J. Phys. Chem. 1987, 91, 2758-2762 Hewlett-Packard 3390A or a Varian 4290 electronic integrator. Temperature measurements were made with a digital RTD platinum thermometer (Omega Engineering, Model 109, fitted with platinum probe). GC-MS analyses on preliminary runs were performed on a Finnigan GC-MS Model 4510.
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. This research was also supported by N S F Grant C H E 84-00706. We are grateful to Mr. John Dekanel for suggesting the use of the Durawax GLC column.
Preparation of Cationic Intermediates by Dissociative Photoionization of van der Waals HCI Mixture Complexes. C,H,Ci+ from a C6H,
+
E. A. Waiters,*+ J. R. Grover,* M. G. White, and E. T. Hui* Department of Chemistry, Brookhaven National Laboratory, Upton, New York 1 1 973 (Received: November 14, 1986)
The ion [C6H6Cl]+(1) has been observed as a product of dissociative photoionizationof a C6H6 + HCl van der Waals complex. It was shown that 1 does not come from the dimer C,&*HCl but from trimers or larger clusters, most likely (C6H6)2HC1. Experiments with C6D6 + HC1 mixtures demonstrate that the departing hydrogen initially belonged to the HCI partner of the complex. The energetics and isotope studies are consistent with an ipso structure for C6H6Clf, i.e., a Wheland u-complex. The overall process of dissociative photoionization does not have an accessible reaction channel leading to C6H6Cl' unless at least one "solvent" molecule is present. Mechanisms by which the transformation may occur are discussed.
Introduction Aromatic electrophilic substitution is one of the most thoroughly studied of organic reactions.' The mechanism is characterized by the formation of a carbocation intermediate, also known as a Wheland intermediate or o-complex. The existence of this intermediate in solution is well established and recent work has confirmed its formation in the gas phaseG2 Evidence has been presented that the mechanism in the gas phase is of the same nature as that in s ~ l u t i o n . ~Significant contributions to an understanding of this process would be thermochemical information on the intermediates and detailed data on the role of solvent. This study was initiated on the inviting premise that dissociative photoionization of a van der Waals cluster molecule offers a new method for the synthesis and study of ionic reaction intermediates, as exemplified in reaction 1. Our goal was to learn whether the C6H6-HCl
+ hv
-+
C6H6Cl+ 1
+H+e
(1)
ion generated in this manner can be identified with the Wheland intermediate' for electrophilic chlorination of benzene. In the course of the study it was learned that 1 does not arise from the C6H6.HCI heterodimer, but comes from larger clusters, most likely trimers.
Experimental Section The experimental apparatus and technique have been described p r e v i o ~ s l y . ~ -Briefly, ~ the apparatus is a photoionizaton mass spectrometer equipped with a supersonic nozzle source. It uses the tunable photon beam available at line U11 of the 750-MeV storage ring at the National Synchrotron Light Source. The light is transmitted with no intervening windows from the electron beam to a normal incidence monochromator, the output of which intersects the molecular beam at the focal point. An osmium-coated laminar grating of 1200 lines mm-I, designed to eliminate second-order light at 800 A, was used. Molecular beams containing the (C6H6),(HCI), target molecules were generated by the +Permanent address: Department of Chemistry, University of New Mexico, Albuquerque, N M 87131. 'Participant in the National Synchrotron Light source/High Flux Beam Reactor (NSLS/HFBR) Faculty-Student Support Program at Brookhaven National Laboratory.
0022-3654/87/2091-2758$01.50/0
skimmed and recollimated nozzle expansion of gaseous HCI saturated with benzene vapor at 23 'C, made by bubbling HCl through room temperature benzene, vapor pressure -80 Torr. The flow of HCl was regulated to maintain a constant head pressure of 800 Torr above the benzene. The technique described earlier8 for optimization of neutral clusters in the molecular beam and a more recent three-wavelength approach for specific analysis of the relative beam density of heterodimersg were applied to the 1:9 C & j HC1 system. Mass spectra were taken by using 584 8, (21.22 eV) light to produce the ions. The secondary production of 1 by ion-molecule reactions is not expected to be observable at the low background pressure of lo-' Torr used here. In addition, energetic grounds can be used to eliminate the possibility of interference from two of the most likely ion-molecule pathways, namely the ion-molecule reactions C1+ C& C6H6C1+ and C6H,+ -k HCI C&&,Cl+ H. The first can be eliminated from consideration because the observed threshold is 2.6 eV below the appearance potential of C1+ from HC1. The second reaction is not likely because the endothermicity
+
-
+
-+
+
(1) For recent reviews see: (a) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic Chemistry, 2nd ed.; Harper: New York, 1980. (b) March, J. Advanced Organic Chemistry; Reactions, Mechanisms, and Structure, 2nd ed., McGraw-Hill: New York, 1977. (c) de la Mare, P. B. D. Electrophilic Halogenation; Cambridge University: Cambridge, England, 1976. (d) Ridd, J. H. Adu. Phys. Org. Chem. 1978, 16, 1. (2) (a) Attin:, M.: Cacace, F.: de Petris, G.; Fornarini, S . ; Giacomello. P. J . Am. Chem. SOC.1985, 107, 2297 and references therein. (b) Miller, D. L.; Gross, M. L. J. Am. Chem. SOC.1983, 105, 3783. (3) Miller, D. L.; Lay, J. O., Jr.; Gross, M. L. J. Chem. Soc., Chem. Commun. 1982, 970. (4) Wheland, G. W. J. Am. Chem. SOC.1942, 64, 900; The Theory of Resonance: Wiley: New York, 1944. ( 5 ) White, M. G.; Grover, J. R. J. Chem. Phys. 1983, 79, 4124. (6) Grover, J. R.; Walters, E. A.; Newman, J. K.; White, M. G. J. Am. Chem. SOC.1985, 107, 7329. (7) Walters, E. A,; Grover, J. R.; White, M. G ; Hui, E. T. J. Phys. Chem. 1985, 89, 3814. (8) Grover, J. R.; Walters, E. A. J . Phys. Chem. 1986, 90, 6201. Since this paper was written the method has been improved. The approximate position of the neutral dimer peak is taken to be where 6/a calculated from eq 15 of this reference first becomes significantly larger than either S/a from eq 12, or S/a from the low-pressure asymptote of eq 15, whichever is larger. (9) Grover, J. R.; Walter, E. A,; Arneberg, D. L.; Santadrea, C. J.. submitted for publication in J . Phys. Chem.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 2759
Preparation of Cationic Intermediates TABLE I: Intensities of C6DcHCI+ and C6D6CI+from 1:9 C a s + HCI at 200 Torr Nozzle Pressure with 584 A Ionizing Radiation" expected intensity for loss of mle exptl intensity H D 0.32 117 0.03 10.53 118 119 1.00 & 0.05 1.oo 1.00 120 0.52 f 0.05 0.57 8.42 121 0.33 f 0.06 0.34 0.63 122 0.42 dz 0.07 0.17 1.58 0.01 0.01 123
.,
,,
I I
"Ion intensities are all normalized to the m / e 119 peak for convenience of comparison.
-
for production of C&,Cl+ in its lowest energy form is more than 1.5 eV. The reaction c6& HCl+ CJ-16Cl+ H is a sensitive probe for interference from ion-molecule reactions since HCl+ forms a t 2.1 eV below the observed onset a t 837 8, and this reaction is exothermic by 1.9 eV to form ground-state C6H6C1+. N o indication of C6H6Cl' production at wavelengths longer than 837 8, was seen, which indication would be easy to recognize because it would reflect the highly structured PIE of HCl+ in this region. Mixtures of C6D6 HC1 (1:9) were prepared as described above. The C6D6 had in excess of 96.6 atom % D as confirmed by mass spectrometry. N o evidence for gas-phase exchange of deuterium between C6D6and HCl was observed, although small amounts of exchange were seen for mixtures held in the stainless steel inlet system for times much longer than the residence time during an experiment. Mass spectra with 584 A ionizing radiation were recorded at several nozzle pressures.
+
,/I
+
a
+
NOZZLE PRESSURE. Ion
n
Results Neutral Cluster Distribution. The pressure at which the beam density of dimers maximizes and the pressure dependence of the relative beam density of trimers in the same pressure region in the jet expansion of a 1:9 mixture of C6H6and HCl were established from the pressure dependence of the mass spectra as described previously.s The results are given in Figure l a , where the solid curve is the dimer function, and in Figure 1b, where the dashed line is the normalized (see Figure l b caption) curve for trimers. An alternate method is applicable to cases in which the dissociative ionization yield of homodimers is considerably smaller than that for heterodimers, as it is in the C6H6 + HCl system. In this method the slope of the threshold curve for dissociative C6H6+ HCl + e, ionization of the dimer, C6H6.HCl hu is determined as a function of total pressure. This slope is directly related to the density of neutral dimers? The results, smoothed from the data given in ref 9, are shown as the dashed line in Figure la. The density of C6H6.HCl dimers rises abruptly with increasing nozzle pressure reaching a maximum at Po N 225-260 Torr. Trimers are first found in significant quantities near Po 175 Torr and their density also rises sharply with increasing pressure. Note that the C6H6Cl+density rise matches that of the trimer curve. That is, C,&CI+ is not detected until independent experiments show that trimers are formed; this is already at or near a nozzle pressure for the maximum density of dimers. In the pressure range of 200-275 Torr the density of dimers changes by a factor of 1.2 while both the trimer and C6H6Cl+densities change by a factor of 5.8. The conclusion drawn is that over this pressure range, 200-275 Torr, C6H&1+ is the product of trimer fragmentation. At higher nozzle pressure, it is possible that still larger clusters are sources of C6H6C1+. Deuterium Isotope Effect. High-resolution mass spectra of a 1:9 mixture of C6D6 HCl at 200 Torr were taken at 584 A. This is near the pressure for the maximum density of dimers, yet it is the lowest pressure a t which measurement of the intensity of C6D6C1+is feasible. The integrated intensities of the ion peaks m l e 117-1 2 3 , corrected for background and normalized to the m / e 119 signal, are presented in Table I. The expected intensity ratios for peaks in this mass range resulting from loss of H or D
+
-
+
-
+
I
I
/
/ I
I
I
NOZZLE PRESSURE (Torr)
Figure 1. (a) Analyses of dimers from the jet expansion of 1:9 C6H6 +
HCI. The solid line is the relative density of C6H6.HC1dimers in the molecular beam collimated from the expansion as obtained by the prescription of ref 8. The dashed line is the result of the three-wavelength method of ref 9. The two sets of data are normalized to the same value at 225 Torr. (b) Distribution of neutral dimers and trimers from the jet expansion of 1:9 C6H6+ HC1 compared with the yield of C6H6C1+ions. The solid line is for the relative beam density of C6H6.HCl,and is the three-wavelength result shown in (a). The three points are the measured intensities of the C6H6Cl+fragment ion from the dissociative photoionization of benzene/HCl clusters at 584 A. The dashed line is the relative beam density of trimers determined by the method of ref 8, normalized
to the second of the three points. At higher pressures the trimer density is overestimated due to interference from the presence of tetramers and larger clusters.8
were estimated from the known isotopic purity of C6D6,the natural abundance of 13C,the natural abundances of 35C1and 37C1,and the observed ratio of the C6H6.HCI+and C,&Cl+ peaks in the expansion of C6Hs HC1. These values are also normalized to m l e 119. Comparison of column 2 with columns 3 and 4 shows
+
2760 The Journal of Physical Chemistry, Vol. 91, No. 11, 1987
Walters et al. is the most effective onset a t such a high energy, 14.81 eV, and not nearer threshold (-13.3 eV)? (2) Why is a third molecule mandatory? (3) Which molecule is the third one, C6H6 or HCI? (4) Why is an HCI hydrogen ejected and not one of the benzene hydrogen atoms? ( 5 ) Is there reason to believe that any particular one of the several energetically accessible isomers of C6H&l+ is favored? The first, second, and fourth questions represent significant constraints on any possible reaction mechanisms, while the fifth represents an important result we should try to extract from this work. Unimportance of C6H6.HC1f o r the Production of c6H6Cl'. Independently of the evidence of Figure 1b there is another strong reason for the surprising conclusion that reaction 1 must be rejected as the source of 1. Since the onset at 837 f 5 A (341 f 2 kcal mol-') would be the effective threshold, it may be combined with the heats of formation of H (51.6 f 0.01 kcal mol-') and C6H6.HCI (-10.4 f 0.5 kcal mol-')' to yield a corresponding heat of formation of 1 of 279 f 2 kcal mol-]. This value is unreasonably large. Such an energetic ion could dissociate spontaneously by the elimination of an H atom, a C1 atom, or an HCl molecule. It seems most unlikely that C6H6Clf would be seen under these conditions. In addition, RRKM calculations using the known dissociation energies predict that the statistical production of C6H&I+ from reaction 1 should be unobservable at any photon energy in this experiment because D(C6H6-HC1) and D(C6H6+-HCI) are both Very Small compared to D(C,&C1+-H). Source of C6H6cl': Identity of the Third Molecule. The parentage of 1 cannot be established unambiguously by using the data presently available. Since it appears that at least in the low-pressure regime, 200-300 Torr, 1 comes from trimers, either (C6H6)2HCl or C6H6(HC1)2, the two likely dissociative photoionization alternatives are
-I
I
WAVELENGTH (Angstroms)
< b l
(C,H,),HCl
+ hu
+
C6H6CI'
+ C6H6 f
+e
(2)
C6H&I+
HC1
+H +e
(3)
H
and 1
720
760 800 WAVELENGTH (Angstroms)
840
880
Figure 2. Observed intensities of the ion C6H6CIt produced by dissociative photoionization of mixed trimers (and larger clusters): (a) over the range 602-922 8, at 5-8, intervals; (b) over the range 700-880 8, at 1-8, intervals, with the region at wavelengths greater than 800 8, expanded and plotted with a displaced zero to highlight the onset at 837
A. that the experimental isotopic abundances are consistent with loss of H from the trimer, either (C6D6)2.HCIor C6D6.(HC1),, to form C6D6CI+. The aromatic ring hydrogens are not removed. Photofragmentation of ( c 6 H 6 ) m ( H c l ) nThe . PIE curve for production of the PIE fragment ion, 1 ( m / e 113), from the parent van der Waals molecules, (c6H6)m(Hcl)n,is shown in Figure 2. This curve was determined at a nozzle pressure of 600 Torr; under these conditions the number density of C6H6-HC1dimers has peaked and is decreasing while higher neutral cluster number densities in the molecular beam have increased to undetermined values. Even at this relatively elevated pressure the counting rates are low, especially for photon energies in the vicinity of the onset. The spectrum in Figure 2b required 130 s per point for a total of 7.2 h of synchrotron beam time distributed over three fills of the storage ring. A total of 2143 counts were collected at 700 8,. Expansion of the vertical scale (and displacement of the zero) in Figure 2b reveals a distinct onset for 1 at 837 -+ 5 8, or 14.81 f 0.09 eV. At wavelengths longer than 840 %, there is still a very low counting rate that is essentially independent of wavelength. Much, if not most, of this intensity is due to scattered light, but a contribution from C6H&I+ cannot be discounted because the actual threshold for its formation from mixed trimers or heterodimers is expected to be around 990-1010 8,. Any contributions of C6H6CI' from second-order radiation should be negligible in the region shown. Discussion The results of this work raise a number of interesting questions to which the remarks in this Discussion are directed. (1) Why
hu
C6&(HC1)2
+
Reaction 3 is excluded from further consideration in this discussion by the following argument. The source gas composition of 1:9 C6H6 HCI was chosen because C6H6 condenses more readily at any given temperature than does HCI, as judged by their respective boiling points. A mixture moderately lean in C6H6 expanded at rather low nozzle pressure should decrease the tendency to form large (C&6), clusters. A careful mass spectrum of this mixture at 600 Torr nozzle pressure, using 584 8, ionizing radiation, gave relative trimer ion intensities, summed over fragments and corrected for transmission efficiency of the quadrupole Of (C6H6)3+:(C6H6)2HCI+:c~H6(HCl),f:(Hcl3)+ = 1.0:6 X 10-2:7 X 10-':4 X At ionizing energies as high as 21 eV it is reasonable to expect that such a systematic and sharp decrease in intensity with respect to the increasing proportion of HCI in the cluster ions means that there is a corresponding systematic decrease of the beam density of neutral clusters with respect to an increasing proportion of HCI. Thus, since the intensity of (C6H6)2+HC1is approximately an order of magnitude higher than that of C6H6(HC1)2f,reaction 2 is the more likely source of 1. Mechanism of C&,CI+ Formation. In considering mechanisms for the production of 1, it is important to know the identity of the chromophore in (C6H6)2HC1and the heats of formation of all the species in reaction 2. The threshold for dissociative ionization of HCI, HCI hu H C1+ + e, is 2.6 eV'O above the observed onset for 1 at 14.81 eV. Therefore reaction 2 does not proceed by a mechanism requiring the formation of C1+ by dissociative ionization of an HC1 partner in the trimer. Another conceivable mechanism is one in which the HC1 is ionized, following which it binds to a benzene molecule releasing sufficient energy to expel the hydrogen atom. However, the observed onset requires the HCI' to be formed with 2.1 eV of excitation, while
+
+
-
+
(10) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T. J . Phys. ReJ Data, Suppl. 1977, 6 , 1.
The Journal of Physical Chemistry, Vola91, No. 11, 1987 2761
Preparation of Cationic Intermediates the photoelectron spectrum of HC1 shows that no HC1+ ions at all are formed with this excitation, the nearest bands being at 0.1 and 3.5 eV." On the other hand, the ionization energy of the C6H6 (1b2J molecular orbital lies at the 14.86 eV" which is the same, within experimental uncertainty, as the onset for C6H6CI+, and, moreover, this band of excitation spreads broadly to higher and lower energies. Since the foosely bound C6H6 and HCI retain substantial individuality, it is therefore reasonable that the chromophore is a benzene rather than an HCl molecule. We also need to know what product species are energetically permissible. Two limiting assumptions can be made for AH? (1). First, 1 is the most stable isomer of C&$I+; i.e., it is paraprotonated chlorobenzene, shown as 2, for which AH? (2) = 196 CI
H
Q H
H
:[
CI
3
2
'
'
C&j.C6H6+(B2,).HCl
L2CeHg t HCI
kcal mol-'.12 Second, the structure of 1 is the ipso isomer 3. The heat of formation for 3 is not available so an estimate was made from the total energy of o-chlorobenzenium ion obtained from double f calculations with contracted Gaussian orbitalsI3 and the difference in total energy between i-C6H6F+ and o-C6H6F+ calculated with extended 4-31G functions after geometry optimization at the STO-3Glevel.14 The resulting value is AH? (3) = 222 kcal mol-'. Heats of formation of the van der Waals complexes are calculated from AHfo(C6H6*HC1)= -10.4 kcal mol-' and the assumption that the dissociation energy of the second van der Waals bond is the same as the first, 4.8 kcal mol-';' thus, AHf0((C6H6),HC1)= 1 kcal mol-'. With these values the heat of reaction of reaction 2 with product 2 is AH20(2) = 263 kcal mol-', and with product 3, AH20(3) = 289 kcal mol-'. For both products the AHo values lie well below the onset at 341 kcal mol-' and are therefore energetically feasible. Formation of 2 requires hydrogen atom migration which suggests that isotopic scrambling in an intermediate or activated complex may be possible. Absence of scrambling was noted experimentally, an observation which tends to support mechanisms for which 2 is not the product, but which is not definitive. We tentatively assume, with more justification later, that reaction 2 with product 3 is the pathway for dissociative photoionization. With product 3, at threshold with emission of zero-energy electrons, final-state products are formed with 52 kcal mol-' of excess energy to distribution between translational and internal degrees of freedom. This amount of energy is insufficient to promote product C6H6, and probably not 3, to an excited electronic state. With the assumption of product 3 and the foregoing estimates of various energies of formation, two alternative mechanisms for reaction 2 are considered further. Both mechanisms are based on the first step being the production of the electronically excited ionic complex C6H6-C6H6+(B2u)-HCI and a zero kinetic energy electron at the observed onset, 14.81 eV, and address the problem of generating C&Cl+ + C6H6 + H from the ionic complex in an internal ion-molecule reaction. Mechanism A is a single-step dissociation into products, reaction 4. We do not see any a priori reason why this mechanism would -+
C6H6CI'
+ C6H6 + H
(4)
be favored over reaction 1. In particular there is no obvious reason why the "extra" C6H6molecule is crucial to make the reaction go, nor does it explain why the ejected H atom comes from HCl. (11) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S. Handbook of HeI Photoelectron Spectra of Fundamental Organic Molecules; Halsted: New York, 1981. (12) Lias, S. G.; Liebman, J. F.; Levin, R. D. J . Phys. Chem. Ref. Data 1984, 13, 695. (13) Binning, R. C.; Sando, K. M. J. Am. Chem. SOC.1980, 102, 2948. (14) Hehre, W. J.; Hiberty, P. C. J. Am. Chem. SOC.1974, 96, 7163.
O -2
t
I _ - -
Figure 3. A possible reaction energy profile for the dissociative photOiOIliZ3tiOn (C&)&CI + hV C6H6C1' + C6H6 + H + e, mechanism B. Values by the arrows are transition energies in electronvolts; heats of formation are found in Table 11. -+
TABLE I 1 Heats of Formation for the Species in Mechanism B AHf'
species
(C6H6)zHC1 C6H6'C6H6'*HCI C6H6'C6H6+'C1 C6H6*C6H6'(B~y).HCI C,&CI''C& C,H&I+
kcal mol-'
1.2 199.8 250.4 342.8 221.4 222
'Estimated as described in the text.
Also, there is no intrinsic reason why reaction 4 should be especially facile just because it has a three-body final state; actually this would be a disadvantage. The alternative mechanism B is but one of many possible stepwise sequences. The mechanism considered is
--
C6H6*C6H6+(B2u).HCl C6H&6H6+'C1
[C6H6Cl+*C6H6]*
C6H,j'C6H6+'CI + H
[C,&jCl+*C6&] * C&jCl+
(Sa) (5b)
+ C6H6
(5c)
This scheme is presented diagrammatically in Figure 3. The heats of formation used in this energy diagram are collected in Table 11. In the first step, Sa, the excitation energy transfers from C6H6' to HC1 in processes that correspond, for unbound molecules, to HCI(X'Z+) C&&(XE,,) HCl(A,a1,311). The C&6+(B2,) HCI(A,a',311) states are rep~lsive,'~ HCl(II) H(2S) Cl(2P3/2), and appear in photon absorption as a continuum with an effective onset at 44000 cm-',I6 or 5.46 eV, essentially the same as the 5.6 eV of excitation energy in C6H6+(BZu), or, independently of any benzene state designation, corresponding to an onset of 14.7 eV, which is within experimental uncertainty of the measured onset of 14.81 f 0.09 eV. Of course one should not expect the onset of photon absorption to be exactly the same as the onset of energy exchange between molecules, but it should be reasonably close if the van der Waals binding perturbs the molecules only a little. Thus the energetic requirements are all remarkably consistent and lead naturally to expulsion of the hydrogen atom from the HCl moiety, in agreement with experiment. Some of the recoiling C1
-
(15) Mulliken, R.
-
+
+
S.Phys. Reu. 1937, 51, 310.
(16) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular
Structure IV. Consrants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979.
2762
The Journal of Physical Chemistry, Vol. 91, No. 11, 1987
atoms will inevitably collide with and form some kind of excited complex C6H6.C6H6+*CIwith the benzene molecule and molecule ion. An estimate for AHfo(C6H6-C6H6+-C1) is obtained, Figure 3, assuming that the energy required to dissociate the hypothetical ionic Complex C6H,j*C6H6+*HCIiS the Sum O f the ( C & j ) * +and (C6H6.HCI)+dissociation energies, 17.0 (ref 17) and 7.3 (ref 7) kcal mol-], respectively. Adding the dissociation energy of HCI, 4.43 eV,Io leaves 1.8 eV of energy to be disposed between the fragments H and C6H&6H6’.C1. The translational portion of this energy, from the repulsive dissociation of the HCI moiety, should be roughly 1 eV (-5.46 - 4.43 eV), resulting in a trsnslationally hot, 11 000 K, H atom. The remaining -0.7 eV is left as excitation energy of the complex Subsequently, step 5b, the ionic complex rearranges with formation of the ipso isomer 3 bound to a neutral benzene “solvent” molecule while retaining the exothermicity as additional internal energy. We believe that the formation of the ipso isomer 3, requring only simple addition of the CI atom to one of the carbon atoms, is more likely than formation of the para isomer 2 or of its meta or ortho analogues. Formation of the latter would also involve the shift of a hydrogen atom from one carbon atom to another, which is a more complicated, and therefore less likely, mechanism. The more likely product is the C6H6CI’ ion rather than the C&CI’ radical, because the ionization potential of the radical is smaller than that of benzene. Finally, the (C6H6CI’*C&) * complex undergoes unimolecular decomposition, step 5c, to give products. By this scheme, about 4 0 4 0 % of the overall heat of reaction will appear as translational energy of the H atom and the remainder, about 1.3 eV, is distributed over internal and translational degrees of freedom of the other tWO Species, C6H6Clf and C6H6. By mechanism B, in contrast with mechanism A, there is a crucial role played by the extra C6H6 molecule, viz., it provides an energy sink for the exothermicity of carbon-chlorine bond formation. The additional vibrational degrees of freedom of the ionic complex intermediate serve as storage for the energy that would otherwise lead to rupture of the newly formed C-C1 bond. Thus, mechanism B explains why 1 can only be produced from a complex at least as large as a trimer. Mechanism B predicts the existence of an ionic complex intermediate C6H6C1+’C6H6 that is internally excited. If the unimolecular decomposition of this ionic complex is reasonably slow, the ion might be detectable in a mass spectrum and the intensity of its signal should depend upon the excitation energy in the ion and the time between its production and detection. A candiate for this ion is clearly seen at m / e 191 and 193 in the mass spectrum of 1.9 C6H6 + HC1 at 600 Torr and 584 f i . In our apparatus the ion ( C ~ H ~ ) Z must C ~ +have a lifetime of at least 5X s or kUni< 2 X los s-I to be observed. Such a low rate coefficient is reasonable for a reactant with a large number of vibrational degrees of freedom, 69 in this case. Of course at a nozzle pressure of 600 Torr, tetramers and larger clusters are produced in abundance, from which this ion could be generated in stable, rather than metastable, form. To test mechanism B as suggested it would be necessary to perform an ancillary lifetime measurement under conditions in which the production of tetramers and larger clusters is suppressed. In this hypothesis it is not clear whether the observed onset is due to the specific formation of C6H6+(B2,)or to the lowest energy of the energy transfer into the repulsive HC1 (n)states, because the C6H6+(E,,) bands at 14 eV are broad and overlap extensively with that of C6H6+(Bzu). In any case, all of the state energies involved, both in C6H6’ and HCI, are somewhat uncertain due to the association of the molecules into a complex. Perhaps theoretical estimates of rates of energy transfer from various states
-
(17) Meot-Ner, M.; Hamlet, P.; Hunter, E. P.;Field, F.H . J . Am. Chem. SOC.1978. 100, 5466.
Walters et al. of C6H6’ into HC1 (xlz+) could help answer this interesting riddle. If mechanism B is valid, step 5a would be expected to take place also with the heterodimer, but the resulting excited C6H6+*Cl would dissociate promptly and thus be unobservable in our experiment. One might, alternatively, look for the production of C6H6CI’ from vacuum-UV irradiation of the C6H6-HC1heterodimer isolated in an argon matrix. If the yield is high enough, one might then be able to observe the IR and visible absorption spectrum of the C6H6CI+produced in this way, compare it with the spectrum of C6H6C1’ produced in the same apparatus by proton capture of chlorobenzene, and thereby show that it is the ipso rather than the para isomer. The original goal of this experiment to prepare and to determine thermodynamic properties of cationic reaction intermediates via dissociative photoionization of van der Waals complexes has been partially successful. The results of photoionization of van der Waals complexes in a molecular beam of C6H6 and HC1 are consistent with the formation of the Wheland intermediate 3, but its thermodynamic properties cannot be determined because the ion is produced from uncharacterized neutral trimers, most probably (C6H6),HC1, and not from the simple dimer, C6H6.HCl. Viewed differently, the result is highly significant because the observed process, reaction 2, is a “half-colli~ion”’~~’~ in which a channel opens for a specific chemical reaction when mediated by a single “solvent” molecule but is closed in the absence of ‘‘solvent”.20 It is interesting that reaction 2 is the reverse of a termolecular ion-molecule reaction. Mechanism B illustrates rather explicitly the function of the third body which is to permit formation of an excited or energized intermediate sufficiently long-lived that, in this instance, atom migration followed by collision with an energetic H atom can plausibly be envisioned. Summary
The ion C6H6C1’ (1) is produced by dissociative ionization of the trimer or larger cluster of C6H6 + HC1. The most likely source is the trimer (C6H6),HC1. It is not detectably produced from the heterodimer C6H6.HCI. Isotopic substitution with deuterium was used to demonstrate that the hydrogen atom lost in forming C6H6CI’ initially belonged to HCI. The onset is at 837 5 A (14.81 0.09 eV), a considerably higher energy than the threshold. The energetics and isotope studies are consistent with the ipso structure 3 for the C6H6Cl’ product; i.e., it is likely to be the Wheland u-complex of electrophilic chlorination of benzene. Direct and stepwise mechanisms for reaction 3 are considered. A stepwise mechanism consistent with known and estimated thermochemistry suggests a crucial role for the “solvent” C& , molecule.
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Acknowledgment. We thank the NSLS/HFBR FacultyStudent Support Program for travel funds to make this work possible. This work was carried out at Brookhaven National Laboratory under contract DE-AC02-76CH00016 with the U S . Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences. We also thank J. K. Newman, P. F. Fernandez, C. Santandrea, B. M. Willard, and D. L. Arneberg for assistance with the experiments. Much on the experimental work was performed while E.A.W. was on sabbatical leave from the University of New Mexico. Part of this manuscript was written while J.R.G. was a guest at the Institute for Molecular Science, Okazaki, Japan, whose fine hospitality is greatly appreciated. (18) Klots, C. D.Kinetics of Ion-Molecule Reactions; Austloos, P., Ed.; Plenum: New York, 1978; p 69. (19) Breckenridge, W. H.; Jouvet, C.; Soep, B. J . Chem. Phys. 1986,84, 1443. (20) Garvey, J. F.; Bernstein, R . B. J . Phys. Chem. 1986, 90, 3577.
