Kinetic study of the reaction of Mn(a6S5/2) with N2O from 448 to 620 K

Article abstract of DOI:10.1063/1.471654

The gas phase reactivity of Mn(a⁶S_5/2) with N₂O in the temperature range 448-620 K is reported. Manganese atoms were produced by the photodissociation of 2-methylcyclopentadienyl manganese tricarbonyl and detected by laser-induced fluorescence. The reaction rate of the a⁶S_5/2 state is very slow and temperature dependent. The rate constants are independent of total pressure indicating a bimolecular reaction. The rate constants are described in Arrhenius form by (2.05±0.45)×10^-10 exp(-44.7±1.0 kJ/mol/RT) cm³ s^-1.

Full text of DOI:10.1063/1.471654

Kinetic study of the reaction of Mn(a 6 S 5/2) with N2O from 448 to 620 K
Mark L. Campbell
Citation: The Journal of Chemical Physics 104, 7515 (1996); doi: 10.1063/1.471654
View online: http://dx.doi.org/10.1063/1.471654
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Kinetic study of the reaction of Mn(a6S5/2) with N2O from 448 to 620 K
Mark L. Campbell
Chemistry Department, United States Naval Academy, Annapolis, Maryland 21402
͑Received 11 December 1995; accepted 16 February 1996͒
The gas phase reactivity of Mn(a6S5/2) with N2O in the temperature range 448–620 K is reported.
Manganese atoms were produced by the photodissociation of 2-methylcyclopentadienyl manganese
tricarbonyl and detected by laser-induced fluorescence. The reaction rate of the a6S5/2 state is very
slow and temperature dependent. The rate constants are independent of total pressure indicating a
bimolecular reaction. The rate constants are described in Arrhenius form by
͑2.05Ϯ0.45͒ϫ10Ϫ10 exp͑Ϫ44.7Ϯ1.0 kJ/mol/RT͒ cm3 sϪ1. © 1996 American Institute of Physics.
͓S0021-9606͑96͒02919-5͔
INTRODUCTION
248 nm photodissociation of 2-methylcyclopentadienyl man-
ganese tricarbonyl, ͑MMT͒. Mn atoms were detected via LIF
using an excimer-pumped dye laser tuned to the
z6P70/2←a6S5/2 transition at 403.076 nm.7 The fluorescence
was detected at 90° to the counterpropagated laser beams
with a three-lens telescope imaged through an iris. The LIF
signal from the photomultiplier tube was sent to a gated box-
car sampling module, and the digitized output was stored and
analyzed by a computer. The MMT precursor was entrained
in a flow of N2 gas. The diluted precursor, N2 buffer gas, and
N2O flowed through calibrated mass flow meters and flow
controllers prior to admission to the reaction chamber. Total
flows were between 400 and 1500 sccm. Typical precursor
pressures inside the reaction chamber ranged from 0.5 to 4.8
mTorr assuming saturated conditions8 in the MMT saturator.
The delay time between the photolysis and dye-laser pulses
was varied by a digital delay generator controlled by a com-
puter. LIF decay traces consisted of 200 points; each point is
an average of 3 laser shots.
The gas-phase chemistry of transition metal ͑TM͒ atoms
in oxidation reactions has recently received considerable
attention.1 In this paper we report a kinetic study of the
ground state of Mn with nitrous oxide,
Mn a6S ͒ϩN O X ⌺ϩ͒→MnOϩN .
͑1͒
1
͑
͑
5/2
2
2
The only other previously reported study of Reaction ͑1͒
involved high velocity Mn atoms produced by laser vapor-
ization in a beam/gas arrangement in which the time resolved
MnO chemiluminescence was observed.2 The production of
excited MnO in reaction ͑1͒ was found to have a large trans-
lational threshold energy. Arguments were presented which
predicted significant barriers should also be present for the
production of ground state MnO, although no quantitative
results were presented.
Recently, Fontijn and co-workers have advanced a reso-
nance interaction model3–6 to predict Arrhenius parameters
and rate constants for metal atoms reacting with N2O. In this
model, the activation barriers are calculated by taking into
account the ionization potential and sp promotion energy of
the metal, the electron affinity of N2O, and the bond energy
of the metal oxide product. Many main group metals have
been studied in reactions with N2O and this model appears to
describe these reactions reasonably well. In contrast, rela-
tively few experimental studies have been performed on TM
atoms; consequently, this model is essentially untested for
TM atoms. Therefore, it is desirable to obtain temperature
dependent studies for the reaction of nitrous oxide with sev-
eral transition metals to determine the applicability of the
resonance interaction model to TM’s.
DATA ANALYSIS AND RESULTS
The decay rates of the ground state (a6S5/2) of Mn as a
function of N2O pressure were investigated as a function of
EXPERIMENT
Pseudo-first order kinetic experiments ͓͑Mn͔Ӷ͓N2O͔͒
were carried out in an apparatus with slowly flowing gas
using a laser photolysis/laser-induced fluorescence ͑LIF͒
technique. The experimental apparatus and technique have
been described in detail elsewhere.1 Briefly, the reaction
chamber is a fourway cross with attached side arms and a
sapphire window for optical viewing. The reaction chamber
is enclosed within a convection oven for temperature depen-
dence experiments. Manganese atoms were produced by the
FIG. 1. Typical plots for determination of k2nd. The pressure in parentheses
is the total pressure at which the experiments were performed. The inset is a
Mn(a6S5/2) first-order decay curve corresponding to the point at 130 Torr
N2O pressure for the 523 K data ͑1/␶ϭ0.020 ␮sϪ1͒.
J. Chem. Phys. 104 (19), 15 May 1996
0021-9606/96/104(19)/7515/3/$10.00
© 1996 American Institute of Physics
7515
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7516
Mark L. Campbell: The reaction of Mn with N2O
TABLE I. Bimolecular rate constants ͑/10Ϫ15͒ for Mn(a6S5/2)ϩN2O in N2
buffer gas.
TABLE II. Experimental Ea’s, calculated resonance interaction model Ea’s,
ionization potentials, sp and sd promotion energies, enthalpy changes for
MϩN2O→MOϩN2 and transition metal oxide bond energies, all in kJ/mol.
Total
pressure
͑Torr͒
Total
flow
͑sccm͒
MMT
pressure
͑mTorr͒
Bimolecular
rate constant
PE
PE
Temperature
Ea
͑expt͒
Ea
͑calc͒
a
b
͑cm3 sϪ1, /10Ϫ15
͒
TM
IPc (sp)d (sd)e
⌬Hf
Oxide BEg
͑K͒
Ti͑3F͒
Cr͑7S͒
Mn͑6S͒
Fe͑5D͒
Cu͑2S͒
Mo͑7S͒
W͑5D͒
12.5h
21.7i
42.3
8.0
20.8
18.4
23.9
39.6l
28.1
22.6
659
653
717
762
745
685
770
190
279
220
231
365
306
232
78
0
204
83
0
Ϫ501
Ϫ291
Ϫ190
Ϫ246
Ϫ118
Ϫ430
Ϫ508
663͑3⌬͒
425͑5⌸͒
357͑6⌺ϩ͒
405͑5⌬͒
269͑2⌸͒
482͑5⌸͒
656͑5⌸͒
448
448
473
498
498
523
548
573
573
598
620
620
250
250
200
125
200
150
100
50
100
100
100
100
650
650
600
1500
1400
450
450
800
800
750
850
400
3.8
4.8
4.0
1.3
2.2
1.0
1.0
0.5
1.0
1.4
1.2
1.5
1.6
1.6
2.9
4.6
4.9
8.1
13.
20.
20.
29.
41.
40.
42.2j
39.6k
39m
0
35
23.8n
aExperimental Ea’s were determined from a fit of the rate constants to the
equation kϭAT1/2 exp(ϪEa/RT) for comparison to the activation barrier
calculated from the resonance interaction model.
bReference 3.
cReference 11.
dEnergy difference ͑determined from Ref. 11͒ of the sxdnϪx ground state
and the lowest energy sxϪ1dnϪxp1 state.
eEnergy difference ͑Ref. 11͒ of the ground state and the lowest energy
s1dnϪ1 state.
temperature and total pressure. First-order exponential de-
cays of Mn in the presence of N2O were observed from
which the first-order rate constant, 1/␶, is determined using
least-squares linear regression. The observed second-order
rate constant is determined from a plot of 1/␶ vs N2O pres-
sure ͑Fig. 1͒. Measured rate constants for the a6S5/2 state at
various temperatures and pressures are listed in Table I. At-
tempts were made to measure the rate constant at lower tem-
peratures than 448 K; however, at lower temperatures, the
rate constant was smaller than the lower limit of our appara-
tus under these conditions. Rate constants were found to be
independent of total pressure, total flow rate, and precursor
partial pressure to within experimental uncertainty. The pres-
sure independence indicates bimolecular kinetics. The rela-
tive uncertainty ͑i.e., the reproducibility͒ of our the rate con-
stants is estimated at Ϯ10%. The absolute uncertainties are
estimated to be Ϯ20% and are based on the sum of the sta-
tistical scatter in the data and uncertainty in the flowmeter
readings ͑5%͒ and the pressure reading ͑1%͒.
fAt 298 K, calculated from Ref. 9 except for the Mn reaction ͑Refs. 9 and
10͒.
gReference 10.
hReference 12.
iReference 13.
jReference 14.
kReference 15.
lThe resonance interaction model uses parameters based on this reaction;
therefore, the calculated Ea matches the experimental value exactly.
mReference 16.
nReference 17.
tainties represent Ϯ2␴.
A
fit to the equation
kϭAT1/2 exp(ϪEa/RT) yields Eaϭ42.3Ϯ1.0 kJ/mol and
Aϭ͑5.9Ϯ1.1͒ϫ10Ϫ12 cm3 KϪ1/2 sϪ1
.
DISCUSSION
The production of MnO from Mn(a6S5/2) reacting with
N2O has a significant barrier ͑Eaϭ44.7 kJ/mol͒ despite the
reaction’s exothermicity of 190 kJ/mol at 298 K.9,10 This
activation energy is over twice the value predicted by the
resonance interaction model ͓Eaϭ18.4 kJ/mol for an equa-
tion of the form kϭAT1/2 exp(ϪEa/RT)͔.3 Clearly, the reso-
nance interaction model does not accurately predict the bar-
rier for this reaction. It is not surprising that this model does
not accurately predict the kinetic behavior of transition met-
als due to the neglect of both the TM’s d electrons and the
product metal oxide’s electronic structure. Previous studies
of TM’s indicate the TM’s electron configuration plays a
significant role in the dynamics of the reactions.1 Table II
presents a compilation of the presently known experimental
values of activation energies for TM’s along with the calcu-
lated values from the resonance interaction model. Obvi-
ously, this theory does not predict Ea’s accurately for tran-
sition metal atoms. Furthermore, it does not appear there is a
correlation between the activation energy and other param-
eters such as ionization potential, sp or sd promotion en-
ergy, the metal oxide bond energy or the exothermicity of the
reaction. A systematic study of the reactions of the remainder
An Arrhenius plot ͑Fig. 2͒ of the rate constants is linear
and yields Arrhenius parameters of 44.7Ϯ1.0 kJ/mol and
͑2.05Ϯ0.45͒ϫ10Ϫ10 cm3 sϪ1 for the activation energy and
frequency factor, A, respectively. The experimental uncer-
FIG. 2. Arrhenius plot for the reaction of Mn(a6S5/2) with N2O. The solid
line is a weighted exponential fit to the equation k(T)ϭA exp(ϪEa/RT).
Error bars represent Ϯ10% uncertainty.
J. Chem. Phys., Vol. 104, No. 19, 15 May 1996
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Mark L. Campbell: The reaction of Mn with N2O
7517
of the transition metals with N2O is currently underway to
gain a better understanding of the dynamics of these reac-
tions.
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