J. Phys. Chem. 1988, 92, 1169-1 178
1169
Energy-Transfer Reactions of N,(A32,+) to SO and Other Diatomic and Polyatomic Molecules De Zhao Cao and D. W. Setser* Department of Chemistry, Kansas State University, Manhattan. Kansas 66506 (Received: June 8, 1987; In Final Form: September 14, 1987)
The total quenching rate constant of N2(A3Z,+) by SO and the formation rate constant of SO(A311) have been measured in a flow reactor by using microwave discharges of dilute SOC12/Ar and SOF2/Ar mixtures as sources of SO(X). The initial vibrational distribution of SO(A) was estimated by extrapolating the steady-state distributions to low pressure. Other excitation-transfer reactions of Nz(A) are summarized and compared with the SO and S, reactions. Total quenching rate constants of Nz(A3Z,+) also were measured for several diatomic halogens and for PF,, PCI,, POCl,, NF,, SOF,, SO,F,, CC12F2, CF4, and HN,. In several instances, effective Nz(A) decay constants also were measured for dilute mixtures of these reagents that ha$ been passed through a microwave discharge. Unsuccessful attempts to excite PF(A311), PC1(A3n), PO(B2Zt), and CF2(A,’Al)from reaction of Nz(A) with the products from the microwave discharges in PF3, PCI,, P0Cl3, and CF2C12 (and CF4) are summarized. The rate constants for formation of I2(D’) from I, and NH(A) from HN, are small. Qualitative experiments demonstrated that the Nz(A) + XeF, reaction does not have a significant branching fraction for XeF(B) formation.
Introduction The first electronically excited state of molecular nitrogen, N2(A3Z,+),which can be generated for systematic study in a flow reactor by the energy-transfer reaction of Ar(3P2,0)with Nz(X), has a long lifetime (-2 s) and small quenching rate constants by rare gases and by N2.’ The energies of Nz(A,vr=O and 1) are 6.17 and 6.35 eV, respectively, but the favored Franck-Condon transitions2 correspond to - 5 eV. These energies are large enough to initiate numerous dissociative excitation and excitation-transfer reactions with a wide range of rate constants depending on the availability of suitable electronically excited acceptor states3-I2 In addition to possible energy storage and laser applications, an understanding of N2(A) chemistry is of importance for the upper a t m o ~ p h e r e . ~The * ~ J present ~ work provides additional information about the excitation-transfer reaction with SO7 using the flow reactor technique. The SO(A) vibrational distribution changed with Ar pressure and estimates were made for the v’ = 0-3 relaxation rate constants. The relative transition probabilities from a given SO(A311,u’)level to a range of v”1evels were measured. These experimental band strengths do not agree with those predicted from Franck-Condon factors. Experiments were done to search for other good excitationtransfer partners with Nz(A) using microwave discharges of various precursor molecules to generate unstable diatomic molecules in the Nz(A) flow reactor. Although fairly strong emission was observed for experiments with PF,, the emission could not
be assigned to PF(A). Discharges with precursors expected to give PCl, PO, and CF2 gave only weak emission. Room temperature quenching rate constants were measured for several of the precursor molecules, as well as for the mixtures that had been passed through the microwave discharge. The dissociative excitation reactions with HN3 and XeF2 also were examined. The homonuclear and intercombination diatomic molecules of group VI elements and the monohalides of group V elements provide a class of molecules with states of possible importance for short wavelength laser applications. The interesting states include the two low-energy metastable singlet states derived from ~*2 configuration and higher energy triplet the u 2 ~ 4 ground-state u* excitation is of particular states. The A3D state from the R* and interest for SO, PC1, and PF; however, the uz~31r*3 configurations give states that are in the same energy range.I4 The excitation-transfer reactions of N2(A) with S2and SO provide efficient formation pathways for Sz(B3Z:,-) and SO(A311).7 Since SO(A311) has a radiative lifetime of -20 ps, slow quenching rates (at least for rare gases), and re’ > rerr,the SO(A311-X3Z-) transition is potentially promising as a l a ~ e r . ~ ~Furthermore, .’~ SO(X) is not reactive and probably can be generated in high concentration^.'^-'^ The S2(B3Z,-) state, which corresponds to a R R* excitationsz4 and is analogous to the Schumann-Runge transition of 02,has been made to lase by using optical pumping.25v26 In this report we provide additional experimental support
( I ) Kolts, J. H.; Setser, D. W. In Reactive Intermediates in the Gas Phase; Setser, D. W., Ed.; Academic: New York, 1979. (2) Lofthus, A.; Krupenie, P. H. J . Phys. Chem. Ref. Data 1977, 6, 113. (3) Dreiling, T. D.; Setser, D. W. J . Chem. Phys. 1983, 79, 5439. (4) Baltayan, P.; Pebay-Peyroula, J. C.; Sadeghi, N. J . Chem. Phys. 1983, 78. 2942. ’ ( 5 ) (a) Piper, L. G.; Caledonia, G. E.; Kennealy, J. P. J . Chem. Phys. 1981, 75, 2847. (b) Piper, L. G. J . Chem. Phys. 1982, 77, 2373. (6) Nadler, I.; Rawnitzkl, G.; Rosenwaks, S. J . Phys. Chem. 1982, 86, 1503. (7) Cao, D. Z.; Setser, D. W. Chem. Phys. Lett. 1985, 116, 363. (8) Richards, D. S.; Setser, D. W. Chem. Phys. Lett. 1987, 136, 9. (9) (a) Iannuzzi, M. P.; Kaufman, F. J. Phys. Chem. 1983,85,2163. (b) Iannuzzi, M. P.; Jeffries, J. B.; Kaufman, F. Chem. Phys. Left. 1982,87, 570. (c) Piper, L. G.; Caledonia, G . E.; Kennealy, J. P. J . Chem. Phys. 1981, 74, 2888. (d) Thomas, I. M.; Kaufman, F. J . Chem. Phys. 1985, 83, 2900. (IO) (a) DeSouza, A. R.; Touzeau, M.; Petitdidier, M. Chem. Phys. Lett. 1985, 121, 423. (b) DeSouza, G . ;Gousset, G.; Touzeau, M.; Khiet, Tu. J . Phys. B. 1985, 18, 166 I . ( 1 1 ) (a) Golde, M. F.; Moyle, A. M. Chem. Phys. Lett. 1985, 117, 375. (b) Thomas, J. M.; Kaufman, F.;Golde, M. F. J. Chem. Phys. 1987,86,6885. (12) Clark, W. G.; Setser, D. W. J . Phys. Chem. 1980, 84, 2225. (13) Thomas, J. M.; Jeffries, J. B.; Kaufman, F. Chem. Phys. Lett. 1983, 102, 50.
70, 947; 1979, 71, 3761.
-
-
(14) Swope, W. C.; Lee, Y-P.; Schaefer 111, H. F. J . Chem. Phys. 1979, (15) (a) Clyne, M. A. A.; McDermid, I. S. J. Chem. SOC.,Faraday Trans. 2 1979, 75,905; our calculated Franck-Condon factors are in close agreement
with the ones listed here. (b) Clyne, M. A. A,; Liddy, J. P. J . Chem. SOC., Faraday Trans. 2 1982, 78, 1127. (c) Clyne, M. A. A,; Tennyson, P. H. J . Chem. SOC.,Faraday Trans. 2 1986, 82, 13 15. (16) Colin, R. J . Chem. SOC.,Faraday Trans. 2 1982, 78, 1139. (17) Clyne, M. A. A.; Macrobert, A. J. Int. J . Chem. Kine?. 1980, 12, 79; 1981. 13, 187.
(18) Black, G.; Sharpless, R. L.; Slanger, T. G. Chem. Phys. Lett. 1982, 90. 55: 1982. 93. 598.
’(19j Brunning, J.; Stief, L. J. J . Chem. Phys. 1986, 84, 4371. (20) Huber, K. P.; Herzberg, G . Molecular Spectra and Molecular Structure IV Constants of Diatomic Molecules; Van Nostrand: New York, 1979. (21) (a) Narasimham, N . A.; Sethuraman, V.; Apparao, K. V. S. R. J . Mol. Spectrosc. 1976, 59, 142. (b) Narasimham, N. A,; Apparao, K. V. S. R.; Balasubramanian, T. K. J . Mol. Spectrosc. 1976, 59, 244. (22) Quick, C. R., Jr.; Weston, R. E., Jr. J. Chem. Phys. 1981, 74, 4951. (23) Matsumi, Y.; Munakata, T.; Kasuya, T. J . Chem. Phys. 1984, 81, 1108. (24) Bondybey, V. E.; English, J. H. J . Chem. Phys. 1978.69, 1865; 1980, 72, 3113. (25) Leone, S. R.; Kosnik, K. G. Appl. Phys. Lett. 1977, 30, 346
0022-3654/88/2092-1169$01.50/00 1988 American Chemical Society
Cao and Setser
1170 The Journal of Physical Chemistry, Vol. 92, No. 5, 1988 N, Inlet
*-
-- .+
-
Reagent Inlet
Pressure Mea8Urement
N, and Reagent Inlet?,
++
f
Reagent Inlet
PI ’u ‘-a **O
I
Figure 1. The flow reactor (with dimensions in mm) used to generate N,(A) and to observe the quenching and excitation reactions. The microwave discharge could be placed at different positions on the reagent line, 8-10 mm Pyrex tubing, to generate SO, S1, PF, etc. For quenching studies the reagents were added at the first inlet and the [N,(A)] was observed at the second window. Excitation spectra usually were taken at the first window. The flow speed was 1 5 and 25 m s-’ for the mechanical pump and mechanical pump plus Roots blower, respectively.
for a branching fraction of > O S for SO(A) formation from N2(A) SO(X). Finding the best precursor source for SO is an important practical concern and microwave discharges in SOCI2/Ar and SOF2/Ar mixtures were investigated as alternatives to discharges in a S 0 2 / A r m i ~ t u r e . ~ Due to the complexity of electronic energy transfer involving molecules, there are no models that permit calculation of rate constants. The product states from S2,7SO,7N0,27-30CS,8 and IF29billustrate the exit channel specificity and the vibrational distributions for CS(a and A), S,(B), and SO(A) show the importance of Franck-Condon factors. Identification of the factors responsible for the electronic product state selectivity is a challenge. For example, the high product branching fraction for IF(B) formation,29bwhich is only 2.4 eV above the I F ground state, is especially puzzling because quenching by the other halogen or interhalogen molecules occurs via dissociative states. The vibrational and electronic state distributions from the SO and S2 reactions will be discussed in an attempt to gain a better understanding of the excitation-transfer reactions of N2(A).
+
Experimental Techniques These experiments employed the Ar(3Po,2)+ N2reaction system for producing N,(A) in the absence of N atoms in the flow reactor’ shown in Figure 1. The metastable Ar(3P2,0)atoms were generated by passing purified Ar through a dc discharge (-250 V). Nitrogen was added into the flow 2 cm downstream from the discharge. The product emission spectra from the N2(A) reactions usually were recorded at the first window. The second observation window, 30 cm downstream from the first window, was used to measure the change in [N2(A)] vs the reagent concentration. The normal flow rates of Ar and N 2 were 150 and 30 mmol min-’, respectively. A wet test meter was used to calibrate the Ar and N2 flow meters (Fischer Porter triflat). The Ar and N, were purified by passing tank gases through two dry ice cooled (high pressure) and a liquid nitrogen cooled (low pressure) molecular sieve traps. Since the cooled molecular sieve traps absorb large (26) Zuev, V. S.; Mikheev, L. D.; Yalovoi, V. I. Sou. J . Quantum Elecrron.
quantities of gas, about 30 min of operation were required to obtain a stable [N,(A)]. The high flow speed and relatively high pressure reduce the loss of [N2(A)]via diffusion to and quenching at the walls. An attempt was made to reduce the wall deactivation rate of N2(A) by coating the Pyrex glass reactor with Teflon. However, the N2(A) decay rate was unchanged from untreated Pyrex glass. Previous attempts’, to use a halocarbon wax to reduce the wall deactivation rate also were unsuccessful. For the standard operating conditions, e.g., 0.7 Torr of N2 and 3.8 Torr of Ar, the N,(A,o’=O to u’=l) ratio was 2:l. If the N, pressure was increased to 1.5 Torr, the u’ = 0:1:2 ratio was 3:2:1. The rate constant measurements were done at -4.5 Torr with a bulk flow speed of 22 m s-’. The distance from the reagent inlet to the observation window was converted to reaction time for a parabolic flow analysis for a reactant that is quenched a t the wall^.'^^^^ The emission spectra were observed with a 0.75-m Jarrell Ash monochromator fitted with a 250-nm blazed grating and equipped with a cooled 9558QB EM1 photomultiplier tube and SSR photon counter. The wavelength response of the detection system was calibrated with standard lamps. For excitation-transfer rate constant measurements, the NO, SO, S2,and N H emission spectra were acquired by computer and stored for subsequent integration to obtain total relative intensities. The 12(D’-A’) emission intensity was too weak to record with the computer and we used hand summing and correction of the wavelength response. In order to generate the unstable molecules, very dilute (typically 0.1-1 .O%) reagent/Ar mixtures (-20 mm 1 m i d ) were passed through the microwave discharge (70 W, 2 50 MHz). The microwave discharge was located 10 cm away from the inlet to the flow reactor in order to reduce scattered light and to allow for the decay of unwanted species. The reagent gases were degassed and purified by vacuum distillation before storage in Pyrex reservoirs as mixtures with Ar. Rigorous purification was not done and the rate constant measurements could be upper limits to the true values for cases with small rate constants, if impurities were in the commercial samples. The reagent flows were measured by observing the pressure rise in a calibrated volume with a pressure transducer, which was calibrated frequently because corrosive gases were being measured. The generation of SO and S2 from microwave discharges of dilute S 0 2 / A r and S2C12/Ar mixtures, respectively, and the measurement of the SO and S2 concentrations were previously described.’ The principle of the method was to assign the degree of dissociation of the precursor molecule. The fractional dissociation of both SO2and S2C12was determined from the diminution of the characteristic chemiluminescence from the Ar(3Po,2)reactions with SO2 or S2C12 upon activation of the microwave d i ~ c h a r g e . ~ , ~The ‘ - ~same ~ strategy was used with SOCl2. The reaction of Ar(3P2,0)with SOCl, gives ArCI* as well as SO(A) The discharge generates some Cl,, as well as dissociating the SOCI,. The generation of C12 was definitely established because the ArCl* emission actually increased and the Cl,(D’-A’) emission could be observed when the microwave discharge was activated. However, the fractional decomposition of SOCl, could be estimated from the emission intensity of the SO(A3II,o’=1) bands at 285 nm with the microwave discharge on and off. The 260-nm C12* emission intensity was used to assign the [Cl,] relative to the fractional dissociation of SOCI,. We assumed that each molecule of SOCl, that was removed gave SO(X). Unfortunately, the quenching of N2(A) by C1, and undissociated SOCl, must be considered in assigning the quenching rate constants by SO. No ArF(B-X) emission (or any other UV or visible emission) was observed from the reaction of Ar(3Po,2)with SOF,,and a
-
B
1975, 5, 442.
(27) Deperasinska, I.; Beswick, J. A,; Tramer, A. J . Chem. Phys. 1979,
-. .
71 , 3411 I
1
I
I
(28) Shibuya, K.; Imajo, T.; Obi, K.; Tanaka, I. J . Phys. Chem. 1984,88, 1457. (29) (a) Piper, L. G.; Cowles, L. M.; Rawlins, W. T. J . Chem. Phys. 1986, 85, 3369. (b) Piper, L. G.; Marinelli, W. J.; Rawlins, W. T.; Green, B. 0. J . Chem. Phys. 1985, 83, 5602. (30) Ottinger, Ch.; Simonis, J.; Setser, D. W. Eer. Bunsen-Ges. Phys. Chem. 1977, 82, 655.
(31) (a) Wu, K. T.; Morgner, H.; Yencha, A. J. Chem. Phys. 1982, 68, 285. (b) Wu, K. T. Chem. Phys. 1984, 87, 109. (32) (a) Setser, D. W.; Drieling, T. D.; Brashears, H. C., Jr.; Kolts, J. H. Faraday Discuss. Chem. SOC.1979, 67, 255. The rB&value quoted here should be 0.1; the weak 260-nm emission assigned to SO(A) and Cl,(A’) from Ar(’P2) + SOCI, was based upon low-resolution spectra. More complete studies, see text, show that SO(A) is the predominant source. (b) Tsuji, M.; Murakami, I.; Nishimura, Y . J . Chem. Phys. 1981, 75, 5373.
The Journal of Physical Chemistry, Vol. 92, No. 5, 1988 1171
Energy-Transfer Reactions of N2(A3Z,+) Molecules
TABLE I: Quenching Rate Constants for Polyatomic Halogen-Containing Molecules
rate const, lo-” cm3 molecule-’ s-l effective molecule u’ = 0 0,b 0.31 f 0.05
SOCI2 SO2 0
4.0
8.0 12. 16. 20, 10l2 MOLECULE/ CM3
Figure 2. Some typical decay plots with [N2(A)]observed at the second window of the reactor; the reaction time was 10.6 ms.
different method must be used to measure the degree of SOF, dissociation. The reaction between Ar(3Po,2)and the products of the discharged SOF2/Ar mixture gave moderately strong SO(A) emission plus some weaker ArF* emission, even though Ar(3Po,2) SO gives no SO(A-X) emission. When N,(A) was added to the discharged SOF2/Ar flow, strong SO(A) emission was observed. The discharged SOF2/Ar flow must contain S O F and SO and the Ar(,P0,,) must react with SOF to give ArF* SO and SO(A) F. It also is possible that some of the ArF* emission was from F2. The SO(A-X) emission intensity ratio from the Ar(3P2,0) SOF and N,(A) SO(X) reactions was 1:6. If the two formation rate constants are similar, [SO]/[SOF] is about 6:1 because [Ar(3Po,2)J [N,(A)]. The SO(A) emission intensity from N2(A) reacting with the discharged SOF2/Ar flow was nearly equal to that from the SOC12/Ar discharged flow for the same initial [SOF2] and [S0Cl2]. The relative SO(A-X) intensities from the Ar/SOF2 and Ar/SOC12 mixtures suggest that about 40% of SOF, was dissociated into SO 2F and about 5% to SOF F. We tried to use He(%) metastable atoms to monitor the degree of dissociation of SOF2 from the microwave discharge in a SOF2/He mixture. However, the microwave discharge in SOF2/He seemed to give some S2as a product, since S2(B-X) emission was observed upon addition of the SOF2/He discharged flow to the‘N2(A)reactor. Thus, no further work was done with the He carrier gas system. Attempts were made to prepare CF2, PF, PCI, and P O molecules by microwave discharges in CF2C12/Ar and CF4/Ar, PF3/Ar, PClJAr, and POC13/Ar mixtures, re~pectively.’~Experimental details will be discussed in the section describing the results of these measurements. This work was exploratory and radical concentrations were not estimated. The PF3 and SOF2 were purchased from Ozark-Mahoning Co. The other chemicals were obtained from standard commercial sources, except for XeF, and HN,, which were prepared in our laboratory. Hydrazoic acid, which is explosive, was handled as recently described.34
+
+
+
+
-
+
-
+
+
Experimental Results A . Total Quenching Rate Constants of N2(A). The quenching reactions of N2(A,u’=O) can be described by the following equations. N2(A) + Q
N2(X) + Q*
b
N2(X)
+ other channels
(la) (lb)
wall
N2(A)
deactivation
(IC)
The rate equation for the loss of N,(A) is d[N2(A)1 /dt = - ( k ~ [ Q l + kwa11)[N2(A)l
+
-
(2)
-
with kQ = kQa k ~ The ~ excitation-transfer . branching fraction is kQ8/kQ. In our experiments [N2(A)] 2 X 1o’O and [QJ
(33) (a) Bielefeld, M.; Elfers, G.; Fink, E. H.; Kruse, H.; Wildt, J.; Winter,
R.;Zabel, F. J. Phorochem. 1984,25,419. (b) Hack, W.; Langel, W. J . Phys. Chem. 1983, 87, 3462. (34) Habdas, J.; Setser, D. W. J. Phys. Chem. 1987, 91, 451.
S2C12 SOF2 SO2F2 CF2C12 CFp PFg PF5 PC13 POCI, NF3 HNg‘
u’= 1
8.7 f 1.5 9.4 f 1.6 3.0 f 0.5 6.0 f 1.5 0.7 & 0.2 0.015 f 0.003 0.015 f 0.005 0.12 f 0.05 0.55 f 0.2 >0.4 f 0.2
5.0 f 2.0 0.004 f 0.001 8.0 f 1.2
0.0002
4.7; 2.5
not observable
3.64
see text
4.12
probably large 0.21 f 0.03
not measd not measd extremely small uncertain not measd
0.40 f 0.06
1.6 2,: 0.23 f 0.06
3.43
4.83
ref 29 7 this 7 this 5c 8 8 this this 38 this this
0.025 f 0.004
very small
small
work
work
work work work worke
'The NO(B211)and SO(B3Z-)states contribute 0.5 with a relatively large total rate eq 9 to the NO(A,u’) distribution and obtained better agreement constant. Since SO has desirable spectroscopic characteristics with experiment than with the FC factor distribution alone. In and since it can be generated from several precursor molecules our opinion, as well as that of others,27 the pr weighting for the (SO,, SOCl,, and SOF2) by electron collision processes or by energy defect is unrealistic for repulsive potential surfaces. chemical reaction (0 S20),it may be possible to design a laser There are, at least, two other general ways to weigh the energy around this excitation-transfer system. The broad SO(A) and defect. A commonly used method is to apply an exponential S2(B)vibrational distributions and the large total formation rate d i s c r i m i n a t i ~ nof~ the ~ ~ ~form ~ exp(-(IAEI/E). Herbleins3 has constants suggest that the N2(A) SO(X) or S,(X) entrance proposed that E = 1/2(wc(N2,X) w,’(CD*)) could be used. channel potentials are more attractive than the N2(X,u’’) + SOSince w,”(N2,X) > uJ(S0,A) using E = 1/2(w,’(S0,A)) also could (A,v’) or S2(B,v’) exit channel potentials. The SO(A) vibrational be appropriate. The results are not so sensitive to e5 and we used distribution largely follows the SO(A,u’+-X,u”-O) Franck-Condon Herblein’s suggestion to exponentially weigh the energy defect factors. On the other hand, the N2(A-+X) Franck-Condon factors with the product of the FC factors. The results for SO(A) are provide an explanation for the selectivity for SO(A) vs SO(B) shown as the second column under theory of Table V. Another formation, which is another way of stating that the entrance method, which is based upon the form obtained from the sudden channel couples to the exit channel for R(N2,X) = R(N2,A). approximation, has a power scaling of the energy defect (AE)-”. Excitation-transfer data to other diatomic acceptor molecules are Given the qualitative nature of the discussion and lack of summarized to show the general versatility of N2(A) as an enknowledge of E or n, it is not possible to distinguish between these ergy-transfer agent. Rate constants for quenching of N,(A) by models. The exponential weighting of the energy defect does /
”
+
+
+
~
(51) Sadeghi, N.; Setser, D. W. J . Chem. Phys. 1983, 49, 2710. (52) Katayama, D. H.; Miller, T. A,; Bondybey, V . E. J. Chem. Phys. 1980, 72, 5469. (53) Herblein, J., private communication, 1987.
(54) Sperlein, R. F.; Golde, M. F.; Jordan, K. D. Chem. Phys. Lett. 1987, 142, 359. This reference reports interesting calculations for CO(a) H2. (55) McGee, T. H.; Weston, R. E., Jr. J . Chem. Phys. 1978, 68, 1736. (56) Yardley, J. T. Introduction to Molecular Energy Transfer, Academic: New York, 1980.
+
J . Phys. Chem. 1988, 92, 1178-1187
1178
inorganic polyatomic chlorine- and fluorine-containing molecules tend to be in the (0.5-5) X IO-" cm3 molecule-' s-l range; the quenching of N2(A) by XeF2 gives a negligible yield of XeF* (as also reported by Young et al.).57
Figure 7. These factors, as well as the probability that electronic quenching rates depend on v' level, make the estimates for the vibrational relaxation rate constants more uncertain than implied in the text.
Note Added in Proof: Extensive LIF m e a ~ u r e m e n t sfor ~ ~a 300 K Boltzmann distribution of SO(A3110 2) rotational states in u' = 0 supports the 35-40-ps lifetime value given in ref 15. Consequently, the steady-state u ' = 0 relative population in Table V and Figure 7 must be increased substantially. However, the k, values in Table V are not affected seriously. The LIF experim e n t also ~ ~ ~give a larger electronic quenching rate constant (8 X cm3 molecule-' s-*) than that employed for the fitting in
Acknowledgment. This work was performed under the auspices of the U S . Department of Energy by the Livermore National Laboratory under contract No. W-7405-ENG-48. We thank Dr. Richards and Mr. Lo of this laboratory for doing the XeF2 experiments and for the SSH calculations, respectively. We thank Dr. Hovis for a preprint of ref 36. Registry NO, N2, 7727-37-9; SO, 13827-32-2;SOC12, 7719-09-7; SOF,, 7783-42-8;Ar, 7440-37-1; PF,, 7783-55-3; PCI,, 7719-12-2; NF,, 7783-54-2; CC12F2, 75-71-8; CF4, 75-73-0; NBH, 7782-79-8; PF, 16027-92-2; PC1, 17167-55-4; PO, 14452-66-5; CF2, 2154-59-8; 12, 7553-56-2;NH, 13774-92-0; S02F2, 2699-79-8;POCI,, 10025-87-3.
(57) Young, R. A,; Blauer, J.; Bower, R. J . Chem. Phys. 1987,87, 3708.
Isomerization and Unimolecular Dissociation Channels of the GlyoxyUc Acid Monomer Charles W. Bock Department of Chemistry and Physical Sciences, Philadelphia College of Textiles and Science. Philadelphia, Pennsylvania 191 44
and Richard L. Redington* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409 (Received: June 15, 1987; In Final Form: August 13, 1987)
Glyoxylic acid and its C203H2isomers are considered in this study of unimolecular reactions evolved from H atom migrations in an intramolecular hydrogen bonded molecule. The sample molecules are investigated by using MO calculations performed at the RHF/6-31G and higher levels of approximation including RHF/6-31G*(5D), RHF/6-31G**(5D), MP2/6-31G, MP2/6-31G*(5D), MP3/6-31G*(5D), and MP4SDQ/6-31G*(5D). A number of STO-3G calculations are also reported in this survey of behavior associated with the C203H2global potential energy hypersurface. Calculations performed on rotamers of the glyoxylic acid monomer agree well with the observed geometry of the intramolecular hydrogen bonded rotamer, the energy difference between the two lowest rotamers, and their observed vibrational spectra. Calculations probing the decarboxylation reaction suggest that the lowest energy pathway involves an intramolecular H atom transfer between 0 atoms to form (HO)HCC02 as an intermediate. A shallow potential energy well is calculated for (HO)HCC02,which dissociates to hydroxycarbene, HOCH, and CO,. The activation energy calculated for this lowest energy route to decarboxylation is 47.9 kcal/mol (excluding zero-point contributions) at the MP4SDQ/6-31GS(5D)//RHF/6-31G*(5D) level. In addition to its mechanistic importance, this result suggests glyoxylic acid monomer dissociation as a possible route to obtain HOCH. H atom migration from OH to the aldehyde C atom of glyoxylic acid leads to a nonplanar transition state for dissociation to H2C0 and C02. The energy of this transition state is 89.9 kcal/mol calculated at the MP2/6-31G*(5D)//RHF/6-31G*(5D) level. Computations at the 6-31G and higher levels are reported for several other previously uninvestigated C203H2isomers. These place dihydroxyketene and singlet carboxyhydroxycarbeneat energies of 39.2 and 5 1.4 kcal/mol, respectively, relative to glyoxylic acid monomer. The dihydroxyketene molecule is found to be nonplanar (C2 point group), with the H atoms out of the heavy-atom skeletal plane. Unimolecular decarbopylation of dihydroxyketene has a calculated activation energy of 20.8 kcal/mol. At the UHF/6-31G level the energy of triplet carboxyhydroxycarbeneis 38.0 kcal/mol, but the calculated value rises to 73.5 kcal/mol, or 14.5 kcal/mol above the corresponding singlet state energy value, at the UMP3/6-31G*(5D)//UHF/6-31G*(5D) level. At the 6-31G level dihydroxyoxireneis calculated to occur as a saddle point at 113.4 kcal/mol, and a potential energy well is found for hydroxyhydroperoxyacetylene at 125.8 kcal/mol.
Introduction Glyoxylic (2-oxoethanoic) acid, OCHCOOH, and formic anhydride are at present the only experimentally known C 2 0 3 H 2 molecules, but several other isomers with conventional electronic structures can be envisioned, and some of these might be sufficiently stable for synthesis in future challenging experiments. In this article several possible isomerizations of the glyoxylic acid (GA) monomer are considered, along with unimolecular dissociation channels that include a channel with a potential for producing hydroxycarbene as a transient product. In general the glyoxylic acid monomer has not been investigated thoroughly. M i c r o ~ a v e l -and ~ infrared4p5spectroscopic studies show that the (1) Marstokk, K. M.; Mollendal, H. J . Mol. Struct. 1973, 15, 137. (2) Christiansen, I.; Marstokk, K. M.; Mollendal, H. J . Mol. Struct. 1976, 30, 137
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intramolecular hydrogen bonded conformer, labeled I below, has the lowest energy among the four GA rotamers. In addition, observations have been attributed to conformer 11, which is obtained from I by internal rotation of the O H group to form a free carboxyl g r o ~ p . Photochemical ~,~ and thermal dissociation experiments on glyoxylic acid vapor were performed by Back and Yamamoto,6 who suggest that transfer of the H atom across the hydrogen bond is important in the decarboxylation of GA. They suggest that transfer of H to the carbonyl 0 atom occurs in the lowest excited singlet state and that transfer to the carbonyl C atom occurs in the ground electronic state. Molecular orbital (3) van Eijck, B. P.; van Duijneveldt, F. B. J . Mol. Srrucr. 1977, 39, 157. (4) Fleury, G.; Tabacik, V. J . Mol. Struct. 1971, 10, 359. (5) Redington, R. L.;Liang, C. K. J. J . Mol. Specrrusc. 1984, 104, 25. (6) Back, R. A.; Yamamoto, S . Can. J . Chem. 1985, 63, 542.
0 1988 American Chemical Society
