Direct investigations of reactions of 1-butoxy and 1-pentoxy radicals using laser pulse initiated oxidation: Reaction with O2 and isomerisation at 293 K and 50 mbar

Article abstract of DOI:10.1039/a903301g

The reactions of 1-butoxy and 1-pentoxy radicals were studied using time-resolved and simultaneous measurement of NO₂ and OH concentrations in laser pulse initiated oxidation studies followed by numerical simulations of the concentration profiles. The alkoxy radicals were produced selectively by the excimer-laser photolysis of 1-butyl bromide and 1-pentyl bromide at 248 nm and subsequent reaction of the 1-alkyl radicals with O₂ and NO. Whereas NO₂ was detected by cw-LIF, OH was monitored by laser long-path absorption at 308 nm. All experiments were performed at 293 ± 3 K and a total pressure of 50 mbar. The reactions with O₂ and the isomerisations via a 1,5-H-shift, viz., CH₃CH₂CH₂CH₂O + O₂ → CH₃CH₂CH₂CHO + HO₂ (5) CH₃CH₂CH₂CH₂O → CH₂CH₂CH₂CH₂OH (6) CH₃CH₂CH₂CH₂CH₂O + O₂ → CH₃CH₂CH₂CH₂CHO + HO₂ (25) CH₃CH₂CH₂CH₂CH₂O → CH₃CHCH₂CH₂CHOH (26) were investigated. Their rate coefficients were varied, utilizing the FACSIMILE integrator, until the best fits were obtained. Whereas in the case of 1- butoxy radicals absolute rate coefficients k(5) = (1.4 ⁺ 0.7) x 10^-14 cm³ molecule^-1 s^-1 and k₆ = (3.5 ± 2) x 10⁴ S^-1 could be derived, only limiting values for the 1-pentoxy radical reactions k₂₅ ≤ 1 X 10^{- 13} cm³ molecule^-1 s^-1 and k₂₆ ≥ 1.0 X 10⁵ S^-1 were obtained.

Full text of DOI:10.1039/a903301g

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Direct investigations of reactions of 1-butoxy and 1-pentoxy radicals
using laser pulse initiated oxidation: reaction with O and
2
isomerisation at 293 K and 50 mbar
H. Hein, A. Ho†mann and R. Zellner
Institut fur Physikalische Chemie, Universitat GH Essen, D-45117 Essen, Germany
Received 26th April 1999, Accepted 7th July 1999
The reactions of 1-butoxy and 1-pentoxy radicals were studied using time-resolved and simultaneous
measurement of NO and OH concentrations in laser pulse initiated oxidation studies followed by numerical
2
simulations of the concentration proÐles. The alkoxy radicals were produced selectively by the excimer-laser
photolysis of 1-butyl bromide and 1-pentyl bromide at 248 nm and subsequent reaction of the 1-alkyl radicals
with O and NO. Whereas NO was detected by cw-LIF, OH was monitored by laser long-path absorption at
2
2
308 nm. All experiments were performed at 293 ^ 3 K and a total pressure of 50 mbar. The reactions with
O and the isomerisations via a 1,5-H-shift, viz.,
2
CH CH CH CH O ] O ] CH CH CH CHO ] HO
(5)
(6)
3
2
2
2
2
3
2
2
2
CH CH CH CH O ] CH CH CH CH OH
3
2
2
2
2
2
2
2
CH CH CH CH CH O ] O ] CH CH CH CH CHO ] HO
(25)
(26)
3
2
2
2
2
2
3
2
2
2
2
CH CH CH CH CH O ] CH CHCH CH CHOH
3
2
2
2
2
3
2
2
were investigated. Their rate coefficients were varied, utilizing the FACSIMILE integrator, until the best Ðts
were obtained. Whereas in the case of 1-butoxy radicals absolute rate coefficients k \ (1.4 ^ 0.7) ] 10~14 cm3
5
molecule~1 s~1 and k \ (3.5 ^ 2) ] 104 s~1 could be derived, only limiting values for the 1-pentoxy radical
6
reactions k O 1 ] 10~13 cm3 molecule~1 s~1 and k P 1.0 ] 105 s~1 were obtained.
25
26
Whereas the reaction of alkoxy radicals with O is chain
terminating, decomposition and isomerisation produce carbon
1
Introduction
2
The oxidative degradation of VOCs (volatile organic
centered radicals. Therefore, the branching ratio of alkoxy
radical reactions is the crucial parameter for the Ðnal distribu-
tion and yield of oxidation products. It a†ects the number of
NO/NO conversions and hence the concentration of NO .
compounds) in the presence of high NO levels is responsible
x
for the generation of tropospheric ozone. Its concentration
during photochemical smog episodes is of great concern
because ozone is highly toxic, not only to humans but also to
animals and plants. As a consequence, VOC oxidation has
been the subject of intensive research in recent decades. This
research activity led to a fairly comprehensive understanding
of various processes involved in VOC oxidation, e.g., the
initial attack of OH radicals or O (during day time) or NO
2
2
Since NO is readily photolysed, the branching ratio of
2
3
3
radicals (during night time) on the VOC.1h3
However, the corresponding database for alkoxy radical
reactions is far from being complete, although alkoxy radicals
represent a major branching point in VOC oxidation mecha-
nisms.
It is known that alkoxy radicals show three distinct com-
petitive reaction pathways, depending on their structure and
on environmental and experimental conditions.4h6 These
reactions are (i) reaction with O , generating an aldehyde or a
2
ketone and HO , (ii) unimolecular decomposition, forming an
2
aldehyde or a ketone and a shorter alkyl radical, and (iii)
isomerisation (1,5-H-shift) via a six-membered and nearly
strain-free transition state. This isomerisation forms a d-
hydroxyalkyl radical, which is further oxidized. After addition
of O and reaction with NO, it produces a d-hydroxyalkoxy
2
radical. These radicals are believed to undergo a second 1,5-
H-shift rather than to react with O or to decompose, ultima-
tely yielding d-hydroxycarbonyl compounds after subsequent
Fig. 1 Reactions of 1-pentoxy radicals under atmospheric condi-
tions. Note that there is a second isomerisation following the forma-
tion of the 4-hydroxy-2-pentoxy radical (in general: d-hydroxyalkoxy
radical).
2
H-abstraction by O .7 The reactions are shown schematically
2
in Fig. 1 using 1-pentoxy radicals as example.
Phys. Chem. Chem. Phys., 1999, 1, 3743È3752
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alkoxy radical reactions has a direct e†ect on the production
of ozone during photochemical smog episodes.
the excitation beam was coupled into a White cell. The detec-
tion limit of this arrangement is approximately 1 ] 1010 mol-
There are a number of di†erences between studies of alkoxy
radical reactions performed previously and the present work.
All investigations of butoxy and pentoxy radicals reported in
the literature have been indirect and/or relative measurements,
e.g., product studies using the FTIR technique.8 Moreover, it
was generally not possible to generate alkoxy radicals selec-
tively. In a typical smog chamber experiment, the oxidation
is initiated by the photolytic production of OH radicals,
which then react with the VOC under investigation. As a
consequence, the analysis of the experiment is complicated
since the multitude of all possible alkoxy radicals has to be
taken into account. Hence the determination of individual
rate constants is difficult and sometimes speculative. In
this paper, we present an investigation of the reactions of
1-butoxy and 1-pentoxy radicals with oxygen [reactions
(5) and (25)] and of their isomerisations by 1,5-H-shift
[reactions (6) and (26)], viz.,
ecules cm~3. It should be noted that NO impurities in the
2
NO gas mixture were measured prior to the oxidation experi-
ment by the NO detection system, thus deÐning the baseline
2
for the LIF measurement. Therefore, only NO produced in
2
the course of the oxidation is observed in the oxidation
experiments.
OH detection was achieved by laser long-path absorption
(LLPA) using an Ar` laser pumped ring-dye laser. The dye
laser was equipped with an intracavity SHG unit which sup-
plied the detection wavelength of 308.417 nm. This corre-
sponds to the Q (4) line of the A2&`ÈX2% (0,0) transition of
1
OH radicals. Again, mirrors in White conÐguration were used
to maximize the detection limit with a typical absorption
pathlength of 40 m, which resulted in a detection limit of
approximately 1 ] 109 molecules cm~3.
The experiments were performed in a slow-Ñow reactor
with the linear Ñow rate (approximately 70 cm s~1) chosen
such that the reaction mixture was completely exchanged
between consecutive experiments (repetition rate 1 Hz) in
order to avoid accumulation of oxidation products. Details of
the experimental arrangement have been described else-
where.9,11
CH CH CH CH O ] O ] CH CH CH CHO ] HO
3
2
2
2
2
3
2
2
2
(5)
(6)
CH CH CH CH O ] CH CH CH CH OH
3
2
2
2
2
2
2
2
The gaseous compounds O , N and NO were directly
CH CH CH CH CH O ] O ]
2
2
3
2
2
2
2
2
taken as delivered by the manufacturer (Messer-Griesheim).
Their purities were 99.995, 99.999 and 99.5%, respectively,
with the main impurities in the NO being NO and N O .
CH CH CH CH CHO ] HO (25)
3
2
2
2
2
2
2 5
CH CH CH CH CH O ] CH CHCH CH CH OH (26)
Since additional NO is accounted for by the NO detection
3
2
2
2
2
3
2
2
2
2
2
system and since these impurities are of the order of 0.5%,
they play no signiÐcant role in the oxidation experiments. Gas
mixtures of 1-bromobutane and 1-bromopentane were pre-
pared by expanding 10È20 mbar bromoalkane vapour into a
(for reaction numbering, see Tables 1 and 2) using direct and
time-resolved monitoring of the formation of NO and OH
2
radicals in the laser Ñash initiated oxidation of 1-butyl and
1-pentyl radicals. This technique has previously been applied
to studies of 2-butoxy radicals.9
20 l storage bulb and Ðlling the bulb with N up to a total
2
pressure of 1000 mbar. The reaction mixture consisted of O ,
2
NO, 1-bromoalkane and N (in the case of 1-bromopentane
2
oxidation only). 1-Bromobutane (Aldrich, P99% purity) and
2
Experimental
1-bromopentane (Fluka, P98% purity) were used without
The general method applied in this work was the time-
further puriÐcation.
resolved and simultaneous monitoring of NO and of OH
radicals during the Ñash initiated oxidation of VOCs in the
Typical
concentrations
were [N ] \ (0È1.2) ] 1018,
2
2
[O ] \ (0.02È1.2) ] 1018, [NO] \ (2.0È5.0) ] 1014, [1-bromo-
2
presence of O and NO . Since the temporal behaviour of the
butane] \ (0.5È2.0) ] 1014 and [1-bromopentane] \ (1.0È1.5)
2
x
experimentally observed concentration vs. time proÐles reÑects
] 1014 molecules cm~3. All experiments were performed at
293 ^ 3 K and a total pressure of 50 mbar.
the rate determining steps of NO and OH producing and
2
consuming processes, absolute rate coefficients can be
extracted from the proÐles using numerical simulations.
The experiments were pulse initiated by the excimer-laser
3
Sensitivity analysis
photolysis of
a
1-bromoalkane, i.e., 1-bromobutane
The method used to analyse the experimentally obtained NO
2
(CH CH CH CH Br) or 1-bromopentane (CH CH CH -
and OH concentrationÈtime proÐles was to simulate numeri-
cally the proÐles with complete chemical mechanisms. The
major advantage of this method is that individual radical
reactions can be investigated even if the reactions of interest
are part of a complex reaction scheme, e.g., an oxidation
mechanism which may consist of more than 20 reactions
occuring simultaneously. In order to use this method, two
conditions have to be fulÐlled: (i) the rate coefficients of the
major part of the mechanism have to be known and (ii) the
reactions under investigation must participate signiÐcantly in
the production and/or consumption of the two species
observed.
The Ðrst condition is largely fulÐlled because the oxidation
of VOCs has been the subject of intensive research for at least
two decades. This activity led to a fairly comprehensive under-
standing of the overall mechanisms of a variety of di†erent
VOC classes, e.g., alkanes and alkenes.12,13 In addition,
unknown rate coefficients can be deduced from analogous
reactions. For instance, the oxygen reaction of 1-propoxy rad-
icals will not be signiÐcantly slower or faster than that of 1-
butoxy radicals. Moreover, there are a number of SARs
(structureÈactivity relationships) for di†erent reactions
3
2
2
2
3
2
2
CH CH Br), at 248 nm. With an initial 1-bromoalkane con-
2
2
centration of (1È2) ] 1014 molecules cm~3, and a laser Ñuence
of typically 30È40 mJ cm~2, the initial concentration of
1-alkyl radicals was estimated to be of the order of 1010È1011
molecules cm~3. In the presence of O and NO, the photo-
2
lytically generated alkyl radicals are converted into primary
alkoxy radicals on a time-scale of \1 ms.
We cannot completely exclude that H atoms are a by-
product of the excimer-laser photolysis of the bromoalkane
radical precursor. However, in a study of the photochemistry
of iodoalkanes, Ross and Johnston10 were able to show that
only I atoms and alkyl radicals are produced at 248 nm. By
analogy, therefore, it may be assumed that the photolysis of
bromoalkanes also produces Br atoms and alkyl radicals
exclusively. However, even if H atoms in small yield were to
be produced in the photolysis of bromoalkanes, this would
not signiÐcantly change the results since the major species to
initiate formation of NO and OH is the alkyl radical.
2
NO was detected by cw-laser induced Ñuorescence (LIF)
2
after excitation at 488 nm by means of the direct output of a
10 W Ar` laser. In order to increase the excitation volume,
3744
Phys. Chem. Chem. Phys., 1999, 1, 3743È3752

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involved in VOC oxidation.14,15 In summary, rate constants
for the majority of reactions occuring in 1-butyl and 1-pentyl
oxidation can be found in the literature.
Of course, if the reactions of 1-butoxy and 1-pentoxy rad-
icals do not participate in the formation or the consumption
whereas our experiments were conducted at 50 mbar. This
makes a comparison difficult because the isomerisation rate
coefficient is expected to be pressure dependent. However, a
recent combined ab initioÈRRKM study of unimolecular reac-
tions of primary and secondary C ÈC alkoxy radicals per-
formed by our group24 indicates rate constants for the
isomerisation at 290 K and 50 mbar of 2.7 ] 104 and
7.5 ] 105 s~1 for 1-butoxy and 1-pentoxy radicals, respec-
tively.
2
5
of NO and OH, no information about their rate constants
2
can be drawn from the proÐles. Whether this condition, i.e.,
sufficient sensitivity of the proÐles towards the rate coefficients
under investigation, is fulÐlled must be veriÐed by simulations.
Such sensitivity tests are based on the mechanisms shown
schematically in Figs. 2 and 3 and in Tables 1 and 2. The
calculations were carried out with the GEPASI program
(version 3.20).16,17
The same theoretical analysis24 reveals a rate constant for
the isomerisation of the d-hydroxy alkoxy radicals occuring in
1-butyl and 1-pentyl oxidation, viz.,
OCH CH CH CH OH ] HOCH CH CH CHOH (12)
2
2
2
2
2
2
2
CH CH(O)CH CH CH OH ]
3.1 Reactions of 1-butoxy and 1-pentoxy radicals
3
2
2
2
CH CH(OH)CH CH CHOH (32)
As shown in Figs. 2 and 3, there are three competitive reaction
channels for 1-butoxy and 1-pentoxy radicals. They can react
3
2
2
(cf., Tables 1 and 2) of 3.2 ] 106 s~1 at 290 K and 50 mbar.
This value is in agreement with the SAR for alkoxy and d-
hydroxyalkoxy radical isomerisations given by Atkinson,7
who estimates 6.4 ] 106 s~1 at 290 K and atmospheric pres-
sure. However, this fast isomerisation prohibits the determi-
with O , yielding butanal and pentanal, respectively, and
2
HO . For this type of reaction a rate constant of 9.5 ] 10~15
2
cm3 molecule~1 s~1 has been predicted previously.7 They
may also isomerize via
a 1,5-H-shift generating a d-
hydroxybutyl or -pentyl radical. These radicals subsequently
nation of k
and k
in our work because the proÐles
12
32
add O and react with NO to form d-hydroxybutoxy or
measured only reÑect the rate determining steps of NO and
OH formation and consumption for which reactions (12) and
2
2
-pentoxy radicals. Although there are a number of studies
dealing with 1-butoxy isomerisation,7,8,21h23 we could not
Ðnd any study on 1-pentoxy isomerisation in the literature.
Moreover, the investigations of 1-butoxy isomerisation report-
ed in the literature were performed at around 1000 mbar,
(32) are not rate determining. Therefore, the proÐles are insen-
sitive towards k and k and no information about these
12
32
rate constants can be extracted.
Under our experimental conditions, i.e., at high NO concen-
trations, 1-butoxy and 1-pentoxy radicals may also react with
NO. This reaction has two pathways, either addition of NO to
form an organic nitrite or H atom abstraction generating
HNO ] butanal or pentanal, respectively. However, the exact
loss process of the 1-alkoxy radicals due to reaction with NO
is of little importance as far as the numerical simulations of
the proÐles observed are concerned. As a consequence, the
two pathways were treated as if they were a single NO reac-
tion.
In principal, primary alkoxy radicals can show a fourth
reaction pathway, namely decomposition to form formalde-
hyde and an alkyl radical containing one C atom less than the
parent VOC. The only primary alkoxy radicals for which
decomposition rate constants at room temperature are avail-
able from the literature are ethoxy, 1-propoxy and 1-butoxy.
Reported decomposition rate constants at 298 K and at
around atmospheric pressure are (in units of s~1)
4.8 ] 10~2,25 3.9 ] 10~3,22 7.7 ] 10~2,26 7.1 ] 10~3 27 and
0.31 7 for ethoxy, 116,25 0.18 22 and 49 7 for 1-propoxy and
0.225 22 for 1-butoxy radicals. These rate constants, if com-
pared with the reaction rates of the other pathways, were con-
sidered to be too slow to be of any importance. Hence the
decomposition was neglected in the mechanisms used for the
simulations.
1-Butoxy radicals. The sensitivity of the proÐles towards k ,
5
the rate constant for the O reaction of 1-butoxy radicals, viz.,
2
CH CH CH CH O ] O ] CH CH CH CHO ] HO (5)
3
2
2
2
2
3
2
2
2
was tested, using the mechanism displayed in Fig. 2 and sum-
marized in Table 1, by varying the rate constant and simulat-
ing the system. The resulting proÐles and changes in absolute
concentration and temporal behaviour are shown in Fig. 4.
This Ðgure reveals that signiÐcant changes in absolute yield
occur for OH only, whereas changes in the NO proÐle are
2
too small to be considered signiÐcant. It follows that no infor-
Fig. 2 Detailed mechanism used for numerical simulations of the 1-
butoxy oxidation. Second-order rate coefficients in cm3 molecule~1
s~1 and Ðrst-order rate coefficients in s~1 are taken from the liter-
mation about k can be drawn from the NO proÐle.
For the simulations the following concentrations and rate
5
2
coefficients were used: [O ] \ 1.2 ] 1018, [NO] \ 5.0 ] 1014
ature. The major Ñuxes predicted for [O ] \ 1.2 ] 1018 and
2
2
and [1-bromobutane] \ 2.0 ] 1014 molecules cm~3, k \ 9.5
[NO] \ 5.0 ] 1014 molecules cm~3 are indicated by the size of the
5
arrows.
] 10~15 cm3 molecule~1 s~1 and k \ 2.7 ] 104 s~1. These
8
Phys. Chem. Chem. Phys., 1999, 1, 3743È3752
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Fig. 4 Sensitivity of the NO (top) and OH (bottom) proÐles
2
towards k , the rate constant for reactions of 1-butoxy with O
5
2
([O ] \ 1.2 ] 1018 molecules cm~3).
2
cannot be formed by reaction (5) followed by reaction (15):
HO ] NO ] OH ] NO
(15)
2
2
Fig. 3 Detailed mechanism used for numerical simulations of the 1-
pentoxy oxidation. Second-order rate coefficients in cm3 molecule~1
s~1 and Ðrst-order rate coefficients in s~1 are taken from the liter-
Rather, the major part of NO is generated by the conversion
2
of organic peroxy radicals into oxy radicals, i.e.,
ature. The major Ñuxes predicted for [O ] \ 1.2 ] 1018 and
2
CH CH CH CH O ] NO ]
[NO] \ 5.0 ] 1014 molecules cm~3 are indicated by the size of the
3
2
2
2 2
arrows.
CH CH CH CH O ] NO (3)
3
2
2
2
2
concentrations and rate coefficients yield branching ratios for
the 1-butoxy radical reactions of 0.31 for reaction with NO,
O CH CH CH CH OH ] NO ]
2
2
2
2
2
OCH CH CH CH OH ] NO (9)
0.52 for the isomerisation and 0.17 for the O reaction. There-
2
2
2
2
2
2
fore, reaction with O plays only a minor role and isomer-
2
for which the rate of formation of NO is independent of k .
A closer look at the OH proÐle reveals that signiÐcant
2
5
isation and reaction with NO are the dominant pathways.
Since k has no signiÐcant e†ect on the NO proÐle, NO
changes in total yield occur only for k P 1.0 ] 10~14 cm3
5
2
2
5
Table 1 Reaction system used to simulate 1-bromobutane oxidation at 293 ^ 3 K and 50 mbar
No.
Reaction
ka
Ref.
1
2
3
4b
5
6
7
8
9
10b
11
12
13
14
15
16
17
18
19
20
CH CH CH CH Br ] hl ] CH CH CH CH ] Br
3
2
2
2
3
2
2
2
CH CH CH CH ] O ] CH CH CH CH O
7.5 ] 10~12
4.9 ] 10~12
1.1 ] 10~14
Varied
18
13
3
2
2
2
2
3
2
2
2 2
CH CH CH CH O ] NO ] CH CH CH CH O ] NO
3
2
2
2
2
3
2
2
2
2
CH CH CH CH O ] NO ] CH CH CH CH ONO
3
2
2
2
2
3
2
2
2
2
2
CH CH CH CH O ] O ] butanal ] HO
3
2
2
2
2
CH CH CH CH O ] CH CH CH CH OH
Varied
3
2
2
2
2
2
2
2
CH CH CH CH O ] NO ] products
4.6 ] 10~11
7.5 ] 10~12
4.9 ] 10~12
1.1 ] 10~14
9.5 ] 10~15
3.2 ] 106
13
18
13
3
2
2
2
CH CH CH CH OH ] O ] O CH CH CH CH OH
2
2
2
2
2
2
2
2
2
2
O CH CH CH CH OH ] NO ] OCH CH CH CH OH ] NO
2
2
2
2
2
2
2
2
2
2
O CH CH CH CH OH ] NO ] O NOCH CH CH CH OH
2
2
2
2
2
2
2
2
2
2
OCH CH CH CH OH ] O ] CHOCH CH CH OH ] HO
7
24
13
19
12
12
12
12
20
20
2
2
2
2
2
2
2
2
2
OCH CH CH CH OH ] HOCH CH CH CHOH
2
2
2
2
2
2
2
OCH CH CH CH OH ] NO ] products
4.6 ] 10~11
3 ] 10~11
8.1 ] 10~12
1.1 ] 10~10
7.6 ] 10~13
2.0 ] 10~12
10
2
2
2
2
HOCH CH CH CHOH ] O ] HOCH CH CH CHO ] HO
2
2
2
2
2
2
2
2
HO ] NO ] OH ] NO
2
2
HO ] OH ] H O ] O
2
2
2
OH ] NO ] HONO
OH ] NO ] HNO
2
3
OH ] di†usion
CH CH CH CH Br ] OH ] products
1.9 ] 10~12
3
2
2
2
a First-order rate coefficients in s~1, second-order rate coefficients in cm3 molecule~1 s~1. b Branching ratio for RO ] NO reactions calculated
2
after ref. 3.
3746
Phys. Chem. Chem. Phys., 1999, 1, 3743È3752

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Table 2 Reaction system used to simulate 1-bromopentane oxidation at 293 ^ 3 K and 50 mbar
No.
Reaction
ka
Ref.
21
22
23
24b
25
26
27
28
29
30b
31
32
33
34
15
16
17
18
19
35
CH CH CH CH CH Br ] hl ] CH CH CH CH CH ] Br
3
2
2
2
2
3
2
2
2
2
CH CH CH CH CH ] O ] CH CH CH CH CH O
7.5 ] 10~12
4.9 ] 10~12
3.1 ] 10~14
Varied
18
13
3
2
2
2
2
2
3
2
2
2
2 2
CH CH CH CH CH O ] NO ] CH CH CH CH CH O ] NO
3
2
2
2
2
2
3
2
2
2
2
2
CH CH CH CH CH O ] NO ] nitrate
3
2
2
2
2 2
CH CH CH CH CH O ] O ] pentanal ] HO
3
2
2
2
2
2
2
CH CH CH CH CH O ] CH CHCH CH CH OH
Varied
3
2
2
2
2
3
2
2
2
CH CH CH CH CH O ] NO ] products
4.6 ] 10~11
1.7 ] 10~11
4.7 ] 10~12
7.5 ] 10~14
8 ] 10~15
3.2 ] 105
13
18
13
3
2
2
2
2
CH CHCH CH CH OH ] O ] CH CH(O )CH CH CH OH
3
2
2
2
2
3
2
2
2
2
CH CH(O )CH CH CH OH ] NO ] CH CH(O)CH CH CH OH ] NO
3
2
2
2
2
3
2
2
2
2
CH CH(O )CH CH CH OH ] NO ] nitrate
3
2
2
2
2
CH CH(O)CH CH CH OH ] O ] CH COCH CH CH OH ] HO
7
24
12
19
12
12
12
12
20
20
3
2
2
2
2
3
2
2
2
2
CH CH(O)CH CH CH OH ] CH CH(OH)CH CH CHOH
3
2
2
2
3
2
2
CH CH(O)CH CH CH OH ] NO ] products
4.1 ] 10~11
3 ] 10~11
8.1 ] 10~12
1.1 ] 10~10
7.6 ] 10~13
2.0 ] 10~12
10
3
2
2
2
CH CH(OH)CH CH CHOH ] O ] CH CH(OH)CH CH CHO ] HO
3
2
2
2
3
2
2
2
HO ] NO ] OH ] NO
2
2
HO ] OH ] H O ] O
2
2
2
OH ] NO ] HONO
OH ] NO ] HNO
2
3
OH ] di†usion
CH CH CH CH CH Br ] OH ] products
3.5 ] 10~12
3
2
2
2
2
a First-order rate coefficients in s~1, second-order rate coefficients in cm3 molecule~1 s~1. b Branching ratio for RO ] NO reactions calculated
2
after ref. 3.
molecule~1 s~1. For smaller values of k the resulting changes
absolute values for k and k , therefore, is to Ðt k to the NO
5
5
6
6
2
in the OH proÐle are too small to be recognized in an experi-
proÐle Ðrst and in a second step to Ðx k using the OH proÐle.
5
mental proÐle (cf., Figs. 12 and 13). As a consequence, an
The relative error of k can be estimated from Fig. 7, in
6
absolute value for k can only be extracted if k P 1.0 ] 10~14
which [NO ] and [OH]
are shown as functions of k .
both increase with increasing k but
apparently reach a limiting value. [NO ] approaches an
5
5
2 =
max
6
cm3 molecule~1 s~1, otherwise only an upper limit can be
derived.
[NO ] and [OH]
2 =
max
6
2 =
To estimate the relative error of k , the NO concentration
upper limit of approximately 6 ] 1011 molecules cm~3 at
5
2
at inÐnite reaction time, [NO ] , and the maximum OH con-
k [ 1 ] 105 s~1. Hence the sensitivity drops, Ðnally reaching
2 =
6
centration, [OH] , are plotted against k in Fig. 5. The
error bars shown represent the 10% uncertainty of the NO
and OH detection systems. Clearly, the NO proÐle is insensi-
tive towards k . The OH proÐle, however, is sensitive towards
but the changes in absolute concentration are within the
errors of the OH detection system. Therefore, the relative
zero, if k is larger than 1 ] 105 s~1. Since the expected value
max
5
6
of k is well below this ““sensitivity thresholdÏÏ, an absolute
2
6
value for k can be extracted from the proÐle.
2
6
5
k
5
error of k is estimated to be 50%.
5
In a similar way as described above, the sensitivity of the
proÐles towards k , the rate constant for the isomerisation of
6
1-butoxy radicals, was veriÐed. The proÐles obtained and the
resulting changes are shown in Fig. 6. As opposed to k , Fig.
5
6 reveals signiÐcant sensitivity of both proÐles towards k if it
6
is varied around its expected value of approximately 2.7 ] 104
s~1. Since k is not reÑected by the NO proÐle, the NO
5
2
2
proÐle can be used to derive k . The procedure to obtain
6
Fig. 5 Dependence of the NO concentration at inÐnite reaction
2
time, [NO ] (left ordinate), and of the maximum OH concentration,
Fig. 6 Sensitivity of the NO (top) and OH (bottom) proÐles
2 =
2
[OH]
(right ordinate), on k . The error bars shown represent 10%
towards k , the rate constant for isomerisation of 1-butoxy ([O ] \
max
5
6
2
uncertainty of the NO and OH detection systems.
1.2 ] 1018 molecules cm~3).
2
Phys. Chem. Chem. Phys., 1999, 1, 3743È3752
3747

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Fig. 7 Dependence of the NO concentration at inÐnite reaction
Fig. 9 Dependence of the NO concentration at inÐnite reaction
2
2
time, [NO ] (left ordinate), and of the maximum OH concentration,
time, [NO ] (left ordinate), and of the maximum OH concentration,
2 =
2 =
[OH]
(right ordinate), on k . The error bars shown represent 10%
[OH]
(right ordinate), on k . The error bars shown represent
max
6
max
25
uncertainty of the NO and OH detection systems.
10% uncertainty of the NO and OH detection systems.
2
2
The dependence of [NO ] and [OH]
on k can be
1-Pentoxy radicals. The mechanism displayed in Fig. 3 and
2 =
max
6
easily understood by considering the mechanism in Fig. 2. As
long as the isomerisation rate is comparable to the rate deter-
mining steps of the straight downward route in Fig. 2, i.e., the
peroxy/oxy conversions [reactions (3), (9) and (15)], the sensi-
in Table 2 was used to test the sensitivity of the proÐles
towards k , the rate constant for the reaction of 1-pentoxy
25
radicals with O , viz.,
2
CH CH CH CH CH O ] O ] pentanal ] HO (25)
tivity of the proÐles towards k is large because the proÐles
3
2
2
2
2
2
2
6
reÑect the rate determining steps of NO and OH production
The concentrations and rate coefficients used are [O ] \ 1.2
2
or consumption. However, if rapid isomerisation leaves the
2
] 1018,
[NO] \ 5.0 ] 1014
and
[1-bromopentane]
\ 9.5 ] 10~15 cm3
peroxy/oxy conversions as the only rate determining steps, the
\ 1.5 ] 1014 molecules cm~3,
k
sensitivity of the proÐles toward k drops, ultimately reaching
25
molecule~1 s~1 and k \ 7.5 ] 105 s~1. They produce
6
zero.
26
branching ratios for the 1-pentoxy radical reactions of 0.05 for
Since the changes in [NO ] are of the order of the experi-
reaction with NO, 0.92 for isomerisation and 0.03 for reaction
2 =
mental uncertainty of the NO detection at around the
with O . Clearly, isomerisation is by far the dominant reac-
2
expected value of k , its relative error is estimated to be 50%
as long as k is in the range (0È5) ] 104 s~1. Otherwise, only a
lower limit for k can be derived.
2
tion pathway.
6
Fig. 8 summarizes the changes in the proÐles if k is varied.
6
25
As can be seen, if k is in the range (0È1.0) ] 10~13 cm3
6
25
Fig. 8 Sensitivity of the NO (top) and OH (bottom) proÐles
2
towards k , the rate constant for reaction of 1-pentoxy with O
Fig. 10 Sensitivity of the NO (top) and OH (bottom) proÐles
25
2
2
([O ] \ 1.2 ] 1018 molecules cm~3). Both proÐles are insensitive
towards k , the rate constant for isomerisation of 1-pentoxy with
2
26
towards k if it is varied around its expected value of 1 ] 10~14 cm3
([O ] \ 1.2 ] 1018 molecules cm~3). No signiÐcant changes occur if
25
2
molecule~1 s~1.
k
P 1.0 ] 105 s~1.
26
3748
Phys. Chem. Chem. Phys., 1999, 1, 3743È3752

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Fig. 11 Dependence of the NO concentration at inÐnite reaction
2
time, [NO ] (left ordinate), and of the maximum OH concentration,
2 =
[OH]
(right ordinate), on k . The error bars shown represent
max
26
10% uncertainty of the NO and OH detection systems. The diagram
2
clearly shows the ““sensitivity thresholdÏÏ of approximately k P 1.0
26
] 105 s~1.
molecule~1 s~1, both proÐles are insensitive towards k . The
25
changes that occur are too small to be recognized in an
experimental proÐle. Only if k is larger than approximately
25
5 ] 10~13 cm3 molecule~1 s~1 is the sensitivity increased
(Fig. 9). The changes are much smaller than the experimental
uncertainties of the NO and OH detection systems, if k is
2
25
varied around its expected value of approximately 1 ] 10~14
Fig. 12 Comparison between experimentally obtained and simulated
concentrationÈtime proÐles for the 1-butoxy system. k and k were
cm3 molecule~1 s~1. As a consequence, only an upper limit
for k can be extracted. Unfortunately, it is not possible to
shift the branching ratio of the 1-pentoxy radical reactions in
5
6
25
varied until the best Ðt was reached. In each case one of these rate
constants was held Ðxed at its best value while the other one was
favour of the oxygen reaction because the assumed [O ] \ 1.2
varied. Experimental conditions: [O ] \ 1.2 ] 1018, [NO] \
2
2
] 1018 molecules cm~3 corresponds to a pressure of 50 mbar,
5.0 ] 1014 and [1-bromobutane] \ 2.0 ] 1014 molecules cm~3,
which is the maximum pressure allowing NO Ñuorescence to
T \ 293 K, p \ 50 mbar, photolysis wavelength j \ 248 nm.
2
be detected.
The sensitivity of the NO and OH proÐles towards k
,
2
26
the isomerisation rate constant of 1-pentoxy radicals, is shown
in Fig. 10. Although there are signiÐcant interrelations, they
only occur if k is in the range (0È1.0) ] 105 s~1. If k is
26
26
larger than 1.0 ] 105 s~1, the changes in the proÐles are much
too small to be considered signiÐcant. This ““sensitivity
thresholdÏÏ also was noticed for k , the 1-butoxy isomerisation
6
rate constant.
Since the expected value of k is 7.5 ] 105 s~1, only a
26
lower limit for that rate coefficient can be derived. Fig. 11
shows the dependence of [NO ] and [OH]
Ðgure reveals large changes, and hence a good sensitivity for
on k . The
2 =
max
26
k
\ 1.0 ] 105 s~1. If k P 1.0 ] 105 s~1, [NO ] and
26 2 =
reach a plateau with no signiÐcant dependence on
26
[OH]
max
. The explanation of the ““sensitivity thresholdÏÏ is the same
k
26
as discussed for the 1-butoxy isomerisation in the preceding
section.
4
Results and discussion
Experiments with a variety of reactant concentrations were
performed. The concentrations of NO and of OH were mea-
2
sured simultaneously, and the FACSIMILE program was
used to reproduce the concentrationÈtime proÐles. For that
purpose again the mechanisms shown in Figs. 2 and 3 and in
Tables 1 and 2 were utilized. In contrast to the sensitivity
tests, in the FACSIMILE simulations the rate coefficients
under investigation were varied, until the simulated proÐles
were best Ðts to the experimentally obtained proÐles.
Typical examples for the agreement between experiment
and simulation are shown in Figs. 12 and 13 for 1-butoxy and
1-pentoxy, respectively. As can be seen, the mechanisms are
able to reproduce the experiments. Also included in these
Ðgures are the sensitivities of the calculated proÐles towards
Fig. 13 Comparison between experimentally obtained and simulated
concentrationÈtime proÐles for the 1-pentoxy system. k and k
were varied until the best Ðt was reached. In each case one of these
rate constants was held Ðxed at its best value while the other one was
25
26
varied. Experimental conditions: [O ] \ 1.2 ] 1018, [NO] \
2
5.0 ] 1014 and [1-bromopentane] \ 1.5 ] 1014 molecules cm~3,
T \ 293 K, p \ 50 mbar, photolysis wavelength j \ 248 nm.
Phys. Chem. Chem. Phys., 1999, 1, 3743È3752
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the rate coefficients varied, as discussed in detail in Section 3.
the enthalpy of the reaction are less certain, contributing to
Note that while k and k were varied in this sensitivity test,
the uncertainty of k calculated by eqn. (37).
5
25
5
k and k were held at their best Ðt values and vice versa.
26
Although in this work we performed a series of simulations
The rate coefficient k determined in the present work may
6
5
also be compared with the rate coefficients of other alkoxy
covering a variety of di†erent experimental conditions, we
present only one comparison for each radical system investi-
gated owing to limited space. These simulations are available
on request either as diagrams or in tabular form. The best Ðts
radicals with O . Direct investigations are available for
2
methoxy,32h38 ethoxy36,39,40 and propoxy41,42 radicals. The
IUPAC recommended13 rate constants at 298
K are
1.9 ] 10~15, 9.5 ] 10~15 and 8 ] 10~15 cm3 molecule~1 s~1
for methoxy, ethoxy and propoxy (1- and 2-propoxy), respec-
tively. In addition, Mund et al.42 determined rate coefficients
of 1.4 ] 10~14 and 6.0 ] 10~15 cm3 molecule~1 s~1 for 1-
propoxy and 2-propoxy, respectively, using LIF measure-
ments for the direct detection of the radicals. Moreover, in a
recent investigation9 of the reactions of 2-butoxy radicals,
applying the same method as described in this work, we
provide the following rate constants: k \ (1.4 ^ 0.7) ] 10~14
5
cm3 molecule~1 s~1, k \ (3.5 ^ 2) ] 104 s~1,
k
O 1.0
6
25
] 10~13 cm3 molecule~1 s~1 and k P 5.0 ] 105 s~1.
26
It should be noted that the discrepancy between the mea-
sured and simulated OH proÐles in the case of 1-pentoxy rad-
icals (Fig. 13) is due to an electronic oscillation superimposed
on the OH signal. The same oscillation, although signiÐcantly
smaller, can be seen in Fig. 12. This oscillation originates from
the electronic processing of the absorption signal. However,
for the determination of k and k only the Ðrst 3 ms of the
obtained
a rate coefficient for the oxygen reaction of
6.5 ] 10~15 cm3 molecule~1 s~1.
The rate constants of C ÈC alkoxy radicals for reaction
25
26
1
4
OH proÐle were used. Hence the oscillation did not perturb
the extraction of the rate coefficients.
with O are summarized in Table 3. The available kinetic data
2
suggest that primary alkoxy radicals tend to have rate con-
In addition to the rate constants extracted from the proÐles,
the simulations resulted in a semi-quantitative determination
of k , k , k and k , the rate constants for reactions of
stants slightly higher than their secondary analogues. This
reactivity trend is conÐrmed by k determined in this work.
5
Whether this conclusion may be valid for even larger alkoxy
3
9
23
29
peroxy radicals with NO. Although having been the subject of
research since the beginning of the 1970s, the rate coefficient
for this reaction has recently been suggested to be a factor of
two larger than previously believed. However, if we used the
values for k , k , k and k as determined by Eberhard and
radicals is the subject of further investigations in our labor-
atory.
A previous study of the O reactions of linear chain C ÈC
2
3
8
alkoxy radicals performed by our group43 indicated rate coef-
Ðcients of (7.3È8.0) ] 10~15 cm3 molecule~1 s~1 at 298 K.
However, in this investigation the oxidation of the parent
VOCs was initiated by Cl atoms which led to the simulta-
neous production of primary and secondary alkoxy radicals
and the results obtained were weighted averages for the two
types of alkoxy radicals. Nevertheless, the result obtained is
consistent with a higher rate coefficient for primary alkoxy
radicals (kO2 B 1 ] 10~14 cm3 molecule~1 s~1) and a corre-
sponding lower rate coefficient for the secondary radicals
(kO2 B 6 ] 10~15 cm3 molecule~1 s~1). Further work is
clearly necessary to conÐrm these conclusions.
3
9
23
29
Howard,28,29 we were unable to simulate the experimentally
obtained proÐles. This applies in particular to the OH proÐle.
Since the simulated proÐles are very sensitive towards the dif-
ferent k(RO ] NO)s, as has been discussed in Section 3, this
2
is a strong indication that the rate constants determined by
Eberhard and Howard are signiÐcantly too high.
4.1 Reactions of 1-butoxy radicals
Reaction with oxygen. Since the value of k derived in this
Isomerisation. Although there are a number of investiga-
tions dealing with 1-butoxy isomerisation,7,8,21h23 no absol-
ute rate constants are available in the literature. All previous
studies provided rate constant ratios k /k from which k can
5
work was determined independently of k , it has to be con-
6
sidered as an absolute determination. To our knowledge, there
is only one previous investigation of the O reaction of 1-
2
butoxy radicals. Morabito and Heicklen21 determined the rate
5
6
6
be deduced. However, a direct comparison between previously
coefficient relative to that of nitrite formation. At 293 K they
reported 1.34 ] 10~14 cm3 molecule~1 s~1, which is in good
agreement with our results.
reported rate coefficients and k as determined in this work
6
should be made carefully for several reasons. First, all studies
reported used the relative rate technique to determine the rate
In his recent review of alkoxy radical reactions, Atkinson7
proposed two SARs for estimating the rate coefficients of reac-
tion with O . In the Ðrst one the only distinction made is that
between primary and secondary radicals. The SAR for
primary alkoxy radicals is
2
Table 3 Summary of available rate constants for alkoxy radical reac-
tions with O at 295 ^ 3 K
2
k
/
O2
k \ 6.0 ] 10~14
Radical
cm3 molecule~1 s~1
Method
Ref.
5
] exp([550/T ) cm3 molecule~1 s~1
(36)
CH O
3
1.9 ] 10~15
9.5 ] 10~15
9.5 ] 10~15
8 ] 10~15
Recommendation
SARa, recommendation
SARa
Recommendation
13
7, 13
7
13
42
7, 13
42
7
C H O
2
5
from which a rate constant at 293 K of 9.2 ] 10~15 cm3
molecule~1 s~1, in satisfactory agreement with the result from
this work, is derived. However, the second SAR,
1-C H O
3
7
1.4 ] 10~14
8 ] 10~15
LIFb
2-C H O
SARa, recommendation
LIFb
3
7
6.0 ] 10~15
9.5 ] 10~15
1.34 ] 10~14
(1.4 ^ 0.7) ] 10~14
8 ] 10~15
k \ 4.0 ] 10~19 ] n ] exp([0.28
1-C H O
SARa
RRSc
LPOd
SARa
LPOd
SARa
LPOd
5
4
9
21
] *HO2/kcal mol~1) cm3 molecule~1 s~1 (37)
This work
7
9
7
2-C H O
4
9
where n is the number of abstractable H atoms, yields a rate
coefficient of 4.0 ] 10~15 cm3 molecule~1 s~1 at 293 K, a
factor of 3.5 lower than that found here. This discrepancy
might be expected because only methoxy, ethoxy and 2-
propoxy radicals were used in the derivation of eqn. (37).
Moreover, only for these three radicals are recommended
values for their heats of formation available.13 For butoxy
and pentoxy radicals, the heats of formation30,31 and hence
6.5 ] 10~15
9.5 ] 10~15
O1.0 ] 10~13
1-C H
O
5
11
This work
a StructureÈactivity relationship; 9.5 ] 10~15 and 8 ] 10~15 cm3 molecule~1
s~1 for primary and secondary alkoxy radicals, respectively. b Direct detection
of the radicals. c Relative rate study; k determined relative to nitrite formation
O2
for which a rate constant of 3.4 ] 10~11 cm3 molecule~1 s~1 was adopted.
d Laser Pulse initiated oxidation.
3750
Phys. Chem. Chem. Phys., 1999, 1, 3743È3752

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of isomerisation. While Morabito and Heicklen21 measured k
6
relative to nitrite formation, all other experimental studies
used the oxygen reaction as reference.8,23 Since the reference
reactions have their own uncertainties, the error of k is
6
potentially enhanced. Second, because of severe difficulties in
producing linear alkoxy radicals with more than three C
atoms, it was hitherto not possible to generate the radicals
selectively. In typical smog chamber experiments, all possible
alkoxy radicals are produced simultaneously, although with
characteristic yields according to the relative reactivity of dif-
ferent H atoms in the parent VOC towards OH radicals. This
complicates the analysis of product yields, making the extrac-
tion of individual rate coefficients difficult and somewhat spe-
culative.
Furthermore, all previous studies were carried out near
atmospheric pressure, whereas our experiments were con-
ducted at 50 mbar. Only the work of Morabito and
Heicklen21 was performed at 533 mbar. Since the fall-o†
behaviour of alkoxy radical isomerisations is not known, a
direct comparison with our results is not possible. For that
Fig. 14 Fall-o† curve of 1-butoxy radicals obtained by RRKM cal-
culations (T \ 290 K)23 and comparison of isomerisation rate con-
stants derived in this work and those calculated from previous studies
using 9.5 ] 10~15 cm3 molecule~1 s~1 as rate coefficient for the O
2
reaction.7
value for k as derived in this work is limited, our experi-
reason, it seems more appropriate to compare k with the
26
6
ments conÐrm the di†erence between these two types of isom-
results from ab initio/RRKM calculations.24 This theoretical
erisations qualitatively.
analysis indicates an isomerisation rate constant at 50 mbar of
2.7 ] 104 s~1, whereas the predicted value for 1000 mbar is
5.0 ] 104 s~1. The agreement between the theoretically
5
Conclusion
obtained and the measured value for k , as determined in this
6
The technique of time-resolved and simultaneous detection of
work, is satisfactory if the fact that an uncertainty of ^1 kcal
NO and OH and subsequent numerical simulation of the
mol~1 in the calculated energy barrier corresponds to an
uncertainty in the predicted rate constant of a factor of
approximately 5 is considered.
2
concentrationÈtime proÐles was used to study the reactions of
1-butoxy and 1-pentoxy radicals. The radicals were produced
selectively by the excimer-laser photolysis of their respective
bromoalkanes in the presence of O and NO . The reactions
The available data on 1-butoxy isomerisation are sum-
marized in Table 4. The detailed fall-o† curve, as provided in
ref. 24, together with experimental data, is shown in Fig. 14.
2
x
investigated are reaction with O and isomerisation. The
2
numerical simulations were carried out with the FACSIMILE
program, which used complete chemical mechanisms, and the
rate coefficients under investigation were varied until the com-
puted proÐles best Ðtted the experimentally obtained proÐles.
Prior to the determination of individual rate constants, the
sensitivity of the simulations towards the rate constants of
interest was tested in detail using the GEPASI code.
4.2 Reactions of 1-pentoxy radicals
As the concentrationÈtime proÐles are insensitive towards k
25
and k (as long as they are varied in their expected ranges),
26
only limiting values for these rate coefficients could be
obtained in this work. Therefore, k and k cannot be dis-
25
26
cussed in detail. To our knowledge, reactions of 1-pentoxy
radicals have not been studied experimentally before.
However, since 1-pentoxy radicals are primary alkoxy rad-
icals, their rate coefficient for reaction with O can be esti-
mated to be approximately 1 ] 10~14 cm3 molecule~1 s~1.
Unfortunately, our result of
In the case of 1-butoxy radicals, this method was found to
be adequate to extract absolute rate constants, whereas the
extremely fast isomerisation of 1-pentoxy radicals led to the
determination of limiting values for k and k only. The rate
2
25
26
coefficients obtained in this work conÐrm the previously
expected trends in reactivity for primary and secondary
alkoxy radicals such that primary radicals have slightly higher
k
O 1.0 ] 10~13 cm3
25
molecule~1 s~1 allows no assessment of the reactivity trends
of primary and secondary alkoxy radicals to be made.
Atkinson7 proposed that isomerisations proceeding via H
rate coefficients for reaction with O than secondary radicals.
2
Moreover, it was shown that isomerisations abstracting H
atom abstraction from a CH group are a factor of approx-
atoms from a CH group are signiÐcantly faster than those
2
2
imately 10 faster than isomerisations abstracting H atoms
which abstract the H atom from a methyl group.
from a CH group. Although the quantitative beneÐt of the
In summary, the technique used in this work must be con-
sidered a useful tool to study reactions which are part of a
complex reaction mechanism. Therefore, further investigations
of alkoxy radical reactions will be carried out in the future in
our laboratory.
3
Table 4 Summary of available isomerisation rate constants of 1-
butoxy radicals at 295 ^ 3 K
p/mbar
k
/s~1
Method
Ref.
Isom
Acknowledgements
1000
1.4 ] 105
RRSa
RRSa
SARb
RRSc
RRKM calculation
LPOd
23
8
7
21
Financial support of this work by BMBF within the German
Troposphere Research Programme (TFS) and by the Commis-
sion of the European Community within the SARBVOC
project (ENV4-CT95-0031) is gratefully acknowledged.
1.8 ] 105
1.6 ] 105
2.0 ] 105
2.7 ] 104
533
50
24
(3.5 ^ 2) ] 104
This work
a Relative rate study; k
determined relative to reaction with O for
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2
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3
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relative to nitrite formation for which a rate constant of 3.4 ] 10~11
cm3 molecule~1 s~1 was adopted. d Laser pulse initiated oxidation.
Phys. Chem. Chem. Phys., 1999, 1, 3743È3752
3751

View Article Online
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