J . Phys. Chem. 1992, 96,9828-9831
9828
+
Kinetics of the Reactions V(a4F3,2,a6D3,2) OX (X = 0, N, and CO) Roy E. McCIeant and Louise Pasternack* Chemistry Division/Code 61 10, Naval Research Laboratory, Washington, D.C. 20375 (Received: June 3, 1992; In Final Form: August 5, 1992) The gas-phase kinetics are reported for the reactions V(a4F3/,)+ OX, where X = 0, N, and CO, in the temperature range 297-643 K. V atoms were produced by the multiphoton dissociation of VC14(g)at 193 nm and were detected by laser-induced fluorescence. Arrhenius rate expressions obtained for these reactions are k(Oz)= (1.20 f 0.26) X exp[-(2.16 f 0.19 kcal/mol)/RT] cm3 s-I, k(N0) = (2.20 f 0.26) X 10-l' exp[-(0.40 f 0.19 kcal/mol)/RT] cm3 s-I, and k(COz) = (4.38 f 0.62) X lo-" exp[-(2.90 f 0.14 kcal/mol)/RT]cm's-'. Also, room temperature rate constants were measured for the disappearance of the excited state V(a6D3/2)due to collisions with OX, Ar, and N2;the V* disappearance rates induced by collisions with the oxidants are found to be considerably faster than those of the ground-state atom, with rate constants k(N0) = (1.00 f 0.16) X (in units of cm3s-I) k(Oz)= (1.34 f 0.21) X k(C02) = (2.17 f 0.35) X 10-l'. k(Ar) = (2.06 f 0.32) X and k(Nz)= (5.62 f 0.83) X Quoted uncertainities are f2a. The rates of collisional quenching are compared to the reaction rates of ground-state vanadium, and quenching mechanisms are discussed.
Introduction Several transition-metal atoms and their corresponding metal oxides are present in the earth's atmosphere,' and thus their reaction kinetics have been of interest to chemists for many years. Transition-metal oxides have also been used for the spectral classification of M-type starsz and have been o k e d in s~nspots.~ The oxidation of transition-metal atoms has been observed to produce metal oxides that exhibit chemilumine~cence."~ Oxidation reactions of 3d transition-metal atoms have been studied by several investigators. Ritter and Wei~shaar*.~ have studied the oxidation kinetics of ground-state Sc, Ti, and V atoms with Oz,NO, and NzO at room temperature. They propose an 0 atom abstraction mechanism involving electron transfer from a neutral M + OX reactant surface to an ion pair M'OX product surface. Although the ordering of the rate constants correlates with the ionization potentials of the metal atoms, the ordering does not correlate with the electron affinities of the oxidants, as would be expected for an electron-trasfer mechanism. They have suggested temperature dependence studies to aid in the elucidation of the reaction mechanism. Reactions of the other 3d transition-metal atoms with oxygen, for which 0 atom abstraction is endothermic, have been investigated.'OJ' Here, reactions of the metal atoms with ground states having s2dp2orbital occupancy (where n is the total number of valence electrons) with oxygen are found to be slow compared to reactions of transition-metal atoms having ground states or low-lying electronic states with s'd"' orbital occupancy. It has been suggested that Cr may react via either an insertion reaction or an association reaction giving a dioxygen complex,1° whereas the later transition-metal atoms give dioxygen complexes.'' Products of vanadium oxidation have been observed by chemi l u 1 n i n ~ n c e . ~From 3 ~ reaction of V + 02,Parson et d.7observed VO(B411) but concluded it came from reactions with metastable V rather than ground-state V atoms. Jones and Gole5observed VO(C4Z-) from reaction of V NOz but were unable to observe it from reaction of V + 02. Here, we report a kinetic study of the oxidation of ground-state vanadium atoms in the temperature range 297-641 K. All reactions were studied by monitoring the disappearanceof vanadium atoms. The reactions studied, with their thermochemistry,I2are as follows:
+
+
V(a4F3/z)+ O2
-
+ V(a4F3,2)+ COz V(a4F312) NO
products
(1)
products
(2)
products
(3)
'NRC/NRL Postdoctoral Research Associate.
Reactions 1 and 2 have previously been studied only at room temperature?^^ and no studies of reaction 3 have been reported. By obtaining Arrhenius parameters for reactions 1-3, we distinguish steric factors and energy barrier effects. In addition, we compare Arrhenius parameters as a function of oxidant in order to test the electron-transfer mechanism. From a measurement of the pressure dependence of these reactions, we also test the association reaction mechanism described above. A room temperature study on the collisional quenching of electronically excited V(a6D3/2)with 02,NO, COz, Ar, and Nz is also reported. This is a low-lying electronic state (2153 cm-' above the ground state) which has a singly occupied s-orbital. The rates of collisional quenching are compared to the reaction rates of groundstate vanadium, and quenching mechanisms are discussed. No previous kinetic studies on quenching of excited states of vanadium have been reported.
Experimental Section Apparatus and Procedure. Kinetic experiments were carried out using a laser photolysis/laser-induced fluorescencetechnique, as described previou~ly.'~The reaction cell was a 5-cm4.d. stainless steel cross with attached glass sidearms and a sapphire window for optical viewing. The cell was contained in a commercial convection oven with ports for the side arms, sapphire window, and manometer line. The cell temperature was measured with a thermocouple, and total pressure was measured with MKS baratron manometers. The =0.09% VCl,/buffer gas mixture, buffer gas (Ar or He), and reactant gases flowed through calibrated mass flow meters and flow controllers (Tylan, FM-360 and FC-260, respectively) prior to mixing and admission to the reaction cell. Each side arm had an attached Brewster window which was purged with a slow flow of buffer gas to prevent deposition of vanadium and other photoproducts. Total flow rates were between 250 and 450 sccm. These flow rates were selected to obtain fresh reactants between laser pulses and minimize secondary product buildup while also allowing time for temperature equilibration. Vanadium atoms were produced by the 193-nm photodissociation of VC14(g) with the focused output of an excimer laser (Lumonics Model Hyperex 420) operating with ArF at 10 Hz. The single-photon absorption cross section14of VC14(g) at 193 cmz. The photolysis laser energy, after passing nm is =6 X through various optical components, was approximately 75 mJ/pulse at the center of the cell. The V atoms were detected via laser-induced fluorescence (LIF) using an excimer-pumped dye laser (Lambda-Physics EMG201 /FL2002) counterpropagating through the cell. Ground-state V atoms were excited at 385.54 nm (fF,/z a4F32)ls using Exciton exalite 389 dye, and detection was at 571 nm (y4PJjz a4D), using a narrow-band interference filter (Baird 11-01-7, 570 f 5 nm) and a longwavelength pass filter (Corning 3-71) to isolate the LIF and to
-
-
This article not subject to US.Copyright. Published 1992 by the American Chemical Society
Kinetics of the V
+ OX Reaction
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9829
0.06 . I
r
'v1
0.04
.
.
3 L
c
0.02.
0.00 . 1
0
3
2
4
CO2 Pressure (Torr)
0
40
20
60
80
100
120
140
Time (ps)
Figure 2. Linear dependence of 1/T(V(a'F3/z)) as a function of COz pressure. T = 349 K and total pressure = 21 Torr. The symbols are data points. The slope of the linear regression line gives a bimolecular rate constant (217uncertainty level) of (6.21 f 0.47)X lo-') cm3s-'. The point at 1.8 Torr of C 0 2 is that obtained from Figure 1.
Figure 1. Typical V(a4F3/z)decay profile. T = 349 K,total pressure = 21 Torr, reactant = 1.8 Torr of COz, pressure of VC4 = 0.2mTorr. The points are experimental data, and the solid line is an exponential fit which gives 1/r = 0.0308 &.
minimize noise due to scattered excimer laser light and prompt emission from excited photofragments. V(a6D3/2)was excited at 410.52 nm (y6D0512 a6D3/2)'5using diphenylstilbene dye, and LIF was isolated with a narrow-band interference filter (Corion P10-420) at 415 f 5 nm. LIF was monitored perpendicular to the laser beams by a photomultiplier tube (Hamamatsu R375). The signal was captured by a gated boxcar sampling module (Stanford Resdrch Systems SR250), and the digitized output was analyzed and stored by a computer. The boxcar averager was triggered by dye laser scattered light incident upon a fast photodiode. Both the photolysis and dye laser beam intensities remained approximately constant during the course of an experiment. The delay time between the photolysis laser pulse and the dye laser probe pulse was varied by a digital delay generator (Stanford Research Systems DG535) controlled by the computer. LIF decay traces consist of 500 points, averaged over 1-5 laser shots. The base line signal was collected prior to the photolysis pulse. The minimum delay time was 1 ps in order to avoid collecting the prompt emission and to allow relaxation of excited-state V atoms to the states under observation. Material& VCl,, obtained from Johnson Matthey Electronics, Inc. (90%), was diluted in either argon or helium in a steel tank. The VC14 sample contained VOC13 as a major impurity, as determined by FIIR analysis. Although the relative concentration of VOC13was reduced by distillation, we were unable to eliminate it entirely. Argon (Air Products Industrial Grade, 99.997%), helium (AirProducts Industrial Grade, 99.995%), O2(Matheson, 99.6%), NO (Matheson, 99.0%), and C 0 2 (AirProducts Industrial Grade, 99.8%) were used as received.
TABLE I:
Vanadium Atom Production and Kinetic Measurements. We observed a higher than first-order dependence of the LIF intensity on pump energy. At least three photons at 193 nm are required to produce V + 4C1 from VCl4.I6 The VOC13 impurity can be dissociated to produce VO 3C1 with only two photons.I6 Although the vacuum-UV absorption cross section of VOCl,, has not been reported, at 220 nm the absorption cross section'' is -6 X IO-'* cm2. The growth of VO from chemical reaction (V + OX) above the background VO signal from photodissociation of the VOC13impurity could not be observed under our experimental conditions. The results presented in this paper are based on the measurement of the disappearance of the vanadium atom reactants. V(a4FJj2)m y . The rates of disappearance of ground-state V(a4F3/2)with 02, NO, and C 0 2were measured at temperatures
+
+
Constants for the Reactions V(II'F~,~)
k(
T (K)
-
RWllb
MeLpured Rate
0x0
297 349 390 445 483 531 577 641
0 2
3.26 f 0.49 5.10 f 0.78 7.04 f 1.05 9.70 1.46 13.2 f 2.0 16.8 k 2.5 17.7 f 2.8 21.6 3.2
cm3 PI)
co2
NO 10.9 f 1.7 11.4k 1.7 14.7 f 2.3 14.0k 2.1
0.329 f 0.049 0.621 f 0.093 1.17 f 0.18 1.55 f 0.24
17.1 2.7 13.6 f 2.2 15.6 k 2.4
2.99 0.47 3.27 & 0.53 4.50 f 0.70
Uncertainties are f2u. See text for discussion.
from 297 to 641 K. Reactions were measured under pseudofirst-order conditions. A typical decay plot of ground-state V atoms in presented in Figure 1. This plot is for the reaction V C 0 2 at 349 K in =19 Torr of argon. The solid line through the data is an exponential fit which covers three lifetimes, T . Although it is likely that V atoms were produced directly (laser pulse width -10 ns), some growth of the LIF signal is observed for =1-5 ps, as shown in the figure. We attribute this growth to collisional relaxation of excited-state vanadium atoms to the ground state. In all experiments, first-order disappearance behavior was observed over a time spanning 3 4 reaction lifetimes. Most of the decay data were collected at a total pressure of 20 Torr using argon buffer gas. Typical VC4 partial pressures were on the order of 0.2 mTorr. Lifetime measurements were independent of the dye laser intensity and the photolysis laser energy. Second-order rate constants are obtained from plots of 1 / vs~ reactant pressure such as that shown in Figure 2. The solid line is a linear least-squares fit to the data. The intercept represents the sum of reaction with precursor, impurities, and other photolysis products and diffusion of V atoms out of the reaction zone. The second-order rate constant is obtained from the slope. In general, such plots extend over a 10-fold reactant pressure range and a 10-fold lifetime range with added reactant. The argon pressure was varied from 20 to 400 and 20 to 200 Torr for the V O2 and V C 0 2 reactions, respectively. The second-order rate constants are found to be independent of pressure over these ranges. The V + NO reaction was studied only at 20 Torr total pressure. The observed rate constants at the various temperatures investigated are listed in Table I. The quoted uncertainities of 2u represent the sum of statistical scatter in the data, uncertainty in the flowmeter readings (5%), the time base of the digital delay generator (2%),and the pressure reading (5%). Figure 3 is an Arrhenius plot of the data in Table I. The solid lines through the data are weighted least-squares linear fits to the Arrhenius expression. Empirical fits obtained are
+
+
+
9830 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
k(N0) = (2.20 f 0.26)
X
lo-" exp(-(0.40 f 0.19 kcal/mol)/RT)
k(O2) = (1.20 f 0.26)
X
exp(-(2.16 f 0.19 kcal/mol)/RT)
k(CO2) = (4.38 f 0.62)
X lo-"
exp(-(2.90 f 0.14 kcal/mol)/RT)
where k( T ) is in units of cm3 and the uncertainities represent f 2 u . We observed no difference to within experimental uncertainty in the rates of reaction of different spin states of V(a4F) with 02.Our room temperature results for the reactions V O2 and V NO agree to within combined error estimates with those reported by Ritter and Weisshaar8 using a hollow cathode sputtering source/LIF technique. V(a6D3,,) Decry. We also measured the rates of disappearance of the lowest lying excited state of vanadium, V(a6D3/2),due to collisions with Ar, N2,0 2 , NO, and C02. For Ar and N2, only physical quenching is energetically allowed. For 02,NO, and COz, both physical quenching to ground-state V atoms and chemical reaction to produce VO are exothermic. In these decay experiments, VC14 was diluted in helium to minimize collisional quenching of V(a6D3/2).A change in the helium pressure (in the absence of reactant gas) from 6.5 to 21.3 Torr did not result in a noticeable change in the decay rate, indicating that quenching due to helium was negligible. Argon is a more efficient quencher of V(a6D3/2)than helium as demonstrated in Figure 4. Also shown is the quenching due to N2. The intercepts are due to quenching by helium, VC14, and photofragments, as well as diffusion of V* out of the reaction zone. The total pressure for these experiments was -1 1 Torr. The disappearance rates of V(a6D3/& are summarized in Table 11. Cross sections, u, determined by u = &/( u ) , where ( u ) is the average relative velocity of the collision partners, are also presented. The rate constants are very large for collisions with the OX gases, on the order of the gas collision rate constant for V* O2and V* + NO, and slightly less for V* C02. The rates for V(a6D3/J quenching by Ar and N2are several orders of magnitude slower, on the order of cm3s-l. The quenching of V(a6DgI2)by N2 was also measured; this bimolecular rate constant is identical, within experimental error, to that for V(a6D3/4+ N2suggesting that, as for V(a4F)reactions, either interconversion among the spin states is fast under these pressure conditions or the rates of the J-states arc indistinguishable under these experimental conditions.
+
+
+
McClean and Pasternack
+
Discus!3ion V(a4F,,,). Since vanadium has a low-lying excited state with sld"-l orbital occupancy,15and other 3d transition metals with this orbital occupancy have been observed to proceed via an assoCiation mechanism, temperature and pressure dependence studies of the V + O2reaction were initiated to determine whether this reaction might proceed via an association mechanism and/or a V02 intermediate at higher pressures. The positive activation energy and the lack of a pressure dependence of the rate constant observed in these experiments suggest that this reaction does not proceed through a long-lived V02 intermediate and that a direct 0 atom transfer is operative, confirming the results of Ritter and Weisshaar,8 who studied this reaction in 0.4-0.8Torr of helium. The rate of the V + C 0 2 reaction was also pressure independent, suggesting a bimolecular 0 atom transfer mechanism. Although, in this experiment, the V + NO reaction was not studied as a function of pressure, the good agreement with the rate constant obtained by Ritter and Weisshaar,8 who studied this reaction in only 0.4-0.8 Torr of helium, confirms their 0 atom transfer mechanism. Table 111 summarizes the reaction exothermicities, AH,and the Arrhenius parameters from our experimental results. There is no correlation between AH and E,, indicating that reaction enthalpy is not the driving force in the oxidation of vanadium atoms. In fact, the reaction with the smallest exothermicity has
1.5
2.5
2.0 1/T
3.0
3.5
K")
Figure 3. Arrhenius plots for the reactions V(a4F3/2)+ OX. Symbols arc experimentalpoints, and the solid lies are weighted least-square fits. Uncertainities are *2u. Scc text for Arrhenius parameters. 0.10 0.087
0.06 -
>
0.04 -
3 h
0.02 0.00 2
0
4
6
8
10
Reactant Pressure (Torr)
FIgm 4. Linear dependence of l / ~ ( v ( a ~ D ~as/ ~a function )) of reactant pressure. T = 297 K and total prcssure = 11 Torr. The symbols arc data points. The solid lines are linear regression fits. Scc text for results of the fits.
the smallest activation energy. The preexponential factors are all less than the gas collision rate constant, indicating the importance of steric factors. Although an electron-transfer mechanism may be involved to some extent in the oxidation kinetics of ground-state vanadium atoms, as suggested by Ritter and Weisshaar,8our results suggest that other factors play a role in the reaction mechanism. The ordering of the room temperature rate constants does not correlate with the electron affmities of the oxidants (shown in Table 11). Additionally, the activation energies and the Arrhenius A factors also do not correlate with the electron affinities, as can be seen in Table 111, as would be expected for an electron-transfer mechanism. Ab initio calculations show that the ground and lower lying states of VO have a mixture of ionic, V+(3dd3d13~4s~)O(2pu'2pr4), and covalent, V( 3da'3dr13d624s1)0(2pu12pr3), character, with the ground state having VO 42- (4s13d6'3d13') orbital occupancy.21 It might be expected that the presence of a low-lying electronic state of V with 3d44s1character might aid in the reaction with oxidants since it would correlate to the d e n t character of ground-state VO. However, Ritter and Weisshaar* observed slower room temperature rates for V 02,NO than NO, although in Sc the lowest sld"-l electronic state for Sc + 02, which is at 11 500 cm-l is much higher in energy than in V.Is Activation energies for the Sc atom (and Ti) reactions are needed to pursue this argument further. An examination of possible electronic-state correlation diagrams for V + O2and V NO offers few symmetry restrictions due to the high spin and orbital angular momentum of V(a4F) and
+
+
Kinetics of the V
+ OX Reaction
collision Dartncr
electron affinitv . (eV\ . , 0.45lC 0.026c -0.6d -1 .6e
0 2
NO ~~
co2 N2 Ar
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9831
k (cm’ P‘) (1.34 f (1.00 f (2.17 f (5.62 f (2.06 f
.
0.21) X 10-lo 0.161 X 10-”
0.35j x
(2.4 f (1.7 f (4.2 (9.5 f (3.9 f
10-11
0.83) X lo-” 0.32) X
.
0.4) X 0.3) X 0.7j x 1.4) X 0.6) X
10-16
1.8 x 10-15 1.6 X 1.3 X 9.9 x 10-16
‘Uncertainties are f 2 u . See text for discussion. bCalculated from an electron-transfer model, as discussed in the text. CReference18. dReference 19. eReference 20.
TABLE Uk Summrry of Arrhenilrm Pulrmeters‘ for Rmctioas of V(r‘F3,d + ox OX -&(kcal/mol) A (cm3 s-I) E, (kcal/mol) 0 2
NO
co2
33 1.3 25
(1.20 f 0.26) X 10-lo (2.20 h 0.26) X 10-” (4.38 h 0.62) X lo-”
2.16 & 0.19 0.40 f 0.19 2.90 f 0.14
‘Uncertainties are f2u. See text for discussion. ‘Enthalpies obtained from ref 12.
the lack of information on the geometry and electronic states of the transition states,’~~ making it difficult to rationalize the different rates and Arrhenius parameters for these reactions. For V C 0 2 ,there may be additional restrictions imposed since the ground state of C02(X’Zg+)does not correlate with ground-state CO(X’Z+) O(3P) fragments. This may lead to an avoided crossing to a surface which correlates to excited state products rather than to the ground state. This is analogous to arguments used to explain the lower reactivity of M + N 2 0 reactions, first proposed by Jonah et aLZ2A theoretical investigation of possible transition states and a measurement of the temperature dependence of other transition-metal oxidation reactions would be helpful. V(a6&D3/2). There are several possible mechanisms for quenching state lies 2153 vanadium a6D(3d44s1) a4F(3d34s2).The a6DD,/2 cm-’in energy above the ground state. E-V,R transfer is possible to molecules which have vibrational frequencies near this excitation energy, including O2(we = 1580 NO (we = 1904 C 0 2 (co? = 2394 ~ m - ’ ) and , ~ ~N2 (we = 2360 c ~ n - ’ ) .Since ~ ~ the quenching rates for Ar and N2 are similar (both on the order of lO-” an3s-’), heavy atom spin-orbit coupling may be as eEcient as E-V,R transfer. Although the electronic transition is forbidden from both spin and orbital considerations, collisional quenching may still be facile due to formation of a collision complex. One possible mechanism for efficient nonadiabatic transitions involves electron transfer to an ionic potential energy According to this model, the distance, R,, at which the electron transfers comsponds to the m e crossing of the ionic and covalent potential curves and can be determined by R, = $/(IP - EA). Using the ionization potential of v a n a d i ~ m ’and ~ the electron affinities listed in Table 11, we calculate an approximate cross section, u = TR:.~’ From the results, shown in Table 11, it can be seen that the trend in the magnitudes of the cross sections is predicted by this simple model. The absolute magnitude of the cross sections is also in good agreement with this model for O2 and NO quenching; however, for C 0 2and N2, which have negative electron affinities, the experimental results are in much poorer agreement. In this model, the cross section does not depend on whether the quenching occws to the ground state or via a chemical reaction. However, for reactive quenching by the oxidants, V(a6D) (3d44s’)correlates orbitally with the covalent part of the ground and low-lying excited states of VO,*’making a direct reaction on an attractive potential surface possible from spin and orbital considerations. Parson et al. observed VO(B411) chemiluminescence from a crassed-beam study of V atoms with 02.’From the dependence of the chemiluminescence signal on V atom source temperature, they suggested that VO(B) was formed from reaction of metastable V atoms (a6D and a‘D). For V(a6D) oxidation by NO and C02, the channel to VO(B411)is endoergic, although lower states of VO are energetically allowed.
+
+
-
s-ry
We have measured rates for the reactions of ground-state vanadium atoms with 02,NO, and C 0 2ovef the temperature range 297-641 K. Reactions with O2 and NO had previously been studied only at room temperature, and no studies of reaction with C 0 2 had been reported. By comparing Arrhenius parameters as a function of oxidant, we tested the electron-transfer mechanism and concluded that other factors are involved in the reaction mechanism. A room temperature study of the disappearance rates of electronically excited V(a6D3/J due to collisions with 02, NO, C02, Ar, and N2 is also reported. The rates of these reactions are compared to those of ground-state vanadium, and quenching the mechanisms are discussed. No previous kinetic studies on these reactions had been reported. Acknowledgmenr. We thank Dr. Charles Douglass for the FTIR analysis of the V C 4 samples. Funding for this study was provided by the Office of Naval Research. R a m NO. V, 7440-62-2; 0 2 , 7782-44-7; NO, 10102-43-9; COZ, 124-38-9; AT,7440-37-1; N2, 7727-37-9.
References and Notes (1) Brown, T. L. Chem. Reo. 1973,73,645. (2) Johnson, H. R.; Sauval, A. J. Asrron. Rrrrophys. Suppl. Ser. 1982.49, 77.
(3) Narasimhamurthy, B.;Rajamanickam, N. J. Asrrophys. Asrron. 1983,
4, 53.
(4) Dubois, L. H.; Gole, J. L. J. Chem. Phys. 1977,66, 779. (5) Jones, R. W.; Gole, J. L. J. Chem. Phys. 1976,65, 3800. (6) Chalek, C. L.; Gole, J. L. J . Chem. Phys. 1976,65, 2845. (7) Parson, J. M.; Geiger, L. C.; Conway, T. J. J . Chem. Phys. 1981.74, 5595. (8) Ritter, D.; Weisshaar, J. C. J. Phys. Chem. 1990,94,4907. (9) Ritter, D.; Weisshaar, J. C. J. Phys. Chem. 1989,93,1576. (10) Pamis, J. M.; Mitchell, S.A.; Hackett, P. A. J . Phys. Chem. 1990, 94,8152. (11) Brown, C. E.; Mitchell, S. A.; Hackett, P. A. J . Phys. Chem. 1991, 95,1062. (12) Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A,; Syverud, A. N. J. Phys. Chem. Ref Dara 1985,I 4 (Suppl. 1). (13) Campbell, M.; McClean, R. E.; Garland, N. L.; Nelson, H. H.Chem. Phys. Leu. 1992,194,187. (14) Iverson, A. A.; Russell, B. R. Specrrochim. Acra 1973,29A, 716. (15) Moore, C. E. Atomic Energy Levels as Derived from the Analysis of Optical Spectra, Vol. 111; Narl. Srand. Ref Dura Ser. (US., Narl. Bur. Srand). 1971,NSRDS-NBS 35. (16) Stein, S.E.; Rukkers, J. M.; Brown, R. L. NISTSrand. Ref. Darabase 1991,25. (17) Dijkgraaf, C. Specrrochim. Acra 1965,21,1419. (181 Travers. M. J.: Cowles. D. C.: Ellison. G. B. Chem. Phvs. Lerr. 1989. 164, 449. (19) Compton, R. N.; Reinhardt, P. W.; Cooper, C. D. J . Chem. Phys. 1975.63,3821. (20) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T. J. Phys. Chem. Ref. Data 1977,6 (Suppl. 1). (21) Bauschlicher, C. W, Jr.; Langhoff, S.R. J . Chem. Phys. 1986,85, 5936. (22) Jonah, C. D.; &re, R. N.; Ottinger, C. J. Chem. Phys. 1972,56,263. ( 2 3 ) Huber, K. P.; Herzberg, G . Molecular Specrra and Molecular Srrucrure IV. Consranrs of Diatomic Molecules; Van Nostrand Reinhold: New York. 1979. (24) Herzberg, 0. Molecular Spectra and Molecular Srrucrure III. Electronic Spectra and Electronic Srrucrure of Polyaromic Molecules; Van Nostrand Reinhold New York, 1966; p 598. (25) Breckenridge, W. H. In Reactions of Small Transient Species; Fontijn, A., Clyne, M. A. A., Eds.; Academic Press: New York, 1983; p 157. (26) Bauer, E.;Fisher, E. R.; Gilmore. F. R. J . Chem. Phys. 1969,51, 4173. (27) Levine, R. D.; Bernstcin. R. B. Molecular Reaction Dynamics and Chemical Reacriuiry;Oxford University Press: New York, 1987; pp 134-137.
