Ab initio MRD-CI study of excited states of formyl chloride HClCO and photofragmentation along Cl–C cleavage

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Chemical Physics 343 (2008) 311–318 www.elsevier.com/locate/chemphys

Ab initio MRD-CI study of excited states of formyl chloride HClCO and photofragmentation along Cl–C cleavage Max Mu¨hlha¨user *, Margret Gruber-Stadler Studiengang Umwelt-, Verfahrens- und Biotechnik, MCI – Management Center Innsbruck Internationale Fachhochschulgesellschaft mbH, Egger-Lienz-Straße 120, A-6020 Innsbruck, Austria Received 3 April 2007; accepted 25 October 2007 Available online 4 November 2007 Dedicated to the 70th anniversary of S.D. Peyerimhoff.

Abstract Large-scale multi-reference configuration interaction (MRD-CI) calculations are carried out for ground and excited states of formyl chloride HClCO to investigate photofragmentation processes relevant to atmospheric chemistry. Four transitions at 6.89 eV (21A 0 ), 8.06 eV (31A 0 ), 8.51 eV (41A 0 ) and 8.80 eV (51A 0 ) are computed with considerable oscillator strengths. The strongest transition (51A 0 ) corresponds with both r ! r* (C–Cl) and p ! p* (CO). The states 31A 0 and 41A 0 correspond to r ! r* (C–Cl). The three lowest singlet 1 0 A and 1A00 states are found to be repulsive upon elongation of the Cl–C bond.  2007 Published by Elsevier B.V. Keywords: Spectroscopy; Excited states; Theoretical; MRCI

1. Introduction The reduction of ozone in the Antarctic atmosphere during the austral spring is strongly related to catalytic reactions involving chlorine substituted hydrocarbons [1–3]. Among the halogens chlorine compounds are believed to be especially effective in promoting ozone loss in the lower stratosphere, and there is a great interest in species which can act as chlorine reservoirs [3–11]. Formyl chloride (HClCO) is an intermediate in the atmospheric degradation of several important chlorinated hydrocarbons, e.g., CH3Cl, CH2Cl2, and C2HCl3 [12–17]. Several theoretical and experimental studies examine the kinetic and photochemical behavior of formyl chloride. Libuda et al. [18] measured the UV absorption cross sections and rate constants for the reactions of formyl chloride with Cl and OH. Anand and Schlegel [19] studied the unimolec*

Corresponding author. Tel.: +43 (0) 51220703210; fax: +43 (0) 51220703299. E-mail address: [email protected] (M. Mu¨hlha¨user). 0301-0104/$ - see front matter  2007 Published by Elsevier B.V. doi:10.1016/j.chemphys.2007.10.026

ular dissociation of formyl halides (X = F, Cl) by an ab initio direct classical trajectory study. Francisco et al. [20] carried out ab initio studies of dissociation pathways on the ground state potential energy surface for HFCO and HClCO whereas Fang and Liu [21] investigated dissociation pathways on the ground- (S0) and the first two low lying excited-states (S1 and T1) of formyl chloride employing ab initio calculations. While all these studies concentrate on the state S0, S1 and T1 of HClCO the present work is focused on the calculation of the electronic spectrum, which requests the calculation of many electronically excited states and the probability of populating these states. Therefore multi-reference configuration interaction MRD-CI calculations are carried out to compute potential energy curves for Cl–C elongation leading to the fragments HCO (2A 0 ) and Cl (2P). The photochemistry of formyl chloride might play an important role for the description of the cycle of chlorine radicals in atmospheric chemistry since the formation of HClCO during polar night could lead to a deceleration of the ozone degradation whereas chlorine re-liberation via

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photofragmentation due to ultraviolet sun light during the day is expected. It is therefore the aim of the present study to investigate this photofragmentation reaction. Quantum chemical calculations are an almost ideal tool to investigate such gas-phase reactions, to characterize short lived intermediates and to detect possible competitive reaction routes. After a brief summary describing the computational techniques used in Section 2, we will present in Section 3 the results obtained and discuss the electronic spectrum of formyl chloride in comparison to formaldehyde, a species which has been well examined theoretically [22–30] and experimentally [31–46]. Furthermore in Section 3 we will discuss the behavior of the lowest six singlet states of HClCO upon Cl–C bond breaking. Finally in Section 4 some conclusions will be summarized to evaluate the significance of photofragmentation of formyl chloride in atmospheric gas-phase reactions. 2. Computational techniques The equilibrium geometry of formyl chloride has been determined both experimentally [47–49] and theoretically [19–21,50–52]. However, as a starting point of our investigation of excited states of formyl chloride we fully optimised the geometry at the QCISD/6-311G(d,p) (quadratic configuration interaction with single- and double-excitations [53]) level of theory using the Gaussian 98 [54] program package. In addition we tested the optimised structure for local minima by vibrational analysis (no imaginary frequencies). For the calculation of excited states we examined several different basis sets, namely cc-pVDZ [55], cc-pVTZ [56], and both basis sets enlarged with s-, p- and d-Rydberg functions (cc-pVDZ + SPD and cc-pVTZ + SPD). While the inclusion of Rydberg functions is very important we find only minor changes between basis sets of double and triple zeta quality. The energetic lowering employing ccpVTZ + SPD instead of cc-pVDZ + SPD is in the order of 0.05 eV. Furthermore it is also very important to compute at least six roots to obtain the correct ordering of states. Therefore the present calculations are based on the more economic cc-pVDZ + SPD basis set. The exponents taken for the s-, p- and d-Rydberg functions located at the carbon centres are as (C) = 0.023, ap (C) = 0.021 and ad (C) = 0.015 [57]. The corresponding exponents for oxygen and chlorine are: as (O) = 0.032, ap (O) = 0.028 and ad (O) = 0.015; as (Cl) = 0.025, ap (Cl) = 0.020 and ad (O) = 0.015 [57]. This basis set is flexible to describe polarisation and electron correlation and is considered to be fairly balanced for all electronic states treated, so that the computed transition energies of the examined energy region should generally be obtained with an error margin of not more than 0.3 eV. The computations of the electronically excited states were performed with the selecting multi-reference singleand double-excitation configuration interaction method MRD-CI implemented in the DIESEL program [58]. The

selection of reference configurations can be carried out automatically according to a summation threshold. We have chosen a summation threshold of 0.85, which means that the sum of the squared coefficients of all reference configurations selected for each electronic state (root) is above 0.85. The number of reference configurations for each irreducible representation was in the range between 13 and 36. An analysis of the molecular orbitals (MO) involved in the selected reference configurations justifies our prior choice of treating the 18 valence electrons active while keeping the remaining electrons in doubly-occupied orbitals (frozen). From this set of reference configurations (mains) all single and double excitations in the form of configuration state functions (CSFs) are generated. From this MRD-CI space all configurations with an energy contribution DE(T) above a given threshold T were selected, i.e., the contribution of a configuration larger than this value relative to the energy of the reference set is included in the final wavefunction. A selection threshold of T = 1 · 107 Hartree was used for the calculation of the excited states of formyl chloride. The effect of those configurations which contribute less than T = 1 · 107 Hartree is accounted for in the energy computation (E(MRD-CI)) by a perturbative technique [59,60]. The contribution of higher excitations is estimated by applying a generalised Langhoff–Davidson correction formula EðMRD-CI þ QÞ ¼ EðMRD-CIÞ ð1  c20 Þ½EðrefÞ  EðMRD-CIÞ=c20 , where c20 is the sum of squared coefficients of the reference species in the total CI wavefunction and E(ref) is the energy of the reference configurations. In total we examined 20 low-lying electronically excited states (the lowest 12 singlet and the lowest 8 triplet states) of HClCO. The number of configuration state functions (CSFs) directly included in the energy calculations is as large as 561,000 (singlet) and 1.07 million (triplet) selected from a total space of 3.7 million (singlet) and 5.3 million (triplet) generated CSFs. 3. Results and discussion The calculated equilibrium geometry of formyl chloride we used for our computations of excited states is shown in Fig. 1. This equilibrium geometry is in reasonable agreement with both, prior experimental work of Davis and Gerry [47] as well as theoretically determined values [20,51]. The deviations of the bond lengths and bond angles ˚ and 0.2 respectively. are less than 0.01 A The calculated electronic transition energies DE (eV) and the corresponding oscillator strengths f from the ground state X1A 0 of formyl chloride HClCO to its electronically excited singlet and triplet states is given in Table 1. The excitation energies are given with respect to the valence electron configuration (7a 0 )2(2a00 )2. Consequently excitations from the highest occupied molecular orbital HOMO 7a 0 and the occupied MOs 6a 0 , 2a00 , to the lowlying virtual orbitals 3a00 (LUMO), 8a 0 , 9a 0 10a 0 , 4a00 and

M. Mu¨hlha¨user, M. Gruber-Stadler / Chemical Physics 343 (2008) 311–318

H 1.097 Å 126.3º

110.2º

C

Cl 1.774 Å

1.185 Å

O Fig. 1. Equilibrium geometry of formyl chloride HClCO obtained with QCISD/6-311G(d,p) optimisation as explained in the text. Table 1 Calculated electronic transition energies DE (eV) and oscillator strengths f from the ground state X1A00 of HClCO to its electronically excited singlet (DE (singlet)) and triplet (DE (triplet)) states State

Excitation

DE (singlet)

f

DE (triplet)

X1A 0 1A00 2A00 2A 0 3A 0 4A 0 5A 0 3A00 6A 0 4A00 5A00 6A00

(7a 0 )2 (2a00 )2 7a 0 ! 3a00 6a 0 ! 3a00 2a00 ! 3a00 7a 0 ! 8a 0 6a 0 ! 8a 0 7a 0 ! 9a 0 7a 0 ! 4a00 7a 0 ! 10a 0 2a00 ! 8a 0 2a00 ! 9a 0 7a 0 ! 5a00

0.00 5.04 6.70 6.89 8.06 8.51 8.80 9.07 9.21 9.21 9.38 9.97

–

– 4.75 6.58 6.05 8.14 8.69 7.57 9.07 8.88 –a –a –a

0.00001 <0.00001 0.03 0.03 0.03 0.28 0.006 0.16 0.015 0.009 0.001

The excitation energies are given with respect to the ground state configuration (7a 0 )2 (2a00 )2 (valence electrons only). The values have been obtained at the MRD-CI + Q/cc-pVDZ + SPDlevel as explained in the text. a Presently not computed.

5a00 can be expected in the energy range up to 10 eV. Contour density plots of these MOs are displayed in Fig. 2. These contour density plots are based on the singlet wavefunctions of the corresponding SCF calculation employing the cc-pVDZ + SPD basis set. As can be seen from Fig. 2 in conjunction with Table 1, the first excitation 7a 0 ! 3a00 is the HOMO–LUMO transition, computed at 5.04 eV. This is in reasonable agreement with the experimental value of 4.76 eV reported by both Libuda et al. [18] and Judge and Moule [61]. QCISD(T) calculations of the vertical excitation energy of formyl chloride by Bent and co-workers [51] resulted in 4.81 eV, while the same authors obtained 4.71 eV with density functional calculations. It can be seen from Fig. 2, in which the molecule is placed in the yz-plane, that the HOMO 7a 0 consists of a linear-combination (LC) of atomic orbitals (AOs) placed in the yz-plane, while MO 3a00 consists of a LC of atomic orbitals with px-character located at the carbon, chlorine and oxygen centres. For such a transition for which the MOs have orthogonal nodal planes a small oscillator strength of f = 0.00001 can be understood. Another explanation of the weakness of this transition is that it is an n ! p* excitation. This type of excitations is exactly

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symmetry forbidden in a molecule with a mirror plane that (i) is orthogonal to the carbonyl plane and (ii) contains the C–O bond. This symmetry property generally manifests itself in terms of small oscillator strengths of n ! p* transitions of carbonyl compounds. The corresponding triplet state 13A00 is calculated about 0.3 eV lower at 4.75 eV, in line with a valence-type transition. In line with such an n ! p* (C–O) excitation is the finding of Fang and Liu [21] that the C–O bond of this excited ˚ . In addition it can be seen state is elongated by 0.179 A from Fig. 2 that the reorganization of charge in MO 3a00 points towards a non-planar structure of the excited 1A00 state in which this 3a00 orbital is populated. This finding has already been discussed by Fang and Liu [21] who present an optimised geometry of the S1 state with an H–C–O– Cl dihedral angle of 133.2. For comparison reasons we also computed the electronic spectrum of pure formaldehyde HCHO. In Table 2 we compare our calculated values to prior experimental work. The electronic spectrum of formaldehyde has been discussed extensively by many authors [23–25,30,32]. Therefore Table 2 is given for completeness, while the present discussion will focus only on such states which are relevant for formyl chloride. The lowest excitation of HCHO is computed with a much lower transition energy of 3.97 eV compared to the 5.04 eV of formyl chloride HClCO. This can be understood on the basis of qualitative MO consideration already: As can be seen from Fig. 2 the HOMO of HClCO consists mainly of a bonding linear-combination of py (Cl) with sp2 (C). Consequently it can be characterized as a bonding r (Cl–C)-type orbital. Because of the larger electronegativity of chlorine compared to hydrogen the HOMO of HClCO (7a 0 ) can be expected to be much lower in energy (more stabilized) than the HOMO of HCHO (5a 0 in Fig. 3). On the other hand the energy of the LUMOs of both HClCO (3a00 ) and HCHO (2a00 ) are comparable in line with the lone-pair character at the oxygen centre (px (O)). Due to the lower HOMO (7a 0 ) of HClCO a larger excitation energy can be expected for the HOMO–LUMO transition 7a 0 ! 3a00 for HClCO compared to HCHO. The 2A00 state originates from 6a 0 ! 3a00 excitation. It is computed with a very weak oscillator strength in line with Fig. 2, showing that the MOs 6a 0 and 3a00 have orthogonal nodal planes. For the 2A 0 state located at 6.89 eV we calculated a much stronger f-value of 0.03. While 2a00 consists of a linear-combination of pCO and a px orbital with lone pair character located at the chlorine centre, the upper MO 3a00 is mainly composed of an antibonding linear-combination of px orbitals located at the carbon, chlorine and oxygen centres. Consequently this 2a00 ! 3a00 excitation can be characterized as p ! p* (C–O) type in line with the f-value of 0.03. The states 3A 0 and 4A 0 originate from the excitations 0 7a ! 8a 0 and 6a 0 ! 8a 0 , respectively. While 7a 0 can be characterized as r (C–Cl) type, MO 6a 0 shows r (C–O) bonding. The upper orbital 8a 0 can be characterized as

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H Cl

C

O

10a‘

H Cl

C O

9a‘

H H C

C

Cl

Cl O

O

3a‘‘

8a‘

H C

H Cl

C

O

Cl

O

7a‘

2a‘‘

H C

Cl

O

6a‘ Fig. 2. Charge density contours of characteristic occupied and virtual orbitals of HClCO.

r* (C–O) and r* (C–Cl) type. Consequently the excitation 7a 0 ! 8a 0 can be considered as r ! r* (C–Cl), while the excitation 6a 0 ! 8a 0 represents r ! r* (C–O). Both r ! r* type excitations are obtained with an f-value of 0.03. As can be seen from Fig. 2, the MO 8a 0 shows also Rydberg character that leads to a mixing of valence-type and Rydberg-type transitions for the states 3A 0 and 4A 0 . Accordingly, a small singlet–triplet splitting can be expected. The finding that our calculation places the triplet states erroneously slightly (<0.2 eV) above the corresponding singlet states for these states emphasizes the small error margin of our present calculation and is in line with the stated error margin of 0.3 eV. In the calculation of the spectrum of formaldehyde the same finding can be observed for the Rydberg-type transitions leading to the states 3A 0 ,

2A00 and 5A 0 where the triplets are also calculated erroneously slightly (<0.08 eV) above the corresponding singlet states, again well within the error margin. The by far strongest transition in the energy range up to 10 eV is obtained for 7a 0 ! 9a 0 . The resulting 5A 0 state is calculated at 8.80 eV with f = 0.28. This remarkable value again is in line with the qualitative MO picture: besides r ! r* (C–Cl) in addition p ! p* (CO) is present in this excitation. The valence character of this transition is underlined by the large singlet–triplet splitting of 1.23 eV. Such p ! p* (CO) type excitation is also present in 0 7a ! 10a 0 leading to the 6A 0 state at 9.21 eV. The somewhat smaller f-value of 0.16 is in line with our finding that the upper MO possesses less p* (CO) character, while py (Cl) becomes more important. In line with the p ! p*

M. Mu¨hlha¨user, M. Gruber-Stadler / Chemical Physics 343 (2008) 311–318

315

H

C O

H

2a‘‘

H H C

C

O

O H

H

1a‘‘

5a‘

Fig. 3. Charge density contours of characteristic occupied and virtual orbitals of formaldehyde HCHO (CS symmetry). Table 2 Calculated electronic transition energies DE (eV) and oscillator strengths f from the ground state X1A 0 of formaldehyde HCHO to its electronically excited states in comparison with experimental values State

DE (singlet) (eV)

f

Experiments (eV)

(5a ) (1a ) 5a 0 ! 2a00

0.00 3.97

– 0.0000

0.00 >3.81 [38] 4.0 [36] 4.07 [39] 4.1a 4.2 [42]

2A 0

5a 0 ! 6a 0

7.26

0.02

3A 0

5a 0 ! 7a 0

8.03

2A00

5a 0 ! 3a00

4A 0

1

XA 1A00

0

Excitation 0 2

00 2

DE (triplet) (eV)

Experiments (eV)

3.63

3.19 [45] 3.2–3.6 [38] 3.3 [43,44] 3.5 [39,43],a

7.09 [32,34,35,40] 7.11 [39] 7.13 [38]

7.23

6.83 [39] 7.09 [38],a

0.03

7.91 [37,41] 7.97 [32] 8.05 [40] 8.14 [34–36] 8.15a

8.07

7.96 [39] 8.11a

8.15

0.0000

8.37 [33]

8.18

8.31a

5a 0 ! 8a 0

8.35

0.003

7.97 [34,35] 8.00 [38] 8.13 [40] 8.14 [32]

8.35

7.79 [39] 7.92a

3A00

5a 0 ! 4a00

9.11

0.001

8.88 [34,35,40] 8.92a

9.13

5A 0

5a 0 ! 9a 0

9.12

0.0004

8.88 [46] 9.03 [34,35] 9.07a

9.17

4A00

5a 0 ! 5a00

6A

0

5A00 3

A

0

9.16

0.002

9.22 [33]

9.16

00

1a ! 2a

9.85

0.18

9.85 [40]

6.03

5.60–6.20 [38] 5.86 [39]

5a 0 ! 6a00

9.92

0.0000

9.59 [40]

9.93

9.59 [33]

00

00

00

1a ! 3a

11.91 0 2

11.60–11.90a

00 2

The excitation energies are given with respect to the ground state configuration (5a ) (1a ) (valence electrons only). The values have been obtained at the MRD-CI + Q/cc-pVDZ + SPD-level. a Cited in Ref. [23] as unpublished results by Chutjian.

character of the states 5A 0 and 6A 0 is the finding that besides the leading configurations 7a 0 ! 9a 0 and

7a 0 ! 10a 0 also the excitations 2a00 ! 3a00 (c2 = 0.05) and 2a00 ! 4a00 (c2 = 0.03) seem to be important.

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M. Mu¨hlha¨user, M. Gruber-Stadler / Chemical Physics 343 (2008) 311–318 11 10 9 8

energy / eV

7 6

1

1

1

3 A', 2 A'', 3 A''

5

2

2

HCO (1 A'') + Cl ( P)

4

1 A', 1 A'', 2 A'

3

HCO (X A') + Cl ( P)

1

1

1

2

2

2 1 0 1

2

3

4

5

6

7

8

9

10

r (C-Cl) / Angström Fig. 4. DIESEL-MRD-CI potential energy curves of the lowest six states of formyl chloride HClCO along a CS symmetric fragmentation pathway breaking the C–Cl bond. The points indicate actually computed distances and resemble relative stabilities (in eV) with respect to the equilibrium structure given in Fig. 1.

The other transitions leading to the states 3A00 , 4A00 , 5A00 and 6A00 are obtained with much weaker oscillator strengths of f = 0.001–0.015. While upper and lower MOs fall into different planes for 3A00 , 5A00 and 6A00 , the 4A00 state results from 2a00 ! 8a 0 excitation. The f-value of 0.015 is in line with the n (Cl) ! r* (C–Cl) type excitation. To examine the photofragmentation of formyl chloride we elongated the Cl–C bond stepwise in the range between ˚ and 10 A ˚ according the equilibrium bond length of 1.774 A to the reaction HClCO ! Cl + HCO

ð1Þ

while all other geometry parameters were kept frozen at the equilibrium values obtained by Anand and Schlegel [19] at the QCISD/6-311G(d,p) level. The results obtained for the three lowest singlet A 0 and A00 excited states are displayed in Fig. 4. All states are found to be repulsive. While the states X1A 0 , 11A00 and 21A 0 lead to a dissociation channel composed of the X1A 0 ground state of HCO and Cl (2P), the states 21A00 , 31A 0 and 31A00 lead to the dissociation channel composed of the first excited 11A00 state of HCO and Cl (2P). The energy difference between these two dissociation channels is computed to be 1.87 eV. This is in agreement with the experimental value of 2.02 eV (613.82 nm) that has been reported by Ramsay [62] for the first UV absorption of the formyl radical. For the fragmentation energy on the ground state potential energy surface we obtained a value of 76.1 kcal/mol in reasonable agreement with the 84.4 kcal/mol obtained by Francisco et al. [20]. Consequently such a fragmentation process on the ground state surface is very unlikely. On

the other hand the cut through the ground state potential energy surface is computed to be barrier free and thus formation of HClCO from its components Cl and HCO is very likely during polar night. The repulsive character of the excited states displayed in Fig. 4 again can be understood on the basis of qualitative MO considerations in conjunction with Fig. 2. In the excited states 11A00 , 21A00 and 21A 0 the MO 3a00 is populated. As discussed above this MO 3a00 consists mainly of an antibonding linear-combination of px-character located at the carbon, chlorine and oxygen centres. Consequently populating this 3a00 MO leads to a fragmentation into Cl and HCO. In addition excitations originating from 7a 0 weaken the r (C–Cl) bond. Thus the states 3A00 (7a 0 ! 4a00 ) and 3A 0 (7a 0 ! 8a 0 ) can be expected to be repulsive. Out of the excited states presented in Fig. 4 most important for the fragmentation according to HClCO ! Cl + HCO are excitations to 21A 0 and 31A 0 due to f-values of 0.03. While an excitation to 21A 0 leads to HCO in its ground state, the excitation to 31A 0 results in the excited 11A00 state of HCO. Although presently not computed, it is obvious from Fig. 2 that the 5A 0 state resulting from 7a 0 ! 9a 0 excitation can be expected to be repulsive, too. The upper MO 9a 0 possesses r* (Cl–C) character. Populating this MO 9a 0 will favour the fragmentation of the Cl–C bond. 4. Summary and conclusions We employed multi-reference configuration interaction (MRD-CI) calculations to compute the electronic spectrum of formyl chloride, a species of interest for atmospheric

M. Mu¨hlha¨user, M. Gruber-Stadler / Chemical Physics 343 (2008) 311–318

chemistry. Four transitions at 6.89 eV (21A 0 ), 8.06 eV (31A 0 ), 8.51 eV (41A 0 ) and 8.80 eV (51A 0 ) are computed with considerable oscillator strengths. The strongest transition 7a 0 ! 9a 0 results in the 51A 0 state calculated at 8.80 eV. This finding is in line with qualitative MO considerations which characterize this transition as both r ! r* (C–Cl) and in addition p ! p* (CO). The valence character of this transition is underlined by a large singlet–triplet splitting of 1.23 eV. The states 31A 0 and 41A 0 correspond to r ! r* (C–Cl). We examined the photofragmentation of formyl chloride into Cl and HCO. The behavior upon elongation of the Cl–C bond of the three lowest singlet 1A 0 and 1A00 states has been explicitly calculated. All excited states are found to be repulsive. Thus the photochemistry of formyl chloride might play an important role for the description of the cycle of chlorine radicals in atmospheric chemistry since chlorine re-liberation via photofragmentation due to ultraviolet sun light during the day is expected. Acknowledgements Margret Gruber-Stadler acknowledges a grant from the Research Council of Norway. Sigrid D. Peyerimhoff is thanked for providing us a version of the DIESELMRD-CI at Innsbruck. In addition we want to thank Michael Hanrath for various improvements of the DIESEL program package. Basis sets were obtained from the Extensible Computational Chemistry Environment Basis Set Database, Version 02/02/06, as developed and distributed by the Molecular Science Computing Facility, Environmental and Molecular Sciences Laboratory which is part of the Pathific Northwest Laboratory, P.O. Box 999, Richlamd, Washington 99352, USA, and funded by the US Department of Energy. References [1] J.G. Anderson, D.W. Tooley, W.H. Brune, Science 251 (1991) 39. [2] S. Solomon, Nature (London) 347 (1990) 347. [3] P.J. Crutzen, R. Mu¨ller, C. Bru¨hl, T. Peter, Geophys. Res. Lett. 19 (1992) 1113. [4] F. Helleis, J.N. Crowley, G.K. Moortgat, Geophys. Res. Lett. 21 (1994) 1795. [5] Y. Li, J.S. Francisco, J. Chem. Phys. 111 (1999) 8384. [6] T.P.W. Jungkamp, U. Kirchner, M. Schmidt, R.N. Schindler, J. Photochem. Photobiol. A 91 (1995) 1. [7] R.N. Schindler, M. Liesner, S. Schmidt, U. Kirchner, T. Benther, J. Photochem. Photobiol. A 107 (1997) 9. [8] J.N. Crowley, F. Helleis, R. Mu¨ller, G.K. Moortgat, P.J. Crutzen, J. Geophys. Res. [Atmos.] 99 (1994) 20683. [9] M. Mu¨hlha¨user, M. Schnell, S.D. Peyerimhoff, Mol. Phys. 100 (2002) 509. [10] M. Schnell, M. Mu¨hlha¨user, S.D. Peyerimhoff, Chem. Phys. Lett. 344 (2001) 519. [11] M. Schnell, M. Mu¨hlha¨user, S.D. Peyerimhoff, J. Mol. Spectrosc. 214 (2002) 124. [12] I.C. Hisatsune, J. Heicklen, Can. J. Spectrosc. 18 (1973) 77. [13] B.W. Gay, P.L. Hanst, J.J. Bulfalini, R.C. Noonan, Environ. Sci. Technol. 10 (1976) 58.

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