J. Phys. Chem. 1994,98, 61406745
6740
Reaction of Chloride Ions with Chlorine Nitrate and Its Implications for Stratospheric Chemistry Bed-Michael Haas, Kevin C. CreUin, Keith T. Kuwata, and Mitchio Okumura' Arthur Amos Noyes Loboratory of Chemical Physics,? California Institute of Technology, Pasadena, California 91 I25 Received: February 16, 1994; I n Final Form: May 3, 19940
We present experimental and theoretical evidence for the rapid gas-phase reaction of C1- with ClONO2 to form C12 and Nos-. The reaction was studied in a Fourier-transform ion-cyclotron-resonance mass spectrometer, and a reaction rate constant of k = (9.2 f 3.0) X 10-lo cm3s-l molecule-' at 298 K was determined. This value was -60% of the rate constant estimated from ion-dipolecollision theory. We also performed ab initiocalculations at the level of second-order Maller-Plesset perturbation theory using diffuse basis sets and at the singlesand-doubles coupled cluster level to examine portions of the potential energy surface for this reaction. We found no barrier for reaction for the approach of C1- toward the C1 atom on ClON02, but we found a minimum along the reaction coordinate corresponding to an ion-molecule complex C12*N03-. The reaction enthalpy remains exothermic with theinclusion of ion hydration enthalpies, indicating that the reaction could proceed in condensedphase water. These considerations suggest that chloride ions may react directly with ClONO2 on water ice films and type I1 polar stratospheric cloud particles. From the rapidity of the reaction, we also infer that gas-phase chloride ion cannot serve as a sink for negative charge or active chlorine in the stratosphere.
Introduction Heterogeneous reactions on aerosols are now held to be responsible for the high degree of activation of chlorine in the polar stratosphere during winter and early spring.1-3 Relatively inert reservoir species such as HCI and C10N02decompogereadiy on the surfacesof polar stratosphericcloud (PSC) particles. These aerosols are believed to be composed of nitric acid hydrates (type I) or water ice (type 11). While laboratory studies have found that a number of reactions occur on model PSC Webster et al.15 conclude from recent field measurements of HCl and C10 concentrations in the stratosphere that the reaction
The proposed existence of solvated C1- on PSC particles17J8 raises the question of whether chlorine nitrate reacts directly with the chloride anion. As a first step, we have examined the following gas-phase reaction, which from known heats of formation21." is exothermic: Cl-
+ CIONO,
-
C1,
+ NO3"(g)
= -24.6 kcal mol-' (2)
The gas-phase rate of reaction 2 has not been measured. C1undergoes analogous exothermicreactions with H N 0 3and Nz05. All three reactions are driven in part by the high electronaffinity21 HCl ClONO, C1, HNO, (1) of NO3. Davidson et aLU have found in ion flow tube experiments that the latter two reactions proceed with near Langevin rates, is critical in determining the winter and spring chlorine budget raising the possibility that reaction 2 is also fast. within the polar vortex. The homogeneous rate of reaction 1 has not been detectable (k< 10-19cm3s-l), but 1 a b o r a t o r y ~ t u d i e s ~ ~ ~ J ~The reactivity of the chloride ion in the gas phase would raise doubts about the proposal by Wong et al.2' to alleviate the threat indicate that this reaction has a high probability on water ice and to stratospheric ozone by removing the accumulation of chlorine water-rich nitric acid trihydrate (NAT). Tabazadeh and Turcol6 in the stratosphere from anthropogenic sources. They propose have attempted to construct a physicochemical model for this to temporarily charge the atmosphere with sufficient electron and other heterogeneous reactions occurring on PSC surfaces, density to trap active chlorine as chloride ion. Their proposal but their models can be improved by a sound understanding of rests on the assumptionthat the resulting chloride ion is chemically the reaction mechanisms. inert; however, if C1-reacts with an abundant (> 109cm-3)neutral Although the form of HCl on PSCs has not been established, speciessuch as chlorine nitrate, such a scheme may not be feasible. laboratory studies on films suggest that the HCI is dissociatively In this paper, we present both experimental measurements of ionized. This has led Molina and co-workers4J4J7J8to propose the reaction rate of C1- with CION02 in the gas phase using a an ionic mechanism in which the chloride ion C1- reacts by fast Fourier-transform ion-cyclotron-resonance (FT-ICR) mass specion-ion recombination with C1+ from dissociatively ionized trometer and theoretical results from a quantum chemical molecules such as CPHO- or CI+NOj. Burley and Johnston19 calculation along the reaction path. We find that the chloride have also hypothesizedthat ionic mechanisms involving concerted ion reacts rapidly with chlorine nitrate. We discuss the implicanucleophilic displacement are important, especially for reactions tions of our results for the proposed ozone mitigation scheme, in sulfuric acid aerosols. and then present thermodynamic considerations which suggest We have investigated gas-phase ion-molecule and cluster ion that this reaction could proceed in the condensed phases of water. reactions in order to provide insights into possible ionic mechanisms of heterogeneous reactions at the molecular level. In an Experiment and Results earlier paper,20 we demonstrated that reactions of CION02 with protonated water clusters produced protonated nitric acid, in the FT-ICR spectrometry is a well-established technique, and its form N02+(H20)., which led us to suggest that the reaction experimental aspects have been discussed in the l i t e r a t ~ r e . ~ ~ * ~ ~ CION02 H20 would proceed by proton catalysis. Only details relevant to this experiment are presented here. A 1-in. cubic trapping cell was located between the poles of a Varian * Author to whom correspondence should be sent. 15-in. electromagnet which was maintained at a field of 1.O T. Contribution No.2922. Data were collected with an Ion-Spec Omega1386 FT-ICR data Abstract published in Advance ACS Absrracrs, June 1, 1994.
+
-
+
+
0022-3654/94/2098-6740SO4.50/0
0 1994 American Chemical Society
Reaction of Chloride Ions with Chlorine Nitrate system and associated electronics. Neutral gases ClONO2 and CC4 were introduced into the cell from ultrahigh-vacuum (UHV) inlet lines by separate leak valves, and their pressures were measured with a Schultz-Phelps ion gauge calibrated against an MKS 390 HA-00001SP05 capacitance manometer. Uncertainties in the absolute pressure limited accuracy of rate constants to f20%. CION02 was synthesized from the reaction of ClzO and N205 and then purified.27 Purity was confirmed by UV absorption spectroscopy of a sample of ClONO2 in a gas cell. Two chlorine nitrate gas handling systems were used. In the first experiments, ClONO2 was introduced through a standard UHV stainless steel inlet line and high-precision leak valve (sapphire/OFHC-copper seal). Because stainless steel surfaces can catalyze ClON02 decomposition,2*the inlet lines and vacuum chamber were treated prior to introduction of ClON02. Halocarbon vacuum grease was used on joints in the inlet system, and the entire system was baked to remove H20. Passivation of the inlet line with ClONOz for 2-3 days was essential to minimize decomposition of chlorine nitrate in the inlet line. During experiments, CION02 was kept in a glass sample bulb held at a temperature of -45 OC by an acetonitrile/dry ice bath to maintain a ClONO2 pressure of -20 Torr in the inlet line. To confirm the results obtained using the stainless steel inlet system and to exclude the possibility of competing reactions with decay products, we repeated the experimentsintroducing ClONO2 through a glass (Pyrex) inlet line with Teflon valves. The drawbacks of this inlet system were coarser control of gas flow by the Teflon leak valve and slighty poorer vacuum seals compared to the UHV seals of the stainless steel line. To compensate for the higher flow rates through the Teflon valve, the stagnation pressure of ClONO2 was kept at 1 Torr by cooling the sample at -80 OC in an acetoneldry ice bath. We used the positive ion mass spectrum as a diagnostic to assess the purity of the CION02 sample in the ICR cell. Spectra were taken 80 ms following a 20-ms pulse of 70-eV electrons (at an emission current of 0.3 NA). Primary ions formed by electron impact ionization underwent 1-2 collisions prior to recording the mass spectrum. We therefore used the Cl2+ signal as a measure of the quality of ClONO2 in the ICR cell, because C12+is produced only by ionization of the possible neutral Cl2 decomposition product and not from fragmentation of ionized CIONO2. In the absence of passivationof the stainless steel line, the mass spectrum consisted almost entirely of C12+;after passivation, the Clz+signal was reduced by 3 orders of magnitude and accounted for less than 5% of the total ion signal. No decomposition was observed using the glass line. Reactant chloride ions were formed by electron attachment to background gas. Chlorine nitrate did not prove to be an efficient source of C1- under any negative ionization conditions. Instead, we chose to generate C1- from CC4. This precursor is ideally suited as a chloride ion source because it has a large cross section for dissociative attachment to form C1- and CC13 productsz9,30 and is inert toward ClON02. CCL also serves to scavenge electrons after the electron beam pul~e.3~ CC14 was added into the ICR cell through a separate leak valve, and CI-reactant ions were formed by electron bombardment of C C 4 at a partial pressure of approximately 3.0 X Torr with an electron energy of 1.7 eV. The C1- ions were then selectivelyisolated (see Figure 1, upper panel) and trapped in the ICR cell by ejecting the unwanted ions NO2- and NO3-. After ejtcting ions of other m / e (defining t = 0 ms), we followed the reaction of the C1- ions with background neutral ClONOz at partial pressures in the range (1-4) X l e 7Torr. All experiments were performed at room temperature (298 K). Mass spectra were then recorded in time intervals of 50 ms to monitor the disappearance of C1- and the appearance of NO3-. Positive ion spectra of ClONO2 were taken after -2 h to assess the extent of decomposition that had occurred by observing the amount of
-
The Journal of Physical Chemistry, Vol. 98, No. 27, 1994 6741
mh
Figure 1. (upper) Isolation of C1- ion at 75 ms after the 1.7-eV electron beam pulse corresponding to t = 0 ms, the time at which all other ions have been ejected. (lower) At t = 200 ms, N o s - products from C1CION02 C12 + NOo- have been formed. The reactant pressures were p(ClON02) = 2.28 X lo-' Torr andp(CCl4) = 3 X le7Torr at 298 K.
-
-2.5
'
0
+
0.1
0.2
0.3
04
Time ( 8 ) Figure 2. Observed decay of the C1- abundance with time for a single run. The line is a fit to the data. Conditions were the same as in Figure 1.
the decompositionproduct C12+present. These spectra indicated that, although the stainless steel gas line had been thoroughly passivated, up to 25% of the CION02 typically had decomposed in the inlet line during a run of experiments. When CIONOz was introduced through the glass inlet line, no sign of decomposition was evident in the mass spectra. Reaction 2 was the only observed reaction of Cl- with ClONOz (see Figure 1). The most likely interference would have come from products of chlorine nitrate decomposition: NO, NO2, and C12. Bakeout of the UHV-sealedchamberand inlet lineminimized H2O contamination, so the products HOC1 and H N 0 3 from C10NOz hydrolysis were absent. Allof theexpectedcontaminants have electron affinities below that of C1and thereforecould neither react with C1- nor produce NO3-. Figure 1 displays mass spectra at (a) t = 0 ms and (b) t = 200 ms and illustrates how the reaction proceeds in time. Figure 2 shows a semilog plot of the decay of the C1- relative abundance for a single experiment. Rate constants for reaction 2 were
6142 The Journal of Physical Chemistry. Vol. 98. No, 27. 1994 determined from the decay of CI- abundance versus time by integrating the pseudc-first-order rate equation d[Cl-]/dt = -k[CIONOzJ[CI-] with [CIONOz] held constant. [Cl-] was calculated as the sum of both Cl- isotopes. Five experiments were performed with the stainless steel inlet line and four with the glass inlet line. Background pressures of CIONOz were in the range ( 1 4 ) X I@' Torr. Therateconstantohtained with thestainlesssteelinlet system, assuming that 25% of the CIONOz decomposed in the inlet line duringeachsetofruns,was(7.4&3.0)X lW'ocmls-~molecule-', where the error bar (Iu) includes the uncertainty in the extent of chlorine nitrate decomposition that had m r r e d during a given runand theaocuracyoftheabsolutepressuremeasurements. Therateconstant obtained whenCIONO2 wasintroduced through theglassinletsystemwas(11.3 &3.7) X I@locm3s-lmolecule-'. The results from the set of runs done with the glass system are somewhat higher. suggesting that we did not account for all the decompositionofCIONOzinthestainlesssteelinletline: however, the two results are still comparable. We calculated an average rate constant for reaction 2 of k = (9.2 X 3.0) X 1WIo cml s-l molecule-I; the two results are within the error ban of their average. We can compare the observed k to a theoretical prediction for thecollisionratebased on averagedipole orientation (ADO) theory?z From the polarizability of C1ONO2calculated at the MP2/6-31G(d) level to be 8.61 A3 (see below) and the experimental ClONOz dipole moment of 0.77 D," we calculate that the ion-dipole collision rate constant is = 1.5 X le9 cm' s-' molecule-'. The reaction efficiency is therefore k / k A m = 0.61.
Haas et al.
c3 3. Ab initio structure of the ClrNO3- complex predicted at the MP2/6-3 I 1 (Z+)C(d) level (scstext). The numbers are bond lengths in A. The diasociation energy of this complex relative to C12 and NO]- is Do = 9.2 kcal mol-' at the CCSD/6-31l+G(d) level, 6.1 kcat mol-l includinga Wncctionfor~sisJctsuperpositionenor. Other parsmetera arc listed in Table 1.
TABLE 1:
Structural Parameters and ch.rge DistriboHoas lor the CI,NOi System obtained from a MP2/ 6-311(2+)G(d) Calculation' ClrN01- wmnlex
CI- + CION02
Clt
+ NO$-
Ab Initio Calculations In order to further characterize the gas-phase activation of CIONOz by CI; we used ab initio calculations to study intermediates [CIzNOIJ- along the reaction coordinate. All calculations were performed using the Gaussian 92 system of programs?' Initial geometry optimizations were performed to identifythe lowest energy isomers on the potential energy surface. Harmonic vibrational frequencieswere then calculated to identify these isomen as minima, transition states, or higher order stationary points. Several geometries corresponding to different approaches of CI- toward CIONOz were examined. The minimum-energy path for approach of the CI- to the chlorine nitrate was toward the chlorine atom along the direction of the CI-0 bond. The lowest energy isomer that was a true minimum was a complex of Clz and NO,; depicted in Figure 3. The next moststableisomer,CIONOzdistortedhythebindingofcl-above the N, was over 20 kcal mol-' higher in energy. In this paper, wesummarize our results on the Clz.NO]-isomer; further details as well as additional work on other isomers will be published cl~ewhere?~ Geometry optimizations of CITNOI-, Clz, and NOI- were performedat therestrictedHartree-Fcck(RHF)andthesecondorder Mdler-Plesset (MP2) levels of theory with a variety of hasis sets. The geometric parameters obtained with the largest basisset aregiven inTable 1. 1naddition.singlbpointcalculations at the singles-and-doubles coupled cluster (CCSD) level were done at the MP2-optimized geometries. All MP2 and CCSD calculations neglected the contribution of excitation of core electrons. All three levels of theory are size-consistent,meaning that 0. may be calculated rigorously as the difference of the electronic energy of the complex and the electronic energies of the fragments Clz and NO,-. At some levels of theory, harmonic vibrational frequencies were calculated to obtain an estimate of Do.
The largest standard basis sets used were 6-311+G(d) and 6-31 1+G(2df)?6 However, LeeandSchaeferl'haveshown that in order to describe anionic species accurately, it is necessary to add a number of diffuses and p functions to the basis set. For
4.29 +0.04 -0.49 4.45 4.47 +0.66
-I
0
+0.25 -0.45 4.23 4.23 +0.66
0 -0.55 -0.55 -0.55
+0.65
The atoms are labeled with a subsmipt, as defined in Figure 3. Bond l e ~ t h s r a n i nAandanglcsEand6indegrccs. Thechargedistributions I were obtained from a natural population analysis of the MPZ density matrim. some basis sets, then, an additional set of diffuse sp functions was added to the standard set of diffuse functions of Clark et al?a for each atom, following the procedure of Lee and Scbaefer?' Basis sets with the two groups of diffuse functions were labcled (2+). Thegeometrywefoundfor theClz-N03-complexattheMP21 6-311(2+)G(d) level is shown in Figure 3, and the parameters are given in Table 1. The subscripts identify specific atoms, as definedin the figure. Thecomplex is nearly planar, with torsional angles varying from planarity by 5 O . I o . The CI-CI bond length is 2.19 A, close to the bond length of 2.02 A that we calculate for molecular chlorine (the experimental value is 1.988 A)?9 On the other hand, the C1-0 bond length has increased significantly from 1.70to2.14A. Thesechangesin bond lengthclearly indicate that the structure is closer to that of molecular chlorine bound to a nitrate ion. The structure of the complex is also reflected in the extent of charge transfer. We obtained charge distributions in CIrNO3fromtheMP2/6-311(2+)G(d)resultsusinga natural population analysismoftheMP2densitymatrix. Thechargeon thechloride
The Journal of Physical Chemistry, Vol. 98, No. 27, 1994 6143
Reaction of Chloride Ions with Chlorine Nitrate
c1- +
'T
CION02
7.. - 90 kcal
r
I
-80t
cl-hyd +
-------..-.--a-
CION02
*
__
Ck"NOs~,d
-1 *.
ClZ + NOrhyd
-loo! -120
While we do not know the temperature dependence of the rate constant for the reaction, exothermicion-molecule reactions with smallbarriers and bound transitioncomplexes typically have either zero or negative temperature dependences. We therefore expect the reaction rate constant to be similar at stratospheric temperatures. For a CION02 mixing ratio of 2 ppb at 100 Torr in the stratosphere, the lifetime of C1- will be on the order of seconds to minutes. The chlorine nitrate densities in the stratosphere are sufficiently high to assure that there is an excess of CION02 relative to C1-, even at charge densities as high as lo9 ~ m - ~ . Furthermore, reaction 2 has the deleterious effect of not only destroying C1- but also activating the reservoir CION02 to form the photoactiveCl2 molecule and sequesteringthe negative change in the inert NO3-ion, thus potentially contributingto stratospheric ozone depletion. In addition, we find that low-energy electron attachment directly to chlorinenitrate does not lead to production of C1-. On the basis of the present results, we conclude that the chloride ion will not be a stable sink of negative charge or atomic chlorine in the stratosphere. The reaction of C1- with CION02 in the gas phase raises the possibility that such a reaction could also occur on a PSC surface. Such a reaction is consistent with the hypothesis that HC1 is dissociatively ionized in stratospheric particles. Our results support the speculationof Molina and ~ ~ - w o r k e r s ~that J ~ the J~J~ chloride ion is involved in the reactions of HCl with ClONO2 and HOC1 on PSCs. However, there does not appear to be a need to invoke explicit ionization of the partner,18 e.g., Cl+NOs- or CPOH-. Chu et have also recently questioned whether such ionization of the partner occurs. The expression Cl+N03-could still be considered formally correct in that it describes the partial charges on neutral chlorine nitrate (see Table 1). A direct ionic reaction is especially plausible if the PSC surfaces have a quasiliquid structure that would allow solvent reorganization as proposed by Molina et al.4J8In the following paragraphs, we examine the effects of going from the gas phase to the condensed phase of water. The reaction enthalpy is modified to first order by the heats of hydration m h y d of the ions. From the literature$2 we find that Mhyd(cl-) = -90 kcal mol-', somewhat higher than AHhyd(N03-) = -79 kcal mol-'. With these corrections, we find AHrX,(hyd) = -14 kcal mol-' (see Figure 4), where we use the notation '(hyd)" to refer to ion hydration. Thus, the reaction remains significantly exothermic. A second-order correction to AH,,, requires solvation energetics for the neutral species. The solvationof neutral chlorinenitrate is not known, but if we assume a crude estimate for the hydration enthalpy of AHhyd(C10N02) = -15 kcal mol-' or roughly 3-4 hydrogen bonds,16 then with Mhyd(c12) = -6.5 kcal mol-' for Clz aqueous hydration,43we find that the heat of reaction would still be exothermic by -5 kcal mol-'. The rate of reaction in the condensed phase will depend on how solvation affects the transition region of the reaction. As is wellknown in S Nr e~a ~ t i o n s , 4 ~the * ~charge ~ . ~ ~ becomes delocalized and the transition region is less stable relative to the reactants and products in polar solvents, especially protic solvents. These effectsdestabilizeion-molecule complexes and increasetransitionstate energies, thus slowing reaction rates. The C1- CION02 system is quite polar, and the ion-molecule complex C12.N03- may remain a stable intermediate in aqueous or solid H20. Our calculated binding enthalpy of Cl2 to the nitrate anion is 6 kcal mol-', somewhat less than that of H20 to NO3- (14.6 kcal An analogous complex, ClzOH-, has been proposed by Eigen and Kustin4' as an intermediate in the aqueous hydrolysis of C12,
-
Figure 4. Reaction coordinate diagram for Cl- + CION02 Clz + Nos-. The upper curve is based on the gas-phase enthalpies. The lower curveincludescorrectionsfor ion solvationenthalpies. The ionic hydration enthalpy of the C12.N03-intermediate is obtained from an estimate of the hydration of the ab initio charge distribution (see text). ion has been reduced to -0.29. The magnitudes of the charges on the oyxgen atoms of the complex are 0.45-0.49, about 20% less than in NO3-,while the chargeon the nitrogen atom is virtually identical to that in the nitrate ion. From the calculated charge distribution, we infer that over 70% of the charge has transferred from the C1- reactant to the NO3- product in the adduct. At the RHF level with the 6-31 1(2+)G(d) basis, the binding energy (including correction for zero-point energy) was DO= 6.6 kcal mol-I, while at the MP2/6-311(2+)G(d) level, DO= 13.6 kcal mol-'. This is the highest level at which the geometry of the structure was optimized. At this geometry, the CCSD/6-3 11+G(d) energy was DO = 9.2 kcal mol-'. Using the statistical mechanical propertiesof an ideal gas and the calculated harmonic frequencies,the C12-N03-binding enthalpy at 298 K was estimated to be A H 2 9 8 = 9.0 kcal mol-I. After accounting for basis set superposition error,35we arrived at a dissociation energy of DO = 6.1 kcal mol-' and a binding enthalpy of A H 2 9 8 = 5.9 kcal mol-'. Discussion
We have found that chloride ions react with chlorine nitrate in the gas phase at near ion-dipole collision rates. Thus, there is little or no activation barrier for this reaction. Our ab initio calculations also indicate the absence of an energetic barrier for the approach of C1- toward the chlorineatom in ClONO2. Instead, they reveal a minimum along the reaction coordinate corresponding to the ion-molecule complex C12.N03-. The reaction pathway, shown in Figure 4 (upper curve), is similar to that of other exothermic ion-molecule reactions. Our results indicate that the anion attacks the C1 atom, which possesses a +0.25 charge, and displaces the NO3- leaving group. The reaction thus proceeds by a nucleophilic displacement mechanism analogous to organic sN2 reactions that have also been studied in the past by gas-phase ion chemistry.4I This mechanism is similar to but more straightforward than the sN2 mechanism proposed by Burley and Johnston.19 An alternative mechanism, long-range electron transfer, is unlikely. NO3- has an electron affinity2' only 0.3 eV (7 kcal mol-') greater than that of C1-; most of the exothermicity arises from the 19 kcal mol-L difference in bond energies22between Cl-CI and Cl-ONO2. Our results have immediate implications for the ozone mitigation scheme proposed by Wong et The large rate constant that we have observed at room temperature suggests that C1- will react with the ClONO2 present in the stratosphere.
-
+
C1-
+ ClOH e C1,OH-
Z=
OH-
+ C1,
(3)
based on experimental evidence from aqueous-phase studies.
6744 The Journal of Physical Chemistiy, Vol. 98, No. 27, 1994
Wecanrougldyestimate theionicpartof the hydrationenthalpy of the intermediate complex by considering the hydration of Clb1-Cl.-N034. The contribktion of each partially charged moiety to the total ionic hydration enthalpy can be estimated by scaling the experimental hydration enthalpies of C1- and NO3using weighting factors obtained from the Born model of ion solvation.42.a The Born equation states that the electrostatic energy of interaction between a charged sphere and a continuous dielectric medium is given by (4) where e is the dielectric constant of the medium, ze is the charge of the sphere, and I is the radius. The model thus predicts that the solvation energy simply scales as the square of the charge, if we assume that the radius r does not change. We treat NO3as if it were composed of four individual charges, rather than regarding it as a sphere with a central charge. With these assumptions, we estimate the solvation enthalpy of the complex by using the interpolation
with z(X,)denoting the partial charges residing on the atom Xi of the C12.N03- complex obtained from the MP2 calculation (see Table 1) and $(Xi) the calculated charge on atom Xi of the separated ions C1- and NO3-. The charge on the center N atom is not included in the summation for NO3-,because it does not change in going from NO3- to CIzNOs-. We obtain an estimate for AHI,,~(C~Z*NO~-) of -65 kcal mol-'. The hydrated complex thus has an enthalpy 7 kcal mol-' below the reactants, as shown in Figure 4. Unfortunately, there is no identifiable Yneutral"moiety, so we cannot readily estimate the total (ion neutral) hydration enthalpy of the complex. By comparison, this equation predicts that the activation barrier for the reaction C1- CH3CI is 48 kcal mol-', far greater than a solution-phase value of 28 kcal mol-' predicted in a recent simulati0n.4~ The z2 dependence thus probably overestimates the effects of solvent destabilization. Even this simple model then predicts that C12-N03- may be a stable complex in condensed-phase H20. From the solvation energetics, we conclude that the reaction C1- ClONOz could be energetically favored in condensed-phase H20. While we cannot estimate the height of the activation barrier in solution or on ice, the above considerationssuggest that the transition state m a y also be stabilized more in reaction 2 than in simple symmetric sN2 reactions such as C1- + CH3Cl. Furthermore, since solvent H20 molecules likely already form hydrogen bonds with the polar nitrate group of neutral ClON02, only small changes in the solvent microstructure are necessary to stabilize NO3- as charge is transferred. The free energy of activation may therefore be low enough for the reaction to proceed on time scales relevant to the stratosphere. Our demonstration that C1- and ClONOz react rapidly in the gas phase is a first step toward an understanding of the molecular mechanism for the heterogeneous reaction 1. We have presented qualitative arguments which suggest that the direct reaction of C1- with ClONO2 is thus a plausible mechanism for the reaction observed to take place on model PSC films, HCl + CION02 Clz HN03. Other mechanisms are possible, e.g., solventassisted reactions of neutral HCI or dissociative ionization of ClONO2 to C10- N02+. However, the proposed mechanism
+
+
+
-
+
+
Haas et al. isconsistent with the high C1-concentrationobserved in laboratory films exposed to HCl and with our understanding of the effects of solvation on sN2 reactions. A number of issues must be clarified before we can establish that this ionic pathway does in fact occur on ice and PSC surfaces. We must understand not only solvation energetics but also the molecular structure of PSC surfaces (crystalline, amorphous, or quasi-liquid), entropic barriers in ice, reactant diffusion, and dissociative ionization of HCl or ClON02. Direct reaction of C1- will also be in competition with ClONO2 hydrolysis followed by the reaction HOC1 HCl. Ultimately, studies of such questions should provide a detailed molecular picture of heterogeneous reactions occurring on or near the surfaces of PSC particles.
+
Acknowledgment. We acknowledge the support of a National ScienceFoundation Presidential Young InvestigatorAward CHE8957243, NSF Graduate Fellowships for K.C.C. and K.T.K., and a Deutsche Forschungsgemeinschaft Fellowship for B.M.H. We are grateful to J. L. Beauchamp for use of the ICR instrument, supported by NSF under Grant CHE-9108318,and for helpful advice. Calculations were supported by the Jet Propulsion Laboratory Supercomputing Project. We acknowledge W. A. Goddard, 111,and the Beckman Institute Materials and Molecular SimulationsCenter for useof the graphics facility used togenerate Figure 3. We thank A.Tabazadeh and M. T. Leu for providing preprints of their work.
Note AddedhProof. WongetaLMhaverecentlydemonstrated a charged-induced recovery of ozone in a chamber experiment, which they attribute to formation of inert C1-. However, the applicability of their results to the atmosphere remains in doubt, because the experiment does not include trace constituents such as ClONO2 and H N 0 3 at relative concentrations [X]/[03] = similar to the lower stratosphere. The rate constants measured in the current work and by Davidson et alF3 indicate that such trace species can rapidly and completely convert the chloride ions back to radical forms, even at the charge densities and time scales used in the chamber experiment. In an elegant flow tube study, van Doren et al. have used cluster chemistry to discover another possible ionic mechanism for the heterogeneous reaction (2). They find that an ion-assisted reaction can occur in which chlorine nitrate and HCl react when one of the two is initially bound to an NOJ- anion." References and Notes (1) Solomon,S.;Garcia, R. R.; Rowland, F. S.;Wuebblca, D. J. Nurure 1986,321,755. (21 Toon. 0.B.: Turco. R. P. Sci. Am. 1991. 264. 68. (3) Brune, W. H.; Ande'mn, J. G.; Toohey,D.'W.;Fahey, D. W. Science 1991,252,1260. (4) Molina, M. J.; Tso, T. L.; Molina, L. T.; Wang, F. C. Y . Science 1987.238. 1253. ( 5 ) Tolbert, M. A,; Rossi, M. J.; Malhotra, R.; Golden, D. M. Science 1987,238, 1258. (6) Leu, M. T. Geophys. Res. Lett. 1988,15, 17. (7) Leu, M. T.; Moore, S. B.; Keyser, L. F. J . Phys. Chem. 1991, 95, 7763. (8) Hanson, D. R.; Ravishankara,A. R. J. Geophys. Res. 1991,96,5081. (9) Hanson, D. R.; Ravishankara, A. R. J. Phys. Chem. 1992,96,2682. (10) Chu, L.T.;Leu,M.T.;Keyser, L. F.J.Phys. Chem. 1993,97,12798. (11) Tolbert,M. A.; Rossi, M. J.; Golden, D. M. Science 1988,240,1018. (12) Tolbert, M. A.; Rossi, M. J.; Golden, D. M. Geophys. Res. Lcrr. 1988,15,847. (13) Leu, M. T. Geophys. Res. Leu. 1988,15,851. (14) Abbatt, J. P.;Molina, M. J. J. Phys. Chem. 1992,96,7674. (15) Webster, C. R.; May, R. D.; Toohey. D. W.; Avallone, L. M.; Anderson, J. G.;Newman, P.; Lait, L.; Schocberl,M. R.; Elkins, J. W.; Chan, K. R. Science 1993,261,1130. (16) Tabazadeh, A.; Turco, R. P. J . Geophys. Res. 1993,98, 12727. (17) Abbatt, J. P.;Beyer, K. D.; Fucaloro, A. F.; McMahon, J. R.; Woolridge, P. J.; Zhang, R.; Molina, M. J. J. Geophys. Res. 1992,97,15819.
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