J . Phys. Chem. 1984, 88, 4759-4764
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Competitive Two-Channel Photodissociation of n-Butylbenzene Ions in the Fourier-Transform Ion Cyclotron Resonance Mass Spectrometer Jyh H. Chen, John D. Hays, and Robert C. Dunbar* Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 441 06 (Received: March 20, 1984)
The photochemistry of n-butylbenzene ions was studied in the ion cyclotron resonance ion trap using Fourier-transform and double-resonance techniques. The branching ratio for formation of the m/e 92 and 91 product ions was determined at several wavelengths. The effect of thermal excitation of the ions was analyzed, and values were derived for the branching ratio as a function of ion internal energy. The results are in agreement with dissociative charge-transfer ionization experiments, after similar correction for thermal excitation, but previous ion-beam photodissociation results are badly divergent as a result of large excess internal energy of the parent ions. RRKM calculations of the branching ratio suggest that the dissociation behavior is consistent with a quasiequilibrium-theory description, and that the activation energy for m/e 91 formation is 0.6 eV higher than that for m/e 92 formation. The actual values of these activation energies are less well determined, but the results are consistent with a value of 1.1 eV for m/e 92 formation, in agreement with existing thermochemical information, assuming a methylenecyclohexadiene product ion structure.
Introduction Fragmentation of molecules prepared with known internal energy provides useful insights into various aspects of energy flow and disposal in polyatomic molecules. Particularly interesting are cases of competitive fragmentation into two or more products. In large molecules such as n-butylbenzene ions, observing such chemistry may indicate whether the excitation energy flows freely throughout the molecule prior to dissociation. The energy dependence of the competition may also be a route to thermochemical conclusions complementary to those available from threshold measurements. The fragmentation behavior of the gas-phase n-butylbenzene parent ion has come to have exceptional interest, by virtue of its use as a standard for the comparison of several different ion dissociation experiments. The chief point of importance is the relative production of the C7H8+( m / e 92) and C7H7+ ( m / e 91) primary fragments from dissociative parent ions of specified internal energy. Photodissociation in the ion cyclotron resonance (ICR) ion trap is exceptionally well suited to quantitative study of such a competitive fragmentation, with the capability of precise energy deposition, effective parent ion thermalization, and virtual freedom from distortions due to kinetic shifts. We have applied the technique using our recently completed Fourier-transform ICR instrument as a means of measuring reliable values for the (91/92) branching ratio, to give some clarification by this independent approach to a somewhat unclear situation. The usual picture of this chemistry, as discussed in detail by Brown,’ is that the formation of C7H7+(91) proceeds by a direct cleavage of C3H7,with a high activation energy and a high frequency factor (or, in other words, a loose transition state). Formation of C7H8+(92) occurs with low activation energy in a McLafferty rearrangement process with low-frequency factor (and tight transition state). This leads naturally to a prediction of dominant C7H8+production at low internal energies, with a switch to dominant C7H7+production at high energies. If the competition is assumed to be governed by purely statistical considerations, statistical theories such as quasiequilibrium theory2 or phase space theory3 can be used to make quantitative predictions of the rates of formation of the two fragments, and their ratio. ‘The qualitative increase in the (91/92) ratio with increasing internal energy of the fragmenting C10H14’ ion has been established in many experiments. Electron-impact ionization experi(1) Brown, P. Org. Mass Spectrom. 1970, 3, 1175. (2) Forst, W. “Theory of Unimolecular Reactions”; Academic Press: New York, 1973. Robinson, P. J.; Holbrook, K. A. “Unimolecular Reactions”;
Wiley-Interscience: Division New York, 1972. (3) Su,T.; Bowers, M. T. In “Gas Phase Ion Chemistry”; Bowers, M. T., E.; Academic Press: New York, 1979; Vol. I, Chapter 4.
0022-3654/84/2088-4759$01.50/0 , I
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ments show this qualitative behavior’ but are so ill-defined in the energy distribution of the parent ions that quantitative conclusions are difficult. Several collision-induced dissociation experiments4 have recently given (91/92) production ratios as a function of energy deposited in the parent ion, but again with no reliable calibration of the energy deposition. A recent ion-beam photodissociation experiment5 gave (91/92) ratios as a function of photon energy, but the largely unknown internal energy distribution of the parent ions in the beam made quantitative analysis difficult. This last study, which yielded fragment kinetic energies as well as (91/92) ratios, was analyzed with a fit of the data to a detailed quasiequilibrium model, with apparent success; however, in view of the unknown (and thus arbitrarily chosen) ion energy distribution, and the use in the modeling of a simple and highly empirical RRK theory with several arbitrarily adjusted parameters, this agreement does not seem to be conclusive evidence for the correctness of the underlying physical picture. The first competitive fragmentation experiment with control over the internal energy of the fragmenting parent ion was the charge-exchange ionization work of Harrison and L i a 6 A series of charge-exchange reagent ions was used to establish (91/92) ratios as a function of parent ion excess energy (as shown in Figure 4). This experiment, showing small but nonzero C7H7+production in the 2-eV region, and a rapidly rising (91/92) ratio above 3 eV, should give a good picture of the overall behavior. Although it is an excellent and convincing experiment, there are several uncertainties in the charge-exchange experiment which make further investigation by the very different photodissociation technique worthwhile: The charge-exchange results were not analyzed allowing for the high-energy thermal tail of hot CloH14 neutrals, which, at the 373 K temperature of the experiment, extends with significant abundance as high as 0.5 eV above zero energy. Moreover, the ion extraction and flight times in the mass spectrometer may well be comparable to or slower than the fragmentation rates, giving various possible distortions of the results. Finally, charge transfer from polyatomic reagent ions may be distorted by excess energy transfer from excited vibrational or rotational modes of the reagent ions or by incomplete energy transfer leaving the reagent neutral product in an excited state. (4)Dawson, P. H.; Sun, W. F., Int. J . Mass Spectrom. Ion Phys. 1982, 44 51. McLurkey, S. A.; Sallans, L.; Cody, R. B.; Burnier, R. C.; Verma, S., Freiser, B. S.; Cooks, R. G. Int. J . Mass Spectrom. Ion Phys. 1982, 44, 215. (5) Mukhtar, E. S.; Griffiths, I. W.; Harris, F. M.; Beynon, J. H. Int. J. Mass Spectrom. Ion Phys. 1981, 37, 159. Griffiths, LIW.; Mukhtar, E. S.; March, R. E.; Harris, F. M.; Beynon, J./H. In?.J. Mass Spectrom. Ion Phys.
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1981. 39. - ,125. (6) Harrison, A. G.; Lin, M. S. Int. J . Mass Spectrom. Ion Processes, 1983, 51, 353. ~
0 1984 American Chemical Society
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The Journal of Physical Chemistry, Vol. 88, No. 20, 1984
Accordingly, we have taken advantage of the capabilities of the Fourier-transform ICR spectrometer,' which is exceptionally well suited to quantitative study of competitive photodissociation chemistry, to conduct a reexamination of this problem.
Experimental Section The study of ion photodissociation chemistry in the ICR ion trap is well-known.s The chief new feature of the present work was the use of the Fourier-transform spectrometer, which has the attractive advantage of being able to display simultaneously all of the ionic species of interest in the cell. The instrument, which is designed around a powerful array processor, has been described in detail.g In these experiments, the electron energy was kept low enough ( 11-eV nominal voltage) to give little primary fragmentation of the parent ions. Trapping voltage was 5 V; other plate voltages were zero. 20 to 100 transients were acquired and averaged to yield spectra like those illustrated below. The broad-band excitation chirp was typically 10 V peak-to-peak, swept over the frequency range in 1 ms. Ions were trapped and irradiated torr (indicated for either 1 or 3 s at pressures around (2-3) X on the ionization gauge). Irradiation used a Coherent Radiation CR- 12 argon-ion laser, with the beam spread to a diameter of -2 cm. Total power was usually -1 W (300 mW in the UV), and accurate neutral density filters were used to give the various intensity levels used. Three visible wavelengths were used (515, 488, and 458 nm), as well as the cluster of UV lines near 358 nm. Double-resonance ejection used an independent oscillator, at an amplitude of 1 V peak-to-peak, gated on during the trapping period and off during the detection period. The complete ejection of C7Hs+was verified by direct observation of the m/e 92 peak, which disappeared in a small fraction of a second with doubleresonance irradiation, while there was little change in the adjacent m l e 91 peak. N
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Results The known photochemistry of n-butylbenzene ion is understood in terms of the straightforward kinetic scheme
(91)
The photodissociation rate constants k , , k2,and k3 are taken as representing kinetics which are simple first order in light intensity, and their magnitudes may be functions both of photon wavelength and of the extent of internal excitation of the dissociating ion. Thus the spectroscopic absorption intensities, the quantum yields of photodissociation, and the competition between the k l and k3 processes are all contained in the ki values. It is straightforward to write the time development of an ion population obeying these kinetics, and having initial concentrations [ 13410, [9210and [9110: [ 1341 = [ 134]0e-r(kl+k3)r
[91] = [134]0 + [92],
(2a)
+ [9110 - [134]oe-(kl+k3)rt-
At fixed light intensity I, eq 2 will describe the ion concentrations as a function of varying time t ; moreover, since t and I (7) See, for instance: Comisarow, M. In "Lecture Notes in Chemistry. Ion Cyclotron Resonance Spectrometry": Springer Verlag: Berlin, 1978; p 136. (8) For a review, see: Dunbar, R. C. In "Gas Phase Ion Chemistry"; Bowers, M. T., Ed.; Academic Press: New York 1979, Vol. I1 Chapter 14; 1984, Chapter 20. (9) Hays, J. D.; Dunbar, R. C. Reu. Sci. Instrum. 1984, 55, 1116.
Chen et al. always appear as the product It, eq 2 describes equally well the ion concentrations at a fxed chosen time tr, as a function of varying I . The present experiments were carried out in the latter mode, with tf fixed at 1 or 3 s, and intensity varied (by accurate neutral-density filters) over a wide range. A constant concern in ion-trap photochemical experiments is the possibility of reaction of one of the photoproduct ions with the parent neutral, which will lead to more complicated kinetic behavior. This was ruled out in the present case by a set of control experiments in which the cell was irradiated for a period of 1 s, with sufficient intensity to yield abundant product ions, following which the ion population was inspected after a delay period (without light) ranging from 0 to 1 s. At the neutral pressure used in these controls (3 X lo-' torr) any reaction of m l e 91 or 92 with parent neutrals proceeding with more than a few percent of collision rate would have shown up clearly as a change in ion abundances with delay after irradiation. No such reactions were seen, and n-butylbenzene appears to be admirably free of complicating ion-molecule reactions. Molecules as large as n-butylbenzene often form more than one isomeric ion with different photochemical behavior. The present experiments, taking advantage of the quantitative well controlled nature of the FTICR instrument, argue against this possibility by showing that the parent ion disappears under irradiation according to eq 2a with a single rate constant, extending to at least 95% dissociation of the initial population of parent ions. Figure 1 illustrates the use of the FTICR instrument to display this ion photochemical behavior. The peaks corresponding to the three ions of interest are shown at several light intensities, and the disappearance of m l e 134, the appearance of m l e 92, and the later appearance of m/e 91 are clearly displayed as the light intensity increases. It is difficult or impossible to separate the contributions to m/e 91 production by the kz and k3 processes using simple kinetic analysis of spectra like those of Fig. ( l ) , and double resonance experiments are essentialOt0A double resonance rf voltage was used to eject the m/e 92 ion rapidly and continuously throughout the ion-trapping period, giving complete suppression of the k2 channel of m/e 91 production, and permitting the separate determination of k3. It is important to be sure that the results are not affected by excess energy in the parent ions. While complete ion thermalization was not guaranteed in these experiments, there are several reasons to believe that this was not a significant problem. The ionizing electron energy was not far above threshold, so that initial energy deposition was expected to be small. Although it has not been measured directly for this ion, the time required for excess energy dissipation by infrared fluorescence is confidently expected to be much less than 1 s.ll Finally, the results (including double resonance) for separate UV experiments using 1- and 3-s ion trapping times were very similar, whereas collisional and IR-radiative relaxation would both be expected to give substantially greater energy dissipation at 3 s, if unrelaxed energy remained after 1 s. Thus, while the possibility of excess internal energy raising the observed (91192) ratios is not ruled out, it seems reasonable to consider the ions as substantially thermal at 300 K. For illustration, plots for two wavelengths are shown: Figure 2 shows the ion abundances at 514.5 nm, along with the best-fitting curves from eq 2; at this wavelength double resonance showed k3 to be relatively small, and the initial buildup and subsequent decay of m/e 92 due to the successive k l and k2 processes are clearly evident. Figure 3 shows the UV results without and with double resonance ejection of mle 92. Comparison of the two sets of curves indicates that k , / k 3 2.0, and k2 is so fast that buildup of m l e 92 is never significant.
=
(10) Comisarow, M. B.; Grassi, V.; Parisod, G. Chem. Phys. Lett. 1978, 57, 413. (1 1) Dunbar, R. C. Spectrochim. Acta, Part A 1975,31,797. Honovich, J. P.; Dunbar, R. C. J. Phys. Chem. 1983,87,3755; J . Am. Chem. SOC.1982, 104, 6220. Dunbar, R. C. J . Phys. Chem. 1983,87, 3105. Dunbar, R. C.,
Chen, J. H. J. Phys. Chem. in press.
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Two-Channel Photodissociation of n-Butylbenzene Ions a
134
I
N-BUTYLBENZENE
3.0 x IO-' torr
514.5 nm
I I eV
I
% 92
100
80
20
120
140
60
40
m/ e 02
00
04
06
08
L a s e r Power ( W )
Figure 2. Ion abundance plot as a function of light intensity at 515 nm. The solid curves are computer-fitted solutions to the kinetics of eq 2.
.8 20
3 5 8 nm
40
m /e
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I
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.6 s9
-
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-
1 20
40
60
80
120
cp P)
140
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Figure 1. FTICR spectra of n-butylbenzene. The top spectrum, without light, shows the predominant parent ion, with some fragmentation to C7HBtand very minor fragmentation to C7H7+. The lower spectra show the increasing production of fragments with increasing light intensity. (3 X torr, trapping time 1.0 s. Each spectrum is the average of 100 transients. The base peak is normalized to 10 for plotting purposes, but the sum of the 134, 92, and 91 peak intensities was actually constant for the spectra in the series.)
TABLE I: Photodissociation Cross Sections (1O-I' cm2) wavelength, nm a1 07 Ul UI + 01 515 3.3 0.73