J . Phys. Chem. 1992, 96, 104-107
104
TABLE V Vibrational Frequencies of p D F B and Its Cation in tbe Ground and Excited States and Their Correspondences (cm-')
6a 16a 17b
ai a, bj,
436 355 125
413 199 145
450 422 158
410 175
120
"From ref 6. bFrom ref 18.
in the ion exactly corresponds to that for the neutral molecule, for which the same mode of 16a decreases its frequency in going from S0(422cm-l) to Sl(a,a*)(175cm-l).l8 Table V summarizes the correspondence of the vibrational frequencies between the ion and the neutral molecule of p-DFB.I8 Therefore, the striking correspondence is quite general for all the difluorobenzenes and their cations. Similar to DFB cation, the great frequency decrease of the particular out-of-plane mode in the S1(*,a*) state of the neutral molecule is due to the vibronic coupling between the SI(*,**)state and the S(a,u*) state lying above SI.The fact that exactly the same out-of-plane mode is responsible for both neutral molecule and its ion indicates that the vibronic coupling scheme is very (18) Knight, A. E. W.; Kable, S. H. J . Chem. Phys. 1988, 89, 7139.
similar. The similarity implies the existence of a close relationship between the electronic states of the neutral molecule and its ion. In the case of the neutral molecule, the S,(r,r*)state arises from the transition of an electron in the highest occupied *-bonding molecular orbital to the lowest vacant **-antibonding molecular orbital. In the corresponding ion, the molecular orbitals associated with the D(a,a) excited state are the two highest a-bonding orbitals. Therefore, the highest a-bonding orbital is common for the neutral molecule and its ion. However, the other orbital is quite Werent. The one for the neutral molecule is the antibonding a* orbital while the one for the ion is the second highest bonding a orbital. This situation is also similar for the S(a,u*) state of the neutral molecule and the D(u,*) state of its ion. The great difference in the molecular orbitals participating in the electronic states of the neutral molecule and ion suggests great difference in their electronic structures and also in the vibronic coupling scheme. If the observed correspondence is not accidental, there must be a close relationship between the different molecular orbitals which are involved in the neutral molecule and its ion. It will be quite interesting to elucidate the origin of the similarity from theory. In the present paper, we showed a striking similarity in the vibronic coupling between each isomer of difluorobenzene and its ion. However, such a similarity seems to exist also for other molecules and their cations. Registry No. m-DFB, 65308-07-8; o-DFB, 65308-08-9.
A Discharge Flow-Photoionization Mass Spectrometric Study of Hydroxymethyl Radicals (H,COH and H,COD): Photoionization Spectrum and Ionization Energy W. Tao, R. B. Klemm,* Brookhaven National Laboratory, BIdg. 81 5, Upton, New York 1 1 973
F. L. Nesbitt, and L. J. Stief NASAjGoddard Space Flight Center, Laboratory for Extraterrestrial Physics, Greenbelt, Maryland 20771 (Received: July 31, 1991; In Final Form: September 6, 1991)
The photoionization spectrum of H2COH was measured over the wavelength range 140-170 nm by using a discharge flow-photoionization mass spectrometer apparatus with synchrotron radiation. Hydroxymethylradicals (H2COHand H2COD) were generated in a flow tube by the reaction of F atoms with CH3OH(D). Ionization energies (IE) were determined directly from photoion thresholds. The IE values, 7.56 f 0.02 and 7.55 f 0.02 eV for HzCOHand H2COD,respectively, are consistent with previous measurements. Also, the dissociative ionization process, presumed to be H3CO* HCO' + H2,was observed with a threshold at 8.61 f 0.06 eV.
-
Introduction The H~COHradical and its isomer, H~CO,are important intermediates in combustion and atmospheric processes while the radical ion, H~COH+,is important in interstellar molecule formation. In hydrocarbon combustion chemistry,IJ H$OH and H ~ C Oare formed in reactions such as O/OH + C H ~ O H+ H~COH+ O H / H ~ Oand c~~+ o2+ H ~ C O+ 0. ln the oxidation of hydrocarbons in polluted atmosphere^,^ H2COH is generated predominantly via the OH + C H ~ O Hreaction while
H 3 C 0 (+NO2) is the product of CH302+ NO. In cold interstellar clouds, ion-molecule reactions predominate; and it has been W3gestd4 that the reactions of c+or CHj+ with C H 3 0 H may yield H2COH' which, on dissociative electron recombination, forms H2CO. Although numerous studies have been performed to characterize these radicals, the information concerning ionization of H2COH and H 3 C 0is limited mainly to indirect studies.s of H2COHf,as derived from The heat of formation appearance potential studies5-10and the proton affinity5*"J2of
(1) Hoyermann, K.; Loftfield, N. S.; Sievent, R.; Wagner, H. Gg. Eighteenth Symposium (International) on Combusrion; The Combustion Institute: Pittsburgh, 1980; p 831 and references therein. ( 2 ) Warnatz, J. In Combustion Chemistry; Gardner, W. C., Jr., Ed.; Springer-Verlag: New York, 1984, and references therein. (3) (a) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Ken, J. A.; Troe, J. J . Phys. Chem. Ref.Data 1989, 18, 881 and references therein. (b) Neshitt, F. L.; Payne, W. A.; Stief, L. J. J . Phys. Chem. 1988, 92, 4030 and references therein.
(4) Huntress, W., personal communication. Cited in: Gottlieh, C. A.; Ball, J. A.; Gottlieb, E. W.; Dickinson, D. F. Astrophys. J . 1979, 227, 422. ( 5 ) Lias, S.G.; Bartmcss, J. E.; Liebman, J. F.; Holmes, J. L.; Lavin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17 (Suppl. No. 1). (6) Refaey, K. M. A,; Chupka, W. A. J . Chem. Phys. 1968,48, 5205 and references therein. (7) Haney, M.A.; Franklin, J. L. Trans. Faraday Soc. 1969.65, 1794 and references therein. (8) Losing, F. P. J . Am. Chem. Soe. 1977, 99, 7526. (9) Berkowitz, J. J . Chem. Phys. 1978, 69, 3044.
~
0022-3654/92/2096- 104$03.00/0 0 1 9 9 2 American Chemical Society
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The Journal of Physical Chemistry, Vol. 96, No. 1, 1992 105
Mass Spectrometric Study of H,COH(D) H,CO, is about 169 kcal mol-'. This value is consistent with an ionization energy (IE) of H2COH of about 7.5 f 0.1 eV. The IE of H,COH measured by Dyke et all3 in a direct photoelectron spectroscopy (PES) study (with HzCOH produced via reaction of F or C1 with CH30H) supports the indirect results: IE(H2COH) = 7.56 f 0.01 eV. Determinations of AHf(H3CO+)and IE(H3CO) from fragmentation studies appear to be susceptible to serious perturbations. For example, Haney and Franklin' attribute reports of AH(H3CO+)= 200-210 kcal mol-', from C H 3 0 H fragmentation, to excess energy (in electron impact studies) combined with rearrangement (isomerization) to H2COH+. By correcting for excess energy, they were able to reconcile their electron impact results with those from the photoionization study of Refaey and Chupkaa6 Similarly, Bouma et al.I4 have recently rationalized dissociative ionization of dimethyl ether in terms of rearrangement (via H atom transfer involving a barrier) followed by fragmentation into H2COH+(or HCO+ H,) CH3. This general problem seems to arise because, unlike H2COH+,there is no low-lying singlet state of H3CO+ that is bound.14-17 The triplet ground state, 3H3CO+,has a heat of formation that is reported's-20to be 245-250 kcal mol-', and thus IE(H3CO) 10.7 eV. On the other hand, Dykez' determined IE(H3CO) = 7.37 eV via PES with pyrolysis of dimethyl peroxide as the source of H3C0. This result is clearly inconsistent with the values just cited and indicates an error in Dyke's analysis. The H 3 C 0 precursor, CH300CH3,is notably unstable, and so this problem might be associated with reagent purity as well as hot band effects. The IE of H&OH has been determined in only one direct (PES) study,13and so this photoionization mass spectrometric study was undertaken as an important check. In the present work, hydroxymethyl (and methoxy) radicals were produced by the reaction of F atoms with methanol and methanol-OD in a flow tube. Subsequently, direct photoionization was accomplished with dispersed synchrotron radiation and detection of the mass-selected ions.
+
+
Experimental Section Experiments were performed by employing a discharge flowphotoionization mass spectrometer (DF-PIMS) apparatus at the beam line U-11 at the National Synchrotron Source (NSLS).22,z3 Briefly, H,COH(D)/H,CO radicals were generated by the reaction of F atoms with CH30H(D).
F + CH30H
-
-
H2COH
H3C0
+ HF
+ HF
(14 (1b)
Fluorine atoms were produced in a microwave discharge of CF4 in helium carrier gas. Methanol, CH30H(D), was introduced (10) Losing, F. P.; Holmes, J. L. J . Am. Chem. SOC.1984, 106, 6917. (11) Tanaka, K.;Mackay, G. 1.; Bohme, D. K. Can. J . Chem. 1978,56, 193. (12) From the measurement of Tanaka et al.," AH = 0.1 kcal mol-', for proton transfer from HCNH' to H2C0, we compute AI-If0298(H2COH+)= 168.8 kcal mol-' from:' AHf0298(H2CO)= -26.0 kcal mol-I; AHroH8(HCN) = 32.3 kcal mol-', and AHf0298(HCNH+) = 227 kcal mol-l (this last value is also discussed in ref 22). (13) Dyke, J. M.; Ellis,A. R.; Jonathan, N.; Keddar, N.; Moms, A. Chem. Phys. Lert. 1984, 111, 201. (14) Bouma, W. J.; N o h , R. H.; Radom, L. Org. MassSpectrom. 1982, 17, -_-. 114
_,
(15) Schleyer, P. V. R.; Jemmis, E. D.; Pople, J. A. J . Chem. Soc., Chem. Commun. 1978, 190. (16) Dewar, M. J. S.; Rzepa, H. S. J. Am. Chem. SOC.1977, 99, 7432. (17) Dewar, M. J. S. Faraday Soc. Discuss. 1976, 61, 197. (18) Burgers, P. C.; Holmes, J. L. Org. Mass Specrrom. 1984, 19, 452. (19) Wodtke, A. M.; Hiutsa, E. J.; Lee, Y . T. J . Chem. Phys. 1986,84, 1044.
(20) Ferguson, E. E.; Roncin, J.; Bonazzola, L.Inr. J. Mass Spectrom. Ion Processes 1987, 79, 215. (21) Dyke, J. M. J . Chem. SOC.,Faraday Trans. 2 1987, 83, 69. (22) Nesbitt, F. L.; Marston, G.;Stief, L. J.; Wickramaaratchi, M. A.; Tao, W.; Klemm,R. B. J . Phys. Chem. 1991, 95, 7613. (23) Klemm, R. 8.; Wickramaaratchi, M. A.; Gleason, J. F. J . Phys. Chem., in press.
1
WAVELENGTH, nm
Figure 1. Photoionization efficiency curve for CH30H (ion counts/light signal vs A); 110 nm IX I 120 nm. Arrow indicates the onset of ionization at 114.3 nm (10.85 i 0.03 eV).
into the flow tube through a movable injector, and the distance from the tip of the injector to the nozzle was typically 4-5 cm. The methanol concentration in the flow tube ((3-5) X 1013 molecules ~ m - was ~ ) in relatively large excess over the F atom concentration (about 5 X lo1, atoms ~ m - as ~ , determined by A[CH30H] measurements). With this [CH30H]/[F] excess and a collision frequency rate constant,' reaction l a (and lb) was complete within 0.5-1.0 cm of the tip of the injector probe, and thus secondary reactions should have been minimized. The Pyrex flow tube was coated with Teflon in order to minimize the loss of radicals on the wall.24 The experiments were conducted at ambient temperature (298 f 2 K), flow velocities were about lo00 cm s-I, and the flow tube pressure was maintained at about 2 Torr. The gaseous mixture in the flow tube was samples as a molecular beam that was produced by expansion (1-mm nozzle, 1.5-mm ski"er/collimator). The source chamber and detection chamber were maintained at about 1 X lo4 and 5 X lv Torr, respectively. Ions were detected with a quadrupole mass filter that was aligned axially with the molecular beam. Measurements of the photoionization spectra were carried out using tunable vacuum-ultraviolet (vacuum-UV) radiation at the NSLS. A monochromator with a normal incidence grating (1200 lines/") was used to disperse the vacuum-UV light, and a LiF fiter (A 1 105 run) was used to eliminate second- and higher-order radiati~n.,~The monochromator slit width was 750 pm, and the resulting spectral bandwidth (fwhm) was about 3 A. No corrections were made to the measured threshold values for the slit function of the monochromator. Methanol (CH30H, Mallinckrodt, analytical reagent, 99.9% purity; CH30D, Fluka A. G., puriss. grade, >99.9% D) was purified via freeze-pump-thaw cycles. Methanol vapor (vapor pressure 4 0 Torr at ambient temperature) was diluted with helium to about 0.05 mole fraction, and the mixture was stored in a 2-L bulb. Helium (MG Industries, 99.9999%) and CF, (MG Industries, 99.95%) were used directly from cylinders without further purification.
Results and Discussion As an example of the quality of data that can be obtained in this PIMS experiment, the photoionization threshold region for methyl alcohol precursor, 110 nm I X I120 nm, is shown in Figure 1. The indicated threshold at 114.3 f 0.3 nm corresponds to an ionization energy of 10.85 f 0.03 eV, which is in excellent agreement with previously reported (photoionization) v a l ~ e s . ~ , ~ - ~ ~ This level of agreement implies that extensive rotational cooling25 was achieved in the nozzle expansion, and thus the photoion threshold is not perturbed, within experimental uncertainty (f0.03 eV), by thermal effects. In the threshold region, photoion sig(24) (a) Sridharan, U. C.; Reimann, B.; Kaufman, F. J. Chem. Phys. 1980, 73, 1286. (b) Lee, J. H.; Tang, I. N . J . Chem. Phys. 1982, 77,4459. (c) Grotheer, H.-H.; Nesbitt, F. L.; Klemm,R. B. J . Phys. Chem. 1986,90, 2512. (25) Grover, J. R.; Walters, E. A,; Newman, J. K.; White, M. C. J. Am. Chem. Soc. 1985, 107, 7329 and references therein. (26) Watanabe, K. J . Chem. Phys. 1957, 26, 542.
Tao et al.
106 The Journal of Physical Chemistry, Vol. 96, No. 1, 1992 I
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WAVELENGTH, nm
WAVELENGTH, nm Figure 2. Photoionization efficiency curve for H2COH (ion counts X 1000/light signal vs A); 140 nm 5 X I 170 nm. Idealized vibrational progression (vI) depicted by the dotted lines. Primary (vi) and secondary (ve) progressions are taken from Dyke et al.” (see text). 2.61
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WAVELENGTH, nm Figure 3. Photoionization threshold region for H2COH (ion counts X 1000/light signal vs A). Arrow indicates the onset of ionization at 164.0 nm (7.56 f 0.02 eV).
nal-to-background is more than 20:1, and the threshold appears to be resolution limited (-0.3 nm fwhm). The small “tail” that extends toward longer wavelengths is probably due mainly to a nonideal slit function but may include a small contribution from residual internal energy. The reaction of F atoms with CH30H produces H 3 C 0 radicals along with H2COH with a branching ratio that is reported to be approximately stati~tical,’,~’ kl,/klb = 3. Although H2COH is thermodynamically more stable than H3co,28-3’the relatively large barrier (30-40 kcal mol-’) for isomerization, H2COH H3C0,30‘32effectively precludes this process at 300 K. We would therefore expect that [H3CO] = 1/3[H2COH]in the flow tube. The photoionization spectrum of HzCOH is shown in Figure 2 as a plot of the photoion yield (ion signal/light signal, arbitrary units) versus wavelength. This figure covers the wavelength region 140-170 nm at 0.5-nmintervals, and it shows a feature of vibrational progressions as the photon energy increases. According to Dyke et al.,I3 the primary vibrational feature corresponds to C-0 stretching in H2COH+in its singlet ground state (X’A,) with a vibrational frequency, vI, of 1650 f 30 cm-’. Thus, the assignment is readily achieved, as depicted in Figure 2. A secondary vibrational series, vII (attributed to CH2 deformation in
-
(27) Dill, B.; Heydtmann, H. Chem. Phys. 1980, 54, 9. (28) Wendt, H. R.; Hunziker, H. E . J . Chem. Phys. 1979, 71, 5202. (29) Radford, H. E. Chem. Phys. Letf. 1980, 71, 195. (30) Batt, L.; Burrows, J. P.; Robinson, G. N. Chem. Phys. Left. 1981,8, 467 and references therein. (31) Saebo, S.; Radom, L.; Schaefer, H. F. J . Chem. Phys. 1983, 78,845. (32) Colwell, S. M. Mol. Phys. 1984, 51, 1217.
Figure 4. Photoionization threshold region for H2COD(ion counts X 1000/light signal vs A). Arrow indicates the onset of ionization at 164.2 nm (7.55 0.02 eV).
H2COH+by Dyke et al.,I3with a frequency of 1370 cm-l), appears (in Figure 2) to perturb the steplie primary series except at vII(2). To obtain the ionization energy, a detailed examination near threshold was carried out, and the results are plotted in Figure 3. The spectrum was obtained in the wavelength region of 162-167 nm at 0.1-nm intervals. The detection sensitivity was optimized by collecting a large number of X scans to improve the signal-to-background ratio. The sharp threshold (seen in Figure 3) was analyzed by applying least-squares analysis to the linear ascending portion of the curve, and the straight line was extrapolated to the background signal level to obtain the ionization energy. The value for IE(H2COH) obtained in this way is 164.0 f 0.3 nm (7.56 f 0.02 eV), as indicated by an arrow in Figure 3. Similarly, the ionization energy for H2COD, 164.2 f 0.3 nm (7.55 f 0.02 eV), was also obtained, as shown in Figure 4. It is important to note that this measurement confirms the structure of the radical with m / z = 32. The quoted uncertainties are primarily due to the instrumental resolution. The threshold values were obtained without correcting for the slit function of the monochromator since this effect should be included in the combined experimental/analytical uncertainty. The ionization thresholds reported in this work should be free of perturbations due to hot bands because vibrational excitation would be expected to residue primarily (if not exclusively) in the H F product molecules. Also, the presence of methoxy radicals should not have perturbed the ionization of H2COH in this study because IE(CH30) = 10.7 eV. The ionization energies for H2COHand H2CODreported here appear to be the adiabatic values for these radicals on the basis of the excellent agreement with the PES measurements of Dyke et al.I3v2l(7.56 f 0.01 eV for H2COH and 7.55 f 0.01 eV for H2COD). Also, as noted above, pronounced steplike features were observed in the photoionization spectrum of H2COH+(Figure 2) that corroborate the vibrational assignment determined from the PES study.I3 Since the completion of this work, a PIMS study has been reported by Ruscic and B e r k ~ w i t in z ~which ~ D2COHand D3C0 were produced by reaction of F atoms with CD30H (Le., reactions la and lb). Their measured ionization energy for hydroxymethyl, IE(D2COH) = 7.540 f 0.006 eV, agrees well with the PES study of Dyke.21 However, their photoionization spectrum for D2COH+ (Figure 1 of ref 33) does not display the steplike detail that is evident in the present work, Figure 2, for H2COH’. This difference might be related to improved signal-to-background achieved with the high light intensity available at the NSLS. In addition, Ruscic and B e r k o ~ i t determined z~~ the IE for D3C0 (33) (a) Ruscic, B.; Berkowitz, J. J . Chem. Phys. 1991, 95, 4033. (b) Ruscic, B.; Bcrkowitz, J. Photoionization Mass Spectrometric Studies of the Isomeric Transient Species CD,OH and CD30. Presented at the ACS/Fourth Chemical Congress of North American, New York, Aug 1991.
The Journal of Physical Chemistry, Vol. 96, No. I, 1992 107
Mass Spectrometric Study of H2COH(D)
-.,,
(285)
_____*______
/-
/'
H,CO+ (n=m+
\
//
248
\
\
',\ 1
--
201 HCOt(+H2)
\
170 -
1
-4
-
Figure 5. Energy level diagram for the H2COH/H3C0system; see text for discussion and references. Energies are indicated in kcal mol-I. The n = 1 level of the H3C0 Rydberg state was estimated42 as 0.751E(3H3CO). Estimated barriers to isomerization of H3CO+ H2COH+ and dissociation of H2COH+ HCO+ H2 are indicated by dashed lines. The "barrier" to isomerization of 'H3CO+ 'H2COH+is proposed to be at the excited singlet level of H3CO+(- 115 kcal mol-' above IH2COH+).'' The barrier to dissociation of H2COH+(+ HCO+ H2)
-
+
-
-
+
is taken to be the 'activation energy" for this process minus the excess energy in the products (80 kcal mol-l - 30 kcal
at 10.726 f 0.008 eV which confirms the indirect measurements of AH(H3CO+)'8-20at about 248 kcal mol-' (see below). Although H3CO+was not observed directly in the present study, HCO+ was detected, with a threshold at 144 f 1 nm (8.61 f 0.06 eV), that was presumably produced via predissociation of H3CO*, Le., from a Rydberg state. The observed HCO+ would not be expected to be produced from H2COH+dissociation (+ HCO' H,) because of a large barrier for this 1,2-elimination proFor a barrier of about 50 kcal mol-I, the threshold to dissociation of H2COH+to form HCO+ (+H2) would be at X < 128 nm (>9.7 eV). The observed photoion threshold for HCO+ (+H,), from H3CO*,is essentially at the thermodynamic threshold for this process (see below). This result is consistent with numerous appearance potential studies5-I0in which H2COH+and HCO+ have been observed, but H3CO+has not been observed below the triplet-state energy level.
+
(34) Williams, D. H.; Hvistendahl, G. J . Am. Chem. Soc. 1974, 96, 6753. (35) Bowen, R.D.; Williams, D. H. J. Chem. Soc., Chem. Commun. 1977, 378. (36) Richard, G. J.; Cole, N. W.; Christie, J. R.;Derrick, P. J. J . Am. Chem. SOC.1978, 100, 2904. (37) Dill, J. D.; Fischer, L. L.; McCafferty, F. W. J . Am. Chem. SOC. 1979, 101, 6531.
A proposed energy level diagram is presented in Figure 5 that summarized what has been discussed thus far along with other pertinent thermodynamic data. Heats of formation for the species involved are as follows: L W ~ ~ ~ ~ ~ ( H = ~C -4 OfH2)kcal mol-', B e n ~ o nas~ reported ~ by Batt et AHf0298(H3CO) = 1f 2 kcal mol-I, Engelking et al.39(this is consistent with the energy difference, between H3C0and H,COH, of 5 kcal mol-l calculated by Saebo et AiYfo298(H2COH+) = 170 f 2 kcal mol-', taking IE(H,COH) = 7.56 eV and Mf0298(H2COH)= -4 kcal mol-'; AHf0298(3H3CO+) = 248 kcal mol-I, taking IE(H3CO) = 10.73 eV, Ruscic and B e r k ~ w i t zand , ~ ~ Mf0298(H3CO)= I kcal mol-'; AHf0298(HCO+)= 201 f 2 kcal mol-I, taking IE(HC0) = 8.27 eV, Dyke et a1.,40and AiYf0298(HCO)= 10.0 kcal mol-I, Chuang et al.'" This energy level diagram43differs little with the essential points proposed by Burgers and Holmes.I8 We have omitted the low-lying singlet energy levells for H3CO+and instead suggest that H3CO* predissociates to form HCO+ (+H,) rather than isomerizes to form H,COH+. Also, we propose a "barrier" to triplet H3CO+isomerization to singlet H2COH+that lies at the excited singlet of H3CO+. In conclusion, we report values for the ionization energies of H2COH and HzCOD (7.56 f 0.02 and 7.55 f 0.02 eV, respectively) that were determined directly from photoion thresholds. These results confirm the values reported by Dyke et al.13 (with PES), and they agree, as well, with the recent results of Ruscic and B e r k ~ w i t z(with ~ ~ PIMS). The observation of HCO+ (+H,) with an onset at about 8.6 eV suggests its formation via predissociation of H3CO*.
Note Added in proof: A recent paper by Bogan et al." provides a current evaluation of the branching ratio for reactions la and lb. The value reported,& kl,/klb = 1.5, is considerably smaller than that given ear1ier.l~~~ Nevertheless, this new branching ratio value has no direct effect on the present spectroscopic study of HZCOH(D). Acknowledgment. We thank Drs. B. Ruscic and J. Berkowitz for communicating their results prior to publication. The work at BNL was supported by the Chemical Sciences Division, Office of Basic Energy Sciences, U.S.Department of Energy, under Contract DE-AC02-76CH00016. The work at GSFC was supported by the NASA Upper Atmosphere Research Program. Registry NO. HZCOH, 2597-43-5; HZCOD, 58456-46-5. (38) Benson, S . W. In Thermochemical Kinetics, 2nd ed.; Wiley: New York. 1976. ..~ (39) Engelking, P. C.; Ellison, G. B.; Lineberger, W. C. J. Chem. Phys. 1978,69, 1826. (40) Dyke, J . M.; Jonathan. N. B. H.; Morris, A,; Winter, M. J. Mol. Phys. 1980, 39, 629. (41) Chuang, M. C.; Foltz, M. F.; Moore, C. B. J . Chem. P h p . 1987,87, 3855. (42) Berkowitz, J. In Phoioabsorption, Photoionization and Photoelectron Spectroscopy; Academic Press: New York, 1979. (43) The stationary electron conventionSis used here along with the assumption that the heat capacities of neutrals and ions change by about the same amount with temperat~re;~ thus, Afff0298(X+)= AHf0298(X)+ IE(X). Errors that result from this assumption are generally small. In addition to Lias et also see: Lias, S.G.; Ausloos, P. J . Am. Chem. SOC.1978, 100, 6027. Traeger, J. C.; McLaughlin, R. G. J. Am. Chem. Soc. 1981,103,3647. (44) Bogan, D. J.; Kaufman, M.; Hand, C. W.; Sanders, W. A.; Brauer, B. E. J. Phys. Chem. 1990, 94, 8128. ~
.-
