Reaction of Oϩ2 with CH4
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J. Chem. Phys., Vol. 114, No. 14, 8 April 2001
TABLE I. Possible reaction channels for the Oϩ2 plus CH4 or CD4 systems
with O2 present. Ion masses are listed both as actual mass and those ob-
served in the experiment after secondary chemistry has occurred as de-
scribed in the text. Thermodynamic data are taken from the NIST Webbook
͑Ref. 31͒, from Van Doren et al. ͑Ref. 13͒, and from Fisher and Armentrout
͑Ref. 21͒.
the hot flow tube walls. The alkalis were nonreactive and,
therefore, did not interfere with the measurements. An ex-
ample of a spectrum has been shown elsewhere.24 The
branching ratios were independent of the O2 concentration,
except that the charge transfer channel is scavenged by the
O2 as described below.
Actual
Observed
CH4 or CD4 was added about 30 cm downstream of the
ion source, 50 cm from the sampling orifice. Signals were
recorded as a function of the CH4 flow. Normally, one ex-
trapolates product branching fractions to zero reactant flow
to obtain the primary branching ratios. However, the primary
reaction was quite slow and some of the secondary chemistry
was fast, making the zero extrapolation difficult to interpret.
Extrapolations to infinite flow were also used. Nevertheless,
all channels could only be separated at 1400 K. The separa-
tion is possible at high temperature because the overall rate
constant is considerably larger, resulting in less CH4 being
added, which in turn makes secondary chemistry less impor-
tant.
The mass spectrometer in the HTFA has little mass dis-
crimination when using low-mass resolution and most of the
data was taken under these conditions. For channels 1 AMU
apart, the low resolution data were divided into individual
peaks using higher resolution data. The rate constants are
accurate to 25%, and relative errors are 15%.29 The branch-
ing ratios are accurate to 25% except where noted.30
Reaction
mass H͑D͒ mass H͑D͒
Primaries observed
͑1͒ Oϩ2 ϩCH4→CH2O2HϩϩHϩ22 kcal/mol
͑2͒ Oϩ2 ϩCH4→CH3ϩϩHO2Ϫ0.4 kcal/mol
͑3͒ Oϩ2 ϩCH4→CH4ϩϩO2Ϫ12.45 kcal/mol
͑4͒ Oϩ2 ϩCH4→H3OϩϩHCOϩ113 kcal/mol
͓5͑a͔͒ Oϩ2 ϩCH4→HCOϩϩHϩH2Oϩ69 kcal/mol
͓5͑b͔͒ →HCOϩϩOHϩH2ϩ54 kcal/mol
͑6͒ Oϩ2 ϩCH4→CH3OϩϩOHϩ78 kcal/mol
͑7͒ Oϩ2 ϩCH4→H2OϩϩCH2Oϩ53 kcal/mol
47͑50͒
15͑18͒
16͑20͒
19͑22͒
29͑30͒
47͑50͒
29͑34͒
17͑22͒
19͑22͒
29͑30͒
31͑34͒
18͑20͒
31͑34͒
18
Primaries not observed
͑8͒ Oϩ2 ϩCH4→CO2ϩϩ2H2ϩ37 kcal/mol
44
͑9͒ Oϩ2 ϩCH4ϩM→O2ϩ͑CH4͒ϩM
48͑52͒
28͑30͒
46͑48͒
͑10͒ Oϩ2 ϩCH4→CH2OϩϩH2Oϩ95 kcal/mol
͑11͒ Oϩ2 ϩCH4→CH2Oϩ2 ϩ2HϪ9.5 kcal/mol
Secondary chemistry
͑12͒ CHϩ4 ϩO2→O2ϩϩCH4
͑13͒ CHϩ3 ϩCH4→C2H5ϩϩH2
→C2Hϩ3 ϩ2H2
32
29͑34͒
27͑30͒
17͑22͒
17͑22͒
?
19͑22͒
31͑34͒
45͑50͒
͑14͒ CHϩ4 ϩCH4→CH5ϩϩCH3
͑15͒ HCOϩϩCH4→CHϩ5 ϩCO
͑16͒ CH3Oϩ2 ϩHe͑CH4͒→products
͑17͒ H2OϩϩCH4→H3OϩϩCH3
͑18͒ H2COϩϩCH4→CH3OϩϩCH3
→C2H5OϩϩH
RESULTS AND DISCUSSION
Table I lists all the processes and potential processes that
can occur in this system involving Oϩ2 , O2 , and CH4 . The
thermodynamic data for all reactions listed are taken from
the NIST Webbook,31 from Van Doren et al.,13 and from
Fisher and Armentrout.21 Reactions ͑1͒–͑7͒ are the primary
channels observed in the present experiments. Reactions
͑5͒–͑7͒ have not been observed elsewhere, but HCOϩ has
been reported as a charge injection device ͑CID͒ product of
CH2O2Hϩ dissociation.16,32 Reactions ͑8͒ and ͑11͒ have been
observed previously at high-kinetic energy21 and reaction ͑9͒
at low temperature.14 Thus, of all the possible channels, only
reaction ͑10͒ has never been observed. We made a careful
search for this channel and cannot rule out a small signal at
high temperature. However, reaction ͑18͒ and a mass coinci-
dence with H13COϩ and C13CHϩ5 prevented a definitive de-
termination. One might expect a small amount of H2COϩ
since it is related to the H2Oϩ channel by charge location
and to the CH2OHϩ channel by proton transfer.
Many of the channels have mass coincidences, and sev-
eral of the primary products react with either CH4 or O2
making quantification by normal means difficult. For ex-
ample, CH2O2Hϩ thermally decomposes above 1000 K. The
evidence for the thermal decomposition is a decrease in the
branching fraction at higher CH4 flows. The large extent of
reaction at large CH4 flows results in an increase in the ef-
fective reaction time for secondary chemistry since more of
the product is formed upstream. The primary reaction time
is, of course, invariant. At all temperatures, the branching
fraction for this channel is reported from the extrapolation to
zero flow, corrected for products not detected at low flow.
This underestimates the nascent branching ratio at high tem-
perature by neglecting contributions from thermal dissocia-
tion, i.e., the reported branching ratios are lower limits.
CHϩ3 reacts rapidly with CH4 to produce C2Hϩ5 ͓reaction
͑13͔͒,33 which has the same mass as HCOϩ. A small amount
of C2Hϩ3 is also observed at 1400 K from the reaction of
CHϩ3 with CH4 . It became clear that the mass 29 AMU
signal is due to both HCOϩ and C2H5ϩ only after studying
the CD4 reaction. In a previous flow tube study,10 it was
assumed that the signal at 29 AMU was due only to C2Hϩ5 .
The fast secondary chemistry of CHϩ3 , coupled with the
small primary reaction rate constant, prevents the HCOϩ and
CHϩ3 channels from being separated. The sum of the two
channels is, therefore, reported, except at 1400 K, where the
overall primary reaction is fast enough to use an extrapola-
tion to zero CH4 flow. Limits can also be placed on the two
channels at 1200 K. The overall rate constant at 1400 K is
over ten times faster than the 500 K rate constant. Additional
complications are that two potential sets of neutral products
can accompany HCOϩ production and that HCOϩ has two
isomeric forms. We list the ion as HCOϩ because CID has
shown that CH2O2Hϩ dissociates into HCOϩ not HOCϩ al-
though we have no evidence as to which ion is formed.32 We
have no information concerning which of the two neutral
product sets is formed either.
The charge-transfer channel is obscured at low CH4 flow
because of the back reaction with the O2 source gas, reaction
͑12͒. At very large CH4 flows, the rate for reaction ͑14͒ is
much faster than that for reaction ͑12͒, and CHϩ4 is much
128.240.225.44 On: Fri, 19 Dec 2014 18:01:31