lo

Ma

itute

of Ch

Univ

3; in

f nitr

sp an

7.04

t less

ng th

discussed.

sociation is of great interest for stratospheric chemistry.

resulted in three broad unstructured bands. The rstabsorption 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

ClNO2 ! ClNO2 DH 34:0 kcal mol 1

ground X2A1 or an electronically excited state, most

likely A2B2. Subsequent experiments at 248 nm by their

group [4], where photofragment translational energy

spectroscopy was used to detect the primary photolysis

products, also conrmed this channel is dominant andincludes the formation of the NO2 fragment in dierent

electronic states. Plenge et al. [8] photolyzed ClNO

s 383*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 pre-dicted 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

X1A1 ground state. The UV absorption spectrum of

gaseous nitryl chloride has been reported rst by Illies

and Takacs [3]. Measurements between 185 and 400 nm

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 reso-

nance uorescence detection of Cl or O atoms [5], and at

350 nm the quantum yields of 0:93 0:15 for Cl and

as frozen-core orbitals.

From this set of reference congurations (mains) all

EMRD-CI Q EMRD-CI 1 c20Eref EMRD-CI=c20;

where c20 is the sum of squared coecients of the refer-ence species in the total CI wavefunction and Eref isthe energy of the reference congurations.

We computed three singlet and three triplet states per

IRREP for ClNO2 of the C2v symmetry. The number ofCSFs 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 congurations. 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

CCSD(T)/6-31G(d) calculation, with the experimental values in para-

hysics Letters 383 (2004) 8488 85single and double excitations in the form of congura-

tion state functions (CSFs) are generated. All congu-

rations of this set with an energy contribution DET above a given threshold T were selected, i.e., the contri-bution of a conguration larger than this value relative

to the energy of the reference set is included in the nal

wavefunction. Selection thresholds of T 107 andT 108 hartrees were used for singlet and triplet states,respectively. The eect of those congurations, which

contribute less than T 107 or T 108 hartrees, isaccounted for in the energy computation (E(MRD-CI))by the perturbative k-extrapolation [12,13]. The contri-bution of higher excitations is estimated by applyingspectroscopy to detect the photolysis product. The Cl(2P)

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 ex-

cited states reported in the literature. It is clear that suchinvestigations 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 ClN

bond cleavage.We have performed the calculations using

the multi-reference conguration interaction method.

2. Computational methods

The equilibrium geometry of nitryl chloride ClNO2was fully optimized using the single and double excita-

tion coupled-cluster method, including a perturbation

estimate of the eects of connected triple excitationsCCSD(T) [9] with the 6-31G(d), 6-311G(d), and

6-31G(2d) basis sets using the GAUSSIANAUSSIAN 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

conguration interaction methodMRD-CI implemented

in the DIESELIESEL program [11]. The selection of the refer-ence congurations by a summation threshold is carried

out automatically. We used a summation threshold of

0.85, which means that the sum of the squared coe-

cients of all reference congurations selected for each

state (root) is above 0.85. The number of reference

congurations per irreducible representation (IRREP)

was in the range between 8 and 17. An analysis of the

molecular orbitals (MO) involved in these selected ref-erence congurations justied the prior choice of treating

the 24 valence electrons as active while the remaining

electrons were kept in doubly occupied orbitals dened

A. Lesar et al. / Chemical PLanghoDavidson correction formulaThe 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 seenthat both values are in reasonable agreement. Further-

more 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

N

O

Cl

1.209 (1.202)

1.885(1.837)

131.9(130.2)

114.0(114.9)

Fig. 1. Equilibrium geometry of nitryl chloride, ClNO2, resulting fromand by a negative ion function for chlorine atom, thus

the cc-pVDZ+ sp and cc-pVTZ+ sp basis sets. The ex-

ponents taken are asN 0:028 and apCl 0:049.The potential energy surfaces of the ground and ex-

cited states were computed with the cc-pVDZ+ sp basis

set. The ClNO2 bond length was changing stepwise in

the range from 1.78 to 10 AA, while all other geometricalparameters were optimized for the ground state at theCCSD(T)/6-31G(d) level of theory.

3. Results and discussiontheses [16]. The bond lengths are given in AA, bond angles in degrees.

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 oscil-

lator strengths are quite similar, therefore we believethat 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 conguration of ClNO2 is

5a124b221a222b12 if the 24 valence electrons aretreated as active in the CI calculations. As can be seen

from Table 1 in conjunction with Fig. 2 the lowest ex-citations 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

1a . As can be seen from Fig. 3, the LUMO 6a can be

somewhat smaller f -value of 0.02 in line with the ob-servation 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 oscillatorstrength mainly from charge transfer. The rst dipole

allowed transition, 4b2 ! 6a1, corresponds to n!r(NCl) and is less intense.

The singlettriplet splitting of most states is relatively

small, up to 0.4 eV. This is expected for transitions from

f to singlet excited states of ClNO2

cc-pVTZ+ sp

DE f DEtrip DEexp

0.0 0.0

0004 4.41 0.0003 4.02 4.14.52 0.0 4.39

01 5.12 0.001 4.72

001 5.07 0.0001 4.82

5.28 0.0 5.39

06 5.74 0.006 5.61

2 5.77 0.02 5.82 5.86.33 0.0 6.37

6 7.04 0.67 4.01 >6.2

8 7.25 0.30 4.31

07 8.95 0.001 8.91

30

20

10

0

E / e

V

2a1

3a1

4a1

5a1

6a17a1

8a1

2b2

3b24b2

5b2

1a2

1b1

2b1

3b1

4b1

2 2v

the SCF level.

86 A. Lesar et al. / Chemical Physics Letters 383 (2004) 84882 1

considered to be an antibonding r(ClN) type MO,while MO 3b1 is p(NO2) antibonding judged on thebasis of a nodal plane between the N and O centers. On

the other hand MO 5a1 shows a r(ClN) bondingcharacter, 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 ox-ygen centers, whereas MO 2b1 is composed of n(Cl)chlorine lone-pair. Consequently, it can be deduced

from qualitative MO analysis that r! r(ClN),namely 5a1 ! 6a1, should be the dominant transition ofthe electronic absorption spectrum. Our calculations

place this transition at 7.04 eV with a large oscillator

strength, f 0:67. Two further transitions are com-puted with sizeable f -values. 31B2 X1A1 correspondto 1a2 ! 3b1 at 7.25 eV. This transition can be consid-ered as a p(O2)! p(NO2) type for which a mediumsize oscillator strength of f 0:30 could be expected.Further, transition 21A1 X1A1 corresponds to 2b1 !3b1 and is computed to be at 5.77 eV. It shows a

Table 1

Calculated vertical excitation energies DE (eV) and oscillator strengths

State Excitation cc-pVDZ+ sp

DE f

11A1 5a14b221a222b12 0.0 0.011B2 4b2 ! 6a1 4.48 0.011A2 4b2 ! 3b1 4.61 0.021B1 5a1 ! 3b1 5.10 0.011B1 2b1 ! 6a1 5.20 0.021A2 1a2 ! 6a1 5.41 0.021B2 3b2 ! 6a1 5.83 0.021A1 2b1 ! 3b1 5.84 0.031A2 3b2 ! 3b1 6.41 0.031A1 5a1 ! 6a1 7.12 0.631B2 1a2 ! 3b1 7.45 0.241B2 4b2 ! 7a1 8.70 0.0

DEtrip is related to the excitation energies for corresponding triplet excita50

40

1a1

1b2

Fig. 2. Schematic diagram of the molecular orbital energy spectrum of

the ground state conguration of ClNO , C symmetry, obtained attions. For comparison the experimental values [3,4] are included.

0

1

2

3

4

5

6

7

8

2 3 4 5 6 7 8 9 10

E / e

V

RClN /

1.62

4.41

4.69

5.26

NO2( 12A1 ) + Cl ( 2P )11A 1, 1

1B2 ,11B1

NO2( 12A2 ) + Cl( 2P )31A1

NO2( 12B2 ) + Cl( 2P )21A 2, 2

1B2 ,31B2

NO2( 12B1 ) + Cl( 2P )11A 2, 2

1B1 ,21A 1

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 ClN bond.

A. Lesar et al. / Chemical Physics Letters 383 (2004) 8488 87ClN

O

O

6a1, LUMO

4b2, LUMO5a1

3b1in-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, and4b2 ! 7a1, for example). Above 5 eV three triplet statesare erroneously obtained somewhat above their corre-

sponding singlet excitations, but this discrepancy is

within the error margin of the present calculations

(0.3 eV). Very large singlettriplet splittings on the orderof 3 eV are observed for the states resulting from

5a1 ! 6a1 and 1a2 ! 3b1 transitions. In both casesupper and lower orbitals have considerable overlap

leading to sizeable exchange integrals which are impor-

tant 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 ST gap for the1B2(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 ClN bond cleavage. Fig. 4 gives the

potential energy surfaces for low-lying singlet excited states

of ClNO2 in the C2v symmetric fragmentation pathway

1a1 2b1

Fig. 3. Charge density contours of characteristic occupied valence

orbitals (5a1, 4b2, 1a2, 2b1) and the lowest unoccupied molecular

orbitals (6a1, 3b1).along the ClN bond. It can be seen that 11B2 and 11B1

states populating the r(NCl) antibonding 6a1 orbitalare highly repulsive, implying that direct and fast photo-

dissociation should occur leading to the ground stateproducts, Cl(3P) + NO2(1

2A1). 31A1 state dissociates to

the NO2 in its rst excited state, while the 31B2 state

correlates with the dissociation channel which corre-

sponds to the NO2 in its second excited state. Between 4.5

and 5.5 eV various crossing of states occur, so that ex-

citation 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-lyingtriplet excited states are presented. Several triplet states,

13A1, 13B2 and 1

3B1, have a repulsive character and

correlate with products in their ground states, while 33A2state dissociates to NO2 in its rst excited state. 0

1

2

3

4

5

6

7

8

2 3 4 5 6 7 8 9 10

E / a

u

RClN /

1.61

5.22

4.39

4.62

NO2(12A1) + Cl(2P)13A1, 1

3B2 ,13B1

NO2(12A2) + Cl(2P)

33B2, 23A 2, 2

3A 1 NO2(12B2) + Cl(2 P)

33A 2

NO2(12B1) + Cl(2P)13A 2, 2

3B2 ,23B1

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 ClN bond.

4. Summary

The computed electronic spectrum of ClNO2 is char-

acterized by two strong transitions at 7.04 eV (31A1 X1A1, r! r type) and 7.25 eV (31B2 X1A1,p(O2)! p(NO2) type). In addition, a further transitionis calculated at 5.77 eV (21A1 X1A1, n(Cl)! p(NO2)type) with a somewhat lower intensity (f 0:02). Thecomputed values nicely match the experimental spec-

trum: the strong band at around 215 nm (5.8 eV) coin-

cides with the calculated excitation energy of the 21A1and 21B2 states around 5.8 eV. Also, a strong increase of

absorption below 200 nm (6.2 eV) supports the calculatedintense 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 path-

ways along ClN cleavage show that 11B2 and 11B1

excited states are highly repulsive, implying that direct

and fast photodissociation should occur leading to the

ground state products, Cl(3P) + NO2(12A1). Pathways

leading to excited NO2 products are also shown.

Acknowledgements

partly by the NATO collaborative linkage grant

EST.CLG.977083. The authors thank M. Hanrath for

assistance in DIESELIESEL program and M. Schnell for

valuable comments.

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Ab initio MRD-CI study on the low-lying excited states of ClNO2IntroductionComputational methodsResults and discussionSummaryAcknowledgementsReferences