J . Phys. Chem. 1985,89, 3617-3622 the electron recombination rate of 2 X lo4 and 5 X lo4 s-l for respectively. The ambipolar diffusion rate is on H 3 0 + and 02+, the order of lo4 s-' from the reduced mobility of 21 cm2/(V s ) . ~ * These destruction rates are smaller than the rates for eq 2 and 3 by a factor -100. (28) R. Johnsen, H. L. Brown, and M. A. Biondi, J. Chem. Phys., 52,5080 (1970).
3617
Although we have no means to measure the concentration of 02+ in the discharge, the above analysis suggests that it is on the order of 1012/cm3. The concentrations of H30+,OH+, and H20+ are 10I0/cm3.
-
Acknowledgment. This work was supported by NSF Grant 84-08316. We also acknowledge the partial support by the Camille and Henry Dreyfus Foundation and the donors of the Petroleum Research Fund, administered by the American Chemical Society.
ARTICLES Fragmentation Thermochemistry of Gas-Phase Ions by Threshold Photodissociation and Charge-Exchange Ionization. Methylnaphthalene and Methylstyrene Ions Jeffrey P. Honovich, Jeffrey Segall,+ and Robert C. Dunbar* Chemistry Department, Case Western Reserve University, Cleveland, Ohio 441 06 (Received: November 1 , 1984; In Final Form: February 20, 1985)
The energies of fragmentation of 1-methylnaphthalene and a-methylstyrene parent ions were studied by threshold photodissociation and charge-exchange ionization. Threshold photodissociation studies in the ICR spectrometer, distinguishing one-photon from two-photon dissociation, examined both pressure dependence with continuous irradiation and pulse-rate dependence with pulsed irradiation. Dissociative charge-exchange ionization was observed with various charge-exchange reagent gases both in the ICR spectrometer and in a magnetic-sectorchemical-ionizationmass spectrometer. The fragmentation threshold for 1-methylnaphthalene ion was found probably to be greater than 4.0 eV and less than 5.8 eV, and is unlikely to be less than 3.4 eV. The heat of formation of the CIIH9+product ion was estimated as 252 kcal/mol, although a considerably lower value is suggested by analogy with benzyl cation and supported by one previous experiment; further study is recommended. The threshold for a-methylstyrene ion was estimated to be 2.2 eV, certainly greater than 2 eV and almost certainly less than 2.75 eV. Such a low threshold value must indicate an extensive rearrangement accompanying the fragmentation process, with the indanyl and vinyltropylium structures being reasonable C9H9+product ion structures.
Introduction The use of monochromatic photons gives the ability to impart a precise amount of internal energy to a molecule of interest. Observing the minimum photon energy sufficient to effect a given fragmentation reaction should accordingly allow accurate threshold measurements and serve as a useful source of accurate thermochemical information. In ion dissociations, the dissociation threshold can normally be considered as giving the dissociation endothermicity, since the large ion-induced-dipole potential well for the separating ion-neutral pair will usually dominate over other energy barriers in the reaction coordinate. For good-sized polyatomic ions, applying this idea has been hampered because the threshold wavelength is not manifested as a transition from dissociation to no dissociation, but rather as a transition from onephoton dissociation to two-photon dissociation. However, the understanding of two-photon dissociation processes and the tools for characterizing them, have advanced to the point that useful thermochemical applications should be practical despite this complication. In this paper we describe the use of photodissociation spectroscopy of trapped ions to determine thermochemical dissociation thresholds for the parent ions of 1-methylnaphthalene and amethylstyrene. In addition to being ions whose dissociation thermochemistry has not been well studied by other precise techniques, these two species share the added point of interest that 'Present address: Chemistry Department, Stanford University.
0022-3654/85/2089-3617$01.50/0
in both cases the dissociation threshold is different from naive predictions, in the first case higher, and in the second case lower. Both thresholds are so surprising as to justify the application of these powerful but difficult photodissociation methods to confirm them with high reliability. Further confidence in the values assigned can be added by confirming evidence from the very different technique of charge-exchange ionization in the fragmentation threshold region. 1-Methylnaphthalene can be thought of as a benzo analogue of toluene, and the dissociation of its parent ion by loss of hydrogen (eq 1) should be analogous to the well-studied dissociation of CH3
I
1''
toluene ion to yield what is believed to be tropylium ion
Recent datal indicate a AH of only 1.8 eV for this latter disso(1) Bombach, R.; Dannacher, J.; Stadelmann, J.-P. J . Am. Chem. SOC. 1983, 105,4205.
0 1985 American Chemical Society
3618
Honovich et al.
The Journal of Physical Chemistry, Vol. 89, No. 17, 1985
ciation, reflecting the large aromatic stabilization of the tropylium ion. (Even if the rearrangement to tropylium does not take place, the benzyl cation is perhaps 0.7 eV less stable, so that AH for this process would still be less than 2.5 eV.) One might expect the methylnaphthalene ion dissociation to reflect a similar stability for the CilH9+ion, which could have the benzotropylium structure. The first careful measurement of this thermochemistry,2 via ionization of C, ,H,. radical, indeed indicated a threshold near 2.7 eV. The energy of C11H9+derived from this measurement has been widely accepted in the literature concerning stabilization of aromatic s y ~ t e m s . ~ However, this consistent picture has been clouded by a series of more recent determinations of the bond cleavage energy of methylnaphthalene ions by appearancf: potential A rough consensus would appear to have emerged in the mass spectrometry literature on a bond energy near 4 eV, a value which indicates little stabilization of C1,H9+relative to the parent radical ion. (This possible higher range of values has not apparently been acknowledged in the aromatic theory literature.) As detailed below, the present results lend more weight to this higher range of bond strengths and emphasize the need for clarification of this situation. In contrast, loss of H- from a-methylstyrene ion (eq 3) has no
-+.
(3)
very obvious source of thermochemical stabilization. The analogous styrene ion requires 4 eV for He cleavage.' The electron-impact measurement6 of 3.35 eV for a-methylstyrene is lower, but still plausible for simple unassisted C-H bond cleavage. However, the much lower value demonstrated by the present results can only be rationalized by a surprisingly extensive structural reorganization concurrent with the fragmentation process.
Techniqqes At wavelengths below the dissociation threshold a two-photon dissociation process can occur by the Freiser-Beauchamp mechanisms shown below:
nl I*!
A'+
-
Hz
4
0
1
2
3
Time ( s e c )
Figure 1. Schematic illustration of the light pulsing in the chopper experiment, for two different chopper rates and also the equivalent neutral-density filter. Ions are assumed to be generated at zero time, and detected after 4 s of trapping and irradiation.
paperssv9 the dissociation is given by the approximate equation (applicable in the high-pressure limit)
y"
A'
Chopper Pulse Sequence
lorf
A"'
*. fragments
102
(4)
Here A+ is the thermal parent ion, Z is the light intensity, ul and u2 are the effective photon absorption cross sections (often assumed to be equal), kp is the collisional relaxation rate constant, Pis the pressure, and k, is the radiative relaxation rate. In this scheme the energy of the first photon is stored as vibrational excitation (A+*) of the ion electronic ground state, following a fast intersystem crossing from the initial excited electronic state (A'+). This vibrationally excited species can either relax by radiation or collisional cooling or can absorb a second photon with subsequent dissociation. The fact that this process involves an intermediate which can be intercepted before dissociation allows it to be distinguished by its pressure dependence from a one-photon process, which has no pressure dependence. As discussed in previous (2) Harrison, A. G.; Lossing, F. P. J . Am. Chem. SOC.1960, 82, 1052. (3) See: Ilic, P.; Trinajstic, N. J . Org. Chem. 1980, 45, 1738. (4) Loudon, A. G . ; Mazengo, R. Z. Org. Muss Spectrom. 1974, 8, 179. (5) Nounou, P. J . Chim. Phys. Phys.-Chim. Biol. 1966, 65, 994. (6) Koppel, C.; Schwarz, H.; Bohlmann, F. Org. Mass Spectrom. 1974, 8, 25. (7) Franklin, J. L.; Carroll, S. R. J . Am. Chem. SOC.1969, 91, 5940. (8) Freiser, B. S.; Beauchamp, J. L. Chem. Phys. Lett. 1975, 35, 35. Orlowsh, T. E.;Freiser, B. S . ; Beauchamp, J. L. Chem. Phys. 1976, 18, 439.
(5) where dissociation D is defined as -In (San/Soff), where Sa, and Soffare the ion signals with and without irradiation, respectively, P is the pressure, and the effective optical absorption rate Z and collisional relaxation rate constant kp are as indicated in eq 4. Therefore plots of 1/D vs. pressure, P, ought, in the case of two-photon dissociation, to give straight lines with slopes related to the cooling rate, kp, while for one-photon dissociation there should be no prequre dependence. Recently another technique has been developed1° to characterize two-photon dissqciation by chopping the laser output at a variable frequency. The product of pulse frequency and pulse width is kept constant so that the average light intensity is always the same, and any observed pulse-rate dependence must be due to the specific interaction between the pulsing irradiation and the relaxation processes; no dependence on chopper frequency is expected for one-photon dissociation. One can also remove the chopper and use an appropriate neutral-density filter to create the equivalent of infinite chopping frequency. These pulse sequences are illustrated in Figure 1 for a 15% duty cycle, although a duty cycle of 10% or less is used in practice. The two techniques are complementary: uncertainty arises in interpreting pressure dependences from the possibility of pressure-dependent ion-molecule reaction sequences, from the possible change in ion-cloud geometry with pressure, and from the possibility that collisional relaxation will not be observably fast compared with radiative relaxation. Observing the chopper-frequency dependence under constant ICR cell conditions eliminates all of these potential sources of error, but as is seen below, can be misleading if relaxation is very slow. Characterization of a dissociation as one-photon o r two-photon by both techniques together should give a very high degree of confidence. Another way of determining ion dissociation thermochemistry is through formation of the parent ions with well-defined internal excitation by charge exchange, looking for subsequent dissociation. Such methods have a long history; recent work has developed a set of reagents and techniques which are convenient and useful for preparation of good-sized hydrocarbon ions with total ionization (9) Dunbar, R. C.; Fu, E . W. J . Phys. Chem. 1977,81, 1531. Kim, M. S.; Dunbar, R. C. Chem. Phys. Lett. 1979,60, 247. Lev, N. B.; Dunbar, R. C. J . Phvs. Chem. 1983. 87. 1924. (10) h, N.B.;Dunbar, R. C. Chem. Phys. Leu. 1981,84,483. Dunbar, R.C. J . Phys. Chem. 1983, 87, 3105.
Fragmentation Thermochemistry of Gas-Phase Ions energy in the 10-16-eV range." Charge exchange offers the advantages of a wide range of accessible ion energies and a rather quick and convenient experiment. Drawbacks are the limited energy resolution due to the limited number of good charge-exchange reagent ions, and inherent uncertainties about excess energy from non-ground-state reagent ions, or energy deficits following incomplete energy transfer from the reagent ion. It seems safe to assume that if very little fragmentation follows charge-exchange ionization, the energy deposition lies below the dissociation threshold; and if nearly complete fragmentation results, the energy deposition lies above threshold. But if partial dissociation is observed, it cannot be safely decided whether the nominal energy deposition of the charge exchange is or is not above dissociation threshold. In the present applications, the charge-exchange experiments serve as a useful confirmation of the photodissociation conclusions, but do not rival them in energy precision.
Experimental Section As described in previous work on photodissociation pressure dependence9 and chopped-laser effects,*Othe ICR instrument used was a Varian ICR-9 modified to operate in the pulsed mode with a McIver trapped cell. Light sources were either a Coherent Radiation CR-12 argon ion laser, operated at visible wavelengths, at UV wavelengths, and as a pump for a tunable dye laser; or a 2.5-kW Canrad Hanovia Hg/Xe arc lamp. Wavelength selection with the arc lamp was with interference filters (- 12-nm bandwidth); and a colored-glass filter combination passing wavelengths between 610 and -710 nm. Samples were commercially obtained, and purified only with a freeze/pump/thaw procedure to remove noncondensible gases. Pressures were determined by the ion current from a previously calibrated VacIon pump. Charge-exchange ionization measurements were carried out both in an ICR spectrometer and in a Dupont Model 490 mass spectrometer equipped with a chemical ionization source. Use of a similar Dupont instrument for charge-transfer ionization has been described," and our instrument was able to reproduce literature results of Harrison and Lin.'* The advantage of using an ICR ion trap for charge-exchange fragmentation experiments is the complete freedom from kinetic shifts: if the parent ion has sufficient energy to dissociate, it is practically certain to do so on the ICR time scale. The experiments were done in the Fourier-transform instrument which has been described,13 but with ion detection by an rf bridge detector. It was found that the experiment could be done very cleanly if, after the electron beam pulse, a double-resonance pulse was used to eject from the cell all the primary parent or fragment ions, after which the buildup of ions resulting from charge transfer from the reagent gas could be easily followed. Sample pressures were of torr, reagent gas pressures of the order of the order of 2 X 10" torr. To minimize excited-state formation, electron energies were the minimum necessary to produce sufficient reagent ions, in the range 12-16 eV nominal. Results Z-Methylnaphthalene. Initial studies on this compound consisted of 1/D vs. pressure plots as a function of wavelength. At wavelengths through the visible the 1/D vs. pressure plots were straight lines with positive slope, as expected for a collisionally quenched two-photon process. Figure 2 shows a few of the results at wavelengths in the UV, where the one-photon threshold might be expected. Values plotted with (X) indicate the behavior at the wavelength of the UV laser, using parent neutral as the collisional quench gas. Values plotted with (e), taken at 366 nm with the arc-lamp source, are representative of a number of experiments done using cyclohexane as the neutral quench gas, in (11) Li, Y. H.; Herman, J. A.; Harrison, A. G. Con.J . Chem. 1981,59. Herman, J. A.; Li, Y. H.; Harrison, A. G. Org. Mass Spectrom. 1982, 27, 143. (12) Harrison, A. G.; Lin, M. S.Int. J . Mass Spectrom. Ion Processes 1983, 51, 353. (13) Hays, J. D.; Dunbar, R. C. Rev. Sei. Intrum. 1984, 55, 1116.
The Journal of Physical Chemistry, Vol. 89, No. 17, 1985 3619
0
20
10 Pressure (XIO-'
torr)
Figure 2. Pressure dependence of 1-methylnaphthalene ion photodissociation at UV wavelengths. The 333-364-nm experiment (UV laser) used parent neutral as the quench gas, while the 366-nm experiment (arc lamp) used cyclohexane as quench gas, and it is evident that cyclohexane is a much less efficient quencher.
1 -Me-Naphthalene
313nm 15
. 0
0
10
/
/ 0 4
5
1
2
3
Pressure ( l O - ' t o r r )
Figure 3. Pressure dependence of 1-methylnaphthalene ion photodissociation at 313 nm (two separate runs). The quench gas was parent neutral. The dashed lines have no significance other than guesses about the pressure behavior.
order to rule out the possibility of a spurious pressure dependence arising from parent ion regeneration through some sequence of ion-molecule reactions. Figure 3 shows the data we were able to obtain in two runs at 3 13 nm with parent neutral quench gas. Although the marginally sufficient light intensity prevented acquiring extensive data, the results shown are sufficient to indicate a very definite pressure dependence and to suggest strongly that the 1/D vs. pressure curve has upward curvature at this wavelength. (The significance of such curvature just below the onephoton threshold is discussed below for the a-methylstyrene case.) The two-photon behavior indicated by the pressure dependences was confirmed by chopper experiments with the visible laser at 458 nm and with the UV laser (333-364 nm). Figure 4 shows the ratio of dissociation at 1 H z to that at 12 Hz. The presence of a strong chopper frequency dependence, and the pressure dependence of the chopper effect, definitely indicate two-photon dissociation. The solid line in the figure results from a numerically calculated simulation of the kinetics of eq 4, including chopped light irradiation, assuming a collisional relaxation rate constant of 3.7 X lo7 s-l torr-I and no radiative relaxation. The simulation can be seen to model the shape of the chopper effect curve very well for the visible wavelength experiment. The relaxation must be considerably slower for the UV wavelengths, leading to both a weaker chopper effect and a weaker pressure dependence of the chopper effect. The chargeexchange ionization observations are shown in Table I for both Dupont and ICR instruments with several reagent gases. a-Methylstyrene. Figure 5 shows 1/D vs. pressure plots at 545 and 560 nm showing no pressure dependence for a-methylstyrene ion, with parent neutral as the quench gas. Figure 6 shows pressure
3620 The Journal of Physical Chemistry, Vol. 89, No. 17, 1985 I
9 /
/
i
/ " h
l
N
V
10
1-Methylnaphthalene
/
8
F
1
1
I
Honovich et al.
-
ti t
A a-Me-Styrene
1
'6'0nmA 0
I
0
2/
/ 1
O/O
2
p r e s s u r e ( x 1 o-'~o r r )
k0. 0
I
I
I
2
4
6
Pressure
J
Figure 6. Pressure dependence for a-methylstyrene ion at longer wavelengths with parent neutral quench gas.
Torr)
Figure 4. Chopper effect and its frequency dependence for l-methylnaphthalene ion. Points (0) are at 458 nm, and points ( 0 ) are at
2
333-364 nm (UV laser). The quench gas was parent neutral. The dashed line is a quantitative numerical simulation of the kinetics (see text). The notation D(1), for instance, denotes the dissociation D at a 1-Hz chopper frequency, so that the quantity plotted is the ratio of dissociation observed at very low and very high pulse repetition rates.
c
I
a-Me-STYRENE
0
c
a-Methylstyrene
r
T 1 2 4
0
3
Pressure
Torr)
Figure 7. Chopper frequency and chopper-effect pressure dependence for
3
1
a
y
0 ,
0
,
o 545nm ,
I
.5
1.5 Pressure '(x10-7 torr)
1
2
Figure 5. Pressure dependence of a-methylstyrene ion photodissociation
at shorter wavelengths with parent neutral quench gas. TABLE I: Extent of Fragmentation of Parent Ion Following Charge-Exchange Ionization
chargeexchange reaeent Ar
co
co2 N2O 0 2
extent of . reagent excess energy fragmentation' recomb energv. eV dewsit.. eV Duwnt ICR 1-Methylnaphthalene, (IP = 8.0 eV) 15.8 7.8 extensive 14.0 6.0 extensive 13.8 5.8 extensive complete 12.9 4.9 small partial 12.1 4.1 none
ocs CH3Br cs* C6F6
o"Small" means
a-Methylstyrene (IP = 8.35 eV) 11.1 2.75 small 10.5
2.15
10.1 10.0
1.75 1.65
-
none none
extensive partial none none
10-20%: ''extensive- means -70-90%.
dependences observed at longer wavelengths. The 580- and 595-nm curves show small but definite pressure effects at lower pressures, possibly leveling off at higher pressures. At the longer wavelengths (620,625,610-710 nm) the curves show a level region at low pressures followed by a sharp rise at higher pressure. Some experiments at these long wavelengths were repeated with a 1-s dark period before the beginning of irradiation to allow any excited ions to relax, with no change in results. There was no observed dependence of dissociation on chopper frequency at any of these
a-methylstyrene ion at 620 nm (0)and 625 nm (a), showing no significant dependence on chopper frequency at any pressure.
wavelengths and at pressures up to 7 X lo-' torr. Figure 7 shows the lack of any chopper effect at 620 and 625 nm. The pressure dependence and lack of chopper effect were sufficiently unexpected to warrant considering some possible explanations other than a transition from one-photon to twcl-photon kinetics. One possibility is a slow, collision-induced isomerization, collisional relaxation of initially excited species, or other slow, progressive, collision-induced change in the ion population; this was ruled out by gated-laser experiments, in which dissociation was effected by a single pulse of laser irradiation at various times during the ion-trapping period. It was found that the extent of photodissociation was completely independent of whether the light pulse came early or late during the trapping period. Another possibility considered was that photodissociation at wavelengths longer than 600 nm is single photon but that the dissociation process after photon absorption is so slow as to make possible collisional quenching of the one-photon process. Evidence against this came from an experiment in which dissociation was brought about by a single 100-ms light pulse, followed by a variable delay of 0-500 ms before ion detection. The extent of photodissociation was found to be independent of delay between light pulse and ion detection. Table I shows the results of the charge-exchange ionization experiments.
Discussion I-Methylnaphthalene. It seems almost certain that the parent ion studied here does have the 1-methylnaphthalene structure, based on the excellent correspondence between the photodissociation spectrum of the ion produced under ICR conditions and the optical spectrum of the 1-methylnaphthalene ion in glassy matrix.I4 Since, according to the pressure dependence, photodissociation requires two photons even at 313 nm, the threshold is apparently greater than 4 eV (92 kcal/mol) (but see the discussion below). (14) Dunbar, R. C.; Klein, R. J . Am. Chem. SOC.1976, 98,
7994.
Fragmentation Thermochemistry of Gas-Phase Ions
The Journal of Physical Chemistry, Vol. 89, No. 17, 1985 3621
SCHEME I: 1-Methylnaphthalene Thermochemistry (AH 80C)
1 lH9
t
(a25Od+lO)
(212)
(76e)
CllHIO
1 lH9’
1
+
(60f)
(28a)
Reference 15. Various techniques ref 15). Present red See Table 11. e Reference 17. ReFerence 16.
sults.
l+
d
TABLE 11: Heat of Formation Values for C I I H ~ + AH,, kal/mol method >250 threshold photodissociation’ 262 electron-impact appearance potential* 254 electron-impact appearance potential (C, ,H9Cl)C 255 theory (structure 1)‘ 250 theory (structure 2)‘ 230 ionization of CIIH9qd 273 f 18 charge exchange“ 27 8 electron-impact appearance potentialC ‘Present results. bReference 5. CReference6. dReference 2 (corrected following ref 16). eReference 4. Both the curvature of the 313-nm pressure plots in Figure 3 and also the weak pressure dependence of the chopper effect at 333-364 nm in Figure 4 are consistent with an approach to threshold around 300 nm, but without data at shorter wavelengths no upper limit can be given for the one-photon threshold. The charge-transfer results of Table I indicate a fragmentation threshold between 4.1 and 5.8 eV. The partial fragmentation observed with N 2 0 in the ICR experiment, with little fragmentation in the Dupont instrument with the same reagent gas, is consistent with a threshold near 4.9 eV, with a strong kinetic shift in the Dupont instrument. Scheme I indicates some reasonable estimates for the thermochemistry of species involved in the C l l H l osystem.ls Under each of the species is given the best estimate of the heat of formation, and along the arrows showing transitions are given the measured reaction enthalpies. AHf of CllHlo should be reliable, and its ionization potential, measured by several accurate techniques, also should be good.’5 The value for the heat of formation of C11H9.radical2 has recently been redetermind,l6 its ionization potential, from an old but apparently careful measurement,2 is of uncertain accuracy. The measurement3q4 of dissociative ionization of C,,Hlo directly to CllHg+is subject to the usual uncertainty of such measurements arising from the possibility of a kinetic shift, so that such values tend to be higher than the true thresholds. Not indicated on the scheme is a similar dissociative ionization measurement” on 1-chloromethylnaphthalene,giving concordant results for CllHg+. Table I1 ties these various measurements together in the form of several experimentally derived AHf values for CllHg+,along with the values estimated by Schwarz and Bohlmann” for the methylnaphthalenium (1) and benzotropylium (2) structures using (15) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T. J . Phys. Chem. Ref.Dura, Suppl. 1977,6(1). “Ionization Potential and Appearance Potential Measurements, 1971-198 1”. Nufl.Srand. Ref Dura NSRDS-NBS
2
semiempirical groupequivalent methods. (Harrison and Lossing’s result from CllH9-has been corrected according to the more recent neutral heat of formation16.) In the absence of conflicting evidence, we would prefer a value of -252 kcal for the heat of formation of CllHg+,corresponding to a homolytic bond cleavage energy of 92 kcal for CloH7CH2-H+. This value is in general accord with the range of the three values from electron impact appearance energy measurements on CloH7CH2-X compounds, agrees with the calculation of Koppel et a1.,6 and is consistent with the present results. The last two values in the table are higher, but the experiments involved in these seem less worthy of confidence than those giving lower values. The problems with this are, first, that there is no obvious reason for the heat of formation to differ so much from the value of 232 kcal from Harrison and Lossing’s (corrected) ionization potential determination from CllH9*,and second, it is highly surprising that this bond cleavage should be some 50 kcal more difficult than for C6H5CH2-Hf. By stretching the interpretation of our data, the disagreement with Harrison and Lossing can be reduced, but the problem with the toluene/methylnaphthalene comparison remains puzzling. It seems clear that the CIIHlo+photodissociation is a two-photon process in the 350-360-nm (3.4 eV) vicinity, where the pressure-dependence and chopper results are all quite strong. The pressure-dependence result at 3 13 nm, and the charge-transfer result with O2at 4.1 eV, seem like weaker evidence, and we would not strongly defend placing the one-photon threshold higher than 3.4 eV, corresponding to AHf (CllHg+) = 239 kcal, or a bond cleavage energy of 19 kcal. This is not unreasonably far from Harrison and Lossing’s result. The threshold dissociation of C7H8+ yields the tropylium structure for C7H7+. When the 15 kcal benzyl/tropylium difference of Stadelmann et a1.I is used, this corresponds to a homolytic bond cleavage energy (yielding benzyl cation) of 56 kcal for toluene. This is 20-30 kcal less than the lower limit energy for methylnaphthalene, even assuming that the latter cleavage gains no benefit from a rearrangement to benzotropylium ion. This difference of 1 eV or more in bond strength is surprising. The threshold dissociation of CloH7CH2-Xcompounds would seem to warrant further study by other precise techniques like photoionization, photoion-photoelectron coincidence, or electron monochromator ionization to lessen the uncertainty surrounding this thermochemistry. a-Methylstyrene. The progression from no pressure dependence at 560 nm to strong pressure dependence at wavelengths longer than 610 nm indicates the crossing of the one-photon dissociation threshold for a-methylstyrene ion in this wavelength region. Around 580 nm, the weak pressure dependence, levelling off at high pressure, is naturally interpretedI8 as reflecting a mixture of one-photon and two-photon ions, with the two-photon component of dissociation being largely suppressed at pressures above torr. 2x Such an admixture of one-photon ions might come from initially excited ions from the electron impact ionization process; however, the same behavior persists with a 1-s dark period following electron impact and preceding light irradiation. It would be expected (particularly at the higher pressures) that this dark period would allow substantial thermalization of initially superthermal ions and that the pressure dependence would become stronger and more linear. Equally strong evidence against collisional relaxation of initially hot ions comes from the independence of the extent of dissociation on the time of the laser pulse in the gated-laser
71.
(16) McMillen, D. F.; Trevor, P. L.;Golden, D. M. J . Am. Chem. SOC. 1980, 102, 7400. (17) Schwarz, H.; Bohlmann, F. Org. Muss Specfrom. 1973, 7, 395.
(18) Dunbar, R. C.; Honovich, J. P. In?.J . Mass Specrrom. Ion Processes 1984, 58, 25.
3622
The Journal of Physical Chemistry, Vol. 89, No. 17, 1985
experiment. Accordingly, it appears instead that the one-photon/two-photon mixture of ions in the population is an equilibrium thermal situation, which in turn locates the dissociation threshold rather precisely. It has been discussed in detail'* how the factors of thermal ion internal energy and kinetic shift each contribute shifts, in opposite directions, of the order of hundredths of an electronvolt to the apparent photodissociation threshold: to an accuracy of the order of h0.05 eV, the wavelength a t which pressure dependence ceases to be noticeable can be taken as the true, 0 K thermochemical dissociation threshold. In this case, this is about 560 nm, or 2.2 eV (51 kcal/mol). The lack of pressure dependence at wavelengths below 560 nm gives a strong indication that this is a one-photon process. The lack of dependence on chopping rate in the chopped-laser experiment was initially puzzling, since any two-photon process which is relaxed by collisions, as in this case at wavelengths greater than 580 nm, ought to show a chopper-rate dependence as well. However, this situation is understandable if the radiative and collisional relaxation rates in the near-threshold wavelength region are exceptionally slow. The slowest chopping rate used was 1 Hz, and calculations indicate that relaxation must exceed 2 s-I for any chopper effect to be observable at 1 Hz and must probably exceed 3 s-I for a substantial effect. On the other hand, given the 4-s irradiation period and the modest extent of dissociation, a relaxation rate of 2 or 3 s-l is sufficient to give a very large decrease in the extent of dissociation compared with the zero-pressure value. Thus a collisional relaxation rate around 2-3 s-I leads, under these conditions, to both a large pressure effect and simultaneously a small chopper effect. Inefficient collisional and radiative relaxation is not surprising at these wavelengths: at 620 nm an ion must be relaxed to within 0.1 or 0.2 eV of the ground state in order to lie below the onephoton threshold, while the first photon deposits about 2 eV of excess internal energy. It is not unexpected that the required removal of more than 90% of the internal energy would require several collisions. The conclusion here that the sum of IR radiation relaxation and collisional relaxation is less than about 3 s-l at 7 X lo-' torr (the highest pressure at which the chopper experiment could be done) is unusual, but not implausible. The unusual shape of the pressure-dependence curves in the red, as in the 6 10-7 10-nm curve in Figure 5, is not surprising in the just-below-threshold wavelength region. As compared with the expected straight, positively sloping line for pure two-photon behavior with single-collision relaxation, these curves show a marked level region at low pressure, followed by a steep rise. The unusual initial level region may arise because several collisions are necessary to relax a photoexcited ion below the one-photon threshold (the "cascaden relaxation model discussed in ref 19), which will give curvature in the pressure dependence as observed. Also, the linear dependence of 1/Don pressure is a high-pressure approximation, and may break down when collisional quenching is inefficient near the one-photon threshold. The charge-exchange ionization results of Table I support these thermochemical conclusions, although they are both less accurate and less reliable than the excellent threshold photodissociation results. The CS2 reagent, giving 1.75 eV of excess energy, does not bring about dissociation, while OCS, giving 2.75 eV of energy, gives extensive dissociation in the ICR instrument, and a small amount of dissociation in the Dupont instrument. CH,Br, depositing 2.15 eV of excess energy, gives partial dissociation in the ICR experiment, supporting assignment of the threshold in this region. (19) Dunbar, R. C.; Chen, J. H. J . Phys. Chem. 1984,88, 1401
Honovich et ai. SCHEME 11: a-Methylstyrene Thermochemistry ( a H f " 2 9 8 , kcal/mol)
T
'gHIO (28")
'
References 6 and 15. Reference 15. Present results. Derived from present results. e Derived from electron impact appearance potential (ref 6). Electron impact, ref 6. a
Scheme I1 shows the thermochemistry of relevance. The best estimate of the ionization potential of a-methylstyrene from photoionization and electron impact is about 8.3 eV,6,15giving a heat of formation of C9HI0+of 219 kcal/mol. The electron-impact appearance potential of C9H9+is 11.8 eV,6 corresponding to a dissociation threshold of 3.5 eV (80 kcal/mol). This is considerably higher than the upper limit of 53 kcal/mol from the above photodissociation results, indicating a large kinetic shift. Koppel et aL6 have made estimates of heats of formation of several possible product C9H9+ions. The most stable structure, indanyl(3), has
3
4
an estimated threshold of 52 kcal/mol, in agreement with our results; the vinyltropylium structure (4), with an estimated threshold of 62 kcal/mol, would also be a plausible product ion structure. It seems clear that the threshold fragmentation of a-methylstyrene ion is accompanied by an extensive structural reorganization to a stable product ion structure;m the time required for rearrangement makes the dissociation slow, leading to a large kinetic shift in the electron-impact appearance potential, but the rearrangement/dissociation is fast enough to give abundant fragmentation on the 1-s time scale of the trapped-ion photodissociation and charge-transfer ionization experiments in the ICR spectrometer.
Acknowledgment. The support of the National Science Foundation and of the donors of the Petroleum Research Fund, administered by the American Chemical Society, is gratefully acknowledged. Construction of the Fourier-transform ICR spectrometer was made possible by a gift from SOHIO. We are grateful to Mr. J. H. Chen for providing them with the results of a chopped-laser experiment on a-methylstyrene ion at 515 nm. Registry NO. CIIHlo, 90-12-0; C9H,o, 98-83-9; CllHIo'., 34475-76-8; CgH,,'., 66824-05-3. ( 2 0 ) Rearrangement of the parent ions prior to photodissociation is a possibility in principle, but photodissociation spectroscopy2] of i he similar @-methylstyreneand 1-phenyl-1-butene ions shows no sign of rearrangement of the nondecomposing ions, and there seems no reason to expect a rearrangement in this case either. (21) Fu, E. W.; Dunbar, R. C. J . Am. Chem. SOC.1978, 100, 2283.
