OH + NO
Figure 1. Schematic energy diagram of the OH + CH3 reaction at the
MRCI+Q/CBS level of theory with the zero-point energy correction
obtained from the CAS(10,10)/cc-pVDZ results. Data from ref 14.
2. Experimental Section
Mass spectrometry was used to follow the decay of methyl
and also to look at products. These authors concluded that
stabilization was the major channel (70-90%) at ambient
temperature and pressures of 0.22-6.82 Torr. At 300 K, they
did not detect the H2O product of reaction R2, and from their
detection limit they concluded that this channel can contribute
at most about 5%. Later work by this group extended the
temperature range to 600 and 700 K.
The reverse of reaction R2 has been studied by Hack et al.15
and by Carstensen et al., who produced the methylene radical
by laser photolysis of ketene.16 Laser-induced fluorescence was
used to follow the disappearance of 1CH2. With the rate constant
they determined, together with an equilibrium constant calcu-
lated from thermodynamic and spectroscopic data, it is possible
to obtain a rate constant for reaction R2.
The experiments were performed in our flow reactor/repetitive
sampled time-of-flight mass spectrometer apparatus. A detailed
description of the apparatus can be found elsewhere, and only
an overview will be given here.20 The tubular quartz reactor (1
cm diameter) was coated with boric acid on the inside to reduce
surface reactions and with a thin layer of platinum on the outside
acting as a conductor used to heat the tube electrically. The
temperature was measured with thin-wire thermocouples placed
at several positions inside the tube. The temperature variation
along the tube was within 7% at 610 K. Precursors, reactants,
and products are sampled continuously through a pinhole in
the wall of the tube. A portion of the sampled gas is
photoionized by the VUV radiation emitted from a hollow
cathode lamp (McPherson, model 630) operated with hydrogen
as discharge gas. The emitted light is passed through a
differentially pumped glass tube, allowing the lamp to be
operated without windows, so that the whole spectrum of the
H2 emission can be used for ionization. By switching voltages
on the collection grid assembly at a high rate, “snapshots” of
the composition of the reaction mixture can be taken, here in
intervals of 48 µs.
The radical precursors were photolyzed by the radiation of
an ArF excimer laser (Lambda Physik, COMPEX 205), creating
a homogeneous distribution of radicals along the tube. Only a
fraction (<5%) of the laser light was actually absorbed by the
precursors in the reactor tube. The rectangular cross section of
the emitted laser radiation was reduced to a square shape by a
Galilean telescope made of cylindrical quartz lenses before it
entered the reactor tube. Typical energies were 120-250 mJ/
pulse exiting the laser. The laser intensity was monitored
separately with two thermoelectric energy meters (Molectron
J50), collecting laser radiation reflected off the quartz entrance
window or exiting the reactor through the rear window. This
intensity was used to calculate shot-by-shot concentrations of
OH radicals (see below). Mass spectra were typically summed
for 20 000-100 000 laser shots per experiment.
De Avillez Pereira et al. carried out an extensive investigation
of the temperature and pressure dependence of the CH3 + OH
reaction.10 In a He bath gas at pressures of 7.6-678 Torr and
temperatures of 290, 473, and 700 K, the rate of OH decay
was followed by laser-induced fluorescence, and CH3 was
monitored by absorption at 216.4 nm. Both radicals were
produced simultaneously by the 193 nm photolysis of acetone
containing traces of water. Master equation calculations com-
bined experimental rate constants for the removal of OH with
RRKM calculations of rate constants for channels with a barrier,
and with literature values for the rate constants of channels R1,
R2, R6, and R7. These calculations gave pressure-dependent
rate constants for the various channels as a function of
temperature.
Lin and co-workers have used ab initio molecular orbital
theory calculations at the G2M level17 to characterize the
potential energy surface for the CH3‚‚‚OH system.18 Rate
constants and branching ratios were obtained from variational
transition-state theory and RRKM calculations.
Most recently, Yu and Muckerman have carried out CASSCF
calculations to determine geometries and vibrational frequencies
of stationary points on the potential energy surface (PES) for
this system.14 The energies were refined by extrapolating
multireference CI (MRCI) calculations employing two large
basis sets to the complete basis set (CBS) limit.19 A schematic
energy diagram is shown in Figure 1. In this paper their results
are used as a starting point in the calculation of rate constants
for the reactions of the CH3‚‚‚OH system for comparison with
experimental findings.
Mass-flow controllers (Tylan General, FC 260) were used to
prepare the gas mixtures introduced into the reactor tube.
Acetone-1,3-13C (Cambridge Isotopes Laboratories, 99 atom %
13C) and acetone-d6 (Aldrich, 99.5 atom % D) diluted in helium
were prepared in advance and stored in 20-L glass vessels. He
(Welco CGI Gas Technologies, UHP grade), H2 (Matheson,
purity 2.1), and the N2O premix (2% in He, Praxair; purity:
N2O 5.5, He 6.0) were used directly from the tank. Water was

Reaction of CH3 with OH Radicals
J. Phys. Chem. B, Vol. 109, No. 17, 2005 8417
introduced by passing He first through a bubbler filled with
purified and degassed water before it entered the mass-flow
controller. The bubbler was placed in a water bath whose
temperature was monitored continuously. The pressure in the
flow reactor was controlled by throttling the gas flow through
a gate valve at the downstream end of the reactor. The flow
velocity employed here was 8 m/s to minimize the pressure drop
along the tube.
Isotopically labeled acetone was used here to separate the
formaldehyde signal (13CH2O, m/e ) 31) from the main two
mass channels associated with ethane (13C2H6, m/e ) 30 and
32) and to distinguish between formaldehyde coming from the
DCOH channel (CHDO, m/e ) 31) or produced directly (CD2O,
m/e ) 32).
Typical concentrations in the reactor were (0.9-2) × 1015
molecules cm-3 of H2O, 1.5 × 1014 molecules cm-3 of N2O,
and (0.4-2) × 1013 molecules cm-3 of acetone, and in some
experiments a quarter of the He was replaced by H2 ([H2] ) 3
× 1016 molecules cm-3). Under the conditions employed, most
of the O(1D) atoms produced in the 193-nm photolysis of N2O
react either with water, generating two OH radicals, or with
H2, giving OH radicals and H atoms. Water is present in all
experiments to rapidly quench vibrationally excited OH radi-
cals.21 By changing the acetone concentration or the laser
intensity, different initial concentrations of methyl or hydroxyl
radicals could be chosen.
For reference measurements, mixtures of C2H2 (Matheson,
with acetone removed by LN2/dry ice distillation), C2H4
(Matheson, >99%), C2H6 (Linde, >99%), CH3OH (Mallinck-
rodt), NO (Matheson, cleaned by LN2/dry ice distillation), O2
(Liquid Carbonic), and CH2O (prepared from paraformaldehyde
(Aldrich)) with He were prepared in 3-L glass bulbs.
Figure 2. Left panel: Experimentally observed traces for the SO +
OH calibration reaction. Experimental conditions and initial concentra-
tions (in cm-3): T ) 302 K, P ) 4 Torr, [H2O] ) 1.6 × 1015, [N2O]
) 2 × 1014, [SO2] ) 3 × 1013, [SO] ) [O(3P)] ) 5.5 × 1012. Right
panel: Fraction of N2O photolyzed as a function of laser intensity
measured at the exit of the flow reactor. The initial SO/O(3P)
concentrations range from (1.0-5.5) × 1012 cm-3. The triangles and
circles depict two measurements performed during and at the end of
this study.
conditions, but with acetone replaced by S18O2 (Stohler, 99.5
atom % 18O, no longer available), S18O radicals and 18O atoms
produced in the photolysis of S18O2 converted OH radicals
quickly into S18O16O and 18O16O:
3. Data Analysis
S18O2 + hν (193 nm) f S18O + 18O(3P)
N216O + hν (193 nm) f 16O(1D) + N2
H216O + 16O(1D) f 216OH
(R8)
(R9)
All data sets came in pairs of experiments: one with and
one without N2O. Because the acetone photolysis at 193 nm
produces not only methyl radicals but also small amounts of
methylene, giving ethylene and ethane as reaction products,
measurements without N2O served as a “baseline” for subse-
quent simulation calculations of all data sets. In addition, in
separate experiments, photolyzing acetone or ketene in He alone
gave values for the loss of methylene due to wall reactions.
Species signals were converted into concentration via calibra-
tion constants, which were obtained for stable species in separate
experiments. Calibration constants for methyl radicals were
obtained directly from the drop in the acetone signal, with a
yield of about 96% for the channel, giving 2CH3 + CO under
the present conditions. A small fraction of the methyl radicals
undergo secondary photolysis, giving CH2 + H. At appropriately
high CH3 concentrations, all methylene radicals can be converted
into ethylene, so that by determining the ethylene concentration
the calibration constant for methylene radicals can be found.
Further descriptions of the calibration procedures and the acetone
photolysis can be found elsewhere.22,23
Unfortunately, OH radicals could not be observed because
of the low sensitivity of the current apparatus toward this
species. Moreover, due to the small absorption coefficient, the
drop in the N2O signal after the laser fired could not be measured
accurately. Therefore, the production of hydroxyl radicals was
checked in two independent ways. First, N2O was photolyzed
in H2, and the H2O product was observed using He instead of
H2 as discharge gas in the hollow cathode lamp. Rate constants
and H2O yields were consistent with a wall loss of OH radicals
between 50 and 100 s-1. Second, under typical experimental
(R10)
S18O + 16OH f S16O18O + H
18O(3P) + 16OH f 16O18O + H
(R11)
(R12)
Therefore, by comparing the SO signal in measurements with
and without N2O, the OH concentration could be deduced via
[SO]n∞o N O - [SO]∞N O kR11 + kR12
2
2
[OH] ) ∆[SO2]N0 O
2
[SO]n∞o N O
2
kR11
The first factor in the right-hand side product is the initial
concentration of SO radicals and O(3P) atoms ([O(3P)]0 ) [SO]0)
produced in the photolysis of SO2. The second factor is the
fraction of SO radicals lost in the reaction with OH radicals.
Values for these two factors were taken from the experiment
(see Figure 2, left panel). The last factor corrects for the loss of
OH radicals with O(3P) atoms with rate constants taken from
the literature.24,25 To find an error estimate for the OH
concentration associated with the rate constants, we assumed
that the relative errors for both rate constants are equal. Then
relative errors of 30% in each rate, a value which we chose
conservatively, would cause a 20% error in the OH concentra-
tion.

8418 J. Phys. Chem. B, Vol. 109, No. 17, 2005
Fockenberg et al.
TABLE 3: Mechanism for the CH3 + OH Reaction: Bimolecular Reactionsa
reaction
A
â
Ea
refs and comments
O(1D) + H2 f OH + H
O(1D) + H2O f 2OH
6.624 × 1013
1.300 × 1014
2.650 × 1012
4.030 × 1014
2.950 × 1014
1.800 × 109
3.870 × 104
1.391 × 1014
1.024 × 108
3.064 × 1016
1.867 × 1014
1.861 × 1014
5.100 × 1014
9.000 × 1012
6.000 × 1014
0.000 × 1012
2.900 × 1014
6.240 × 1015
6.000 × 1010
6.000 × 1010
6.000 × 1012
6.000 × 1012
1.144 × 1014
9.000 × 1014
5.000 × 1012
5.800 × 1012
2.400 × 1012
6.022 × 1012
4.000 × 1014
4.000 × 1014
1.200 × 1014
7.080 × 1014
1.350 × 1014
0
0
0
0
0
0
2.7
0
1.6
-1.42
0
0
0
0
0
0
0
0
0
0
0
28
28
28
28
28
O(1D) + H2O f O + H2O
O(1D) + N2O f NO + NO
O(1D) + N2O f N2 + O2
O(1D) + He f O + He
upper limit, see ref 55
56
25
57
rate constant from ref 58
59
rate constant from ref 60
300 K, rate constant from ref 61
300 K, yield from ref 62
600K, see above
600 K, see above
rate constant from ref 63
20% deactivation
assumed
O + H2 f H + OH
6260
-218
3299
0
3279
O + OH f O2 + H
OH + H2 f H2O + H
CH + H2O f CH2O + H
CH + H2 f CH2 + H
CH + N2O f products
-511.5
CH2(S) + H2 f CH3 + H
CH2(S) + H2 f CH2(V) + H
CH2(S) + H2 f CH3 + H
CH2(S) + H2 f CH2(V) + H
CH2(S) + H2O f CH2(V) + H2O
CH2(S) + H2O f CH3 + OH
CH2(V) + He f CH2(S) + He
CH2(V) + He f CH2 + He
CH2(V) + H2O f CH2 + H2O
CH2(V) + H2 f H2 + H2
CH2 + CH3 f C2H4 + H
CH2 + CH2 f C2H2 + H + H
CH2 + O2 f CO + OH + H
CH2 + O2 f 2H + CO2
0
0
0
0
0
0
0
0
0
0
0
0
0
-0.7
-0.7
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
compare to NH2 in ref 37
assumed
assumed
23
here, see also references in ref 32
56
56
56
1500
1500
1500
-1788
CH2 + O2 f O + CH2O
CH2 + H f CH + H2
CH2 + OH f CH2O + H
CH2 + OH f CO + H2 + H
CH2 + O f CO + H2
CH3 + O f CH2O + H
CH3 + O f CO + H2 + H
CH3 + OH f CH3OH
CH3 + OH f CH2(S) + H2O
CH3 + OH f HCOH + H2
CH3 + OH f CH2O + H2
CH2O + OH f HCO + H2O
CH2O + H f HCO + H2
HCO + M f H + CO + M
HCO + H f H2 + CO
57
0
0
0
0
0
assumed, compare to CH3 + O reaction in ref 31
assumed
57
31
31
see text
see text
see text
see text
64
57
65
65
65
65
65
57
66
57
67
67
56
65
65
5.811 × 106
1.265 × 108
1.566 × 1014
9.033 × 1014
1.018 × 1014
3.000 × 1014
3.000 × 1014
2.168 × 1014
7.527 × 1014
7.226 × 106
1.441 × 106
6.279 × 106
5.000 × 1014
1.807 × 1014
1.018 × 1014
7.838 × 1014
2.403 × 100
1.150 × 1014
0.230 × 1014
2.03
1.62
0
0
0
0
0
0
0
2
2
2.0
0
0
-1263.8
2170
15759
0
0
0
0
0
HCO + OH f H2O + CO
HCO + O f OH + CO
HCO + O f H + CO2
HCO + HCO f CH2O + CO
HCO + CH3 f products
C2H6 + OH f C2H5 + H2O
CH3OH + OH f CH2OH + H2O
CH3OH + OH f CH3O + H2O
CH2CO + H f HCCO + H2
CH2CO + H f CH3 + CO
CH2CO + OH f CH2OH + CO
CH2CO + CH2(S) f C2H4 + CO
CH3COCH3 + OH f H2O + CH3COCH2
CHD2 + D f CD3 + H
0
864.4
-840
1500
8000
3378
0
0
-0.33
4.0
0
0
61
68
-900
0
0
69; used for the CD3 + OH reaction
assumed
CD3 + H f CHD2 + D
0
a Data are given in Chemkin II format: k ) ATâ exp(-Ea/RT), with A in mol cm3/s, T in K, and Ea in cal/mol. CH2(S) stands for singlet
methylene; CH2(V) stands for vibrationally excited triplet methylene. Not listed explicitly are CH2(V) reactions, which are the same as those for
CH2.
Isotopically labeled S18O2 was used here to separate the
S18O16O produced in the reaction from precursor S18O2 so that
the absolute radical concentration could be determined cleanly
from the drop in the S18O2 concentration after photolysis. The
dependence of the OH concentration and thereby ultimately the
O(1D) production on the laser intensity was measured by varying
the laser power (see Figure 2, right panel). The O(1D) production
as a function of laser intensity as well as the upper limit for the
wall loss of OH of 50 s-1 agreed well with the results obtained
with the first method. In addition, similar yield-intensity
relationships could be determined for SO2 and acetone as well,
whereby the ratios of the resulting proportionality factors
equaled the ratios of the respective absorption coefficients at
193 nm,26-28 which is consistent with the notion that the fraction
of photolyzed precursors is a product of absorption coefficient,
laser intensity, and an empirical common factor (e.g., {∆[N2O]}/
{[N2O]} ) Cσ1N93OInte1x9c3imer). This relationship was used further
on to determine2 the O(1D) concentration for all other experi-
ments.
Simulation Calculations. With a new fitting program,
concentration profiles calculated using the Chemkin II program

Reaction of CH3 with OH Radicals
J. Phys. Chem. B, Vol. 109, No. 17, 2005 8419
TABLE 4: Mechanism for the CH3 + OH Reaction: Pressure-Dependent Reactionsa
reaction
A
â
Ea
refs and comments
CH + H2 (+ M) T CH3 (+ M)
LOW/4.820 × 1025 - 2.80 590.0/
TROE/0.578 122.0 2535.0 9365.0/
CH3 + H (+ M) T CH4 (+ M)
LOW/6.528 × 1023 -1.80 0.00/
TROE/ 0.63 61.00 3315.00/
CH3 + CH3 (+ He) f C2H6 (+ He)
LOW/8.05 × 1031 -3.75 981.6/
TROE/0.00 570.00 1000.00/
CH3 + O2 (+ M) T CH3O2 (+ M)
LOW/4.406 × 1024 -3.00 0.00/
SRI/0.60 1.0 1.0/
C2H4 + H (+ M) f C2H5 (+ M)
LOW/4.714 × 1018 0.00 760.00/
TROE/0.76 40.0 1025/
C2H4 + OH (+ M) f C2H4OH (+ M)
LOW/3.477 × 1021 -0.80 0.00/
SRI/0.60 0.001 0.001/
1.970 × 1012
0.43
-370
56
57
2.108 × 1014
2.500 × 1015
1.762 × 1016
3.974 × 109
5.300 × 1012
2.500 × 1015
0
0
175
0
-0.69
-1.7
1.28
0
54, 10% higher A∞
28
1292
0
57
28
CD3 + CHD2 (+ He) f C2HD5 (+ He)
LOW/8.05 × 1031 -3.75 981.62/
TROE/0.00 570.00 1000.00/
-0.69
175
same as CH3 + CH3; used for the CD3 + OH reaction
a Data are given in Chemkin II format: low- and high-pressure limit rate constants k ) ATâ exp(-Ea/RT) with A∞ in mol cm3/s, A0 in mol2 cm6/s,
T in K, and Ea in cal/mol. Further information on the format can be found at, e.g., http://www.me.berkeley.edu/gri_mech/data/k_form.html.
3
TABLE 5: Mechanism for the CH3 + OH Reaction:
similar. However, with the overall rate constant for the CH2
+ OH reaction chosen to be the same as the one for the CH3 +
O(3P) reaction, a 50% yield for the CH2O channel is large
enough to explain all of the formaldehyde observed. The CO
channel was assumed to contribute the remaining 50% of the
overall rate constant.
Unimolecular and Wall Reactionsa
reaction
A
â
Ea
refs and comments
O(1D) f O
3.000 × 103
8.000 × 101
5.000 × 101
3.000 × 103
0
0
0
0
0
0
0
0
0
0
determined independently
see text
used here
used here
cumulative loss
HCOH f CH2O 9.000 × 103
OH f wall
CH2 f wall
CH2(S) f wall
With respect to the CH3 + OH reaction, the calculated barrier
heights14 leading to HCOH + H2 products make these channels
more likely than the CH2O + H2 channel. Here we assume that
the isomerization of HCOH to CH2O is almost instantaneous.
However, no experimental data that could confirm or refute this
assumption are available.
a Data are given in Chemkin II format: k ) ATâ exp(-Ea/RT) with
A in s-1, T in K, and Ea in cal/mol.
package29 were fitted iteratively to selected mass traces from
multiple data sets simultaneously. The current version allows
rate constants (A, â, and Ea; see Tables 3-5 for definition),
initial concentrations, calibration constants, and signal offsets
to be varied. The minimization routine consists of a genetic
algorithm followed by a Simplex algorithm. We chose the
genetic algorithm for its robustness in a multidimensional
parameter space and its capability to explore this space
efficiently. The quality of the concentration profiles at each
iteration is evaluated according to a merit function, e.g., ø2. The
overall goal is, as usual, to minimize the value of this merit
function.
Surprisingly, the simulated concentration profiles (see below)
for the methylene radicals predict a higher methylene signal
than actually measured in most measurements at 300 K, and
this also causes a higher prediction for the ethylene (m/e ) 30)
concentration (see Figure 3). This discrepancy is more pro-
nounced at lower CH3 concentrations and higher OH concentra-
tions; however, it disappears if hydrogen is used to generate
OH radicals or at the higher temperature of 612 K. Significantly,
higher loss rates for methylene (e.g., wall rates up to 700 s-1
)
or hydroxyl radicals on the reactor wall or lower concentrations
of OH radicals could be ruled out as reasons for this discrepancy
because these would be incompatible with observations made
in the control measurements. To test if the OH concentrations
as determined via the calibration measurements (see above) were
too high, fits were performed on two datasets (one with H2O
and one with H2/H2O), where the input concentrations for OH
radicals were chosen to be half their original value. Fewer OH
radicals lead to smaller methylene concentrations, shifting the
simulated profiles into the “right” direction. However, the fit
for the H2O case is still far from being satisfactory. With respect
to the product distribution, the methanol yield had to be raised
The reaction mechanism is summarized in Tables 3-5. We
wanted to focus our attention on the product distribution of the
CH3 + OH reaction, such as that of methanol and formaldehyde.
In earlier measurements we failed to detect methanol at room
temperature; this was attributed to the relatively low concentra-
tion of hydroxyl radicals used. To clarify this point, higher OH
concentrations were employed here, which, however, made it
necessary to include many more reactions in the mechanism.
The analysis of the reaction mechanism revealed that the 3CH2
+ OH reaction played a major role with respect to the
formaldehyde signal observed at 300 K. In the absence of
3
experimental or theoretical data on the CH2 + OH reaction,
1
by a factor of 3, which still leaves CH2 + H2O as the major
one can only speculate about possible product channels.
Presuming that an adduct is formed first, it is interesting to note
that this [H2C-OH]q adduct is connected via a low-lying barrier
to isomerization to the [H3C-O]q complex obtained in the CH3
+ O(3P) reaction, which has two major product channels: CH2O
+ H (∼84%) and CO + H2 + H (∼15%).30,31 One might assume
that the product distributions for the two reactions would be
product channel with a yield of at least 70%.
In addition, an overall rate constant for the CH3 + OH
reaction of less than 3 × 10-11 cm3/s would be necessary to
explain the behavior observed; this would be in contradiction
to literature data, which places the rate constant around (5-6)
× 10-11 cm3/s at 300 K and 4 Torr of helium.32-34 For the fit

8420 J. Phys. Chem. B, Vol. 109, No. 17, 2005
Fockenberg et al.
constant at 300 K and 4 Torr of helium. Because methanol
doesnot suffer the difficulties associated with multiple sources,
a yield of 6 ( 3% can be given, where the uncertainty was
estimated by considering the uncertainty in determining the rate
1
constant for the CH2 + H2O channel as described above.
Above 600 K the situation is completely different. Because
the CH2O and CH3OH channels of the title reaction have higher
yields, fewer OH radicals needed to be generated. In combina-
tion with higher methyl radical concentrations used, uncertain
reactions such as the 3CH2 + OH reaction played a lesser role.
Again, setting the two product channels alternately to 100%
and 0% changed the yield of the formaldehyde channels of the
CH3 + OH reaction only by 5%. At 612 K and 8 Torr of helium,
the total rate constant and yields determined for the various
channels are
CH3 + OH (+ He) f products (5.8 ( 1.2) × 10-11 cm3/s
f CH3OH (+ He)
20 ( 7%
f 1CH2 + H2O
46 ( 14%
Figure 3. Experimentally observed traces and simulated signal profiles
for the 13CH3 + OH reaction. The three calculated profiles in the lower
panel were obtained by changing the rate constant for the quenching
of vibrationally excited triplet methylene by helium from 2 × 10-13
cm3/s (upper line) to 1 × 10-13 cm3/s (middle line) to 0 cm3/s (lower
line). T ) 302 K, P ) 4 Torr. Initial concentrations (in cm-3) used in
the simulation calculations: [H2O] ) 9.4 × 1014, [N2O] ) 1.53 × 1014,
[O2] ) 2.3 × 1011, [OH] ) 5.5 × 1010, [O(1D)] ) 4.0 × 1011, [13CH2]
) 4.4 × 1010, [13CH3] ) 1.76 × 1012, [H] ) 1.1 × 1011, [13CH2CO] )
2.3 × 1010, [13CH3CO13CH3] ) 1.12 × 1013.
f CH2O + H2/HCOH + H2 34 ( 8%
The main uncertainties at this temperature reside with
determining initial radical concentrations and the associated
calibration constants used to convert counts into concentrations.
The two are directly connected, and variations in calibration
measurements for stable as well as radical species typically lie
within 20%. To get an estimate for the error in the rate constant
of the CH3 + OH reaction, the best value was varied by (50%,
keeping the product distribution as it is. While this does not
have a large influence on the profiles of the main species (methyl
radicals and ethane), the 50% profiles for methanol and
formaldehyde lie on the outer limits of the signal noise.
Therefore, an error of 20% was adopted for the overall rate
constant. Furthermore, a 10% change in the yield of formalde-
hyde and a 20% change in the yield of methanol would place
their simulated concentration profiles just within the noise of
the data. Together with the uncertainty in the calibration
constants, this leads to the error quoted for the yields.
of the data obtained at 300 K, the rate constant was fixed at 4
× 10-11 cm3/s.
Recent measurements from this laboratory35 on the quenching
of 1CH2 showed that it is possible for vibrationally excited 3CH2
1
to be converted back to CH2. In the absence of vibrational
deactivation, a steady-state concentration of a few percent is
possible, implying that the reactivity of such an ensemble could
be dominated by reactions of singlet methylene. To explore this
3
possibility, vibrationally excited triplet methylene, CH2(V),
which could only be produced in deactivation reactions of singlet
methylene, was introduced to the reaction mechanism. Rate
constants for vibrational deactivation of 3CH2(V) were assumed
to be similar to those measured for NH2.36,37 Instead of listing
individual reactions, a cumulative first-order loss for 1CH2 was
chosen to account for reactions on the reactor wall and with
other molecules such as acetone. As can be seen in Figure 3, a
reactivation of 1CH2 can have measurable effects, in particular
on the observed methylene traces. Within the range of reasonable
values for steady-state concentration (here 3%) and vibrational
deactivation by He atoms, it was not possible to describe the
experimental methylene traces satisfactorily. However, all traces
acquired with H2 present and above 600 K showed little effect
of this recrossing, as expected, because the deactivation of
singlet methylene or its reaction with H2 outweigh all other
reactions.
To distinguish between the two formaldehyde-producing
channels at 612 K and 8 Torr of helium, perdeuterated acetone
was photolyzed, giving two CD3 radicals. For the simulation
calculations, the same mechanism was used as in the all-H-
atom system with a few new reactions added (see Tables 3 and
4). Experimental data and simulated profiles are shown in Figure
4. If hydroxymethylene is produced in the CD3 + OH reaction,
then in the absence of isotope scrambling the isomerization
should yield singly deuterated formaldehyde, CHDO (m/e )
31), in contrast to CD2O (m/e ) 32) resulting directly from an
HD loss from the [CD3OH]q complex. Indeed, both products
are found, and interestingly the CD2O yield is larger than (or
at least similar to) the one for DCOH, which is surprising
considering that the corresponding barrier height as calculated
by Yu and Muckerman14 for the dissociation of CH3OH lies
above both the CH3 + OH entrance channel and the barrier
leading to HCOH + H2. In general, the measured product
distributions of the CD3 + OH and the 13CH3 + OH reactions
According to the simulations, all the formaldehyde generated
3
at 300 K can be attributed to the CH2 + OH reaction. To
establish an upper limit for the formaldehyde production in the
3
CH3 + OH reaction, the second channel of the CH2 + OH
reaction, CO + H2 + H, was set to 100%, giving a value for
the HCOH + H2 channel of at most 10% of the total rate

Reaction of CH3 with OH Radicals
J. Phys. Chem. B, Vol. 109, No. 17, 2005 8421
calculations with various models for weak energy-transferring
collisions.38,39 The MULTIWELL suite is particularly well
adapted to systems such as the one of interest here, where radical
recombination leads to an energetic product that can dissociate
through more than one reaction channel. The energy-grained
master equation resulting from the combination of energy-
transfer steps and reactions is solved using a stochastic method
originally developed by Gillespie.40-42 Barker and Ortiz39 have
successfully applied this method to the reactions of vibrationally
excited 2-methylhexyl radicals, which can isomerize by multiple
pathways to form any of six stable isomers, can fragment by
C-C or C-H bond fission, or can be stabilized by collision.
These methods have also been developed by Vereecken et al.
and applied to the reaction H + HNCO.43 The vibrationally
excited H2NCO thus formed can dissociate either to reactants
or to H2N + CO.
We have applied MULTIWELL to the CH3 + OH reaction
by calculating rate constants for the unimolecular reaction of
vibrationally energized CH3OH, formed with the appropriate
bond formation energy, to yield the products of reactions R1-
R7. The rate constants for the recombination reactions can be
obtained from the equilibrium constants and the unimolecular
dissociation rate constants. Our major interest is in branching
ratios, which are given directly by the MULTIWELL calcula-
tions. At temperatures up to 1000 K, reactions R6 and R7 are
found to be essentially negligible because of high energy
barriers.
RRKM rate constants for channels R3-R5 can be calculated
using the properties of the corresponding transition states (cis-
TS3, trans-TS3, and TS2) given by Yu and Muckerman (Table
7).14 Table 2 indicates that the Yu and Muckerman values for
enthalpies of reactions for these processes are in good agreement
with experimental values, although the Yu and Muckerman
enthalpies for dissociation of methanol to form either CH3 +
Figure 4. Experimentally observed traces and simulated signal profiles
for the CD3 + OH reaction. The dashed lines are simulated signal
profiles without the contributions of CD2O, CD3OH (top panel), and
CHDO (bottom panel). T ) 612 K, P ) 8 Torr. Initial concentrations
(in cm-3) used in the simulation calculations: [H2O] ) 2.3 × 1015,
[N2O] ) 1.4 × 1014, [O2] ) 3 × 1011, [OH] ) 9.2 × 1010, [O(1D)] )
2.53 × 1011, [CD2] ) 9.6 × 1010, [CHD2] ) 7.0 × 1010, [CD3] ) 3.35
× 1012, [H] ) 9.2 × 1010, [D] ) 2.0 × 1011, [CD2CO] ) 4.1 × 1010,
[CD3COCD3] ) 1.4 × 1013.
1
OH or CH2 + H2O are lower than experimental values by
∼1-2 kcal mol-1. Nevertheless, for the sake of consistency,
and because of some disagreement between values from the
two experimental databases, the values of Yu and Muckerman
were used in most of the present calculations. Also, from the
standpoint of determining the major branching ratios at ambient
temperatures, the important factor is the relative barrier heights
(intrinsic endothermicities) for channels R1 and R2. The Yu
and Muckerman calculations predict that the R2 barrier is higher
by 0.5 kcal mol-1. The Sandia reference data11 give a difference
of 0.8 kcal mol-1, whereas the NIST values12 indicate a
difference of 0.6 kcal mol-1 in the opposite direction. The data
analysis of de Avillez Pereira et al. indicated that there was
very little difference between these barrier heights, and their
are in good agreement, with somewhat more methanol and a
little less singlet methylene generated in the deuterated case.
CD3 + OH (+ He) f products
(5.8 ( 2.4) × 10-11 cm3/s
f CD3OH (+ He)
32 ( 8%
f 1CD2 + HDO
f DCOH + D2
f CD2O + HD
35 ( 8%
10 ( 3%
23 ( 16%
1
calculations were made with the barrier for the CH2 + H2O
channel higher by 0.4 ( 0.5 kcal mol-1. The calculations of
Here, the error in the rate constant is twice as high as in the
all-hydrogen case; this is a consequence of uncertainties in the
calibration constants, which were in general adopted from the
hydrogen-containing species. Because C2D6 was a major product
species in these reactions, its calibration constants were taken
from the analysis of the CD3-CD3 recombination under low
laser fluences, minimizing the production of CD2. The error
for the CD2O + HD channel was chosen to be twice its
1
Lin and co-workers18 lead to an energy for the CH2 + H2O
channel that is lower than that for the CH3 + OH channel by
1.6 kcal mol-1. This was criticized by Miller,44 who pointed
out that Ruscic45 had deduced a value of 0.39 ( 0.20 kcal mol-1
in the opposite direction, using experimental values for relevant
enthalpies of formation. Therefore, the use of the Yu and
Muckerman value for this energy difference appears to be valid.
determined value because the CD2O signal overlapped with the
Channels R1 and R2 proceed through “loose” transition states
because there is no barrier for the recombination reaction. In
such cases of loose transition states, it is customary to carry
out variational transition-state theory (VTST) calculations to
determine the parameters of the TS that provide the smallest
predicted unimolecular dissociation rate constant. Jordan et al.
have applied this procedure to reaction R1, using two models
+
much larger signal from the main fragmentation channel, C2D4
of the C2D6 ionization.
,
4. RRKM and Master Equation Calculations
A. MULTIWELL. Barker has developed a suite of computer
programs that combine RRKM unimolecular rate constant

8422 J. Phys. Chem. B, Vol. 109, No. 17, 2005
Fockenberg et al.
TABLE 6: Parameters for Reactions R1 and R2
R1
R2
(CH3OH f CH3+OH)
(CH3OH f CH2 + H2O)
T, K
a
k
bi (cm3 molecule-1 s-1
diss (molecule cm-3
uni (s-1
)
8.0 (-11)
5.3 (-11)
3.1 (-11)
3.38 (-39)
3.08 (-13)
9.66 (6)
2.70 (-49)
1.64 (-23)
2.99 (-4)
1.77 (15)
3.20 (15)
4.17 (15)
1.6 (-10)
1.6 (-10)
1.6 (-10)
1.03 (-39)
1.46 (-13)
5.66 (6)
1.65 (-49)
2.33 (-23)
9.06 (-4)
2.49 (15)
7.52 (15)
1.63 (16)
300
500
1000
300
500
1000
300
500
1000
300
500
1000
K
)
k
)
A∞ (s-1
)
a Reference 12.
TABLE 7: Properties of Molecules and Transition Statesa
for the CH3‚‚‚OH interaction,46 and their calculated rate constant
at 300 K is about twice as large as the experimental value
obtained by de Avillez Pereira et al.
An alternative to the VTST method has been suggested and
implemented for several recombination reactions by the Pilling
group.47 It was shown many years ago by Forst48 that the
Arrhenius form of the rate constant
CH3OH
300, 1065, 1098, 1179, 1425, 1497, 1536, 1551, 2936,
2982, 3280, 3797; 4.05 (1,1), 21.2 (1,2).
580, 1424 (2), 3071, 3314 (2); 3.62 (3,1), 1.81 (2, 2).
3613; 0.915 (1,2); electronic degeneracy, 4.
1468, 2868, 3129; 0.87 (2,1), 1.91 (1,2).
1709, 3734, 3851; 0.67 (2,1), 1.44 (1,2).
2374i, 924, 927, 1142, 1177, 1204, 1303, 1523, 1652,
2185, 2893, 3169; 5.08 (1,1), 19.3 (1,2).
Barrier height, 89.26 kcal/mol.
CH3
OH
CH2(1A1)
H2O
TS2
k∞ ) A∞ exp(-E∞/kT) )
cis-TS3
1338i, 522, 636, 888, 1112, 1228, 1296, 1454, 1480,
2415, 2866, 3711; 5.65 (1,1), 21.7 (1,2).
Barrier height, 86.97 kcal/mol.
∞ k(E)F(E) exp(-E/kT) dE/Q (1)
∫
0
trans-TS3
1419i, 516, 602, 951, 1118, 1201, 1318, 1499, 1536,
2138, 3199, 3746; 5.57 (1,1), 21.77 (1,2).
Barrier height, 85.36 kcal/mol.
is related by a Laplace transform to the function f(E) ) k(E)F(E),
where F(E) is the state density of reacting molecules at a specific
energy E and k(E) is the corresponding rate constant. In the
above expression, Q is the partition function. Thus, with s )
1/kT as the inversion parameter,
a Data are given as vibrational levels, cm-1 (degeneracy, if * 1);
moments of inertia, amu Å2, (symmetry, dimension).
total collision rate was calculated with a Lennard-Jones model,
and the probability function for energy loss was a single
exponential with ∆Edown ) 250 cm-1. To test the importance
of collisional deactivation, a few calculations were also made
using ∆Edown values of 125 and 500 cm-1. The energy
maximum was 40 000 cm-1, with a grain size of 20 up to 5000
cm-1 and a grain size of 160 cm-1 thereafter. Doubling the
energy maximum did not change the results calculated for 300
K.
f(E) ) k(E)F(E) ) L -1{Q(s)A∞ exp(-sE∞)}
(2)
With the added fact that L -1{Q(s)} ) F(E), the energy-
dependent rate constant becomes
k(E) ) A∞F(E - E∞)/F(E)
) 0 E < E∞
E g E∞
(3)
Thus, if the rate constant can be expressed in the Arrhenius
form, it is possible to obtain k(E) using only properties of the
reacting molecule and without invoking properties of a TS. De
Avillez Pereira et al. have applied this method to reactions R1,
R2, R6, and R7, using experimental determinations of the
recombination rate constants and calculated equilibrium con-
stants. MULTIWELL has a provision for using this method to
obtain k(E), which can then be used in further calculations. This
procedure is not completely straightforward: Although the
recombination rate constant, kbi(T), for both channels is almost
temperature independent and can easily be fitted to an Arrhenius
expression, the equilibrium constant, Keq(T), does not exactly
fit such an expression over a wide temperature range. Hence,
the resulting unimolecular rate constant, kuni(T), does not exactly
conform to the requirements of the inverse Laplace transform
(ILT) method. In the present calculations, A∞ was obtained using
the value of kuni calculated at each temperature and with the
energy barrier as the value of E∞ (this choice of the energy has
been suggested by Barker38). The equilibrium constants and rate
constants for these two channels are shown in Table 6. The
collisional data required for the ME calculations are those used
by de Avillez Pereira et al. and are as follows: σ[CH3OH], 3.626
Å; σ[He], 2.551 Å; ꢀ[CH3OH], 481.8 K; ꢀ[He], 10.22 K. The
The results of the MULTIWELL calculations in terms of
relative product yields at 300 K using the ILT method are shown
in Figure 5a. For comparison with experiment, where redisso-
ciation of methanol to reactants (Channel 1) is not observed,
and with Variflex calculations, which do not include this
channel, the relative yields were renormalized with this channel
omitted (Figure 5b). It should be noted in passing, however,
that R1 was the major product channel in all our calculations
except those at high temperature or pressure. By and large, the
relative yields are very similar to the relative product yields
that can be calculated from the pressure- and temperature-
dependent expressions given by de Avillez Pereira et al., but
with a sharper dropoff in the CH3OH channel at low pressures.
The present calculations indicate much larger relative yields of
the HCOH + H2 channels, which is a consequence of a barrier
height of 85.4 kcal/mol for t-HCOH formation, much lower than
the value of 91.0 kcal/mol used by de Avillez Pereira et al. Xia
et al. calculated a barrier height of 85.5 kcal/mol,18 almost
exactly the same as that used here. The relative yield of
formaldehyde is so low that the values given by the MULTI-
WELL calculations are probably accurate only to an order of
magnitude, given the time limit placed on the integration
process, but the de Avillez Pereira et al. expressions predict a

Reaction of CH3 with OH Radicals
J. Phys. Chem. B, Vol. 109, No. 17, 2005 8423
Figure 5. Relative product yields at 300 K in helium for the
dissociation of CH3OH produced by the reaction CH3 + OH. These
were calculated using MULTIWELL and the inverse Laplace transform
(ILT) method for channels R1 and R2 discussed in the text: (a) with
channel R1 included; (b) with channel R1 excluded and renormalized.
The parameters used in the calculation were taken from ref 14 (YM
set). In this and subsequent figures, the symbols for a given product
channel are fixed: methanol, squares; R1, pentagons; R2, circles; R3,
inverted triangles; R4, triangles; and R5, diamonds.
Figure 6. Relative product yields calculated with MULTIWELL and
the inverse Laplace transform (ILT) method for channels R1 and R2.
The experimental values of the dissociation energy for CH3OH f CH3
+ OH (91.2 kcal/mol) and for 1CH2 + H2O (91.6 kcal/mol) were used,
and the barrier for TS2 was reduced to 87.26 kcal/mol (YMexp2 set):
(a) 300 K; (b) 612 K. The solid square with error bars is the
experimental methanol yield obtained in this work.
by 2 kcal/mol is hereafter denoted as YMexp2. The results
obtained using the YMexp2 set are shown in Figure 6.
much lower relative yield of ∼10-8. The barrier height they
assumed is several kilocalories per mole larger than that used
in this work. The temperature dependence was determined using
the ILT method at 300, 500, and 1000 K, and at pressures of 1
and 760 Torr. Even at the highest temperature, the relative yield
of the HCOH products is probably below the detection limit.
The relative yields of channels R6 and R7, given the high
barriers for these channels, are found to be negligible even at
1000 K.
B. VARIFLEX. Klippenstein et al. have developed a suite
of programs suitable for various types of RRKM and ME
calculations.49 One of the examples included in this suite is the
CH3 + OH reaction, which was set up to model reactions R1
and R2 with the parameters used in the calculations of de Avillez
Pereira et al. The relative yields at 300 K and at various
pressures obtained in the VARIFLEX calculations are in good
agreement with the values obtained by de Avillez Pereira et al.
We extended these calculations to include reactions R3-R5
using the same input data, based on the Yu and Muckerman
PES, that were used in the MULTIWELL calculations. VARI-
FLEX includes an option for use of the ILT method, with the
recombination rate constant as the input parameter. The values
derived by de Avillez Pereira et al. were used: k-1 ) 8.0 ×
In an attempt to obtain better agreement between the present
calculations and the experimental results reported here, calcula-
tions were also made using a value of 91.2 kcal/mol for the
energy of dissociation of methanol to methyl and hydroxyl
radials. This value was obtained by extending the 298 K
experimental value11 of 93.1 kcal/mol to 0 K. To maintain the
balance between the R1 and R2 channels, the dissociation energy
10-11(T/300)-0.79 and k-2 ) 1.6 × 10-10 cm3 molecule-1 s-1
.
The same collision model was used, with a value of 230 cm-1
for ∆Edown . Rates were evaluated at an energy-resolved level,
using a Boltzmann distribution for the rotational energies. The
energy range was that used in the original example, from -6000
to 5000 cm-1 relative to the CH3 + OH fragments, and the
grain size was also that used in the example, 30 cm-1.
1
for the CH2 + H2O channel was set 0.39 kcal/mol above the
value of the R1 channel, in accordance with the experimental
difference between the two channels suggested by Ruscic. This
means that all other transition states are effectively lowered by
roughly 1.8 kcal/mol with respect to the R1 channel. This set
of energies is hereafter denoted as YMexp (YM refers to Yu
and Muckerman). An additional set of energies in which the
barrier height for formaldehyde formation was decreased further
The relative product yields calculated with VARIFLEX as a
function of pressure at 300 K are found to be very similar to
those obtained with the MULTIWELL calculations. Compared

8424 J. Phys. Chem. B, Vol. 109, No. 17, 2005
Fockenberg et al.
trations.50 At 298 K, their average value of the rate constant is
7.3 × 10-11 cm3 molecule-1 s-1, in good agreement with the
de Avillez Pereira et al. value. The present RRKM calculations
are in good agreement with these experimental values, as a result
of using the ILT method to obtain rate constants from
experimental data.
The experimental results for the CH3 + OH reaction at 300
K have to be considered carefully because it was not possible
to fit the methylene radical profiles satisfactorily. The most
probable explanation for the discrepancy between simulations
and data is an unidentified loss of OH radicals. Nevertheless,
most of the formaldehyde observed could be ascribed to other
sources, while a yield of 6 ( 3% for the methanol channel can
be given. This is in agreement with measurements by Deters et
al.,32 who measured a 1CH2 yield of around 90% at 298 K and
1 Torr of helium. However, it contradicts results by Oser et
al.,51 who stated that methanol is the prevalent channel under
the conditions used here. The experimental yield of methanol
(6 ( 3%) is much lower than the value at 4 Torr calculated
with MULTIWELL (54%, renormalized) or given by de Avillez
Pereira et al. (65%). MULTIWELL calculations using the
experimental value for the methanol dissociation energy decrease
the fraction of methanol formation to 30% by favoring other
channels. For the 1CH2 + H2O channel, there is good agreement
between the MULTIWELL calculations (41%) and those of de
Avillez Pereira et al. (35%). The higher dissociation energy
(YMexp set) reduces the MULTIWELL results slightly (32%).
All of these are much lower than our experimental value of
g70%, which is in reasonably good agreement with the value
of 89 ( 9% obtained by Deters et al. It should be noted that
the measurements of this relative yield by Deters et al. are based
on measurements of the 3CH2 radical using laser magnetic
resonance spectroscopy. This requires the assumptions that the
triplet radical is not formed in a direct reaction and that the
singlet methylene is rapidly converted to the triplet state by
collisions. Wilson and Balint-Kurti have used ab initio methods
to obtain a PES for the direct triplet formation,52 and they obtain
a barrier height of 6 kcal/mol. VTST methods gave a rate
constant of 4.0 × 10-16 cm3 molecule-1 s-1 at 298 K, so the
neglect of this possible channel is justified.
Figure 7. Bimolecular rate constants calculated using MULTIWELL
(ILT with YM set), solid line; VARIFLEX (long dashed line; compared
with those of de Avillez Pereira et al., short dashed line): (a) CH3 +
OH f CH3OH; (b) CH3 + OH f CH2 (1A1) + H2O.
with the MULTIWELL calculations, the pressure drop-off of
the CH3OH channel is not as rapid. The relative yield of the
1CH2 + H2O channel agrees quite closely with the MULTI-
WELL calculations. At low pressures, VARIFLEX predicts
slightly lower yields of the HCOH products, but the agreement
in the extremely small H2CO yield is very close. Barker38 points
out that bimolecular recombination rate constants for each
channel as a function of pressure can be obtained from
MULTIWELL results by using the expression
Our experiments do not distinguish between the two isomers
of HCOH, so the total calculated yield for both isomers should
be used for comparison. Experimentally, HCOH was not
detected under these conditions, in agreement with a MULTI-
WELL value of 5%. In contrast, the YMexp energies give barrier
heights for the HCOH + H2 channels that are 4.2 and 5.8 kcal/
mol lower than the energy for dissociation to reactants, leading
to a predicted yield of 38%, due to the reduced yield in other
channels. The VARIFLEX calculations of Xia et al. use similar
threshold differences of 3.8 and 5.6 kcal/mol. Their calculations
do not extend to pressures below 1 atm, but at this pressure the
yield of HCOH products is predicted to be 14%. Because de
Avillez Pereira et al. assumed a higher barrier for these channels
in their calculations, they predict essentially no yield of these
products. None of the calculations predict a measurable yield
of formaldehyde under these conditions, in agreement with
experiment.
k
bi,R ) (k∞uni,R/Keq,R) f(P,R)
(4)
where f(P,R) is the fraction of dissociations leading to channel
R at pressure P. A comparison of MULTIWELL and VARI-
FLEX values for channels R1 and R2 with those calculated from
the expressions in Appendix B in the work of de Avillez Pereira
et al. is given in Figure 7.
5. Comparison of Experimental and Calculated Results
300 K, 4 Torr. Under these conditions, the major products
(apart from redissociation to CH3 + OH) obtained from the
calculations are methanol and 1CH2 + H2O. Over the pressure
range used in the experiments of de Avillez Pereira et al., the
rate constant for methanol formation is essentially constant near
the high-pressure limit (8.0 × 10-11 cm3 molecule-1 s-1 at 300
K), within experimental error. Deters et al. made similar
measurements at pressures of 45-467 Torr of helium, using
transient UV absorption spectroscopy to follow radical concen-
612 K, 8 Torr. In contrast to the experiments carried out at
300 K, at 612 K the simulated profiles fit the experimental data
very well. The total rate constants measured here agree
reasonably well with the high-pressure value of 4.6 × 10-11
cm3/s given for this temperature by de Avillez Pereira et al.,10
keeping in mind that their measured rate constants showed little
pressure dependence above 10 Torr. However, as mentioned

Reaction of CH3 with OH Radicals
J. Phys. Chem. B, Vol. 109, No. 17, 2005 8425
above, the conditions used here were not very sensitive to
absolute rate constants. With regard to the product distribution,
it could be shown that the methanol, singlet methylene, and
formaldehyde channels are all important. For methanol, the
experimental value of 20 ( 7% is in good agreement with the
de Avillez Pereira et al. calculated value (31%), and MULTI-
WELL gives a value of 18%, which is reduced by a factor of
2 with the (higher) experimental dissociation energies (YMexp
set). Humpfer et al.9 obtained a value of 8% for this channel at
700 K and 0.5 Torr. For the 1CH2 + H2O channel, there is again
good agreement between MULTIWELL (77%) and de Avillez
Pereira et al. (69%); the MULTIWELL calculation with higher
dissociation energies (YMexp set) decreases this value to 56%.
The experimental value obtained in this work (46 ( 14%) is
somewhat lower. Humpfer et al. obtain a value of 38%, in good
agreement with the value found in this experimental work. For
HCOH, the experimental value of 10 ( 3% (DCOH) is in
reasonable agreement with the value obtained with MULTI-
WELL (5%), but the higher dissociation energy calculations
give a value of 32%. Humpfer et al. obtained a value of 46%
in their experiments with OD in place of OH. Calculations still
predict very low yields of formaldehyde; even the higher
dissociation energy MULTIWELL model predicts only ∼1%.
This is in distinct contrast to the experimental yield of 23 (
16% (CD2O), which is about 3 times the yield observed by
Humpfer et al. (7%). MULTIWELL calculations with the
YMexp2 parameters, which decrease the barrier for TS2 by 2
kcal/mol in the higher dissociation energy model, resulted in
an increased yield that was still only about 5% (Figure 6b).
As suggested above, the experimental result that the methanol
yield increases with temperature in the pressure range between
1 and 10 Torr is in apparent disagreement with the calculations.
This may arise from the choice of parameters in the energy-
transfer model employed in the ME calculations. Here we have
employed a temperature-independent ∆Edown , but Figure 8
shows the calculated methanol yield at 300 and 612 K for three
different ∆Edown values (125, 250, and 500 cm-1). There is
considerable precedent for invoking a model in which ∆Edown
increases with temperature, especially for recombination reac-
tions (see, e.g., the methyl-methyl recombination reaction).53,54
As seen in Figure 8, the present experimental yield of methanol
is in excellent agreement with the choice of ∆Edown ) 150
cm-1 at 300 K and 500 cm-1 at 612 K. This is qualitatively in
accord with the predictions of a biased random walk (BRW)
model that predicted values of 220 and 360 cm-1, respectively.
Figure 8. Methanol yield calculated with MULTIWELL and the
inverse Laplace transform (ILT) method for channels R1 and R2 for
three values of the ∆Edown parameter: 125, 250, and 500 cm-1. The
experimental values of the dissociation energy for channels R1 and
R2 were used, and the barrier for TS2 was reduced to 87.26 kcal/mol
(YMexp2 set): (a) 300 K; (b) 612 K. The solid square with error bars
is the experimental value obtained in this work.
of the methanol channel is rapidly varying and, as shown by
comparison of the MULTIWELL and VARIFLEX results, is
sensitive to the parameters of the RRKM calculation and the
implementation of the ME solution. In addition, the product
yields show a high sensitivity toward the relative position of
the transition states in each set.
Despite these sensitivities to unknown parameters, general
trends of pressure and temperature dependence of the product
distribution can be captured theoretically. At 300 K, qualitative
agreement between the present calculations and experimental
results was obtained with the Yu and Muckerman energy values
leading to 1CH2 + H2O as the dominant products, with methanol
being of lesser importance. At 612 K, isomers of formaldehyde,
1CH2 + H2O, and methanol are all important channels in the
calculated results when the experimental dissociation energies
for R1 and R2 were chosen. While the yield of isomers of
formaldehyde calculated with the experimental dissociation
energies for R1 and R2 at 612 K (identified as mass 30 isomers
in Figure 9) is in excellent agreement with experiment, the
calculated yield of formaldehyde per se falls well below that
observed. It appears that the energy of the transition state to
CH2O would have to be significantly lower than the energy of
6. Conclusions
The CH3 + OH reaction has been investigated both experi-
mentally and theoretically. Isotopic substitution (12C f 13C and
H f D) facilitated a clean distinction between the main product
species in the experimental system: formaldehyde isomers,
methanol, and ethane. While the analysis of the experimental
data at 300 K and 4 Torr remains inconclusive, the observations
1
indicate that CH2 + H2O is the main product channel and
stabilization to methanol is of minor importance. At 612 K and
8 Torr, the data show that three channels (1CH2 + H2O, CH2O/
HCOH + H2, CH3OH) are operating with similar yields, with
the direct formation of formaldehyde being more efficient than
the production of HCOH.
Two different types of RRKM methods (VARIFLEX and
MULTIWELL) have been applied to calculate the product
distribution of this reaction using different sets of theoretically
and experimentally obtained energies of the potential energy
surface. In the pressure range of interest (1-10 Torr), the yield
1
the entrance channel, CH3 + OH, or the CH2 + H2O product
channel, and it is unlikely that errors in the ab initio calculations
of Yu and Muckerman could be large enough to justify such
an assertion.

8426 J. Phys. Chem. B, Vol. 109, No. 17, 2005
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Figure 9. Relative product yields of CH2, CH3OH, and isomers of
CH2O (mass 30 isomers) at 612 K calculated with MULTIWELL and
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Choosing a constant value for ∆Edown could not explain the
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From an experimental point of view, further experiments,
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Acknowledgment. The authors thank Dr. Matthew Eager
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National Laboratory under Contract DE-AC02-98CH10886 with
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