J. Phys. Chem. 1996, 100, 15821-15826
15821
Gas Phase Reactions of Hydrated Halides with Chlorine John V. Seeley,† Robert A. Morris, and A. A. Viggiano* Phillips Laboratory, Geophysics Directorate, Ionospheric Effects DiVision (GPID), 29 Randolph Road, Hanscom AFB, Massachusetts 01731-3010 ReceiVed: April 12, 1996; In Final Form: June 18, 1996X
A selected ion flow tube (SIFT) equipped with a supersonic expansion ion source was used to study the reactions of hydrated halides X-(D2O)n (X ) F, n e 6; X ) Cl, n e 8; X ) Br, n e 16; X ) I, n e 13) with Cl2 at temperatures ranging from 140 to 400 K. The bare halide reactions were observed to produce Clwith rate constants ranging from one-third to two-thirds the collision rate. At low hydration levels the reactions proceeded at nearly the collision rate Via a ligand switching mechanism, displacing one or more water molecules to produce a trihalide (i.e., XCl2-(D2O)m + (n - m)D2O where m < n). The rate constants decreased for the fluoride and chloride clusters with each increase in hydration level at n g 4 and n g 6, respectively. The bromide and iodide clusters continued to react at nearly the collision rate over the entire range of hydration levels studied. The implications these results have on the plausibility of halide surface reactions occurring on atmospheric aerosols and the structures of halide clusters is discussed.
Introduction Water molecules can profoundly influence the chemistry of gas phase ions. For reactions such as OH- + CH3Br f CH3OH + Br- the addition of water molecules to the ionic reactant causes the rate constant to decrease drastically.1,2 However, some reactions, such as OH- + SO2 f HSO3-, have rates which increase upon hydration and remain at the collision rate even when as many as 50 waters are added to the ionic reactant.3 We have recently added a supersonic expansion source to our selected ion flow tube which allows the rate constants and product distributions of hydrated ion reactions to be directly determined. Here we report our studies of hydrated halide reactions with chlorine:
X-‚(D2O)n + Cl2 f products where X- is F-, Cl-, Br-, and I- and n e 11. Less accurate information is also discussed for up to n ) 16 for Br- and n ) 13 for I-. Reactions of halides with chlorine have previously been studied in the gas phase and in solution. The gas phase studies have focused on the bare halide reactions.4-9 At room temperature the chloride displacement channel, X- + Cl2 f XCl + Cl-, dominates.6-9 At high energies the charge transfer channel, X- + Cl2 f X + Cl2-, also becomes accessible.4 In solution, the reactions form trihalide complexes, X- + Cl2 f XCl2-.10 The rate constant for Br- + Cl2 f BrCl2- has been recently measured by Margerum and co-workers to proceed at the diffusion limit,11 while it is believed that the F- and Clreactions with Cl2 are substantially slower.12 The stabilities of the trihalide complexes in aqueous solutions have been found to increase with increasing halogen size.10,11 Halide-halogen reactions are believed to be rapid on aqueous surfaces. Hu et al.13 have observed the uptake of Cl2 and Br2 on aqueous droplets (100-200 µm diameter) to be enhanced by the addition of NaBr or NaI to the droplet. The magnitude and character of the enhancement could not be explained by bulk phase chemistry alone. Thus, they have proposed that the * Author to whom correspondence should be addressed. † Air Force Geophysics Scholar. X Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)01094-5 CCC: $12.00
gas phase halogen molecules react rapidly with Br- or I- in the interfacial region of the droplet (i.e., within 10 Å of the gas/liquid surface) to form a trihalide complex which is then readily incorporated into the bulk liquid. This study provides kinetic data to help fill the gap between bare ion reactions and the reactions in solution. The results are used to assess the plausibilty of the surface reaction mechanism proposed by Hu et al. and to the provide information on the structure of halide clusters. Experimental Section Rate constants and product distributions were determined with a variable-temperature selected ion flow tube (SIFT). The SIFT apparatus has been thoroughly described.14 Only the features which are pertinent to the present experiments will be discussed. In order to study ionic clusters, the standard electron impact ion source was replaced by a supersonic expansion source. The design and operation of this ion source have recently been described in detail.15 Briefly, a small stagnation cell containing 4 atm of Ar and several milliliters of D2O was housed inside a vacuum chamber. Gas was expanded from the stagnation cell through a 25 µm orifice into the vacuum chamber. A small flow of halide precursor was added immediately downstream of the orifice. The resulting gas mixture was ionized with a negatively biased hot filament (ThO2/Ir) to produce halide clusters, X-(D2O)n. D2O was used instead of H2O to minimize mass coincidences with the product ions. The following halide precursors were used: NF3 for fluoride clusters, CH3Cl or CF2Cl2 for chloride clusters, CH3Br for bromide clusters, and CF3I for iodide clusters. The resulting distribution of ionic clusters was sampled with a blunt skimmer and passed into a quadrupole mass filter. A single ionic cluster was mass selected and injected into the flow tube Via a Venturi inlet. The ions were transported down the tube by a fast flow of buffer gas (either He or H2) with pressure ranging from 0.2 to 0.4 Torr. A portion of the clusters dissociated after injection. The extent of dissociation depended on the flow tube temperature, the identity of the carrier gas, and the size of the cluster. At room temperature only clusters with n e 2 were observed. In order to study larger clusters, the flow tube temperature was lowered to 140 K and H2 was © 1996 American Chemical Society
15822 J. Phys. Chem., Vol. 100, No. 39, 1996
Seeley et al. TABLE 1: Rate Constants for the Bare Halide Reactions with Cl2 Obtained at Temperatures near 298 K k, 10-10 cm3 s-1
b
Figure 1. Typical plot of reactant and product ion signals as a function of molecular chlorine concentration. This particular plot was obtained for the reactions Cl-(D2O) + Cl2 and Cl-(D2O)2 + Cl2 at T ) 140 K with H2 as a carrier gas.
used as the carrier gas. Hydrogen was used to minimize breakup by lowering the injection center of mass energy.16 Under these conditions clusters with n as large as 11 could be produced with sufficient signal intensity for determining rate constants. Smaller intensities could be made up to n ) 16. Small clusters (typically n e 4) were injected as X-(D2O)n with the majority of the ion intensity observed as X-(D2O)n-1 and smaller amounts of X-(D2O)n and X-(D2O)n-2 also observed. For larger clusters, injecting X-(D2O)n resulted in a distribution of three to five smaller hydrates with the main peaks being X-(D2O)n-1 and X-(D2O)n-2. Chlorine (99.5% purity from AGA Gas, Inc.) was added to the gas flow through a heated finger inlet17 located 50 cm from the downstream end of the flow tube. The reactant ions and the product ions were sampled through a 0.2 mm orifice mounted on a blunt sampling cone and were mass analyzed in a second quadrupole mass spectrometer. To avoid excessive mass discrimination, the resolution of the downstream quadrupole was set as low as possible while still completely separating the reactant and product peaks. The reaction time was obtained directly from measurements of the ion time of flight. Rate constants were determined by recording the reactant ion signal as a function of the Cl2 flow rate. Where possible, the rate constant of a given hydrate was determined for several different initial distributions of reactant ions by varying the hydrate that was injected. The rate constants were found to be independent of the hydrate distribution. Product distributions were determined by monitoring the product ion signal intensities as a function of Cl2 flow rate. The broader distribution of reactant ions observed for large clusters limited our ability to accurately determine the product branching ratios for hydrates with n g 5. Figure 1 contains a typical plot of reactant and product ion signals as a function of Cl2 concentration. We estimate that the reaction rate constants determined with this apparatus have an absolute uncertaintity of (25% and the relative error of (15%.14 Results and Discussion Bare Halide Reactions: X- + Cl2 f Products. The bare halide reactions were studied at temperatures between 130 and 500 K. The room temperature rate constants are listed in Table 1 along with the previously reported rate constants and the Langevin collision rates. A Cl2 polarizability of 4.61 × 10-24 cm3 was used for the calculation of Langevin collision rates.18 For the F- + Cl2 reaction we obtained a room temperature rate constant of 7.9 × 10-10 cm3 molecules-1 s-1. This value
halide reactant
Langevin rate constanta
this work
Spanel et al.b
Fussen et al.c
FClBrI-
13.0 10.4 8.22 7.45
7.9 6.4 3.4 5.7
8.3 6.4 2.6 5.1
8.0
others 8.8d 5.2e
3.3 5.5
a Calculated using a Cl polarizability of 4.61 × 10-24 cm3. 2 Reference 9. c Reference 8. d Reference 6. e Reference 7.
is in excellent agreement with the previous measurements. The rate constant was found to be independent of temperature between 130 and 500 K. At each temperature, the only ionic product was Cl-, indicating that the reaction proceeds Via chloride displacement:
F- + Cl2 f FCl + Cl-
(1)
Reaction 1 is exothermic by 7.0 kcal mol-1.19 For the Cl- + Cl2 reaction we determined the chlorine isotope exchange rate by measuring the rate constant for the reaction 37
Cl- + 35Cl2 f 37Cl35Cl + 35Cl-
(2)
Our experimental approach was as follows: The upstream quadrupole was adjusted to inject a 20 to 1 ratio of 37Cl- to 35Cl-. The resolution of the downstream quadrupole was increased so the 35Cl and 37Cl isotopes could be clearly distinguished. The 37Cl- signal was monitored as a function of the Cl2 concentration, [Cl2]. The injected Cl2 had a natural isotopic distribution. The resulting data were fit with the expression S ) A exp[-(kγ[Cl2]treacn)] + B, where S is the signal intensity, k is the rate constant for reaction 2, A and B are arbitrary fitting parameters, treacn is the reaction time, and γ is a constant equal to 1.02. The fitting function, variable parameters, and constants account for the natural isotopic distribution of the injected Cl2, back-reactions which produce 37Cl- (e.g., 35Cl- + 37Cl35Cl f 35Cl + 37Cl-), and the slight 2 differences in collision rates for the different isotopes.7 We determined the room temperature rate constant for reaction 2 to be 6.4 × 10-10 cm3 molecules-1 s-1. Our result is in excellent agreement with the result of Spanel et al.9 and approximately 25% greater than the result of Van Doren et al.7 but is within the combined uncertainty. Under the conditions of this study we did not observe the formation of Cl3-. Babcock and Streit6 reported that reaction 2 has a termolecular association channel with a rate constant of 0.9 × 10-29 cm6 molecule-2 s-1 in helium; however, at the pressures of this study, this channel is negligible (1.1 × 10-13 cm3 molecule-1 s-1) when compared to the isotope exchange channel. The rate constant for reaction 2 was found to be independent of temperature for 130-500 K. For the Br- + Cl2 reaction we obtained a room temperature rate constant of 3.4 × 10-10 cm3 molecules-1 s-1. This value is in excellent agreement with the results of Fussen et al.8 and slightly higher than the results of Spanel et al.9 The rate constant was found to have a slight positive temperature dependence (see Figure 2). The data were fit with the Arrhenius expression k ) 4.3 × 10-10 exp (-72/T) cm3 molecule-1 s-1, which corresponds to an activation energy of 0.14 kcal mol-1. At each temperature the only observable ionic product was Cl-, indicating that the reaction proceeds Via chloride displacement:
Br- + Cl2 f BrCl + Cl-
(3)
Reactions of Hydrated Halides with Chlorine
J. Phys. Chem., Vol. 100, No. 39, 1996 15823 TABLE 2: Rate Constants for the Reactions X-(D2O)n + Cl2 f Products at T ) 140 K (Rate Constants in 10-10 cm3 s-1) X)F
Figure 2. Arrhenius plot for the reaction Br- + Cl2.
According to the JANAF tables reaction 3 is exothermic by 0.1 ( 0.3 kcal mol-1.19 The small activation energy found in this study for reaction 3 then implies either that there is a small (0.2 kcal) barrier or that the reaction is slightly endothermic, a possibility allowed by the uncertainty in the thermochemistry. The thermoneutral reaction of Cl- with Cl2 has no barrier and is nearly a factor of 2 faster than reaction 3, and therefore we favor the endothermic argument. The values for Br- and Clare well-established, and therefore the heat of formation of BrCl would be too low by about 0.2 kcal mol-1, within the stated uncertainty. Huber and Miller5 and Lee et al.20 have reported that the Br- + Cl2 reaction can also produce BrCl2-; however, under the conditions of our study, this channel was found to be negligible. For the I- + Cl2 reaction we obtained a room temperature rate constant of 5.7 × 10-10 cm3 molecules-1 s-1. This value is in excellent agreement with the results of Fussen et al.8 and slightly higher than the results of Spanel et al.9 The rate constant was found to be independent of temperature between 130 and 500 K. At each temperature, the only observed ionic product was Cl-, indicating that the reaction proceeds via chloride displacement:
I- + Cl2 f ICl + Cl-
(4)
Reaction 4 is exothermic by 5.2 kcal mol-1.19 Two different mechanisms have been proposed for the bare halide reactions. Van Doren et al.7 proposed that reaction 2 forms a linear trihalide complex at the collision rate which then separates into the two possible halide ion/dihalogen products with equal probabilities. Their model is supported by the rate constant they obtained for reaction 2 that was approximately half of the collision rate and by theoretical studies which indicate that Cl3- has a linear ground state structure.21-27 However, the results obtained in this work and by Fussen et al.8 and Spanel et al.9 for reactions 1, 2, and 4 indicate that the rate constants are closer to two-thirds of the collision rate. Spanel et al.9 explained these results by proposing that the reactions form a triatomic halogen complex in which all of the atoms become equivalent. The complex then separates into the three possible halide ion/dihalogen products with equal probabilities. Due to the linear structure of ground state Cl3-, the randomization of the atoms would likely be the result of multiple additions and cleavages occurring during the Cl3- lifetime. Such a mechanism is plausible for isotope exchange reactions such as Cl- + Cl2, where each possible product channel is energetically identical; however, for reactions 1 and 4, the chloride displacement channel is exothermic by greater than 5 kcal mol-1 and should be heavily favored due to the additional phase space. Thus, at present we have no good explanation for why the reactions are
X ) Cl a
X ) Br
X)I
n
kexp
kexp/kc
kexp
kexp/kc
kexp
kexp/kc
kexp
kexp/kc
0 1 2 3 4 5 6 7 8 9 10 11
9.4 8.6 8.3 6.4 3.6 1.8 0.86
0.72 0.86 0.94 0.77 0.46 0.24 0.12
6.3 7.3 7.6 8.0 7.9 6.5 3.4 1.2 7 we did not observe any reaction. The addition of Cl2 did not lead to any changes in the n > 7 halide clusters peak shapes, so we concluded that interference from possible Cl5-(D2O)n production was negligible. When a single D2O is added to Br-, the rate constant increases to the collision rate and BrCl2- is formed:
f FCl2- + 2D2O
The importance of each channel is a function of temperature: at T g 298 K the bare trihalide is the major product (reaction 6b), while at T < 200 K FCl2-(D2O) is the only observable product (reaction 6a). Using the calculated heat of formation for FCl2-, we estimate reaction 6b is endothermic by 6 kcal mol-1, indicating that reaction 6 mainly proceeds through channel 6a, and the FCl2-(D2O) product subsequently decomposes at higher temperatures.15,17,28 Data for n g 3 were only obtained at 140 K. For n ) 3, the rate constant remained near the collision limit and a ligand switching mechanism was observed:
F-(D2O)n + Cl2 f FCl2-(D2O)n-1 + D2O
(7)
For n > 3, the same ligand switching mechanism was observed but the rate constants decreased with each increase in the hydration number. In each case association was not observed. We found that the F-(D2O)7 and F-(D2O)8 peaks broadened and increased in intensity with the addition of Cl2. This effect was probably the combination of a very slow removal of F-(D2O)7,8 and the formation of FCl4-(D2O)0,1 (which overlap with F-(D2O)7,8 peaks) from the secondary reactions
FCl2-(D2O) + Cl2 f FCl4- + D2O FCl2-(D2O)2 + Cl2 f FCl4-(D2O) + D2O
(8)
Unfortunately this interference kept us from determining the reaction rate constants for n g 7. When a single D2O is added to Cl-, the rate constant increases and the only observed product for 140 K e T e 400 K is Cl3-:
Cl-(D2O) + Cl2 f Cl3- + D2O
(9)
Wincel et al.32 have recently reported a similar result: they observed Cl-D2O + Cl2 to proceed at the collision rate,
Cl-(D2O)n + Cl2 f Cl3-(D2O)n-1 + D2O
(50%) (50%) (10)
Br-(D2O) + Cl2 f BrCl2- + D2O
(11)
We also observed production of small amounts of Cl3-. Plots of fractional product intensity Vs [Cl2] indicated that the Cl3was formed from a secondary reaction, presumably
BrCl2- + Cl2 f BrCl + Cl3-
(12)
The occurrence of reaction 12 sets a limit on the Br--Cl2 bond strength of e23.8 kcal mol-1. The Br--Cl2 bond strength has not been directly measured; however, Lee et al. have studied the photodestruction of both Cl3- and BrCl2- and estimated the Br--Cl2 bond strength to be roughly equivalent to the Cl-Cl2 bond strength, which we calculated to be 23.7 kcal mol-1. Thus, our ab initio calculations, the occurrence of reaction 12, and the photodestruction studies of Lee et al. are all consistent with one another. For n ) 2, the rate constant remains at the collision limit and the only observed product for 140 K e T e 400 K is the bare trihalide:
Br-(D2O)2 + Cl2 f BrCl2- + 2D2O
(13)
Using an association enthalpy of -23.7 kcal mol-1 for Br-+ Cl2 f BrCl2-, reaction 11 is exothermic by 12.0 kcal mol-1 and reaction 13 is exothermic by 0.4 kcal mol-1. For 3 e n e 11, the rate constants are all at the collision limit and the major product observed is BrCl2-(D2O)n-2, indicating that the reactions
Reactions of Hydrated Halides with Chlorine
J. Phys. Chem., Vol. 100, No. 39, 1996 15825
proceed by the formation of the trihalide and the displacement of two D2O molecules:
Br-(D2O)n + Cl2 f BrCl2-(D2O)n-2 + 2D2O
(14)
Smaller amounts of n - 1 and n - 3 were also observed but were hard to quantify due to more than one primary cluster in the flow tube. In each case association was not observed. For n g 3 we observed minimal secondary production of Cl3-. The signal intensity for n > 11 was insufficient for accurate rate constants measurements. However, mass spectra obtained as a function of [Cl2] for a broad distribution of bromide hydrates indicated that clusters with n as high as 16 (the largest observable with this system) reacted at approximately the same rate as the smaller clusters (n e 11). We estimate these rates to be collisional to within a factor of 2. The rate constants and product distributions of the iodide clusters are similar to those obtained for the bromide clusters with the exceptions that each of the first three hydrates were found to produce ICl2- and the higher hydrates mainly produced trihalide clusters that “boiled-off” three D2O molecules instead of two:
I-(D2O)n + Cl2 f ICl2-(D2O)n-3 + 3D2O
(15)
In each case an association channel was not observed. The secondary reactions for the iodide clusters were also similar to those seen for the bromide clusters. Reactions of the small iodide clusters (n e 3) lead to the secondary formation of Cl3-, presumably due to
ICl2- + Cl2 f ICl + Cl3-
(16)
The occurrence of reaction 16 sets a limit on the I--Cl2 bond dissociation energy of e28.9 kcal mol-1. We were unable to determine accurately the rate constants for clusters with n > 9 due to insufficient signal intensity. However, the mass spectra indicated that hydrates as large as n ) 13 (the largest observable with this system) reacted rapidly with Cl2. We estimate these rates to be within a factor of 2 of collisional. Reactivity on Aqueous Droplets. Our study shows that Brand I- react rapidly with Cl2 over a wide range of hydration levels. This result gives support to the mechanism proposed by Hu et al.; i.e., the uptake of Cl2 on droplets of NaBr or NaI solutions is enhanced by reactions such as Cl2(g) + I-(interface) f ICl2-(interface), which occur rapidly near the surface of the liquid droplets. Although it is uncertain how applicable results from gas phase studies are to aerosol surface reactions, it is interesting to note that other reactions which have been proposed to lead to surface enhanced uptake of species also occur rapidly in the gas phase at high levels of hydration. Examples include
SO2(g) + OH-(interface) f HSO3-(interface)
(17)
Reaction 17 was proposed to explain the anomolously large
NH3(g) + H+(interface) f NH4+(interface)
(18)
uptake of SO2 on droplets with high pH,34,35 while reaction 18 has been proposed to explain the large uptake of NH3 on droplets of low pH.36 In the gas phase, OH-(H2O)n + SO2 forms HSO3-(H2O)m at the collision rate for n up to at least 50 3 and H+(H2O)n + NH3 forms NH4+(H2O)m at the collision rate for n up to at least 11.17 Structures of the Hydrated Halides. To date, the stabilities of halide-water clusters have been determined using high-
pressure mass spectrometry31,37,38 and photoelectron spectroscopy.39,40 Cluster structures have been generated using ab initio calculations,41-45 molecular dynamics simulations,46-51 and classical electrostatic solvation models.52 Each theoretical method can reproduce the experimental stabilization energies, but the predicted equilibrium geometries are often in conflict. The main difference is whether the halide ion is located on the surface or within the interior of the cluster. Ab initio calculations and molecular dynamics simulations predict that fluoride clusters will be in surface states for n e 3 and in interior states for n g 4.42,45,47,51 However, for the cases of Cl-, Br-, and I-, the ab initio calculations41,43 predict surface states will be favored for n e 5 and interior states will be favored for n g 6, while the molecular dynamics simulations46,47,49-51 predict surface states will be favored up to at least n ) 15 and classical electrostatic solvation models for I- clusters indicate that surface states will be favored for n as large as 60.52 It should be noted that each theoretical method predicts that the energetic differences between surface and interior states will be quite small (i.e.,
