Kinetic study of gas-phase Y(a 2D3/2) and La(a 2D3/2) with O2, N2O, CO2 and NO

Article abstract of DOI:10.1016/S0009-2614(98)00905-1

The second-order rate constants of gas-phase Y(a ²D_3/2) and La(a ²D_3/2) with O₂, N₂O, CO₂ and NO as a function of temperature are reported. In all cases, the reactions are relatively fast. For Y(a ²D_3/2), the bimolecular rate constants (in cm³ s^-1) are described in Arrhenius form by k(O₂)=(2.2±0.1)×10^-10 exp(-3.5±0.6 kJ mol^-1/RT), k(N₂O)=(1.9±0.2)×10^-10 exp(-4.0±0.8 kJ mol^-1/RT), k(CO₂)=(1.0±0.1)×10^-10 exp(-2.3±0.6 kJ mol^-1/RT), where the uncertainties are ±2σ. The rate constants for Y reacting with NO are temperature insensitive with a value of 1.0×10^-10 cm³ s^-1. For La(a ²D_3/2), the bimolecular rate constants for all the reactants are near the gas-kinetic collision rate.

Full text of DOI:10.1016/S0009-2614(98)00905-1

18 September 1998
Ž
.
Chemical Physics Letters 294 1998 339–344
2
2
ž
/
ž
/
Kinetic study of gas-phase Y a D3r2 and La a D3r2 with O2,
N2O, CO2 and NO
)
Mark L. Campbell
Chemistry Department, United States NaÕal Academy, Annapolis, MD 21402 USA
Received 3 June 1998; in final form 13 July 1998
Abstract
The second-order rate constants of gas-phase Y a D3
2
2
Ž
.
Ž
.
and La a D3r2 with O2 , N2O, CO2 and NO as a function of
r2
2
3
Ž
.
Ž
temperature are reported. In all cases, the reactions are relatively fast. For Y a D3 , the bimolecular rate constants in cm
r2
1
y
10
y1
sy are described in Arrhenius form by k O2 s 2.2"0.1 =10
.
Ž
.
Ž
.
Ž . Ž . Ž
exp y3.5"0.6 kJ mol rRT , k N2O s 1.9"
10 1
y
10
y
1
y
y
.
Ž
.
Ž
.
Ž
.
Ž .
exp y2.3"0.6 kJ mol rRT , where the
10
0.2 =10
exp y4.0"0.8 kJ mol rRT , k CO2 s 1.0"0.1 =10
uncertainties are "2s. The rate constants for Y reacting with NO are temperature insensitive with a value of 1.0=10y
cm3 sy1. For La a D3 , the bimolecular rate constants for all the reactants are near the gas-kinetic collision rate. q 1998
2
Ž
.
r2
Published by Elsevier Science B.V. All rights reserved.
1. Introduction
reported. In addition, LIF detection of the TM oxide
product established that the Group-3 metals even
abstract oxygen from aldehydes, ketones and car-
The kinetics of gas-phase reactions involving
transition metal TM atoms and oxygen-containing
oxidants has recently received considerable attention
Ž
.
w x
w
x
boxylic acids 6 . Gole and co-workers 7–9 ob-
served YO and LaO chemiluminscence in beam-gas
studies involving gas-phase yttrium and lanthanum
oxidation in the presence of O2 , NO2 , N2O and O3.
Again, bimolecular abstraction processes were indi-
cated. The only previous rate constant measurements
involving either yttrium or lanthanum in the gas
phase under multiple collision conditions reported on
the reactions of yttrium with small hydrocarbons
w x
1 . TM chemistry is an intriguing field of study due
to the high multiplicities of the atomic ground states
and the large number of low-lying metastable states.
In this Letter we report a gas-phase kinetic study
of the ground a 2 D3r2 states of yttrium and lan-
thanum with O2 , N2O, CO2 and NO. Parson and
w
x
co-workers 2–5 have studied the Group-3 metals
reacting with O2 , NO, SO2 and ROH in crossed-beam
experiments and observed the chemiluminescence of
the metal oxide product. Bimolecular abstraction was
established as the reaction mechanism, although ab-
solute rate constants for these processes were not
w
x
10,11 . We are unaware of any rate constant mea-
surements involving these metals in the gas phase
with oxygen-containing oxidants. This study contin-
ues our efforts to further understand the dynamics of
gas-phase transition metal reactions. Although pri-
marily phenomenological, the increase in the data
base of these reactions should yield greater insight
)
E-mail: campbell@brass.nadn.navy.mil
0009-2614r98r$ - see front matter q 1998 Published by Elsevier Science B.V. All rights reserved.
Ž
.
PII: S0009-2614 98 00905-1

(
)
340
M.L. CampbellrChemical Physics Letters 294 1998 339–344
into the important factors affecting transition metal
chemistry. We hope these new kinetic results will
inspire further theoretical work in an effort to gain a
better understanding of these reactions.
precursors at room temperature, the precursors re-
quired heating to get enough molecules into the gas
phase. Thus, the lowest temperature at which kinetic
experiments could be performed corresponded to
348 K for yttrium and 373 K for lanthanum. Below
these temperatures the LIF signal was too weak to
measure reliable values of the rate constant.
2. Experimental
The heated transition metal precursor was en-
trained in a flow of nitrogen buffer gas. The precur-
sor carrier gas, buffer gas and reactant gases flowed
through calibrated mass flow meters and flow con-
trollers prior to admission to the reaction chamber.
Each sidearm window was purged with a slow flow
of nitrogen buffer gas to prevent deposition of the
transition metal and other photoproducts. Pressures
were measured with MKS Baratron manometers, and
chamber temperatures were measured with a thermo-
couple. The delay time between the photolysis pulse
and the dye-laser pulse was varied by a digital delay
Žw
x
Pseudo-first-order kinetic experiments TM <
w
x.
oxidant were carried out in an apparatus with
slowly flowing gas using a laser photolysisrlaser-in-
duced fluorescence LIF technique. The experimen-
tal apparatus and technique have been described in
detail elsewhere 12 . Briefly, the reaction chamber
is a stainless-steel reducing 4-way cross with at-
tached side arms and a sapphire window for optical
viewing. The reaction chamber is enclosed within a
Ž
.
w
x
Ž
.
convection oven Blue M, model 206F, Tmax s623 K
for temperature dependence experiments.
Ž
.
Yttrium and lanthanum atoms were produced by
generator Stanford Research Systems DG535 con-
trolled by a computer. The trigger source for these
experiments was scattered pump laser light incident
upon a fast photodiode. LIF decay traces consisted of
200 points, each point averaged for 4 laser shots.
The following reagents were used as received:
Ž
.
the 248 nm photodissociation of yttrium III hexaflu-
w Ž . x
Ž
oroacetylacetonate Y hfa
and tris 6,6,7,7,8,8,8-
heptafluoro-2,2-dimethyl-3,5-octanedionate lan-
thanum III La FOD , respectively, using the fo-
cused output of an excimer laser Lambda Physics
3
.
Ž
. w
Ž
. x
3
Ž
.
Ž
. Ž
.
Ž
. Ž
.
Lextra 200 . The photolysis laser output was focused
Y hfa Strem, 99.9% , La FOD Strem, 99% , O2
MG Industries, 99.8% , N2O MG Industries, elec-
tronic grade, 99.999% , CO2 MG Industries, anaer-
obic grade, 99.9% , NO MG Industries, 99.0% and
3
3
Ž
.
Ž
.
Ž
using a lens fs564 mm positioned approximately
one focal length from the detection zone. Yttrium
atoms were detected via LIF using an excimer-
.
Ž
.
Ž
.
Ž
Ž
.
pumped dye laser Lambda Physics Lextra
N2 Potomac Airgas, 99.998% .
2
50rScanMate 2E tuned to the y D5r28 § a 2 D3r2
.
transition at 403.983 nm and the fluorescence at
412.831 nm was isolated with an interference filter
3. Data analysis and results
2
w
x
13 . Some experiments utilized the y F5r28 §
a 2 D3r2 transition at 407.738 nm 13 . Experimental
results were found to be independent of the optical
transition employed. Lanthanum atoms were detected
The decay rates of the a 2 D3r2 states of yttrium
and lanthanum as a function of reactant pressure
were investigated at various temperatures and total
pressures. The loss of ground state atoms is de-
scribed by the first-order decay constant, k1:
w
x
utilizing the y D5r28 § a 2 D3r2 transition at
2
515.869 nm and the fluorescence at 545.515 nm was
w
x
isolated with an interference filter 13 . The fluores-
cence was detected at 908 to the counterpropagated
laser beams with a three-lens telescope imaged
w
x
k1 s1rtsko qk2 oxid ,
1
Ž .
where t is the first-order time constant for the
Ž
through an iris. A photomultiplier tube Hamamatsu
removal of the transition metal under the given
.
Ž
.
R375 was used in collecting the LIF which was
subsequently sent to a gated boxcar sampling module
Stanford Research Systems SR250 , and the digi-
tized output was stored and analyzed by a computer.
experimental conditions, ko s1rto is the loss
term due to diffusion out of the detection zone and
reaction with the precursor and precursor fragments,
and k2 is the second-order rate constant. Typical
decay profiles are shown in Fig. 1. A time constant,
Ž
.
Because of the low vapor pressure of the two

(
)
M.L. CampbellrChemical Physics Letters 294 1998 339–344
341
are based on the sum of the statistical scatter in the
data, uncertainty in the flowmeter and flow con-
Ž
.
troller readings 5% and the total pressure reading
Ž
.
1% , and uncertainties due to incomplete gas mixing
and incomplete relaxation of excited states produced
in the photolysis event.
2
(
)
3.1. Reactions of Y a D3r 2
Second-order rate constants for the a 2 D3r2 state
reacting with O2 , N2O, CO2 and NO at a total
pressure of 5 Torr from 373 to 548 K are listed in
Table 1. Rate constants could not be measured above
548 K due to thermal decomposition of the precursor.
The rate constants at 373 K for these reactions were
also measured at a total pressure of 20 Torr and are
independent of total pressure within experimental
uncertainty.
2
2
Ž
.
Ž
.
Fig. 1. Typical Y a D3r2 and La a D3r2 decay curves in the
Ž . Ž .
a La, P O2 s117
presence of O2 at 498K, Ptotal s5.0 Torr:
Ž .
Ž
.
mTorr, t s23.3 ms; and b Y, P O2 s124 mTorr, t s32.4 ms.
The solid lines through the data are exponential fits. The inset is a
ln plot of the data.
3
y1
Ž
.
The rate constants in cm s
Arrhenius form by:
are described in
t, for each decay profile was determined using a
linear least-squares procedure. The second-order rate
constant is determined from a plot of 1rt vs. reac-
tant number density. Typical plots for obtaining sec-
ond-order rate constants are presented in Fig. 2; the
slope yields the observed rate constant. The relative
k O s 2.2"0.1 =10y10
Ž
.
Ž
.
2
=exp y3.5"0.6 kJ moly1rRT ,
2
Ž .
Ž
.
k N O s 1.9"0.2 =10y10
Ž
.
Ž
.
2
=exp y4.0"0.8 kJ moly1rRT ,
3
Ž .
Ž
.
Ž
.
uncertainty i.e., the reproducibility of the second-
order rate constants is estimated at "20% based on
repeated measurements of rate constants under iden-
tical temperature and total pressure conditions. The
absolute uncertainties are estimated to be "30% and
k CO s 1.0"0.1 =10y10
Ž
.
Ž
.
2
=exp y2.3"0.6 kJ moly1rRT ,
4
Ž .
Ž
.
where the uncertainties are "2s. The reaction with
NO is temperature independent within experimental
error.
Table 1
y10
Second-order rate constantsa r10
cm3 sy1 for the a2 D3r2
.
Ž
state of yttrium with O2 , N2O, CO2 and NO at a total pressure of
5 Torr
T
O2
N2O
CO2
NO
Ž .
K
348
373
398
423
448
473
498
523
548
0.61
0.73
0.75
0.78
0.88
0.88
0.92
0.95
0.99
0.45
0.54
0.55
0.60
0.70
0.69
0.72
0.73
0.76
0.43
0.49
0.49
0.52
0.56
0.53
0.58
0.58
0.59
1.0
1.1
1.0
1.0
0.92
1.0
1.0
1.0
1.0
2
Ž
.
Fig. 2. Typical plots for determining k2nd for Y a D3r2 and
2
Ž
.
La a D3r2 . The solid line for each set of data is a linear
aUncertainties are "30%.
regression fit from which k2nd is obtained.

(
)
342
M.L. CampbellrChemical Physics Letters 294 1998 339–344
Table 2
portant. We did not attempt to directly observe the
YO product molecule; however, YO has been ob-
served previously for these reactions by other groups
y10
Second-order rate constantsa r10
cm3 sy1 for the a2 D3r2
.
Ž
state of lanthanum with O2 , N2O, CO2 and NO at a total pressure
of 5 Torr
w
x
2,3,5,7,9 . Thus, bimolecular abstraction of an oxy-
T
O2
N2O
CO2
NO
gen atom to produce YO is the indicated reaction
mechanism. The exothermicities for production of
YO on the ground state surface for the reactions of
yttrium with O2 , N2O, CO2 and NO are 211, 542,
Ž .
K
373
398
423
448
473
498
523
1.5
1.5
1.7
1.7
1.6
2.0
2.0
2.3
2.1
2.2
2.3
2.1
2.6
2.8
1.9
1.8
1.9
2.1
1.9
2.2
2.4
1.9
1.8
1.9
1.9
1.7
2.1
2.3
177, and 78 kJ moly1, respectively 14 ; thus, no
thermodynamic barriers exist for these reactions. The
w
x
experimentally measured kinetic barriers for all these
y1
Ž
.
reactants are small -5 kJ mol , and in every
case reaction occurs on average in fewer than 10
hard-sphere collisions. The 4d series appears to be
the most reactive of the transition metal series. This
reactivity has been ascribed to the similar size and
relative energies of the 5s and 4d subshells. These
similarities allow the d orbitals to be more involved
in the bonding yielding a more attractive potential in
aUncertainties are "30%.
2
(
)
3.2. Reactions of La a D3r 2
Second-order rate constants for the a 2 D3r2 state
reacting with O2 , N2O, CO2 and NO at a total
pressure of 5 Torr from 373 to 523 K are listed in
Table 2. The rate constants at 373 K for these reac-
tions were also measured at a total pressure of 20
Torr and are independent of total pressure within
experimental uncertainty.
w
x
the entrance channel 10,15 .
4.2. LaqOX
The rate constants for lanthanum with all the
reactants are near the gas-kinetic collision rate, indi-
cating barriers to reaction are negligible. The slight
increase in the rate constants for all the reactions can
be accounted for as arising from the increased colli-
sion frequency of the reactants at higher tempera-
tures.
As with the yttrium reactions, bimolecular ab-
straction of an oxygen atom to produce LaO is
indicated for these oxidants based on the pressure
independence of the rate constants and the detection
w
x
of the LaO product by other groups 2,5,8 . The
production of LaO is exothermic for all the oxygen-
containing oxidants reported here. The exothermici-
ties for the reactions of ground state lanthanum with
O2 , N2O, CO2 and NO are 303, 634, 269 and 170 kJ
4. Discussion
moly1, respectively 14 ; thus, no thermodynamic
barriers exist for these reactions. The reactions of
lanthanum with all the reactants studied here are near
the gas-kinetic collision rate. Thus, lanthanum is one
of the most reactive of the TMs in reactions with
oxygen-containing molecules, and its reactions with
oxygen-containing oxidants are considerably faster
than the reactions of the other TMs in the 5d series
w
x
Unfortunately, little is known about the potential
energy surfaces for the interaction of transition metal
atoms with even the simplest of molecules. A com-
plete understanding of these reactions awaits ad-
vances in theoretical dynamics calculations for these
systems. Nevertheless, it is still useful to assess the
experimental data reported thus far to look for gen-
eral trends and correlations between the reaction
rates and the physical properties of the transition
metal.
w x
1 .
The only TM more reactive with these oxidants
w
x
than lanthanum is niobium 16 . Niobium’s reactivity
has been attributed to its lone valence s electron in
the 5s subshell. Lanthanum is the most reactive of all
the TMs with s2 dny2 configurations.
4.1. YqOX
The pressure independence observed for these
reactions indicates termolecular processes are unim-
The reactivity of lanthanum might be due to its
small ionization energy, i.e., lanthanum has the low-

(
)
M.L. CampbellrChemical Physics Letters 294 1998 339–344
343
w
x
est ionization energy of all the TMs 17 . An electron
transfer model has been proposed to explain rate
constants of TMs in O-atom abstraction reactions
configurations of niobium and molybdenum. Both
atoms have s1 dny1 configurations which make these
atoms exceptionally reactive. The only exception to
this trend in the 5d series is rhenium, the least
reactive of all the TMs which involve exothermic
abstraction processes. Hopefully, advances will be
made in dynamics calculations so that the variations
in the rate constants for these reactions can be better
understood.
w
x
18 . The small ionization energy of lanthanum might
facilitate electron transfer from the metal atom to the
oxidant which consequently results in fast reactions.
Lanthanum is unique among the 5d series TM
atoms because of its lack of a filled 4f subshell.
Lanthanum is not affected by the lanthanide contrac-
tion so its size is considerably larger and its ioniza-
tion energy smaller than the other 5d TM atoms.
Thus, it is not surprising that the unique nature of the
electronic structure of lanthanum relative to the other
5d TM metals results in its anomalous reactivity.
With the completion of these kinetic measure-
ments for yttrium and lanthanum, all the exothermic
abstraction reactions involving TMs with O2 have
now been reported. Table 3 tabulates the room-tem-
perature rate constants for these reactions along with
the ionization energy and reaction enthalpies. There
appears to be no relation to the kinetics of these
processes and the enthalpy of reaction. However,
within a TM series there appears to be a modest
correlation between the ionization energy and the
rate constant. The three atoms of the 3d series show
an inverse relationship between ionization energy
and the rate constant. The deviations from this trend
in the 4d series are complicated by the electronic
Fontijn and co-workers have advanced a reso-
w
x
nance interaction model 29 to predict barriers to
reaction and rate constants for metal atoms reacting
with N2O. In this model, the activation barriers are
calculated by taking into account the ionization en-
ergy and sp promotion energy of the metal, the
electron affinity of N2O, and the bond energy of the
metal oxide product. The resonance interaction model
predicts the energy barriers for the reactions of yt-
trium and lanthanum with N2O to be 2.2 and 0.0 kJ
moly1 29 , respectively. These values are very close
to the measured values, although the predictive abil-
ity of this model has not been as accurate for other
w
x
w x
transition metals 1 .
5. Summary
We have measured bimolecular reaction rate con-
stants and Arrhenius parameters for the reactions of
Table 3
Ž .
Ž .
Ž .
Ž .
Ionization energies, enthalpies of reaction and room-temperature rate constants for the reaction: TM g qO2 g ™ TMO g qO g
Metal
IEa
D H o b
k2nd
Ref.
y1
y11
cm3 sy1
.
Ž
.
Ž
.
Ž
eV
kJ mol
r10
2
1 2
Ž
Ž
Ž
Ž
Ž
Ž
.
w
w
w
x
Sc s d
D
6.561
6.828
6.746
6.217
6.634
6.759
7.092
5.577
6.825
7.89
y183
y170
y139
y211
y297
y285
y99
y303
y320
y340
y177
y125
y34
0.61
0.16
0.31
5.4c
2.0
18,19
3r2
2
Ti s d2 3F2
.
x
20–22
2
3 4
.
x
18,23
V s d
F
D
3r2
2
1 2
.
Y s d
this work
3r2
2
Zr s d2 3F2
1
w
.
w x
1
4 6
.
x
Nb s d
D
25
16
x
24,25
1r2
Mo s d5 7S3
11
1
Ž
.
w
2
1 2
La s d
D
13.c
3.7
this work
w x
Ž
Ž
Ž
.
3r2
.
2
Hf s d2 3F2
1
2
3 4
.
w
26
w
12
w
27
w
28
x
x
x
x
Ta s d
F
0.73
3r2
2
W s d4 5D0
7.98
7.88
8.7
0.16
0.018
0.071c
Ž
.
2
5 6
Ž
.
Re s d
S
5r2
.
Os s d6 5D4
2
Ž
a
w
x
Ref. 17 .
b
w
x
Ref. 14 .
c Value obtained by extrapolation to 298 K.

(
)
344
M.L. CampbellrChemical Physics Letters 294 1998 339–344
the a 2 D3r2 state of yttrium and lanthanum with O2 ,
N2O, CO2 and NO. In each case, abstraction of an
oxygen atom to produce the transition metal oxide is
Ž
.
P.E.M. Siegbahn, M. Svensson, J. Phys. Chem. 99 1995
13955.
w
w
x
12 M.L. Campbell, R.E. McClean, J. Chem. Soc., Faraday
Ž
.
Trans. 91 1995 3787.
the reaction channel. The activation barriers for all
x
13 W.F. Meggers, C.H. Corliss, B.F. Scribner, Tables of Spec-
tral-Line Intensities, Part 1 Arranged by Elements, Natl. Bur.
Stand. Monogr. 145, U.S. Gov. Print. Off., Washington, DC,
1975.
y1
Ž
.
.
these reactions are small Ea -5 kJ mol
w
x
14 D.D. Wagman, W.H. Evans, V.B. Parker, R.H. Schumm, I.
Acknowledgements
Halow, S.M. Bailey, K.L. Churney, R.L. Nuttall, The NBS
tables of chemical thermodynamic properties, J. Phys. Chem.
Ž
.
Ref. Data 11 1982 Suppl. 2.
This research was supported by a Cottrell College
Science Award of Research. Acknowledgment is
made to the Donors of the Petroleum Research Fund,
administered by the American Chemical Society, for
partial support of this research. The author is a
Henry Dreyfus Teacher–Scholar.
w
w
w
x
Ž
.
15 J.J. Carroll, J.C. Weisshaar, J. Phys. Chem. 100 1996
12355.
x
16 R.E. McClean, M.L. Campbell, E.J. Kolsch, J. Phys. Chem.
A 101 1997 3348.
17 D.R. Lide Ed. , CRC Handbook of Chemistry and Physics,
75th ed., CRC Press, Boca Raton, FL, 1995.
18 D. Ritter, J.C. Weisshaar, J. Phys. Chem. 94 1990 4907.
19 M.L. Campbell, K.L. Hooper, E.J. Kolsch, Chem. Phys. Lett.
¨
Ž
.
x
Ž
.
w
w
x
x
Ž
.
¨
Ž
.
274 1997 7.
w
w
x
Ž
.
20 D. Ritter, J.C. Weisshaar, J. Phys. Chem. 93 1989 1576.
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.
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.
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