CASSCF and CASPT2 Study on O- and Cl-Loss Predissociation

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CASSCF and CASPT2 Study on O- and Cl-Loss Predissociation Mechanisms of OClO (A 2A2) Qingyong Meng and Ming-Bao Huang* College of Chemistry and Chemical Engineering, Graduate University of the Chinese Academy of Sciences, P. O. Box 4588, Beijing 100049, People’s Republic of China ABSTRACT: For studying O- and Cl-loss predissociation mechanisms of OClO (A 2A2), we calculated O- and Cl-loss dissociation potential energy curves (adiabatic minimum-energy dissociation paths) of several low-lying doublet and quartet states at the CASPT2 level and located the MECPs (minimum energy crossing points) for many pairs of the potential energy surfaces (PESs) at the CASPT2 and CASSCF levels. On the basis of our calculation results (including the spinorbit couplings at the MECPs), we predict three processes for O-loss predissociation of A 2A2 and four processes for Cl-loss predissociation of A 2A2. The most favorable process for O-loss predissociation is OClO (A 2A2) f A 2A2/1 2B2 MECP f 1 2B2 (1 2A0 ) f O (3Pg) þ ClO (X 2Π) (the first O-loss limit), and the needed energy for this process from X 2B1 is 2.92 eV. The most favorable process for Cl-loss predissociation is OClO (A 2A2) f A 2A2/1 2B2 MECP f TS1 (1 2B2) f 1 2B2/1 2A1 MECP f Cl (2Pu) þ O2 (X 3Σg) (the first limit), and the needed energy is 3.08 eV. In the previously suggested mechanisms (processes), the A 2A2 state was considered to go to the important 1 2B2 state via 1 2A1 (A 2A2 f 1 2A1 f 1 2B2). In the present study we have found that the A 2A2 state can directly go to 1 2B2 (at the located A 2A2/1 2B2 MECP the CASPT2 energy (relative to X 2B1) and CASSCF spinorbit coupling are 2.92 eV and 61.3 cm1, respectively). We have compared our processes (A 2A2 f 1 2B2 f ...) with the processes (A 2A2 f 1 2A1 f 1 2B2 f ...) suggested in the previous MRCI studies and rewritten by us using our calculation results. Energetically the MRCI process for O-loss predissociation (to the first limit) is only slightly (0.13 eV) more favorable than our process, and the MRCI processes for Cl-loss predissociation (to the first and second limits) need the same energies as our processes. By considering the probabilities of radiationless transitions, the MRCI processes are less favorable than our processes since the MRCI processes proceed via more PES/PES crossings (more MECPs). The experimental facts concerning the photodissociation are explained.

1. INTRODUCTION It is now a well-known fact that the polar stratospheric ozone destruction is caused on a large scale by the photochemistry of chlorine dioxide (OClO).13 The main purpose of the present theoretical work is to study O- and Cl-loss dissociation processes from the A 2A2 state of chlorine dioxide. In the past 2 decades, the OClO (A 2A2) radical has been the focus of numerous experimental421 and theoretical2230 studies. Theoretical studies using proper calculation methods and treatments could produce comprehensive pictures for photodissociation mechanisms of small molecules and offer support to available experimental facts. In 1992 and 1996, Peterson and Werner22,23 studied O- and Cl-loss photodissociation of OClO (A 2A2) using the multireference configuration interaction (MRCI) method (one of the proper calculation methods for excited electronic states at the present stage). In 1992 they22 calculated two-dimentional potential energy functions (contour plots) and several potential energy cuts for the first four electronic states of OClO and predicted the O-loss predissociation mechanisms of the A 2A2 state. The 1 2A1 state was considered to predissociate the A 2A2 state. After this initial crossing to 1 2A1 the molecule dissociated through the linear configuration to ClO þ O, or crossed onto the 1 2B2 surface and dissociated to ClO þ O from the 1 2B2 state. r 2011 American Chemical Society

Their predictions could be expressed by the following two simple equations (processes): (I) A 2A2 f 1 2A1 f the linear configuration f O (3Pg) þ ClO (X 2Π) and (II) A 2A2 f 1 2A1 f 1 2B2 f O (3Pg) þ ClO (X 2Π). Apparently the internal conversion (vibronic coupling) between the A 2A2 and 1 2A1 states is not possible, and the authors22 considered that the A 2A2/1 2A1 interaction involved in the first steps of processes I and II was the spinorbit coupling (we calculated the spinorbit coupling values in the present work). In 1996 Peterson and Werner23 studied Cl-loss predissociation mechanisms of the A 2A2 state of OClO. On the basis of their calculation results (potential energy surface contour plots and potential energy cuts for the low-lying electronic states), they predicted the Cl-loss predissociation mechanisms of the A 2A2 state (described below). The 1 2B2 potential energy surface (PES) was intimately tied to the production of Cl þ O2 (a 1Δg), and the 1 2B2 state was accessed from the A 2A2 state via vibronic interaction with the 1 2A1 surface, which was itself coupled to the A 2A2 state by spinorbit coupling. The 1 2A1 surface crossed the Received: November 15, 2010 Revised: February 20, 2011 Published: March 16, 2011 2692

dx.doi.org/10.1021/jp110894v | J. Phys. Chem. A 2011, 115, 2692–2701

The Journal of Physical Chemistry A 1 2B2 surface relatively late in the exit channel, and this 1 2A1 surface correlated to the ground-state products Cl (2Pu) þ O2 (X 3Σg). Their predictions could be expressed by the following two simple equations (processes): (III) A 2A2 f 1 2A1 f 1 2B2 f Cl (2Pu) þ O2 (a 1Δg) and (IV) A 2A2 f 1 2A1 f 1 2B2 f 1 2A1 f Cl (2Pu) þ O2 (X 3Σg). Peterson and Werner22,23 emphasized that a quantitative description of OClO photodissociation would require extensive dynamic calculations on multiple three-dimentional sufaces and it was not possible to compute the global surfaces due to the extreme costs. Actually the methods they used for electronic structure calculations were good (are still good now). We think that for the dynamic calculations one would need good techniques for the treatments: for example, the automatic full geometry optimization technique for locating minima and transition states in different PESs, automatic partial geometry optimization technique for the calculations of adiabatic minimum-energy dissociation potential energy curves (PECs), and the technique of automatic searching for minimum energy crossing point (MECP) between the PESs of two specific electronic states. With these techniques one does not need to calculate many contour plots of potential energy surfaces and too many potential energy cuts, and with help of these techniques one may reach predictions for the predissociation mechanisms. In the present theoretical work we study O- and Cl-loss dissociation processes from the A 2A2 state of OClO using the multiconfiguration second-order perturbation theory (CASPT2)31,32 and complete active space self-consistent-field (CASSCF)33 methods. As with the MRCI calculations, the CASPT2 calculations are based on the CASSCF calculations and incorporate both static and dynamic electron correlation. The calculations were carried out using MOLCAS v7.4 quantum chemistry softwares,34 and the above-mentioned three techniques are incorporated in the MOLCAS v7.4.34 We have considered the four low-lying doublet states (X 2B1, A 2A2, 1 2A1, and 1 2B2) and also the 1 4A1, 1 4A2, 1 4B1, and 1 4B2 quartet states. On the basis of our calculation results (the CASPT2 O- and Cl-loss dissociation PECs and the results at the located MECPs), we will present a comprehensive picture for the O- and Cl-loss predissociation mechanisms of OClO (A 2A2). We will discuss the predissociation mechanisms predicted in the previous MRCI studies22,23 in comparison with those predicted in the present study. We will also try to explain available experimental facts. Here we will only mention the experimental work of Davis and Lee10 in 1996 for a dissociation dynamic study within a range of vibrational states of the A 2A2 state using photofragment translational energy spectroscopy, which was also discussed in the previous MRCI papers.22,23 The main experimental facts concerning the photodissociation observed in their experiments10 are the following: (i) the dominant (g96%) channel is O-loss; (ii) at E > 3.1 eV, the Cl-loss channel was found to diminish strongly with the O þ ClO yield, reaching approximately unity at E > 3.27 eV; and (iii) at E < 3.1 eV A 2A2 could interact via spinorbit coupling to nearby 2A1, which dissociates to O þ ClO via a nearly linear transition state, and the O2 was primarily formed in the a 1Δg state (≈80%). Before and after this important experimental work, other two experimental groups (Vaida et al.4,5 and Stert et al.18) suggested primary photochemistry dynamics of the A 2A2 state involved the couplings of A 2A2 to the close lying 1 2A1 and 1 2B2 states.

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Figure 1. Atom labelings for the OClO radical (in the C2v symmetry) used in the present work. r denotes the distance between the Cl atom and the midpoint of the O1O2 distance.

2. CALCULATION DETAILS Geometry and atom labeling used for the OClO radical are shown in Figure 1, and r denotes the distance between the Cl atom and the midpoint of the O1O2 distance. In O-loss dissociation the OClO system remains in the Cs symmetry and in Clloss dissociation the OClO system in the C2v symmetry. The Clloss dissociation of the OClO system in the C2v symmetry is an assumption, and it is supported by the potential energy surface calculations in the previous MRCI study.23 A contracted atomic natural orbital (ANO-L) basis set,3538 Cl[6s5p4d3f]/O[5s4p3d2f], was used. With a CASSCF wave function constituting the reference function, the CASPT2 calculations were performed to compute the first-order wave function and the second-order energy in the full CI space. The OClO radical in the ground state has an electron configuration of ... (2b1)2 (7a1)2 (4b2)2 (1a2)2 (5b2)2 (8a1)2 (3b1)1 (9a1)0 (6b2)0 (10a1)0 (4b1)0 (11a1)0 (7b2)0. In the CAS calculations for OClO, 13 electrons were active and the active space included the 7a111a1, 2b14b1, 4b27b2, and 1a2 orbitals (the same active space as in the previous MRCI studies2224). Compared to the full valence active space for OClO, the 5a1, 6a1, and 3b2 (mainly consisting of 2s(O1), 2s(O2), and 3s(Cl) orbitals) were not included and four virtual orbitals (10a1, 4b1, 11a1, and 7b2) were added. In the calculations of O-loss dissociation PECs, the 13 active orbitals were labeled using the irreducible representations in the Cs point group. In the calculations for electronic states of OClO in the linear configuration (D¥h for equilibrium geometries and C¥v for O-loss dissociation) the 13 active orbitals were labeled using the irreducible representations in the subgroups of D¥h and C¥v. In the CAS calculations for the related species (O2, ClO, Cl, and O) we always used the full valence active spaces. In all CASPT2 calculations, the weight values of the CASSCF reference functions in the first-order wave functions were larger than 0.85. In the CASPT2 geometry optimization calculations for the doublet and quartet states, we obtained the equilibrium geometries and adiabatic relative energies (denoted as T0) of these states to the X 2B1 state. The C2v PECs (the potential energy cuts) of the OClO states, as functions of the OClO bond angle (ranging from 60 to 180), were calculated at the CASPT2 level (with the ClO bond distance fixed at the experimental value of 1.470 Å in the X 2B1 geometry39). The CASPT2 O-loss dissociation PECs (minmum energy paths) of the OClO states were obtained on the basis of the CASPT2 partial geometry optimization calculations performed at a set of fixed R(ClO1) (see Figure 1) values ranging from small starting values to 5.0 Å. The OClO systems in the different states at the R(ClO1) value of 5.0 Å are called O-loss asymptote products. The CASPT2 Cl-loss dissociation PECs (minmum 2693

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Table 1. CASPT2 Adiabatic Relative Energies (T0, eV) and Geometries (R(ClO), — OClO) of the Doublet and Quartet Electronic States of the OClO Radical a T0 state X2B1

b

geometry c

d

b

CASPT2

MRCI

CASPT2

expt

CASPT2

MRCI

CASPT2c

expt

0.0

1.476, 117.7

1.476, 117.9

1.477, 116.1

1.470, 117.4 e

1.592, 89.4

1.597, 89.7

1.608, 89.6

1.630, 106.4 1.605, 121.5

1.633, 106.2 1.612, 120.0

1.646, 104.5 1.630, 116.4

0.0

0.0

0.0

2

1 B2

2.10

1.98

2.20

A2A2 1 2A1

2.67 2.76

2.65 2.60

2.68 2.76

1 2Πu g

2.15

2 2A1

2.86

1 4B1 h

2.39

1 4B2

3.08

3.08

1.757, 155.6

1.863, 152.2

1 4A2

3.11

3.22

1.753, 163.1

1.923, 157.0

1 4A1

4.02

2.69

1.627, 106.2 f

1.582, 180.0 2.78

1.608, 136.0

1.690, 132.4

1.615, 180.0

1.780, 86.4

a R(ClO) values are given in angstroms and — OClO values in degrees; for notations, see Figure 1). b References 22 and.24. c Reference 26 (using the MS-CASPT2 method). d Evaluated using the experimental adiabatic ionization energy data reported in ref 40. e Reference 39. f Reference 15. g The globe minimum of 1 2A1 is in the linear configuration (1 2Πu). h In the linear configuration (1 4Σg).

energy paths) of the OClO states were obtained on the basis of the CASPT2 partial geometry optimization calculations performed at a set of fixed r (see Figure 1) values ranging from small starting values to 5.0 Å. The OClO systems in the different states at the r value of 5.0 Å are called Cl-loss asymptote products. For studying nonadiabatic O- and Cl-loss dissociation processes, we performed the CASSCF MECP calculations (automatic searching) for selected state/state pairs in the C2v and Cs symmetries and then calculated the CASPT2 energies and CASSCF spinorbit coupling values at the located MECPs. The state/ state pairs were selected mainly on the basis of the PEC crossings in figures of the C2v PECs, O-loss dissociation PECs, and Cl-loss dissociation PECs, though a MECP for a state/state pair is a special crossing point (a geometry) between the two PESs. For the A 2A2/1 2A1, A 2A2/1 2B2, and 1 2A1/1 2B2 state/state pairs, we searched the MECPs by performing (time-consuming) CASPT2 calculations and calculated the CASSCF spinorbit coupling values at the located CASPT2 MECPs. For the equilibrium geometries of all of the calculated electronic states, all of the transition states along the O-loss dissociation PECs, and some important transition states (see below) along the Cl-loss dissociation PECs, we performed the CASSCF frequency analysis calculations at the respective CASSCF optimized geometries (they are close to the respective CASPT2 geometries). In the present article, the evaluated energy differences (relative energies) were not corrected for zero-point energies.

3. RESULTS AND DISCUSSION 3.1. Electronic States of the OClO Radical. 3.1.1. CASPT2 Adiabatic Relative Energies and Geometries. In Table 1 are given

the CASPT2 T0 (adiabatic relative energy to X 2B1) values and optimized geometries for the X 2B1, A 2A2, 1 2A1, 1 2B2, 2 2A1, 1 4A1, 1 4A2, 1 4B1, and 1 4B2 states of the OClO radical. The 1 4B1 state is predicted to be in a linear geometry and denoted as 1 4Σg. The calculations for the 1 2A1 state indicate that the bent geometry is just a local minimum, and the globe minimum is at the linear configuration (denoted as 1 2Πu). The 1 2Πu globe minimum is predicted to be 0.61 eV lower than the 1 2A1 local minimum.

Figure 2. CASPT2 C2v potential energy curves for the doublet and quartet states of the OClO radical, as functions of the OClO angle (the R(ClO) value is fixed at the experimental value of 1.470 Å in the X 2B1 geometry).

For the A 2A2 state, our CASPT2 T0 value of 2.67 eV is very close to the experimental value (2.69 eV),40 as are the MRCI value24 and the previous MS-CASPT2 value.26 Our CASPT2 geometries for the X 2B1 and A 2A2 states are very close to the experimental geometries,15,39 as are the MRCI geometries.22,24 There are no available experimental data for the other states. Our CASPT2 T0 values for the 1 2A1 (bent) and 1 2B2 states are slightly larger than the MRCI values, while our CASPT2 geometries are very close to the MRCI geometries.22 For the A 2A2, 1 2A1 (bent), 1 4A2, and 1 4B2 states, the previous MS-CASPT2 T0 values26 are quite close to our CASPT2 T0 values, but the previous MSCASPT2 geometries26 had longer (or much longer) ClO bond lengths and smaller OClO angles than our CASPT2 geometries. 3.1.2. C2v Potential Energy Curves: E( — OClO). The CASPT2 C2v PECs (E( — OClO)) for the X 2B1, A 2A2, 1 2A1, 1 2B2, 2 2A1, 2 2 B1, 1 4A2, 1 4B1, and 1 4B2 states are given in Figure 2. The C2v PEC for 1 4A1 was not calculated (too high in energy). As a consequence of the RennerTeller effect, the X 2B1 and 1 2A1 PECs converge to 1 2Πu at — OClO = 180 and the 1 2B2 and A 2A2 PECs converge to 1 2Πg. The 1 4B2 and 1 4A2 PECs converge to 1 4Πg, and the 1 4B1 2694



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Table 2. First and Second O-Loss Dissociation Limits (Product Groups) of the OClO Radical, Together with the CASPT2 and Experimental Sum Energy Values (ΔE, eV) for the Product Groups Relative to OClO (X 2B1) ΔE product group O (3Pg) þ ClO (X2Π) (the first limit) O ( Dg) þ ClO (X Π) (the second limit) 1

a

2

b

a

CASPT2

MRCI

exptb

exptc

exptd

states of OClO in Cs symmetrye

2.74

2.30

2.56

2.49

2.58

three 2,4A0 , three 2,4A00 (X2B1, A2A2, 1 2A1, 1 2B2, 1 4B2, 1 4B1)

4.68 c

five 2A0 , five 2A00

4.45 d

e

Reference 22. References 10 and.17. Reference 14. Reference 18. In parentheses are the OClO states calculated in the present work.

Table 3. First, Second, and Third Cl-Loss Dissociation Limits (Product Groups) of the OClO Radical, Together with the CASPT2 and Experimental Sum Energy Values (ΔE, eV) for the Product Groups Relative to OClO (X 2B1) ΔE product group

CASPT2

MRCIa

exptb

exptc

exptd

Cl (2Pu) þ O2 (X3Σg) (the first limit)

0.24

0.26

0.17

0.18

0.03

Cl ( Pu) þ O2 (a Δg) (the second limit)

1.23

2

1

Cl ( Pu) þ O2 (b 2

a

Σgþ)

1

(the third limit)

1.90

states of the OClO radicale 2,4

A1, 2,4B1, 2,4A2 (X2B1, A2A2, 1 2A1, 1 4A1, 1 4B1, 1 4A2)

1.17

two 2A1, two 2B1, 2A2, 2B2 (1 2B2)

1.82

2

b

A1, 2B1, 2B2

Reference 22. Evaluated using the experimental energy data mentioned in ref 22. References 6 and.7. d Reference 18. e In parentheses are the OClO states calculated in the present work.

PEC converges to 1 4Σg. The 2 2A1 and 2 2B1 PECs converge to 1 2 Δg. The CASPT2 C2v PECs for the doublet states have features similar to the previously reported MRCI C2v PECs.22 The CASPT2 geometry optimization calculation results for 1 2Πu and 1 4Σg were reported in Table 1. There exists an avoided crossing region between the 1 2A1 (a component of 1 2Πu) and 2 2A1 (a component of 1 2Δg) PECs at the OClO angle value of around 130. Thus, there should be a small barrier between the bent and linear configurations of the 1 2A1 state. The CASPT2 geometry optimization calculations predict that the barrier is 0.03 and 0.64 eV higher in energy than the bent (1 2A1) and linear (1 2Πu) configurations, respectively. We would mention that the CASPT2 C2v PECs (E( — OClO)) of the OClO states, given in Figure 2, were calculated at the fixed ClO bond distance value of 1.470 Å (see section 2). 3.2. O- and Cl-Loss Dissociation Potential Energy Curves. On the basis of the CASPT2 geometry optimization calculations for ClO (X 2Π), O2 (X 3Σg), O2 (a 1Δg), and O2 (b 1Σgþ) (the optimized bond length values being 1.571, 1.206, 1.215, and 1.226 Å, respectively) and the CASPT2 energy calculations for O (3Pg), O (1Dg), and Cl (2Pu), we evaluated the CASPT2 sum energies for the product groups of O-loss dissociation (the first and second limits) and Cl-loss dissociation (the first, second, and third limits). The CASPT2 sum energies of the low-lying O- and Cl-loss dissociation product groups relative to the X 2B1 reactant (“sum energy of the product group relative to the X 2B1 reactant” will be abbreviated to “sum energy of the product group” in the rest of the paper) are given in Tables 2 and 3, respectively. The experimental6,7,10,14,17,18 and MRCI22 sum energy values (relative to X 2B1) are also listed in the tables. The CASPT2 sum energy values of 2.74, 4.68, 0.24, 1.23, and 1.90 eV for the O (3Pg) þ ClO (X 2Π), O (1Dg) þ ClO (X 2Π), Cl (2Pu) þ O2 (X 3 Σg ), Cl (2Pu) þ O2 (a1Δg), and Cl (2Pu) þ O2 (b 1Σgþ) product groups are in reasonable agreement with the respective experimental sum energy values (the deviations from the experimental values or the average experimental values being smaller than 0.20 eV for the O-loss product groups and 0.11 eV for the Cl-loss product groups). The MRCI sum energy value22 for

c

Figure 3. CASPT2 O-loss dissociation potential energy curves from the X 2B1 (1 2A00 ), A 2A2 (2 2A00 ), 1 2B2 (1 2A0 ), 1 2A1 (1 2Π), 1 4B1 (1 4Σ), and 1 4B2 (1 4A0 ) states of the OClO radical. Values in parentheses are the CASPT2 energies (eV) of the reactants (at their CASPT2 equilibrium geometries) and asymptote products relative to the X 2B1 reactant, and values in square brackets are the CASPT2 sum energies (eV) of the product groups relative to the X 2B1 reactant (the experimental sum energies of the O-loss product groups are given in Table 2).

O (3Pg) þ ClO (X 2Π) was 0.24 eV smaller than the average experimental value, and the MRCI sum energy value22 (0.26 eV) for Cl (2Pu) þ O2 (X 3Σg) had a wrong sign. In the following two subsections we will report the CASPT2 O- and Cl-loss dissociation PECs from the doublet and quartet states of OClO. In the previous MRCI papers22,23 no minimumenergy PECs were reported. 3.2.1. O-Loss Dissociation PECs. The CASPT2 O-loss dissociation PECs of the X 2B1 (1 2A00 ), 1 2B2 (1 2A0 ), 1 2A1 (1 2Π), A 2A2 (2 2A00 ), 1 4B1 (1 4Σ), and 1 4B2 (1 4A0 ) states are given in Figure 3. The O-loss PECs for 1 4A1 and 1 4A2 were not calculated (too high in energy). The CASPT2 energies of the 2695



Figure 4. CASPT2 Cl-loss dissociation potential energy curves from the X 2B1, A 2A2, 1 2B2, 1 2A1, 1 4A2, 1 4B1, and 1 4A1 states of the OClO radical. Values in parentheses are the CASPT2 energies (eV) of the reactants (at their CASPT2 equilibrium geometries) and asymptote products relative to the X 2B1 reactant, and values in square brackets are the CASPT2 sum energies (eV) of the product groups relative to the X 2 B1 reactant (the experimental sum energies of the Cl-loss product groups are given in Table 3).

reactants and the asymptote products relative to X 2B1 are given in parentheses in Figure 3 (“CASPT2 energy of the reactants (the asymptote products) relative to X 2B1” will be abbreviated to “CASPT2 energy of the reactants (the asymptote products)” in the rest of the paper). The CASPT2 energy values (2.782.82 eV) for the X 2B1 (1 2A00 ), 1 2B2 (1 2A0 ), 1 2A1 (1 2Π), A 2A2 (2 2A00 ), 1 4B1 (1 4Σ), and 1 4B2 (1 4A0 ) asymptote products are close to the CASPT2 sum energy value (2.74 eV) of the O (3Pg) þ ClO (X 2Π) product group. In all six asymptote products, the Mulliken charge values at the O1 center are around 0.000 e and the ClO2 bond length values are equal to the CASPT2 bond length value of ClO (X 2Π). These facts indicate that the six OClO states correlate with O (3Pg) þ ClO (X 2Π) (the first O-loss dissociation limit). There are transition states (TSs) along the A 2A2 (2 2A00 ), 1 2A1 (1 2Π), and 1 4B1 (1 4Σ) PECs. The CASPT2 full geometry optimization calculations predict that TS (2 2A00 ), TS (1 2Π), and TS (1 4Σ) are 3.23, 3.02, and 2.90 eV higher in energy than X 2B1, respectively, and the R(ClO1) values in the optimized geometries are 2.025, 1.896, and 2.211 Å, respectively. There are intermediates (IMs, shallow minima) along the 1 2A1 (1 2Π) and 1 4B2 (1 4A0 ) PECs. The A 2A2 (2 2A00 ) PEC crosses the 1 4B2 (1 4A0 ) PEC. The most important configurations in the CASSCF wave functions change at TS (2 2A”) and TS (1 2Π) along the O-loss PECs. The changes in the most important configurations at TSs imply the interactions with higher lying states. 3.2.2. Cl-Loss Dissociation PECs. The CASPT2 Cl-loss dissociation PECs of the X 2B1, 1 2B2, A 2A2, 1 2A1, 1 4A2, 1 4B1, and 1 4A1 states are given in Figure 4. The Cl-loss PEC for 1 4B2 was not calculated (too high in energy). The CASPT2 energies of the reactants and the asymptote products (relative to X 2B1) are given in parentheses in Figure 4. In all seven asymptote products the Mulliken charge values at the Cl center are around 0.000 e. The CASPT2 energy values (0.370.42 eV) for the X 2B1, 2 A A2, 1 2A1, 1 4A2, 1 4B1, and 1 4A1 asymptote products are quite close to the CASPT2 sum energy value (0.24 eV) of the Cl (2Pu) þ O2 (X 3Σg) product group. In these asymptote products the

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O1O2 bond length values are all around 1.206 Å (the CASPT2 bond length value of O2 (X 3Σg)). These facts indicate that the X 2 B1, A 2A2, 1 2A1, 1 4A2, 1 4B1, and 1 4A1 states of OClO correlate with Cl (2Pu) þ O2 (X 3Σg) (the first Cl-loss dissociation limit). On the basis of similar analyses of our calculation results, we conclude that the 1 2B2 state of OClO correlates with Cl (2Pu) þ O2 (a 1Δg) (the second Cl-loss limit). As shown in Figure 4, there are TSs along all seven Cl-loss PECs. The CASPT2 full geometry optimization calculations predict that TS (A 2A2) (the TS along the A 2A2 Cl-loss PEC) is 4.33 eV higher in energy than X 2B1. The r value is 1.705 Å in the CASPT2 optimized geometry of TS (A 2A2) (1.712 Å in the CASSCF optimized geometry). TS (A 2A2) is 1.10 eV higher in energy than TS (2 2A00 ) along the A 2A2 O-loss PEC (see above). The TSs along the 1 2A1 and 1 4B1 Cl-loss PECs are also much higher in energy than the TSs along the 1 2A1 and 1 4B1 O-loss PECs, respectively (see above). There are two TSs and one shallow minimum between the TSs along the 1 2B2 Cl-loss PEC, and we denote the TSs at smaller and larger r values as TS1 (1 2B2) and TS2 (1 2B2), respectively. TS1 (1 2B2) was located in the CASPT2 and CASSCF full geometry optimization calculations, and TS2 (1 2B2) was located by performing the CASPT2 and CASSCF partial geometry optimization calculations at a set of fixed r values. Full geometry optimization calculations for TS2 (1 2B2) were difficult due to the avoided crossing between 1 2B2 and 2 2B2 in that region. We performed the CASSCF calculations for obtaining the r values in the CASSCF optimized geometries of the TSs (to be used in section 3.4.2). Our CASPT2 calculations predict that the r values in TS1 (1 2B2) and TS2 (1 2B2) are 1.486 and 1.950 Å, respectively, and that TS1 (1 2B2) and TS2 (1 2B2) are 2.96 and 3.15 eV higher in energy than X 2B1, respectively. Our CASSCF calculations predict that the r values in TS1 (1 2B2) and TS2 (1 2B2) are 1.502 and 2.000 Å, respectively, and that TS2 (1 2B2) is 0.20 eV higher in energy than TS1 (1 2B2). The CASSCF r values in the two TSs and the energy difference between the two TSs are close to the CASPT2 results. The CASPT2 relative energy (to X 2B1) and r value for TS1 (1 2B2) are close to the MRCI results23 (2.86 eV and 1.485 Å). The results for TS2 (1 2B2) were not reported in the MRCI paper.23 The most important configurations in the CASSCF wave functions change at TS (X 2B1), TS2 (1 2B2), TS (A 2A2), TS (1 2A1), TS (1 4A2), TS (1 4B1), and TS (1 4A1) along the Cl-loss PECs but do not change at TS1 (1 2B2) and all of the intermediates. For TS (A 2A2), TS1 (1 2B2), and TS2 (1 2B2), we performed the CASSCF frequency analysis calculations at the respective CASSCF optimized geometries (they are close to the respective CASPT2 geometries). 3.3. MECP Calculations. Our MECP search was carried out on the basis of the PEC crossings in the C2v PEC figure (Figure 2) and in the O- and Cl-loss dissociation PEC figures (Figures 3 and 4). For all the selected state/state pairs in the C2v and Cs symmetries we performed CASSCF MECP calculations (automatic searching). For three of the selected state/state pairs, we performed the CASPT2 calculations for locating the MECPs (not automatic searching). 3.3.1. CASSCF MECP Calculations. In Figure 2, the A 2A2 PEC crosses the 1 2A1, 1 2B2, 1 4B1, 1 4A2, and 1 4B2 PECs. We located the A 2A2/1 2A1, A 2A2/1 2B2, and A 2A2/1 4B1 MECPs but failed to find MECPs for the A 2A2/1 4A2 and A 2A2/1 4B2 state pairs. Since Peterson and Werner22,23 considered the 1 2A1/1 2B2 interaction in their suggested predissociation mechanism for A 2A2 (see Introduction), we also located the 1 2A1/1 2B2 MECP. 2696



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Table 4. Geometries of the Minimum Energy Crossing Points (MECPs) for Selected State/State Pairs (See Text) Calculated at the CASSCF and CASPT2 Levels (Bond Lengths, Å; bond angles, deg), Together with the CASSCF SpinOrbit Coupling Values (cm1) and CASPT2 State/State Energies (ΔE/ΔE (eV/eV), Relative to X 2B1) Calculated at the Located MECPs (the A 2A2/1 2 A1, A 2A2/1 2B2 (1), and 1 2A1/1 2B2 (1) MECPs Were Located in the CASPT2 Calculations) CASPT2 ΔE/ΔE a

MECP geometry spinorbit state/state MECP (method)

r

R(ClO1)

R(ClO2)

— OClO

coupling

E[TS (2 2A00 )] = 3.23

b

E[TS (A 2A2)] = 4.33 c

O-loss A2A2/1 2A1 (CASPT2)

0.870

1.620

1.620

115.0

10.8

2.79/2.79

A2A2/1 2B2 (1) (CASPT2)

0.824

1.700

1.700

122.0

61.3

2.92/2.92

1 2A1/1 2B2 (1) (CASPT2)

0.798

1.620

1.620

121.0

64.2

2.77/2.77

A2A2/1 4B1d (CASSCF)

0.498

1.806

1.806

148.0

66.6

3.44/3.29 (3.36)

1.853

1.606

127.2

112.0

2.97/3.17 (3.07)

2 2A00 /1 4A0 (A 2A2/1 4B2) (CASSCF) Cl-loss A2A2/1 2B2 (1) (CASPT2)

0.824

1.700

1.700

122.0

61.3

1 2B2/X 2B1e (CASSCF)

1.930

2.062

2.062

40.9

0.0

1 2B2/A2A2 (2) (CASSCF)

1.943

2.074

2.074

40.9

69.4

3.20/3.22 (3.21)

1 2B2/1 2A1 (2) (CASSCF)

1.922

2.054

2.054

41.3

124.9

3.01/3.14 (3.08)

1 2B2/1 4A1e (CASSCF)

2.059

2.126

2.126

28.9

0.5

4.24/3.96 (4.10)

1 2B2/1 4A2 (CASSCF)

2.105

2.185

2.185

31.1

120.5

3.27/2.55 (2.91)

1 2B2/1 4B1e (CASSCF)

1.848

1.989

1.989

43.4

9.8

2.98/2.97 (2.98)

2.92/2.92 3.15/3.11 (3.13)

a

Values in parentheses are the averages. b Barrier energy value (eV) along the CASPT2 O-loss dissociation PEC of the A 2A2 state. c Barrier energy value (eV) along the CASPT2 Cl-loss dissociation PEC of the A 2A2 state. d The A 2A2/1 4B1 MECP in O-loss predissociation of A 2A2 was not considered (due to the high energy). e The 1 2B2/X 2B1, 1 2B2/1 4A1, and 1 2B2/1 4B1 MECPs in Cl-loss predissociation of A 2A2 were not considered (due to the small spinorbit couplings).

In Figure 3, the A 2A2 (2 2A00 ) PEC crosses the 1 4B2 (1 4A0 ) PEC, and we located the 2 2A00 /1 4A0 MECP in the Cs symmetry. In the “O-loss” section of Table 4 we report the geometries of the A 2A2/1 4B1 and 2 2A00 /1 4A0 MECPs located in the CASSCF calculations and the CASPT2 state/state energies (relative to X 2B1) at the CASSCF MECPs (the important A 2A2/1 2A1, A 2A2/1 2B2, and 1 2A1/1 2B2 MECPs were located in the CASPT2 calculations, and the results will be described in section 3.3.2). The small r value and the large OClO angle values in the geometries of the A 2A2/1 4B1 and 2 2A00 /1 4A0 MECPs imply that the two MECPs are “on the way” to the O-loss dissociation. At the two located MECPs, the CASSCF energy differences between the states in the pairs are smaller than 0.02 eV and the CASPT2 energy differences are smaller than 0.2 eV. We will use the averages of the CASPT2 energies in the discussion. It is noted that the CASPT2 average energy value (3.36 eV) at the A 2A2/1 4B1 MECP is larger than the CASPT2 energy value (3.23 eV) of TS (2 2A00 ) (representing the barrier along the A 2A2 (2 2A00 ) adiabatic O-loss dissociation path). We will not consider the A 2A2/1 4B1 MECP in the suggestion of O-loss predissociation mechanisms of A 2A2. In Figure 4 the 1 2B2 Cl-loss PEC (correlating to the second Cl-loss limit) crosses the X 2B1, A 2A2, 1 2A1, 1 4A2, 1 4B1, and 1 4 A1 Cl-loss PECs (correlating to the first limit) at the r values of around 2.0 Å. We located the 1 2B2/X 2B1, 1 2B2/A 2A2, 1 2B2/1 2 A1, 1 2B2/1 4A2, 1 2B2/1 4B1, and 1 2B2/1 4A1 MECPs in the CASSCF calculations. In the “Cl-loss” section of Table 4 we report the geometries of the six CASSCF MECPs and the CASPT2 state/state energies (relative to X 2B1) at the CASSCF MECPs. In the geometries of these six MECPs, the r values are larger than 1.8 Å and the OClO angles are smaller than 45, indicating that these six MECPs are “on the way” to the Cl-loss dissociation and correspond to the six PEC crossing points at the

r values of around 2.0 Å in Figure 4. At these CASSCF MECPs, the CASSCF energy differences between the states in the pairs are smaller than 0.001 eV and the CASPT2 energy differences are smaller than 0.3 eV, except the 1 2B2/1 4A2 MECP at which the CASPT2 energy difference is 0.72 eV (large CASPT2 energy difference is due to the fact that the CASSCF MECP geometry is quite different from the CASPT2MECP geometry). The averages of the CASPT2 energies will be used in the discussion. We will locate the A 2A2/1 2B2 and 1 2A1/1 2B2 MECPs “on the way” to the O-loss dissociation (see section 3.3.2). We denote the A 2A2/ 1 2B2 and 1 2A1/1 2B2 MECPs “on the way” to the O-loss dissociation (with the r values smaller than 1.0 Å; see below) as A 2 A2/1 2B2 (1) and 1 2A1/1 2B2 (1), and the A 2A2/1 2B2 and 1 2A1/1 2B2 MECPs “on the way” to the Cl-loss dissociation (with the r values larger than 1.0 Å; see above) as A 2A2/1 2B2 (2) and 1 2A1/1 2B2 (2). It is noted that the CASPT2 average energy values at the MECPs listed in the “Cl-loss” section of Table 4 are smaller than the CASPT2 energy value (4.33 eV) of TS (A 2A2) (representing the barrier along the A 2A2 adiabatic Cl-loss dissociation path). 3.3.2. CASPT2MECP Calculation. The A 2A2/1 2B2, A 2A2/1 2 A1, and 1 2A1/1 2B2 MECPs, involved in the previously suggested predissociation mechanisms of A 2A2 (see Introduction), were located in the CASPT2 calculations (the CASPT2 C2v PEC (E( — OClO)) calculations at a set of fixed R(ClO) values). In the “O-loss” section of Table 4 we report the geometries of the A 2A2/1 2B2, A 2A2/1 2A1, and 1 2A1/1 2B2 MECPs located in the CASPT2 calculations and the CASPT2 energies (relative to X 2B1) at the MECPs. The small r values and large OClO angle values in the CASPT2 geometries of the A 2A2/1 2B2, A 2A2/1 2A1, and 1 2A1/ 1 2B2 MECPs imply that the three MECPs are “on the way” to the O-loss dissociation, and the A 2A2/1 2B2 and 1 2A1/1 2B2 MECPs should be denoted as A 2A2/1 2B2 (1) and 1 2A1/1 2B2 (1) (with the 2697


The Journal of Physical Chemistry A r values smaller than 1.0 Å; see above). The A 2A2/1 2B2 (1) MECP is also listed in the “Cl-loss” section of Table 4 since it is very important in our theoretical prediction for both the O- and Cl-loss predissociation mechanisms of OClO (A 2A2) (see below). 3.3.3. SpinOrbit Couplings at the Located MECPs. The two crossing states may have two kinds of interaction (vibronic and spinorbit couplings) both related to the nonradiative transitions. One may estimate strengths of the interactions between the two states by calculating the couplings at their MECP. For all of the state/state pairs (Table 4), except 12A1/12B2, the vibronic coupling should not be considered due to the different spin multiplicities (doublet vs quartet) or unmatched space symmetries (direct products being not in the a1 or b2 symmetry). In the present study we will consider only the spinorbit coupling (in the previous MRCI studies,22,23 spinorbit coupling was also expected to be the dominant predissociation mechanisms for A 2 A2). We calculated the CASSCF spinorbit coupling values at all of the (CASSCF and CASPT2) MECPs listed in Table 4, and the calculated values are given in Table 4. Since the calculated spinorbit coupling values at the 1 2B2/X 2B1, 1 2B2/1 4B1, and 1 2B2/1 4A1 MECPs in “Cl-loss” section are too small (smaller than 10.0 cm1), we will not consider these three MECPs in the suggestion of Cl-loss predissociation mechanisms of A 2A2. 3.4. Predicted Predissociation Mechanisms of the A 2A2 State. 3.4.1. O-Loss Predissociation Mechanisms of the A 2A2 State. On the basis of the MECP results (energies and spinorbit couplings) in the “O-loss” section of Table 4 and the O-loss PECs (energies at the stationary points and dissociation limits) in Figure 3, we suggest the O-loss predissociation mechanisms of the A 2A2 state as follows.

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is called the needed energy for this process. The needed energies for processes 13 are 3.02, 2.92, and 3.07 eV, respectively. As shown in Table 4, the spinorbit coupling values at the A 2A2/1 2B2 (1) and 2 2A00 /1 4A0 MECPs are large (larger than 60 cm1) and the value at the A 2A2/1 2A1 MECP is not too small (10.8 cm1). On the basis of the CASPT2 needed energies, we conclude that process 2 (2.92 eV) is the most favorable process for dissociation to the first O-loss limit. The 1 2A1/1 2B2 (1) MECP does not appear in our suggested processes, and it will be mentioned in the discussion on the mechanisms suggested in the previous MRCI studies22,23 (see the following). As mentioned in section 3.3.1, the A 2A2/1 4B1 MECP cannot be considered due to its high energy. 3.4.2. Cl-Loss Predissociation Mechanisms of the A 2A2 State. On the basis of the MECP results (energies and spinorbit couplings) in “Cl-loss” section of Table 4 and the Cl-loss PECs (stationary points and dissociation limits) in Figure 4, we suggest the Cl-loss predissociation mechanisms of the A 2A2 state as follows. process 4: OClO ðA 2 A 2 Þ ð2:67 eVÞ f A 2 A 2 =1 2 B2 MECP ð2:92 eVÞ f TS1 ð1 2 B2 Þ ð2:96 eVÞ f TS2 ð1 2 B2 Þ ð3:15 eVÞ f 1 2 B2 f Cl ð2 Pu Þ þ O2 ða 1 Δg Þ ð1:23 eV, the second Cl-loss limitÞ; process 5: OClO ðA 2 A 2 Þ ð2:67 eVÞ f A 2 A 2 =1 2 B2 ð1Þ MECP ð2:92 eVÞ

f TS1 ð1 2 B2 Þ ð2:96 eVÞ

process 1:

f 1 2 B2 =A 2 A 2 ð2Þ MECP ð3:21 eVÞ f A 2 A 2

OClO ðA 2 A 2 Þ ð2:67 eVÞ f A 2 A 2 =1 2 A 1 MECP ð2:79 eVÞ

f Cl ð2 Pu Þ þ O2 ðX 3 Σg Þ ð0:24 eV, the first Cl-loss limitÞ;

f 1 Π f TSð1 ΠÞ ð3:02 eVÞ 2

2

f IM ð1 2 ΠÞ ð2:64 eVÞ f O ð3 Pg Þ

process 6: OClO ðA 2 A 2 Þ ð2:67 eVÞ f A 2 A 2 =1 2 B2 ð1Þ MECP ð2:92 eVÞ

þ ClO ðX 2 ΠÞ ð2:74 eV, the first O-loss limitÞ;

f TS1 ð1 2 B2 Þ ð2:96 eVÞ

process 2: f 1 2 B2 =1 2 A 1 ð2Þ MECP ð3:08 eVÞ f 1 2 A 1

OClO ðA 2 A 2 Þ ð2:67 eVÞ f A 2 A 2 =1 2 B2 ð1Þ MECP ð2:92 eVÞ

f Cl ð2 Pu Þ þ O2 ðX 3 Σg Þ ð0:24 eV, the first Cl-loss limitÞ;

f 1 2 B2 ð1 A 0 Þ f O ð3 Pg Þ 2

þ ClO ðX 2 ΠÞ ð2:74 eV, the first O-loss limitÞ; process 7: process 3: 2 00

4 0

OClO ðA A 2 Þ ð2:67 eVÞ f 2 A =1 A MECP ð3:07 eVÞ 2

OClO ðA 2 A 2 Þ ð2:67 eVÞ f A 2 A 2 =1 2 B2 ð1Þ MECP ð2:92 eVÞ

f TS1 ð1 2 B2 Þ ð2:96 eVÞ f TS2 ð1 2 B2 Þ ð3:15 eVÞ

4 0

f 1 A f O ð3 P g Þ

f 1 2 B2 =1 4 A 2 MECP ð2:91 eVÞ f 1 4 A 2 f Cl ð2 Pu Þ

þ ClO ðX 2 ΠÞ ð2:74 eV, the first O-loss limitÞ Values in parentheses are the CASPT2 energies of the species relative to X 2B1 (abbreviated to “CASPT2 energies of the species”). The maximum CASPT2 energy value along a process

þ O2 ðX 3 Σg Þ ð0:24 eV, the first Cl-loss limitÞ Values in parentheses are the CASPT2 energies of the species relative to X 2B1 (the CASPT2//CASSCF average energies for 2698



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Table 5. O- and Cl-Loss Predissociation Processes of OClO (A 2A2) Predicted in the Present Study [Dissociation Products, Needed Energies (ΔE (eV), Maximum CASPT2 Relative Energy to X 2B1 along a Process), and Brief Descriptions of the Processes] process

ΔE a

dissociation products

brief description of process

O-loss 1

first limit

3.02 (TS (1 2Π))

2

first limit

2

2

A 2A2/1 2B2 (1) f 1 2A0 (1 2B2)

2 00

4 0

3 Cl-loss 4

a

A 2A2/1 2A1f 1 2Π (1 2A1) f TS (1 2Π)

2.92 (A A2/1 B2 (1) MECP)

first limit

3.07 (2 A /1 A MECP)

2 2A00 (A 2A2)/1 4A0 (1 4B2)

second limit

3.15 (TS2 (1 2B2))

A 2A2/1 2B2 (1) f TS1 (1 2B2) f TS2 (1 2B2)

5

first limit

6

first limit

7

first limit

2

2

A 2A2/1 2B2 (1) f TS1 (1 2B2) f 1 2B2/A 2A2 (2)

2

2

A 2A2/1 2B2 (1) f TS1 (1 2B2) f 1 2B2/1 2A1 (2)

3.21 (1 B2/ A A2 (2) MECP) 3.08 (1 B2/ 1 A1 (2) MECP)

A 2A2/1 2B2 (1) f TS1 (1 2B2) f TS2 (1 2B2) f 1 2B2/1 4A2

2

3.15 (TS2 (1 B2))

In the parentheses is the species along the process, at which the maximum CASPT2 relative energy value appears.

the 1 2B2/A 2A2 (2), 1 2B2/1 2A1 (2), and 1 2B2/1 4A2 MECPs). As mentioned in section 3.3.3, we do not consider the 1 2B2/X 2B1, 1 2B2/1 4A1, and 1 2B2/1 4B1 MECPs in the “Cl-loss” section of Table 4 due to their small spinorbit couplings (smaller than 10.0 cm1). The spinorbit coupling values at the A 2A2/1 2B2 (1), 1 2B2/A 2A2 (2), 1 2B2/1 2A1 (2), and 1 2B2/1 4A2 MECPs (involved in processes 57) are large (see Table 4). Along each of processes 57 there are two MECPs. We should decide that the three (second) MECPs (1 2B2/A 2A2 (2), 1 2B2/1 2A1 (2), and 1 2B2/1 4A2) appear before, after, or between TS1 (1 2B2) and TS2 (1 2B2) along the 1 2B2 PEC (actually the MECPs are not along the 1 2B2 PEC) by checking the r values in the two TSs and MECPs. Since we have only the CASSCF geometries of the MECPs, we use the r values in the CASSCF geometries of TS1 (1 2B2) (r = 1.502 Å) and TS2 (1 2B2) (r = 2.000 Å) (see section 3.2.2). On the basis of the r values in the three (second) MECPs in Table 4, we have decided that the 1 2B2/A 2A2 (2) and 1 2B2/1 2A1 (2) MECPs (in processes 5 and 6, respectively) appear between the two TSs and the 1 2B2/1 4A2 MECP (in process 7) appears after TS2 (1 2B2). The needed energies for processes 47 are 3.15, 3.21, 3.08, and 3.15 eV, respectively. On the basis of the CASPT2 needed energies, we conclude that process 6 (3.08 eV) is the most favorable process for dissociation to the first Cl-loss limit. In Table 5 we list all seven suggested (O- and Cl-loss) dissociation processes of OClO (A 2A2), including the dissociation limits, needed energies, and brief descriptions. 3.5. Discussion on the Mechanisms Suggested in the Previous MRCI Studies. The mechanisms for O-loss predissociation of OClO (A 2A2) suggested in the previous MRCI study of Peterson and Werner22 were expressed by processes I and II in the Introduction. Their process I (A 2A2 f 1 2A1 f 1 2Π f O (3Pg) þ ClO (X 2Π)) is just our process 1. Process 1 is energetically less favorable than process 2 (see section 3.4.1) and the spinorbit coupling (10.8 cm1) at the A 2A2/1 2A1 MECP involved in process 1 is quite small. Their process II (A 2A2 f 1 2A1 f 1 2B2 f O (3Pg) þ ClO (X 2Π)) proceeds via the A 2A2/1 2A1 and 1 2A1/1 2B2 PES crossings. The CASPT2 energy and CASSCF spinorbit coupling at the 1 2A1/1 2B2 (1) MECP (located in the CASPT2 calculations; see section 3.3.2) are 2.77 eV and 64.2 cm1, respectively. By using our calculation results, process II can be written as follows: OClO (A 2A2) (2.67 eV) f A 2A2/1 2A1 MECP (2.79 eV, 10.8 cm1) f 1 2A1/1 2B2 (1) MECP (2.77 eV,

64.2 cm1) f 1 2B2 (1 2A0 ) f O (3Pg) þ ClO (X 2Π) (2.74 eV, the first O-loss limit), and the CASPT2 needed energy is 2.79 eV. We would compare process II (rewritten using our results and notations) with our process 2. Though the CASPT2 needed energy value (2.79 eV) for process II is 0.13 eV smaller than the value (2.92 eV) for process 2, process II proceeds via two MECPs (two PES/PES crossings) and the spinorbit coupling at the first MECP is quite small. Considering the probabilities of radiationless transitions,41 the process via two PES/PES crossings should be less favorable than the process via one PES/PES crossing. Our calculations indicate that the A 2A2 state can directly go to 1 2B2 via one MECP (A 2A2/1 2B2 (1) MECP). That is process 2. The mechanisms for Cl-loss predissociation of OClO (A 2A2) suggested in the previous MRCI study23 were expressed by processes III (A 2A2 f 1 2A1 f 1 2B2 f the second limit) and IV (A 2A2 f 1 2A1 f 1 2B2 f 1 2A1 f the first limit) in the Introduction. In their processes, the A 2A2 state was considered to go to the important 1 2B2 state via 1 2A1. On the basis of our calculation results (MECPs, Cl-loss PECs, and TSs), process III can be written as follows: A 2A2 (2.67 eV) f A 2A2/1 2A1 MECP (2.79 eV) f 1 2A1/1 2B2 (1) MECP (2.77 eV) f TS1 (1 2B2) (2.96 eV) f TS2 (1 2B2) (3.15 eV) f 1 2B2 f Cl (2Pu) þ O2 (a 1Δg) (1.23 eV, the second Clloss limit), and the CASPT2 needed energy is 3.15 eV (at TS2 (1 2B2)). Process IV can be written as follows: A 2A2 (2.67 eV) f A 2A2/1 2A1 MECP (2.79 eV, 10.8 cm1) f 1 2A1/1 2B2 (1) MECP (2.77 eV) f TS1 (1 2B2) (2.96 eV) f 1 2B2/1 2A1 (2) MECP (3.08 eV) f 1 2A1 f Cl (2Pu) þ O2 (X 3Σg) (0.24 eV, the first Clloss limit), and the CASPT2 needed energy is 3.08 eV (at 1 2B2/1 2 A1 (2) MECP). We would compare processes III and IV (rewritten using our results and notations) with our processes 4 and 6, respectively. The first two steps (A 2A2 f A 2A2/1 2A1 MECP (2.79 eV) f 1 2A1/1 2B2 (1) MECP (2.77 eV)) in processes III and IV seem to be energetically more favorable than the first step (A 2A2 f A 2A2/1 2B2 (1) MECP (2.92 eV)) in processes 4 and 6. However, the CASPT2 needed energies for these processes are determined by the later species along the processes (TS2 (1 2B2) and 1 2B2/1 2A1 (2) MECP, respectively). Therefore, process III has the same needed energy (3.15 eV) as process 4, and process IV has the same needed energy (3.08 eV) as process 6. However, processes III and IV have to pass through one more MECP (PES/PES crossing) than processes 4 and 6, respectively. In the CASPT2 C2v PEC figure (Figure 2) and in the MRCI C2v PEC figure22 (calculated at the fixed ClO bond distance value of 1.470 Å), the A 2A2/1 2B2 PEC crossing point is far away 2699


The Journal of Physical Chemistry A from and higher in energy than the A 2A2 PEC minimum, which might lead to the ignorance of the interaction between the A 2A2 and 1 2B2 PESs. The A 2A2/1 2B2 (1) MECP located in our CASPT2 calculations is close in CASPT2 energy to the A 2A2 (PES) minimum (see Tables 1 and 4). 3.6. Discussion on the Mechanisms Suggested in the Previous Experimental Study. We would give our explanations for the main conclusions (summarized in points iiii by us in the Introduction) in the experimental study of Davis and Lee10 on the O- and Cl-loss photodissociation mechanisms of OClO (A 2A2). The reasons for point i (“the dominant channel is O-loss”) are that the needed energy (2.92 eV) of the most favorable process (process 2) for O-loss dissociation is 3.7 kcal/ mol lower than that (3.08 eV) of the most favorable process (process 6) for Cl-loss dissociation and that process 6 undergoes one more MECP (one more PES/PES crossing) than process 2. Our CASPT2 calculations predict that TS (2 2A00 ) along the A 2A2 (2 2A00 ) O-loss PEC is 3.23 eV higher in energy than X 2B1, and this calculated barrier value is very close to the experimental value of 3.27 eV10 mentioned in point ii (see Introduction). We explain point ii just by considering that at E > 3.27 eV (3.23 eV) the OClO (A 2A2) system undergoes the adiabatic dissociation along the A 2A2 (2 2A00 ) O-loss PEC (see Figure 3). Point iii was concerning the photodissociation mechanisms at E < 3.1 eV. The process described in the first half of point iii is just our process 1 (the needed energy being 3.02 eV), and the process mentioned in the second half is just our process 4 (the needed energy value of 3.15 eV being close to the value of 3.1 eV).

4. CONCLUSIONS For studying O- and Cl-loss predissociation mechanisms of OClO (A 2A2), we calculated the CASPT2 O- and Cl-loss dissociation PECs for the four doublet (X 2B1, A 2A2, 1 2A1, and 1 2B2) and four quartet states of OClO and carried out the CASPT2 and CASSCF MECP calculations for the selected state/state pairs. On the basis of our calculation results (the energies and geometries of the TSs and MECPs and the spinorbital couplings at the MECPs), we predict three processes (13) for O-loss predissociation of A 2A2 and four processes (47) for Cl-loss predissociation of A 2A2. The most favorable process for O-loss predissociation (to the first limit) is process 2 {OClO (A 2A2) f A 2A2/1 2B2 (1) MECP f 1 2B2 (1 2A0 ) f O (3Pg) þ ClO (X 2Π) (with a needed energy of 2.92 eV)}, and the most favorable process for Cl-loss predissociation (to the first limit) is process 6 {OClO (A 2A2) f A 2A2/1 2B2 (1) MECP f TS1 (1 2B2) f 1 2B2/1 2A1 (2) MECP f 1 2A1 f Cl (2Pu) þ O2 (X 3Σg) (with a needed energy of 3.08 eV)}. In the previous MRCI papers22,23 the authors suggested two mechanisms for O-loss predissociation of A 2A2 (called processes I and II) and two mechanisms for Cl-loss predissociation of A 2A2 (called processes III and IV). We rewrite processes IIIV using our calculation results and notations (process I is less important), and the (rewritten) processes IIIV are comparable with our processes 2 (O-lss), 4 (Cl-loss), and 6 (Cl-loss), respectively. The first two steps in processes IIIV describe that the A 2A2 state goes to 1 2B2 via the A 2A2/1 2A1 and 1 2A1/1 2B2 MECPs, while the first steps in our processes 2, 4, and 6 describe that the A 2 A2 state directly goes to 1 2B2 via the A 2A2/1 2B2 MECP. For O-loss predissociation, process II is 0.13 eV energetically more favorable than process 2, but process II proceeds via two MECPs. For Cl-loss predissociation, processes III (to the second limit)

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and IV (to the first limit) have the same needed energies as processes 4 and 6, respectively, but processes III and IV have to pass through one more MECP (PES/PES crossing) than processes 4 and 6, respectively. In the explanation of the experimental facts of Davis and Lee10 concerning photodissociation, only part of our calculation results were used. The experimental workers4,5,18 suggested that primary photochemistry dynamics of the A 2A2 state involved the couplings of A 2A2 to the close lying 1 2A1 and 1 2B2 states. The A 2A2/1 2B2 coupling is involved in our processes 2 and 47, and the A 2A2/1 2A1 coupling is involved in our process 1.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: 86-10-88256129. Fax: 86-10-88256129. E-mail: mbhuang1@ gucas.ac.cn.

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CASSCF and CASPT2 Study on O- and Cl-Loss Predissociation

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