J . Phys. Chem. 1986, 90, 4002-4006
4002
Photoionization and Fragmentation of 0 - , m-, and p-Dlbromobenzene: Isomer Scrambling and Product Thermochemistry' Mehdi Moini? and George E. Leroi* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 (Received: December 23, 1985: In Final Form: March 11, 1986)
The ionization and fragmentation of 0-,m-,and p-dibromobenzeneupon excitation in the 8.5-13-eV range have been studied by photoionization mass spectrometry, and threshold energies for formation of C6H4Br2+,C6H4Br+,and C6H4+from each precursor have been measured. The ionization energies of 0-,m-, and P - C ~ H ~ Bare Q distinct, but the appearance energies of C6H4Br+agree for all three isomers and those of C6H4+are also the same (higher in value but within experimental error of Br loss), independent of the substituent distribution on the parent neutral. The threshold energies have been combined with other thermochemical data to provide upper limits for the cation enthalpies of formation. A similar calculation yields an estimate for the heat of formation of neutral o-benzyne: Afff0298(C6H4)= 420 20 kJ mol-'. The thermochemistry of decomposition of C6H4Br2+to give C6H4+is consistent only with the formation of Brz as the neutral product. The results show that substituent isomerization occurs in C6H&2+* prior to dissociation to form either C6H4Br+or C6H4+.
*
Introduction
In contrast to their monohalo counterparts, very little information is available regarding the ionization and fragmentation of the dihalobenzenes. Thus, although the ionization energies of the three dibromobenzene isomers have been reported from photoelectron spectroscopy (PES) measurements:-4 neither the energetics nor the dynamics of fragmentation processes involving the 0-, m-, or p-dibromobenzene cations have been studied. We have investigated the interaction of vacuum ultraviolet radiation with 0-, m-,and p-dibromobenzene at low sample pressures in order to obtain thresholds for their ionization and fragmentation. In addition to monitoring the effects of the substituent geometry, the measurements can be combined with tabulated thermochemical data for the neutral fragmentation partners to provide good estimates of bond dissociation energies and of heats of formation for the parent and daughter cations. .The interaction of ionizing radiation with gases leads first to molecular ions with a distribution of internal energies. In competition with other deactivation channels, these ions may isomerize and/or dissociate if their internal energy exceeds the thresholds for such reactions. Isomerization among dihalobenzenes has been observed both in solution5 and in rare gas matrices!*' We report here the results of a photoionization mass spectrometric (PIMS) study of o-, m-,and p-dibromobenzene in the gas phase, in which both isomerization and dissociation are observed. From the measured appearance energies and ancillary thermochemical data, upper limits for the heats of formation of the parent ions and for C6H4Br+and C6H4+have been obtained which are more reliable than previously reported values. The enthalpy of formation of neutral o-benzyne (C6H4),a likely intermediate in many organic reactions.* is also estimated. Experimental Section
The photoionization mass spectrometer has been described p r e v i o ~ s l y . ~Briefly, the vacuum UV radiation from a Hinterregger-type windowless discharge lamp is dispersed and focused with a 1-m near-normal incidence monochromator (McPherson 225) into a static-type ion source into which the sample gas flows. The monochromator is equipped with a 600 L/mm, MgF,-overcoated aluminum concave diffraction grating and provides 1.68-8, (fwhm) photon band widths when used with 100-km slit widths. The transmitted photon intensity is measured by monitoring the fluorescence of a sodium salicylate coated quartz disk with an R C A 8850 photomultiplier. The signal from the photomultiplier is amplified with a picoammeter and digitized with a voltage-to-frequency converter and computer-interfaced counter. Present address: Department of Chemistry, University of Florida, Gainesville. FL 326 1 1.
The hydrogen many-line spectrum, generated from dc discharge of low-pressure hydrogen, serves as the light source for these experiments. Ions formed in the ion source are accelerated at 90° to the photon beam, focused by an ion lens system, mass selected with a quadrupole mass filter (Extranuclear 324-9), and detected with a channeltron electron multiplier. The ion pulses from the channeltron are amplified and discriminated;I0 they are then processed with another computer-interfaced counter. The data collection employs a variable integration time technique that permits the data to be acquired with a constant signal-to-noise ratio in a minimum amount of time." Photon and ion counts are accumulated a t each wavelength until the desired signal-to-noise ratio is attained; the monochromator wavelength is then changed by a predetermined amount via a computercontrolled stepping motor, and the process is repeated. The relative photoionization efficiency (PIE = ion count/transmitted photon count) is calculated at each wavelength and plotted as a function of photon wavelength or energy to produce a PIE curve. All photoionization efficiency data were acquired at 1-8, intervals. PIE curves were recorded with a high signal-to-noise ratio, which required integration times of 50-600 s per point. Data collection times varied from 12 to 48 continuous hours per PIE. The PIE curves were corrected for sample pressure and instrument drift, as well as for stray light. 0- and m-dibromobenzene were obtained from the Aldrich Chemical Co., Inc.; p-dibromobenzene was purchased from the Eastman Kodak Co. All samples were thoroughly degassed by repeated freeze-pumpthaw cycles and used without further purification. For the PIE measurements the sample pressure in Torr. the ion source was maintained below 5 (fl) X Monochromator entrance and exit slit widths of 100 pm were employed for parent ions and 300-pm slits were used for the (1) Presented in part at the 33rd Annual Mecting of the American Society for Mass Spectrometry, San Diego, CA, May 1985. (2) Streets, D. G.; Ceasar, G. P. Mol. Phys. 1973, 26, 1037. (3) CvitaS, T.; Klasinc, L. Croat. Chem. Acta 1977, 50, 291. (4) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T. Monogr. Ser. Res. Inst. Appl. Electr., Hokkaido Univ. 1978 25, 1. (See also Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S. Handbook ofHeI Photoelectron Spectra of Fundamental Organic Molecules, Japan Scientific Societies Press: Tokyo, 198 l .) (5) Bunnett, J. F.; Moyer, C. E., Jr. J . Am. Chem. Soc. 1971, 93, 1183. (6) Friedman, R. S.; Kelsall, B. J.; Andrews, L.J . Phys. Chem. 1984, 88, 1944. (7) Friedman, R. S.;Andrews, L. J . Am. Chem. SOC.1985, 107, 822. (8) Hoffman, R. W.Lkhydrobenzene and Cycloalkynes; Academic Press: New York, 1967; Levin, R. H.In Reactiue Intermediates, Jones, M., Jr., Moss, R. A,, Ed.;Wiley: New York, 1978. (9) Darland, E.J.; Rider, D. M.; Tully, F. P.; Enke, C. G.; Leroi, G. E. Int. J . Mass Specrrom. Ion Phys. 1980, 34, 175. (10) Dadand, E. J.; Hornshuh, J. M.; Enke, C. G.; Leroi, G . E. Anal. Chem. 1979, 51. 245. (1 1) Darland, E. J.; Leroi, G. E.; Enke, C. G. Anal. Chem. 1980, 52, 714.
0022-3654/86/2090-4002$0 1.5010 0 1986 American Chemical Society
Photoionization of
0-,m-,and
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 4003
p-Dibromobenzene
TABLE I: Threshold Energies and Thermochemical Data Relevant to the Photoionization and Fragmentation of P-. m-.and o-Dibromobenzene ~~~
~
234-238
ion p-C6H4Br2+
155-157
C6H4Br'
mlz
neutral
re1 int"
IE or AE,
100
8.85
eV
kJ/mol
* 0.02
966.9 f 7 113 f 5b 1175.3 f 10 11 1.884' 1271.8 f 15 30.907'
P-C6H4Br2
17
12.17 f 0.05
6
12.33 f 0.1
Br
m-C6H4Br2+
234-2 38
100
8.95 f 0.02
C&Br+
15
12.15 f 0.05
5
12.44 f 0.1
Br 76
C6H4+
Brz
o-C6H4Br2+
234-238
8.91 f 0.02
14
12.16 f 0.05
5
12.40 f 0.1
Br C6H4+
76
309 f 7
976.7 f 7 117 f 5b 1178.3 f 10 11 1.884c 1282.5 f 15 30.907c
o-C6H4Br2
C6H4Br+
155-157
100
320 f 7
916.5 f 1 113 f 5b 1173.4 f 10 111.884' 1282.4 f 15 30.907'
m-C6H4Br2 155-157
Cb,-Br bond energy, kJ/mol
2989
Br2
313 f 7
"Under full illumination of the hydrogen discharge; f5%. bCalculated by using group equivalents, ref 13. cReference 14, pp 1-780-1 Woveleng th (Angstroms)
Wavelength (Angstroms) 1370.00
1420.00
~
8.70
8.90
~ ,
1450.00
~ ~
9.1 0
~ '
-
E
Energy (eV)
Figure 1. PIE curves for
0-,m-,
1350.00
and p-dibromobenzene parent ions.
daughter ions. Sample temperature for all measurements was room temperature (-22 "C), except for the C6H4+cation produced from p-dibromobenzene where the sample reservoir temperature was raised to about 70 O C . The energy range (8-13 eV) and intensity of the hydrogen lamp enables the threshold PIES for the parent ions of the isomeric dibromobenzenes to be easily measured. However, with this light source only two of the fragments, C6H4Br+and C6H4+,could be measured, the latter with some difficulty. The higher AEs or lower intensities of other fragments precluded observation of their threshold behavior.
Results Photoionization efficiency (PIE) curves in the 9-eV region for the o-,m-,and pdibromobenzene parent ions are shown in Figure 1. The ionization energies (IEs) for the molecular ions were determined by expanding the threshold region and extrapolating the straight-line portion of the appropriate PIE curve just above the ionization onset back to the base line. The IEs of the isomers are distinct and distinguishable. An expanded plot of the p-dibromobenzene cation PIE in the 8.5-9.5-eV photon energy range is shown in Figure 2. Steplike structure is observed which is attributable to a vibrational progression in the ionic state; the
8.50
8.70
8.90
9.10
9 30
9 50
Energy (eV)
Figure 2. Fine structure in the threshold region of the PIE curve of
p-dibromobenzene. vibrational separation is approximately 0.025 eV = 200 cm-I. The PIE curves of the C6H4Br+ and C6H4+daughter ions generated by dissociative ionization of each of the isomeric dibromobenzenes are collected in Figure 3. The appearance energy (AE) of a given fragment cation was chosen as the energy at which the PIE first rises perceptibly from the base line. Because of possible kinetic shift, reverse activation energy, and other factors,'* this value must be considered an upper limit to the desired adiabatic AE. The IE and AE values obtained in this work are listed in Table I, where the relative intensities of the cations from each precursor, obtained under full illumination of the hydrogen discharge and verified by 12-20-eV electron bombardment, are also given. (The E1 study revealed that the threshold for formation of C6H3+[ m / z 751 is only slightly higher than that of C6H4+.However, we were unable to measure the PIE of C6H3+ under our experimental conditions.) In addition to other factors which vary little from one PIE curve to another, the error in an (12) Vestal, M. In Fundamental Processes in Radiation Chemistry, Ausloos, P. Ed.; Interscience: New York, 1968.
4004
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 Wavelength (Angstroms) 1050.00 l
I
TABLE II: Comparison of First Ionization Energies of the Dibromobenzene Isomers Obtained by PIMS and PES IE, ev neutral PIMP PESb PESc PESd
950.00 ,
,
,
l
,
,
,
,
,
,
Moini and Leroi
~~~
,
* . . B
~~
o-C6H4Br2 m-C6H4Br2 p-C6H4Br2
~
~
8.91 8.95 8.85
~~
8.99 9.05 8.90
9.02 9.10 8.91
9.05 9.07 8.90
"This work. bReference3. CReference2. dReference4. Within the photon energy range of the hydrogen lamp, the ion signal at m / z 154 is negligible, and that at m / z 156 can be. accounted for by the natural distribution of I3C in the C6H4Br2 precursors. Thus there is no evidence of the production of C&Br+ under our experimental conditions, probably because the photon energy is below the activation energy for HBr loss. Moreover, the absence of ions at m / z 77 (C6H5+)indicates that the samples are not contaminated by C6HSBr,a plausible impurity.
I 1 50
12 50
12.00
13 00
t13 50
Energy ( e v )
Figure 3. PIE curves of the C.&Br* and C6H4+,daughterions generated by photofragmentation of each of the isomeric dibromobenzenes.
extrapolated IE or AE depends on the available photon intensity. With the hydrogen lamp the intensity is high in the vicinity of the parent ion thresholds, and rather low in the AE range of the daughter ions. Wide slits were employed in the latter cases, which leads to larger error in the measurements. Also, the PIE curves of the parent ions rise more sharply and uniformly than those of the daughters, which permits more accurate extrapolation to the base line. These factors are responsible for the lower error reported for the parent ion threshold energies. The threshold energies obtained in this work were combined with literature thermochemical data to obtain the room temperature enthalpies of formation for the cations studied. The identities of the neutral partners, the values of their heats of formation used in the calculations, and the ionic heats of formation determined therefrom are also listed in Table I. Because experimental values are not available, the enthalpies of formation of the neutral parents were calculated from group additivity tab l e ~ ;those ' ~ of Br and Br2 are rather precisely k n 0 ~ n . lThe ~ bond dissociation energy for removing a Br atom from the benzene ring in C6H4Br2+can be determined directly from the difference in the threshold energies of the fragment and molecular ions. [Bond energy = AHf0(C6H4Br+) AHfo(Br) - mfo(C6H4Br2+) = AE(C6H4Br+)- IE(C6H4Br2+).] These bond energies are given in the last column of Table I. The error limits for fragment ion thermochemistry reported in the last two columns of Table I represent the experimental errors of our measurements plus the uncertainties in the auxiliary data used in the calculations. They do not include the unknown, and partially off-setting, systematic uncertainties in determining PIE thresholds due to the kinetic shift or to the thermal internal energy content of the p r e c ~ r s o r . ' ~
+
(13) Franklin, J. L.; Dillard, J. G.; Rosenstock, H. M.; Herron, J. T.; Draxl, K.; Field, F. H. Natl. Stand. Ref Data Ser., Natl. Bur. Stand. 1969, No. 26, Appendix 7. (14) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T. J. Phys. Chem. Ref Data 1977, 6 , Supplement No. 1.
Discussion The first ionization process in substituted benzenes involves removal of a a-electron from the ring; the value of the ionization energy is sensitive to the nature and symmetry of the substituents.16 The vertical ionization energy of bromobenzene is 9.041 eV, and those of the dibromine derivatives are slightly lower. The order and the numerical values of the IEs of the three isomers determined in this PIMS study are compared with those obtained by PES2" in Table 11. The agreement is excellent. Photoionization gives IEs about 0.054.1 eV lower than the vertical photoelectron values. Although the ortho isomer I E is lower than that of the meta isomer in each report, their difference is within the combined experimental errors of the determinations. However, the p C6H4BrZIE is lower than those of the other isomers by more than the experimental error. The enthalpy of formation of the para cation (Table I) is also the lowest of the three. The highest-occupied r-molecular orbital in the benzene cation is split upon monosubstitution into 2BI and 2A2 components. Para- (L4-) disubstitution is predicted2*I6to destabilize the 2BIorbital (2B2, for identical substituents), and this is reflected in our experimental results. Vibrational structure is clearly evident in the threshold region of the p-C6H4Br2+PIE curve, Figure 2. Prominent steps, uniformly separated by approximately 0.05 eV, are evident above 8.85 eV. Less-pronounced steps can be identified at the midpoints of these intervals, as designated by the vertical lines in the figure. Similar vibrational structure is seen in the high-resolution [Ne I] PES spectrum of p-C6H4Br2,I7where a progression with separation of about 0.025eV is observed and identified as v6(ag)," the symmetric C-Br stretching vibration in the ground ionic state. We report the IE of p-C6H4Br2as 8.85 eV. Another step is apparent at 8.80 eV in Figure 2. We consider the higher value to be more reliable because the height of the lower energy step is within the noise level of the data below the threshold and because it could be due to ionization of the neutral in a thermally excited vibrational state (i.e. due to hot bands). The PIE curves of the C6H4Br+and C6H4+fragment ions from the isomeric dibromobenzenes (Figure 3) have been measured for the first time. Within experimental error, the AEs for the bromine-loss daughter ions are the same (12.16 f 0.05 eV) for all three isomers (see Table I). This does not necessarily lead to the conclusion that the energy required to break the first C,,-Br bond is independent of the position of the second Br substituent. We suggest that the dissociation energy for the loss of one bromine atom is above the energy required for substituent isomerization (scrambling) in the ion. (Additional evidence in support of this suggestion is presented below.) Thus, regardless of the substituent (15) Traeger, J . C.; McLoughlin, R. G. J . Am. Chem. SOC.1981, I03, 3647. (16) Baker, A. D.; May, D. P.; Turner, D. W. J . Chem. SOC.B 1968,22. (17) Potts, A. W.; Lyus, M. L.; Lee, E. P. F.; Fattahallah, G. H. J . Chem. Soc., Faraday Trans. 2 1980, 76, 556. (18) Green, J. H. S . Spectrochim. Acta 1970, 26A, 1503.
Photoionization of
0-,m-,and
p-Dibromobenzene
geometry in the neutral, the same ionic precursor would dissociate in each case, with the same threshold energy. The calculated bond dissociation energy is the same for each isomer within experimental error. Although decomposition of C6H4Br2+to form C6H4Br+ Br involves simple bond cleavage and probably passes through a loose transition state with little reverse activation energy,I9 the experimental threshold may involve a significant kinetic shift.20 For example, it has recently been shown by elegant time-resolved photoionization mass spectrometry (TPIMS) measurements that a kinetic shift on the order of 0.4 eV accompanies a normal PIMS determination of the AE of C6H5+from C6H5Br." Thus the C6H4Br+AEs given in Table I must be considered upper limits to the desired 0 K threshold energy. If we assume that the kinetic shift in our threshold measurement of C6H4Br+is similar to that accompanying Br loss from C6H5Br+ (-0.4 eV2'), then the energy required to break the first Cb,-Br bond in C6H4Br2+would be 275 f 12 kJ mol-'. This value is consistent with the Cb,-Br bond energy in C6H5Br+,which has been m e a s ~ r e d ~tol -be ~ ~266 f 5 kJ mol-'. If a kinetic shift of 0.4 eV is adopted, the activation energy for loss of one bromine atom from C6H4Br2+is 2.85 f 0.1 eV, and the heats of formation of the C6H4Br+cations given in Table I should be reduced by 38.6 kJ mol-'. The threshold portions of the PIE curves for C6H4+produced from 0-,m-, and p-dibromobenzene are also shown in Figure 3. Due to the weakness of the ion signal, competition with Br-loss fragmentation, and kinetic shift effects, the A b can be determined less precisely than those of the parent ions and C6H4Br+. Not only are the values for the three isomers the same, well within experimental error (12.4 f 0.1 eV), but the appearance energy for production of C6H4+from C6H4Br2is only very slightly higher than that for formation of C6H4Br+. This result is consistent with photoionization results from benzene, where the threshold energies of C6H5+and C6H4+are the same within experimental uncertaint^,^^,^^ as well as with multiphoton ionization studies on It is worth noting that, compared to AE(C6H4Br+), the experimental AE of C6H4+is likely to be a less reliable upper limit to the threshold energy. (For C6H4+from C6H6 the experimental - thermochemical difference is suggested to be about 1 eV.24928) With these considerations in mind, we propose that within experimental error the activation energies for the formation of C6H4Br+ and C6H4+from each of the isomeric C6H4Br2+ cations are the same. Thermochemical information can be used to determine that C6H4+is produced from 0-C6H4BrZby the reaction
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 4005 SCHEME I Br
+
+
+
O-CgH4Br2 hv -* C6H4+ Br2 AE = 12.40 f 0.1 eV
+ e-
If the neutral partner is Br2 we find for C6H4+,most likely the 1,2-dehydrobenzene (benzyne) cation, to be 1283 f 15 kJ mol-'. This is slightly lower than the value [ 1311 f 10 kJ mol-I] suggested by Rosenstock and c o - w o r k e r ~ it ; ~is~ probably a better upper limit. If two Br atoms were formed instead, the heat of formation of C6H4+derived from the experimental AE would be (19) Dannacher, J.; Rosenstock, H. M.; Buff, R.; Parr, A. C.; Stockbauer, R. L. Chem. Phys. 1983, 75, 23. (20) Lifshitz, C. Mass Spectrom. Rev. 1982, 1, 309. (21) Malinovich, Y.;Arakawa, R.; Haase, G.; Lifshitz, C. J. Phys. Chem. 1985,89, 2253. (22) Rosenstock, H. M.; Stockbauer, R. L.; Parr, A. C. J . Chem. Phys. 1980, 73, 773. (23) Baer, T.; Kury, R. Chem. Phys. Lett. 1982, 92, 659. (24) Rosenstock, H. M.; Larkins, J. T.; Walker, J. A. Int. J. Mass Spectrom. Ion Phys. 1973, 11, 309. (25) Sergeev, Y . L.; Golovin, A. V.; Akopyan, M.E.;Vilesov, F. I. High Energy Chem. (Engl. Transl.) 1977, 11, 88. (26) Pandolfi, R. S.; Gobeli, D. A.; El-Sayed, M. A. J . Phys. Chem. 1981, 85, 1779. (27) Koplitz, B. D.; McVey, J. K. J . Phys. Chem. 1985, 89, 2761. (28) Levin, R. D.; Lias, S . G. Natl. Stand. Re$ Data Ser., Natl. Bur. Stand. 1982, No. 71.
(29) Rosenstock, H . M.; Stockbauer, R. L.; Parr, A. C. J. Chim. Phys. Phys. Chrm. Biol. 1980, 77, 745.
+
6' br
TABLE 111: Vertical Ionization Energies (in eV)O and Suggested Symmetry Assignmentsbof Dibromobenzene Cation Electronic States o-C6H4Br2+
m-C6H4Br2+
p-C6H4Brz+
assignmt
IE
assignmt
IE
assignmt
IE
lA2 2B1
9.05 9.45 10.33 11.08 (11.3) 11.38 12.15
2A2 2BI 2B2 'BI 'Al 'A2 2Al
9.07 9.51 10.73 10.86 11.15 11.95 12.28
2Bzg 2BIg
8.91 9.79 10.64 10.94 10.95 11.94 12.34
''42
2B2 *B, 2Al 2Al
2B3g
'Bj, 'B2" 2B2g
2Ag
OThe energy above the ground electronic state of the neutral for those states below 12.5 eV; ortho and meta isomers from ref 4, para isomer from ref 17. analogy with the dichlorobenzenes (ref 41 and 42) and from PES peak shapes. Some of the assignments differ from those given in ref 17 on the basis of S C F calculations; except for the last row, they coincide with assignments given in ref 3.
1088 kJ mol-', a value well below the expected range.14 The same thermochemical arguments can be applied to m- and p-dibromobenzene. If one combines the heat of formation for C6H4+determined from the o-CSH4Br2precursor with the experimental AEs and the enthalpy of formation of the appropriate parent molecule, the heat of formation of the accompanying neutral fragment(s) is calculated to be on the order of 30 kJ. This is consistent only with the formation of Br2, not two Br atoms. The loss of Br2 upon fragmentation of C6H4Br2to form C6H4+ thus is not restricted to the isomer initially possessing adjacent Br ring substituents. This result is consistent with the earlier conclusion that isomer scrambling reactions take place prior to or concurrent with the fragmentation. Isomerization accompanying fragmentation of benzene cations is now well e s t a b l i ~ h e d . ~Ion ~ fragments produced from deute~-ated,~l I3C-labeled, and doubly labeled specie^'^-^^ retain no "memory" of the position at which the labels were placed. Mass spectrometric measurements on deuterium-labeled chlorobenzenes also show that label scrambling occurs.36 The results of the dissociative ionization of the dibromobenzenes are summarized in Scheme I. The mechanism of the isomer scrambling cannot be determined from this PIMS investigation (30) Rosenstock, H. M.; Dannacher, J.; Liebman, J. F. Radiat. Phys. Chem. 1982, 20, 7.
(31) Jennings, K. R. Z . Naturforsch. 1967, 22a, 454. (32) Horman, J.; Jeo, A. N.; Williams, D. H. J . Am. Chem. SOC.1970, 92, 2131. (33) Perry, W. 0.;Beynon, J. H.; Baitinger, W. E.; Amy, J. W.; Caprioli, R. M.; Renaud, K. N.; Leitch, L. C.; Meyerson, S. J. Am. Chem. SOC.1970, 92, 7236. (34) Dickinson, R. J.; Williams, D. H. J . Chem. Soc. B 1971, 249. (35) Beynon, J. H.; Caprioli, R. M.; Perry, W. 0.;Baitinger, W. E. J . Am. Chem. SOC.1972, 94, 6828. (36) Pollack, S . K.; Hehre, W. J. Tetrahedron Lett. 1980, 21, 2483.
4006
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
alone. Photoelectron spectroscopy shows that several excited ionic states exist below the photofragmentation thresholds determined here. Their energies and probable symmetry assignments are given in Table 111. The state assigned with A symmetry which lies above 12 eV involves removal of a LT C-Br bonding electron, and it is tempting to implicate it as a localized state from which unimolecular decomposition occurs, or at least to consider it a “doorway” state to fragmentation on a lower-lying potential energy surface. However, the true thresholds for Br loss, and probably for Br, loss as well, likely lie at slightly lower energies. Moreover, it is now established from photoelectron-photoion coincidence (PEPICO) measurements and time-resolved PIMS that fragmentation of the monohalobenzene parent ions to form C6H5+ occurs through a loose transition state on the ground ionic state and perhaps surface in the cases of C6H51i9and C6H5Br,21922*37 for C6H5C1,also.22,38339The ionic fragmentation of benzene to form C6H5+and C6H4+occurs competitively on the ground-state C,H6+ although as noted earlier the isotopic scrambling of ring and substituent labels reveals that surface crossings have occurred prior to f r a g m e n t a t i ~ n .It~ is ~ unlikely that photofragmentation of C6H4Br2to form C6H4Br’ and C6H4+occurs by a different mechanism, although experiments where the threshold kinetic shift and the internal energies of the fragments can be measured are required to confirm this c o n c l u ~ i o n . ~ ~ The vertical ionization of the dibromobenzenes can produce parent ions in any electronic state below the excitation energy. Thus the ions which subsequently fragment may be formed initially with electronic rather than vibrational excitation, and competition among various deactivation channels (e.g. direct decomposition, internal conversion, radiation) will ensue. Our results show that the 0-, m-, and p-dibromobenzene parent ions are distinguishable at the ionization threshold, but that the nature of the ion which fragments to form either C6H4Br+or C6H4+is independent of the substituent distribution on the neutral precursor. Substituent scrambling is likely to occur concomitant with internal conversion surface crossings, and in this case electronic states which correlate with the 2B1excited state of bromobenzene near 11.2 eV may be involved. This state, which involves ionization from the Br lone-pair out-of-plane .rr orbital, splits into two components upon disubstitution, the lower of which lies near 10.9 eV for the para (37) Dunbar, R. C.;Fu, E. W. J . Phys. Chem. 1977, 81, 1531. (38) Rosenstock, H. M.; Stockbauer, R. L.; Parr, A. C. J . Chem. Phys. 1979, 71, 3708. (39) Pratt, S. T.;Chupka, W. A. Chem. Phys. 1981, 62, 153. (40) Such experiments are in progress; Baer, T., personal communication.
Moini and Leroi (,B3J and meta (zB1)cations, and near 10.3 eV (2Az)for the ortho isomer [see Table 1111. Elegant photoionization matrix experiments on isomeric di- and trichlorobenzenes by the Andrews group6v7have demonstrated that photochemical rearrangements of the molecular cations can be induced. For example, d2,FC12+ is converted first to m-C6H4Cl2+and then to p-CsH4C12 upon continued visible irradiation of an o-C6H4C12+/Armatrix; similarly, matrix-isolated m-c6H4Cl2+rearranges to yield the p C6H4C12+isomer.7 The relative order of the corresponding excited electronic states for the three dichloro isomers differs from the order mentioned above in the dibromo case, and the observed matrix stability coincides with the electronic state energies. Again, which leaves a chlorine a halogen lone-pair 7r electron is lone pair hole in the parent ion. It is suggested that an excited, bridged chloronium cation then forms, and that isomerization occurs via a “chloronium ion walk” around the ring to the most stable isomeric form, which relaxes to the ground cationic state in the matrix.’ In the case of the gaseous dibromobenzene cations the same mechanism, a bromonium ion walk, could account for the observed isomeric scrambling prior to fragmentation, and for the substituent proximity required for the concerted loss of Br, which accompanies the formation of C6H4+. The enthalpy of formation of neutral o-benzyne, C6H4,can be calculated from the C6H4+appearance energy if the other necessary thermochemical data are known. If we accept 8.95 eV as the adiabatic ionization energy of b e n ~ y n e , ~then ~ ~ AHHf0298(C6H4) ’ = AE(C&4+) - IE(C&+) 4- &Yfo(C6H4Br2) - AHfo(Br2) is calculated to be 420 f 20 kJ mol-’. This value is at the low end of the range suggested previously in the literature on the basis of other e ~ p e r i m e n t a l ~and ~ *t ~h e~ o, ~r e t i ~ a lconsiderations, ~~,~ but we believe that its accuracy is within the quoted uncertainty limits. Acknowledgment. We thank Dr. T. V. Atkinson and M. Rabb for help with instrumentation, and K.-K. Ng who provided the chemicals and helpful discussion concerning reactivity of benzyne in solution. Registry No. o-C6H4Br2, 583-53-9; o-C6H4Br2+; 74350-74-6; mC6H4Br2, 108-36-1; m-C6H4Br2+.,74336-44-0; p-C6H4Br,, 106-37-6; p-C6H4Br2+.,74365-38-1; o-benzyne, 462-80-6. (41) Maier, J. P.; Marthaler, 0. Chem. Phys. 1978, 32, 419. (42) RuSEiE, B.; Klasinc, L.; Wolf, A.; Knop, J. V. J . Phys. Chem. 1981, 85, 1486. (43) Grutzmacher, H.-F.; Lohmann, J. Ann. Chem. 1967, 705, 81. (44) Natalis, P.; Franklin, J. L. J . Phys. Chem. 1965, 69, 2935. (45) Noell, J. 0.; Newton, M. D. J . Am. Chem. SOC.1979, 101, 51. (46) Dewar, M.J. S.; Ford, G. P.; Reynolds, H. R. J . Am. Chem. SOC. 1983, 105, 3162.
