J. Chim. Phys. (1998) 95, 2129-2142
C EDP Sciences. Les Ul~s
Rate constants for the reactions of CH30
with CH20, CH3CH0 and i-C4Hqo
C. ~ittschen*,B. Delcroix, N. Gomez and P. Devolder
Laboratoire de Cinetique et Chimie de la Combustion, URA 876 du CNRS, Centre d'Etude
et de Recherche Lasers et Applications (CERLA), Universite de Sciences
et Technologies de Lille 1, 59655 Villeneuve-d'Ascq cedex, France
(Received 8 April 1998; accepted 26 June 1998)
Correspondenceand reprints.
F&SUME
Les constantes de vitesse des reactions du radcal methoxyle (CH30) avec 3 reactifs
ont ete mesurees par deux techques diffbentes: la photolyse laser (LP) et le reacteur
a ecoulement rapide (W), toutes deux associees a une detection de CH30 par
fluorescence induite par laser (LIF). La reaction avec le formaldehyde CHzO a ete
etudiee dans la gamme de tempkratures 295 - 450 K. Ces deux techques conduisent a
des valeurs de kl bien coherentes et representees par I'expression kl = (1.1k0.3) X 10-l2
exp(-9.6k1.0 kJ mol-' / RT) cm3 S-', independante de la pression. La reaction avec
l'acetaldehyde CH3CH0 a ete etudiee dans la gamme de temperatures 286 - 493 K.
L'ensemble des valeurs est bien represente par kz = (5.7k1.3) X 10-l3exp(-5.2f0.7 kJ
mole' 1 RT) cm3 S-', independante de la pression. Avec la t e c h q u e de LP aucune
reaction n'a pu etre detectee jusqu'a 356 K pour la reaction de CH30 avec l'isobutane iC4HI03,)l(
d'ou la valeur k t e : k3 1 2.5 X 10-15cm3S-'.
Mots cles: cinetique, chunie atmospherique, rahcaux methoxyle, aldehyde
ABSTRACT
The rate constants for the reactions of CH30 radlcals with 3 reactants have been
measured by two Merent techques : laser photolysis (LP) and fast flow reactor (FT),
both coupled with a detection of CH30 radcals by laser induced fluorescence. The
reaction with formaldehyde CH20 has been measured in the temperature range 295 450 K. Both sets of results (LP and FF) are in excellent agreement and lead to a
pressure independent rate constant of kl = ( l .li0.3) X 10'12 exp(-9.6k1.0 kJ mol-' /
RT) cm3 S-'. The reaction with acetaldehyde CH3CHO has been measured between
286 and 493 K. Both sets of experiments are again in excellent agreement and lead to
a pressure independent rate constant of kz = (5.7k1.3) X 10-l3exp(-5.2k0.7 kJ mol" 1
C. Fittschen et a/.
2130
RT) cm3 S-'. No reaction could be detected with isobutane i-C4Hlo at temperature up to
356 K, leading to an upper limit for k31 2.5 X 10-l' cm%d1.
Key words: lunetics, atmospheric chemistry, methoxy radcals, aldehyde
INTRODUCTION
Alkoxy rachcals are important species both in atmospheric and in combustion
chemistry. They are formed during oxidation of hydrocarbons. In a £irst step, akyl
radcals react with O2 to form peroxy radlcals R02, whlch may then react to alkoxy
radcals via two reaction paths, whether NO is present or not:
R02 + NO
+ R 0 + NO2
R02 + R'O2
+RO+RYO+02
Isomerisation of alkoxy radcals by intrarnolecular H-atom transfer plays an
important role in atmospheric chemistry [l ,2]. It is believed that hgher alkoxy radicals
react in the atmosphere m d y by either unirnolecular decomposition
CxHyCH20
+ C,H! + CH20
C,H,CH20
+ C,Hy.~CH20H
or isomerisation
whle the fate of smaller alkoxy radcals up to C3 is m d y by reaction with 0
C,HyCH20 + 0
2
+ C,H,CHO
2
+ H02
Decomposition of alkoxy radcals as well as their reaction with 0 2 has already been
subject of several h e c t rate constant measurements, especially for smaller radcals up
to C4H90 [3]. Reactions of hlgher alkoxy radcals are more =cult
to observe, due to
difficulties in the clean preparation of the radicals as well as the lack of a selective
detection of the radcals and the reaction products. This is especially true for
isomerisation reactions, since hydroxyalkyl radcals are not easily detectable.
Rate constants for the reactions of CHjO with CH20, CH3CH0 and i-C4Hlo
2131
Viskolcz et al. [4] have recently shown by systematic ab-initio calculations, that the
banier heights for isomerisation reactions of different alkyl radcals (C3 to C6) are the
sum of two independent factors : the ring strain energy of the correspondmg cyclic
transition state and the activation energy of the relevant bimolecular abstraction by the
CH3 radical. For example the barrier height for the 1,5-isomerisation of the l-pentyl
rahcal is computed to be very close to that of the corresponding bimolecular reaction
CH3LC& since the relevant ring strain energy is negligibly small. In his recent review
on alkoxy rahcals, Atkinson [5] has suggested, that this additive rule should also be
valid for alkoxy radicals. Since various theoretical calculations conclude that the ring
strain energy is m d y determined by the size of the ring and not by its content, it is
important to measure the other parameter, i.e. the activation energy of the H-atom
abstraction reaction by alkoxy radcals (CH30 and CH3CH20 for example).
There are only very few published data upon such abstraction reactions, most of
them indirect.
For the reactions of CH30 with formaldehyde
CH30+ CH20+ products
(1)
and acetaldehyde
CH30 A CH3CH0 + products
(2)
there are only a few relative studies:
Reaction (l) has been measured by Toth et.al.[6] relative to the reaction
CH30 + CH3OOCH3 + CH3OH + CH200CH3
Tley determined at 298 K a ratio of k , k
=
151. L
(4)
has been estimated by Barker
et.a1.[7] to l ~ , = 1 . 7 ~ 1 0cm3
- ' ~ S-' in the temperature range 391 - 432 K. Hoare et.al.[8]
have determined the rate constant of reaction (1) relative to
CH30 + CH3COCH3+ CH30H + CHzCOCH3
(5)
C . Fittschen et a/
2132
gving a ratio of kl/ks=2.84 exp(16.7kJ mol" 1 RT). The rate constant of reaction (5)
has to our knowledge not yet been determined, such that a reliable deduction of kl
fiom literature data seems difficult.
The rate constant for the reaction of CH30 with acetaldehyde (2) has been
determined by Kelly et.a1.[9]relative to the reaction
CH30 + 0
+ CH20 + H01
(6)
leadmg to a ratio of k2&=14 at 298 K. The reference reaction has been investigated
2
several times[] 0, l l]. Takmg a mean value of
=
2x10-15cm3s-l,one obtains a value
k2=2.8x10-l4cm3s-l. On the other hand Moortgat et al.[12] investigated the photooxidation of acetaldehyde and proposed a complex mechanism to account for their
experimental data. It turned out that the product dstribution was strongly dependent on
the rate constant for the reaction between CH30 and acetaldehyde. They deduced fiom
the complex mechanism a rate constant of k2 = 1.5 X 10-13cm3 S-' at room temperature.
Concerning the reaction of CH@ radicals with i-C4Hlo
CH30 + I-C4H10+ products
(3)
there are 3 studes: Berces et al. [l31 have measured the rate constant at 464 - 533 K
and determined the following Arrhenius expression: k3=3.32x10-I3exp(-17.15 kJ mol-I
R T ) cm3 S-'. Batt et al. [l41 have measured the same rate constant in the temperature
range 383
-
433 K and determined k3=6.61x10~13exp(-10.04
kJ mol" /RT) cm3 S-'.
Recently, Biggs et al.[15] reported an upper limit of k3 2 1 . 7 ~ 1 0 "cm3
~ S-' at 298 K.
T h g into consideration the scarcity of experimental results of H-atom abstraction
reactions, we have investigated the reaction of the most simple alkoxy radical CH30
with 3 molecules possessing an easily abstractable H-atom: CH20, CH3CH0 and iC4H10by two cllfferent absolute techmques, laser photolysis and fast flow reactor, both
coupled to a detection of CH30 by laser induced fluorescence.
Rate constants for the reactions of CH30 with CHpO, CH3CH0 and i-CaHlo
21 33
EXPERIMENTAL SECTION
Experiments have been performed by two Merent techques: laser photolysis and
hscharge flow, both coupled with a detection by laser induced fluorescence.
Laser Photofysis:
The reaction cell for the laser photolysis experiment is made of stamless steel and
can be heated to up to 600 K. The temperature is measured by a thermocouple, whch
can be moved into the reaction zone.
We have generated CH30 radicals by laser photolysis (Lambda Physik LPX 202i)
of CH30NO at 351 nm.The absorption cross section of CH30N0 is hlgher at 248 nm
[16], but it is well known that acetaldehyde undergoes at t h ~ swavelength photohssociation ( o = 8 X 10-~'cm2) with a quantum yleld of W0.51 to yield CH3-rahcals.
Methyhtrite CH3ONO has been synthesized accordmg to a well known procedure
[17]: H2S04 (30%) has been added slowly to a saturated solution of NaN02 in
CH30WH20,cooled down to 0°C. The methyl nitrite formed is carried away in situ by
a slow flow of nitrogen and passes first over KOH, than over CaC12. The product is
then trapped at -80°C, degassed and used without further purification. CH3ON0,
highly hluted in He, could be stored in a darkened pyrex balloon for weeks. Typical
methyhtrite concentrations were 2x10'~ cm-3. Because of the small absorption
coefficient of CH30N0 at 351 nm ( o
=
1.35x10-'~cm2) [l81 we have focussed the
excirner laser by two cylindncal lenses, leadmg to a beam profile of 0.6 X 0.6 cm2.
Typical laser energy was 40 mJ cm-2 in the focused beam, leadmg to a CH30concentration of 4x l0 l l cm-3.
The relative concentration of CH30 radcals was determined from the integrated LIF
intensity. The probe laser (beam profile: 0.4
X
0.3 cm2, 5-10 dlpulse) was a
frequency doubled dye laser (Quantel TDL 50, Rhodamine 610 / methanol) pumped by
a frequency doubled YAG laser (Quantel YG 781C). The probe beam runs anti-
21 34
C . Fittschen et al.
parallel to the photolysis beam through the cell. CH30 radicals were excited at 2,
=
292.6 nm and fluorescence was detected through a cut-off filter (photo-multiplier
Hamamatsu R928) at wavelengths longer than 305 nm, perpendicular to the laser
beams. The fluorescence signal is integrated in a boxcar (EG&G 4121B) and averaged
in a computer. A typical decay consists of 20 - 50 points at different delays between
the two lasers, each averaged over typically 30 laser shots. Different delajrs are
provided by a digtal delay generator (EG&G 9650), which is computer controlled. All
experiments have been performed at a repetition rate of 2 Hz.
The canier gas He ('N 45, &r Liquide) was used without further purification. The
gas flows were regulated by calibrated mass flow-meters (Tylan FC-260). To change
the concentration of the reactant the appropriate flow-meter was controlled by the
computer. The other flow meters could be read by the computer for concentration
calculation. A typical total flow rate of 2000 cm3/mn STP leads to a gas velocity
through the cell of 3 c d s , perpendicular to the laser beams.
Acetaldehyde was prepared for both experiments in the same way : commercially
available acetaldehyde (Aldrich, 99%) was degassed and a diluted mixture in Helium
was prepared and stored in darkened balloons. The concentration was calculated from
measured flows through calibrated flow meters.
Formaldehyde was prepared by two Merent methods : for the laser photolysis
experiments paraformaldehyde was depolymerized at 70°C and the CH20 was canied
away with a small flow of He. To avoid polymerization, the main flow of He was
added imrnedately. The concentration was measured by W-absorption at 312.5 nm
(Hg lamp). Because of the h & l y structured UV-absorption spectrum [19], the
absorption coefficient under our conditions has been determined by measuring the
absorption as a function of CH20 pressure, measured by a calibrated 0-100 Torr
pressure gauge.
Rate constants for the reactions of CH30with CH20, CH3CH0 and i-C4Hlo
0 5-]
21 35
Fig. 1 shows a plot ln(b/r) =
I
f([CH20]), linear over the
concentration
range.
The
entire
derived
absorption coefficient at 3 12.5 nm,
1
0.0
0.00
0.25
0.50
075
1.00
1.25
i
1.50
[CH,O] U 10.'~ cm?
Figure l : Plot of In (Id0 = f/[CH20]) -for the
measzrrement of the absorption coeficient of
C H 2 0 at 312.5 nm.
employed
for
our
measurements, was a
concentration
=
(6.9
+ 0.7) X
I O - ~cm2.
'
the flow
paraformaldehyde
tube experiments,
was
heated
to
120°C. To avoid polymerization, the CH20 monomer was passed through a trap with
P20j. The undiluted CHzO flow rate was measured by the rate of pressure increase in
a calibrated reservoir. The excellent agreement between both series of experiments
confirms, that complications due to CHzO loss on walls can be neglected.
Experiments using laser photolysis have been performed in 100 Torr of Helium for
CH20 and 200 Torr of He for CH3CH0. All experiments in the discharge flow set-up
have been performed between 1 and 2 Torr He.
FastJlow reactor :
The CH30 radicals are generated by the fast reaction :
F + CH30H + CH30 (CH2OH) + HF
The detection of CH30 was similar to that employed in the laser photolysis
experiments. To avoid excessive loss of CH30 on walls (injector or flow tube), both
surfaces were covered by a thm Teflon film. Typical wall decay rates are in the order
of kW0= 30
S-'.
For variable temperature experiments, the flow tube temperature is
controlled by a thermocouple placed in the injector.
It is well known that both CH30 and CH20H rahcals are formed in roughly equal
ylelds in a large range of temperature [20]. To make sure that CH20H radicals were
C . Fittschen et a1
21 36
not interfering in the lunetic investigations, a few control experiments have been
conducted with two other sources of CH30, free of CHlOH :
FtC&+CH3+- W
CH3 + NO2 + CH30 + NO
F + CHtONO + CH30 + FNO.
followed by
or
Rate constants obtained with the lfferent CHtO sources were equal w i h the
experimental error.
Ltke in our previous work [21] two minor corrections have been applied to the
observed pseudo first order decay rate k,,b, to derive the real pseudo first order rate kl :
(i) a correction for the pressure gralent along the flow tube (Poiseuille law)
(ii) a correction for the axial and ralal dlfision :
kl = b b s ( l ~ l ~ , b ~ ~ / d + ~ ~ ~ ~ / 4 8 ~ )
with v (cm S-') and R (cm) respectively the flow velocity and tube radus (a value of
D = 457 cm2 S-' at 297 K and 1 Torr has been adopted as in our previous work [21]).
RESULTS AND DISCUSSION
Reaction of CH30 + aldehydes
Typical plots of first order decays of LE-signals from CH30 are shown in fig. 2 for
the reaction with CH20 (LP experiments) and in fig. 3 for the reaction with CH3CHO
(FF experiments).
2.0
-
9
1 . 7 ~ 1 0cm"
'~
3 0x10'~cm"
4 . 4 ~ 1 0cm.'
'~
1.5
g
5
" l0
1.0
S
$ 0.5
/
5 . 6 0 ~ 1 0cmJ
'~
9 22x10" cm.'
14.4x10~cm~~
00
t (-c)
Figure 2: Typical deca-v rates o f CHjO-LIF
signals for the reaction o f C H 3 0 with CH20
resultingfrom LP experiments at 293 K and
100 Torr-fordifferent C H 2 0 concentrations.
t
(msec)
Figure 3: Tvpical decqv rates of CHjO-LIF
signals for the reaction of C H 3 0 with
CHjCHO resulting from FF experiments, at
3'3 K and 2 Torr for drfreree CH3CHO.
Rate constants for the reactions of CH30 with CH20. CH3CH0 and i-CaHlo 2137
The decays have been found truly exponential in all experiments. The derived firstorder rate constants k' have been plotted against reactant concentration. Typical
curves are shown in fig. 4 for CH20, resulting from LP experiments and in fig. 5 for
CH3CH0, resulting fiom FF experiments at hfferent temperatures (error bars represent
an estimated error of 10%).
Figure I : Plot o f f r s t order rate constants for
the reaction of CH30 with CH20 as afunction
of [CH201 for dzfferent temperatures. resulting
from LP experiments.
Figure 5: Plot o f f r s t order rate constants for
the reaction of CH30 with CHjCHO as a
function
of [CHjCHO] for
dzferent
temperatures. resulting from FF experiments.
The intercepts (i.e. [reactant] = 0) of these plots are due to reactions of CH30 with
impurities and self reaction and, in the case of LP experiments, diffusion out of the
observation volume or, in the case of FF experiments heterogeneous losses on the
reactor wall. A comparison of the intercept of such a plot k1= f([reactant]) with the
rate constant measured in the absence of reactant is a characteristic for the quality of
the experimental data. In all cases these two values have been found close.
The slopes of these plots ylelded
the birnolecular rate constants for the
reactions of CH30 with CHzO or
CH3CHO.
The
experimental
conhtions are listed in table (1).
Table 1: Experimental conditions for LP
and FF experiments.
C . Fittschen et a1
2138
Fig. 6 shows the temperature dependence of the obtained bimolecular rate constants
as Arrhenius plots. The agreement between the results of the two dfferent
experimental techmques is excellent and leads to the following Arrhenius expressions :
CH20
kl = (l .lf0.3) X 10-l2exp(-9.6k1.0 kJ mol-' 1 RT) cm3 S"
CH3CHO
k,
=
(5.7k1.3) X 10-l3exp(-5.2f 0.7 kJ mol-' / RT) cm'
-12.5
S-'
Both rate constant values are
LP i LIF
pressure independent and show a
positive temperature coefficient,
r -13 0-
9"
as
-13.5
is
expected for
classical
abstraction reactions. NevertheCHzO
2.00
2.25
less, the preexponentiel factors are
2.50
2.75
3.00
3.25
3.50
much lower than one would
1000 1 T (K-')
expect for a drect reaction with a
Figure 6: -4rrhenius plot for the rate constants o f the
reactions o f C H 3 0 with CH20 and CH30 with
CHjCHO, resulting .fr-om both experimental
techniques. Error bars represent statistical error.
Smallbarrier 1221,
The reactivity of CH30 radicals
can be compared with the reactivity of OH radtcals and 0 atoms versus the same
reactant. Table 2 shows a compilation of the correspondmg parameters and the
reaction enthalpies computed from the NIST data base[23].
Table 2: Arrhenius parameters and reaction enthalpies for the reactions of CH,O and
CH,CHO with different radicals.
--
CH3CHO ( A )
CHzO (F)
&[23]
kl mol-'
CH''
Jthis work]
OH
,
[24]
0
[24]
,
A
Ea
cm' S ' kJ mol-'
m,[23]
kl m0l-l
A
cm' S-'
E,
kJ mol-'
A F J A AE ~ F - E ~ A
W mol-'
- 63 5
1.05 X 10-12
9.5
- 75.7
5.75 X 10-l~ 5.2
1.8
4.3
- 127.1
1.0 X 10-"
0
- 139.2
5.6 X 10-12
-2.2
1.8
2.2
- 56.8
3 . 4 1~ ~ " 13 3
- 69 0
1.8 X 10-"
9.15
1.9
4.15
Rate constants for the reactions of CH30 with CH20, CH3CH0 and i-C4Hlo
2139
For all three radicals the pre-exponential factor is roughly 2 times hgher for the
reactions with CH20 than with CH3CH0. This could be explained by the fact, that
CH20 has two easily abstractable H-atoms whlle CH3CHO has only one.
In the case of an abstraction reaction, the difference in the activation energy
between CH20 and CH3CH0 reactions should correspond roughly to the difference in
the bond strength of the correspondmg C-H bond. McMiUen and Golden [25] have
calculated the bond strength of the C-H bond in both molecules to 364 kJ mol-I
(CH20) and 359.8 kJ mol-' (CH3CHO), pving a ddference of 4.2 kT mol-l. The
experimentally obtained differences between the activation energies are in excellent
agreement for the CH30 radical (4.3 kJ mol") and 0 atoms (4.15 kJ mol-l), while for
the OH radical the difference is somewhat smaller (2.2 kJ mol"). It can be noted that a
negative activation energy as is observed in the case of the reaction of OH + CH3CHO
is an unusual behavior for an abstraction reaction. Taylor et al. [26] have recently
proposed, on the basis of experiments and QRRK calculations, that this reaction is
m a y an addition reaction up to 550 K and that the abstraction path gets predominant
only at higher temperatures.
Also, the reaction between CH20 and OH, temperature independent over a large
range, has been the subject of many studies [f.e.27,28]. Employing ab-initio
calculations, Soto et a1.[29] have considered two reaction paths for reaction (1) : either
a direct abstraction path
OH + CH20 + H0.--HCHO+ H20 + HCO
(84
or an addition - elimination path
OH
CH20 + HO..-CH20 -+ H + HC(0H)O
(8b)
Their calculations resulted in a banier of 43.5 kJ mol-' for path (8b) and 15.0 kJ
mol-' for path (8a), in line with the experimental findmgs of pressure independence and
a lack of a detection of fonnic acid as a product.
C . Fittschen et a1
2140
Preliminary quantum chemistry calculations of the reaction of CH30 with CH20
[30] indicate a very low barrier for the abstraction path (-4 kJ mol") and an early
transition state, quite similar to the calculations relevant to reaction (8a).
Besides abstraction reaction, leading to the formation of CH30H and HCO, reaction
( 1 ) exlubits another exothermic set of products : CHlOH + CH20 (AHr = -33.4 kJ
mol-l). However, the corresponclmg 5-member cyclic intermediate should extubit a
banier well above the experimentally observed activation energy.
As a conclusion of the above &scussion we have not retained the possibhty of
addition-elimination in the reactions of CH30 with CH20 or CH3CH0 for the
following reasons :
- we have observed no pressure effect on the rate constant between 2 and 200 Torr;
- we observe a positive temperature coefficient;
-
prehmary quantum chemistry computations [30] with sophsticated methods
(MRSDCI and CASPT2) indicate a very low banier for the abstraction channel.
CH30 + i - C a l 0
l k s reaction has been investigated by the laser photolysis technique at temperatures
up to 356 K (LP). No reaction could be observed. The loss of CH30 radicals without
reactant, due to reaction with impurities, self reaction and dfision, is k0 = 50 - 70 S-'.
An increase of this loss of 50
S-'
due to a reaction after addtion of reactant can be
easily observed. On thls basis, considering the hghest employed reactant
concentration ([i-C4Hlo],
=
2.0 X 1016 cm"), we estimate an upper limit of the rate
constant of
k312.5x10-15cm3s-'
(at356K).
The use of higher reactant concentration was prohibited by unacceptable q u e n c h g
of the CH30 fluorescence.
Rate constants for the reactions of CH30 with CH20, CH3CH0 and i-C4Hlo 2141
An extrapolation of the Arrhenius expression of Berces et al. [l31 for reaction (3)
down to 356 K leads to a rate constant of k3
=
9.6 X 10-16 cm3 S-', well below our
upper limit, while an extrapolation of the expression proposed by Batt et al. [l41 leads
to k7 = 1.7 X 10-l4cm3 S-l, roughly 10 times hlgher than our upper limt. The recently
published [l 51 upper l h t for k3 I 1.7 X 10-l~cm3 S-' at 295 K is in agreement with our
result.
Conclusion
We have reported here the first direct measurements of the rate constants for the
reactions of CH30 radicals with two aldehydes : CHzO and CH3CH0. Experiments
have been performed by two different techmques, laser photolysis and fast flow
techmque. No pressure dependence could be detected and both reactions show a slight
positive temperature dependence, consistent with hydrogen abstraction being the
dominant mechanism. For the reaction of CH30 radicals with ~ - C ~ Honly
I O an upper
lirmt for the rate constant could be given.
Acknowledgment
The Centre dYEtudeset de Recherches Lasers et Application (CEIUA) is supported
by the Mmistere Charge de la Recherche, the Region N o r m a s de Calais and the
Fonds Europeen de Developpement Economique des Regions (FEDER). The authors
thank Prof. Horst Hippler, University of Karlsruhe, for helpful discussions.
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