Photodissociation of Chlorobenzene Molecular Ion: Investigation of

Photodissociation of Chlorobenzene Molecular Ion: Investigation of Entropy Bottleneck in Ionic Systems. 2. Yong Hyeon Yim, and Myung Soo Kim. J. Phys...
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J. Phys. Chem. 1994,98, 5201-5206

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Photodissociation of Chlorobenzene Molecular Ion: Investigation of Entropy Bottleneck in Ionic Systems. 2 Yong Hymn Yim and Myung So0 Kim' Department of Chemistry and Research Institute of Molecular Sciences, Seoul National University, Seoul 151-742, Korea Received: November 29, 1993; In Final Form: February 23, 1994"

Dissociation of the chlorobenzene molecular ion has been investigated on a nanosecond time scale by photodissociation mass-analyzed ion kinetic energy spectrometry. The present rate-energy data together with the previous microsecond data have been compared with theoretical results. A thermochemically reasonable analysis requires that the reaction occurs via a non-totally loose transition state. The AHrOo(CaHs+) of 1141 kJ/mol, which leads to a 3.29-eV energy barrier for this reaction, allows consistent and successful analysis of rate-energy data for dissociations of chloro- and iodobenzene ions. It has been confirmed that transition-state switching (TSS) does not occur at high internal energy for these reactions. Possible occurrence of TSS near the threshold is also discussed,

Introduction Recently, variational transition state theory (VTST)'-' has been widely used in the theoretical calculation of the dissociation rate constants for ions and neutrals. In VTST, transition state is determined by the variational criterion which locates the transition state at the position of the minimum flux from reactant to product. Application of this criterion is particularly interesting for a reaction without any pronounced maximum in the potential energy surface. This is because the location of the transition state in such a case is expected to be determined by delicate balance between enthalpic and entropic effect^.^ When the internal energy of the reactant is only slightly above the reaction threshold, the enthalpic effect is expected to dominate and the transition state is likely to be located at the orbiting transition state (OTS). As the internal energy increases, however, the entropic effect becomes important and eventually transition-state switching (TSS)' to the non-totally loose transition state may occur. That is, the position of minimum flux from reactant to product may be affected by the entropy bottleneck at higher internal energy. TSS for a reaction that proceeds on the potential energy surface without any pronounced maximum has been a matter of interest for many years.&'' For neutral reactions, excellent experimental and theoretical studies supporting TSSIave been made by Zewail and co-workers16 and Marcus and co-workers,' respectively. However, Occurrence ofTSS in ion fragmentation is still a matter of controversy.* Although Bowers and co-workers7 have been successful in applying a TSS model for several ionic systems, all of these systems possess multiple-well potential surfaces. To determine whether entropy bottleneck also plays an important role in ionic systems, it is important to choose fragmentation reactions which are known definitely to proceed via a single-well potential. Lifshitz and co-~orkers2,3,14suggested that the halogen radical loss reactions of halobenzene radical cations should be good candidates. They performed microcanonical VTST calculation for the dissociation of bromobenzene ion and compared the result with the experimental rate constants in the range of 103-106 s-'.I4 It was concluded that the reaction proceeded via Author to whom correepondenceshould be addressed. Ahtract published in Advance ACS Abstracts, April IS, 1994.

0022-3654/94/2098-5201$04.50/0

OTS over the internal energy range considered and TSS did not occur. Also, accurate measurement of the rate constant at higher internal energy was called for to check the possible Occurrence of TSS well above the threshold. Accordingly, we measured the rate constant for the dissociation of iodobenzene molecular ion using the photodissociationtechniqueon a nanosecond time scale.'' Theoretical analysis of the rate-energy data on a nanosecond time scale together with the microsecond data reported previously found no evidence to support TSS at the internal energy well above the threshold. Instead, as in the cases of neutral systems,5J6 occurrence of TSS near the threshold was found possible when the best literature value of 2.38 eV was taken as the reaction critical energy. Such a possibility could be removed, however, when the critical energy was adjusted upward to 2.46 eV; namely, the reaction could be viewed as to occur via OTS over the entire energy range investigated with the latter critical energy. As a continuation of our effort to study the influence of the entropy bottleneck in the halogen loss from halobenzene molecular ions, we have investigated the photodissociation kinetics of chlorobenzene ion.

This reaction looks particularly interesting in view of various conflicting reports on its kinetics and energetics. If the halogen loss from C6HsX*+(X = C1, Br, I) occur via OTS, the heat of formation of C&+ at zero absolute temperature, AHf00(C6H5+), obtained from the kinetic analysis should be the same regardless of the substitutent. Pratt and Chupka17 reported that a higher value of AHfoo(C6Hs+)was required to fit the experimental rateenergy data on a microsecond range for C&Cl'+ with the OTS model than for C6H$r*+ and C&I*+. Hence, it was suggested that the dissociation of the former occur via a tighter transition state than that of the latter ions. By measuring the rate constant on a nanosecond time scale using the present technique, the rateenergy data over expanded internal energy range becomes available. This, we expect, will enable more reliable kinetic analysis for reaction 1. 0 1994 American Chemical Society

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5202 The Journal of Physical Chemistry, Vol. 98, No. 20, 1994

b

1 5400

5600

5800

6000

TRANSLATIONAL ENERGY (eV) Figure 1. Field-on PD-MIKE spectrum for reaction 1 obtained at 1.5 kV appliedvoltagewith 5 14.5-nmexcitation. Experimental andcalculated resultsareshownasfilledcirclesandsolidlines,respectively.Thepressures of the reagent gas used for charge exchange are (a) 0.03 and (b) 0.06 Torr in the ion source.

Experimental Section The experimental setup has been described in detail elsewhereIsJ8 and will be reviewed only briefly here. A doublefocusing mass spectrometer with reversed geometry (VG Analytical Model ZAB-E) modified for PD study was used. Ions generated by charge exchange in the ion source and accelerated to 8 keV were mass-analyzed by the magnetic sector. Then, the ion beam was crossed with the chopped laser beam perpendicularly in the field region of an electrode assembly. The 5 14.5 and 488.0nm lines of an argon ion laser (Spectra Physics Model 16449) were used. The translational kinetic energy of the fragment ions was analyzed by the electric sector. That is, mass-analyzed ion kinetic energy spectrometry (MIKES) was used to select the reactant ion and to analyze the kinetic energy of the product ion. Since the MIKE spectrum contains contributions from metastable unimolecular dissociation and collision-induced dissociation by residual gas, phase-sensitive detection was adopted to record the MIKE spectrum originating from photodissociation, namely the PD-MIKE spectrum. Xe was used as the reagent gas for charge exchange ionization. The temperature of ion source chamber was maintained at 110 "C. The influence of the collisional relaxation occurring in the ion source was corrected by the method devised previ0us1y.I~ Data Analysis To measure the photodissociation rate constant, a high voltage was applied on the electrode assembly and molecular ions were photoexcited above the dissociation threshold in the field region of the assembly. The translational kinetic energy spectrum for the resulting photofragment ion will be called the field-on PDMIKE spectrum. Field-on PD-MIKE spectra for chlorobenzene molecular ion obtained at 1.5 kV applied voltage using 5 14.5-nm excitation are shown as filled circles in Figure 1. Photofragment ions formed at different positions in the field region appear at different translational energies in the PD-MIKE spectrum. In Figure 1, for example, peak A and its tail correspond to such ions. Fragment ions generated after exiting the electrode assembly appears as peak B. The method to obtain the rate constant or its distribution by analyzing the PD-MIKE band shape have been described in detail previously.l~J~The overall field-on PD-MIKE peak shape is expressed as weighed sum of h(K,t), which is the peak shape function for the dissociation occurring at time t.

H ( K ) = l P ( t ) h ( K , r )dr

0.88

(2)

h(K,t)sfor each t were derived from the field-off PD-MIKE peak shape. Here, K is the translational energy scale in the MIKE

0.00

b

0.04

0.08

0.12

ION SOURCE PRESSURE (torr) Figure 2. log(1og k)vs the pressure of reagent gas used for charge exchange ionization;filled circles represent experimental data and solid line is obtained from linear regression of six low-pressure data.

spectrum. The probability density for dissociation occurring at time t , P ( t ) , is taken as follows considering the random lifetime distribution19 and the approximate linear relation between log k and E.

P ( t ) a SPE(k(E))k(E)sk'"' d E = sPe(k(E))e-k(E)'d k (3) Pe(k(E)) is an energy distribution function of molecular ions from which the dissociation was observed. The following analytical expression has been found adequate in the previous work.15

PE(k(E))= 2[~~/2]"'exp(-a(1og k - log k,)')

(4)

Here, k, is the most probable rate constant and a is a constant related to the width of the distribution. Pe(k(E)),that is, kcand a,can be determined via regression. In the case of PD-MIKE data shown in Figure 1, a and b, for example, the best k, values were 6.4 X lO7and 4.5 X lo7s-l, respectively. Thecorresponding a values were 17.5 and 8.8, respectively. The PD-MIKE band shapes calculated with best-fit k, and a values are also shown in Figure 1. In the previous study on the photodissociation of iodobenzene molecular i0n,l5 internal energy lowering of the molecular ion due to the collisional relaxation in the ion source was observed and the method to correct for the influence of collisionalrelaxation on the rate constant was devised. A similar trend in collisional relaxation has been observed in the present study also. The influence of the collisional relaxation can be easily seen by comparing two PD-MIKE band shapes shown in Figure 1, a and b, which were obtained for the molecular ion generated at 0.03 and 0.06 Torr of ion source pressure, respectively. The stronger B peak relative to A at higher source pressure indicates that the rate constant is smaller at higher source pressure due to more significant collisional energy relaxation of the molecular ions before they exit the ion source. Hence, the collisional relaxationfree k, was estimated by linear extrapolation of the high-pressure data to the zero-pressure limit as in the previous study. Considering the decent linear relation betwene log k and E and the suggestion of Faulk and co-workers" that the internal energy should decrease exponentially as the number of collisions, or pressure, increases, log(1og k,) was plotted versus ion source pressure (Figure 2). Data in the pressure range 0 . 0 3 4 0 6 Torr were extrapolated to the zero-pressurelimit to obtain the collisional relaxation-free k,. The average rate constants obtained at the 95% confidence limit were (7.7 f 3.4) X 107 and (8.4 f 3.2) X 107s-1, respectively, for PD with 514.5- and 488.0-nm laser lines. The internal energy of the molecular ion generated by charge exchange ionization and then photoexcited may be estimated by

Photodissociation of Chlorobenzene Molecular Ion

hu (514.5nm)

The Journal of Physical Chemistry, Vol. 98, No. 20, 1994 5203

t I

TABLE 1: Thermochemical Data (kJ/mol) CaHsX

2.41

X=I

x = c1

Eo

u 1

charge exchange

.................____ ____.....

C6HsCI" Figure 3. Schematic potential energy diagram for the dissociation of chlorobenzene molecular ion. Energies in electronvolts.

3.5

4.0

4.5

5.5

5.0

6.0

INTERNAL ENERGY (eV) Figure 4. Rate-energy dependence for reaction 1: Result of (A)Pratt (0) Stanley et (0)Durant et al.,23 (- - -) initial and Ch~pka,'~ report by Rosenstock et and ( 0 )the present experimental data; vertical bars represent random errors. PSTcalculationsin the zero angular momentum limit; (-) with Eo = 3.29eV and (- -) with EO= 3.44eV.

.

the following relation provided that the collisional relaxation is not involved.

Ei, = RE + E t h -4- h~ - IE

(5)

Here, R E is the recombination energy of Xeand IE is the ionization energy for chlorobenzene. The best literature values are 12.13 and 9.06 eV, respectively.21 Even though charge exchange with Xe*+in the 2Pl/2 state (RE, 13.44 eV) is also possible, almost all of the molecular ions thus generated dissociate before arriving at the photoexcitation region according to the previous rateenergy data.17,22-24& is the thermal vibrational internal energy of the molecular ion at 110 O C . The thermal internal energy distribution is estimated as previously18 and the most probable value is found to be 0.1 1 eV. The internal energy acquired by the molecular ion is shown diagramatically in Figure 3. There is some uncertainty involved in the estimation of the internal energy using eq 5 related to the possibility that some of the excess energy in the charge exchange process is released as the kinetic energy of products through momentum transfer.25 According to a recent study on charge exchange of Are+ with aliphatic hydrocarbons such as ethylene, as much as 95% of the excess energy appeared as the internal energy of the hydrocarbon ions.25c Our own photodissociation study of the thermometric n-butylbenzene ion supports a near-quantitative conversion of the excess energy into the internal energy once the effect of the collisional relaxation is accounted for.26 Errors in the estimation of the internal energy are expected to be fO.l eV or less. Results and Discussion The photodissociation rate constants for reaction 1 by 488and 5 14.5-nm excitation obtained by the procedure described in the previous sections are shown in Figure 4. Since the kinetics of this reaction has been studied extensively, various information on the rate-energy relation and the thermochemical data is

non-totally loose TS 229.6b(2.38) 317.4c(3.29) loose TS 237.3b(2.46) 331.9c(3.44) mr0z9s(CsHsX) 164.9 f 5.9d9r 54.4f 0.9' mr'o(C6HsX) 180.9f 5.9'J 69.3 f 0.V IE(CsH5X)g 838.0 874.2f 1.9 ~f'O(X')~ 107.2 119.6 mr'~(CsHs+)~ non-totally loose TS 1141 1 I41 loose TS 1149 1156 mf'zss(C6Hs')' 330.5 f 8.4 mr'o(C6Hs'Y 343.5 f 8.4 IE(CsH5'Y 781.5 f 9.6 mf'~(CsHs+)~ 1125 f 18 a Numbers in theparenthesesaregivenin eV. Reference 15. Present result. Reference 34. Reference 35. ICalculated from the room temperature value using the statistical method as in ref 36.g Reference 21. Calculated from the values given in this table. Reference 37. f Reference 38. Calculated from data immediately above.

available.17,22-24-27-31 The experimental data of Pratt and C h u p ka,I7 Durant and co-workers,23 and Stanley and co-workers24 obtained on a microsecond time scale are shown also in Figure 4. Rosenstockandco-workers22alsodeterminedthe rate constant on a similar time scale by analyzing the breakdown patterns measured by photoelectron-photoion coincidence mass spectrometry through RRKM (Rice-Ramsperger-Kassel-Marcus) m0de1ing.I~ Rate-energy relation in their initial report228 is reproduced in Figure 4 which seems to display the same general tendency as other data. The same authors reported that a refined treatment of the experimental data had been made.22b However, the refined rate-energy relation was not reported and cannot be included in Figure 4. There are other reports on the rate-energy relation for reaction 1 which deviate quite significantly from the general tendency in the experimental data shown in Figure 4. The earlier result by Baer and ~o-workers2~ is not included here following the arguments by later in~estigators.3~J3The more recent measurement of rate-energy relation by resonanceenhanced two-photon ionization reported by Ripoche and cow o r k e r ~is~not ~ included either because RRKM modeling of the data required a negative value for the entropy of activation at 1000 K which is quite unrealistic for a simple bond cleavage reaction. In addition, extension of their RRKM calculation to the nanosecond range deviates remarkably from the present experimental data. Another complication in the analysis of the rate-energy data arises from the fact that the thermochemical data for the chemical species involved has been upgraded over the years. The most recent and hopefully the most reliable thermochemical data available from the literature are listed in Table 1. Since the earlier investigators used various sets of thermochemical data available at the time of study, their findings from the kinetic analysis such as AHfoo(C6Hs+)are recalculated here. In our earlier investigation of photodissociation kinetics of iodobenzene molecular ion,I5 the main difficulty arose from the uncertainty in the value of reaction critical energy. In particular, the values reported in the literature were obtained by analyzing kinetic data and hence were model-dependent. Situation is similar in the present case also. In this regard, it is important to note that AHfoo(CaH5+)evaluated from the ionization energy and the heats of formation of phenyl radical in Table 1 is 1125 f 18 kJ/mol. Even though the heat of formation of phenyl radical in Table 1 was also obtained from a kinetic experiment, a more reliable result is expected because the thermochemical kinetic experiment for neutrals was carried out at energies near the threshold. In addition, it is known that the dissociative thermal electron attachment method used to obtain the AHf02g8(C6H5')

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5204 The Journal of Physical Chemistry, Vol. 98, No. 20, 1994

TABLE 2: Molecular Parameters Used in PST and VTST Calculations Vibrational Frequencies," cm-I 3070(2), 3050(2), 3030, 1580(2), 1480,1445, CtjHSCl'+ (reactant ion)b 1325,1265, 1175, 1157,1085,1070,1025,

CaH5+(0TS)'

C&CI'+ C6H5+

1002,985,965,902,830,740,702,E 682, 615,467,416,400,297,d 196d 3060(3), 3050(2), 1600(2), 1490(2), 1330, 1310, 1180(2), 1150, 1040, 1010,990(2), 975,850(2), 700,670,600(2), 410(2)

Rotational Constants! cm-I 0.0765 0.1517

Polarizability, l k Z 4cm3 Clg

2.2

Numbers in parentheses denote the degeneraciesof vibrational modes. Reaction coordinate. Transitional modes. Reference 7c. /Estimated values. 8 Reference 36.

b Reference 27.

value in Table 1 provides an upper bound to the bond dissociation energy.37 Then, the upper limit for AHroo(C&+) is not likely to be significantly higher than the upper limit of the above value, namely 1143 kJ/mol. If the reaction occurs via a loose transition state, namely OTS, over the entire energy range covered experimentally, the rate constant can be evaluated using the phase space theory (PST).39 In analogy with our previous study on the dissociation of iodobenzene molecular ion, PST calculation has been carried out to fit the experimental rate-energy data using the critical energy as an adjustable parameter. Molecular parameters used in the calculation are listed in Table 2. A good fit has been achieved with the critical energy (EO)of 3.44 eV and the result is shown in Figure 4. Since PST assumes OTS as the transition state, AHfoo(C6H5+) can be readily evaluated from the energy barrier. However, Azffoo(C&+) thus obtained is 1156 kJ/mol which is well outside the error limits of the value obtained from the ionization energy of phenyl radical and related thermochemical data. Hence, 3.44 eV is not an acceptable value for the critical energy and PST does not provide a good description for this reaction. That is, the bottleneck exists before OTS even though this reaction is a simple bond cleavage. It is to be noted that Pratt and ChupkaI7also discarded the loose-transition-state model on the similar energetics ground in the interpretation of their microsecond data. In the initial report by Rosenstock and co-workers22a on PEPICO study of chlorobenzene ion, a good RRKM modeling was achieved using 3.40-eV critical energy and 9.995-eu entropy of activation at 1000 K, which is indicative of a rather loose transition state. As was mentioned earlier, however, the same authors reported Eo of 3.19 eV and AHf0o(C,&+) of 1130 f 5 kJ/mol from a refined treatment.22b The reported AHf0o(C6Hs+) value is well within the error limits for AHfoo(C&+) determined from the thermochemical kinetics of neutrals. We could not reproduce the RRKM modeling by Rosenstock and co-workers because the refined experimental data are not available from the literature. Our own attempt to fit theexperimentaldata in Figure 4 showed that Eo of 3.19 eV was too low to result in a satisfactory fit. This means that EoandAHfoo(C6Hs+)reported by Rosenstock and co-workersz2b should be considered as minimum values for each properties. In our earlier photodissociation study of iodobenzene molecular ion,I5 Eo of 2.38 eV was found adequate to fit the experimental data over the high microsecond to low nanosecond time range using VTST modeling. AHfoo(CaH5+)evaluated from this energy barrier assuming negligible reverse barrier is 1141 kJ/mol (Table 1) which is well within the error limitsof AHH,~O(C~H~+) mentioned earlier. In addition, this is in good agreement with AHfoo(C&+) reported by Dannacher and co-workers33 in their benchmark measurement of iodobenzene ion which becomes 1137 f 5 kJ/ mol after adjustment with more recent thermochemical data.

-

8 -

0

2 .-E

6 -

A

-

A-

M 0

4 -

2 -

3.5

4.0

4.5

5.5

5.0

6.0

INTERNAL ENERGY (eV) Figure 5. Rate-energydependencefor reaction 1: -)modified RRKM calculation with EO= 3.29 eV and LW*= 7.47 eu. (-) VTSTcalculation with b = 2.9 using eq 8. Other symbols are the same as in Figure 4. (-e

When 1141 kJ/mol is used to evaluate Eo for reaction 1,3.29 eV is obtained. Using this value, PST calculation has been carried out to check the looseness or rather nonlooseness of the reaction. The result is also shown in Figure 4. Comparing the PST calculation with the experimental data, it is apparent that the reaction proceeds via a non-totally loose transition state over the entire energy and time range covered by the experimental data shown in Figure 4. More importantly, it is obvious that TSS at high internal energy does not occur in this reaction. The same conclusion has been made for the dissociation of iodobenzene molecular ion. In the photodissociation study of iodobenzene molecular ion, it was found that the experimental rate-energy data could be explained adequately by a loose-transition-state model (PST) when 2.46 eV was taken as the critical energy. Thecorresponding AHfoo(C6Hs+)value of 1149 kJ/mol results in the critical energy of 3.37 eV for reaction 1. The PST rate-energyrelation calculated with this critical energy value lies halfway between the two calculated PST curves in Figure 4 and cannot explain the experimental data for chlorobenzene ion. That is, one and the same AHf0o(CsH5+)cannot explain the rate-energy data for the dissociations of chloro- and iodobenzene simultaneously when the loose-transition-state model is used. Since the general trend observed in the present analysis is similar to the case of iodobenzene molecular ion reported previously,I5 RRKM and VTST calculations have been attempted here also using EOof 3.29 eV. The rate-energy relation obtained by RRKM modeling of reaction 1 is shown in Figure 5. The frequencies of five halogen-dependent modes other than the reaction coordinate (C-Cl stretch, 702 cm-I) have been lowered following the model I1 developed by Rosenstock and co-workers.22 The vibrational frequencies of the transition state used to calculate the displayed rate-energy relation are listed in Table 2. The equivalent entropy of activation at 1000 K evaluated from these parameters is 7.47 eu. The rate-energy relation obtained by RRKM modeling with adjusted value follows the experimental data very closely. VTST calculations have been carried out according to the simplified method proposed by Lifshitz and co-workers.14 As in the case of iodobenzene ion, two bending vibrations of chlorobenzene were taken as the transitional modes (Table 2) and treated as hindered rotors with barrier height Vo(r). Here, Vo(r) is a loosening function for the transitional modes along thereaction coordinate r. The rate constant was calculated as usual:3

Here, p(E) is the density of states of the reactant ion a t energy E and IV is the state sum at the VTST transition state

Photodissociation of Chlorobenzene Molecular Ion

a -W(E - V(r),r) = 0 ar

The Journal of Physical Chemistry, Vol. 98, No. 20, 1994 5205

(7)

W(E - V(r),r)is the state sum along the reaction coordinate r and V(r) is the potential energy at r. As was the case of iodobenzene ion, a good fit with the experimental data could not be achieved when Gaussian-type Vo(r) originally proposed by Lifshitz and co-workers was used. As before,lS however, a decent fit was possible (Figure 5) when a Lorentzian-type function was used.

Here, V, and re are the equilibrium barrier height and the equilibrium bond length, respectively. b is an adjustable parameter. Since the influence of transitional mode loosening on the state sum becomes important late in the reaction with the Lorenzian type function, this may be called a late loosening model compared to the Gaussian type which is early loosening. Most importantly, the fact that the simplified VTST model can fit the experimental data and that the VTST rate constant is lower than the PST rate constant over the entire experimental energy range confirm that TSS at high internal energy does not occur. Pratt and Chupkal' also carried out an analysis of their rateenergy data using the non-totally loose-transition-state (RRKM) model and the loose-transition-state model of Klots (PST).39a Their result could be fit with both of the models for iodo- and bromobenzene, resulting in slightly highervaluesof AHf0o(C6H5+) for Klots's model. However, a significantly higher valueof p H r o o (C6H5+)was required to fit the data for chlorobenzene with Klots's model. From this, they concluded that the non-totally loosetransition-state model was appropriate for chlorobenzene ion decomposition reaction, while the other two halobenzene decomposition reactions could be fit within experimental error by either of the two models. Our own conclusion on the dissociation kinetics of iodo- and chlorobenzene molecular ions are similar to but slightly different from that of Pratt and Chupka.I7 Based on the thermochemical argument, it is thought that chlorobenzene ion dissociates definitely via a non-totally loose transition state and iodobenzene ion likely via a non-totally loose transition state; i.e., dissociation processes of these ions, and probably also bromobenzene ions which cannot be investigated with the present technique due to some experimental difficulty at the moment, can be viewed on equal terms. All these reactions occur via rather loose but nontotally loose transition states over the experimentally accessible energy range. Assuming negligible reverse barriers, this means that dynamical bottlenecks with entropic origin exist along the reaction coordinates well before OTSs. Accepting thevariational criterion in VTST derived from dynamical consideration, the above reactions are expected to proceed via OTSs at internal energy very near the threshold. Then, the fact that non-totally loose transition state is involved in the energy range investigated experimentally suggests that TSS occurs as the internal energy increases slightly above the threshold. Such a picture is in general agreement with experimental and theoretical findings in neutral reactions.sJ6 In the present investigation, the value of AHf0o(C6Hs+) has been taken as a guideline to judge the reasonable value of the reaction critical energy. Recently, Klippenstein and co-workers40 carried out a combined theoretical and experimental study on the hydrogen loss from benzene molecular ion. A rigorousvariational evaluation resulted in roughly an order of magnitude smaller number of states at TS than that obtained by PST type calculations. This compares very favorably with the present case. A satisfactory agreement between the theoretical and experimental results was reported for an assumed dissociation energy of 3.88 eV to the lowest triplet state of CsHs+ which was expected to lie

approximately 0.2 eV above the ground state. This dissociation energy results in 1150 kJ/mol of AHfo~(C6Hs+). Accepting thisvalue means that thedissociation of C6HsI*+occurs via a loose transition state, unlikely the hydrogen loss from benzene cation, to the triplet state of C&+. On the other hand, assumption of a non-totally loose transition state is still needed to describe the dissociation of C6HsCl". The difference in the nature of the transition states among the similar dissociation reactions is difficult to comprehend. Considering that both the singlet and triplet states of C ~ H Sare + energetically accessible, differences in the final state distributions of C6H5+ may be a possible source of variation among AHfoo(CaH5+) determined from different reactions. As suggested by the above investigators, a detailed consideration of the interaction energies with highlevel a b initio calculations and a rigorous variational evaluation of the rate-energy relation seem to be the next logical step to further understand the halogen loss reactions of halobenzene cations. Conclusion The rate constant for the dissociation of chlorobenzene molecular ion has been measured on a nanosecond time scale with reasonable accuracy using the PD-MIKES technique. When combined with previous microsecond data, the rate-energy data over an wide internal energy range become available. This, we believe, enables more reliable kinetic modeling of the reaction. The main difficulty in the theoretical analysis of the above data arises from the fact that the reaction critical energy is not known definitely. Depending on the value adopted, either the non-totally loose-transition-state model or the loose-transitionstate (OTS) model can account for the experimental rate-energy data. When AHfoo(CbHs+)evaluated from the heat of formation of C6H5' is taken as the guideline, however, the non-totally loosetransition-state model looks more appropriate. In addition, A H f O o (C6Hs+)needed to fit the rate-energy data for chlorobenzene ion using the loose-transition-state model differs from that for iodobenzene ion. Even though this difference is smaller than the errors involved in the measurement of other relevant thermochemical parameters, it seems to be meaningful that one and the same value of AHfoo(C6Hs+)can not explain the rate-energy data for the two systems simultaneously under the loose-transitionstate model. On the other hand, the reaction critical energies evaluated from one and the same AHf0o(C6Hs+)are adequate to explain the rate-energy data for the two systems when VTST is used. The fact that the experimental rate constants, and VTST ones also, are lower than the loose-transition-state rate constants means that the reactions occur via non-totally loose transition state over the entire internal energy ranges investigated. This is also confirmed by successful RRKM fitting of the rate-energy data for these reactions. The main impetus for the present study has been derived from the suggestion by Lifshitz and co-workers14 to measure the rate constant at high internal energy to check the possible occurrence of TSS. In this regard, one may think that the outcome of the present study can be inconclusive because the internal energy scale estimated in this work may be erroneous by about 0.1 eV. However, once the internal energy scale of the data points is lowered by such a magnitude, the rate-energy data become more contradictory with the occurrence of TSS a t high internal energy. Hence, the present results together with the previous findings on iodobenzene ion confirm that TSS at high internal energy does not occur. The experimental data considered here does not tell whether TSS occurs near the reaction threshold even though VTST suggests such a possibility. An accurate measurement of the rate-energy relation near the threshold together with reliable thermochemical data is needed to check such a possibility.

5206 The Journal of Physical Chemistry, Vol. 98, No. 20, 1994 Acknowledgment. This work was supported financially by Yukong Ltd. and Ministry of Education, Republic of Korea. References and Notes (1) (2) 315. (3) (4) (5)

Wardlaw, D. M.; Marcus, R. A. Adu. Chem. Phys. 1988, 70, 231. Lifshitz, C. Int. J. Mass Spectrom. Ion Processes 1992, 118f119,

Lifshitz, C. Ado. Mass Spectrom. 1989, 11, 713. Hu, X.; Hase, W. L. J. Chem. Phys. 1989, 93, 6029. (a) Klippenstein, S.J.; Khundkar, L. R.; Zewail, A. H.; Marcus, R. A. J . Chem. Phys. 1988,89,4761. (b) Klippenstein, S.J.; Marcus, R. A. J . Chem. Phys. 1989,91,2280. (c) Klippenstein, S.J. Chem. Phys. Lett. 1990, 170,71. (d) Klippenstein,S. J.; Marcus, R. A. J . Chem. Phys. 1990,93,2418. (e) Klippenstein, S.J. J . Chem. Phys. 1991, 94, 6469. (6) (a) Song, K.; Chesnavich, W. J. J. Chem. Phys. 1989,91,4664. (b) Song, K.; Chesnavich, W. J. J . Chem. Phys. 1990, 93, 5751. (7) (a) Bowers, M. T.; Jarrold, M. J.; Wagner-Redeker, W.; Kemper, P. R.; Bass, L. M. Faraday Discuss. Chem. SOC.1983,75,57. (b) Chesnavich, W. J.; Bass, L.; Su,T.; Bowers, M. T. J . Chem. Phys. 1981, 74, 2228. (c) Jarrold, M. F.; Wagner-Redeker, W.; Illies, A. J.; Kirchne, N. J.; Bowers, M. T. Int. J . Mass Spectrom. Ion Processes 1984, 58, 63. (8) (a) Dodd, J. A.; Golden, D. M.; Brauman, J. I. J. Chem. Phys. 1984, 80, 1894. (b) Dodd, J. A.; Golden, D. M.; Brauman, J. I. J . Chem. Phys. 1985,82,2169. (c) Chesnavich, W. J.; Bowers, M. T. J . Chem. Phys. 1985, 82, 2168. (d) Truhlar, D. G. J. Chem. Phys. 1985,82,2166. (e) Booze, J. A.; Schweinberg, M.; Baer, T. J . Chem. Phys. 1993, 99, 4441. (9) Chesnavich, W. J. J . Chem. Phys. 1986, 84, 2615. (10) Hase, W. L. Chem. Phys. Lett. 1987, 139, 389. (11) Rai, S.N.; Truhlar, D. G. J . Chem. Phys. 1983, 79, 6046. (12) Pollak, E.; Baer, M.; Abu-Salbi, N.; Kouri, D. J. Chem. Phys. 1985, 99, 15. (13) Berblinger, M.; Pollak, E.; Schlier, Ch. J . Chem. Phys. 1988, 88, 5643. (14) Lifshitz, C.;Louage, F.; Aviyente, V.; Song, K. J . Phys. Chem. 1991, 95, 9298. (15) Yim, Y. H.; Kim, M. S.J . Phys. Chem. 1993, 97, 12122. (16) (a) Khundkar, L. R.; Knee, J. L.; Zewail, A. H. J. Chem. Phys. 1987, 87, 77. (b) Potter, E. D.; Gruebele, M.; Khundkar, L. R.; Zewail, A. H. Chem. Phys. Lett. 1989,164,463. (17) Pratt, S.T.; Chupka, W. A. Chem. Phys. 1981, 62, 153. (18) (a) Choe, J. C.; Kim, M. S.J. Phys. Chem. 1991,95,50. (b) Choe, J. C.; Kim, M. S.Int. J. Mass Spectrom. Ion Processes 1991, 107, 103. (c) Choe, J. C.; Kim, M. S.J . Phys. Chem., 1992, 96, 726.

Yim and Kim (19) Robinson, R. J.; Holbrook, K. A. Unimolecular Reactions; Wiley: New York, 1972. (20) Faulk, J. D.; Dunbar, R. C.; Lifshitz, C. J . Am. Chem. SOC.1990, 112, 7893. (21) Lias, S.G.;Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J . Phys. Chem. Ref. Data, 1988, 17 (Suppl. No. 1). (22) (a) Rosenstock, H. M.; Stockbauer, R.; Parr, A. C. J . Chem. Phys. 1979,71,3708. (b) Rosenstock, H. M.; Stockbauer, R.; Parr, A. C. J . Chem. Phys. 1980, 73, 773. (23) Durant, J. L.; Rider, D. M.; Anderson, S.L.; Proch, F. D.; Zare, R. N. J . Chem. Phys. 1984,80, 1817. (24) Stanley, R. J.; Cook, M.; Castleman, A. W. Jr. J . Phys. Chem. 1990, 94, 3668. (25) (a) Futrell, J. H. Int. J . Quantum Chem. 1987,31,133. (b) Horning, S. R.; Kotiaho, T.; Dejarme, L. E.; Wood, J. M.; Cooks, R. G.Int. J . Mass Spectrom. Ion Processes 1991,110,l. (c) Tsuji, M.; Kouno, H.; Matsumura, K.; Funatsu, T.; Nishimura, Y.; Obase, H.; Kugishima, H.; Yoshida, K. J . Chem. Phys. 1993, 98, 201 1. (26) Oh, S.T.; Kim, M. S. Manuscript in preparation. (27) Baer, T.; Tsai, 8. P.; Smith, D.; Murray, P. T. J. Chem. Phys. 1976, 64, 2460. (28) Dunbar, R. C. J. Phys. Chem. 1987, 91,2801. (29) Ripoche, X.;Dimicoli, I.; Le Calv6, J.; Piuzzi, F.; Botter, R. Chem. Phys. 1988, 124, 305. (30) Nikami, N.; Miyata, Y.; Sato, S.;Sasaki, T. Chem. Phys. Lett. 1990, 166, 470. (31) Ripoche, X.; Dimicoli, I.; Botter, R. Int. J . Muss Spectrom. Ion Processes 1991, 107, 165. (32) Brand, W. A.; Baer, T. Int. J. Mass Spectrom. Ion Processes 1983, 49, 103. (33) Dannacher, J.; Rosenstock, H. M.; Buff, R.; Parr, A. C.; Stockbauer, R. L.; Bombach, R.; Stadelmann, J. P. Chem. Phys. 1983, 75, 23. (34) Malinovich, Y.; Lifshitz, C. J . Phys. Chem. 1986, 90, 2200. (35) Platonov, V. A.;Simulin, Y. N. Russ. J . Phys. Chem. 1985,59, 179. (36) Benson, S.W. Thermochemical Kinetics; Wiley: New York, 1976. (37) Chen, E. C. M.; Albyn, K.; Dussack, L.; Wentworth, W. E. J . Phys. Chem. 1989, 93, 6827. (38) Sergeev, Yu. L.; Akopyan, M. E.; Vilesov, G. I. Opt. Spektrosk. 1972, 32, 230. (39) (a) Klots, C. E. 2.Naturforsch. 1972,27A, 553. (b) Chesnavich, W. J.; Bowers, M. T. J . Am. Chem. SOC.1977,99,1705. (c) Chesnavich, W. J.; Bowers, M. T. J . Phys. Chem. 1977, 66, 2306. (40) Klippenstein, S.J.; Faulk, J. D.; Dunbar, R. C. J . Chem. Phys. 1993, 98, 243.