- Email: [email protected]
Ab initio MRD-CI study on the low-lying excited states of ClNO2
Chemical Physics Letters 383 (2004) 84–88 www.elsevier.com/locate/cplett
Ab initio MRD-CI study on the low-lying excited states of ClNO2 Antonija Lesar
a,*
, Milan Hodoscek
a,b
, Max M€ uhlh€ auser c, Sigrid D. Peyerimhoff
c
a
Department of Physical and Organic Chemistry, Institute Jozef Stefan, Jamova 39, SI-1000 Ljubljana, Slovenia Centre for Molecular Modeling, National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia Institut f€ur Physikalische und Theoretische Chemie der Universit€at Bonn, Wegelerstrasse 12, 53115 Bonn, Germany b
c
Received 12 September 2003; in final form 30 October 2003 Published online:
Abstract A theoretical study on the electronic absorption spectrum of nitryl chloride, ClNO2 , has been carried out using multi-reference configuration interaction, MRD-CI, methods with cc-pVDZ + sp and cc-pVTZ + sp basis sets. The electronic spectrum is characterized by two very strong transitions (f from 0.30 to 0.67 ) at 7.04 eV (31 A1 X1 A1 ) and 7.25 eV (31 B2 X1 A1 ). Further, the 1 1 transition at 5.77 eV (2 A1 X A1 ) is predicted to be somewhat less intense (f ¼ 0:02). In addition, the potential energy curves for the ground and low-lying singlet excited states are examined along the Cl–N bond cleavage. Also, triplet excited states of ClNO2 are discussed. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Halogenated compounds play an important role in the ozone depletion catalytic cycle in the atmosphere. It is well-known that reactive chlorine atoms are released in the stratosphere by solar UV photolysis of chlorine species; therefore, an understanding of their photodissociation is of great interest for stratospheric chemistry. Over the past two decades nitryl chloride, ClNO2 , is recognized as a trace gas in the troposphere and the stratosphere [1,2]. Photolysis of nitryl chloride is predicted to be rapid by sunlight during the day and the dominant loss mechanism, yielding primarily atomic chlorine. ClNO2 is a planar molecule of C2v symmetry with the X1 A1 ground state. The UV absorption spectrum of gaseous nitryl chloride has been reported first by Illies and Takacs [3]. Measurements between 185 and 400 nm resulted in three broad unstructured bands. The first absorption band, centered around 300 nm, is weak, but absorptions for the second band at around 220 nm and the third, below 185 nm, are strong. Recent work of Huber and coworkers [4] on the absorption spectrum *
Corresponding author. Fax: +386-1-2519385. E-mail address: [email protected] (A. Lesar).
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.11.011
recorded for the 190–380 nm range using photofragment translational energy spectroscopy is in general agreement with previous findings [3,5]. An important photodissociation channel of nitryl chloride following absorption in the visible and ultraviolet bands is ClNO2 ! Cl þ NO2
DH ¼ 34:0 kcal mol1
ð1Þ
The reaction enthalpy associated with this channel refers to the products in their electronic ground states [6]. The photolysis of ClNO2 has been investigated by resonance fluorescence detection of Cl or O atoms [5], and at 350 nm the quantum yields of 0:93 0:15 for Cl and <0.02 for O have been derived. Huber and coworkers [7] studied the photolysis at 235 nm by applying the resonance-enhanced multiphoton ionisation in combination with time-of-flight mass spectroscopy. The authors have found that the photoproduct NO2 is formed in the ground X2 A1 or an electronically excited state, most likely A2 B2 . Subsequent experiments at 248 nm by their group [4], where photofragment translational energy spectroscopy was used to detect the primary photolysis products, also confirmed this channel is dominant and includes the formation of the NO2 fragment in different electronic states. Plenge et al. [8] photolyzed ClNO2 at 240 and 308 nm, employing photoionization mass
A. Lesar et al. / Chemical Physics Letters 383 (2004) 84–88
spectroscopy to detect the photolysis product. The Cl(2 P) product dominates at the long wavelength regime, and at 308 nm the quantum yield of 0:93 0:10 was deduced. There have been no theoretical studies of ClNO2 excited states reported in the literature. It is clear that such investigations can support the spectral assignment and understanding of the photodissociation processes of ClNO2 . This gives us the primary motivation for the present work in which we have examined the low-lying excited states and the potential energy curves for the ground and the lowest excited states along the Cl–N bond cleavage. We have performed the calculations using the multi-reference configuration interaction method.
2. Computational methods The equilibrium geometry of nitryl chloride ClNO2 was fully optimized using the single and double excitation coupled-cluster method, including a perturbation estimate of the effects of connected triple excitations CCSD(T) [9] with the 6-31G(d), 6-311G(d), and 6-31G(2d) basis sets using the GA U S S I A N 98 program package [10]. The computations of the electronically excited states were performed using a CCSD(T)/6-31G(d) geometry with the multi-reference single and double excitation configuration interaction method MRD-CI implemented in the DI E S E L program [11]. The selection of the reference configurations by a summation threshold is carried out automatically. We used a summation threshold of 0.85, which means that the sum of the squared coefficients of all reference configurations selected for each state (root) is above 0.85. The number of reference configurations per irreducible representation (IRREP) was in the range between 8 and 17. An analysis of the molecular orbitals (MO) involved in these selected reference configurations justified the prior choice of treating the 24 valence electrons as active while the remaining electrons were kept in doubly occupied orbitals defined as frozen-core orbitals. From this set of reference configurations (mains) all single and double excitations in the form of configuration state functions (CSFs) are generated. All configurations of this set with an energy contribution DEðT Þ 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. Selection thresholds of T ¼ 107 and T ¼ 108 hartrees were used for singlet and triplet states, respectively. The effect of those configurations, which contribute less than T ¼ 107 or T ¼ 108 hartrees, is accounted for in the energy computation (E(MRD-CI)) by the perturbative k-extrapolation [12,13]. The contribution of higher excitations is estimated by applying Langhoff–Davidson correction formula
85
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 EðrefÞ is the energy of the reference configurations. We computed three singlet and three triplet states per IRREP for ClNO2 of the C2v symmetry. The number of CSFs directly included in the energy calculations are as large as 2.9 and 4.4 million for the singlet and triplet, respectively, selected from a total space of 4.4 and 17.4 million, respectively, generated configurations. For the calculations of excited states, we used the correlation consistent AO basis sets of Dunning of double and triple zeta quality [14,15]. In addition both basis sets were enlarged by s-Rydberg functions located at the nitrogen and by a negative ion function for chlorine atom, thus the cc-pVDZ + sp and cc-pVTZ + sp basis sets. The exponents taken are as ðNÞ ¼ 0:028 and ap ðClÞ ¼ 0:049. The potential energy surfaces of the ground and excited states were computed with the cc-pVDZ + sp basis set. The Cl–NO2 bond length was changing stepwise in , while all other geometrical the range from 1.78 to 10 A parameters were optimized for the ground state at the CCSD(T)/6-31G(d) level of theory.
3. Results and discussion The geometry of ClNO2 is given in Fig. 1 in which an optimized values at the CCSD(T)/6-31G(d) level are compared to the experimental values [16]. It can be seen that both values are in reasonable agreement. Furthermore our values nearly coincide with those of CCSD(T)/ TZP calculations previously reported by Lee [17]. In Table 1 we summarized the calculated vertical excitation energies and oscillator strengths of present investigations. We included the computed values of the
O 114.0 (114.9) Cl
N
131.9 (130.2)
1.885 (1.837) 1.209 (1.202) O
Fig. 1. Equilibrium geometry of nitryl chloride, ClNO2 , resulting from CCSD(T)/6-31G(d) calculation, with the experimental values in para, bond angles in degrees. theses [16]. The bond lengths are given in A
A. Lesar et al. / Chemical Physics Letters 383 (2004) 84–88
cc-pVDZ + sp and cc-pVTZ + sp basis sets for the singlet states and the latter basis set for the corresponding triplet excitations. As can be seen from the table, the calculated excitation energies and corresponding oscillator strengths are quite similar, therefore we believe that our calculated excitation energies have an error well below 0.3 eV. In Fig. 2 we present the SCF-MO energy scheme of valence orbitals of ClNO2 and in Fig. 3 some important molecular orbital contour plots are shown. The ground state configuration of ClNO2 is ð5a1 Þ2 ð4b2 Þ2 ð1a2 Þ2 ð2b1 Þ2 if the 24 valence electrons are treated as active in the CI calculations. As can be seen from Table 1 in conjunction with Fig. 2 the lowest excitations of ClNO2 populate the lowest unoccupied molecular orbital LUMO 6a1 and the virtual MO 3b1 . They originate from the valence MOs 4b2 , 5a1 , 2b1 , and 1a2 . As can be seen from Fig. 3, the LUMO 6a1 can be considered to be an antibonding r (Cl–N) type MO, while MO 3b1 is p (NO2 ) antibonding judged on the basis of a nodal plane between the N and O centers. On the other hand MO 5a1 shows a r(Cl–N) bonding character, while MO 4b2 corresponds to n(Cl) and n(O) type lone-pair orbitals at the chlorine and oxygen atoms. MO 1a2 corresponds to a negative linear combination (p character) of n(O) type lone-pair orbitals at the oxygen centers, whereas MO 2b1 is composed of n(Cl) chlorine lone-pair. Consequently, it can be deduced from qualitative MO analysis that r ! r (Cl–N), namely 5a1 ! 6a1 , should be the dominant transition of the electronic absorption spectrum. Our calculations place this transition at 7.04 eV with a large oscillator strength, f ¼ 0:67. Two further transitions are computed with sizeable f -values. 31 B2 X1 A1 correspond to 1a2 ! 3b1 at 7.25 eV. This transition can be considered as a p (O2 ) ! p (NO2 ) type for which a medium size oscillator strength of f ¼ 0:30 could be expected. Further, transition 21 A1 X1 A1 corresponds to 2b1 ! 3b1 and is computed to be at 5.77 eV. It shows a
5b 2
8a 1
0
4b 1
7a 1 6a 1
3b 1
–10
E / eV
86
5a 1
4b 2 3b 2
4a 1
2b 2
–20
1a 2
2b 1
1b 1
3a 1
–30 2a 1
–40
1b 2
1a 1
–50 Fig. 2. Schematic diagram of the molecular orbital energy spectrum of the ground state configuration of ClNO2 , C2v symmetry, obtained at the SCF level.
somewhat smaller f -value of 0.02 in line with the observation that in the low 2b1 MO the charge density is located largely at the chlorine, leading to a n(Cl) ! p (NO2 ) type transition, which gets its oscillator strength mainly from charge transfer. The first dipole allowed transition, 4b2 ! 6a1 , corresponds to n ! r (N–Cl) and is less intense. The singlet–triplet splitting of most states is relatively small, up to 0.4 eV. This is expected for transitions from
Table 1 Calculated vertical excitation energies DE (eV) and oscillator strengths f to singlet excited states of ClNO2 State
11 A1 11 B2 11 A2 21 B1 11 B1 21 A2 21 B2 21 A1 31 A2 31 A1 31 B2 41 B2
Excitation
cc-pVDZ + sp
2
2
2
ð5a1 Þð4b2 Þ ð1a2 Þ ð2b1 Þ 4b2 ! 6a1 4b2 ! 3b1 5a1 ! 3b1 2b1 ! 6a1 1a2 ! 6a1 3b2 ! 6a1 2b1 ! 3b1 3b2 ! 3b1 5a1 ! 6a1 1a2 ! 3b1 4b2 ! 7a1
cc-pVTZ + sp
DE
f
DE
f
DEtrip
0.0 4.48 4.61 5.10 5.20 5.41 5.83 5.84 6.41 7.12 7.45 8.70
0.0 0.00004 0.0 0.001 0.0001 0.0 0.006 0.02 0.0 0.66 0.28 0.007
0.0 4.41 4.52 5.12 5.07 5.28 5.74 5.77 6.33 7.04 7.25 8.95
0.0 0.0003 0.0 0.001 0.0001 0.0 0.006 0.02 0.0 0.67 0.30 0.001
– 4.02 4.39 4.72 4.82 5.39 5.61 5.82 6.37 4.01 4.31 8.91
DEexp 4.1
5.8 >6.2
DEtrip is related to the excitation energies for corresponding triplet excitations. For comparison the experimental values [3,4] are included.
A. Lesar et al. / Chemical Physics Letters 383 (2004) 84–88
87
8 11A 2, 21B 1 ,2 1A 1 2 2 NO 2( 1 B1 ) + Cl( P
7 6 4.69 4.41
31A1
E / eV
5.26
6a1, LUMO
5 4
)
NO 2( 1 2A 2 ) + Cl( 2P )
3
3b1
2
11A 1, 11B 2 ,11B 1 2 NO 2( 1 A1 ) +
1.62
1 O Cl
)
21A 2, 21B 2 ,31B 2 NO 2( 12B 2 ) + Cl( 2P
2
Cl ( P )
0
N
2
3
4
5
6 7 8 RCl–N / Å
9
10
O
1a1
4b2, LUMO
2b1
Fig. 3. Charge density contours of characteristic occupied valence orbitals (5a1 , 4b2 , 1a2 , 2b1 ) and the lowest unoccupied molecular orbitals (6a1 , 3b1 ).
in-plane to out-of-plane orbitals (from 4b2 or 5a1 to 3b2 , for example) and in particular if charge transfer occurs from one part of the molecule to another, so that both MOs involved have a small overlap (2b1 ! 3b1 , and 4b2 ! 7a1 , for example). Above 5 eV three triplet states are erroneously obtained somewhat above their corresponding singlet excitations, but this discrepancy is within the error margin of the present calculations (0.3 eV). Very large singlet–triplet splittings on the order of 3 eV are observed for the states resulting from 5a1 ! 6a1 and 1a2 ! 3b1 transitions. In both cases upper and lower orbitals have considerable overlap leading to sizeable exchange integrals which are important in the description of this energy gap. In the ozone molecule, for example, in which 1a2 and 3b1 are very similar to the present MOs, the S–T gap for the 1 B2 (a2 ! b1 ) is 3.4 eV [18]. To examine the role of ClNO2 as a possible source for Cl and NO2 radicals in the atmospheric chemistry we studied excited states for the Cl–N bond cleavage. Fig. 4 gives the potential energy surfaces for low-lying singlet excited states of ClNO2 in the C2v symmetric fragmentation pathway
Fig. 4. Calculated MRD-CI potential energy curves of the low-lying singlet states of the ClNO2 along a C2v symmetric fragmentation pathway breaking the Cl–N bond.
along the Cl–N bond. It can be seen that 11 B2 and 11 B1 states populating the r(N–Cl) antibonding 6a1 orbital are highly repulsive, implying that direct and fast photodissociation should occur leading to the ground state products, Cl(3 P) + NO2 (12 A1 ). 31 A1 state dissociates to the NO2 in its first excited state, while the 31 B2 state correlates with the dissociation channel which corresponds to the NO2 in its second excited state. Between 4.5 and 5.5 eV various crossing of states occur, so that excitation in this energy range can lead to NO2 products in various excited states as was found experimentally. In addition, in Fig. 5 potential energy surfaces for low-lying triplet excited states are presented. Several triplet states, 13 A1 , 13 B2 and 13 B1 , have a repulsive character and correlate with products in their ground states, while 33 A2 state dissociates to NO2 in its first excited state.
8 7 6 5.22
E / au
5a1
5
4.62
4
4.39
13A 2, 23B 2 ,23B1 NO 2(12B1) + Cl( 2P) 33B 2, 23A 2, 23A 1 NO 2(12B 2) + Cl( 2 P) 33A 2
3
2
2
NO 2(1 A 2) + Cl( P)
2
1.61
1
13A1, 13B 2 ,13B 1 NO 2(12A1) + Cl( 2P)
0 2
3
4
5
6
7 8 RCl–N / Å
9
10
Fig. 5. Calculated MRD-CI potential energy curves of the low-lying triplet states of the ClNO2 along a C2v symmetric fragmentation pathway breaking the Cl–N bond.
88
A. Lesar et al. / Chemical Physics Letters 383 (2004) 84–88
4. Summary The computed electronic spectrum of ClNO2 is characterized by two strong transitions at 7.04 eV (31 A1 X1 A1 , r ! r type) and 7.25 eV (31 B2 X1 A1 , p (O2 ) ! p (NO2 ) type). In addition, a further transition is calculated at 5.77 eV (21 A1 X1 A1 , n(Cl) ! p (NO2 ) type) with a somewhat lower intensity (f ¼ 0:02). The computed values nicely match the experimental spectrum: the strong band at around 215 nm (5.8 eV) coincides with the calculated excitation energy of the 21 A1 and 21 B2 states around 5.8 eV. Also, a strong increase of absorption below 200 nm (6.2 eV) supports the calculated intense transitions above 6.2 eV. The weak absorption band centered at around 300 nm (4.1 eV) might originate from the lowest triplet excited state. The calculated photofragmentation reaction pathways along Cl–N cleavage show that 11 B2 and 11 B1 excited states are highly repulsive, implying that direct and fast photodissociation should occur leading to the ground state products, Cl(3 P) + NO2 (12 A1 ). Pathways leading to excited NO2 products are also shown. Our MRD-CI study not only confirms the experimental findings but contributes to understanding the important role of ClNO2 as a source of Cl and NO2 radicals in atmospheric chemistry.
Acknowledgements This work was funded by the Ministry of Education, Science and Sport of Slovenia, Grant No. P-544 and
partly by the NATO collaborative linkage grant EST.CLG.977083. The authors thank M. Hanrath for assistance in DI E S E L program and M. Schnell for valuable comments.
References [1] M.A. Tolbert, M.J. Rossi, D.M. Golden, Science 240 (1988) 1018. [2] B.J. Finlayson-Pitts, F. Livingston, H. Berko, Nature 343 (1990) 622. [3] A.J. Illies, G.A. Takacs, J. Photochem. 6 (1976) 35. [4] A. Furlan, M.A. Haeberli, J.R. Huber, J. Phys. Chem. A 104 (2000) 10392. [5] N.H. Nelson, H.S. Johnston, J. Phys. Chem. 85 (1981) 3891. [6] NIST Chemistry Webbook, NIST Standard Reference Database Number 69, March 2003, Release. Available from http://webbook.nist.gov/chemistry. [7] R.T. Carter, A. Hallou, J.R. Huber, Chem. Phys. Lett. 310 (1999) 166. [8] J. Plenge, R. Flesh, M.C. Sch€ urmann, E. R€ uhl, J. Phys. Chem. A 105 (2001) 4844. [9] J.A. Pople, M.H. Gordon, K. Raghavachari, J. Chem. Phys. 87 (1987) 5968. [10] M.J. Frisch et al., GA U S S I A N 98, Revision A.7, Gaussian, Inc., Pittsburgh PA, 1998. [11] M. Hanrath, B. Engels, Chem. Phys. 225 (1997) 197. [12] R.J. Buenker, S.D. Peyerimhoff, Theor. Chim. Acta 35 (1974) 33. [13] R.J. Buenker, S.D. Peyerimhoff, Theor. Chim. Acta 39 (1975) 217. [14] T.H. Dunning Jr., J. Chem. Phys. 90 (1989) 1007. [15] D.E. Woon, T.H. Dunning Jr., J. Chem. Phys. 98 (1993) 1358. [16] J. Durig, Y.H. Kim, G.A. Guirgis, Spectrochim. Acta 50A (1994) 463. [17] T.J. Lee, J. Phys. Chem. 98 (1994) 111. [18] A. Banichevich, S.D. Peyerimhoff, Chem. Phys. 174 (1993) 93.
Ab initio MRD-CI study on the low-lying excited states of ClNO2 Antonija Lesar
a,*
, Milan Hodoscek
a,b
, Max M€ uhlh€ auser c, Sigrid D. Peyerimhoff
c
a
Department of Physical and Organic Chemistry, Institute Jozef Stefan, Jamova 39, SI-1000 Ljubljana, Slovenia Centre for Molecular Modeling, National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia Institut f€ur Physikalische und Theoretische Chemie der Universit€at Bonn, Wegelerstrasse 12, 53115 Bonn, Germany b
c
Received 12 September 2003; in final form 30 October 2003 Published online:
Abstract A theoretical study on the electronic absorption spectrum of nitryl chloride, ClNO2 , has been carried out using multi-reference configuration interaction, MRD-CI, methods with cc-pVDZ + sp and cc-pVTZ + sp basis sets. The electronic spectrum is characterized by two very strong transitions (f from 0.30 to 0.67 ) at 7.04 eV (31 A1 X1 A1 ) and 7.25 eV (31 B2 X1 A1 ). Further, the 1 1 transition at 5.77 eV (2 A1 X A1 ) is predicted to be somewhat less intense (f ¼ 0:02). In addition, the potential energy curves for the ground and low-lying singlet excited states are examined along the Cl–N bond cleavage. Also, triplet excited states of ClNO2 are discussed. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Halogenated compounds play an important role in the ozone depletion catalytic cycle in the atmosphere. It is well-known that reactive chlorine atoms are released in the stratosphere by solar UV photolysis of chlorine species; therefore, an understanding of their photodissociation is of great interest for stratospheric chemistry. Over the past two decades nitryl chloride, ClNO2 , is recognized as a trace gas in the troposphere and the stratosphere [1,2]. Photolysis of nitryl chloride is predicted to be rapid by sunlight during the day and the dominant loss mechanism, yielding primarily atomic chlorine. ClNO2 is a planar molecule of C2v symmetry with the X1 A1 ground state. The UV absorption spectrum of gaseous nitryl chloride has been reported first by Illies and Takacs [3]. Measurements between 185 and 400 nm resulted in three broad unstructured bands. The first absorption band, centered around 300 nm, is weak, but absorptions for the second band at around 220 nm and the third, below 185 nm, are strong. Recent work of Huber and coworkers [4] on the absorption spectrum *
Corresponding author. Fax: +386-1-2519385. E-mail address: [email protected] (A. Lesar).
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.11.011
recorded for the 190–380 nm range using photofragment translational energy spectroscopy is in general agreement with previous findings [3,5]. An important photodissociation channel of nitryl chloride following absorption in the visible and ultraviolet bands is ClNO2 ! Cl þ NO2
DH ¼ 34:0 kcal mol1
ð1Þ
The reaction enthalpy associated with this channel refers to the products in their electronic ground states [6]. The photolysis of ClNO2 has been investigated by resonance fluorescence detection of Cl or O atoms [5], and at 350 nm the quantum yields of 0:93 0:15 for Cl and <0.02 for O have been derived. Huber and coworkers [7] studied the photolysis at 235 nm by applying the resonance-enhanced multiphoton ionisation in combination with time-of-flight mass spectroscopy. The authors have found that the photoproduct NO2 is formed in the ground X2 A1 or an electronically excited state, most likely A2 B2 . Subsequent experiments at 248 nm by their group [4], where photofragment translational energy spectroscopy was used to detect the primary photolysis products, also confirmed this channel is dominant and includes the formation of the NO2 fragment in different electronic states. Plenge et al. [8] photolyzed ClNO2 at 240 and 308 nm, employing photoionization mass
A. Lesar et al. / Chemical Physics Letters 383 (2004) 84–88
spectroscopy to detect the photolysis product. The Cl(2 P) product dominates at the long wavelength regime, and at 308 nm the quantum yield of 0:93 0:10 was deduced. There have been no theoretical studies of ClNO2 excited states reported in the literature. It is clear that such investigations can support the spectral assignment and understanding of the photodissociation processes of ClNO2 . This gives us the primary motivation for the present work in which we have examined the low-lying excited states and the potential energy curves for the ground and the lowest excited states along the Cl–N bond cleavage. We have performed the calculations using the multi-reference configuration interaction method.
2. Computational methods The equilibrium geometry of nitryl chloride ClNO2 was fully optimized using the single and double excitation coupled-cluster method, including a perturbation estimate of the effects of connected triple excitations CCSD(T) [9] with the 6-31G(d), 6-311G(d), and 6-31G(2d) basis sets using the GA U S S I A N 98 program package [10]. The computations of the electronically excited states were performed using a CCSD(T)/6-31G(d) geometry with the multi-reference single and double excitation configuration interaction method MRD-CI implemented in the DI E S E L program [11]. The selection of the reference configurations by a summation threshold is carried out automatically. We used a summation threshold of 0.85, which means that the sum of the squared coefficients of all reference configurations selected for each state (root) is above 0.85. The number of reference configurations per irreducible representation (IRREP) was in the range between 8 and 17. An analysis of the molecular orbitals (MO) involved in these selected reference configurations justified the prior choice of treating the 24 valence electrons as active while the remaining electrons were kept in doubly occupied orbitals defined as frozen-core orbitals. From this set of reference configurations (mains) all single and double excitations in the form of configuration state functions (CSFs) are generated. All configurations of this set with an energy contribution DEðT Þ 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. Selection thresholds of T ¼ 107 and T ¼ 108 hartrees were used for singlet and triplet states, respectively. The effect of those configurations, which contribute less than T ¼ 107 or T ¼ 108 hartrees, is accounted for in the energy computation (E(MRD-CI)) by the perturbative k-extrapolation [12,13]. The contribution of higher excitations is estimated by applying Langhoff–Davidson correction formula
85
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 EðrefÞ is the energy of the reference configurations. We computed three singlet and three triplet states per IRREP for ClNO2 of the C2v symmetry. The number of CSFs directly included in the energy calculations are as large as 2.9 and 4.4 million for the singlet and triplet, respectively, selected from a total space of 4.4 and 17.4 million, respectively, generated configurations. For the calculations of excited states, we used the correlation consistent AO basis sets of Dunning of double and triple zeta quality [14,15]. In addition both basis sets were enlarged by s-Rydberg functions located at the nitrogen and by a negative ion function for chlorine atom, thus the cc-pVDZ + sp and cc-pVTZ + sp basis sets. The exponents taken are as ðNÞ ¼ 0:028 and ap ðClÞ ¼ 0:049. The potential energy surfaces of the ground and excited states were computed with the cc-pVDZ + sp basis set. The Cl–NO2 bond length was changing stepwise in , while all other geometrical the range from 1.78 to 10 A parameters were optimized for the ground state at the CCSD(T)/6-31G(d) level of theory.
3. Results and discussion The geometry of ClNO2 is given in Fig. 1 in which an optimized values at the CCSD(T)/6-31G(d) level are compared to the experimental values [16]. It can be seen that both values are in reasonable agreement. Furthermore our values nearly coincide with those of CCSD(T)/ TZP calculations previously reported by Lee [17]. In Table 1 we summarized the calculated vertical excitation energies and oscillator strengths of present investigations. We included the computed values of the
O 114.0 (114.9) Cl
N
131.9 (130.2)
1.885 (1.837) 1.209 (1.202) O
Fig. 1. Equilibrium geometry of nitryl chloride, ClNO2 , resulting from CCSD(T)/6-31G(d) calculation, with the experimental values in para, bond angles in degrees. theses [16]. The bond lengths are given in A
A. Lesar et al. / Chemical Physics Letters 383 (2004) 84–88
cc-pVDZ + sp and cc-pVTZ + sp basis sets for the singlet states and the latter basis set for the corresponding triplet excitations. As can be seen from the table, the calculated excitation energies and corresponding oscillator strengths are quite similar, therefore we believe that our calculated excitation energies have an error well below 0.3 eV. In Fig. 2 we present the SCF-MO energy scheme of valence orbitals of ClNO2 and in Fig. 3 some important molecular orbital contour plots are shown. The ground state configuration of ClNO2 is ð5a1 Þ2 ð4b2 Þ2 ð1a2 Þ2 ð2b1 Þ2 if the 24 valence electrons are treated as active in the CI calculations. As can be seen from Table 1 in conjunction with Fig. 2 the lowest excitations of ClNO2 populate the lowest unoccupied molecular orbital LUMO 6a1 and the virtual MO 3b1 . They originate from the valence MOs 4b2 , 5a1 , 2b1 , and 1a2 . As can be seen from Fig. 3, the LUMO 6a1 can be considered to be an antibonding r (Cl–N) type MO, while MO 3b1 is p (NO2 ) antibonding judged on the basis of a nodal plane between the N and O centers. On the other hand MO 5a1 shows a r(Cl–N) bonding character, while MO 4b2 corresponds to n(Cl) and n(O) type lone-pair orbitals at the chlorine and oxygen atoms. MO 1a2 corresponds to a negative linear combination (p character) of n(O) type lone-pair orbitals at the oxygen centers, whereas MO 2b1 is composed of n(Cl) chlorine lone-pair. Consequently, it can be deduced from qualitative MO analysis that r ! r (Cl–N), namely 5a1 ! 6a1 , should be the dominant transition of the electronic absorption spectrum. Our calculations place this transition at 7.04 eV with a large oscillator strength, f ¼ 0:67. Two further transitions are computed with sizeable f -values. 31 B2 X1 A1 correspond to 1a2 ! 3b1 at 7.25 eV. This transition can be considered as a p (O2 ) ! p (NO2 ) type for which a medium size oscillator strength of f ¼ 0:30 could be expected. Further, transition 21 A1 X1 A1 corresponds to 2b1 ! 3b1 and is computed to be at 5.77 eV. It shows a
5b 2
8a 1
0
4b 1
7a 1 6a 1
3b 1
–10
E / eV
86
5a 1
4b 2 3b 2
4a 1
2b 2
–20
1a 2
2b 1
1b 1
3a 1
–30 2a 1
–40
1b 2
1a 1
–50 Fig. 2. Schematic diagram of the molecular orbital energy spectrum of the ground state configuration of ClNO2 , C2v symmetry, obtained at the SCF level.
somewhat smaller f -value of 0.02 in line with the observation that in the low 2b1 MO the charge density is located largely at the chlorine, leading to a n(Cl) ! p (NO2 ) type transition, which gets its oscillator strength mainly from charge transfer. The first dipole allowed transition, 4b2 ! 6a1 , corresponds to n ! r (N–Cl) and is less intense. The singlet–triplet splitting of most states is relatively small, up to 0.4 eV. This is expected for transitions from
Table 1 Calculated vertical excitation energies DE (eV) and oscillator strengths f to singlet excited states of ClNO2 State
11 A1 11 B2 11 A2 21 B1 11 B1 21 A2 21 B2 21 A1 31 A2 31 A1 31 B2 41 B2
Excitation
cc-pVDZ + sp
2
2
2
ð5a1 Þð4b2 Þ ð1a2 Þ ð2b1 Þ 4b2 ! 6a1 4b2 ! 3b1 5a1 ! 3b1 2b1 ! 6a1 1a2 ! 6a1 3b2 ! 6a1 2b1 ! 3b1 3b2 ! 3b1 5a1 ! 6a1 1a2 ! 3b1 4b2 ! 7a1
cc-pVTZ + sp
DE
f
DE
f
DEtrip
0.0 4.48 4.61 5.10 5.20 5.41 5.83 5.84 6.41 7.12 7.45 8.70
0.0 0.00004 0.0 0.001 0.0001 0.0 0.006 0.02 0.0 0.66 0.28 0.007
0.0 4.41 4.52 5.12 5.07 5.28 5.74 5.77 6.33 7.04 7.25 8.95
0.0 0.0003 0.0 0.001 0.0001 0.0 0.006 0.02 0.0 0.67 0.30 0.001
– 4.02 4.39 4.72 4.82 5.39 5.61 5.82 6.37 4.01 4.31 8.91
DEexp 4.1
5.8 >6.2
DEtrip is related to the excitation energies for corresponding triplet excitations. For comparison the experimental values [3,4] are included.
A. Lesar et al. / Chemical Physics Letters 383 (2004) 84–88
87
8 11A 2, 21B 1 ,2 1A 1 2 2 NO 2( 1 B1 ) + Cl( P
7 6 4.69 4.41
31A1
E / eV
5.26
6a1, LUMO
5 4
)
NO 2( 1 2A 2 ) + Cl( 2P )
3
3b1
2
11A 1, 11B 2 ,11B 1 2 NO 2( 1 A1 ) +
1.62
1 O Cl
)
21A 2, 21B 2 ,31B 2 NO 2( 12B 2 ) + Cl( 2P
2
Cl ( P )
0
N
2
3
4
5
6 7 8 RCl–N / Å
9
10
O
1a1
4b2, LUMO
2b1
Fig. 3. Charge density contours of characteristic occupied valence orbitals (5a1 , 4b2 , 1a2 , 2b1 ) and the lowest unoccupied molecular orbitals (6a1 , 3b1 ).
in-plane to out-of-plane orbitals (from 4b2 or 5a1 to 3b2 , for example) and in particular if charge transfer occurs from one part of the molecule to another, so that both MOs involved have a small overlap (2b1 ! 3b1 , and 4b2 ! 7a1 , for example). Above 5 eV three triplet states are erroneously obtained somewhat above their corresponding singlet excitations, but this discrepancy is within the error margin of the present calculations (0.3 eV). Very large singlet–triplet splittings on the order of 3 eV are observed for the states resulting from 5a1 ! 6a1 and 1a2 ! 3b1 transitions. In both cases upper and lower orbitals have considerable overlap leading to sizeable exchange integrals which are important in the description of this energy gap. In the ozone molecule, for example, in which 1a2 and 3b1 are very similar to the present MOs, the S–T gap for the 1 B2 (a2 ! b1 ) is 3.4 eV [18]. To examine the role of ClNO2 as a possible source for Cl and NO2 radicals in the atmospheric chemistry we studied excited states for the Cl–N bond cleavage. Fig. 4 gives the potential energy surfaces for low-lying singlet excited states of ClNO2 in the C2v symmetric fragmentation pathway
Fig. 4. Calculated MRD-CI potential energy curves of the low-lying singlet states of the ClNO2 along a C2v symmetric fragmentation pathway breaking the Cl–N bond.
along the Cl–N bond. It can be seen that 11 B2 and 11 B1 states populating the r(N–Cl) antibonding 6a1 orbital are highly repulsive, implying that direct and fast photodissociation should occur leading to the ground state products, Cl(3 P) + NO2 (12 A1 ). 31 A1 state dissociates to the NO2 in its first excited state, while the 31 B2 state correlates with the dissociation channel which corresponds to the NO2 in its second excited state. Between 4.5 and 5.5 eV various crossing of states occur, so that excitation in this energy range can lead to NO2 products in various excited states as was found experimentally. In addition, in Fig. 5 potential energy surfaces for low-lying triplet excited states are presented. Several triplet states, 13 A1 , 13 B2 and 13 B1 , have a repulsive character and correlate with products in their ground states, while 33 A2 state dissociates to NO2 in its first excited state.
8 7 6 5.22
E / au
5a1
5
4.62
4
4.39
13A 2, 23B 2 ,23B1 NO 2(12B1) + Cl( 2P) 33B 2, 23A 2, 23A 1 NO 2(12B 2) + Cl( 2 P) 33A 2
3
2
2
NO 2(1 A 2) + Cl( P)
2
1.61
1
13A1, 13B 2 ,13B 1 NO 2(12A1) + Cl( 2P)
0 2
3
4
5
6
7 8 RCl–N / Å
9
10
Fig. 5. Calculated MRD-CI potential energy curves of the low-lying triplet states of the ClNO2 along a C2v symmetric fragmentation pathway breaking the Cl–N bond.
88
A. Lesar et al. / Chemical Physics Letters 383 (2004) 84–88
4. Summary The computed electronic spectrum of ClNO2 is characterized by two strong transitions at 7.04 eV (31 A1 X1 A1 , r ! r type) and 7.25 eV (31 B2 X1 A1 , p (O2 ) ! p (NO2 ) type). In addition, a further transition is calculated at 5.77 eV (21 A1 X1 A1 , n(Cl) ! p (NO2 ) type) with a somewhat lower intensity (f ¼ 0:02). The computed values nicely match the experimental spectrum: the strong band at around 215 nm (5.8 eV) coincides with the calculated excitation energy of the 21 A1 and 21 B2 states around 5.8 eV. Also, a strong increase of absorption below 200 nm (6.2 eV) supports the calculated intense transitions above 6.2 eV. The weak absorption band centered at around 300 nm (4.1 eV) might originate from the lowest triplet excited state. The calculated photofragmentation reaction pathways along Cl–N cleavage show that 11 B2 and 11 B1 excited states are highly repulsive, implying that direct and fast photodissociation should occur leading to the ground state products, Cl(3 P) + NO2 (12 A1 ). Pathways leading to excited NO2 products are also shown. Our MRD-CI study not only confirms the experimental findings but contributes to understanding the important role of ClNO2 as a source of Cl and NO2 radicals in atmospheric chemistry.
Acknowledgements This work was funded by the Ministry of Education, Science and Sport of Slovenia, Grant No. P-544 and
partly by the NATO collaborative linkage grant EST.CLG.977083. The authors thank M. Hanrath for assistance in DI E S E L program and M. Schnell for valuable comments.
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