Ion - Molecule Reaction Studies of Hydroxyl Cation and Ionized Water with Ethylene

Article abstract of DOI:10.1021/jp990226s

Rate coefficients and product branching ratios for the ion - molecule reactions of the hydroxyl cation, ionized water, and their deuterated analogues with ethylene have been determined using a selected ion flow tube (SIFT) at room temperature and in 0.5 Torr of helium buffer gas. In all cases, reactions proceed at or near the collision rate. The major product is always charge transfer: 79% for L₂O^?+and 66% for LO⁺ and does not depend on the isotopic form of hydrogen present (L = H or D). For the L₂O^?+ reactions, the remaining 21% of products are from proton or deuteron transfer, with no evidence of an isotope effect on this step even in the HOD^?+ reaction. The greater exothermicity of the initial charge transfer in the LO⁺ reaction is revealed by the observation of additional product channels, forming the vinyl cation and protonated carbon monoxide. Multistep mechanisms that proceed through rate-determining charge-transfer, followed by a product-determining step, are postulated to explain these observations.

Full text of DOI:10.1021/jp990226s

J. Phys. Chem. A 1999, 103, 4879-4884
4879
Ion-Molecule Reaction Studies of Hydroxyl Cation and Ionized Water with Ethylene
Vyacheslav N. Fishman and Joseph J. Grabowski*
Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260
ReceiVed: January 19, 1999; In Final Form: April 28, 1999
Rate coefficients and product branching ratios for the ion-molecule reactions of the hydroxyl cation, ionized
water, and their deuterated analogues with ethylene have been determined using a selected ion flow tube
(
SIFT) at room temperature and in 0.5 Torr of helium buffer gas. In all cases, reactions proceed at or near the
•
+
+
2
collision rate. The major product is always charge transfer: 79% for L O and 66% for LO and does not
depend on the isotopic form of hydrogen present (L ) H or D). For the L
of products are from proton or deuteron transfer, with no evidence of an isotope effect on this step even in
the HOD reaction. The greater exothermicity of the initial charge transfer in the LO reaction is revealed
by the observation of additional product channels, forming the vinyl cation and protonated carbon monoxide.
Multistep mechanisms that proceed through rate-determining charge-transfer, followed by a product-determining
step, are postulated to explain these observations.
•
+
2
O
reactions, the remaining 21%
•
+
+
Introduction
flange14 is capable of allowing high-quality quantitative data
to be collected for both rate coefficients and branching ratios,
by characterizing the reactions of L2O and LO , where L )
H or D, with H2CdCH2.
Detailed information about gas-phase ion-molecule reactions
can lead to a better understanding of the chemical composition
•
+
+
1
,2
3
of interstellar gas clouds, planetary atmospheres, combustion
4-6
7
processes, and even chemical ionization mass spectrometry.
+
•+
Experimental Section
The ions HO and H2O , and their deuterated analogues, are
extremely reactive species that contribute to each of these
All of the measurements reported here were performed with
the University of Pittsburgh’s selected ion flow tube (SIFT),
chemical environments. Ethylene is a known component of
circumstellar shells1,2 and is produced by photochemical and
14
the details of which have been previously described; only those
experimental details unique to this study are reported here.
thermal reactions involving methane in the hydrogen-rich
envelope on the Jovian planets (Jupiter, Saturn, Uranus, and
3
15
Reactant ions were produced in a Brinks type ionizer using
Neptune). The contribution of ethylene in combustion processes
of hydrocarbon fuels such as gasoline and kerosene has long
+
•+
electron ionization on water vapor: H2O for HO and H2O ;
+
•+
4
D2O for DO and D2O ; and the vapor from a 3:1 liquid-phase
been known.
•
+
mixture of D2O:H2O for production of HOD . The potential
difference between the filament and grid was selected (20-30
eV) to minimize production of excited states in the reactant ion.
At the same time, the potential of the grid with respect to the
grounded reaction tube was kept as low as possible both to
minimize the kinetic energy of the ions produced (in order to
improve ion separation in the first quadrupole) and to decrease
the likelihood of collision-induced dissociation (CID) during
the injection process. All ion-molecule measurements were
carried out in the presence of helium buffer gas at a pressure of
0.46-0.53 Torr and a temperature of 298 ((1) K. High-purity
helium (99.997%) was further purified by passage through a
liquid nitrogen cooled molecular-sieve trap before use.14 Eth-
ylene (technical grade, 98%) was used as received. The error
bars reported are one standard deviation based upon repeated
measurements taken over at least two experimental days. We
estimate the absolute error limits on a reported rate coefficient,
kobs, to be 20%, due to systematic errors involved in measure-
Isotopic fractionation of H-to-D in various molecules is
important when interpreting cosmic origins and abundances.
Some molecules are known to show an enhanced D-to-H ratio
that exceeds the conventional ratio by an order of magnitude
8,9
or more. Previous tandem mass spectrometry studies of several
ion-molecule reactions support the hypothesis that ion chem-
1
,2
istry is important in understanding nonconventional ratios.
Isotopic labeling, of course, is also an important tool for gaining
1
0
insight into a large variety of chemical reactions.
•
+
The reaction of H2O with C2H4 has been examined
previously; Dotan et al.,11 using a flow-drift tube, were able to
measure the rate coefficient as a function of kinetic energy
-
10
3
(
0.03-2 eV) in helium with k300 K ) 16 ((4.8) × 10
cm
-
1
-1
molecule s , but were unable to determine the reaction
products. Rakshit and Warneck, using a drift chamber mass
spectrometer, examined H2O with C2H4 in CO2 and reported
a rate coefficient (at undefined interaction energy and accuracy)
12
•
+
-
9
3
-1 -1
of 1.5 × 10 cm molecule s , and stated that the reaction
proceeds exclusively by charge transfer. Ion-molecule studies
16,17
ments of neutral and buffer gas flows and pressures.
Reaction
•
+
+
of extremely reactive species such as an H2O and HO can
efficiency, Eff, is the ratio kobs/kcoll and is based on a collision
rate coefficient, kcoll, calculated according to the variational
be challenging; Shul et al.13 were able to investigate several
+
•+
18
such systems only by simultaneously injecting HO , H2O ,
and H3O and at the cost of both being able to determine reliable
collision complex theory developed by Su and Chesnavich.
+
The absolute error limit on the primary product yields is
estimated to be no more than 5%. For the very fast reactions
being examined in this study, with equally fast secondary
branching ratios and increased error limits on rate coefficients.
Here, we demonstrate how our recently described SIFT injector
1
0.1021/jp990226s CCC: $18.00 © 1999 American Chemical Society
Published on Web 06/06/1999

4880 J. Phys. Chem. A, Vol. 103, No. 25, 1999
Fishman and Grabowski
reactions, trace ion products (e2% of the total products) are
difficult to quantify and therefore are omitted from consider-
ation.
Results
Due to the high reactivity of the ions under study with water,
and the difficulty of eliminating all adventitious water from the
16
helium, we tested our instrument for its ability to cleanly inject
•
+
D2O by first creating, selecting, and injecting m/z 20 in the
16
absence of the He buffer gas. The spectra shown in Figure 1a
and b demonstrate that clean injection of only m/z 20 can be
achieved in our instrument. After just a single m/z value is
selected and injected into the He-filled reaction tube, it may
react with the 0.4 ppm of H2O16 still present in the purified
•
+
helium. Thus when we inject D2O into the He-filled reaction
tube, we typically see spectra similar to Figure 1c. The reaction
•
+
of D2O (m/z 20) with H2O has been studied by Anicich and
19
Sen in an ICR; they found a rapid reaction (kobs ) 1.56 (+0.78,
-
9
3
-1 -1
-
0.22) × 10 cm molecule s ; Eff ) 54%) gives 21%
m/z 18, 19% m/z 19, and 59% m/z 21. In our experiment, (e.g.,
Figure 1c) we attribute the m/z 21, m/z 19, and a portion of the
m/z 18 signal to the reaction of D2O with adventitious water.
The m/z 18 peak also reflects a small amount of a CID process
•
+
(eq 1) that we are unable to completely eliminate while retaining
adequate signal.
He
•
+
+
•
-1
D O
8 DO + D + ∆H ) 130.3 kcal mol (1)
rxn
2
CID
The presence of the small amounts of these other ions
contributes to the larger than normal error bar on the branching
ratios reported. However, their contribution is minimal due to
their trace nature and the added fact that upon addition of
•+
ethylene the H2O will preferentially react with the much more
abundant ethylene (∼35 ppm).
The bimolecular rate coefficients obtained in the present study
are summarized in Table 1. The error bars for kobs indicate the
precision of our experimental data, one standard deviation.
The most challenging aspect of characterizing many ion-
molecule reactions is a determination of its branching ratio, the
relative yield of the competitive product channels. The branching
ratios summarized in Table 1 are the average of 3-6 indepen-
dent measurements, each of which consists of data collected
by varying either the reaction distance at fixed ethylene
concentration or by varying ethylene concentration at fixed
reaction distance (time).16 In all cases the reactions were
followed to between 30 and 60% completion in order to
Figure 1. Mass spectra demonstrating the injection capability of the
•+
SIFT for D
2
O
and its subsequent reaction with ethylene. (a) The mass
spectrum of ions produced in the ion source region as observed via the
detection system when the selection quadrupole is operated in total
transmission mode and there is no buffer gas in the 150 cm long flow
tube. (b) A mass spectrum obtained under the same conditions as in
20,21
minimize complications from facile secondary reactions.
An
(
a) wherein the only difference is that the selection quadrupole transmits
example of one typical experimental measurement of the
•+
2
only m/z 20, D O . (c) An example of a typical “reactant ion spectrum”
•+
branching ratio for the reaction of H2O with ethylene is shown
in Figure 2. All branching ratios reported in the Table 1 represent
a compilation of data from similar plots.
•
+
for the D
0 and the flow tube is filled with 0.5 Torr of He. As described in the
text, the m/z 18, 19, and 21 ions result principally from reaction of
2
O
studies with the transmission quadrupole selecting m/z
2
•+
•+
•+
L2O + H2CdCH2. Formal charge (CT) and proton transfer
PT) leads to the observed m/z 28 and 29 product ions with the
D
2
O
with adventitious water and partially from CID of D
2
O . (d)
An example of a mass spectra obtained during the early portion of the
(
•
+
•+
•
+
reaction of D
2 4
2
O
with H
2
CdCH
2
; m/z 28 is C
2
H
4
and m/z 30 is
relative ratio of 78:22 for the reaction of H2O with H2Cd
+
C H D . All spectra shown here are single scans (no averaging)
obtained at a scan rate of 0.5 amu s
•
+
CH2. The reaction of D2O with H2CdCH2 similarly shows
m/z 28 (CT) and 30 (deuteron transfer, DT) as 83:17 (see Figure
-
1
.
•
+
1
d), while the reaction of HOD with H2CdCH2 gives m/z 28
vinyl cation, formed via hydride transfer. The remaining 22%
+
(
CT), 29 (PT), and 30 (DT) as 77:12:11. The efficiency for
of the products appear to be from PT in the HO reaction but,
•
+
+
H2O + H2CdCH2 reported as 106% in Table 1 might be
misleading; the larger than 100% value likely reflects errors in
as revealed by the DO reaction, actually result from a
combination of proton transfer (15%) and fragmentation to yield
1
8
+
+
kobs and error in the assumptions used to derive kcoll.
LCO (7%). For these LO reactions, any trace primary product
ions (e2% of the total ion product yield) at m/z 30, 31, and/or
32 are difficult to reproducibly quantify due to their low
abundance and fast secondary reactions and therefore are omitted
LO + H2CdCH2. The reactions of HO+ and DO with
H2CdCH2 are essentially identical and show CT as the major
channel (66%). Both reactions also yield 12% of m/z 27, the
+
+

Reaction of Hydroxyl Cation and Ionized Water with C2H4
J. Phys. Chem. A, Vol. 103, No. 25, 1999 4881
TABLE 1: Summary of Kinetic Data for the Reaction of L
x
O•+ (x ) 1, 2) with Ethylene in 0.5 Torr of Helium at 298 K
b
∆Hrxna
k
obs
a
(cm molecule s-1
3
-1
# t1/2c
m/zc
ion productd
IP (eV)
)
efficiency (%)
106%e
yield
78%
(kcal/mole)
H
2
O•+
HOD•+
12.62
15.5 ((0.4) × 10
-10
6.2
28
29
28
29
30
28
30
27
28
29
C
2
C
2
C
2
C
2
C
2
C
2
C
2
C
2
C
2
C
2
H
4
H
5
H
4
H
5
H
4
H
4
H
4
H
3
H
4
H
5
•+
-48.7
-20.9
-48.9
-20.8
-17.7
-49.1
-17.6
-123.3
-63.1
-52.2
-41.0
-118.6
-58.1
-34.6
-44.1
+
2
2%
-10
•+
+
12.63
11.0 ((0.2) × 10
77%
1.5
77%
1
1
2%
1%
+
D
D
2
O•+
12.64
13.02
9.48 ((0.5) × 10
-10
67%
99%
6.8
5.5
83%
7%
13%
•+
+
1
D
HO+
14.8 ((0.9) × 10
-10
+
•
+
6
2
6%
1%
+
+
and/or HCtO
DO+
13.03
-10
109%e
+
15.9 ((1.2) × 10
3.3
11%
27
28
29
30
C
C
2
H
H
3
•
+
6
7%
2
4
+
7
%
HCtO
C H D
2 4
+
1
5%
a
NIST Chemistry WebBook, NIST Standard Reference Database Number 69; Mallard, W. G., Linstrom, P. J., Eds.; National Institute of Standards
and Technology (http://webbook.nist.gov): Gaithersburg, MD; March 1998. b The error reported is one standard deviation of the average. See text
c
for a discussion of the kinetic models used to extract these bimolecular rate coefficients from the observed data. The average number of half-lives
used to determine the rate coefficient. Only reaction products that are g2% of the overall yield are reported. e See text for a discussion of Eff >
d
1
00%.
process. We next examined D2O•+ with H2CdCH2 and observed
a significant drop in the reaction efficiency, to 67%, but an
experimentally indistinguishable product yield of 83% formal
•
+
CT and 17% formal DT. The data for D2O (as compared to
•
+
that for H2O ) suggest that the collision is no longer the sole
rate-determining step, and that the rate-determining step for both
reactions is not the same as the product-determining step.
•
+
The branching ratio we determined for H2O with H2Cd
CH2 is somewhat different than that determined by Rakshit and
Warneck who observed only the CT channel. We are unsure of
the reasons for the difference but two possibilities suggest
themselves. The first is that the interaction energy in their
experiment is suprathermal; for a multistep chemical transfor-
mation (as we suggest below) this would favor products earlier
in the reaction sequence (i.e., would enhance CT over PT, vide
2
2
infra). The second possibility is that the complicated reaction
sequence and fitting procedure employed in their study was
unable to accurately identify all the products.
To gain additional information, we next examined the reaction
•
+
of HOD with H2CdCH2. This presents a challenge as there
•
+
+
are two reactive isobaric ions at m/z 19, HOD and H3O , and
Figure 2. An example of one branching ratio experiment for the
•+
no convenient way to exclusively prepare and study just HOD .
The reaction of H3O with H2CdCH2 has been studied before
•
+
reaction of H
2
O
with H
2
CdCH
2
in 0.5 Torr of He at 298 K. The fit
+
of the observed points to a straight line demonstrates that secondary
reactions are not yet contributing to the observed ion yields. The slopes
of the lines for this one experiment give the product yield of 80% of
m/z 28 and 20% of m/z 29.
2
3,24
24
in helium
carrier gas; the study by Matthews et al. in 0.50
Torr of helium indicates that reaction proceeds according to eq
2.
kobs ) 7.8 × 10-11
+
+
from consideration. As for the H2O•+ reaction, the 109%
efficiency in Table 1 for the DO reaction reflects errors in
kobs and/or kcoll.
H O + H CdCH
8 65% C H + H O (2a)
3
2
2
2
5
2
0
.5 Torr He
+
+
3
5% C H O
(2b)
2
7
Discussion
To ensure the major reactant ion injected at m/z 19 in our
experiment is HOD , we electron ionized a 3:1 (liquid phase)
The rate coefficient reported here, 1.55 ((0.04) × 10 cm3
-
9
•+
-
1
-1
•+
molecule s , for the reaction of H2O with H2CdCH2, is
mixture of D2O: H2O. This should ensure a low relative
1
1
+
in excellent agreement with that reported by Dotan et al. as
abundance of H3O but will not completely eliminate its
-9
3
-1 -1
1
.6 ((0.5) × 10 cm molecule s , at 300 K, as determined
contribution in the injected signal. Because of the relatively slow
+
•+
•+
in a flow-drift tube, as well as that reported by Rakshit and
reaction of H3O compared to D2O , H2O , and (presumably)
Warneck12 as 1.5 × 10 cm molecule
-9
3
-1 -1
s
as determined in
HOD , as well as the fact that the branching ratio measurements
•+
a drift chamber mass spectrometer. The unit efficiency clearly
indicates that the rate-determining step for the reaction is the
collision rate. In addition, we are able to identify the products
as 78% from a formal CT process and 22% from a formal PT
are determined from the first 40% of reaction or less (for this
reaction), we are able to determine the yields reported in Table
1 in good confidence. This confidence in the branching ratio
+
data is reinforced by the lack of observation of C2H7O under

4882 J. Phys. Chem. A, Vol. 103, No. 25, 1999
Fishman and Grabowski
Figure 3. Experimental data for the observed decay of m/z 19 (points)
•+
2 2
for the reaction of HOD with H CdCH for five different experiments.
In each case, the straight line is a fit to a standard pseudo-first-order
kinetic model that assumes only one reactant is present at m/z 19. The
intensity data represents average counts per second determined for a
total counting time of 5 s.
Figure 4. Representative experimental data (points) and fits to various
models for the determination of the bimolecular rate coefficient for
•
+
the conditions used. As shown in Table 1, the reaction of HOD•+
the reaction of HOD with H
2 2
CdCH using a two-component kinetic
model. The line represents a fit to a specific kinetic model. (a) Assumes
the rate coefficient for HOD is the collision rate coefficient and the
rate coefficient for the H
ref 24); the only adjustable parameter is the relative concentration of
H O in the m/z 19 signal. (b) Like (a) but allows the rate coefficient
of the HOD reaction to vary as well.
with H2CdCH2 yields the same product distribution as the
previous two reactions, 77% CT, and 23% proton/deuteron
transfer. The identical product yield with the previous reactions,
along with equivalent yields of proton and deuteron transfer,
are consistent with the absence of any isotope effect in the
product-determining step.
•
+
+
3
O reaction as measured by Matthews et al.
(
+
3
•
+
•
+
The kinetic characterization of the HOD with H2CdCH2
reaction is more problematic; normally our kinetic measurements
in the SIFT follow reactions for ∼5 or more half-lives. However,
the presence of two reactive, isobaric ions at m/z 19, in unknown
concentrations, precludes our standard measurement since all
pseudo-first-order kinetics plots followed over 5 or more half-
lives are severely curved, indicating two distinct reacting species.
We therefore obtained an estimate for the rate coefficient for
We therefore reanalyzed the kinetic data for the HOD•+
H2CdCH2 reaction using a two-component kinetic model that
allowed two reactive isobaric ions to be present: fraction f of
+
•
+
which is HOD reacting with an unknown rate coefficient and
+
-11
fraction (1 - f) of which is H3O reacting with k ) 7.8 × 10
3
-1 -1
24
cm molecule
s
(0.5 Torr He). Figure 4b displays the
results of the best fit of this model to the same data as presented
in Figure 4a. This model clearly reproduces the experimental
observations much better. In addition, this more correct model
allows us to extend the useful range of the kinetic data beyond
the first 50% decay. Nine separate kinetic runs, collected over
two different experimental days, are all well fit by this two
•
+
the HOD + H2CdCH2 reaction by using data for only the
•
+
first ∼50% decay of the initial HOD signal (the average
number of half-lives used is 0.9 averaged over 8 individual
experiments) and a standard pseudo-first-order kinetic model.
This analysis rigorously provides a lower limit to kobs of 9.0 ×
•
+
component model and return kobs for the HOD reaction of
-
10
3
-1 -1
1
0
cm molecule
s
and visually looks to adequately fit
-
9
3
-1
-1
•+
1.1 ((0.1) × 10
cm molecule
s . For these nine
the observations (Figure 3). However, a kobs for HOD + H2Cd
-10
experiments, the average number of half-lives over which data
was collected was 1.5, and was held low in order to maximize
our sensitivity to the reaction we care about (the faster
CH2 of 9.0 × 10 corresponds to a reaction efficiency of only
6
3%, which is significantly less than that observed for the
•
+
comparable H2O reaction.
+
component). The relative concentration of H3O in the m/z 19
signal varied from a low of 9% to a high of 44%, and averaged
20%. We therefore conclude that the best rate coefficient to
A more correct treatment of the data would recognize that
the observed decay of m/z 19 is due to the sum of the two kinetic
•
+
processes, a fast reaction due to DOH + H2CdCH2 and a
•
+
-9
+
describe the reaction of HOD with H2CdCH2 is 1.1 × 10
slower reaction due to H3O + H2CdCH2 (eq 2). If one assumes
3
-1 -1
•
+
cm molecule
s
which corresponds to a reaction efficiency
that the DOH reaction occurs at the collision limit (i.e.,
•
+
of 77%.
identical to what is observed for the H2O reaction) and the
+
The reaction of D2O•+ with H2CdCH2 potentially has a
kinetic complication similar to that found in the HOD reaction
as m/z 20 could also be H2OD . However, in the D2O reaction,
the neutral precursor added to the ion source is pure D2O, thus
H2OD should present a much smaller problem than the
corresponding complication in the HOD study. Nonetheless,
we treated the kinetic data for the D2O reaction using the same
two-component model as for the HOD reaction (we kept the
H3O reaction occurs with the rate coefficient reported by
2
4
•+
Matthews et al., then the only variable needed to fit the
observed kinetic data to the model is the relative concentration
+
•+
+
of H3O in the m/z 19 signal. One of our data sets, analyzed in
+
exactly this fashion, is shown in Figure 4a. Note that in this
•
+
figure, the only variable is the fraction of the m/z 19 signal that
+
•+
is H3O and this fraction was varied in order to obtain the best
•
+
fit (shown by the solid line). As is clearly discernible in Figure
+
4
a, the model poorly reproduces the experimental data leading
rate coefficient for the H2OD fixed at the same value as for
•
+
+
to the conclusion that the DOH reaction is not proceeding at
the collision limit.
the H3O reaction). Data sets (8) collected over two different
experimental days were fit and returned kobs for D2O• + H2Cd
+

Reaction of Hydroxyl Cation and Ionized Water with C2H4
J. Phys. Chem. A, Vol. 103, No. 25, 1999 4883
Figure 5. Mechanistic hypothesis for the reaction of H
2
O•+ with ethylene at 300 K. Exothermic reaction channels are indicated by a positive
energy yield.
+
Figure 6. Mechanistic hypothesis for the reaction of OH with ethylene at 300 K. Exothermic reaction channels are indicated by a positive energy
yield.
CH2 as 9.45 ((0.51) × 10-10 cm3 molecule-1 s-1 which
the branching ratio. This two step mechanism is also consistent
1
2
corresponds to a reaction efficiency of 67%.
with the Rakshit and Warneck observation of only CT
provided their interaction energy was suprathermal. In a
multistep mechanism, one expects that as the interaction energy
is raised, products that correspond to an earlier portion of the
•+
To summarize the L2O + H2CdCH2 reaction data: all three
isotopic variants react to give 79 ((5)% charge transfer and 21
(
(5)% proton transfer with no distinction between proton or
•
+
22
deuteron transfer in the case of HOD . Furthermore, there is a
small, but reproducible reduction in reaction efficiency upon
deuterium incorporation into the reactant ion; the L2O reaction
mechanism should increase. The simplest mechanistic picture,
that of competitive CT and PT from the initial collision complex
(Figure 5), seems to be inconsistent with a kinetic isotope effect
on the rate of the reaction but not on the product distribution.
•+
•+
proceeds with unit efficiency for H2O , but only 77% efficiency
•
+
•+
+
+
for HOD and 67% for D2O . Based on these observations, it
seems likely that the reaction is kinetically controlled by an
initial charge transfer, one that is subtly affected by isotope
effects brought about by deuterium substitution. After this highly
energetic CT reaction, there is a partitioning between separation
and hydrogen (deuterium) atom transfer that does not measur-
ably display an isotope effect.
The reactions of HO and DO with H2CdCH2 display
•+
behavior similar to that described above for the L2O reaction.
+
Both LO reactions proceed predominantly to give formal CT
+
product (66%). Both LO reactions give a minor yield (12%)
of formal hydride transfer product, producing the vinyl cation
(m/z 27). The remaining 22% of the reaction products appear
+
+
at m/z 29 for the HO reaction. However, the DO reaction
We therefore propose the reaction mechanism shown in
Figure 5. In this mechanism, all reaction channels are initiated
by CT. The ion-dipole complex formed from CT, [H2O H2Cd
reveals that this product mixture is a 2:1 mixture of formal
+
+
proton (deuteron) transfer, giving C2H5 (C2H4D ), and a
+
+
fragmentation reaction giving HCO (DCO ).
•+
•+
CH2 ], can then partition between separation or further reaction.
Unlike the L2O reactions, there is neither an isotope effect
-
1
As the CT is so exothermic, (49 kcal mol , excluding ion-
dipole complexation energy) there is no measurable isotope
effect on the 21% of hydrogen (deuterium) atom transfer that
follows. Deuterium substitution can thus impact the CT step
on the branching ratio nor on the observed rate coefficient. Also,
+
there are additional product channels for the LO reactions. In
•
+
analogy to the mechanism proposed for the L2O reaction in
+
Figure 5, we propose a mechanism for the LO reaction in
(by a subtle change in energy-level matching) while not affecting
Figure 6. As in the earlier case, we envision the reaction

4884 J. Phys. Chem. A, Vol. 103, No. 25, 1999
Fishman and Grabowski
proceeding by initial CT with the product selection occurring
(4) Hucknall, D. J. Chemistry of Hydrocarbon Combustion; Chapman
and Hall: London, 1985.
from the so-formed ion-molecule complex. The details of the
(
5) Fialkov, A. B.; Kalinich, K. Y.; Fialkov, B. S. In: 24th Symp. (Int.)
+
+
complex fragmentation channels (leading to C2H3 and HCO )
must be considered speculative at this point due to lack of
Combust. 1992, 785.
6) Tanner, S. D.; Goodings, J. M.; Bohme, D. K. Can. J. Chem. 1981,
59, 1760.
(7) Harrison, A. G. Chemical Ionization Mass Spectrometry, 2nd ed.;
CRC Press: Boca Raton, FL, 1992.
8) Duley, W. W.; Williams, D. Interstellar Chemistry; Academic
(
+
detailed information (such as whether the neutrals for the C2H3
+
channels are H + OH or H2O). The HCO pathway is consistent
with our observations and interpretation of the same product
formed in the O + H2CdCH2 reaction. The highly exo-
(
•
+
16
Press: New York, 1984.
•
+
thermic nature of the CT, even more so than in the L2O
+
(9) Lequeux, J.; Roueff, E. Phys. Rep. 1991, 200, 241.
10) Lehman, T. A. Mass Spectrom. ReV. 1995, 14, 353.
11) Dotan, I.; Lindinger, W.; Rowe, B.; Fahey, D. W.; Fehsenfeld, F.
(
(
H2CdCH2 reaction, helps understand the greater product
complexity here.
C.; Albritton, D. L. Chem. Phys. Lett. 1980, 72, 67.
12) Rakshit, A. B.; Warneck, P. J. Chem. Soc., Faraday Trans. 2 1980,
76, 1084.
(
Conclusion
(
13) Shul, R. J.; Passarella, R.; DiFazio, L. T.; Keesee, R. G.; Castleman,
The 298 K ion-molecule reaction products and rate coef-
A. W. J. Phys. Chem. 1988, 92, 4947.
(14) Fishman, V. N.; Grabowski, J. J. Int. J. Mass Spectrom. Ion
Processes 1998, 177, 175.
•
+
+
ficients for the reaction of both L2O and LO with H2Cd
CH2 are proposed to proceed initially by charge transfer. For
most reactions the rate-determining step is the collision step;
(
15) Brink, G. O. ReV. Sci. Instrum. 1966, 37, 857.
(16) Fishman, V. N.; Graul, S. T.; Grabowski, J. J. Int. J. Mass Spectrom.
•+
•+
for reactions of HOD and D2O , the CT step is partially rate-
determining. The product-determining steps are after the CT
and, as a result, no isotope effects are found. Multiple-step
mechanisms are thus proposed for both reaction series that
explain all observations.
Ion Processes 1999, 185/186/187, 477.
17) Grabowski, J. J.; Melly, S. I. Int. J. Mass Spectrom. Ion Processes
1987, 81, 147.
(
(
(
18) Su, T.; Chesnavich, W. J. J. Chem. Phys. 1982, 76, 5183.
19) Anicich, V. G.; Sen, A. D. Int. J. Mass Spectrom. Ion Processes
1
998, 172, 1.
(
20) Ikezoe, Y.; Matsuoka, S.; Takebe, M.; Viggiano, A. Gas-Phase
Acknowledgment. We gratefully acknowledge support of
the National Science Foundation in carrying out this study.
Ion-Molecule Reaction Rate Constants Through 1986; Maruzen Company,
Ltd.: Tokyo, 1987.
(21) Anicich, V. G. J. Phys. Chem. Ref. Data 1993, 22, 1469.
(
22) Bierbaum, V. M.; Grabowski, J. J.; DePuy, C. H. J. Phys. Chem.
References and Notes
1
984, 88, 1389.
(
(
(
1) Smith, D. Chem. ReV. 1992, 92, 1473.
(23) McIntosh, B. J.; Adams, N. G.; Smith, D. Chem. Phys. Lett. 1988,
2) Smith, D.; Spanel, P. Mass Spectrom. ReV. 1995, 14, 255.
148, 142.
(24) Matthews, K. K.; Adams, N. G.; Fisher, N. D. J. Phys. Chem. A
1997, 101, 2841.
3) Lewis, J. S.; Prinn, R. G. Planets and Their Atmospheres; Academic
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