Kinetic study of the reactions of gas phase Pd(a1S0), Ag(5s 2S1/2), Au(6s 2S1/2), Cd(5s2 1S0, and Hg(6s2 1S0) atoms with nitrous oxide

Article abstract of DOI:10.1021/jp021957m

The reactivity of gas-phase Pd(a¹S₀), Ag(5s ²S_1/2), Au(6s ²S_1/2), Cd(5s^{2 1}S₀), and Hg(6s^{2 1}S₀) with N₂O as a function of temperature and pressure is reported. The transition metal atoms were produced by the photodissociation of an appropriate precursor molecule and detected by laser-induced fluorescence. The reaction of palladium with N₂O is pressure dependent indicating adduct formation. The room-temperature limiting low-pressure third-order rate constant is (2.5 ± 0.8) × 10^-30 molecule^-2 cm⁶ s^-1 in argon buffer. The limiting low-pressure third-order rate constant can be expressed as log k₀(T) = -219.06 + 152(log T) - 30.5(log T)² molecule^-2 cm⁶ s^-1 over the temperature range 294-523 K. The reactions of ground-state silver, gold, cadmium, and mercury with N₂O are very slow, and only upper limits for the rate constants can be reported for these reactions.

Full text of DOI:10.1021/jp021957m

3048
J. Phys. Chem. A 2003, 107, 3048-3053
2
2
Kinetic Study of the Reactions of Gas Phase Pd(a1S0), Ag(5s S1/2), Au(6s S1/2), Cd(5s2 1S0),
and Hg(6s2 1S0) Atoms with Nitrous Oxide
Mark L. Campbell†
Chemistry Department, United States NaVal Academy, Annapolis, Maryland 21402
ReceiVed: August 27, 2002; In Final Form: February 26, 2003
2
2
The reactivity of gas-phase Pd(a1S0), Ag(5s S1/2), Au(6s S1/2), Cd(5s2 1S0), and Hg(6s2 1S0) with N2O as a
function of temperature and pressure is reported. The transition metal atoms were produced by the
photodissociation of an appropriate precursor molecule and detected by laser-induced fluorescence. The reaction
of palladium with N2O is pressure dependent indicating adduct formation. The room-temperature limiting
low-pressure third-order rate constant is (2.5 ( 0.8) × 10-30 molecule-2 cm6 s-1 in argon buffer. The limiting
low-pressure third-order rate constant can be expressed as log k0(T) ) -219.06 + 152(log T) - 30.5(log T)2
molecule-2 cm6 s-1 over the temperature range 294-523 K. The reactions of ground-state silver, gold, cadmium,
and mercury with N2O are very slow, and only upper limits for the rate constants can be reported for these
reactions.
Introduction
reaction chamber is enclosed within a convection oven (Blue
M, model 206F, Tmax ) 623 K) with holes drilled to allow for
the exiting sidearms and the telescoping of the LIF signal to
the PMT.
The reactions of gas-phase metal atoms with nitrous oxide
have recently received considerable attention because of a
number of both fundamental and practical reasons.1,2 The
reactions of transition metal atoms, TM, with N2O
Transition metal atoms are produced by the 248 nm photo-
dissociation of the appropriate precursor with the output of an
excimer laser (Lambda Physics Lextra 200). Some precursors
require the focusing of the beam to observe the atomic signal.
When focusing is required, a lens (f ) 564 mm) is placed
approximately one focal length from the reaction zone. The
preferred arrangement is without a focusing lens because
focusing tends to produce a greater percentage of excited state
atoms. Transition metal atoms are detected via LIF using an
excimer-pumped dye laser (Lambda Physics Lextra 50/Scan-
Mate 2E). The fluorescence is detected at 90° to the counter-
propagated laser beams with a three-lens telescope imaged
through an iris. Interference filters are used to isolate the LIF.
A photomultiplier tube (Hamamatsu R375) is used in collecting
the LIF which is subsequently sent to a gated boxcar sampling
module (Stanford Research Systems SR250), and the digitized
output is stored and analyzed by a computer.
TM(g) + N2O(g) f TMO(g) + N2(g)
(1)
are all exothermic3,4 because of the formation of the stable N2
and metal oxide molecules. Despite this exothermicity, metal
atom reactions with N2O have been observed to have significant
energy barriers. These barriers have been attributed to the
requirement of a nonadiabatic transition along the reaction
pathway.5
Transition metal chemistry is an intriguing field of study
because of the high multiplicities of the atomic ground states
and the large number of low-lying metastable states. For many
TM atoms, states resulting from s2dn-2 and s1dn-1 configurations
(where n refers to the number of valence electrons) are nearly
degenerate so the electronic structure of the atom plays an
important role in its reaction dynamics. The kinetics of the
transition metals reacting with N2O have previously received
considerable attention.6-33 Here, we report the kinetics of the
remainder of the nonradioactive transition metals reacting with
N2O. With the completion of this study, rate constants for the
reactions of all of the transition metals with the exception of
Tc have now been reported.
The transition metal precursor is entrained in a flow of buffer
gas. The diluted precursor, buffer gas, and N2O flow through
calibrated mass flow meters and flow controllers prior to
admission to the reaction chamber. Each sidearm window is
purged with a slow flow of buffer gas to prevent deposition of
metal atoms and other photoproducts. Pressures are measured
with MKS Baratron manometers, and chamber temperatures are
measured with a thermocouple.
Experimental Section
The delay time between the photolysis pulse and the dye-
laser pulse is varied by a digital delay generator (Stanford
Research Systems DG535) controlled by a computer interfaced
through a Stanford Research Systems SR245 computer interface.
The trigger source for these experiments is scattered pump laser
light incident upon a fast photodiode. LIF decay traces typically
consist of 200 data points, each point averaged from 3 to 5
laser shots.
Kinetic experiments in our laboratory utilize pseudo-first-
order conditions ([TM] , [N2O]) in an apparatus with slowly
flowing gas. The experimental apparatus and technique have
been described in detail elsewhere.34 Briefly, the reaction
chamber is a stainless steel reducing 4-way cross with attached
sidearms and a sapphire window for optical viewing. The
† Henry Dreyfus Teacher-Scholar.
10.1021/jp021957m This article not subject to U.S. Copyright. Published 2003 by the American Chemical Society
Published on Web 04/08/2003

Reactions of Gas Phase Pd, Ag, Au, Cd, and Hg
J. Phys. Chem. A, Vol. 107, No. 17, 2003 3049
Figure 1. Typical decay curves with added N2O in N2 buffer at 623
K: (a) Au(a2S1/2), Ptotal ) 100.0 Torr, P(N2O) ) 27.9 Torr, τ ) 8.6
ms; (b) Cd(a1S0), Ptotal ) 100.0 Torr, P(N2O) ) 93.8 Torr; τ ) 48 ms.
The solid lines through the data are least-squares fits.
Figure 2. Typical Pd(a1S0) decay curves with added N2O in argon
buffer at 294 K and Ptotal ) 20.0 Torr: (a) P(N2O) ) 0.559 Torr, τ )
69 µs; (b) P(N2O) ) 0.420 Torr; τ ) 89 µs; (c) P(N2O) ) 0.280 Torr,
τ ) 120 µs. The solid lines through the data are least-squares fits. The
inset is a ln plot of the data.
Data Analysis and Results
Under pseudo-first-order conditions in which no production
processes occur after the initial photolysis event, the decrease
in the [TM] with time following the photolysis laser pulse is
given by
-d[TM]/dt ) (kobs[N2O] + kd)[TM] ) (1/τ)[TM] (2)
where kobs is the observed second-order rate constant due to
N2O at fixed buffer gas pressure, kd represents the depletion
rate constant due to the reaction of transition metal with
precursor molecules and fragments and diffusion out of the
detection zone, and τ is the observed time constant for transition
metal depletion in the presence of a given [N2O], precursor,
and buffer gas pressure. The LIF signal at time t after photolysis
is proportional to [TM]. Thus
Figure 3. Typical plots for determining kobs for Pd(a1S0) + N2O at
294 K. The solid line for each set of data is a linear regression fit from
which kobs is obtained.
LIF ) LIF0 exp(-t/τ)
(3)
where LIF0 is the initial LIF signal immediately following the
photolysis laser pulse. LIF decays which exhibit single-
exponential behavior are fitted using a least-squares procedure
to determine τ. The decays for silver, cadmium, and mercury
exhibited single-exponential behavior. Plot b in Figure 1
illustrates the exponential behavior exhibited by cadmium.
As a result of the photolysis of the transition metal precursor,
atoms in excited states are often produced in significant
quantities. These excited state atoms possibly complicate the
temporal profile of the ground state. For a ground state in which
quenching of excited states after the initial photolysis laser pulse
is important, the time dependence of the ground state’s
concentration will exhibit biexponential behavior. If this situ-
ation is not handled properly, significant errors in the determi-
nation of the values of the rate constants can occur. For cases
where the decay rate of an excited state down to the ground
state is fast compared to the removal rate of the ground state,
cascade processes are unimportant at long times, and the ground-
state removal rate is determined by analysis of the decay at long
times. Biexponential behavior of this type has been observed
for numerous transition metals.21-24,27,28,33 Figures 1a and 2
illustrate this type of behavior which was observed for gold
and palladium, respectively. When analyzing data with a
biexponential component, it is critical that the time constant
does not include contributions from the growth due to decay of
excited states. In Figures 1a and 2, the solid lines indicate the
range of data used to calculate the value of the time constant.
Once the reaction time constant has been determined for a
series of nitrous oxide concentrations, the slope of a plot of 1/τ
vs [N2O] yields the observed second-order rate constant, kobs
.
Figures 3 and 4 illustrate examples of these types of plots for
palladium. 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 tem-
perature and total pressure conditions. The absolute uncertainties
are conservatively estimated to be (30% and are based on the
sum of the statistical scatter in the data, uncertainty in the
flowmeter and flow controller readings (5%) and the total
pressure reading (1%), uncertainties due to incomplete gas
mixing, and uncertainties due to incomplete relaxation of the
excited electronic states to the ground state.
Many of the reactions of transition metal atoms with nitrous
oxide are very slow. In some cases, the rates are slower than
the detection limit of our experimental method. For our
experimental arrangement, the detection limit for the measure-
ment of rate constants depends on the transition metal being
studied. This variation in the detection limit as a function of
transition metal atom is due to two factors: (1) the conditions

3050 J. Phys. Chem. A, Vol. 107, No. 17, 2003
Campbell
Figure 5. Temperature dependence of the limiting low-pressure third-
order rate constant k0. The solid line is a polynomial fit in log T. See
text for the results of the fit.
Figure 4. Typical plots for determining ksecond as a function of
temperature for Pd(a1S0) + N2O at 20.0 Torr. The solid line for each
set of data is a linear regression fit from which kobs is obtained.
higher pressures at higher temperatures. The reaction of pal-
ladium with O2 remained in the linear regime well past 100
Torr total pressure.35 We were unable to measure rate constants
in the falloff region because of signal detection problems at
the high pressures required. The third-order rate constants
decrease dramatically with increasing temperature. The limiting
low-pressure third-order rate constants are fit by the expression
TABLE 1: Second-Order Rate Constantsa and Limiting
Low-Pressure Third-Order Rate Constants for Pd(a1S0)
Reacting with N2O
temp pressure
kobs
k0
(K)
(Torr) (10-13 molecule-1 cm3 s-1
)
(10-31 molecule-2 cm6 s-1
)
log k0(T) ) -219.06 + 152(log T) -
30.5(log T)2 molecule-2 cm6 s-1 (4)
294b
2.5
5.0
2.3
3.8
7.4
11.
16.
5.0
9.4
4.1
7.9
5.6
3.3
1.7
0.98
0.41
0.24
25.
10.0
15.0
20.0
10.0
20.0
10.0
20.0
20.0
20.0
20.0
20.0
20.0
20.0
over the temperature range 294-523 K. The experimental data
and a fit to the data using eq 4 are shown in Figure 5.
348
373
17.
15.
Ag(5s 2S1/2). Rate constant measurements were attempted up
to a temperature of 623 K in N2 buffer utilizing Ag(CF3-
COCHCOCF3)P(CH3)3 as precursor. Detection of silver atoms
was accomplished by both exciting and detecting the fluores-
398
423
448
473
498
523
12.
7.2
3.9
2.4
1.1
0.65
2
2
cence from the 5p P1/2°-5s S1/2 transition at 338.289 nm. A
UG-1 filter was utilized to isolate the fluorescence. The rate of
this reaction is extremely slow so that we can only report an
upper limit. Detection of silver atoms could be observed at 623
K in partial pressures of N2O up to 26 Torr. The lifetime of
silver under these conditions was approximately 1400 µs. Based
on these data, we report an upper limit for the reaction of
Ag(2S1/2) at 623 K of 2 × 10-15 molecule-1 cm3 s-1. Assuming
a preexponential factor of approximately the gas kinetic collision
rate, we can place a lower limit on the activation energy of
approximately 60 kJ/mol.
a Absolute uncertainties are estimated at (30%. b Room-temperature
varied from 293 to 295 K.
necessary to attain a measurable LIF signal and (2) the lifetime
of the transition metal in the detection zone in the absence of
oxidant. The factors affecting our detection limit have been
described in detail elsewhere,33 and the same arguments apply
for these experiments. For the transition metals reported here,
all but palladium exhibited reaction rates too slow at the
temperatures studied such that the rate constants are smaller
than the detection limit of our system; therefore, for these atoms,
only upper limits for the rate constants could be determined at
these temperatures.
Pd(a1S0). Pressure and temperature-dependent rate constants
were measured utilizing Pd(CF3CO2)2 as a precursor. Detection
of palladium atoms was accomplished by exciting the 5p 3D1°-
a1S0 transition at 247.642 nm and detecting the 5p 3D1°-5s 1D2
transition at 348.977 nm. A broadband interference filter
centered at 350 nm was utilized to isolate the fluorescence.
Second-order rate constants in argon buffer as a function of
temperature and pressure are listed in Table 1. The reaction of
palladium with N2O is pressure dependent indicating adduct
formation. The measured rate constants at 294, 348, and 373 K
measured at pressures of 20 Torr and below fell within the linear
third-order regime. The calculated limiting low-pressure third-
order rate constants are listed in Table 1. Rate constants above
373 K were only measured at 20 Torr and are expected to be in
the linear regime as the linear regime is expected to extend to
Au(6s 2S1/2). Temperature-dependent rate constant measure-
ments were attempted up to a temperature of 623 K in N2 buffer
utilizing (CH3)2(C5H7O2)Au as a precursor. Detection of gold
2
2
atoms was accomplished by exciting the 6p P3/2°-6s S1/2
transition at 242.795 nm and detecting the 6p P3/2°-6s2 2D5/2
2
transition at 312.278 nm. A broadband interference filter
centered at 300 nm was utilized to isolate the fluorescence. The
rate of this reaction is extremely slow so that we can only report
an upper limit. Detection of gold atoms could be observed at
623 K in partial pressures of N2O up to 27.9 Torr. The lifetime
of gold under these conditions was approximately 8600 µs.
Based on these data, we report an upper limit for the reaction
of Au(2S1/2) at 623 K of 3 × 10-16 molecule-1 cm3 s-1
.
Assuming a preexponential factor of approximately the gas
kinetic collision rate, we can place a lower limit on the activation
energy of approximately 70 kJ/mol.
Cd(5s2 1S0). Temperature-dependent rate constant measure-
ments were attempted up to a temperature of 623 K in N2 buffer
utilizing Cd(acac)2 as a precursor. Detection of cadmium atoms
was accomplished by both exciting and detecting the fluores-
1
cence from the 5p P1°-5s2 1S0 transition at 228.802 nm. A

Reactions of Gas Phase Pd, Ag, Au, Cd, and Hg
J. Phys. Chem. A, Vol. 107, No. 17, 2003 3051
narrowband interference filter centered at 228 nm was utilized
to isolate the fluorescence. The rate of this reaction is extremely
slow so that we can only report an upper limit. Detection of
cadmium atoms could be observed at 623 K in partial pressures
of N2O up to 93.8 Torr. The lifetime of cadmium under these
conditions was approximately 48 ms. Based on these data, we
report an upper limit for the reaction of Cd(5s2 1S0) at 623 K of
2 × 10-17 molecule-1 cm3 s-1. Assuming a preexponential
factor of approximately the gas kinetic collision rate, we can
place a lower limit on the activation energy of approximately
80 kJ/mol.
but inhibits PdO bond formation required for the abstraction
reaction. The unusually small bond energy of PdO has been
attributed to the unique closed-shell structure of the palladium
atom.36
Various mechanisms have been proposed to explain transition
metal/N2O reactions. Initial studies by Weisshaar and co-
workers8 of Sc, Ti, and V with simple oxygen-containing
oxidants (including N2O) indicated a relationship between the
rate constant and the ionization energy of the transition metal
and the electron affinity of the reactant. They proposed what is
now termed the electron-transfer mechanism. In this mechanism,
it is envisioned that as the metal atom approaches the N2O
molecule an electron from the transition metal transfers to the
N2O molecule similar to the harpoon mechanism. However,
because of the relatively high ionization energies of the transition
metals and the relatively low electron affinity of the N2O, the
metal-N2O distance at which the electron “jumps” is much
closer than in the prototypical alkali metal/halogen reaction.
Further experimental results indicated this mechanism does not
adequately describe these reactions, although there does appear
to be a loose connection between the ionization energies of the
transition metals and the experimentally measured energy
barriers (Table 2).
Hg(6s2 1S0). Temperature-dependent rate constant measure-
ments were attempted up to a temperature of 423 K in N2 buffer
utilizing Hg(CH3)2 as a precursor. Detection of mercury atoms
was accomplished by both exciting and detecting the fluores-
3
cence from the 6p P1°-6s2 1S0 transition at 253.652 nm. A
narrowband interference filter centered at 250 nm was utilized
to isolate the fluorescence. The rate of this reaction is slow so
that we can only report an upper limit. Detection of mercury
atoms could be observed only up to a temperature of 423 K in
partial pressures of N2O up to 18.6 Torr. The lifetime of mercury
under these conditions was approximately 6200 µs. Various
precursor molecules were tried for mercury, but a satisfactory
precursor which was stable above 423 K was not found. Based
on these data, we report an upper limit for the reaction of
Another mechanism proposed to describe these reactions is
the direct abstraction model.7 This model supposes the ease with
which an oxygen atom is abstracted from the N2O is dependent
upon the electron configurations of the metal atom and the metal
oxide product. A large majority of the metal oxide products
have one electron in the metal atom’s valence s orbital, i.e., the
highest occupied σ molecular orbital. In this model, the barrier
to the reaction is related to the promotion energy of the ground
state to the lowest lying state with a single electron in the
valence s orbital. The fact that many of the transition metals
with ground-state s1 electron configurations have large energy
barriers (Table 2) indicates this mechanism does not fully
explain the entire transition metal series.
Hg(6s2 1S0) at 423 K of 5 × 10-16 molecule-1 cm3 s-1
.
Assuming a preexponential factor of approximately the gas
kinetic collision rate, we can place a lower limit on the activation
energy of approximately 45 kJ/mol.
Discussion
The group 11 metal atoms studied here exhibit extremely low
reactivities with nitrous oxide despite their ground-state s1
electron configurations. Two groups have previously reported
on the reaction of copper with N2O. Fontijn and co-workers10
found an activation energy of 39.6 kJ/mol, whereas Vinckier
and co-workers14 determined an activation energy of 48.6 kJ/
mol. Thus, copper was found to be only slightly reactive
although copper is much more reactive with N2O than both silver
and gold. Copper’s reactivity is unusual in that the 3d transition
metals are generally less reactive than their 4d and 5d
congeners.33
All of the group 12 metals are unreactive with N2O. In a
previous study, zinc was reported to react very slowly with N2O
with a reported upper limit rate constant of 1 × 10-16
molecule-1 cm3 s-1 at 623 K.33 Thus, the closed valence s and
d subshells in these atoms renders these atoms unreactive toward
abstracting an oxygen atom from N2O.
The group 10 transition metals all exhibit pressure-dependent
kinetics with N2O. The only other transition metal to exhibit
termolecular kinetics with N2O is iridium;26 however, iridium’s
limiting low-pressure third-order rate constants are almost 2
orders of magnitude smaller than the least reactive of the group
10 metals (nickel). The room-temperature limiting low-pressure
third-order rate constants for ground-state Ni, Pd, and Pt are
8.2 × 10-32, 2.5 × 10-30, and 3.7 × 10-31 molecule-2 cm6
s-1, respectively.31,33 Thus, palladium is the most reactive toward
adduct-formation of the group 10 atoms. Palladium is unique
among the group 10 atoms by not exhibiting an abstraction
channel. Both nickel and platinum exhibited an abstraction
channel with a barrier; that is, the low-pressure second-order
rate constants increase with increasing temperature for these
atoms. The lack of an s electron in the valence shell of palladium
may help in adduct formation because of decreased repulsion
Fontijn and co-workers have proposed a semiempirical
method to calculate energy barriers for transition metal/N2O
reactions called the SECI (semiempirical configuration interac-
tion) model.37,38 In SECI, the Arrhenius parameters are estimated
by taking into account the ionization energy and sp promotion
energy of the transition metal atom, the electron affinity of N2O,
and the bond energy of the metal oxide product. The original
formulation of the model was based primarily on main group
reactions and when applied to TM atoms gave inconsistent
results.37 Blue and Fontijn have formulated an improved model
specifically for TMs.38 In this formulation, the experimental
activation barrier for one “basis” reaction is used to determine
activation barriers for an entire series. Transition metal series
based on valence electron structures gave six groups, each
differing only by the contiguous number of d electrons. Table
2 lists the calculated values of the energy barriers (ESECI) along
with the experimentally determined activation energies. ESECI
is calculated based on k(T) having a T1/2 factor included in the
preexponential term, so comparisons with activation energies
calculated from the Arrhenius form cannot be directly made.
However, the differences in the two forms result in only small
variations in the calculated energies. Including a correction for
the different equation forms, Blue and Fontijn report the
calculations for the six TM groups yield an average deviation
of |ESECI - Eexp| of only 3.8 kJ/mol,38 although the calculated
values for Ni, Mo, and Re show an alarmingly large deviation
from the experimental values. Although not as general as the
original formulation, the results are considerably more accurate

3052 J. Phys. Chem. A, Vol. 107, No. 17, 2003
Campbell
TABLE 2: Transition Metal Ionization Potentials and sp Promotion Energies, Transition Metal Oxide Bond Energies,
Enthalpies of Reaction, and Experimental and Calculated Activation Barriers for the Reaction TM(g) + N2O(g) f
TMO(g) + N2(g)
c
IPa
(kJ/mol)
sp PEa
(kJ/mol)
TMO BEb
(kJ/mol)
∆H° b
(kJ/mol)
ESECI
Ea(exp)
(kJ/mol)
metal
(kJ/mol)
ref
Sc(s2d1 2D3/2
Ti(s2d2 3F2)
)
631
658
651
653
187
193
196
279
677
668
621
425
-514
-501
-470
-291
10.4
12.4
12.0
12.0
14.3
10.7
21.1
20.5
44.7
44.4
49.4
45.7
48.8
11.3
39.6
48.6
>65
4.0
21
11
33
33
13
15
16
17
32
33
33
10
14
33
29
28
22
18
19
25
this work
this work
this work
29
28
23
24
30
20
26
31
this work
this work
V(s2d3 4F3/2
)
Cr(s1d5 7S3)
[20.8]
Mn(s2d5 6S5/2
Fe(s2d6 5D4)
)
717
762
220
231
399
386
-190
-248
42.3
47.3
Co(s2d7 4F9/2
Ni(s2d8 3F4)
)
759
737
745
282
308
365
380
377
266
-214
-215
-113
55.1
56.5
[39.6]
Cu(s1d10 2S1/2
)
Zn(s2d10 1S0)
906
600
640
652
684
710
720
804
731
868
538
659
761
770
760
843
873
865
890
1007
387
179
177
199
306
302
324
609
354
360
159
215
208
232
227
281
315
361
447
450
<267
715
772
765
556
>-103
-542
-628
-595
-431
-346
-254
-72
[51.7]
1.0
1.7
0.0
8.0
0.0
0.0
[>59.3]
[41.3]
[49.3]
0.0
Y(s2d1 2D3/2
Zr(s2d2 3F2)
)
3.3
Nb(s1d4 6D1/2
Mo(s1d5 7S3)
Ru(s1d7 5F5)
)
∼0
41
524
38
1.3
e
Rh(s1d8 4F9/2
Pd(d10 1S0)
)
401
234d
217
Ag(s1d10 2S1/2
Cd(s2d10 1S0)
)
-53
>60
>80
∼0
∼232
∼-68
-634
-651
-672
-508
∼-455
∼-365
-244
-225
-53
La(s2d12D3/2
Hf(s2d2 3F2)
)
797
797
795
668
∼623
∼594
410
387
219
9.5
11.3
13.6
25.6
>50
38.1
>45
-6f
>70
>45
Ta(s2d3 4F3/2
W(s2d4 5D0)
)
14.8
19.3
17.2
29.8
35.5
[46.2]
[56.4]
[60.9]
Re(s2d5 6S5/2
Os(s2d6 5D4)
)
Ir(s2d7 4F9/2
)
Pt(s1d9 3D3)
Au(s1d10 2S1/2
Hg(s2d10 1S0)
)
∼209
∼-90
a Reference 37. b References 3 and 4. c Calculated in refs 37 and 38 for an equation of the form k(T) ) AT1/2 exp(-ESECI/RT), values in brackets
are from ref 37. d Reference 36. e No bimolecular component measured. f Calculated from the rate constants reported in ref 31.
than the original formulation. The best results are obtained by
direct comparison to the measurements of a closely similar
element.
Table 2 lists the experimental activation energies for all of
the nonradioactive transition metals reacting with N2O. The wide
range of barrier heights indicates a complex relationship between
the way the reactant potential energy surfaces evolve into
products. To understand the reactions of transition metal atoms
with N2O, attention must be given to multidimensional intersec-
tions among diabatic potential energy surfaces arising from both
ground and low-lying excited states of the reactants. The large
number of low-lying states for many transition metals means
numerous potential energy surfaces are accessible in the entrance
channel. Many of the reactions of transition metals with N2O
have large exothermicities so that various product channels may
be accessed. Consequently, the activation barrier is a complex
interplay of how a transition metal’s reactant surfaces evolve
into the accessible product channels. Because all transition
metal/N2O reactions require transitions from reactant surfaces
of O(1D2) character to surfaces of O(3P) character to access low-
energy product states, barriers are expected unless other factors
make alternative excited state channels (with possibly lower
barriers) accessible. Transition metals with a large number of
low-lying atomic states and a large number of low-lying excited
states in their oxides are expected to increase the likelihood of
a smaller barrier because alternate pathways with possible lower
barriers than the ground-state reaction may become accessible.
Furthermore, the greater the magnitude of the exothermicity,
the more energy available to access metal oxide excited states.
Thus, a reaction with a large exothermicity should improve the
likelihood of a smaller barrier. Loose correlations between
ionization energies, sp promotion energies, and metal oxide bond
Recently, two groups have attempted to use calculations to
determine the energy barriers in 3d transition metal/N2O
reactions. Vinckier and co-workers39 used density functional
theory and the coupled cluster method to calculate transition
state geometries and energy barriers in these reactions. The
calculations were unable to quantitatively reproduce the ex-
perimental barriers, but a good correlation between the activation
barrier and the binding energy of the formed transition metal
oxide was found. They also found that the 3dn+1s1 electron
configuration deviated from this trend; that is, this configuration
is more reactive than the 3dns2 configuration.
Recently, Stirling40 has performed density functional calcula-
tions for the ScfNi reactions with N2O. The calculations
support “the electron-transfer-assisted oxygen abstraction”
mechanism in which the reactions are initiated by an electron
transfer from the transition metal to the N2O molecule. This
electron transfer assists the N-O bond weakening and the
O-(2P) abstraction to the metal atom without surface crossing.
Charge back-donation from the N2O to the metal follows,
forming additional bonds between the reactants. Significant 4s-
3d hybridization takes place on the metal atom, facilitating the
electron transfer to the N2O molecule. The result is that the
reactant and product channels connect on a single potential
energy surface. Thus, this mechanism exhibits the basic features
of the simple electron transfer and direct abstraction mecha-
nisms.

Reactions of Gas Phase Pd, Ag, Au, Cd, and Hg
J. Phys. Chem. A, Vol. 107, No. 17, 2003 3053
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of the mechanisms in these reactions awaits more accurate
theoretical calculations beyond the rudimentary attempts re-
ported thus far.
Acknowledgment. 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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