Gas Phase Reactions of Some Positive Ions with Atomic and Molecular Oxygen and Nitric Oxide at 300 K

Article abstract of DOI:10.1021/jp9913719

Rate coefficients and product ion distributions have been determined for the reactions of HCNH⁺, HC₃N⁺, H₂C₃N⁺, H₂O⁺, N₂+, HCO⁺, and HCO₂+ with O atoms in a selected ion flow tube at 295 ± 5 K. Only two of these ions, HC₃N⁺ and N₂⁺, were found to react with atomic oxygen, and both ions exhibited multiple reaction channels. Reactions with O atoms may be relevant to synthetic processes occurring in some interstellar clouds. Because of the application of the N + NO titration to determine O atom concentrations, the rate coefficient and product distribution for the reaction of each ion with O₂ and NO was also measured.

Full text of DOI:10.1021/jp9913719

7470
J. Phys. Chem. A 1999, 103, 7470-7473
Gas Phase Reactions of Some Positive Ions with Atomic and Molecular Oxygen and Nitric
Oxide at 300 K
Graham B. I. Scott, David A. Fairley, Daniel B. Milligan, Colin G. Freeman, and
Murray J. McEwan*
Department of Chemistry, UniVersity of Canterbury, PriVate Bag 4800, Christchurch, New Zealand
ReceiVed: April 26, 1999; In Final Form: July 21, 1999
Rate coefficients and product ion distributions have been determined for the reactions of HCNH+, HC3N+,
+
+
H2C3N+, H2O+, N2 , HCO+, and HCO2 with O atoms in a selected ion flow tube at 295 ( 5 K. Only two
+
of these ions, HC3N+ and N2 , were found to react with atomic oxygen, and both ions exhibited multiple
reaction channels. Reactions with O atoms may be relevant to synthetic processes occurring in some interstellar
clouds. Because of the application of the N + NO titration to determine O atom concentrations, the rate
coefficient and product distribution for the reaction of each ion with O2 and NO was also measured.
1. Introduction
Several studies of ion-O atom reactions have subsequently
been reported,6-11 again with aeronomy providing the principal
Relatively few reactions involving cations and atomic oxygen
have been investigated,1-3 despite more than thirty years having
elapsed since the first measurement of an ion-O atom reaction
was reported.4 Indeed the paucity of reliable experimental
measurements for cation-O atom reactions is a large and
obvious gap in the database of ion-neutral reactions.
The principal reason for the lack of experimental data on
cation-O atom reactions is that the experiments are quite
difficult to perform. Atomic oxygen is not a trivial reagent to
generate and monitor. Generally O atoms have been produced
by forming atomic nitrogen in a microwave discharge of N2
and then titrating the N atoms (∼1% of the N2 flow) with
measured flow rates of nitric oxide to form known fluxes of
atomic oxygen, viz.3,5
motivating force for the investigations.
Fehsenfeld in 197612 and Viggiano et al. in 198013 undertook
investigations of ion-O atom reactions with a different motiva-
tion: interstellar chemistry. These workers reported the reactions
of several hydrocarbon ions with O atoms. The hydrocarbon
ions they examined were found to react with O atoms via
multiple channels including C+ transfer, C atom transfer, CH
transfer, CH+ transfer, and other fragmentation pathways. Most
recently Le Page et al.14 examined the reactions of the
prototypical polycyclic aromatic hydrocarbon (PAH) cations
C10H6+, C10H7+, and C10H8+ with O atoms. Of these, only the
+
C10H8 species was found to react.
In the present work we extend the existing database by
examining for the first time the reactions of atomic oxygen with
nitrile cations, protonated nitriles, and protonated carbon oxides,
N + NO w O + N2
(1)
+
as well as reporting a reexamination of the N2 + O reaction.
As an adjunct to the primary task of establishing the rate
coefficients and product distributions for the reactions of the
selected cations with O atoms, experimental data are also
obtained for the reactions of all cations studied with molecular
oxygen and nitric oxide.
k ) (3.4 ( 1.0) × 10-11 cm3 s-1
The first experimental study of a reaction between an ion
and atomic oxygen was reported by Ferguson and colleagues
in 1965.4 These workers measured a rate constant and deter-
mined the product channels for the reaction between N2+ cations
and O atoms, viz.4
2. Experimental Section
The well-established NO titration procedure13,15 was used to
determine rate coefficients and product branching ratios for all
of the O atom reactions reported in this paper.
N2+ + O w NO+ + N
(2a)
(2b)
w O+ + NO
Figure 1 of ref 16 shows a schematic diagram of the neutral
probe used for this study. Essentially, the experimental meth-
odology involved establishing a microwave discharge in pure
(100%) nitrogen, thereby generating a small but sufficient flux
of atomic nitrogen, (∼0.5% fractional dissociation was typical).
Vibrationally excited N2 molecules and metastable N atoms (2D
k ) (2.5 ( 1.0) × 10-10 cm3 s-1
The impetus for studying process 2 was the suggestion that
it played an important role in the E and F1 regions of the earth’s
ionosphere.4 Aeronomy therefore sparked the first efforts to
characterize cation-O atom reactions.
2
and P) were removed from the neutral flow by a small glass
wool plug located a short distance (∼5 cm) downstream of the
* E-mail address: m.mcewan@chem.canterbury.ac.nz.
discharge region.3,15 Measured flow rates of nitric oxide were
10.1021/jp9913719 CCC: $18.00 © 1999 American Chemical Society
Published on Web 08/31/1999

Gas Phase Reactions of Some Positive Ions
J. Phys. Chem. A, Vol. 103, No. 37, 1999 7471
TABLE 1: Reaction Rate Coefficients and Product
Distributions for the Reaction of the Specified Cations with
Atomic Oxygen
reactant
ion
products
(branching ratio)
∆Ho
a
b
c
d
kobs
kprev kcoll (kJ mol-1
)
HCNH+ no reaction
<0.25
6.6
6.0
HC3N+
C3NO+ + H (0.50)
4.1
HC2N+ + CO (0.40)
-301.4
HC3NO+ (0.10)
-651.1e
H2C3N+ no reaction
<0.25
<0.57
1.4
6.0
7.2
6.6
H2O+
N2
no reaction
NO+ + N (0.95)
O+ + NO (0.05)
no reaction
1.4f
-295.3
-98.3
+
2.5g
HCO+
HCO2
<0.25
<0.25
6.5
6.1
Figure 1. A set of typical data for the reaction of N2+ with N atoms,
no reaction
+
O atoms, and NO. The endpoint of the NO titration is clearly visible
a Observed rate coefficient in units of 10-10 cm3 s-1 b Rate coef-
.
+
as the intersection of the two solid lines. The difference in the N2
signal at zero NO flow indicates a very slow reaction between N2
ficients determined in other laboratories, in units of 10-10 cm3 s-1
.
+
c Langevin collision rate in units of 10-10 cm3 s-1. kcoll values were
calculated according to the parameterized theory of Su and Chesnav-
ich.17 d The listed exothermicities are taken from Mallard.18 e Ther-
mochemistry for the NCCHdCdO+ cation. f References 7, 9, and 19.
g Reference 4.
and N atoms.
admitted through a secondary reactant inlet protruding into the
neutral flow immediately downstream of the probe’s elbow; in
this way known flow rates of atomic oxygen were formed via
reaction 1. Sufficient flow time was available for the N + NO
titration reaction to proceed to completion in the side tube.
The endpoint of the titration reaction, Fendpoint, was readily
apparent for most systems as the intersection between two linear
decays on a semilogarithmic plot of Ln[Cation+] versus NO
flow rate. Figure 1 illustrates this phenomenon for the NO
titration of N2+. Additionally, for many reactions the endpoint
of the titration reaction may be discerned by monitoring the
appearance of product ion(s), provided they have been un-
equivocally identified as arising from the reaction of N or O
atoms. Precisely at Fendpoint, the recorded NO flow was equal
to the original flux of N atoms, (i.e., the atomic nitrogen flow
before any NO had been introduced). Moreover, at the titration
endpoint all of the N atoms were converted to O atoms.
Determination of Fendpoint thus facilitated the calculation of N
and O atom fluxes for any recorded NO flow. Simply put, any
NO flow rate less than the Fendpoint was equal to the flow of O
atoms, while the N atom flux for such a point was given by the
difference between the original flux of N atoms and the flow
of NO. This last statement is only approximately correct as
atomic oxygen recombines (thereby forming O2) more rapidly
on surfaces than does atomic nitrogen. Oxygen atom recombina-
tion may cause the rate coefficient data reported in this study
to be low by as much as ∼20%.
TABLE 2: Reaction Rate Coefficients and Product
Distributions for the Reaction of the Specified Cations with
Molecular Oxygen
reactant
ion
HCNH+ no reaction
HC3N+ HCO+ + CO +
CN (0.55)
products
(branching ratio)
∆Ho
a
b
c
d
kobs
<0.005
0.05 0.025e
kprev
kcoll (kJ mol-1
)
7.6
6.6
-322.7f
-51.5
HC3N+‚O2 (0.45)
H2C3N+ no reaction
<0.005
2.5
6.6
8.6
H2O+
O2+ + H2O (1.0)
1.5g, 4.3h
5.2i, 3.3j
12.9k, 2.0l
4.6m
+
N2
O2+ + N2 (1.0)
0.51 0.98n, 0.49o 7.6
0.51p, 0.47q
-338.1
0.457r, 0.50s
0.90t, 0.60u
0.65V, 1.0x
1.1y, 0.42z
HCO+ no reaction
<0.005 <0.5A
7.5
6.8
<0.002B
+
HCO2
no reaction
<0.005
a Observed rate coefficient in units of 10-10 cm3 s-1
.
b Rate coef-
ficients determined in other laboratories, in units of 10-10 cm3 s-1
.
c Langevin collision rate in units of 10-10 cm3 s-1. kcoll values were
calculated according to the parameterized theory of Su and Chesnav-
ich.17 d The listed exothermicities are taken from Mallard et al.18
e Reference 20. f Thermochemistry based on the HCO+ cation, (not
HOC+). g Reference 21. h Reference 22. i Reference 1. j Reference 23.
k Reference 24. l Reference 25. m Reference 26. n Reference 27. o Ref-
erence 28. p References 29 and 30. q References 31, 32, 33, 34, and
35. r Reference 36. s Reference 37. t Reference 38. u Reference 39.
V Reference 40. x References 15 and 40. y Reference 41. z Reference 42.
A Reference 43. B Reference 44.
The simultaneous presence of atomic nitrogen and oxygen
precludes the determination of accurate product ratios except
for systems in which the ionic products produced by the O atom
reaction are unreactive with N atoms. This situation arises
because as the O atom flux is extrapolated to zero flow, the N
atom flux tends to its maximum value. This deleterious
complicating feature of the NO titration procedure has not been
previously discussed in the chemical literature but it may
potentially skew product distribution data.
the inherent difficulties in accurately determining O atom fluxes,
the rate coefficients for the reactions of the various cations with
atomic oxygen are considered accurate to (40%. In contrast,
the appropriate error for the reactions of these same species
with either NO or O2 is ( 15%.
3. Results
Summaries of all of the results obtained in this work are
presented in Tables 1, 2, and 3. Table 1 details the results
obtained for the reactions between atomic oxygen and seven
non-hydrocarbon cations. Tables 2 and 3 tabulate the data
obtained for the reactions of these same ions with O2 and NO,
respectively.
Previous measurements, where they exist, are listed in column
4 of each table.
4. Discussion of Results
All data were obtained at room temperature, 295 ( 5 K, and
at flow tube pressures between 0.3 and 0.35 Torr. Owing to
(i) The nitrile and Protonated Nitrile Cations, (HCNH+,
HC3N+, and H2C3N+), with O/O2/NO. HCNH+ was formed

7472 J. Phys. Chem. A, Vol. 103, No. 37, 1999
Scott et al.
TABLE 3: Reaction Rate Coefficients and Product
Distributions for the Reaction of the Specified Cations with
Nitric Oxide
branching ratios obtained in the two measurements are in
reasonable agreement.20
(ii) H2O+ with O/O2/NO. The water cation appears unreac-
tive with atomic oxygen, although presumably some O atom
exchange occurs which is of course, not visible without isotopic
labeling. H2O+ also participates in moderately rapid charge
transfer reactions with both O2 and NO.
reactant
ion
products
(branching ratio)
∆Ho
a
b
c
d
kobs
<0.14
kprev
kcoll (kJ mol-1
)
HCNH+ no reaction
8.5
HC3N+ NO+ + HC3N (1.0) 1.8
7.4 -228.0
H2C3N+ no reaction
<0.23
3.6
7.4
+
(iii) N2 with O/O2/NO. The molecular nitrogen cation
H2O+
NO+ + H2O (1.0)
4.4e, 4.5f
5.9g, 3.6h
11i
9.6 -323.4
reaction is perhaps the best studied of all ion-O atom reactions.
+
N2 undergoes a moderately rapid two-channel reaction with
NO+ + N2 (1.0)
3.6
3.0j, 4.40k 8.5 -610.0
atomic oxygen viz.
+
N2
4.8l, 3.3m
5.0n
+ 2
+
N2 ( Σg ) + O(3P) w NO+(1Σ) + N(4S) (0.95)
(5a)
(5b)
HCO+ no reaction
<0.005
8.4
7.6
HNO+ + CO2 (1.0)
0.18 <1.00
-2.4p
.
b Rate coef-
+
HCO2
w O+(4S) + N2(1Σ+ ) (0.05)
g
a Observed rate coefficient in units of 10-10 cm3 s-1
ficients determined in other laboratories, in units of 10-10 cm3 s-1
.
k ) 1.4 × 10-10 cm3 s-1
c Langevin collision rate in units of 10-10 cm3 s-1. kcoll values were
calculated according to the parameterized theory of Su and Chesnav-
ich.17 d The listed exothermicities are taken from Mallard et al.18
e Reference 21. f References 23 and 45. g Reference 22. h Reference 13.
i Reference 24. j Reference 46. k Reference 34. l Reference 38. m Ref-
erence 7 and 19. n Reference 15 and 40. o Reference 47. p Themochem-
istry based on the HNO+ cation, (not NOH+).
The rate coefficient and product distribution obtained from
the current study are in excellent agreement with the two most
recent prior investigations of this reaction carried out in the early
1970s.7,9,19
Spin is conserved in both the predominant channel, 5a, and
the minor charge transfer pathway, 5b. Furthermore, the
observed rate coefficient for both channels is < 4/6 kL, this being
the statistical weight of the spin allowed quartet channel.
The major channel, 5a, involves transfer of a N+ fragment
from the primary cation to the reactant neutral atom, thereby
forming the stable NO+ species and a nitrogen atom. The minor
pathway, 5b, proceeds by simple charge transfer from cation to
atom. Reaction 5b is one of the few molecular ion-atom charge
transfer processes known.1-3 There is some uncertainty in the
literature as to the rate coefficient for reaction 5b.48 McFarland
et al.9 quote a value for the rate coefficient of reaction 5b of
k5b < 1 × 10-11 cm3 s-1. Our value is k5b ) 7 × 10-12 cm3 s-1
which is consistent with their measurement.
via electron impact on hydrogen cyanide, HCN, while HC3N+
and H2C3N+ were both derived from cyanoacetylene, HC3N.
The protonated ions, HCNH+ and H2C3N+, are unreactive with
all three neutral species considered in the current study, namely
O atoms, O2, and NO.
Atomic oxygen and HC3N+ react via a rapid three-channel
process, viz.
HC3N+(2Π) + O(3P) w C3NO+ + H(2S) (0.50)
w HC2N+(2A) + CO(1Σ+) (0.40)
(3a)
(3b)
(3c)
w NCCCHO+(2A′′) (0.10)
k ) 4.1 × 10-10 cm3 s-1
N2+ also undergoes charge transfer reactions with O2 and NO;
the results tabulated in the present work are in accordance with
the evaluated data of Anicich for gas phase bimolecular ion-
molecule reactions.1
The ground electronic state of C3NO+ is unknown, and in
view of its even number of electrons may well be a singlet. If
so, then spin is conserved in all three channels and the observed
rate coefficient for all channels is greater than 2/6 kL. The two
major channels, (3a and 3b), proceed via H-O exchange and
C atom transfer from cation to neutral, respectively, within the
(HC3N+‚O)* complex. Carbon monoxide, being a particularly
stable neutral molecule, is generated in the 40% pathway, 3b.
It is interesting to note from the observation of a 10%
NCCCHO+ channel that the lifetime of the association complex
is sufficiently long for some stabilization to occur (i.e., within
the time scale for stabilizing collisions with the bath gas which
is ∼0.1 µs).
(iv) HCO+ and HCO2+ with O/O2/NO. Both the HCO+ and
+
HCO2 cations are unreactive with atomic and molecular
oxygen. HCO+ is also unreactive with NO; however, HCO2
+
reacts with this neutral via a slow proton transfer reaction. This
finding is in agreement with the sole previous measurement of
the HCO2 + NO reaction by Roche and co-workers.47
+
5. Conclusions
An examination of the data in Table 1 indicates that few of
the seven non-hydrocarbon species examined in the current study
are reactive with O atoms. Specifically, HCNH+, HCO+,
HCO2+, H2O+, and H2C3N+are unreactive with atomic oxygen.
The first three of the ions are observed interstellar species,49
and interestingly this trio are also unreactive with H2, N2, N,
and O,1,2 (as also is H2C3N+). It appears then that these cations
are less reactive, particularly when located in an environment
where the four neutrals cited above are among the most plentiful
HC3N+ also undergoes a moderately rapid charge transfer
reaction with nitric oxide, NO, and a slow two-channel reaction
with molecular oxygen, i.e.
HC3N+ + O2 w HCO+ + CO + CN (0.55)
w HC3N+‚O2 (0.45)
(4a)
(4b)
neutral species.49,50 Only HC3N+ and N2 undergo reactions
+
with O but neither of these cations have been detected in the
interstellar medium to date.49 N2 is unlikely to be detected
+
k ) 5.0 × 10-12 cm3 s-1
because of its rapid reaction with H2 but HC3N+ exhibits only
very slow reaction with H2,1 indicating that a barrier may be
present on the potential surface making the reaction with H2
prohibitively slow at the temperatures of interstellar clouds.
The rate coefficient obtained from the present study is double
that previously reported by Fox et al.; however, the product

Gas Phase Reactions of Some Positive Ions
J. Phys. Chem. A, Vol. 103, No. 37, 1999 7473
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Cyanoacetylene, (HCCCN), has, however, been detected in
interstellar clouds.49
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The two cations that were observed to react with atomic
oxygen in the current study, HC3N+ and N2+, did so relatively
rapidly, via multiple product channels. This finding suggests
that the potential energy surfaces for the transition states may
be complex. Moreover, in the case of the HC3N+ + O reaction
there is no clearly predominant product channel. Notwithstand-
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for the HC3N+ + O reaction, the systems examined in the
present work show little propensity to participate in termolecular
association processes. In other words, the association complex
formed in the ion-atom collision is short-lived (,0.11 µs) on
the time scale of a flow tube experiment. This outcome is
different from that observed for the reactions between various
cations and either H or N atoms;51,52 however, the reasons for
the divergent behavior are unclear. Note also that the two viable
cation-O atom reactions encountered in this work demonstrate
a tendency to generate particularly stable neutral molecules such
as CO and NO.
Three of the cations examined participate in reactions with
O2, namely HC3N+, H2O+, and N2+. These last two reactions
proceed by charge transfer, generating O2+, which, while not
yet detected in the interstellar medium,49 is very likely to be
present. These same cations, along with HCO2+, also react with
NO mainly via moderately rapid charge transfer processes. A
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reaction of positive ions with both O2 and NO and the reactions
reported here further enlarge this database.1,2
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support.
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