The spectroscopy of OClO in polar liquids

SpectmchimicaActa. Vol. 48A. No. 9, pp. 1293-1301. 1992 Fvintedin Great Britain

The spectroscopy of OCIO in polar liquids ROBERT

C. DUNN, BKET N. FLANDERS,VERONICAVAIDA*~ and JOHN D. SIMON*

The Department Abstract-The

near-UV

‘AI electronic

maximum

models.

from different of OCIO

University

A ‘A ?+X’B, on dielectric

Changes

at 355 nm indicate between

involvement

of this OCIO

of OCIO

properties

is examined

(to form Cl0

heterogeneous

La Jolla. CA 92093-0341.

in polar solutions.

of the solvent can be qualitatively

U.S.A.

Both the 1,,;,, of the

described

of

by continuum

widths are discussed in terms of. inhomogeneity

resulting

UV

photolysis

Picosecond time resolved absorption

a solvent dependence

bond breakage

at San Diego,

vihronic widths vary significantly with solvent. The dependence

in the inhomogeneous

solvation geometries.

competition

of California

transition

band and the inhomogeneous

the absorption solvation

of Chemistry.

on the excited and 0)

photochemistry

studies following the

state reactivity.

and isomerization in stratospheric

In many solvents.

(to form CIOO).

ozone depletion

there is

The possible

is described.

TCIE heterogeneous chemistry of chlorine compounds is emerging as an important problem as observations of the Antarctic stratosphere suggest that chemistry on cloud surfaces significantly contributes to the stratospheric balance of ozone [l-3]. The uniquely large depletion of ozone in the Antarctic spring seems to result from a combination of extreme cold temperatures which produce stratospheric clouds (PSCs) along with the presence of sun-light needed to drive photochemical reactions [l]. Detailed studies of the stratospheric composition performed in September 1987 revealed that chemistry over the Antarctic had been greatly disturbed by the presence of clouds resulting in large chlorine radical abundances which readily destroy ozone in the sun-lit atmosphere [4]. These studies established firmly the link between ozone loss and halocarbon chemistry [5]. Current models [l] of ozone loss include heterogeneous reactions which convert chlorine from the rather inactive compounds HCI and CIONOl into far more reactive species such as Cl? and HOCI. These will, in turn, dissociate in the sun-lit atmosphere to give atomic chlorine which can destroy ozone. Traditionally, the Earth’s atmosphere has been treated as a gas-phase system and due to low densities, only bimolecular and a few three-body processes were included in models. Recent studies established that the highly unusual heterogeneous chlorine chemistry occurring in the surface of polar stratospheric clouds plays an important role in ozone depletion [l-4]. The magnitude of such condensed-phase processes cannot be evaluated until a quantitative data base is obtained. In this paper we report results of condensed-phase spectroscopic measurements aimed at understanding the properties of the photoreactive excited states of OCIO in solution. The motivation for this study is the role that OCIO may play in polar ozone depletion [6]. In the stratosphere, OCIO results from the reaction of Cl0 and BrO [7]. These precursor molecules are formed from reaction of ozone and halide radicals from the photochemical decomposition of CFCs [8,9]. The concentration of OCIO in the: Antarctic stratosphere has been measured and is found to anticorrelate with the ozone concentration during the Austral spring [4]. The link of OCIO to ozone depletion is built upon recent gas-phase studies which investigated the spectroscopy and photoreactivity of the isolated molecule [6, IO, Ill. Prior to the above gas-phase studies, OCIO was not expected to lead to new ozone loss. Photolysis was believed to only lead to formation of oxygen atoms and CIO, which could then react with oxygen molecules to form ozone. The picture that emerged from our studies is that in the near-UV, there are closely spaced photoreactive electronic states. Excitation in the near-UV can result in either bond breakage to form vibrationally excited Cl0 and atomic oxygen, or isomerization to form the reactive Cl00 molecule * Authors

to whom correspondence

f Permanent

U(A)

address: Department

should bc addressed. of Chemistry.

University

10:9-G 1203

of Colorado.

Boulder.

CO xO3W.

U.S.A.

1294

R. C. DUNN et al.

[6, 10, 111. The isomerized product fragments into oxygen and atomic chlorine. The quantum yields for these two pathways is still controversial [ 12. 131. However, to the extent that the isomerization pathway to form Cl00 occurs, OCIO can become a player in the chemistry of ozone destruction. In matrices of argon [ 14, 151, nitrogen [ 151 and sulphuric acid [16], excitation at 360nm (near the maximum of the near-UV absorption band of OCIO), OCIO isomerization proceeds with a quantum yield of nearly 1.0. As is evident from the literature [6, 14-161, the complex photoreactivity of OCIO is affected by the surrounding environment. We report preliminary findings on the condensed-phase chemistry of this molecule. The spectroscopy and dynamics are found to depend on the polar solvent environment. Time resolved studies of the photochemistry in methanol and water suggest that isomerization to form Cl00 readily occurs in solution with an appreciable quantum efficiency (3 10%) [17]. gas-phase

EXPERIMENTAL

Steady state spectra were recorded using a Perkin-Elmer The data were digitized

by an IBM-PC/286

using a l-cm pathlength

cell. The absorption

arc lamp.

To determine

using a non-linear Time

resolved

mode-locked, tunable

absorption

Q-switched

detected

to a Lock-in repetition

amplifier

(EG&G

amplifier.

This computer

Nd

using a low pressure Hg distribution

IR

integrated Model

laser system [18]. A

using a kHz picosecond

: YAG laser was used to synchronously

ranging from 280 to 440 nm were generated

The photolysis

YAG

by a sample-and-hold 5209).

also controlled

of the pump and probe All

oscillator.

The probe beam was

circuit (SRS Model

The pump beam was chopped

250). and sent

at 500 Hz (half the

this instrument

laser pulses. Delay

was generated

by reaction

coherence

changing the relative

properties

artifacts

in this

and any other signals

of the sample. A detailed

description

of

[ 181.

of oxalic acid and potassium

OCIO and

does not absorb in the spectral

which enabled

signal for computer.

times of up to 8 ns could be achieved

to remove

dependent

can be found elsewhere

[19]. The gaseous products,

was processed by an IBM-PC/AT

a digital delay line (Velmex)

laser beams were depolarized

pump a

by frequency

and probe beams at 355 and 266 nm. respectively,

pulse from the

The output from the Lock-in

which might arise from polarization

further

was calibrated

rate of the laser); the digital output from the chopper was used as the reference

the Lock-in

OCIO

spectrometer.

the spectra were fit to a Gaussian

kinetics were recorded

amplified,

3B UV-vis

fitting algorithm.

from the cavity dumped

by a PMT.

Lambda

All steady state spectra were recorded

spectrometer

maxima,

and cavity dumped

the dye laser output.

were generated

manner.

least-squares

dye laser. Probe wavelengths

doubling

timing

the absorption

AT computer.

chlorate

as previously

described

CO?, were collected and bubbled through the solvent. CO?

range probed.

All

solvents were spectrograde

and used without

purification.

RESULTS AND DISCUSSION

Solubility und stability of OCIO in solution Before presenting results on the spectroscopy of OCIO in solution, some general remarks on the solubility and stability of these solutions are needed. OCIO is very soluble in polar solvents. At room temperature, the solubility of OCIO in water is on the order of 2.0 mol/l [20]; consequently, OCIO is approximately two orders of magnitude more soluble than gases such as CO1 [20]. The high solubility of this molecule in water is significant, suggesting the potential heterogeneous chemistry of OCIO in PSCs. Our studies show that OCIO is very stable in dark solutions. The stability in water was determined by monitoring the absorption spectrum of dissolved OCIO over a period of several weeks. The solution was kept in the dark so that only thermal processes could occur. In a room temperature solution, the half-life of OCIO was found to be -2 weeks. Other solvents, such as methanol, were stable for even longer periods of time.

1295

Spectroscopy of OCIO in polar liquids

400 350 Wavelength (nm) Fig. I. Room temperature gas-phase absorption spectrum of OCIO. The spectrum exhibits a strong progression in the vibrational modes.

Steady state absorption spectroscopy Figures 1-4 show the absorption spectra of OCIO in the gas-phase and water, 2-pentanol, and acetonitrile solutions, respectively. All spectra were recorded at 22°C. In all solvents, residual vibronic structure is observed. A comparison of the vibrational spacings in the near-UV spectrum of OCIO in solution and gas-phase finds similar vibrational spacings. These results suggest that the solvent does not have a large effect on the shape of the excited state potential. However, the maximum of the absorption spectrum, Table 1, shifts significantly in the solvents studied. These results can be understood in terms of the solvation effects on the absolute free energy of the electronic states. The free energy change associated with dissolving a molecule of dipole moment ,B in a solvent of dielectric constant E is given by Eqn (1) [21.22]: AC = 2(p’la”)((& - 1)/(2c+ 1)).

(1)

The variable a is the radius of the assumed spherical solute. If we consider the dissolution of OCIO in polar solvents, the free changes associated with the ground and excited state will differ as the dipole moment is not the same in these two states. Within this model.

250

300

400 350 Wavelength (nm)

450

500

Fig. 2. Room tcmpcrature absorption spectrum of OCIO in water solution. Very little structure is observed in the spectrum.

12%

R. C. DUNN et al.

340

420 380 Wavrlrngth (nm)

0

460

Fig, 3. Room temperature absorption spectrum of OCIO in 2-pentanol solution. The spectrum exhibits well-defined vibronic structure. The spacings are comparable to that observed in the gasphase spectrum (see Table 1).

the absorption spectrum should correlate with the difference associated with the two electronic states, given by Eqn (2): v,,, a 2((~~V-j&)/a3)((~-

in the free energies

(2)

1)/(2~+ 1)).

In Fig. 5, the absorption maximum is plotted as a function of the dielectric function, (E - 1)/(2s + 1). With the exception of trifluoroethanol (TFE), a linear correlation is observed. These data were fit using a linear least-squares analysis. The slope of the resulting line (also shown in Fig. 5, regression coefficient of 0.91) gives (,& -@a3 = 1.12 D2/A3. The solvent dependence of the absorption spectrum indicates that the dipole moment of the ‘A2 state is slightly smaller than that of the ‘B, ground state, in agreement with ab initiocalculations [23]. The ground state dipole moment of OClO is 1.78 D [24]. The absolute value of the excited state dipole moment depends on the value used for the spherical cavity which represents the OClO molecule. OClO is a small molecule and the concept of a dielectric cavity is expected to break down as solute/solvent sizes become comparable. In this limit, molecular models of the liquid are needed to understand solute/solvent interactions. The energy gap between the ground state and the first excited state of OClO in solution is effected by both bulk dielectric effects and specific interactions with the c

400 350 Wavolongth (nm) Fig. 4. Room temperature absorption spectrum of OCIO in acetonitrile solution. The solution spectrum shows essentially no vibronic structure.

Spectroscopy of OCIO in polar liquids

1297

Table 1. Absorption maxima for the zA2+2B2 transition of OCIO in various solvents; the dielectric constant, E, is also given.

Solvent Formamide Water Acetonitrile Methanol Trifluoroethanol Ethanol 2-Pentanol

Dielectric constant (E)

Absorption maximum (cm-‘)

111.0 78.4 37.5 32.1 26.7 24.5 13.9

27,976 27,999 27.840 27,834 27.977 27,762 27,705

solvent. The observation that polar aprotic and protic solvents fall on the same line shows that specific hydrogen bonding interactions between the dissolved OClO and protic solvents are either not occurring, or only weakly affect the energy gap when compared to bulk dielectric effects. For example, methanol (protic) and acetonitrile (aprotic) have similar dielectric properties and OClO exhibits a similar absorption spectrum in the two solvents. However, trifluoroethanol forms stronger hydrogen bonds than normal alcohols (e.g. methanol and ethanol). Since the ground state dipole moment of OClO is larger than the excited state moment, specific hydrogen bonding interactions stabilize the ground state more than the excited state. This stabilization would lead to a blue shift of the absorption spectrum. The data point for TFE shown in Fig. 5 exhibits a strong blue shift compared to solvents of similar dielectric constant (methanol). This observation suggests that the blue shift results from the specific hydrogen bonding interactions with TFE solvent. This conclusion is consistent with studies that have examined hydrogen bonding interactions between protic solvents and molecular solutes PIThe effect of pH on the absorption spectrum in water was examined by adding HCl. The stability of the OClO solution is not affected by the addition of acids. Spectra obtained in pure water and pH = 3 and 5 solutions were identical within experimental error. In light of the above discussion of the effects of specific hydrogen bonding on the

Fig. 5. The absorption maximum of the near-UV transition of OCIO in room temperature solutions is plotted as a function of the solvation free energy for molecular dipoles. The solid line is a least-squares fit to all the data except trifluoroethanol (TFE, 17). The slope of the line is related to the difference in permanent dipole moment of the ground and excited *A2 state (see Eqn 2). The deviation of TFE from the observed correlation is discussed in the text in terms of a specific hydrogen bonding interaction on the energies of these two electronic states. Solvents plotted: 1, 2pentanol; 2, ethanol; 3, methanol; 4. acetonitrile; 5. water; 6, formamide; and 7, tritluomethanol.

R. C. DUNN et al.

1298

absorption properties acidic solution.

of OClO, this result suggests that the solute is not protonated

in

Time-resolved picosecond absorption studies Although the primary purpose of the present paper is to present solution phase spectra of OClO, the photoreactivity of this molecule in solution is of great interest [17,26]. Excitation of OClO in the near-UV accesses the 2A2state which lies in close proximity to two other electronic states, 2B2and 2A,. These states are not observed spectroscopically but can participate in the excited state chemistry of this molecule [6,11-17,27,28]. Two photoreactions are observed. One pathway involves direct dissociation, via coupling to the low lying 2A, excited state, into 0 and vibrationally hot Cl0 [lo, 291. The second photoreactive path involves formation of O2 and Cl presumably via the photoisomerized intermediate Cl00 [6,17]. The relative partitioning between these two pathways is

I.ww n2L -500

0

lsoe

2ooo

!2soo

Fig. 6. Time resolved absorption signals following the photolysis of OCIO at 355 nm in room temperature methanol solutions. Top: 410 nm; middle: 300 nm; bottom: 266 nm.

Spectroscopy of OCIO in polar liquids

Lb

-Om

ll....,....,....,....,....l 0 so0 1000 nm

lmo

2wa

1299

2500

w

Fig. 7. Time resolved absorption signals following the photolysis of OCIO at 355 nm in room temperature water solutions. Top: 410 nm; middle: 300 nm; bottom: 266 nm.

found to depend on excitation wavelength. This complex chemistry occurs as a result of coupling between the Franck-Condon accessed *A2excited state and the nearby *B2and *A, states [ll, 17, 27, 281. Photochemical studies of OClO in argon, nitrogen, and sulphuric acid matrices found exclusive formation of the Cl00 photoproduct [14-161. In matrices, cage effects can dominate product formation; the rigidity of the matrix can prevent the formation of stable bimolecular products as expected for the photodissociation to form Cl0 and 0. In solution, the solvent cage is dynamic and a branching ratio between the two pathways is expected. This branching depends on both static and dynamic properties of the liquid. A complete discussion is beyond the scope of this paper but will be reported elsewhere [30]. The relative efficiency of the reaction channels are strongly dependent upon both the position of the curve crossing between the two states and the magnitude of their coupling. Since the charge distributions of the reactive *B, and *A, states are different from both each other and the Franck-Condon populated *A2 state [27,28], Eqn (1) predicts that as the solvent is varied, the relative free energies between these two

1300

R. C. DUNNet al.

surfaces is changed. This would directly affect the partitioniong of excited OClO molecules between the two photochemical pathways. Figure 6 shows time dependent absorption signals for the photolysis of OClO at 355 nm in methanol. Figure 7 gives analogous results in aqueous solution. The same probe wavelengths (410, 300 and 266nm) are shown for the two solvents. These particular wavelengths were chosen based on the known absorption spectra of the major photoproducts for the two reaction pathways. For example, Cl0 absorbs light between -230 and 310 nm [31,32]. Cl is characterized by a broad absorption in water between -240 and 290 nm [32], and Cl00 absorbs from -230 to 290nm (33,341. Thus the wavelengths chosen monitor the time evolution of these three chemical species. In methanol, a slow bleach is observed at 410nm which is much longer than the instrumental response (80 ps). The signals at 300 and 266 nm show a fast absor$tion rise that is within the instrument response time followed by a decay in absorption that reaches a constant value in -1.5 ns. A detailed discussion of these dynamics have recently been reported [17]; here we summarize the chemistry. The data indicate a partitioning between dissociation (to form Cl0 and 0) and isomerization (to form Cl00 and subsequently O2 and Cl). Using gas-phase cross-sections for the absorbances of Cl00 and Cl0 [31,32], the dynamic data indicate that > 10% of the photoexcited molecules undergo isomerization. The fast decay at 300 and 266 nm was assigned to the disappearance of the Cl00 absorption (t1,2- 400 ps). The delayed bleach observed at 410nm reflects either a low lying absorption of Cl00 or the formation of free Cl in solution. Steady state spectra of these reactive intermediates in this spectral range are required to make a definitive assignment. In water, the dynamics following excitation are very different from those of OClO in methanol. While the dynamics at 266 nm look similar, the other two probe wavelengths reveal different behavior. The bleach at 410 nm is now within the instrument response time and the absorption at 300 nm now has a slower component after the initial fast rise instead of the decay that was observed in the methanol solution. In interpreting the data it is important to consider that the absorption spectra of the photoproducts may also be sensitive to changes in the dielectric properties of the fluid. Our analysis of these time dependent data support a partitioning between the two photochemical pathways [30]. Since the absorption cross-sections of OClO [30], Cl0 [32] and Cl [32] are known in water, it is possible to kinetically model the dynamics and obtain an accurate measure of the branching ratio between the two pathways. In water solution, 10% of the excited molecules generate chlorine atoms [30]. The formation of Cl upon photolysis of OClO in water suggests that heterogeneous chemistry may be important in catalytic ozone depletion in the stratosphere. Acknowledgemenrr-This work is supported by grants from the National Science Foundation to V. V. (ATM-89-13231) and J.D.S. (CHE-90-13138). We thank Professors J. Kraut and D. Kearns for use of the absorption spectrometers, and Professor M. Thiemens for use of his vacuum equipment.

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