J . Phys. Chem. 1994, 98, 5644-5649
5644
Structures, Relative Stabilities, and Spectra of Isomers of HClOz Joseph S. Francisco' and Stanley P. Sander Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91 109 Timothy J. Lee NASA, Ames Research Center, Moeffet Field, California 94035 Alistair P. Rendell SERC Daresbury Laboratory, Warrington WA4 4AD, U.K. Received: September 13, 1993; I n Final Form: February 3, 1994"
Ab initio molecular orbital calculations have been carried out to determine the relative stabilities, and the spectroscopy of isomers of HC102. Two straight-chain isomers, HOOCl and HOClO, and one branched species, HC102, have been identified as energy minima on the HClO2 potential energy surface. The highest level of calculation performed ( C C S D ( T ) / A N 0 4 ) suggests that HOOCl is the lowest energy form. The branched structure is 51.0 kcal mol-' higher in energy. The HOClO is 8.3 kcal mol-' higher in energy compared to the lowest energy structure. Intense infrared absorptions for HOOCl are calculated at 1399 and 392 cm-l, a t 1186 and 249 cm-I for HOClO, and 2168 and 1009 cm-' for HC102. The heat of formation of HOOCl calculated using perturbation theory is similar to that determined using the CCSD(T) level 'of theory.
Introduction
C10
The role of chlorine-containing species in stratospheric processes has been actively investigated since 1974 when Rowland and Molina drew attention to the potential impact of chlorofluorocarbons on stratospheric ozone.' Since that time, the major focus of attention regarding stratospheric chlorine has been on the ozonedepleting catalytic cycle involving atomic chlorine and chlorine monoxide (ClO), namely
-c1+ -
c1+0, c10 + 0, c10 + 0 net 0
+ 0,
0,
20,
(1) (2) (3)
Atmospheric modeling studies have demonstrated the critical importance of processes that control the partitioning between C1 and C10 (ClO, species).2 The partitioning of C10, species is controlled in part by the reaction with other species such as H O and HO2 (HO, species) or N O and NO2 (NO, species). The first experimental measurement of the Cl HO2 reaction was made by Leu and DeMore3 using a flow discharge mass spectrometric method at room temperature. Lee and Howard4 pointed out that there are two channels that the C1+ HO2 reaction proceeds along,
+
C1+ H 0 2
-.
-
HO
+ C10
HC1+ 0,
(4) (5)
Using a discharge flow system for infrared laser magnetic resonance detection of H02, HO, and C10, Lee and Howard4 determined the branching ratio for these reactions. For the reaction of H O radicals with C10 radicals, there have been several detailed experimental studies. The two product channels that have been studied are Abstract published in Aduance ACS Abstracts, May 1, 1994.
0022-3654/94/2098-5644%04.50/0
+ HO
-.
-
+ C1
(6)
HC1+ 0,
(7)
HO,
Barrows et al.5 have suggested that an HOOCl intermediate may be involved in the C10 + H O reaction system. Lee and Howard4 also suggested the involvement of an HOOCl complex in the C1 + HO2 reaction. This was based on the thermochemical analysis of Benson and co-workers.6 Benson reasoned that the strong attraction of the chlorine to the terminal oxygen could only suggest that a HOOCl complex is formed. Presently, there are no experimental studies which have isolated the HOOCl intermediate. The only theoretical study of the HOOCl intermediate is the work of Lee and Rendell? who examined the stability of the HOOCl intermediate relative to dissociation. A question we raised is whether there are other stable forms of the HOOC1. It is possible that HOOCl or one of its isomers may be an important chlorine reservoir species in the stratosphere. Consequently, in the present work, ab initio molecular orbital calculations are used to examine the minimum energy structural forms for HClOz species. The heat of formation is also examined at several levels of theory. To aid in the spectroscopic characterizations, harmonic vibrational frequencies and infrared intensities are presented along with the vertical electronic excitation energies for the HClO2 species.
Computational Methods Ab initio molecular orbital calculations were performed using the GAUSSIAN 88 and 92s and the TITAN9 systems of programs. Geometries were fully optimized at the second-order M~ller-Plesset perturbation level of theory,'o with analytical gradients," using 6-31G(d)I2 and 6-31 lG(d,p)" basis sets. A second set of polarization functions supplements the latter basis set to comprise the 6-31 1G(2d,2p) basis set. The equilibrium geometries were also determined with the TZ2P basis set using the CCSD(T) level of theory14 (singles and doubles coupledcluster theory plus a perturbational estimate of the effects of connected triple excitations using analytical gradient meth0 d s ~ ~ J 6The ) . TZ2P basis set was described in detail previo~s1y.l~ Energies were also evaluated at the CCSD(T) level of theory 0 1994 American Chemical Society
Isomers of HClOz
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5645
TABLE 1: Equilibrium Geometries for HClO2 Isomers' coorMP2/ MP2/ MP2/ CCSD(T)/ molecule dinate 6-31G(d) 6-31lG(d,p) 6-311G(2d,2p) TZ2P 0.965 HOOC1, HO 0.980 0.966 0.968b 1.423 skew 00 1.408 1.443 1.437 1.751 1.750 c10 1.739 1.746 101.3 100.7 HOO 100.6 100.6 109.2 108.1 ClOO 108.1 108.6 89.7 88.2 HOOCl 90.2 89.1 0.965 HOOCI, HO 0.98 1 0.966 0.968 1.462 1.475 trans 00 1.488 1.493 1.706 1.705 CIO 1.701 1.703 96.7 96.6 HOO 96.3 97.0 103.8 104.4 ClOO 103.8 104.8 180.0 180.0 180.0 IHOOC1 180.0 HOOCI, HO 0.980 0.966 0.965 0.967 cis 1.451 1.465 00 1.478 1.486 1.722 1.720 c10 1.717 1.718 103.2 102.4 HOO 103.3 102.0 108.6 108.9 109.8 ClOO 109.6 0.0 HOOCl 0.0 0.0 0.0 0.966 HOClO HO 0.983 0.968 0.968 OCI 1.754 1.774 1.752 1.755 C10' 1.513 1.500 1.503 1.536 101.9 HOC1 104.1 102.9 102.3 115.3 112.5 114.6 OC10' 115.6 74.3 78.7 HOCIO' 75.7 79.1 1.336 HC102 HCI 1.353 1.344 1.351 1.472 CIO 1.488 1.480 1.497 101.3 100.9 101.4 HCIO 101.5 117.3 118.1 116.5 OClO 118.7 a Bond distances are in angstroms and bond angles in degrees. Taken from ref 7.
using atomic natural orbital (ANO) basis sets.18 The Gaussian exponents and contraction coefficients for the A N 0 basis sets were also described in detail in ref 17. The A N 0 1 basis set consists of 5s4p2d, 4s3p2d, and 4s2p ANOs on the C1, 0, and H atoms, respectively, while the A N 0 2 basis set consists of 5s4p2dlf, 4s3p2d,lf, and 4s2plfANOs. The A N 0 3 basis set consists of 6s5p3d2f and 4s3p2d ANOs, and the A N 0 4 basis set consists of 6s5p3d2flg, 5s4p3d2flg, and 4s3p2dlfANOs. Extended electron correlation was calculated with fourth-order Merller-Plesset perturbation theory19 in the space of single, double, triple, and quadruple excitations using the optimized geometries obtained at the MP2 level. The accuracy of the ab initio calculation of the energies decreases in the order CCSD(T) > MP4 > MP2. However, in the present work we use the less accurate methods to examine their usefulness in predicting properties of the HC102 potential energy surface that can lead to the experimental characterization of HClO2 species. Harmonic vibrational frequencies and intensities were calculated with the MP2 and CCSD(T) methods using finite differences of analytical gradients.11,1s,16,20 For the estimation of the vertical energies of low-lying excited electronic states, the singles configuration interaction method is used.*I
Results and Discussion Structural and Energetic Properties of the HClOl Species. Equilibrium Geometryfor HOOCl. The ground-state geometry for HOOCl is given in Table 1. The equilibrium conformation of this molecule has not been determined experimentally. However, from the computations, the minimum energy structure is skewed (Figure la,b). The HOOCl dihedral angle a t the MP2/ 6-31 1G(2d,2p) level of theory is predicted to be 88.1'. Lee and Rendell'obtained a similar result (89.1 ") at theCCSD(T)/TZZP level of theory. At the MP2/6-3 11G(2d, 2p) level, the HOO and ClOO angles are 100.7' (100.6') and 108.1' (108.6'), which are also similar to the CCSD(T)/TZ2P results. [The numbers in parentheses are the CCSD(T)TZ2P results.] The calculated C10 bond length is usually more difficult to describe at the MP2
1
0
1
1.408 1.423' .
3
1.751 p 1.750'
100.7'
II
l n \
88.2'
89'7
Figure 1. (a) Minimum energy structure of HOOCl [MP2/6-311G(d,p)-optimizedgeometry, no asterisk; MP2/6-3 1 lG(2d,2p)-optimized, asterisk; see Table 1 for complete list of geometrical parameters]. (b) View of the minimum energy structure of HOOCl along the 00 axis
[MP2/6-31 lG(d,p)-optimizeddihedralangle,noasterisk;MP2/6-311G(2d,2p)-optimized, asterisk]. 1.774 1.752'
1.500 1.503'
0.968 0.968'
II
A
74.3 78.7'
Figure 2. (a) Minimum energy structure of HOClO [MP2/6-311G(d,p)-optimized geometry, no asterisk; MP2/6-3 1 lG(2d,2p)-optimized,
asterisk; see Table 1 for complete list of geometrical parameters]. (b) View of the minimum energy structure of HOClO along the C10 axis [MP2/6-3 1 lG(d,p)-optimizeddihedral angle, noasterisk; MP2/6-311G(2d,2p)-optimized, asterisk]. level of theory. However, a comparison with the CCSD(T) value shows that MP2 performs reasonably well in this case. The geometry trends predicted for the cis and trans structures parallel those of the skewed structure both at the MP2 and CCSD(T) levels of theory. The significant changes in the geometrical parameters upon rotation occur in the C10 and 00 bond lengths. In general, the C10 bond decreases from the skew configuration to the cis and trans configurations. The 00 bond length, on the other hand, is smaller in the skew configuration. The calculated barriers for hindered rotation are given in Table 2. The addition of diffuse functions and the inclusion of higher order correlation a t the MP4SDTQ level decreases the cis barrier from 7.5 to 6.0
5646 The Journal of Physical Chemistry, Vol. 98, No. 22, 1994
n 1.344
W
W
Figure 3. Minimum energy structure of HClOz [MP2/6-311G(d,p)optimized geometry, no asterisk; MP2/6-311G(2d,Zp)-optimized, asterisk: see Table 1 for complete list of geometrical parameters].
kcal mol-'. However, there is no significant increase in the trans barrier with basis set size and correlation. At the MP4SDTQ/ 6-31 1++G(3df, 3pd)//MP2/6-311G(2d, 2p) level the trans barrier is 3.7 kcal mol-'. A similar trend can be seen in the CCSD(T) results. The cis barrier decreases from 5.5 to 5.3 kcal mol-1 from the TZ2P to A N 0 2 basis sets. A decrease of 0.3 kcal mol-' in the trans barrier occurs on going from the TZ2P to A N 0 2 basis set. Structure and Energetics of Isomers of HOOC1. There are three isomeric forms of HClO2: (1) HOOC1, (2) HOC10, and (3) HC102. The second isomeric form, HOClO, was first calculated by McGrath et a1.2a using the MP2 level of theory was a 6-3 1G(d) basis set. We have reexamined this structure using 6-31 lG(d,p) and the 6-311G(2d, 2p) basis sets. At the MP2/ 6-3 1G(d) level, our calculated structure agrees with that reported by McGrath et a1.22 However, significant structural changes occur as a result of increasing basis set size. The H O bond distance decreases by 0.017 A, and the HOC1 angle decreases by 2.2' on going from the 6-31G(d) basis to the 6-31 1G(2d,2p) basis. The CCSD(T)/TZ2P and MP2/6-3 11G(2d,2p) results for these parameters are in good agreement. However, the difference between the MP2 and CCSD(T) C10' bond distances and the OClO' bond angles is more significant. The MP2 level of theory predicts a bond that is shorter than CCSD(T) by 0.033 A, and an angle that is larger than CCSD(T) by 2.1'. The third isomeric form is of C,symmetry; it has an HCl bond length of 1.35 1 %, and a C10 bond length of 1.497 A. The C10 bond in HClO2 is perhaps the shortest of the isomeric forms. This is due to the C1=0 multiple bonding characteristics of HC102, which do not occur for HOOCl and only occur for the terminal oxygen of HOC10. The C1-0 bond distance in HClO2 is shorter than the terminal C1-0 bond distance in HOClO, probably because of resonance effects (in HClO2), and is similar to the situation for C1C102.17 In our computational exploration of isomeric forms of HC102, an OOHCl structure was found on the MP2 surface. At the MP2/6-311G(2d,2p) level of theory, the 00 bond length was found to be 1.323 A. The H O and C10 bond lengths were 0.981 and 2.378 A, respectively. The C100, HO0,andHOClangleswere 118.0, 105.4,and71.8°,respectively. The unusually long C1-0 bond length suggested that this species may be a weakly bound complex which lies ca. 20.0 kcal mol-l above the HOOCl structure. However, further exploration revealed that this structure may be an artifact of the MP2 surface, since no true minimum structure of this form was found on the CCSD(T) surface. Calculated relative energies (see Table 2) for the three stable HC102 isomers show that the lowest energy structure is the HOOCl straight-chain structure. The next lowest energy structure is the HOClO isomer, which is not surprising since the H-0 bond energy is usually quite large. The highest energy isomer is HClO2. The relative energetic stability of HOClO and HC102 are sensitive to basis set size, which is due to the valence expansion at the chlorine center; Le., polarization functions are more important than for HOOC1. For HOClO, with the 6-31G(d) basis set, HOClO is 20.9 kcal mol-' higher in energy than HOOCl (MP2 level of theory), but this decreases to 5.5 kcal mol-' using the 6-31 1G(2d,2p) basis set. A similar basis set dependence occurs at the CCSD(T) level; the difference between
Francisco et al. TZ2Pand thelargeAN02 basisset is 5.3 kcal mol-I. The relative energies for HClOz a t the MP2/6-311G(2d,2p) and CCSD(T)/ TZ2P levels are in reasonable agreement (only differing by 4 kcal mol-'). We have also calculated CCSD(T) energies with the A N 0 3 and A N 0 4 basis sets to examine the convergence of the relative energetics of the three isomers with basis set and electron correlation. According to the series extrapolation, the basis set limit result should be obtained by subtracting the A N 0 2 - A N 0 4 difference from the A N 0 4 result. The uncertainity is obtained by halving this difference. For HClO2, our estimated basis set limit CCSD(T) energy difference is 47.5 f 1.8 kcal mol-1, and for the HOClO, the energy difference is 6.3 f 1 kcal mol-'. The CCSD(T) results should bevery close to the correlation and basis set limit. We do note that the relative energetic ordering of the three structures is not altered by the choices of correlation method and basis set. SpectroscopicCharacterizationofHa02Isomers. Vibrational Frequencies and Intensities. The calculated vibrational frequencies for the three isomeric forms of HClOz are provided in Table 3. Allvibrational frequencies presented in the table are calculated at the MP2/6-31G(d) and CCSD(T)/TZ2P levels of theory. To establish the degree of confidence in the various levels of calculation, we havecalculated thevibrational spectrum of HOOH at the MP2/6-3 1G(d) and CCSD(T)/TZZP levels and compared the results with experimental data. The harmonic frequencies for HOOH agree reasonably well: the vibrational frequencies are 5.0 and 3.8% (rms) in overestimation of the experimental frequencies for the MP2 and CCSD(T) levels of theory, respectively. In the prediction of the vibrational frequencies for HOOC1, the most intense bands are predicted to be u2 (1399 cm-I) and us (392 cm-l). The u2 mode, which is the HOO bend, is similar to the bend in HOOH. The torsion mode, Yg, is quite analogous to the torsional frequency of HOOH. This information should be useful in the assignment of the experimental spectrum of HOOC1. Moreover, it also points to some potential experimental problems that could obscure the assignment of HOOC1. If HOOH is used as a precursor for the production of HOOC1, the absorption bands for u2 and u4 of HOOH could overlap the most intense bands of HOOC1, thus obscuring its identification. In this case, the band which would allow the two species to be clearly distinguished is the u5 (633 cm-I) mode, which is the ClOO bend of HOOC1. This mode is, unfortunately, predicted to be a weak IR absorber. Intense bands for HOClO that are consistently predicted at the MP2 and CCSD(T) levels of theory are the torsion and the OC1 stretch modes, u4 and u5 (see Table 3). The modes that are most useful in experimentally distinguishing HOClO from HOOCl are u2, u3, and us. Low-Lying Excited Electronic States of He102 Isomers. Another spectroscopic property that is useful in the characterization of HClO2 isomers is the vertical excitation energies for each of the isomers. This information is important in the characterization of the electronic spectrum. Configuration interaction with all singly excited determinants, CIS, has been shown to be an effective method of surveying the excited states of closed-shell molecules with reasonable expense for polyatomic molecules.21 However, we note that the CIS results are qualitative, but nevertheless, it provides an avenue for identifying the most distinguishing features among the isomeric forms. Table 4 contains the calculated CIS vertical excitation energies and oscillator strengths U, for HC102 isomers obtained by using the 6-311G(2d, 2p) and 6-311++G(3df, 3pd) basis sets. The geometries used are the MP2/6-311G(2d, 2p) calculated structures. The energies were calculated with and without diffuse functions. Diffuse functions wereadded toobtain a morecomplete manifold of excited states for each molecule. Included in Table 4 are the first four singlet low-lying excited states for each isomer. Focusing on the excited states of HOOCl and HOClO isomers,
Isomers of HClOz
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5647
TABLE 2. Total and Relative Energies of HClO2 Isomers’ level of Theory
HOOCI, skew HOOC1. trans HOOCI, cis HOClO HClO, -610.115 03 (0) -610.019 37 (3.1) -610.103 02 (7.0) -610.081 04 (20.9) -609.984 87 (70.6) MP2/6-3 1G(d) MP2/6-31 lG(d,p) -610.299 10 (0) -610.292 83 (3.4) -610.286 76 (7.2) -610.259 27 (24.6) -610.16445 (83.4) MP2/6-31 lG(2d.2~) -610.379 13 (0) -610.372 19 (3.8) -160.368 96 (5.9) -610.354 28 (15.2) -610.274 82 (64.3) MP4/6-31 l++G(2df,Zp)//MP2/6-31 lG(2d,2p) -610.413 04 (0) -610.407 14 (3.2) -610.403 58 (5.4) -610.399 42 (8.1) -610.325 55 (53.8) MP4/6-31 l++G(3df,3pd)//MP2/6-31lG(2d,2~) -610.427 98 (0) -610.42200 (3.2) -610.418 40 (5.5) -610.418 61 (5.5) -610.352 17 (46.5) CCSD(T)/TZZP -160.350 22 (0)b -610.343 80 (3.6) -610.34068 (5.5) -610.324 72 (15.6) -610.239 42 (68.5) CCSD(T)/ANOl -610.372 68 (0) -610.366 13 (3.7) -610.362 79 (5.7) -610.350 79 (13.3) -610.271 35 (62.5) CCSD(T)/ANO2 -610.46000 (0) -610.453 97 (3.3) -610.450 82 (5.3) -610.443 00 (10.3) -610.371 48 (54.5) CCSD(T)/AN03 -610.487 75 (0) -610.481 83 (3.3) -610.478 89 (5.1) -610.473 64 (8.5) -610.403 43 (51.9) CCSD(T)/AN04 -610.511 19 (0) -610.505 18 (3.4) -610.502 32 (5.1) -610.497 33 (8.3) -610.428 28 (51.0) 0 Values in the parentheses are relative energies corrected for zero-point energy given in kcal mol-’. All other values are in hartrees. b Taken from ref 7. ~
~~~
TABLE 3: Harmonic Frequencies and Intensities of HClO2 Isomers
molecule HOOH
vibrational mode no. 1
2 3 4 5
6 HOOCl
1
2 3 4 5 6 HOClO
1
2 3 4 5 6 HClOz
1
2 3 4 5
6 0
approx vibrational description HO sym. stretch HOO sym. deformation 00 stretch torsion HO antisym. stretch HOO antisym. deformation HO stretch HOO bend 00 stretch ClOO bend torsion CIO stretch HO stretch HOC1 bend CIO’ stretch OCI stretch torsion OClO bend HCI stretch C102 sym. stretch HOC1 sym. bend OClO bend ClOz asym. stretch HOC1 asym. bend
exp‘ 3614 1393 863 3615 1269
intensities (kmmol-1) MP2/ CCSD(T)/ 6-31G(d) TZ2P 24 1 lb 0 0.2 2 1 236 175 89 52 119 103 37 38c 60 46 41 30 17 20 94 79 23 19 58 67 88 43 85 68 149 86 153 111 2 1 226 123 69 0.1 36 60 10 13 224 75 56 127
Fundamental frequencies taken from ref 17. Taken from ref 23. Taken from ref 7.
TABLE 4 ~
harmonic frequencies (cm-I) MP2/ CCSD(T)/ 6-31G(d) TZ2P 3738 3802b 1465 1428 929 882 338 377 3740 3798 1324 1310 3669 3744c 1418 1399 870 835 685 633 395 392 379 361 3654 3755 1206 1186 1172 935 584 540 422 393 302 249 2243 2168 1100 1022 1065 905 424 415 1227 1093 1101 1009
Calculated Vertical Excitation Energies and Oscillator Strengths
~~
AE (eV)‘ HClOz species state 6-311G(2d,2p) 6-31l++G(2df,2p) HOOCl lla 4.3 4.3 21a 5.5 5.5 31a 7.3 7.3 41a 1.5 7.5 HOClO lla 3.1 3.1 2la 3.8 3.8 3la 6.9 6.7 4la 7.9 7.9 HClOz 1la’ 6.5 6.4 1la” 7.6 7.5 21a‘ 7.8 7.6 21a” 8.3 8.2
f (rei)' 6-31l++G(3df,3pd) 4.3 5.5 7.3 7.5 3.2 3.9 6.7 7.9 6.4 7.5 7.6 8.2
6-311G(2d,2p) 6-31l++G(2df,2p) 0.0003 0.0004 0.0000 0.0001 0.0541 0.0568 0.0175 0.0166 0.0000 0.0001 0.0024 0.0027 0.0017 0.0015 0.0202 0.0342 0.0324 0.03 1 1 0.0013 0.0022 0.0463 0.0418 0.0273 0.0409
6-31l++G(3df,3pd) 0.0003 0.0001 0.0555 0.0157 0.0001 0.0027 0.0017 0.0353 0.0252 0.0021 0.04 1 1 0.0349
‘Obtained at the CIS level of theory using MP2/6-311G(2d,2p) geometries.
we are interested in knowing if the electronic spectra of these isomers are distinguishable. We find that the vertical excitation energies of the states of HOOCl are insensitive to the basis set used. This may be due to a deficiency in the CIS method, which neglects higher than single excitations beyond the single excitation in the configuration interactions. The 31a state of HOOCl at 7.3 eV (170 nm) has a relatively large oscillator strength and may correspond to the most intense peak in the optical spectrum of HOOC1. The lowest lying singlet state of HOOCl is the 1 ‘A state which occurs at 4.3 eV (289 nm). The oscillator strength is very low, which suggests that the absorption band attributed
to this state may appear weak in the optical spectrum. The first excited singlet state of HOClO appears at 3.1 eV (394 nm), but it has a small oscillator strength. The 2 ‘A state at 3.8 eV (326 nm) should appear stronger. Of the four low-lying singlet states, the most intense band that should appear in the optical spectrum should be the 4 la state appearing a t 7.9 eV (156 nm). Heats of Formation. To assess the stability of HOOC1, it is useful to know the heat of formation of this species. Isodesmic reactions, which have been used to obtain heats of formation for many molecules, are those in which the reactants and products contain the same types of bonds. Because of this property, errors
5648 The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 TABLE 5
Francisco et al.
Isodesmic Heats of Reaction and Heats of Formation for HOOCl (kcal mol-')
level of theory MP2/6-31G(d) MP2/6-31 lG(d,p) MP2/6-311G(2d,2p) MP4/6-311++G(2df,2p) MP4/6-31 l++G(3df,3pd) CCSD(T)/ANO2"
HOC1 -535.169 44 -535.303 18 -535.362 40 -535.379 31 -535.390 51 -535.413 45
total energies (hartress) HOOH HOH -151.13493 -76.199 24 -151.268 89 -76.282 89 -151.314 46 -76.307 44 -151.351 37 -76.329 29 -151.364 00 -76.338 65 -151.382 57 -76.346 58
HOOCl -610.115 03 -610.299 10 -610.379 13 -610.413 04 -610.427 98 -610.46000
relative energies (kcal mol-') Mo,.o(HOC1 + HOOH HOH + HOOCI) Mor.a(HOOC1) -7.0 1.8 -7.0 1.8 -6.9 1.9 -8.1 0.7 -8.3 0.5 -7.4 1.6
-.
Taken from ref 7. in the energy due to defects in the basis set and electron correlation cancel to a large extent. The isodesmic scheme used here is HOOCl HOH HOOH HOCl. In the calculation of the heat of formation of HOOCl from the isodesmic scheme, literature values of the heats of formation for theother species in theequation are used. The values of AHof,olisted in the JANAF table23 are -57.10f0.01 forHOHand-31.02f0.00forHOOH,allinkcal mol-'. A recent theoretical study by Francisco and Sanderz4 supports the JANAF-derived A@o (HOCl) of -17.1 f 0.5 kcal mol.-l Vibrational frequencies and zero-point energies calculated at the MP2/6-3 l G * level of theory are used. The heat of reaction for the isodesmic reaction scheme is insensitive to both basis set and correlation effects, as shown in Table 5. The difference between MP2 and MP4 results, with variation in basis set size, is less than 1.4 kcal mol-]. We havecalculated the heat of reaction using the QCISD(T)/6-31G(d) level of theory. The results are not consistent with those obtained using the MP2 method. Lee and Rendell' using the CCSD(T)/TZ2P level of theory found A@,o (HOOCl) to be 1.5 kcal mol-'. Our MP2/6-311G(2d,2p) calculations, which are comparable to the CCSD(T)/TZ2P calculations, results in a heat of formation of 1.9 kcal mol-'. This is consistent with Lee and Rendell's result at that level. It is also interesting to note that Colussi and Grela, using bond additivity arguments, derived a heat of formation value for HOOCl of 1.O kcal mol-l at 298 K. This is consistent with the present result and that of Lee and Rendell. We note that the MP4/6-3 1I++G(3df,3pd) level of theory underestimates the heat of formation of HOOCl relative to the CCSD(T)/AN02 calculation by 1.1 kcal mol-I. This results from an overcorrection of the energy for HOOCl in the isodesmic scheme. The heats of formation of HOClO and HC102 can be obtained by adding to the AH& of HOOCl our best estimates for AEl and AEz, at 0 K. Performing this step, we obtain 11.9 and 56.1 kcal mol-' for the heats of formation of HOClO and HC102, respectively. The heats of formation at 0 K for HOOH, HOOC1, and ClOOCl are -3 1.O, -1.6, and +34.1 kcal mol-], respectively. This suggests that successive chlorination of the hydrogen peroxide destabilizes the peroxidestructure. Anexamination of the stability of HOOCl with respect to dissociation into HO + C10, HOz C1, and H C102 also shows that it is stable to thermal decomposition. Moreover, these results suggest that HOOCl should be more stable than ClOOCl, which has been isolable. AtmosphericImplications. Reactions 4-7 play important roles in stratospheric chemistry in the coupling between the HO, and C10, families. If isomers of HC102 were to form by recombination reactions, e.g.
-
+
+
+
+
OH+ClO+M~HClO,+M
(8)
+M
(9)
C1+ HO,
+M
-
HC10,
or by heterogeneous reactions occurring on the surface of polar stratospheric clouds or sulfuric acid aerosols, then the partitioning of species within the C10, family might be affected, particularly in the lower stratosphere. The most significant impact would occur if the species produced in reactions 8 and 9 were kinetically
and photochemically inert. Stratospheric models have shown that a 5% branching for HCl formation from the C10 O H reaction has a major effect on the net production rate of HC1. A reasonable estimate for kg is 1 X le3'cm6 molecule-2 s-1. Formation of stabilized HOOCl by reaction 8 may therefore proceed at about 5% of the total rate of the OH + C10 reaction since kb is about 2 X 10-l' cm3 molecule-I s-l at 220 K. By the same reasoning, HOOCl formation by reaction 9 reduces the net HCl production by reaction 5 by the same fraction. The effect of HOOCl formation in the daytime is expected to be minor, however, due to rapid removal by photolysis, HOOCl hv -* OH C 1 0 (10)
+
+
-
+
+
HO, C1 (11) While the absorption spectrum and photolysis pathways for HOOCl are unknown, the lifetime with respect to photolysis is expected to be between those of HOOH and ClOOCl, which are about a day and a few minutes, respectively. Under these conditions, the daytime equilibrium concentrations of HOOCl would be toosmall to influence thepartitioningof ClO,. Removal of HOOCl may also occur by reaction with C1 and OH, i.e. OH HOOCl H,O ClOO (12)
+
-
+ H C l + ClOO
C1+ HOOCl (13) Neither of these reactions can compete with photolysis but may be important in other ways. For example, reaction 12 results in the termination of HO,. The possibility remains, however, that the formation of HOOCl during the onset of nighttime, via reactions 8 and 9, may be important. Given the thermal stability of HOOCI, it is very likely that at least some will form in the stratosphere, but additional work will need to be done in order to determine whether these concentrations are indeed significant. Conclusions The equilibrium structures, vibrational and electronic spectra, relative energetics, and heats of formation of HOOC1, HOC10, and HC102 isomers have been investigated with the MP2 and CCSD(T) ab initio electronic structure methods. The lowest energy isomer is the branched-chain HOOCl skew structure. Its heat of formation is estimated as 1.6 kcal mol-', while that for HOClO and HClOz are 1 1.9 and 56.2 kcal mol-', respectively. Distinguishing spectroscopic features of the HClOl isomers have been identified, which should allow these species to be identified experimentally.
Acknowledgment. Support for this computing research was provided by the JPL Supercomputing Project. The JPL Supercomputing Project is sponsored by JPL and the NASA office of Space Science and Applications. The research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract to the National Aeronautics and Space Administration. References and Notes (1) Rowland, F. S.; Molina, M. J. Rev. Geophys. Space Phys. 1975.13, 1.
( 2 ) Froidevaux, L.; Allen, M.; Yung, Y. L. J . Geophys. Res. 1985, 90, 12999. ( 3 ) Leu, M.-T.; DeMore,
W.B. Chem. Phys. Lett. 1976, 41, 121.
Isomers of HClOz (4) Lee, Y.-P,; Howard, C. J. J . Chem. Phys. 1982, 77, 756. Thrush, B. A.; Wilkinson, (5) Burrows, J. P.; Cliff, D. I.; Harris, G. W.; J. P. T. Proc. R. Soc. London A 1979,368,463. ( 6 ) Wissman, M.; Shum, L. G. S.;Heneghan, S.P.; Benson, S.W. J . Phys. Chem. 1981,85,2864. (7) Lee, T. J.; Rendell, A. P. J . Phys. Chem. 1993, 97, 6999. (8) GAUSSIAN 88; Frisch, M.J.; Head-Gordon, M.;Schlegel, H. E.; Raghavachari, K.; Binkley, J. S.;Gonzalez, C.; DeFrees, D. J.; Fox, D. J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.; Baler, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Fluder, E. M.; Topiol, S.;Pople, J. A. Gaussian, Inc.; Pittsburgh, PA, 1988. (b) GAUSSIAN 92, Revision A,; Frisch, M.J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M.W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.;Gonzalez, C.; Martin, R. L.; Fox, D. J.; DeFrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian, Inc.; Pittsburgh, PA, 1992. (9) TITAN is a set of electronic structure programs written by T. J. Lee, A. P. Rendell, and J. E. Rice. (10) (a) Pople, J. A,; Binkley, J. S.;Seeger, R. Int. J . Quantum Chem. Symp. 1976, SIO,1. (b) Pople, J. A. In Geometrical Derivatives ofEnergy Surfaces and Molecular Properties; Jorgensen, P., Simons, J., Eds.; Reidel: Dordrecht, The Netherlands, 1986; p 109.
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5649 (11) Schlegel, H. B. J . Comput. Chem. 1982, 3, 214. (12) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2251. (13) Krishnan, R.; Binkley, J. S.;Seeger,R.; Pople, J. A. J . Chem. Phys. 1980, 72, 650. (14) Raghavachari, K.; Trucks, G. W.; Pople, J. A,; Head-Gordon, M. Chem. Phys. Lett. 1989, 157,479. (15) Rendell, A. P.; Lee, T. J. J. C k m . Phys. 1991, 94, 6219. (16) Lee, T. J.; Rendell, A. P. J . Chem. Phys. 1991, 94, 6229. (17) Lee, T. J.; Rohlfing, C. M.; Rice, J. E. J . Chem. Phys. 1992,97,6593. (18) Almlof, J.; Taylor, P. R. J. Chem. Phys. 1987, 86, 4570. (19) Schlegel, H. B. J . Chem. Phys. 1%6,84,4530. (20) Pople, J. A.; Krishnan, R.; Schlegel, H. B.; Binkley, J. S. Int. J. Quantum Chem. Symp. 1979,513, 225. (21) Foresman, J. B.; Head-Gordon, M.; Pople, J. A.; Frisch, M. J. J. Phys. Chem. 1992, 96, 135. (22) McGrath, M. P.; Clemitshaw, K. C.; Rowland, F. S.; Hehre, W. J. J . Phys. Chem. 1990.94, 6126. (23) JANAFThermochemical Tables, 3rd ed.;Chase, M. W., Jr., Davies, C. A., Downey, J. R., Frurip, D. J., McDonald, R. A., Syverud, A. N., Eds.; J . Phys. Chem. Ref. Data, Suppl. 1985, 14. (24) Francisco, J. S.;Sanders, S. P. J . Chem. Phys. 1993, 99, 6219.
