J. Phys. Chem. 1993,97, 6999-7002
6999
Ab Initio Characterization of C100H: Implications for Atmospheric Chemistry Timothy J. Lee' NASA Ames Research Center, Moffett Field, California 94035- 1000
Alistair P. Rendell SERC Daresbury Laboratory, Warrington WA4 4AD, U.K. Received: March 18, 1993
The equilibrium structure, dipole moment, harmonic vibrational frequencies, and infrared intensities of ClOOH are determined using the CCSD(T) (singles and doubles coupled-cluster theory plus a perturbational estimate of the effects of connected triple excitations) electronic structure method in conjunction with a T Z 2 P (triple { plus double polarization) basis set. The heat of formation of ClOOH is determined (using two different isodesmic reactions) to be +1.5 f 1 kcal/mol a t 0 K or +0.2 f 1 kcal/mol a t 298.15 K. Using the computed heat of formation, we examined the stability of ClOOH with respect to the C10 OH, ClOO H, and H O O C1 dissociation limits. Since ClOOH is found to be quite stable, it is argued that the chemistry of ClOOH should be included in any accurate modeling of the stratosphere.
+
+
Introduction There has been considerable interest over the last 20 years in examining the chemical reactions that take place in the stratosphere, particularly those involving chlorine species as chlorine has been shown to be involved in the catalytic destruction of ozone.14 Since the total amount of chlorine in the stratosphere will continue to rise for several years to come, it is important to have a detailed understanding of stratospheric chlorine chemistry. As part of these investigations,there have been several st~dies5-~ that have focused on the products of the O H + C10 reaction. C10 and OH are important minor constituents of the stratosphere.1° Two product channels have been studied OH OH
+ C10 + C10
--.
C1+ OOH HCl + 0,
(a) (b) It has been well established that the major reaction product is C1 + OOH, Le., reaction a. Burrows et aL7 have discussed the possible intermediates in reactions a and b and have suggested a C100H* species as a common intermediate for both reactions. However, considering that all of the species involved in reactions a and b are in their ground electronic states, it is important to note that reaction a can proceed along a singlet or triplet potential energy surface (PES) while reaction b must proceed along the triplet PES, provided that spin-orbit and nonadiabatic couplings are neglected. Moreover, since ClOOH is most likely a stable peroxide with a singlet ground state, it would seem that the triplet pathway would be inherently higher in energy, which is consistent with the much smaller rate constant for reaction b relative to reaction a. Burrows et al. suggest that although reaction b is much more exothermic than reaction a, it is hindered due to a potential barrier-possibly the rearrangement of triplet C100H* or OClOH*-which would explain why reaction b has such a small absolute rate con~tant.S-~Thus, to summarize, it seems most likely that reaction a will proceed along the singlet pathway, and that the explanation of Burrows et al. for the small rate constant of reaction b is plausible. To our knowledge, there has not been any high-level ab initio investigations of ClOOH reported in the literature. Moreover, we are not aware of any experimental information for C100H. Thus in the present investigation we examine the ClOOH molecule in its 'A ground electronic state, determining its equilibrium geometrical structure, dipole moment, harmonic vibrational frequencies,and infrared (IR) intensities. The heat of formation
+
of ClOOH is also accurately determined, and from these data the possible importance of ClOOH in stratospheric chemistry is discussed. It is shown that ClOOH is the most likely intermediate species involved in reaction a. It is also shown that ClOOH is thermally quite stable, much more so than ClOOC1, and it is therefore argued that ClOOH is potentially an important chlorine reservoir species, particularly during the Antarctic and Arctic night when ultraviolet radiation from the sun is at a minimum. Computational Methods The equilibrium geometry of ClOOH was determined with a TZ2P basis set at the CCSD(T) level of theory11 (singles and doubles coupled-cluster theory plus a perturbational estimate of the effects of connected triple excitations). The TZZP basis set consists of Dunning'slz [ 5s3p/3s] contraction of Huzinaga's13 (1Os6p/5s) primitive Gaussian functionsfor 0 and H, respectively, augmented with two sets of polarization functions14(ad = 2.314 and 0.645 for 0; ap = 1.407 and 0.388 for H). The C1 TZZP basis set is composed of McLean and Chandler's15 [6s5p] contracted functions supplemented with two sets of polarization functions (ad = 1.072 and 0.357). A previous study16 has shown that little is gained by further uncontraction of the sp basis set. For the TZ2P basis set, all six Cartesian components of the d functions were included in the basis set. Coupled-cluster analytical gradient methods17J8were used to locate the equilibrium structure. Quadratic force constants, harmonic frequencies, and IR intensities were determined by finite differencesof analytical gradients. The dipole moment was determined as the derivativeof the energy with respect to an external electric field. In order to determine accurately the heat of formation of ClOOH, two isodesmic reactions have been used (these will be given later) and the reaction energies have been evaluated at the MP2 (second-order Merller-Plesset perturbation theory), CCSD, and CCSD(T) levelsof theory using atomic naturalorbital (ANO) basis sets.lg The H and 0 A N 0 basis sets are those of AlmlBf and Taylor,19 while the C1 A N 0 basis set is taken from Bauschlicher and Roos.*O For C1 the density matrices of the neutral atom and the negative ion were averaged. The primitive basis sets are van Duijneveldt1szl(13s8p/8s) sets augmented with an even tempered sequence of (6d4f16p4d) polarization functions for 0 and H, respectively. The polarization function orbital exponents are obtained from a = 2.5"ao;n = 0, ..., k with = 0.13 and 0.39 for the 0 d and f functions, respectively; a0 = 0.10 and 0.26 for the H p and d functions. The C1 primitive basis set
0022-365419312097-6999%04.00/0 0 1993 American Chemical Society
Lee and Rendell
7000 The Journal of Physical Chemistry, Vol. 97, No. 27, 1993
(a),
TABLE I: Total Energy Equilibrium Structure, Rotational Constants (MHz), Dipole Moment (D), and Harmonic Frequencies (cm-l) of CIOOH, Obtained at the CCSD(T)/TZZP Level of Theory. 3744 wl;0-H str E 0.350217 1399 w2; OOH bend 1.74 P TIb
roo rclo ron LClOO LOOH 7e A
B C
0.016 1.443 1.746 0.968 108.6 100.6 89.1 47516 6020 5440
w3; 0-0 str w4; C1-0 str w5; torsion w6;
II I2
I3 14
15 16
cloo bend
835 633 392 361 38.2 45.9 30.0 20.0 78.8 19.3
The energy is reported as -(E + 610). Bond lengths rare in A and angles L are in degrees. IR absorptionintensitiesZare in km/mol. See ref 28 for a description of the TIdiagnostic. The torsional angle of ClOOH in degrees. is Partridge's22 (19s14p) set augmented with a (5d4f) set of polarization functions with (YO = 0.06 and 0.19 for the C1 d and f functions. The basis set denoted A N 0 1 consists of 5s4p2d, 4s3p2d, and 4s2p ANOs on C1, 0, and H , respectively. The A N 0 2 basis set is composed of 5s4p2dlf, 4s3p2dlf, and 4s2pld ANOs on C1,0, and H. For the A N 0 basis sets, only the spherical harmonic components of the d- and f-type functions were included. The coupled-cluster geometry optimization was performed with the TITAN23 program system. The MP2 and coupled-cluster single-point energies were performed with the parallel TITAN coupled-cluster interfaced to the SEWARDZ7 integral program. These latter calculations were performed on Intel Gamma hypercubes located at Daresbury Laboratory and at the NAS Facility of the NASA Ames Research Center. Results and Discussion A. Equilibrium Structure and Vibrational Frequencies. The equilibrium geometry, dipole moment, rotational constants, harmonic vibrational frequencies, and infrared intensities of ClOOH are listed in Table I. The equilibrium geometry of ClOOH is close to that expected based on examination of the CCSD(T)/TZ2P geometries of ClOOCl and HOOH.16 That is, the C1-0 and 0-H bond distances are similar to the values obtained for the ClOOCl and HOOH species, respectively, while the ClOO and OOH angles are also similar to thevalues obtained for the ClOOCl and HOOH molecules. The other two geometrical parameters, the 0-0bond distance and the torsional angle, are somewhat different from the values in ClOOCl and HOOH. The 0-0bond distance for ClOOH is intermediate between the ClOOCl(l.411 A) and HOOH (1.474 A) values, but the ClOOH torsional angle is much closer to the value in ClOOCl (84.7O) than to the value in HOOH (114.1'). Thus the torsional angle of ClOOH is perhaps the one geometrical parameter that would have been difficult to predict a priori. It is expected that the theoretical geometry prediction for ClOOH will exhibit similar errors to those found for ClOOCl and HOOH (at the CCSD(T)/TZ2P level of theory) in ref 16. The dipole moment of ClOOH is very similar to the CCSD(T)/TZZPvalue for HOOH (1.77 D), but much larger than the value for ClOOCl (0.76 D), while the ClOOH T I diagnostic28 value is intermediate between the ClOOCl and HOOH values.16 It should be noted that the ClOOH T Idiagnostic value (0.016) indicates that the CCSD(T) electron correlation method should perform very well for this molecule. The harmonic vibrational frequencies of ClOOH follow similar trends, in comparison to the CCSD(T)/TZZPvalues for ClOOCl and HOOH, as found for the molecular geometry. That is, the 0-H stretch and OOH bend harmonic frequencies are similar to the analogous quantities in the HOOH species while the C1-0
TABLE 11: Quadratic Force Constants of ClOOH Obtained at the CCSD(T)/TZZP Level of Theow. . ClOOH I -
fll fi2 f22 fi3
f23 f33
h4 f24
f34 f 4 4
f15
3.7870 0.7283 2.3593 -0.0227 -0,0189 7.8208 0.3556 0.1339 -0.0117 1.3221 0.5453
0.0135 -0.0352 0.0151 0.9887 0.0041 -0.0226 -0.0172 -0.0298 -0.0362 0.0750
f25 f35
f45 f55 fi6 f26 A 6 f46 f56 f66
Units are aJ/A2, aJ/A.rad, and aJ/rad2. See text for definition of the internal coordinates. stretch and ClOO bend frequencies are similar to the analogous quantities in the ClOOCl species. Similarly, the ClOOH 0-0 stretch harmonic frequency is intermediate between the ClOOCl and HOOH values. The one exception again is the torsional mode. While the torsional angle is much closer to the value in ClOOCl, the torsional frequency is much closer to the value in HOOH. The fundamental vibrational frequencies for ClOOH may be estimated by taking the difference between theory (harmonic) and experiment (fundamental) for ClOOCl and HOOH for the appropriate modes and then adjusting the ClOOH harmonic frequencies. For the 0-0 stretch we have taken the average between the ClOOCl and HOOH corrections. Performing this procedure yields 3559, 1361, 818, and 640 for V I , v2, v3, and v4, respectively (for modes that have symmetric and antisymmetric parts in ClOOCl and HOOH, we have taken the average of these two). The lack of experimental values for the ClOO bends of ClOOCl precludes such an estimate for Vg. However, it is expected that this procedure is mainly correcting for anharmonicity effects, which should be small for the ClOO bend, and therefore the harmonic frequency given in Table I should be close to the actual fundamental frequency. It is more difficult to assess the anharmonic correction for the torsional mode because it is probably small', for ClOOCl, while it is fairly substantia129(=80 cm-l) for HOOH. Since the torsional harmonic frequency for ClOOH is more similar to that of HOOH than to that of ClOOCl, it may be that the anharmonic correction follows a similar trend, but a full anharmonic analysis is needed to be certain. Since the CCSD(T)/TZ2P quadratic force field is expected to be quite reliable, the CCSD(T)/TZZP quadratic force constants for ClOOH are presented in Table I1 in simple internal coordinates. These internal coordinates are given by
S, = LClOO, S , = LOOH, S, = 7 where T is the ClOOH torsional angle. It is hoped that the data in Tables I and I1 will aid in the experimental observation and characterization of C100H. B. Heat of Formation and Dissociation Energies. In order to assess the possible importance of ClOOH in atmospheric chemistry it is important to know the stability of ClOOH with respect to possible dissociation pathways. The first step in this assessment is to determine the heat of formation of C100H. This is easily done using a combination of experimental and theoretical information. It is well-known that it is relatively easy to determine the reaction energies of isodesmic reactions using ab initio quantum mechanical methods. The key is to set up an isodesmic reaction involving ClOOH in which the heats of formation of all the other species involved in the reaction are known experimentally. Two possible isodesmic reactions present themselves, HOC1 + HOOH
-
ClOOH + H,O
+ AE,
(1)
Ab Znitio Characterization of ClOOH
The Journal of Physical Chemistry, Vol. 97, No. 27, 1993 7001
TABLE 111: Energies (kcal/mol) for Reactions 1 and 2 (See Text). CCSD(T)/TZ2P Geometries Were Used' AE1
AE2
MP2/ANO1 MP2/AN02
-7.1 -7.0
-11.1 -10.9
CCSD/ANOl CCSD/ANO2 CCSD(T)/TZZP CCSD(T)/ANOl CCSD(T)/ANO2
-5.7 -5.5 -6.6 -6.8 -6.6
-10.4 -10.2 -1 1.6 -11.6 -11.4
ClOOH ClOOH ClOOH
a Zero-point vibrational energies not included-see text for energy differences where these are included.
C120 + 2 H O O H
-
2C100H
+ H,O + AE2
and experimental data used to determine the heat of formation of ClOOH are given in Table IV. It is also of interest to determine the stability of ClOOH with respect to the following dissociation channels
(2)
The experimental heats of formation of HOCl, HOOH, and H 2 0 are taken from ref 30, while the heat of formation of ClzO is taken from ref 31 (note that the value given in ref 30 is not reliable). The species HOC1, HOOH, HzO, and C120 have all been studied at the CCSD(T)/TZZP level of theory previously.l6~32 Thus using the CCSD(T)/TZ2P geometries and computing MP2, CCSD, and CCSD(T) energies with the A N 0 1 and A N 0 2 basis sets, we have determined values for MI and AE2, which are presented in Table 111. Note that the quantities in Table I11 do not include zero-point vibrational energy (ZPVE) effects-these will be discussed below. The values reported in Table I11 show that the AE1 and AEz reaction energies are fairly insensitive to improvements in both the one-particle and n-particle treatments. As indicated above, this insensitivity is quite common for isodesmic reactions and is due to a balanced cancellation of errors. Taking -6.6 and -1 1.4 kcal/mol as our best estimates of the electronic contribution and correcting these for ZPVE effects (using CCSD(T)/TZ2P harmonic frequencies), we obtain -7.4 and -1 2.2 kcal/mol as our best estimates for AEl and AEz at 0 K. Combining these with the experimental AH? values,30Jl we obtain AHP(C100H) = +1.6 and +1.4 kcal/mol from reactions 1 and 2, respectively. The excellent agreement between the two values indicates the internal consistency of the chlorine oxide thermochemistry. (It is interesting to note that if the MP2/AN02 MIand A E 2 values had been used rather than the CCSD(T)/ANO2 quantities, the difference in the AHfo(C1OOH)value determined from reactions 1 and 2 would be 0.7 kcal/mol, indicating that even when using isodesmic reactions it is desirable to use the highest level of theory possible.) We take +1.5 kcal/mol as the best estimate of the heat of formation of ClOOH at 0 K. It is difficult to place an uncertainty on this value because the largest source of error is probably due to the experimental heats of formation of HOCl and C120. However, it is expected that our estimate of A H r O (C100H) should be accurate to within f 1 kcal/mol. Correcting the AEI and AE2 values for temperature effects to 298.1 5 K, and using the experimental AH?gs.15values, we obtain +0.2 f 1 kcal/ mol for AHf298.l5(ClOOH). For completeness, the raw theoretical
-
-
C10
+ OH + AE3
(3)
+ AE,, ClOO + H + AE, C1+ OOH
(4)
(5)
Accurate heats of formation are known for C10, OH, C1, OOH, and H , (C10 from ref 33; OOH from ref 34; and C1, OH, and H from ref 30) while the value for ClOO is less certain.33 Nonetheless, using these experimental AH?98J5 quantities together with the computed AHH298.15 value for ClOOH we obtain 33.4, 31.4, and 73.4 kcal/mol for AE3, A E 4 , and A E 3 , respectively. Thus it is evident that ClOOH is quite stable to thermal dissociation. (Note that since we have used heats of formation to determine the reaction energies, vibrational effects are automatically included.) In order to assess the probability that ClOOH will be formed in the stratosphere, it is necessary to have some knowledge of the concentrations of the species in reactions 3-5. Some limited data are available in ref 10, which suggest that the reverse of reaction 3 would be the most likely source of ClOOH (the concentration of H and C1 atoms is fairly low in the lower stratosphere). If C10 and OH react to form ClOOH, then clearly ClOOH will be in a highly excited ro-vibrational state-with the vibrational energy probably concentrated in the 0-0 stretch. Thus the formation of ClOOH will depend on how efficiently this internal energy is transferred to the C1-0 stretch and ultimately lost to the C1 OOH product channel (reaction a) or whether the vibrationally hot ClOOH will survive long enough to have some of the internal energy dissipated by collisions with other molecules (i.e., species with vibrational degrees of freedom). As only 2 2 kcal/mol of internal energy must be lost to make the C1 OOH product channel inaccessible, it seems likely that some ClOOH will survive. Note that 1 quanta of the 0-0 stretch for small u is more than 2 kcal/mol, although it is not known how close the 0-0 energy levels become near dissociation. Thus it would seem that accurate models of the stratosphere should take into account the presence and chemical reactions of ClOOH, especially in models of the nighttime chemically perturbed region of the Antarctic ozone hole where radiation from the Sun is at a minimum. This is due to the possibility that, similar to ClOOCl, ClOOH may readily photodissociate. However, since the UV absorption cross sections of HOOH are considerably smaller than those for ClOOC1,IO the UV spectrum and transition probabilities of ClOOH need to be determined in order to assess fully its stability with respect to the Sun's radiation. The existence of other singlet isomers of chemical formula ClOOH has been briefly considered. The equilibrium geometry
+
+
TABLE IV: Theoretical and Experimental Data Used To Determine the Heat of Formation of C100H' method MP2/ANOl MP2/AN02 CCSD/ANOl CCSD/ANOZ CCSD(T)/TZ2P CCSD(T) / A N 0 1 CCSD(T)/ANO2 ZPVEb AHf O
AJp98.15
ClOOH -610.329 -610.407 -610.345 -610.427 -610.350 -610.372 -610.459 10.527
342 16 615 97 420 26 384 31 216 81 679 04 997 91
HOOH -1 5 1.309 658 40 -151.355 565 50 -151.316 330 89 -151.362 814 08 -151.318 749 55 -151.333 917 16 -151.382 566 26 16.519 -129.808 -136.106
HOC1 -535.317 -535.373 -535.334 -535.394 -535.333 -535.349 -535.413 8.263 -71.5 -74.5
610 20 608 36 567 64 188 98 481 38 661 62 448 57
H20
-76.309 203 36 -76.332 690 20 -76.314 573 85 -76.338 410 19 -76.312 457 89 -76.321 701 68 -76.346 575 01 13.514 -238.921 -241.826
c120 -994.330 -994.419 -994.356 -994.451 -994.356 -994.380 -994.483 2.257 83.2 81.4
921 19 346 50 169 56 314 65 91 1 28 781 65 292 16
a All electronic energies (&) determined at the CCSD(T)/TZZP equilibrium geometries. Experimental heats of formation (kJ/mol) taken from ref 30 for HOCI, HOOH, and H20; ref 31 for C120. A conversion factor of 4.184 kJ/kcal was used. Zero-point vibrational energies using the CCSD(T)/TZZP harmonic frequencies (kcal/mol).
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Lee and Rendell
The Journal of Physical Chemistry, Vol. 97, No. 27, 1993
of HOClO has been determined at the CCSD(T)/TZZP level of theory, and HOClO has been found to be 16.0 kcal/mol higher in energy than ClOOH (not including zero-point vibrational energies, which should not contribute very much). Since higherangular momentum polarization functions will preferentially favor HOClO, where there is valence expansion at the chlorine center, this energy difference has also been determined at the CCSD(T)/ A N 0 2 level of theory. This value, 10.7 kcal/mol, shows that ClOOH is indeed much more stable than HOClO, as would be expected from Lewis dot structure arguments. A more detailed investigation of the various minima and transition states on the ClOOH singlet potential energy surface will be presented at a later date. Conclusions The equilibrium structure, dipole moment, vibrational spectrum, and IR intensities of ClOOH have been determined using the CCSD(T) electronic structure method in conjunction with a TZ2P basis set. The heat of formation of ClOOH has been determined to be 1S f 1 kcal/mol at 0 K or +0.2 f 1 kcal/mol at 298.15 K. The computed AH?98,15 value has been used to show that ClOOH is thermally quite stable, with the lowest reaction channel, C1 OOH, being endothermic by 3 1.4 kcal/ mol and the second lowest, C10 + OH, being endothermic by 33.4 kcal/mol. Based on the thermal stability of ClOOH and the likelihood of its formation in the stratosphere, especially in the Antarctic or Arctic nighttime, it is argued that ClOOH is a potentially important reservoir for chlorine. It is therefore hoped that the ab initio predictions presented herein will encourage and simplify the laboratory observation and characterization of ClOOH, so that its presence and concentration in the stratosphere may be determined. The importance of ClOOH in stratospheric chemistry may then be more fully assessed.
+
+
Acknowledgment. T.J.L. thanks the NAS Facility at NASA Ames Research Center for access to the 128 node Intel i860 hypercube. The authors also would like to thank NATO for the award of a Collaborative Research Grant. References and Notes (1) Molina, L. T.; Molina, M. J. J . Phys. Chem. 1987, 91, 433.
(2) Molina, M.J.; Tso, T.-L.; Molina, L. T.; Wang, F. C.-Y. Science 1987, 238, 1253. (3) Barrett. J. W.: Solomon, P. M.: de Zafra. R. L.: Jaramillo. M.: Emmons, L.; Parrish, A. Nature 1988, 336, 455. (4) Rowland, F. S.Annu. Rev. Phys. Chem. 1991,42,731. (5) Leu, M. T.; Lin, C. L. Geophys. Res. Lett. 1979, 6, 425. (6) Ravishankara, A. R.; Eisele, F. L.; Wine, P. H. J . Chem. Phys. 1983, 78, 1140. (7) Burrows, J. P.; WaUington,T. J.; Wayne, R. P.J. Chem.Soc.,Faraday Trans. 2 1984,80, 957. (8) Hills, A. J.; Howard, C. J. J . Chem. Phys. 1984, 81, 4458. (9) Poulet, G.; Laverdet, G.; Le Bras, G. J . Phys. Chem. 1986,90,159. (10) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampon, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina, M. J. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling, Evaluation Number 10, August 15,1992; NASA JPL Publication 92-20; Jet Propulsion Laboratory: Pasadena, CA, 1992. (1 1) Raghavachari, K.; Trucks, G. W.; Pople, J. A.; Head-Gordon, M. Chem. Phys. Lett. 1989, 157,479. (12) Dunning, T. H. J . Chem. Phys. 1971, 55, 716. (13) Huzinaga, S.J . Chem. Phys. 1965,42, 1293. (14) Dunning, T. H. J . Chem. Phys. 1989, 90, 1007. (15) McLean, A. D.; Chandler, G. J . Chem. Phys. 1980, 72, 5639. (16) Lee, T. J.; Rohlfing, C. M.; Rice, J. E.J. Chem. Phys. 1992,97,6593. (17) Rendell, A. P.; Lee, T. J. J. Chem. Phys. 1991,94,6219. (18) Lee, T. J.; Rendell, A. P. J. Chem. Phys. 1991, 94, 6229. (19) Alml&f,J.; Taylor, P. R. J. Chem. Phys. 1987, 86, 4070. (20) Bauschlicher, C. W.; Roos, B. 0. J . Chem. Phys. 1989, 91, 4785. (21) van Duijneveldt, F. B. IBM J. Res. Deu. 1971, 945. (22) Partridge, H. J . Chem. Phys. 1987,87,6643. (23) TITAN is a set of electronic structure programs written by T. J. Lee, A. P. Rendell, and J. E. Rice. (24) Rendell, A. P.; Lee, T. J.; Komornicki, A,; Wilson, S . Theor. Chim. Acta 1993, 84, 271. (25) Rendell, A. P.; Lee, T. J.; Lindh, R. Chem. Phys. Lett. 1992,194, 84. (26) Rendell, A. P.; Guest, M. F.; Kendall, R. A. J . Comput. Chem., submitted for publication. (27) Lindh, R.; Ryu, U.;Liu, B. J . Chem. Phys. 1991, 95, 5889. (28) Lee, T. J.; Taylor, P. R. Int. J . Quantum Chem., Symp. 1989, 23, 199. (29) Willetts, A.; Gaw, J. F.; Handy, N. C.; Carter, S.J . Mol. Spectrosc. 1989,135, 370. (30) Chase,M. W.;Davies,C. A.; Downey, J.R.:Frurip, D. J.;McDonald, R. A.; Syverud, A. N. J . Phys. Chem. Re$ Data 1985, 14 (Suppl. 1). (31) Alqasmi, R.; Knauth, H.-D.; Rohlack, D. Ber. Bunsen-Ges. Phys. Chem. 1978,82, 217. (32) Lee, T. J.; Rice, J. E. J . Phys. Chem., in press. (33) Abramowitz, S.;Chase, M. W. Pure Appl. Chem. 1991.63, 1449. (34) Bauschlicher, C. W.; Partridge, H. Chem. Phys. Lett., submitted for publication. '
