J. Phys. Chem. 1984, 88, 2455-2459 make the reaction much less efficient on the semiconductor than on glass. This is resolved if we assume that the photochemistry takes place in unrelaxed excited states and that injection only occurs subsequently, from a relaxed excited state. Interestingly, the dye fluorescence intensity has been found to decrease when an appropriate bias potential is applied to the semiconductor electrode.lsJ9 This has been attributed to an enhancement of the escape of the injected electrons into the bulk of the semiconductor and, therefore, to a decrease in the population of the unoxidized (fluorescent) dye on the surface. The results of Iwasaki et a1.18 are puzzling, however, in view of our observations about the fluorescence of deposited dye molecules. These authors reported 7 = 6.9 ns for sodium fluorescein at the electrode-solution interface, a lifetime which is similar to the solution value.20 This means that the fluorescence monitored in these (18) T. Iwasaki, T. Sawada, H. Kumudu, A. Fujishima, and K. Honda, J . Phys. Chem., 83, 2142 (1979). (19) J. S. Pflug, L. R. Faulkner, and W. R. Seitz, J. Am. Chem. SOC.105, 4890 (1983).
2455
experiments comes from molecules whose excited state can neither inject nor be quenched by energy transfer but which, somehow, respond to the applied bias voltage. In conclusion, we have estimated the injection and energy transfer quenching rate constants for rhodamine B deposited on indium oxide. No indication was found of an insulating layer on the aged indium oxide surfaces used in our experiments. From the long-term fluorescence decays we concluded that most molecules deposited on indium oxide can inject, but that some do so near surface traps at which fast recombination takes place.
Acknowledgment. This research was supported by the Office of Basic Energy Sciences, U.S. Department of Energy, under Contract No. DE-AC02-81ER10881. Support by Temple University under a Grant-in-Aid for Research is also acknowledged. Registry No. In203, 1312-43-2; Rhodamine B, 81-88-9. (20) M. M. Martin, Chem. Phys. Left.,35, 105 (1975).
Chemiluminescence and Laser Fluorescence Study of Several Mg(1S,3P0)Oxidation Reactions: On the MgO Dissociation Energy John W. Cox?and Paul J. Dagdigian* Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218 (Received: November 14, 1983)
-
Chemiluminescence and laser fluorescence studies have been carried out on the reactions of Mg( 1S,3P0)with NO2, 03,C02, and SG,. For Mg(3F"')+ NO2,continuum emission was observed and ascribed to an E E energy transfer forming excited N02(A2B2).Weak MgO chemiluminescence in the arc bands and B-X system was seen in M ~ ( ~ p + 0 )O3 ; the photon yield was determined to be -0.003%. Ground-state MgO(X'Z+) product was detected by laser fluorescence in the Mg(3Po)+ C02 and SO2reactions and in Mg('S) + NO2and N20. These reactions were used to set lower bounds to the MgO dissociation energy, for which there is a disagreement between theoretical and experimental determinations. The highest lower bound of Doo(MgO) Z 3.1 eV was obtained from the Mg('S) + NO2 reaction. This result is consistent with a recent ab initio value of Doo(MgO) = 2.65 & 0.16 eV if this reaction has a translational energy barrier. Such a barrier is likely in view of the small MgO signals seen in the present experiments.
Introduction Reactions of Mg with simple oxidants have aroused interest in the past few years.'-" From a fundamental viewpoint, new experimental techniques and theoretical advances have allowed detailed study of the dynamics of reactions involving multiple potential energy surfaces. Study of Mg reactions is especially attractive because it is one of the simplest metal atoms for which experimental studies are convenient and a reasonable number of product oxide electronic states are energetically accessible. From a practical point of view, such reactions are of interest because of the possibility of creating chemically large concentrations of electronically excited atoms, as in Mg/N,O/CO flames.' Unfortunately, the MgO dissociation energy has thus far eluded definitive determination. In a review of earlier experimental work, the 1978 supplement to the JANAF tabled8 recommended a value of 3.47 A 0.26 eV. In a very recent reanalysis of all available experimental data, Pedley and M a r ~ h a l lderive '~ a slightly revised value of 3.72 f 0.13 eV. By contrast, a recent extensive ab initio calculationm disagrees with the available experimental results and yields a value of 2.65 h 0.16 eV for the MgO dissociation energy. The wave functions in this calculation are expected to be of high quality. Indeed, a similar calculation2' for the CaO dissociation +Present address: Chemistry Division, Code 61 10, Naval Research Laboratory, Washington, D.C. 20375.
0022-3654/84/2088-2455$01.50/0
energy yields a result in agreement with the accepted experimental va1~e.l~ (1) D. J. Benard, W. D. Slafer, and J. Hecht, J . Chem. Phys., 66, 1012 (1977); D. J. Benard and W. D. Slafer, ibid., 66, 1017 (1977). (2) G. Taieb and H. P. Broida, J . Chem. Phys., 65, 2914 (1976). (3) R. P. Blickensderfer, W. H. Breckenridge; and D. S. Moore, J . Chem. Phys., 63, 3681 (1975). (4) F. Engelke, R. K. Sander, and R. N. Zare, J. Chem. Phys., 65, 1146
, - ~-.,.. 1197h\
(5) R. J. Malins and D. J. Benard, Chem. Phys. Left., 74, 321 (1980). (6) W. H. Breckenridge and W. L. Nikolai, J . Chem. Phys., 73, 2763 (1980); W. H. Breckenridge and H. Umernoto, ibid., 77, 4469 (1982). (7) W. H. Breckenridge and H. Umemoto, J. Chem. Phys., 75, 4153 (1981). (8) W. H. Breckenridge and H. Umemoto, J. Chem. Phys., 75,698 (1981). (9) A. Kowalski and J. Heldt, Chem. Phys. Lett., 54, 240 (1978). (10) A. Kowalski and M. Menzinger, Chem. Phys. Lett., 78,461 (1981); J . Chem. Phys., 78, 5612 (1983). (11) P. J. Dagdigian, J . Chem. Phys., 76, 5375 (1982). (12) J. W. Cox and P. J. Dagdigian, J . Phys. Chem., 86, 3738 (1982). (1982). (13) B. Bourguignon, J. Rostas, and G. Taieb, :b, J. Chem. Phys., 77, 2979 (\____,. I 9117) (14) W. H. Breckenridge and H. Umemoto, J. Phys. Chem., 87,476, 1804 (1983). (15) H. H. Michels and R. A. Meinzer, Chem. Phys. Lett., 98, 6 (1983). (16) D. R. Yarkony, J . Chem. Phys. 78, 6763 (1983). (17) N. Adams, W. H. Breckenridge, and J. Simons, Chem. Phys., 56,327 (1981).
0 1984 American Chemical Society
Cox and Dagdigian
2456 The Journal of Physical Chemistry, Vol. 88, No. 12, 1984
6
-I c Mg(3P')+03 cMg(3Po)+N20
cz w z
-
w
B 1x+
tMg('S)+03
2-
c Mg(lS 1 t N 2 0 c / M ~ ( ~ P ' ) +0 2
.
l -
- Alii 0
XIL+
-I
b3Ztc/Mg(3P0)+C02 c M g ( 3 P o )+ S O 2
- a3n
2Mg(1S)+N02
as scattering geometry and light collection optics have been given earlier.12q33For dark reactions, laser-induced fluorescence was employed to detect ground-state MgO, as described in detail for our study" of the Ms(~P")+ N 2 0 and O2reactions. In the present work, Molectron DL200 dye laser, pumped by an NRG 0.7-5-200 nitrogen laser, was employed. The fluorescence detection zone was 0.1-cm diameter X 1 cm long, located 3.8 f 0.2 cm downstream from the beam collimator. The fluorescence was detected with a Hamamatsu R928 photomultiplier. The dye laser intensity was attenuated with neutral density filters to ensure linear fluorescence signals. MgO B-X fluorescence was detected through a 500 f 50 nm interference filter to block atomic emission lines from the metastable Mg source. The atomic beam source for production of ground 3s2 'S and/or metastable 3s3p 3P0atoms has been dscribed in detail previous1y.12,27,32,33 A typical operating temperature for Mg was 1130 O C (corrected optical pyrometer reading); the metal effusion rate (source orifice 0.06-cm diameter) was usually 0.3 g/h. Metastable atoms were created by a dc discharge (-18 Vdc, 2.0 A) within the source. Emission at 457.1 nm due to radiative decay of metastable 3P10atoms in the beam was easily visible by eye and was observed with a photometer to monitor beam stability during runs. Magnesium turnings were from Fisher. All gases were supplied by Matheson, except 03,which was prepared from O2 in an ozonator prior to a run and stored as a 1:0.0650 f 0.0005 02/03 mixture, as previously described.33 Laser-induced fluorescence and time-of-flight techniquesgswere used to characterize the metastable beam, as described in detail elsewhere for the heavier alkaline-earth atom^.^^,^^*^^,^^ Mg has one metastable state, 3s3p 3P0,lying 2.714 eV above the ground state.36 We have not directly measured the metastable conversion efficiency since the Mg 3s3p lPo 3s2 'S resonance line lies beyond the range of our dye laser at 285.21 nm. Our earlier rough estimateI2 on the Mg conversion efficiency depended directly on the 3P10radiative lifetime. Recent work37suggests that the lifetime is about a factor of 2 less than that used.38 We therefore modify our estimate of the conversion efficiency to 20%. This estimate involved a comparison of Ca and Mg effusion rates, and the error may be considerable. Fortunately, the conversion efficiency is not required for the results reported here. The J-state distribution in the Mg(3Po,l,20)multiplet was determined by laser fluorescence excitation of the 3s4s 3s3p 'PJ" lines at 516.73, 517.27, and 518.36 nm for J = 0, 1, and 2, re~pectively.~~ Using available transition pr~babilities,~~ we found the distribution to be 1:0.51 f 0.12:0.30 f 0.07 for J = 2, 1, and 0, respectively. By comparison, a statistical distribution at the source would yield the distribution 1:0.47:0.20 at the detector when account is taken of the Mg(3P10)level radiative d e ~ a y . ~ ~ , ~ * Time-of-flight (TOF) measurements were also performed in order to determine Mg(3P?) velocity distributions. For J = 0 and 2, which have very long radiative lifetimes,&the distributions were fit using the usual two-parameter form f(u) = Nu2 exp[-(u u 0 ) * / u 2 ] . The fitted parameters were nearly identical for these two levels: vo = (1.26 f 0.04) X lo5 cm/s and u = (3.83 f 0.10) X lo4 cm/s. While the Mg('S) velocity distribution was not measured, prior investigations of the alkaline earths'2~27~32J3 suggest that the Mg('S) and Mg(3Po) velocity distributions are very similar. The atomic beam velocity distribution was used to calculate relative translational energy distributions appropriate to the scattering experiments. Convoluting the beam and target NOz (294 K) scattering-gas distribution~,2~*~~ we find the average relative velocity and energy to be (1.40 f 0.03) X lo5 cm/s and +-
Experimental Section Low-resolution (1.5-nm fwhm) MgO chemiluminescence spectra were observed over 250-890 nm with a previously described beam-gas scattering apparatus.12*23~32~33 Higher resolution scans employed a 1-m m o n o ~ h r o m a t o r . ~Experimental ~ details such
(18) M. W. Chase, Jr., J. L. Curnutt, R. A. McDonald, and A. N. Sy-
verud, J . Phys. Chem. Ref. Data, 7, 793 (1978). (19) J. P. Pedley and E. M. Marshall, J . Phys. Chem. Ref. Data, 12,967 (1983). (20) C. W. Bauschlicher, Jr., B. H. Lengsfield, and B. Liu, J . Chem. Phys., 77,4084 (1982). (21) C . W. Bauschlicher, Jr., and H. Partridge, Chem. Phys. Lett., 61, 366 (1983). (22) R. N. Zare, Ber. Bunsenges. Phys. Chem., 78, 153 (1974). (23) L. Pasternack and P. J. Dagdigian, J . Chem. Phys., 67, 3854 (1977). (24) R. C. Estler and R. N. Zare, Chem. Phys., 28, 253 (1978). (25) T. Kiang, R. C. Estler, and R. N. Zare, J. Chem. Phys., 70, 5925 (1979). (26) F. Engelke, Chem. Phys., 39, 279 (1979). (27) J. A. Irvin and P. J. Dagdigian, J . Chem. Phys., 73, 176 (1980). (28) K. P. Huber and G. Herzberg, "Molecular Spectra and Molecular Structure", Van Nostrand Reinhold, New York, 1979, Vol. IV. (29) T. Ikeda, N. B. Wong, D. 0. Harris, and R. W. Field, J. Mol. Speczrosc., 68, 452 (1977). (30) P. C. F. Ip, R. W. Field, and K. Cross, to be submitted for publication. (31) C. W. Bauschlicher, Jr., B. H. Lengsfield, D. M. Silver, and D. R. Yarkony, J . Chem. Phys., 74, 2379 (1981). (32) J. A. Irvin and P. J. Dagdigian, J. Chem. Phys., 74, 6178 (1981). (33) J. W. Cox and P. J. Dagdigian, J . Chem. Phys., 79, 5351 (1983). (34) J. H. Moore and J. P. Doering, Phys. Reo., 174, 178 (1968); 182, 176 (1969).
-
(35) L. Pasternack and P. J. Dagdigian, Reu. Sci. Instrum., 48, 226 (1977). (36) C. E. Moore, Natl. Stand. ReJ Data Ser. (US., Natl. Bur. Stand.), NSRDS-NBS 35 (1971). (37) D. Husain and J. Schifino, J . Chem. SOC.,Faraday Trans. 2,78,2083 (1982). (38) P. S. Furcinitti, J. J. Wright, and L. C. Balling, Phys. Reu. A , 12, 1123 (1975). (39) K. Ueda, M. Karasawa, and K. Fukuda, J . Phys. SOC.Jpn, 51, 2267 ( 1982). (40) R. H. Garstang, J . Opt. SOC.Am., 52, 845 (1962).
MgO Dissociation Energy
The Journal of Physical Chemistry, Vol. 88, No. 12, 1984 2457 I
I
0 420
-
I
I
I
I
500
600
700
800
;
I
I
900 100
I
I
50-
-
>-
i,
370
0 370
375
380
385
390
WAVELENGTH ( n m )
Figure 2. Single-collision chemiluminescence spectrum for the reaction of M g ( f p ) with N O z a t 0.8 mtorr. (a) Source discharge on and N O z present. The Mg(3Plo+1S) signal is 7343 counts/s in magnitude. This spectrum has been displaced upward by one tic mark for clarity. (b) Background due to thermal radiation from the source: oven on, discharge off, no scattering gas. (c) Near-UV scan with 1-m monochromater. The atomic lines are due to scattered light from the source. No spectral correction has been made in (c).
0.17 f 0.01 eV, respectively. The other scattering gases have essentially identical averages since their molecular weights are similar.
380
390
490
500
WAVELENGTH ( n r n )
Figure 3. Single-collision chemiluminescence spectrum for the reaction Only the spectral regions spanning the MgO arc of Mg(3Po) with 03. bands and B-X system are shown. (a) Background: discharge off, scattering gas absent. (b) Source discharge on, scattering gas absent. The feature at 384 nm is the Mg 3s3d 'DJ 3s3p 3PJ0multiplet, and the feature at 486 nm is the tail of another Mg atomic line. (c) Source discharge on, 1.6 mtorr of O2present. (d) Source discharge on, 1.6 mtorr of the 02/03 mixture present (0, partial pressure 0.1 mtorr). The features at 373 and 496 nm are the MgO arc bands and B-X band system, respectively. Scans b, c, and d have been displaced upward by 3 tic marks for clarity. No spectral corrections have been made in the figures.
-
energy transfer, which we find to be 1.4 f 0.2 AZ over 0.201.60-mtorr NO2 pressure range. This cross section is a lower ChemiluminescenceStudies bound since the radiative lifetime5*'52 of NO2*is sufficiently long Figure 2 presents the emission spectrum observed in the reaction such that some of the excited molecules will move out of the of Mg(3Po) with NO2. None of the strong MgO bands in the detector's field of view before emitting. Fitting the integrated near-UV region414s were detected. The broad continuum comNOz* signal to the forms3peap yields an attenuation cross section mencing at 440 nm in Figure 2 does not correspond to any known for Mg(3p0)of 68 f 16 A*; thus, energy-transfer efficiency is >2%. system of MgO. Since the strongest MgO BIZ+-X'B+ bands lie MgO chemiluminescence was observed in the reaction of between 501 and 496 nm,46the presence of the continuum does Mg(3Po) with 03.Figure 3 shows MgO bands at 373 and 496 not permit a definite statement regarding their production. As nm. The latter is the well-known B ' Z + - X ' F system.46 The for the continuum, the Mg(3P") multiplet lies 2.71 eV above the system at 373 nm, while not rotationally analyzed, has been ground stcte and is therefore nearly energetically resonant with tentatively identified44as the d3A-a311 transition. This system the NOz(AZB2)excited ~ t a t e . 4 ~ We believe that the continuum is usually the most intense UV MgO feature observed in flames.'J3 arises from resonant electronic energy transfer: No other UV systems of Mg041-45are present in our spectrum. The Mg atomic lines arise from light scattered from the discharge Mg(3Po) + N02(%zA,) Mg('S) N02*(AZB2) (1) source. No chemiluminescence was observed from the groundstate Mg('S) + O3reaction. Absolute chemiluminescence cross sections for production of Several investigators have observed NO2 fluorescence in laser MgO B'Z' and d3A from Mg(3P") O3 were found to be (4.9 irradiation experiments having similar excitation energies. Senum i 2.0) X lV3 and (3.4 f 1.5) X A', respectively, by a method and S c h w a r t ~irradiated ~~ NOz at 7 mtorr with the 441.6-nm previously presented in detai1.12,32This calculation utilized a He-Cd laser line and recorded a continuum spanning 440-800 Mg(3PIo)radiative lifetime of 2.1 The decay of the Mg(3P10 nm. Donnelly and K a ~ f m a excited n ~ ~ NOz at pressures of 0.09-10 'S) emission vs. scattering-gas pressure was monitored in order mtorr with the 532-nm second harmonic of a Nd:YAG laser, to derive total reaction cross sections qat. For NO, and the 02/03 observing a 532-890-nm continuum. The NO2* spectra observed mixture, we find atotequals 86 f 9 and 34 f 10 A2, respectively. in these two studies are quite similar to the spectrum of Figure Taking into account the attenuation of the latter due to 0, (atot 2. The spectrally corrected NO2*and Mg('PI0 IS)integrated emission signals can be used to estimate a cross s e c t i ~ n for ' ~ ~ ~ ~ = 19.3 f 5.5 A2I1),we estimate ut, for O3is 254 f 75 A2. This implies that the photon yield for the Mg(3Po) O3reaction is (41) L. Brewer and R. F. Porter, J. Chem. Phys., 22, 1867 (1954);L. -0.003%. We note that utotfor NO2 includes both chemical Brewer and S.Trajmar, ibid., 36, 1585 (1962). reaction and nonreactive energy transfer.
-
+
+
-
-
+
(42) D.Pesic and A. G. Gaydon, Proc. Phys. Soc., London, 73,244(1959); D. Pesic, ibid., 76, 844 (1960). (43) L. Brewer, S. Trajmar, and R. A. Berg, Astrophys. J., 135, 955 (1962);S.Trajmar and G. E. Ewing, ibid., 142, 77 (1965). (44)J. Schamps and G. Gandara, J . Mol. Spectrosc., 62, 80 (1976). (45) P. J. Evans and J. C. Mackie, Chem. Phys., 5, 27 (1974); J. Mol. Spectrosc., 65, 169 (1977). (46) R.W.B. Pearse and A. G. Gaydon, "The Identification of Molecular Spectra", 4th ed., Chapman and Hall, London, 1976. (47) D. Hsu, D. L. Monts, and R. N. Zare, "The Spectral Atlas of Nitrogen Dioxide", Academic Press, New York, 1978. (48) G. I. Senum and S.E. Schwartz, J. Mol. Spectrosc., 64, 75 (1977). (49) V. M. Donnelly and F. Kaufman, J. Chem. Phys., 67,4768(1977).
Laser Fluorescence Studies Ground-state MgO product was detected in several Mg reactions by laser fluorescence excitation of the BIBf - X I Z + Av = (50) R. Solarz and D. H. Levy, J . Chem. Phys., 60, 842 (1974). (51) V. M. Donnelly and F. Kaufman, J. Chem. Phys., 69, 1456 (1978). (52) D. L.Monts, B. Soep, and R. N. Zare, J . Mol. Spectrosc., 77, 402 (\1-979) -.-/.
(53) C.R.Dickson, S. M. George, and R. N. Zare, J . Chem. Phys., 67, 1024 (1977).
2458
The Journal of Physical Chemistry, Vol. 88, No. 12, 1984
Cox and Dagdigian
TABLE I: Calculation of Doo(MgO)a
reacn Mg(3w + 0 3 Mg(3Po)+ O2 Mg(3Po)+ C02 Mg(lS) + NO,
highest MgO state d3Ae X'Z, u = 38 X'Z, u = l e X'Z, u = 2e
E(Mg0) 3.65' 0.28* 0.1oh 0.19*
5.115 f 0.002 5.452 f 0.002
E(Mg)C 2.714 2.714 2.714
3.116 f 0.009
0.000
Doo(OX)b 1.075 f 0.017
E(0X) 0.043 0.026 0.033 0.041
E/ d 0.17 f 0.01 0.13 i 0.01 0.17 f 0.01 0.17 & 0.01
Do0 (MgO) 21.80 22.53 22.64 23.10'
"All energies given in eV. bReference 56. CReference36. dAverage over incident relative translational energies. 'This work. fThe sum of To (d-a) (ref 28) and To(a-X) (ref 29 and 30). EReference 11. "Reference 28. 'See text.
0 sequence. The Franck-Condon factorss4for these bands are nearly unity. For Mg(3p0) SO2,it was possible to detect a very weak signal near the (0,O)band, but no structure was observed at pressures up to 5.5 mtorr. Distinct structure, just within the 0.02-nm dye laser bandwidth, was seen in the (0,O)and (1,l) bands for the Mg(3p0) CO, reaction at 5.5 mtorr. Breckenridge and U m e m ~ t owere ' ~ unable to detect MgO XIZtin the Mg(3Po) SO, and CO, reactions. Figure 4 presents MgO laser fluorescence spectra for the NOz and N 2 0 reactions. For Mg('S) NOz, the Mg('S) highest vibrational level observed was u = 2 although the (2,2) band is very weak. At 2.5 mtorr, the ratio of the integrated intensities for the NO, vs. the N,O reaction is approximately 0.75. When the discharge in the source is turned on to produce metastable Mg(3p0),the integrated MgO (1,l) band fluorescence signal in the NzO reaction increases by -30% at 2.5 mtorr; the Mg(1S,3P0) N,O spectrum is also shown in Figure 4. In our previous MgO laser fluorescence study," MgO XIZ+was not detected in the Mg('S) N 2 0 reaction. The detection sensitivity in the present experiment is significantly higher, as evidenced by the fact that MgO could be observed in the Mg(3p0) N20 reaction at 0.80 mtorr, as opposed to a minimum NzO pressure of 4.5 mtorr in the earlier study." The present work shows that there was some collisional rotational relaxation at 4.5 mtorr and that the nascent MgO distribution for Ms(~F@)+ N20 is probably somewhat hotter than that derived earlier." Quantification of the differences between the two studies is difficult because the metastable conversion efficiency is not accurately known for our source. Breckenridge and Urnemotol4 were able to detect MgO from Mg('S) N,O in a flame apparatus; they gave arguments for a greater sensitivity, primarily because of the low flow velocity, than in our molecular beam experiment."
+
+
+
+
+
+
+
+
+
499.0
499.5
500.0
560.5
WAVELENGTH ( n m )
Figure 4. Laser fluorescence excitation spectra of the BIZt-X'Zt band system for MgO formed in the reactions (a) Mg('S) + NO2, (b) Mg('S) + N20,and (c) Mg('S?P") + N20. The scattering-gas pressure was 2.5 mtorr for all scans.
The bracketed term in eq 4 represents the reactant energy and in a typical experiment encompasses an energy spread, particularly for E / . Average energies are usually employed in eq 4. However, for a reaction channel with an activation barrier, averaging should include only those encounters which actually result in reaction.55
Table I summarizes the calculation of a lower bound to Doo(MgO)from a number of Mg('S,3P) reactions. The reactant energies E(Mg), E ( O X ) , and E,' are reported as averages over the incident distributions. Thus, the resulting bounds on Doo(MgO) would need to be reduced if there were an activation barrier in a reaction. It should be noted that the Mg('F@) reactions studied by Breckenridge and U m e m ~ t o yield ' ~ very low bounds on Doo(MgO). It can be seen in Table I that the highest bound arises from the Mg('S) NO2reaction and that this appears to contradict the ab initio calculationz0 for Doo (MgO). However, it is quite possible that in this reaction only the high-energy tail of the translational energy distribution is responsible for the formation of MgO product. Thus, a higher value for E/ should be employed in eq 4, which would reduce our bound on Doo(MgO). This possibility is suggested by our observation of MgO fluorescence signals of comparable intensity for the Mg('S) NO, and N,O reactins (Figure 4). Ab initio calculations by Yarkony16 indicate that the latter reaction, though substantially exothermic, has an activation barrier. While these calculations were not able to quantify the barrier height, experiments by Breckenridge and Umemoto14 suggest a 0.7-0.9-eV barrier. This barrier arises because of the orbital rearrangement required for the electron transfer to form the ionic MgO product.I6 The MgO is thus formed only in collisions with energy in excess of this barrier.
(54) R. W. Nicholls, J . Res. Narl. Bur. Stand., Sect. A , 66, 227 (1962); P. S. Dube, Indian J . Pure Appl. Phys., 11, 45 (1973); F. S. Ortenburg, V. B. Glasko, and A. I. Duitriev, Sou. Astron. (Engl. Transl.), 8, 258 (1964). ( 5 5 ) B. A. Thrush, J . Chem. Phys., 58, 5191 (1973).
(56) D. R. Stull and H. Prophet, Natl. Stand. Ref: Data Ser. (U. S., Natl. Bur. Stand.), NSRDS-NBS 37 (1971); H. W. Chase, Jr., J. L. Curnutt, J. R. Downey, Jr., R. A. McDonald, A. N. Syverud, and E. A. Valenzuela, J. Phys. Chem. R e j Data, 11, 695 (1982).
-
MgO Dissociation Energy For a reaction Mg OX MgO + X, conservation of energy for individual states of the reactants and products can be written
+
as4,22-27
E(Mg)
+ E ( 0 X ) + E: + DoO(Mg0) - DoO(0X) = E(Mg)
+ Jm) + E t (3)
Here E: and E,' are the initial and final relative translational energies; E(OX), E(Mg), and E(Mg0) are the relevant internal energies; and Doo(MgO) and Doo(OX)are dissociation energies (at 0 K). In practice it is not possible to directly determine Ef' and E ( X ) , if X is not an atom. These terms can be minimized by setting E(Mg0) equal to that for the highest MgO level detected. The resulting determination of Doo(MgO)yields a lower bound: DoO(Mg0) 1 D o " ( 0 X )
+ E(Mg0) [E(Mg) + E ( O X ) + Ei'l (4)
+
+
J . Phys. Chem. 1984, 88, 2459-2465 The large electron affinity of N0257suggests that there will be no dynamical restrictions (e.g., steric effects) in the Mg('S) NO2 reaction. The reactive cross sections for the heavier alkaline earth reacting with NO2 are all s i ~ a b l e . ' ~ > Thus, ~ * * ~the ~ small MgO signals observed for this reaction must arise for a similar reason as in the N 2 0 reaction, Le., an energy threshold for this reaction. Our lower bound for Doo(MgO) is consistent with the ab initio value20if this reaction had an energy threshold of -0.5 eV. For the relative translational energy distribution appropriate to our beam-gas experiment, only 1% of the collisions have energy in excess of 0.5 eV. While our conclusions about the MgO dissociation energy are tenuous, the present study is nevertheless significant in that it is
+
-
(57) E. Herbst, T.A. Patterson, and W. C. Lineberger, J . Chem. Phys., 61, 1300 (1974).
(58) C. D. Jonah, R. N. Zare, and Ch. Ottinger, J . Chem. Phys., 56, 263 (1972). (59) H. Wang and M. Menzinger, Can. J . Chem., in press.
2459
the first experiment to offer support for the low ab initio value.20 Further experiments are clearly warranted. With the present technique, a quantitative estimate of the energy threshold in the Mg('S) + NO2 reaction can be made with a crossed-beam geometry using a seeded supersonic Mg('S) beam.
Acknowledgment. We have benefited from conversations with D. R. Yarkony, K. Kirby, and B. Liu about the ab initio calculations. We also thank E. Murad for his encouragement and for providing a copy of ref 19, R. W. Field and P. C. F. Ip for communicating their unpublished results on MgO spectroscopy (ref 30), J. P. Doering for the loan of a monochromator, and D. W. Robinson for the use of an ozonator. This work has been sumorted by the Army Research Office under Grant DAAG29-8 l--K-O102, the-National Science Foundation under Grant CHE80-25614, and the North Atlantic Treaty Organization under Grant 232.81. Registry No. Mg, 7439-95-4; NOz, 10102-44-0; O,, 10028-15-6; C 0 2 , 124-38-9; SOZ, 7446-09-5; MgO, 13094-48-4.
Theoretical Investigation of Lithium and Sodium Complexes with COP Kenneth D. Jordan* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: November 14, 1983)
The geometries, stabilities,and vibrational frequencies of LEO2, NaC02, Li2C02,Na2C02,LiC204,and Li2C204are calculated by using the self-consistentfield Hartree-Fock method. LiCOz is found to exist in both C , and C, structures, with the former being more stable. When basis sets utilizing only s and p functions are employed, NaCO, is also found to have C, and C2, forms, with the latter being only weakly bound. With a larger basis set, including d polarization functions, only the C2, form of NaCO, is predicted to be stable. For the LizC02and Na2COzmolecules, the lowest energy structures are found to have C, symmetry. Two different DZhstructures are found for both Li2C204and Na2C204,and a C2, form of LiC204 is also characterized.
Introduction The interactions of alkali atoms with C02 molecules have been studied in gas-phase collision experiments' and in matrix isolation ~pectroscopy.~,~ The collision studies give an adiabatic electron affinity (EA) of -0.6 A 0.2 eV for C02. This should be compared with the -3.6 eV value for the vertical EA determined from electron scattering measurement^.^ The large difference between the vertical and adiabatic EA'S may be understood from the large stabilization of C02- upon bending. Indeed, theoretical calcul a t i o n ~predict ~ an OCO angle of 130-135' for the equilibrium structure of C02-. This leads one to expect that alkali atoms will interact with COF to form ionic complexes with the COz portion of the complexes adopting a bent structure as in C02-. This picture has been confirmed for LiCOz and N a C 0 2 by matrix isolation experiments2v3and by theoretical studies carried out by our groups6 In this earlier theoretical study it was found that the potential energy surfaces of the LiC02 and N a C 0 2 molecules possess both C, and C,minima. In the former the alkali atom lies on the C , axis bridging the oxygen atoms, and in the latter the alkali atom binds to one of the oxygen atoms of the bent C02entity. Although both forms of LiC02 were found to be strongly bound with respect to Li + COz, the C, form of N a C 0 2 was predicted to be bound by only 0.14 eV. This small binding could, in fact, be a result of deficiencies in the atomic basis sets employed. In the present study the LiC02 and NaCOz species are reexamined using basis sets with d polarization functions, which were not included in the basis sets utilized in the previous study. These new calculations should provide a more reliable determination of the stability of 'John Simon Guggenheim Memorial Fellow.
the C,form of NaC02 We also extend the earlier work to include the Li2C02,NaZCO2,LiC204,Li2C204,and Na2C204molecules. Vibrational frequencies are calculated in the normal-mode approximation for these species (with the exception of LiC204)as well as the monoalkali complexes. The resulting frequencies are compared with the experimental values of Kafafi et aL3
Computational Details The spin-restricted and spin-unrestricted versions of the Hartree-Fock self-consistent field (SCF) method were employed for the calculations on the closed-shell and open-shell species, respectively. An analytical first-derivative program was utilized to optimize the structures, and a finite-difference method was employed to obtain the second derivatives from the analytically determined first derivatives. The SCF iterations and the geometry au in the optimizations were converged to at least 1.0 X energy. All calculations were performed using the GAUSSIAN 83 programs7 on the Chemistry Department's Harris 800 minicomputer. (1) R. N. Compton, P. W. Reinhardt, and C. D. Cooper, J . Chem. Phys., 63, 3821 (1975).
(2) M. E. Jacox and D. E. Milligan, Chem. Phys. Lett., 28, 163 (1974). (3) Z. H. Kafafi, R. H. Hauge, W. E. Billups, and J. L. Margrave, J. Am. Chem. SOC.,105, 3886 (1983). (4) M. J. Boness and G. J. Schulz, Phy. Rev. A , 9, 1969 (1974). ( 5 ) J. Pacansky, V.Wahlgren, and P. S. Bagus, J . Chem. Phys., 70,3008 (1979), and references therein. (6) Y. Yoshioka and K. D. Jordan, Chem. Phys. Lert., 84, 370 (1981). (7) The GAUSSIAN 83 program was developed by W. Hehre and co-workers at the University of California, Irvine, CA: D. J. DeFrees, B. A. Levi, S. K. Pollack, R. F. Hout, Jr., W. J. Pietro, E. A. Blurock, and W. J. Hehre, to be submitted for publication to the Quantum Chemistry Program Exchange, Indiana University, Bloomington, IN.
0022-3654/84/2088-2459$01 .SO10 0 1984 American Chemical Society
