Gas-phase kinetics of ground-state platinum with O2, NO, N2O and CH4

Article abstract of DOI:10.1039/a705892f

The gas-phase reactivity of ground-state platinum with O₂, NO, N₂O and CH₄ is reported. Platinum atoms were produced by the photodissociation of [Pt(CH₃)₃(C₅H₄CH₃)] and detected by laser-induced fluorescence. The reaction rates of platinum with all the reactants are pressure dependent indicating adduct formation; however, the reaction with N₂O has a bimolecular component. The room-temperature limiting low-pressure third-order rate constants in argon buffer are (2.3 ± 0.2) × 10^-31 molecule^-2 cm⁶ s^-1, (4.3 ± 0.4) × 10³¹ molecule^-2 cm⁶ s^-1, (3.7 ± 0.5) × 10^-31 molecule^-2 cm⁶ s^-1 and (2.1 ± 0.9) × 10^-28 molecule^-2 cm⁶ s^-1 for O₂, NO, N₂O and CH₄, respectively, where the uncertainties are ±2σ. The limiting high-pressure second-order rate constants are (2.5 ± 0.5) × 10^-12 molecule^-1 cm³ s^-1, (2.3 ± 0.8) × 10^-11 molecule^-1 cm³ s^-1, (2.3 ± 0.3) × 10^-12 molecule^-1 cm³ s^-1 and (6.3 ± 0.3) × 10^-12 molecule^-1 cm³ s^-1 for O₂, NO, N₂O and CH₄, respectively. The second-order rate constant for the abstraction channel for the reaction with N₂O at 296 K is approximately 1 × 10^-13 molecule^-1 cm³ s^-1.

Full text of DOI:10.1039/a705892f

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Gas-phase kinetics of ground-state platinum with O , NO, N O and
2
2
CH
4
Mark L. Campbell¤
Chemistry Department, United States Naval Academy, Annapolis, MD 21402, USA
The gas-phase reactivity of ground-state platinum with O , NO, N O and CH is reported. Platinum atoms were produced by
2
2
4
the photodissociation of [Pt(CH ) (C H CH )] and detected by laser-induced Ñuorescence. The reaction rates of platinum with
3 3
5
4
3
all the reactants are pressure dependent indicating adduct formation; however, the reaction with N O has a bimolecular com-
2
ponent. The room-temperature limiting low-pressure third-order rate constants in argon bu†er are (2.3 ^ 0.2) ] 10~31
molecule~2 cm6 s~1, (4.3 ^ 0.4) ] 10~31 molecule~2 cm6 s~1, (3.7 ^ 0.5) ] 10~31 molecule~2 cm6 s~1 and (2.1 ^ 0.9) ] 10~28
molecule~2 cm6 s~1 for O , NO, N O and CH , respectively, where the uncertainties are ^2p. The limiting high-pressure
2
2
4
second-order rate constants are (2.5 ^ 0.5) ] 10~12 molecule~1 cm3 s~1, (2.3 ^ 0.8) ] 10~11 molecule~1 cm3 s~1,
(2.3 ^ 0.3) ] 10~12 molecule~1 cm3 s~1 and (6.3 ^ 0.3) ] 10~12 molecule~1 cm3 s~1 for O , NO, N O and CH , respectively.
2
2
4
The second-order rate constant for the abstraction channel for the reaction with N O at 296 K is approximately 1 ] 10~13
2
molecule~1 cm3 s~1.
Gas-phase transition metal (TM) chemistry is an intriguing
Ðeld of study due to the high multiplicities of the atomic
ground states and the large number of low-lying metastable
states. The early gas-phase work of TMs focused on the reac-
tions of the cations due to the ability of mass-spectrometric
methods to control the kinetic energy and electronic state of
the reactant ion selectively and to detect the products sensiti-
vely. An understanding of how the low-lying M` electronic
states govern chemical reactivity is now fairly well estab-
lished.1h5
The kinetics of the reactions of neutral TM atoms has
recently been an active Ðeld of study.6h48 The cumulative data
reported thus far indicate the electronic state is a very impor-
tant factor in the dynamics of these reactions. For example,
for both the termolecular association and abstraction chan-
and S are not rigorously good quantum numbers in platinum
due to the large spinÈorbit coupling.
The only other kinetic studies involving gas-phase platinum
chronicled the reactions of ground-state platinum with small
hydrocarbons.19,20 We are unaware of any previous kinetic
studies involving oxygen-containing oxidants. The reporting
of these reactions for platinum will further update the data-
base of TM reactions and allow comparison of platinumÏs
kinetic behavior with other TMs. Unfortunately, theoretical
calculations involving the 5d series are extremely difficult
owing to the complications arising from electron correlation,
relativistic and spinÈorbit e†ects. Moderately successful theo-
retical studies involving iridium and platinum with methane
have been reported.19 We hope these new kinetic results will
inspire further theoretical work in an e†ort to understand
these reactions further.
nels with O , TMs with s1dn~1 conÐgurations have been
2
found to be more reactive than their s2dn~2 counter-
parts.26,32,47
Experimental
Of the TMs, the 3d and 4d series have garnered the most
experimental and theoretical attention6h18,21h43 while the 5d
series remains relatively unstudied. Quite recently, however,
studies involving a few of the 5d TMs have been report-
ed.18h20,44h48 The most complete study of the 5d series has
involved reactions with hydrocarbons.18h20 In the reactions of
TMs with hydrocarbons, the 5d series metals are generally
more reactive than the 3d metals. This trend has been
explained as arising from the better size match between the 6s
and 5d orbitals relative to the 4s and 3d orbitals of the 3d
series.19 This better size match helps drive the chemical reac-
tion due to the increased bond energy of the metalÈcarbon or
metalÈhydrogen bonds in the product. For reactions with
oxygen-containing molecules, the 5d series again appears to
be more reactive than the 3d series, although this trend is
based on only a few 5d metals.44h48 More experimental
studies are required to substantiate this trend.
Pseudo-Ðrst-order kinetic experiments ([Pt] @ [oxidant])
were carried out in an apparatus with slowly Ñowing gas using
a laser photolysis/laser-induced Ñuorescence (LIF) technique.
The experimental apparatus and technique have been
described in detail elsewhere.44 BrieÑy, the reaction chamber
is a stainless-steel reducing four-way cross with attached side
arms and a sapphire window for optical viewing. The reaction
chamber is enclosed within a convection oven (Blue M, model
206F) for temperature dependence experiments.
Platinum atoms were produced by the 248 nm photo-
dissociation of trimethyl(methylcyclopentadienyl)platinum(IV)
[Pt(CH ) (C H CH )] using the output of an excimer laser
3 3
5
4
3
(Lambda Physics Lextra 200). Platinum atoms were detected
via LIF using an excimer-pumped dye laser (Lambda Physics
Lextra 50/ScanMate 2E) with a KDP doubling crystal tuned
to the [(d )4(d )5(p )1]¡ ^ a 3D transition at 292.979
3@2
5@2
1@2
3
3
In this paper we report a temperature- and pressure-
nm.49 The Ñuorescence was detected at 90¡ to the counter-
propagated laser beams with a three-lens telescope imaged
through an iris. A photomultiplier tube (Hamamatsu R375)
was used in collecting the LIF which was subsequently sent to
a gated boxcar sampling module (Stanford Research Systems
SR250), and the digitized output was stored and analyzed by a
computer.
dependent kinetic study of the (6s)1(5d)9 a 3D state of plati-
3
num with oxygen, nitric oxide, nitrous oxide and methane.
Here we use the more compact RussellÈSaunders term sym-
bolism to describe the ground state of platinum rather than
the more correct term, [(d )4(d )5(s )] , based on jj coup-
3@2
5@2 1@2 3
ling. Thus, we use the more familiar symbolism even though L
The platinum precursor was entrained in a Ñow of argon
bu†er gas. The precursor carrier gas, argon bu†er gas, and
¤ Henry Dreyfus Teacher-Scholar.
J. Chem. Soc., Faraday T rans., 1998, 94(3), 353È358
353

View Article Online
reactant gases Ñowed through calibrated mass Ñow meters
and Ñow controllers prior to admission to the reaction
chamber. Each sidearm window was purged with a slow Ñow
of bu†er gas to prevent deposition of platinum and other
photoproducts. Total Ñows were between 150 and 4000 sccm.
Pressures were measured with MKS Baratron manometers,
and chamber temperatures were measured with a thermocou-
ple.
The delay time between the photolysis pulse and the dye-
laser pulse was varied by a digital delay generator (Stanford
Research Systems DG535) controlled by a computer. The
trigger source for these experiments was scattered pump laser
light incident upon a fast photodiode. LIF decay traces con-
sisted of 200 points, each point averaged for four laser shots.
Materials
Trimethyl(methylcyclopentadienyl)platinum(IV) (Strem, 98%),
O
(MG Industries, 99.8%), N O (MG Industries, electronic
2
2
grade, 99.999%), CH (Linde, ultrahigh purity grade, 99.99%)
and Ar (Potomac Airgas, 99.998%) were used as received. The
4
Fig. 2 Typical plots for determining k
for Pt(a 3D ) ] O at 296
obsd
3
2
K illustrating the dependence of the bimolecular rate constants on
total pressure. The solid line for each set of data is a linear regression
NO (Liquid Carbonic, 99%) was passed through a liquid
nitrogen/n-pentane trap at ca. [100 ¡C to condense impurities
Ðt from which k
is obtained.
(primarily NO ) before entrance into the reaction chamber.
obsd
2
order rate constants are presented in Fig. 2È5; the slope yields
the observed rate constant. The relative uncertainty (i.e. the
reproducibility) of the second-order rate constants is estimated
at ^20% based on repeated measurements of rate constants
under identical temperature and total pressure conditions.
The absolute uncertainties are conservatively estimated to be
^40% and are based on the sum of the statistical scatter in
the data, uncertainty in the Ñowmeter and Ñow controller
readings (5%) and the total pressure reading (1%), and uncer-
tainties due to incomplete gas mixing.
Data analysis and results
The decay rates of the ground state of platinum as a function
of reactant pressure were investigated in Ar bu†er gas at
various temperatures and total pressures. The loss of ground-
state platinum is described by the Ðrst-order decay constant,
k
:
1st
k
\ 1/q \ k ] k [oxid]
(1)
1st
o
obsd
where q is the Ðrst-order time constant for the removal of
platinum under the given experimental conditions, k is the
Measured rate constants for the a 3D state reacting with
3
o
O , NO, N O and CH in argon bu†er at various pressures
loss term due to di†usion out of the detection zone and reac-
tion with the precursor and precursor fragments, and k
the second-order rate constant. Typical decay proÐles are
2
2
4
are listed in Table 1. The second-order rate constants were
also measured at temperatures up to 423 K. Rate constants
could not be measured reliably above 423 K owing to the
is
obsd
shown in Fig. 1. A time constant, q, for each decay proÐle was
increase in k for platinum at higher temperatures; i.e. plati-
determined using
a linear least-squares procedure. The
o
num reacts rapidly with the precursor at temperatures [423
second-order rate constant is determined from a plot of 1/q vs.
reactant number density. Typical plots for obtaining second-
K. For the reactions with O , NO and CH , the rate con-
2
4
stants at 423 K and 5 Torr are 3.4 ] 10~14, 6.6 ] 10~14 and
5.1 ] 10~12 molecule~1 cm3 s~1, respectively. These rate con-
stants are either lower or equal to the room-temperature rate
Fig. 1 Typical Pt(a 3D ) decay curves with added oxidant in argon
3
bu†er at 296 K: (a) P \ 20.0 Torr, P(NO) \ 0.67 Torr, q \ 86 ls;
total
(b) P \ 20.0 Torr, P(O ) \ 1.18 Torr; q \ 130 ls; (c) P \ 10.0
Fig. 3 Typical plots for determining k
for Pt(a 3D ) ] NO at 296
total
2
total
obsd
3
Torr, P(NO) \ 0.78 Torr; q \ 197 ls. The solid lines through the data
K. The solid lines are linear regression Ðts from which k
is
obsd
are least-squares Ðts. The inset is an ln plot of the data.
obtained.
354
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

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Table 2 Second-order rate constants for Pt(a 3D ) ] N O at 2.5
3
2
Torr in argon bu†er
k
/
obsd
T /K
1013 molecule~1 cm~3 s~1
296
373
398
423
1.3
1.9
2.1
2.8
second-order rate constants increased slightly with tem-
perature. Rate constants as a function of temperature for this
reaction are shown in Table 2.
For the reaction of platinum with methane, the rate con-
stants are only slightly pressure dependent over the pressure
range studied here. Furthermore, it appears the fall-o† to the
high-pressure limit occurs at a relatively low pressure (ca. 20
Torr). It is reasonable to question whether the pressure depen-
dence is real or simply the result of statistical Ñuctuation in
the measurements. The rate constants at 5 and 20 Torr were
each measured seven times on six di†erent days. The standard
deviations for the measurements for each of these two pres-
sures are both 0.6 ] 10~12. Thus, using the t-test, the di†er-
ence between the rate constants is signiÐcant to the 99%
conÐdence level. However, a more convincing argument than
a statistical analysis is the result of a Ñow-tube study for this
reaction done by Carroll et al. at very low pressures.19 The
rate constants measured at 1.1 Torr or less are signiÐcantly
lower than the rate constants measured here. Their use of
helium as the bu†er gas is not expected to cause such a dra-
matic di†erence in the rate constants. Thus, the cumulative
evidence supports the conclusion that the reaction is pressure
dependent.
Fig. 4 Typical plots for determining k
for Pt(a 3D ) ] N O at
obsd
3
2
296 K. The solid lines are linear regression Ðts from which k
is
obsd
obtained.
constants at this pressure within experimental error. Thus, the
pressure dependence indicates termolecular processes for these
reactants while the temperature results indicate either small or
no barriers to reaction. For the reaction with N O, the
2
Discussion
Pt + O
2
Results for the reaction of platinum with O indicate a termol-
2
ecular reaction mechanism:
*Ar+
Pt ] O ÈÈÈ PtO ; *H¡ \ [394 kJ mol~1 (ref. 50) (2)
2
2
The bimolecular abstraction channel to produce PtO is endo-
thermic by 106 kJ mol~1;50,51 thus the thermodynamic
barrier prohibits this pathway at the temperatures studied
here. The variation with total pressure of the second-order
rate constants at room temperature in argon bu†er is shown
in Fig. 6. The solid lines through the data in Fig. 6 are
weighted Ðts to the simpliÐed LindemannÈHinshelwood
expression52
Fig. 5 Typical plots for determining k
for Pt(a 3D ) ] CH at 296
obsd
3
4
K. The solid lines are linear regression Ðts from which k
is
obsd
obtained.
k [Ar]
o
k
\
(3)
obsd
1 ] k [Ar]/k
o
=
Table 1 Second-order rate constants (10~12 molecule~1 cm3 s~1) for
Pt(a 3D ) with O , NO, N O and CH in argon bu†er at 296 K
where k is the limiting low-pressure third-order rate constant,
3
2
2
4
o
k
is the limiting high-pressure second-order rate constant,
and [Ar] is the bu†er gas number density. k and k for
=
total
pressure
/Torr
o
=
K
the reaction of
O
in argon bu†er at 296
O
NO
N O
CH
2
2
2
4
are (2.3 ^ 0.2) ] 10~31 molecule~2 cm6 s~1 and
(2.5 ^ 0.5) ] 10~12 molecule~1 cm3 s~1, respectively. The
listed uncertainties are ^2p. The value of the limiting low-
pressure third-order rate constant is relatively large, consistent
2.5
5
10
20
30
50
75
100
150
200
250
300
[
0.043
0.089
0.17
0.20
0.32
[
[
0.080
0.16
0.32
[
0.13
0.18
0.23
0.36
0.42
0.55
0.78
0.86
1.2
4.9
5.0
5.3
6.3
[
with other termolecular reactions with O involving s1dn~1
2
transition metals. Previous studies of the 3d transition metal
0.72
[
6.2
[
association reactions with O indicate that TMs with ground
2
0.54
0.77
0.96
1.1
1.3
1.9
2.4
3.1
6.2
[
state or low-lying s1dn~1 conÐgurations are reactive whereas
TMs with s2dn~2 ground-state conÐgurations are unre-
active.26 This behavior has been explained in terms of simple
1.3
1.4
1.5
6.3
[
molecular orbital concepts. In the approach of the TM to O ,
1.2
3.7
6.4
2
the nature of the potential in the region of the onset of orbital
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
355

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the same rationale used to explain the reactivity dependence
on electron conÐguration in the termolecular reactions of 3d
TM atoms with O . Even if the d-electrons of the TM atom
2
are involved in the bonding, the reactivity of the s2 conÐgu-
ration would still be expected to be lower than that of the s1
conÐguration because of its more di†use nature. The greater
efficiency of platinum relative to iron and manganese may be
due to the better size match between the 6s- and 5d-orbitals
compared to the 4s- and 3d-orbitals of the 3d series. The
better size match might help drive the chemical reaction due
to the increased incorporation of the d-orbitals in the bonding
to yield larger metalÈoxygen bond energies in the product.
Again, a more thorough quantitative understanding of the
dynamics of this reaction awaits accurate theoretical calcu-
lations.
Pt + N O
2
Fig. 6 Pressure dependence of the reaction of Pt(a 3D ) with O and
The pressure dependence of the rate constants for the reaction
of platinum with N O indicate the termolecular channel is
important. However, a Ðt of the rate constants to eqn. (3) gave
3
2
NO in argon bu†er at 296 K. Error bars represent ^20% uncertainty.
The solid lines are Ðts to eqn. (3).
2
unsatisfactory results (see below). Furthermore, the rate con-
stants (Table 2) increase with temperature; thus, a bimolecular
abstraction channel is also indicated. The abstraction channel:
overlap will be determined by the interaction between the s-
orbital of the TM atom (which has a larger spatial extent than
the d-orbitals) and the in-plane p*-antibonding orbital of O .
2
For a TM atom with an s1 conÐguration, an electron-pair
Pt ] N O ] PtO ] N ;
2
2
bond may be formed from the overlap of these orbitals, as in a
radicalÈradical recombination process. In the case of an s2
conÐguration, one of the s-electrons occupies the antibonding
*H¡ \ [225 kJ mol~1 (ref. 50 and 51) (5)
is exothermic so there are no thermodynamic barriers for this
reaction. However, the pressure dependence of the rate con-
stants indicates the termolecular component is also operative.
The variation with total pressure of the second-order rate con-
stants at room temperature in argon bu†er gas is shown in
Fig. 7. The dashed line in Fig. 7 is the Ðt to eqn. (3). A better
Ðt to the data is obtained by Ðtting the rate constants to the
equation:
molecular orbital between the TM atom and O , which desta-
2
bilizes the complex and leads to a less attractive or a repulsive
potential.26
The value of the low-pressure limiting third-order rate con-
stant for platinum is approximately an order of magnitude
smaller than another Group 10 atom, nickel.26 k for platinum
o
is more similar to the Group 11 copper atom21,26 than to
nickel. Thus, this reaction is a case where the 3d TM is more
reactive than the 5d TM of the same group. The smaller reac-
tivity of platinum relative to nickel is surprising considering
the better ability of the 5d orbitals in platinum to be involved
in bonding.
Unfortunately, a more thorough quantitative understanding
of the dynamics of these reactions awaits accurate theoretical
calculations.
k [Ar]
o
k
\
] k
(6)
obsd
abs
1 ] k [Ar]/k
o
=
where k is the rate constant for the bimolecular abstraction
abs
channel. The solid line through the data in Fig. 7 is a weighted
Ðt to eqn. (6). k , k and k for the reaction of N O in argon
o
=
abs
2
bu†er at 296 K are (3.7 ^ 0.5) ] 10~31 molecule~2 cm6 s~1,
(2.3 ^ 0.3) ] 10~12 molecule~1 cm3 s~1 and (1.1 ^ 0.2) ]
10~13 molecule~1 cm3 s~1, respectively. In Ðtting the rate
constants to eqn. (6), the assumption is made that the
abstraction channel is independent of the termolecular
Pt + NO
Results of the reaction of Pt(a 3D ) ] NO indicate a termole-
3
cular reaction mechanism.
*Ar+
Pt ] NO ÈÈ PtNO
(4)
Production of PtO from the abstraction channel is endo-
thermic by 239 kJ mol~1.50,51 The variation with total pres-
sure of the second-order rate constants at room temperature
in argon bu†er is shown in Fig. 6. The solid lines through the
data in Fig. 6 are weighted Ðts to eqn. (3). k and k for
o
=
K
the reaction of NO in argon bu†er at 296
are (4.3 ^ 0.4) ] 10~31 molecule~2 cm6 s~1 and
(2.3 ^ 0.8) ] 10~11 molecule~1 cm3 s~1, respectively.
Termolecular processes are expected for the majority of TM
atoms reacting with NO since only the Group 3, 4 and 5
atoms along with tungsten have exothermic abstraction reac-
tions with NO. Thus far, however, extensive pressure-
dependent rate constants have been reported only for
chromium,25 iron,24,43 manganese,43 molybdenum37 and
ruthenium.43 The two atoms in the 3d series with s2dn~2 elec-
tron conÐgurations are extremely inefficient processes with
NO whereas the TMs with s1dn~1 conÐgurations have much
more efficient reactions. In the s1 conÐguration, the singly
occupied orbital of the TM might overlap favorably with the
unpaired electron on NO, forming a bond. This argument is
Fig. 7 Pressure dependence of the reaction of Pt(a 3D ) with N O in
3
2
argon bu†er at 296 K. Error bars represent ^20% uncertainty. The
solid line is a Ðt to eqn. (6). The dotted line is the Ðt determined from
eqn. (3).
356
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

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the metal, the electron affinity of N O, and the bond energy of
2
channel. This may not, however, be the case. There is the
possibility that the mechanism for this reaction resembles the
the metal oxide product. The resonance interaction model
mechanism proposed for the reaction of Al ] CO ; i.e. an
predicts a room-temperature rate constant of 2 ] 10~19
molecule~1 cm3 s~1 for platinum,57 a value over Ðve orders of
magnitude smaller than the smallest experimentally measured
rate constant here.
2
energized intermediate is formed which can decompose along
three di†erent paths.53,54 If the mechanism for the reaction of
platinum with N O involves an energized intermediate
2
complex (PtON *):
2
Pt + CH
4
k1
k3*Ar+
Pt ] N O E8F PtON * ÈÈ PtON
(7)
(8)
The two most plausible reaction pathways for the reaction of
platinum with methane are the termolecular insertion channel
to produce HwPtwCH and the bimolecular H elimination
2
k2 2
2
Â
k~1
Â
ÈÈ PtO ] N
2
3
2
reaction to produce PtwCH . Our experimental results com-
then the experimentally observed rate constant is:53
2
bined with the previously reported Ñow tube results19 indicate
this reaction follows a termolecular reaction mechanism:
A
k ] k [Ar]
B
2
3
k
\ k
obsd
1
k
] k ] k [Ar]
*Ar+
~1
2
3
Pt ] CH ÈÈ HwPtwCH ;
4
3
According to this mechanism the Pt and N O react to form
an energized complex, PtON *, with rate constant k . This
complex can then be stabilized by collisions (k [Ar]) or can
2
*E \ [134 kJ mol~1 (ref. 19) (10)
2
1
The variation with total pressure of the second-order rate con-
stants at room temperature in argon bu†er is shown in Fig. 8.
The solid lines through the data in Fig. 8 are weighted Ðts to
eqn. (3). The open squares in Fig. 8 are rate constants from ref.
19 and were not included in the Ðt to eqn. (3). k and k
3
either react to produce PtO product (k ) or decompose back
2
to reactants (k ). Thus, the physical interpretation of eqn. (8)
~1
is that the experimentally observed rate constant is equal to
the rate of energized complex formation multiplied by the
fraction of energized complexes which do not decompose back
to reactants. The only rate constant which can be determined
o
=
determined from the rate constants reported here for the reac-
tion of CH in argon bu†er at 296 K are (2.1 ^ 0.9) ] 10~28
4
absolutely from the data is k ; only relative values of the other
molecule~2 cm6 s~1 and (6.3 ^ 0.3) ] 10~12 molecule~1 cm3
1
constants can be determined. A least-squares Ðt of the rate
s~1, respectively.
constants to eqn. (8) yields (2.3 ^ 0.3) ] 10~12 molecule~1
Of the four reactions reported here, extensive calculations
have only been reported for the reaction of platinum with
methane. These calculations make a clear prediction that the
product of this reaction is the collisionally stabilized, long-
lived HwPtwCH insertion complex rather than H elimi-
cm3 s~1 for k . The ratios k /k and k /k are 2.1 ^ 2.1 and
1
2
3
~1 3
51 ^ 20 molecule cm~3, respectively. k /k is obviously not
2
3
determined very well from our data. These parameters yield
essentially the same goodness of Ðt to the data as the param-
eters reported previously for the Ðt to eqn. (6). In the limit of
zero bu†er gas pressure, eqn. (6) reduces to:
3
2
nation. A condensation of the primary Ðndings reported by
Carroll et al. follows.19
The calculations by Carroll et al. indicate the electronic
structure of platinum is an important factor in the dynamics
of this reaction. The calculations investigated both the lowest
triplet and singlet potential-energy surfaces along paths
A
k
B
2
k
\ k
(9)
obsd
1
k
] k
~1
2
which corresponds to the rate of the abstraction reaction in
the absence of bu†er gas. Substitution of the Ðtted parameters
from eqn. (8) to eqn. (9) yields a value of 9.0 ] 10~14
molecule~1 cm3 s~1 for the rate constant of the abstraction
channel in the absence of bu†er gas. Further experiments in
which the pressure dependence of the yield of PtO product is
determined would help to resolve the correct mechanism.
The only other termolecular reaction involving a TM with
N O thus far reported is the a 4F state of iridium. IridiumÏs
leading from Pt ] CH to the CwPtwH bond insertion inter-
4
mediate and Ðnally on to elimination of H . The s1d9 ground
2
state has the proper conÐguration to form two covalent bonds
in the HwPtwCH insertion complex. This contributes to the
3
large binding energy (134 kJ mol~) of the insertion complex
relative to the ground-state asymptotes. Equally important,
however, is the presence at relatively low energy of the d10 1S
0
state which diminishes long-range repulsion and correlates in
spin to the deep HwPtwCH insertion complex. When com-
2
9@2
reaction efficiency, however, is over two orders of magnitude
smaller than the termolecular parameters reported here for
platinum.48 Recent experiments in our laboratory indicate the
3
bined, these two features allow ground-state Pt to access the
reaction of nickel with N O is also pressure dependent with
rate constants similar in magnitude to those reported here for
2
platinum.
Abstraction reactions of all TM atoms with N O are exo-
2
thermic owing to the formation of the stable N and metal
2
oxide molecules. Despite this exothermicity, metal-atom reac-
tions with N O have been observed to have signiÐcant energy
2
barriers. These barriers have been attributed to the require-
ment of
a non-adiabatic transition along the reaction
pathway.55 In order to conserve electron spin, the 1&` ground
state of N O correlates adiabatically to O(1D ) ] N (1&`),
2
2
2
15868 cm~1 above the O(3P ) ] N (1&`) asymptote.56 Thus,
2
2
the lowest-energy reaction pathway requires a transition from
reactant surfaces of O(1D ) character to surfaces of O(3P)
2
character in order to access the low-energy product states.
Recently, Fontijn and co-workers have advanced a reso-
nance interaction model57h60 to predict barriers to reaction
and rate constants for metal atoms reacting with N O. In this
model, the activation barriers are calculated by taking into
account the ionization potential and sp promotion energy of
Fig. 8 Pressure dependence of the reaction of Pt(a 3D ) with CH in
2
3
4
argon bu†er at 296 K. Error bars represent ^20% uncertainty. The
solid line is a Ðt to eqn. (3). Open squares are data points from ref. 19.
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
357

View Article Online
19 J. C. Carroll, J. C. Weisshaar, P. E. M. Siegbahn, A. M. Wittborn
deep insertion well. The considerable strength of the covalent
and M. R. A. Blomberg, J. Phys. Chem., 1995, 99, 14388.
20 J. J. Carroll and J. C. Weisshaar, J. Phys. Chem., 1996, 100,
12355.
PtwH and PtwCH bonds causes the tripletÈsinglet crossing
3
to occur at a low energy. The intersection between triplet and
singlet surfaces is calculated to be only 5 kJ mol~1 above
ground-state reactants, which enables the triplet ground-state
reactants to access the singlet insertion well by collisions at
thermal energies.
21 C. Vinckier, J. Corthouts and S. DeJaegere, J. Chem. Soc.,
Faraday T rans. 2, 1988, 84, 1951.
22 D. Ritter and J. C. Weisshaar, J. Phys. Chem., 1989, 93, 1576.
23 D. Ritter and J. C. Weisshaar, J. Phys. Chem., 1990, 94, 4907.
24 S. A. Mitchell and P. A. Hackett, J. Chem. Phys., 1990, 93, 7822.
25 J. M. Parnis, S. A. Mitchell and P. A. Hackett, J. Phys. Chem.,
1990, 94, 8152.
26 C. E. Brown, S. A. Mitchell and P. A. Hackett, J. Phys. Chem.,
1991, 95, 1062.
27 A. S. Narayan, P. M. Futerko and A. Fontijn, J. Phys. Chem.,
1992, 96, 1290.
The calculations further exclude H elimination as the reac-
2
tion pathway. The calculations indicate a large potential
barrier separates HwPtwCH from H elimination products
3
2
which are substantially (30 kJ mol~1) endoergic from the
ground-state asymptote. The possibility that the platinum
reaction proceeds all the way to H elimination becomes
2
28 R. E. McClean and L. Pasternack, J. Phys. Chem., 1992, 96, 9828.
29 M. L. Campbell and R. E. McClean, J. Phys. Chem., 1993, 97,
7942.
highly unlikely when the barrier to a-elimination is con-
sidered. The calculations Ðnd this barrier to lie an additional
99 kJ mol~1 higher than the Ðnal products. This barrier is
attributed to the bonding in the transition state which is of the
four-center type. Since the Pt atom cannot form more than
two bonds using the s1d9 state, the H PtCH complex is
30 D. E. Clemmer, K. Honma and I. Koyano, J. Phys. Chem., 1993,
97, 11480.
31 M. Helmer and J. M. C. Plane, J. Chem. Soc., Faraday T rans.,
1994, 90, 31.
2
2
32 M. Helmer and J. M. C. Plane, J. Chem. Soc., Faraday T rans.,
1994, 90, 395.
formed with two covalent bonds to the carbene and a molecu-
larly bound hydrogen molecule. The formation of a dihydride
product would require a very large promotion energy to the
excited s1d8p1 atomic state and is therefore unfavorable.
Our results indicate the saturated termolecular limit for the
reaction of platinum with methane is reached at a relatively
low pressure (ca. 20 Torr). This implies a slow unimolecular
decay rate of the collision complex; i.e. the complex lifetime is
very long. Arguments for the plausibility of a long lifetime for
this complex have been advanced previously by Carroll et
al.19 The necessity for singlet complexes to regain access to
the triplet asymptotic surface before dissociating presumedly
increases the lifetime of this complex compared to the case of
a reaction which occurs on a single potential surface.
33 A. Fontijn, A. S. Blue, A. S. Narayan and P. N. Bajaj, Combust.
Sci. T echnol., 1994, 101, 59.
34 M. L. Campbell, R. E. McClean and J. S. S. Harter, Chem. Phys.
L ett., 1995, 235, 497.
35 M. L. Campbell and J. R. Metzger, Chem. Phys. L ett., 1996, 253,
158.
36 M. L. Campbell, J. Chem. Phys., 1996, 104, 7515.
37 R. E. McClean, M. L. Campbell and R. H. Goodwin, J. Phys.
Chem., 1996, 100, 7502.
38 M. L. Campbell, J. Chem. Soc., Faraday T rans., 1996, 92, 4377.
39 R. Matsui, K. Senba and K. Honma, Chem. Phys. L ett., 1996,
250, 560.
40 R. E. McClean, M. L. Campbell and E. J. Kolsch, J. Phys. Chem.
A, 1997, 101, 3348.
41 M. L. Campbell, K. L. Hooper and E. J. Kolsch, Chem. Phys.
L ett., 1997, 274, 7.
I thank Professor Gary Fowler of the USNA Mathematics
42 R. Matsui, K. Senba and K. Honma, J. Phys. Chem. A, 1997, 101,
Department for performing the calculations to Ðt the N O
179.
2
43 R. E. McClean, M. L. Campbell, M. D. Vorce, in preparation.
44 M. L. Campbell and R. E. McClean, J. Chem. Soc., Faraday
T rans., 1995, 91, 3787.
45 J. S. S. Harter, M. L. Campbell and R. E. McClean, Int. J. Chem.
Kinet., 1997, 29, 367.
46 M. L. Campbell, J. Phys. Chem., 1996, 100, 19430.
47 M. L. Campbell and K. L. Hooper, J. Chem. Soc., Faraday
T rans., 1997, 93, 2139.
48 M. L. Campbell, J. Phys. Chem. A, 1997, 101, 9377.
49 W. F. Meggers, C. H. Corliss and B. F. Scribner, T ables of
Spectral-L ine Intensities, Part I, Arranged by Elements, NBS
Mono., 145, U.S. Government Printing Office, Washington, DC,
1975.
50 D. D. Wagman, W. H. Evans, V. B. Parker, R. H. Schumm, I.
Halow, S. M. Bailey, K. L. Churney and R. L. Nuttall, The NBS
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51 J. B. Pedley and E. M. Marshall, J. Phys. Chem. Ref. Data, 1983,
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52 P. J. Robinson and K. A. Holbrook, Unimolecular Reactions,
Wiley-Interscience, New York, 1972.
rate constants to eqn. (8). This research was supported by a
Cottrell College Science Award of Research Corporation.
Acknowledgment is made to the Donors of the Petroleum
Research Fund, administered by the American Chemical
Society, for partial support of this research.
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J. Chem. Soc., Faraday T rans., 1998, V ol. 94