J . Phys. Chem. 1984, 88, 4242-4246
4242
Time-Resolved Kinetic Energy Releases for Metastable Phenol Ions C. Lifshitz,* S. Gefen, and R. Arakawat Department of Physical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91 904, Israel (Received: February 1 , 1984)
-
Time-resolved kinetic energy releases (KERs) were determined from metastable ion peak shapes for the reaction C6H50H+ c-C5H6++ CO in phenol. Ion beam pulsing was combined with an ion trapping device and with an MS/MS technique of time-resolved magnetic dispersion. The part of the KER which is due to reverse activation energy, F,was observed to increase with increasing ion lifetime.
Ion trapping in an electron space charge has been combined with an MS/MS technique and applied to the phenol ion problem. The resultant time-resolved KERs are going to be presented here.
Introduction The phenol ion undergoes a unimolecular dissociation in the gas phase to C-CSH~'and CO, which has been the subject of considerable controversy.ls2 Russell et al.' have observed that T I / , , the kinetic energy release (KER) deduced from the metastable ion peak width a t half-height, decreases with increasing ion lifetime, as is expected for a KER of statistical origin. Maquestiau et al.2 have found the KER to be constant or even increasing with increasing ion lifetime. The mechanism for the CO loss is not clear and may or may not involve the intermediate formation of the keto form of phenol.'" The energetics of the reaction have recently been determined by time-resolved electron impact7 and photoionization: as well as by photoelectron photoion coincidence? The energy threshold is 11.4 f 0.1 eV at 298 Ks and 11.59 eV at 0 Ke9 From the known heats of formation of dissociation products, a reverse activation energy of 1.8 eV7s8 has been deduced. In a unimolecular dissociation of statistical origin the KER may have two contributi0ns:'O P, KER due to a potential energy barrier in the exit channel, Le., due to reverse activation energy, and KER originating from the nonfixed energy along the reaction coordinate. The phenol ion dissociation is very slow at threshold energies having a specific microcanonical rate coefficient k ( E ) 5 lo-' Metastable ians which dissociate in the field-free regions of magnetic sector mass spectrometersI0 have lifetimes of the order of microseconds. The nonfixed energy E* available for release as kinetic energy of the products is of the order of 1 eV in the microsecond r a ~ ~ gase is~ evidenced ?~ by a kinetic shift'," of this order of magnitude. On the basis of statistical theories of unimolecular dissociation, such as the quasiequilibrium theoryl2.l3 the nonfixed energy E* along the reaction coordinate should decrease as the lifetime of the ion increases, Le., as ion dissociations nearer to the threshold should depend on the ion energy are probed. As a result, lifetime and should in fact decrease with increasing ion lifetime as has been observed by Russell et al.' for KER in the phenol ion reaction. The contrary results of Maquestiau et al? have, however, been obtained on several different instruments and have been confirmed also in a review article by Holmes and T e r l ~ u w . ' ~ Previous determinations of time-resolved KERs in metastable ions involved variation of the accelerating voltage in order to vary the ion lifetime. Greater discrimination against off-axis ions is expected, the lower the accelerating voltage2 leading to increased "dishing" and, possibly, to erroneous data. KER data can be obtained for energy selected ions, from their flight time distributions, but only at relatively high energies, such that the distribution is no longer asymmetric due to the slow dissociation kinetics. The kinetic energy release distribution (KERD) for phenol ions was determined9 at 13.6 eV, an energy at which k N 4 X lo6 s-I. We have developed recentlyI5-I6an ion trapping device which allows the determination of time-resolved KERs for metastable ions over a wide range of ion lifetimes (from microseconds to milliseconds) without introducing discrimination effects.
Experimental Section The technique of trapped ion mass spectrometry (TIMS) has been described in detail recently.'' In the present study, a continuous electron beam of 3-5-eV energy and FA current, provided by thermionic emission from a rhenium filament, is used to trap the ions produced when a pulse of -27 V (negative with respect to the ionization chamber) and 2-5-ps duration is applied to the filament. At a known and variable time after the ionizing pulse a positive voltage pulse is applied to the repeller electrode to remove ions for mass analysis. Ion trapping with the TIMS technique has been combined in the present study with a relatively new mass spectrometry/mass spectrometry (MS/MS) method described recently by Enke and co-worker~.'~MS/MS by time-resolved magnetic dispersion utilized ion beam pulsing and time-resolved detection techniques in a magnetic sector mass spectrometer (a MAT CH4). Simultaneous momentum and velocity analysis of the ions is achieved and allows the separation, in a single-focussing magnetic instrument, of normal fragment ions from metastable ions having the same nominal mass m*, due to their different flight times. The technique was originally developed17for CAD (collisionally activated dissociations); we have employed it for the study of unimolecular dissociations. Time-resolved detection is presently achieved through a digitizer and transient recorder (Nicolet series 1170 signal-averaging system and Biomation Model 8 100 waveform recorder). A 1.8 X lo3 n resistor is employed to monitor
-
e,
-
(1) D. H. Russell, M. L. Gross, and N. M. M. Nibbering, J . Am. Chem. Soc., 100, 6133 (1978). (2) A. Maquestiau, R. Flammang, G. L. Glish, J. A. LaramBe, and R. G. Cooks, Org. Mass Spectrom., 15, 131 (1980). (3) D. H. Russell, M. L. Gross, J. van der Greef, and N. M. M. Nibbering, Org. Mass Spectrom., 14, 474 (1979). (4) A. Maquestiau, Y.van Haverbeke, R. Flammang, C. De Meyer, K. G. Das, and S . Reddy, Org. MassSpectrom., 12, 631 (1977). (5) J. S. Splitter and M. Calvin, J . Am. Chem. Soc., 101, 7329 (1979). (6) H. J. Walther, H. Eyer, U. P. Schlunegger, C. J. Porter, E. A. Larka, and J. H. Beynon, Org. Mass Spectrom., 17, 81 (1982). (7) C. Lifshitz and S. Gefen, Org. Mass Spectrom., 19, 197 (1984). (8) C. Lifshitz and Y . Malinovich, Int. J . Mass Spectrom. Ion Processes,
in press. (9) M. L. Fraser-Monteiro. L. Fraser-Monteiro, J. de Wit, and T. Baer, J . Phys. Chem., in press. (10) R. G. Cooks, J. H. Beynon, R. M. Caprioli, and G. R. Lester, "Metastable Ions", Elsevier, Amsterdam, 1973. (11) C. Lifshitz, Mass Spectrom. Rev., 1, 309 (1982). (12) C. Lifshitz, A h . Mass Spectrom., 7A, 3 (1978). (13) C. E. Klots, J . Chem. Phys., 64, 4269 (1976). (14) J. L. Holmes and J. K. Terlouw, Org. Mas Spectrom., 15,383 (1980). (15) C. Lifshitz, P. Gotchiguian, and R. Roller, Chem. Phys. Lett., 95, 106 (1983). (16) S. Gefen and C. Lifshitz, Int. J . Mass Spectrom. Ion Processes, 58, 251 (1984). (17) J. T. Stults, C. G. Enke, and J. F. Holland, Anal. Chem., 55, 1323 (1983).
Permanent adress: Institute of Chemistry, College of General Education, Osaka University, Osaka, Japan.
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1
1984 American Chemical Society
Kinetic Energy Releases for Metastable Phenol Ions
-
TABLE I: Time-Resolved Kinetic Energy Releases (meV) for the Phenol Ion Reaction C6H50H+. C5H,+* -I-CO
= 2.0- 6 . 5 ~ 5
A
1
0
I = 12.0- 1 6 . 5 ~ 5
0 t = 22.0
The Journal of Physical Chemistry, Vol. 88, No. 19, 1984 4243
storage time, ps
-26.5~~
TII 7 550 540 490
Tmin’’
0
95 120 220
10 20
T,,, 1300 1250 1020
T,i, KER from the width at the top of the peak; KER from the width at half-height; T,,, KER from the width at the base, slightly above background noise level. I O 0
8’8
Q o oo
0 00
o
k , sec-‘
Figure 1. Field-free region metstable ion fractional abundances as a function of rate coefficient; these were calculated for reaction 2 in phenol and for the MS/MS experiment with a single-focusing magnetic sector instrument (the CH4) at three storage times: 0, 10, and 20 ps. The corresponding reaction times (entrance into and exit out of the field-free
region) are indicated in the figures. These abundance curves can serve as rate constant distribution functions, relevant to the present experiments. the output from the electron multiplier. The magnetic field strength, B, is selected by means of a Hall probe and gaussmeter combination and the Nicolet is triggered by the ion repeller pulse or, alternatively, by the ionizing pulse. Ion time-of-flight distributions are accumulated for a preselected number of scans (typically 8192) and the intensity at the peak maximum is read off. The field strength B is advanced and the whole process is repeated until a complete metastable ion peak shape is obtained. Metstable peak shapes are obtained for different ion trapping times and these yield time-resolved KERs. Since the accelerating potential is held constant there is no problem of variable instrumental discrimination against off-axis ions. Ion-source pressures were held at 10” torr in order to prevent bimolecular processes which could cause collisional quenching at long ion-storage times. In a typical experiment at zero delay time, at an accelerating voltage of 2960 V, the arrival time of the parent ion of phenol, m l z 94, is 16.36 ps; the arrival time of the daughter ion, C,H,+.,m/z 66, is 13.78 ps and that of the metastable at m* = 46.34 is 16.4 ps, equal to that of the parent ion as is expected.” When the delay time between the ionization pulse and the repeller pulse is increased, the arrival time with respect to the ionization pulse is observed to increase by the expected amount, while it remains unchanged with respect to the repeller pulse. The KER T is obtained from1°
-
-
T=-
m I 2eVd2 16mZ3m3
where ml is the mass of the reactant ion, I/ is the accelerating voltage, d is the metastable peak width in mass units (corrected for the normal peak width), m2 is the mass of the product ion, and m3 is the mass of the product neutral. Metastable ions dissociating in the field-free region of our magnetic sector instrument do not possess a well-defined energy; as a result a range of rate coefficients, k, is sampled. Figure 1 represents the calculated fractional abundance of ions detected as metastable phenol ions of nominal mass 46.34 in the CH4, as a function of k, the rate constant for unimolecular decomposition, at three different storage times in the ion trap. These calculations were performed on the basis of the known residence times in the
m/z
Figure 2. Metstable ion peak shapes for reaction 2 at two storage times: (0) 10 ps; (0)20 ps. The intensities are arbitrary to scale. The rn/z abscissa has been calibrated with Hall-probe measurements of B, the magnetic field strength for normal ions at rn/z 46. Each point is due to 8 192 time-of-flight scans and eight consecutive smoothings by the Nicolet
transient recorder. various regions of our mass spectrometer for a m l z 94 ion, in a fashion explained by Chupka in his classical paper.l* The distribution of sampled rate coefficients shifts to lower k’s as is to be expected, the most probable k being 2 X lo5, 7 X lo4, and 4 X lo4 s-l for t(storage) = 0, 10, and 20 ps, respectively.
Results and Discussion Table I summarizes time-resolved KERs for the phenol ion reaction. The T1,Zvalues (KERs from the peak widths at halfheight) are in good agreement with previously published data.1-4,6J4~’9,20 Time-resolved metastable peaks are shown in Figure 2. They do not demonstrate any “dishing” at the top even for the longer storage times, possibly due to the relatively large collector slit height of 10.0 mm employed.21 Although ions are trapped efficiently, the relative intensity of the metastable (Figure 2) decreases with increasing storage time, reflecting the fact that sampling of high rate coefficients is no longer possible (see Figure l), and storage times in considerable excess of 20 pus gave very low-intensity peaks. The most striking effect we have observed for the metastable peak shapes is the flattening out at the top with increasing storage time (Figure 2) and a concommitant increase in Tmin(see Table I). This may be a real effect or an artifact due to improved energy resolution with increased storage time. We have investigated this effect further by comparing our metastable peak shape with a calculated one, based on the PEPICO data for energy selected phenol ions at 13.6 eV9 (see Figure 3). The calculation was (18) W. A. Chupka, J . Chem. Phys., 30, 191 (1959). (19) M. K. Hoffman, M. D. Friesen, and G. Reichmond, Org. Mass Soectrom.. 12. 150 (19771. ‘ (20) C’J. Porter, A. G.’Brenton, and J. H. Beynon, Int. J. Mass Spectrom. Ion Phys., 35,353 (1980). (21) S.Howells, A. G. Brenton, J. H. Beynon, and R. P. Morgan, Znt. J . Mass Spectrorn. Ion Phys., 32,35 (1979).
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The Journal of Physical Chemistry, Vol. 88, No. 19, 1984
Lifshitz et al. TABLE 11: Tunneling Corrections to the Unirnolecular Rate Constants for the Phenol Ion Reaction 2
E , eV
I
I I
I
81 I
I
I
I I I I I
O O O O
01
I
10
01
I
I
r--L
2.90 3.00 3.05 3.10
p w a 1.049 X 9.34 x 10-2 2.394 X lo-' 1
d E ) , I/cm-' 1.32 X lOI3* 2.49 x 1 0 1 3 3.40 X 10" 4.62 x 1013
k , s-' 2.4
X
1.1 x 10-4
2.1
X
6.5 x 10-4
a Tunneling probability. *Vibrational frequencies of the phenol ion were taken from ref 9.
in fact decrease with decreasing lifetime of the ion and/or with increasing internal energy. Let us first examine the possibility of quantum-mechanical tunneling through the potential barrier. Since the reaction
O
_ - - - - - - I-- - - - - - - - I - -1 %
I
_9- _ _ _ _ _k-. -_- *_I - - - - _ - -+ - - - _ 1L_- - -8
r+
45
46
47
I
48
m/z Figure 3. Comparison of experimental metastable peak shape at a storage time of 10 ps (0)with a calculation (dashed rectangles) based on the experimental distribution at Ephoron = 13.6 eV of ref 9; see text for further explanations.
carried out by using eq 1 and assuming a rectangular peak shape for single-valued KER's in our instrument. The assumption of a rectangular peak shape for relatively low KER's is commonly made for magnetic instrument^.^^-^^-*^ Since it assumes no discrimination against off-axis ions, it is no longer valid at higher KER's of -0.5 eV. The general shapes of the peaks are similar (Figure 3), although the calculated one based on the PEPICO data is clearly narrower, especially at the top. This is not surprising since the lowest KER needed to fit the PEPICO time-of-flight distributiong was 70 meV (compared with our Tmi,, for 1 0 - r ~ storage time, of 120 meV, see Table I). If, in fact, one calculates a TIl2value from the computed peak of Figure 3, a value of -330 meV is obtained, which is considerably lower than the ones we have obtained (Table I), and also lower than the literature Tl12 values for metastable ions having lifetimes of the order of 2 to -30 P S . ~The value from the PEPICO data may not be directly comparable to data from magnetic instruments. The half-width of the PEPICO peak9 corresponds to -600 meV. While the assumption of rectangular peak shapes for single-valued KER's underestimates the calculated peak width at the base of the peak (Figure 3), T l 1 2is certainly less than 600 meV when calculated for the magnetic instrument, since the PEPICO peaks are considerably more dish-topped. The rate coefficient corresponding to an ionizing energy of 13.6 eV is k 1 4 X lo6 s-l (see ref 9) and is thus in considerable excess of the highest rate coefficients sampled by our metastable ions (see Figure 1). We conclude therefore that the kinetic energy release, due to reverse activation energy in the phenol ion reaction, decreases with increasing internal energy of the decomposing ion. Let us examine the above conclusion more closely. It seems to us that it has been implicitly assumed in the past that Te,KER due to a potential barrier along the exit channel, should be independent of the internal energy of the dissociating ion. Also, since should increase with increasing internal energy of the ion, the sum of T" fi has implicitly been assumed to increase with increasing internal energy. We think that the controversy concerning the phenol ion reaction may be resolved if we allow fl to increase with increasing internal energy and Te to decrease with increasing internal energy. Since reflects contributions from both and Te it may either stay constant or decrease slightly or even increase, depending upon which distribution of rate coefficients is sampled by the metastable ions. It remains for us to justify the observation that Tmlnwhich reflects T" only, i.e., KER due to reverse activation energy may
+
(22) D. T. Terwilliger, J. H. Beynon, and R. G. Cooks, Proc. R. SOC. London, Sec. A , 341, 135 (1974). (23) J. L. Holmes and A. D. Osborne, Znt. J . Mass Spectrom. Ion Phys., 23, 189 (1977).
involves a hydrogen shift from the hydroxyl group to an adjacent carbon atom, quantum-mechanical tunneling cannot be ruled out. Tunneling through a rotational barrier is responsible for the slow fragmentation in methane cations, which leads to the observation of a metastable transition in the microsecond range.24 Tunneling corrections to unimolecular rate constants for neutral systems have been applied by Miller and co-workers.25,26 They have been employed25 for a reaction system which is quite similar to the present one, namely, the photochemistry of formaldehyde which involves isomerization, trans-HCOH H 2 C 0 , and C O elimination, H 2 C 0 H2 CO. Calculations for these reactions indicate25that tunneling effects are quite significant for rates of lo9 s-l or slower. The microcanonical transition-state expression for the unimolecular rate constant which includes tunneling is25,26
-
+
-
(3) where P is the tunneling probability for one-dimensional motion along the reaction coordinate, E is the total energy, eo$ is the zero-point energy of the transition state, and p ( E ) is the density of states of the reactant. Both P(E - cot) and p ( E ) rise with increasing internal energy E , below the classical barrier for dissociation. For the small molecules investigated by Miller,25,26the tunneling probability rose much faster with energy than the density of states, so that k ( E ) was a monotonically rising function of E . With large polyatomic molecules a situation is conceivable, at least in principle, where p ( E ) rises faster with E than the tunneling probability. If this were to occur, then below the classical barrier k ( E ) would decrease with increasing E , reach a minimum value at the barrier, and start rising again above the barrier, where the number of quantum states of the transition state rises above 1. Furthermore, if this type of tunneling were to occur in the microsecond range, then the possibility would present itself for an increased kinetic energy release with increasing lifetime of the metastable ion, as is observed experimentally for Te in the phenol ion reaction. We have investigated the possibility of tunneling in reaction 2 by calculating k(E) according to eq 3. The tunneling probability was calculated by assuming that an Eckart potential is a good representation of the barrier.25 The barrier height relative to the phenol ion was taken as 3.1 eV (71.5 kcal/mol) and relative to CO, as 1.g3 eV (41.9 kcal/m01).~The microcanonical C-C&+ rate coefficients were calculated as several energies below the barrier and the results are summarized in Table 11. Several conclusions transpire from these calculations: (a) The microcanonical rate coefficient increases with increasing E ; (b) If tunneling takes place, it leads to rate coefficients which are much too low to contribute to field-free-region metastable ion disso-
+
(24) A. J. Illies, M. F. Jarrold, and M. T. Bowers, J. Am. Chem. Soc., 104, 3587 (1982). (25) W. H. Miller, J . Am. Chem. SOC.,101, 6810 (1979). (26) Y. Osamura, H. F. Schaefer, 111, S . K. Gray, and W. H. Miller, J . Am. Chem. SOC.,103, 1904 (1981).
The Journal of Physical Chemistry, Vol. 88, No. 19, 1984 4245
Kinetic Energy Releases for Metastable Phenol Ions
X’Z‘
co
C~H,OH?
Figwe 4. Schematic suggested surface crossing between ground (g2B1) and A2A2states of the phenol cation along the reaction pathway to the cyclopentadienyl cation plus CO products.
ciations. The metastable ion dissociation in the microsecond time range has to be due to ions having an excess energy above the classical barrier. There is an additional piece of evidence favoring the above conclusion. The comparison of the presently measured KERs for the field-free-region metastables with PEPICO data (Figure 3) has shown not only that F decreases with decreasing ion lifetime (Figure 2 and Table I) but that it actually decreases with increasing internal energy, an effect which tunneling below the barrier cannot explain. We feel that an explanation for the behavior of the phenol ion reaction lies in the theoretical treatments of Derrick and cow o r k e r ~and ~ ~of Lorquet and c o - w o r k e r ~ . ~ *The - ~ ~ phenol ion dissociation is an endoergic reaction and as such should have a “late” barrier.27 Energy releast along the minimum energy pathway will initially appear almost entirely as kinetic energy of separation, and the absence of curvature in the path means that little of this energy will be lost subsequently to other modes of product motion.27 However, a model which represents a molecule as a one-dimensional dissociation coordinate provides a poor picture of a unimolecular dissociation, as has been pointed out by Lorquet.2s Curvature is introduced into the reaction pathway because additional degrees of freedom, such as out-of-plane bendings, provide a low-energy trough around genuine intersections. The complexity of reaction 2, which involves rearrangement of a hydrogen atom prior to, or concomittant with the C O loss, can certainly introduce curvature into the reaction pathway. The degree of curvature depends on a symmetry-lowering coordinate30 and may therefore depend on the degree of internal energy excitation in the ion. The system passes from phenol of C, symmetry to cyclopentadiene CO having C, symmetry via an intermediate region of C, symmetry. It is worthwhile to inspect the electronic states of the ions which are involved. Both phenol and cyclopentadiene have been carefully studied by photoelectron spectroscopy. The outer degenerate molecular orbital of benzene, le,,, is split into two components in phenol, the difference in energies being 0.91 eV. The electrons occupying these orbitals are R electrons and they must be delocalized over the whole composite molecule.32 The outermost orbital in phenol is b, and the one further in is a2.32,33 In cyclopentadiene the situation is re-
+
~ e r s e d , ~the ” ~outermost ~ orbital being a2 and the one with a higher ionization energy is b, and the difference in ionization energies between the two is 2.054 eV. It would thus seem that the ground state of the phenol cation, which is 2B,, correlates with the first excited state of the product cyclopentadienyl cation, while the first excited state of the phenol cation, 2A2,correlates with ground-state products. This is drawn schematically in Figure 4, where the relevant ionized molecular orbitals are included. The two potential energy surfaces, 2Azand 2B1,thus have to undergo a crossing along the reaction path, either a genuine one, if C2”symmetry is conserved, or a conical intersection due to out-of-plane bending motions, as has been predicted on theoretical grounds for H2CO+.29 It is interesting to note that the excess energy at the dissociation threshold above ground-state products of reaction 2 (1.8 eV) corresponds_very closely to the difference in energy between the X2A2 and A2B, states of the cyclopentadienyl cation. A true crossing may in fact occur at the dissociation threshold. There have been suggestions that the release of energy as translation is a consequence of the orbital symmetry of the transition ~tate.~*.~’ The argument advanced was that, subsequent to surface crossing in an orbital symmetry “forbidden” process, a molecular orbital is occupied which is characterized by a mutual repulsion between the products. We are not advancing this argument for the phenol ion reaction. The a2 orbital is nonbonding to CO and the b, orbital is repulsive between carbon and oxygen. We would argue that the high reverse activation energy encountered in this reaction is due to surface crossings, but that the degree to which this excess energy shows up as translation of the products depends upon the degree of curvature along the reaction path. The total energy above the product ground states, at a photon energy of 13.6 eV, is 3.8 eV.’ The average KER at this energy, 0.74 eV, is in considerable excess above the statistically calculated oneg yet it is certainly only 19.5% of the total available energy. Thus, while the true crossing produces a repulsive energy release type of surface with a late barrier, at higher energies we suggest that the system follows a curved pathway around the apex of the cone in a conical type intersection. This would lead to channeling of some of the internal energy into internal degrees of freedom of the products. The highest minimum KER T,,, obtained for field-free metastable ions (0.22 eV) is also only -12% of the reverse activation energy, indicating again that excess energy is channeled into internal vibrational motions. Conclusion A barrier along the path of reaction 2 channels a fraction of the reverse activation energy into translation of the products. This fraction of back-activation energy which is deposited into translation decreases with decreasing ion lifetime (Table I) and with increasing nonfixed energy along the reaction path. The minimum energy path at threshold possibly involves a genuine surface crossing and a stiff, symmetry conserving, configuration of the transition state, while at higher energy, most of the reactive trajectories prefer to follow a curved path around the apex of the cone. The possibility that the change in KER is due to two decomposing structures, to decomposition occurring from two different energetic species, or to a decomposition leading to two final different structures (cyclic and linear, for example) cannot be ruled out. Further work is in progress to clarify some of the remaining discrepancies among existing data.
(1978).
(34) P. J. Derrick, L. &brink, 0. Edquist, B.-0. Jonsson, and E. Lindholm, Int. J . Mass Specrrom. Ion Phys., 6, 203 (1971). (35) P. J. Derrick, L. Asbrink, 0. Edquist, and E. Lindholm, Spectrochim. Acta, Part A , 27, 2525 (1971). (36) C. Fridh, L. Asbrink, and E. Lindholm, Chem. Phys. Lett., 15, 408
(1973).
(1972). (37) P. Dechant, A. Schweig, and W. Thiel, Angew. Chem., 85, 358 (1973). (38) D. H. Williams and G. Hvistendahl, J . A m . Chem. SOC.,96, 6753 (1974). (39) G. Hvistendahl and D. H. Williams, J . Chem. SOC.,Perkin Trans. 2, 881 (1975).
(27) J. R. Christie, P. J. Derrick, and G. J. Rickard, J. Chem. Soc., Faraday Trans. 2, 74, 304 (1978). (28) J. C. Lorquet, Org. Mass Spectrom., 16, 469 (1981). (29) M. Vaz Pires, C. Galloy, and J. C. Lorquet, J . Chem. Phys., 69, 3242
(30) M. Desouter-Lecomte. C. Galloy, J. C. Lorquet, and M. Vaz Pires, J . Chem. Phys., 71, 3661 (1979). (31) J. C. Lorquet, Adu. bfam Spectrom., SA, 3 (1980). (32) J. W. Rabalais, Principles of Ultraviolet Photoelectron Spectroscopy”, Wiley, New York, 1977. (33) T. P. Debies and J. W. Rabalais, J . Electron Spectrosc., 1, 355
4246
J . Phys. Chem. 1984, 88, 4246-4251
Acknowledgment. This research was partly supported by a grant from the United States-Israel Binational Science Foundation (BSF), Jerusalem, Israel R. Arakawa thanks the Faculty of Mathematics and Sciences of The Hebrew University of Jerusalem for a postdoctorate fellowship. We thank Professors Haas and Ottolenghi for the loan of the Biomation-Nicolet setup and Professor M. L. Fraser-Monteiro and co-workers for a preprint
of ref 9 prior to publication. Mr. A. Moss and Mr. M. Peres were instrumental in setting up the necessary electronics for the new MS/MS experimental setup. We thank Professors T. Baer, R. B. Gerber, J. L. Holmes, J. C. Lorquet, and H. Schwarz for valuable discussions and comments. Registry No. C6H50H+,40932-22-7.
Yields of Excited States of Solutes in Irradiated Benzene and Cyclohexane Hae Tak Choi,? Fumio Hirayama,t and Sanford Lipsky* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received: February 13, 1984)
The yields of lowest excited singlet states of diphenyloxazole and p-terphenyl in benzene and of diphenyloxazole,p-terphenyl, and biphenyl in cyclohexane have been measured for excitation by 85Kr 0 particles. The dependence of the yield on solute concentration for benzene solutions is shown to be accurately represented by a Stern-Volmer function from 5 X lo4 to M and to extrapolate at infinite solute concentration to the yield of excited singlet states of neat liquid benzene. The presence of oxygen in the solution does not affect the extrapolation. The absolute efficiencies of energy transfer from irradiated benzene to the solutes are in good agreement with previous measurements made by using optical excitation below the ionization threshold. These results provide additional confirmation that the mechanism of formation of excited solute states in fast-electron-irradiated benzene does not significantly involve electron or hole capture by the solute. They also demonstrate that the inhomogeneity of energy deposition does not affect the ratio of probabilities of the decay of excited benzene by photon emission to its decay by nonradiative energy transfer to the solute. For cyclohexane solutions, we confirm that the yields of excited solute states are lower than in benzene solutions at comparable concentration, but larger than would be expected were the same “nonionic” mechanism to apply as it does in benzene. The consequences of this are briefly explored.
Introduction The mechanism for the production of solute excited states in irradiated cyclohexane solutions appears to be quite complex, involving numerous ionic intermediates, and has not yet been fully elucidated.’-14 Compounding the difficulty is the fact that the yields of these states are often found to be quite disparate when obtained by different techniques. For example, in the case of 1 X lo-) M biphenyl in cyclohexane, Baxendale and Wardman,I5 using a “comparison” technique, obtain a yield of biphenyl molecules excited to SI (per 100 eV of absorbed energy, Le., G(S,)) equal to 0. 14,16whereas with an ”absolute” technique, Dainton, Ledger, May, and Salmon17report G(SJ = 0.38. Similarly, large discrepancies exist for benzene and toluene as and still significant, although somewhat smaller discrepancies, for naphthalene.I5*l7For anthracene, however, the two techniques agree to within approximately 20%.159’7 Clearly, whatever is the cause of the discrepancy, it is strongly solute dependent. The comparison technique employed by Baxendale and coworkers1sdetermines the ratio of the solute emission intensity from a cyclohexane solution to that from a benzene solution of the same solute but extrapolated to infinite concentration. Using, as standard, a G value of 1.6 for the primary yield of SI states of benzene, the technique assumes that solute excitation in a benzene solution derives exclusively from nonradiative electronic energy transfer from the excited SIstates of benzene with a probability that can be represented by a Stern-Volmer function that extrapolates to unity as solute concentration approaches infinity. Thus, in the “extrapolated” solution, the yield of solute excited states is taken to be the primary yield of excited S1 states of benzene (i.e., GB = 1.6). On the other hand, the absolute technique utilized by Dainton et a l l 7 relies on no other G value and requires no significantly questionable physical assumption. But it is, act Present address: Photosystems and Electronic Products Department, E. I. du Pont de Nemours and Company, Towanda, PA 18848. 3 Permanent address: Department of Physics, Miyazaki Medical College, Kiyotake, Miyazake 889-16, Japan.
0022-3654/84/2088-4246$01.50/0
cordingly, a more complex technique and, therefore, somewhat more subject to experimental error. In this investigation we attempt to evaluate the validity of the assumption utilized by B a ~ e n d a l e . ’ ~The first part of the assumption, namely that excited solute states derive from electronically excited SI states of benzene, has been repeatedly verified (1) Singh, A. Radiat. Res. Reu. 1972,4, 1 and references cited therein. (2) Thomas, J. K. Znr. J . Radiat. Phys. Chem. 1976,8, 1 and references
cited therein. (3) Salmon, G. A. Znt. Radiat. Phys. Chem. 1976,8, 13 and references cited therein. (4) Walter, L.; Hirayama, F.; Lipsky, S. In?.J . Radiat. Phys. Chem. 1976,
8,231. (5) Brocklehurst, B. J . Chem. SOC.,Faraday Trans. 2 1976, 72, 1869. Brocklehurst, B. Discuss.Faraday Soc. 1977,63,96.Brocklehurst, B. Radiat. Phys. Chem. 1983,21,57. ( 6 ) Busi, F.; Flamigni, L.; Orlandi, G. Radiat. Phys. Chem. 1979,13, 165. (7) Jonah, C. D.; Sauer, M. C., Jr.; Cooper, R.; Trifunac, A. D. Chem. Phys. Lett. 1979,63,535. (8) Sauer, M. C., Jr.; Jonah, C. D. J . Phys. Chem. 1980,84,2539. (9)Katsumura, Y.; Taaawa, S.:Tabata, Y. J . Phys. Chem. 1980.84,833. (10) Hermann, R.; Brcde, 0.;BOs, J.; Mehnert, R.Ber. Bunsenges. Phys. Chem. 1980,84, 814. (11) Busi, F. Radiat. Phys. Chem. 1980,16,101. Busi, F.;Casalbore, G. Gaz. Chim. Ztal. 1981,111, 443. (12) Tagawa, S.;Katsumura, Y.; Tabata, Y. Radiat. Phys. Chem. 1982, 19,125. Tagawa, S.;Tabata, Y.; Kobayashi, H.; Washio, M. Zbid. 1982,19, 193. Katsumura, Y.;Tagawa, S.;Tabata, Y. Zbid. 1982,19,243. Katsumura, Y.; Tabata, Y.; Tagawa, S. Zbid. 1982,19, 261. Tagawa, S.;Washio, M.; Tabata, Y.; Kobayashi, H. Ibid. 1982,19,277. (13) Luthjens, L. H.; Codee, H. D. K.; De Leng, H. C.; Hummel, A. Radiat. Phys. Chem. 1983,21, 21. (14) Katsumura, Y.; Yoshida, Y.; Tagawa, S.;Tabata, Y. Radiat. Phys. Chem. 1983,21,103. Tabata, Y.; Katsumura, Y.; Kobayashi, H.; Tagawa, S. Zbid. 1983,21, 123. (15) Baxendale, J. H.; Wardman, P. Trans. Faraday Soc. 1971,67,2997. (16) This yield is interpolated from values of 0.091 at 0.5 X M and 0.197 at 2.0 X 10” M in ref 15. (17) Dainton, F. S.; Ledger, M. B.; May, R.; Salmon, G. A. J . Phys. Chem. 1973,77,45. (18) Baxendale, J. H.; Mayer, J. Chem. Phys. Lett. 1972, 17,458. (19) Walter, L.; Lipsky, S. Int. J . Radiat. Phys. Chem. 1975, 7, 175.
0 1984 American Chemical Society
