4311
J. Phys. Chem. 1986, 90,431 1-4317 results than for any other case. The JW has the most centrally located potential barrier and hence its adiabatic barriers are more centrally located than corresponding barriers on the other surfaces. Thus the first excited adiabatic barrier is not exceptionally early and agreement with exact results is good. However, if recrossing becomes more important as barrier location becomes earlier, kPRP-ICVT(n,T) would be expected to become more and more an overestimation as n increases beyond 1, since on most surfaces with a potential barrier adiabatic barriers in the entrance channel become earlier as the initial excitation increases. It would be interesting to perform further tests of ICVT and PRP-ICVT calculations to study this possibility in general. C. Implications for Three-Dimensional Reactions. The results presented here indicate that the ICVT/LAG method should be reliable for the thermal 0 H2reaction, and the PRP-ICVT/LA method should be reliable for the 0 H2(n=l) reaction. In a separate paper, application of these two theories to the M2 potential energy surface, which reduces to the ModPolCI surface of collinear geometries but also includes a fit to a b initio bend potentials,1° yielded excellent agreement with experiment2v3for both k(T) and k(n=l,T). Similar good agreement was obtained by a reduced-dimensionality study employing the collinear ModPolCI potential energy surface modified by adiabatic
+
+
ground-state bending energy eigenvalues.1° For the DIM-RMOS surface, in contrast, TST rate constants with CEQ transmission coefficients predict a k(n=l,T) value that is 1.5 orders of magnitude larger than e ~ p e r i m e n t .The ~ present calculations provide a greater understanding of this result and lead to greater confidence in the conclusion that the M2 potential energy surface is consistent with experiment, whereas the DIM-RMOS one is not. V. Conclusions We conclude that variational transition-state theory with semiclassical tunneling corrections can be applied to calculate state-selected reaction rates of vibrationally excited systems provided one ignores adiabatic dynamical bottlenecks that occur after the first Occurrence in proceeding from reactants to products of an appreciable local maximum in the reaction-path curvature. Acknowledgment. This work at Chemical Dynamics Corp. was supported in part by the Army Research Office through Contract No. DAAG29-84-3-0011 and that at the University of Minnesota, Illinois Institute of Technology, and Argonne National Laboratory was supported in part by the U S . Department of Energy, Office of Basic Energy Sciences. Registry No. 0, 17778-80-2;H2,1333-74-0.
Time-Dependent Mass Spectra and Breakdown Graphs. 8. Dissociative Photoionization of Phenol Y. Malinovich and C. Lifshitz* Department of Physical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel (Received: February 26, 1986)
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Time-resolvedphotoionization mass spectrometry in the millisecond range has been employed to study the reaction C6HSOH’+ c-C5H,’+ + CO in phenol. Experimental photoionization efficiency curves and breakdown graphs at t = 6 ws and 2 ms were compared to those predicted by the statistical theory (RRKM/QET) and by previous photoelectron photoion coincidence spectrometry results. The experimental breakdown curves for 2 ms are the first to be obtained by photoionization for such a long reaction time in any system. A sensitivity analysis yielded the following activation parameters: critical energy of activation, E , = 67.6 0.9 kcal/mol, and entropy of activation, AS*(1000 K) = 2.2 1.2 eu.
*
Introduction Activation parameters (activation energy and activation entropy) are known for only a small number of unimolecular ionic dissociation reactions. A major problem in determining the critical energy (or activation energy) for several interesting reactions of some complexity has been the fact that the ions spend only a short time (several microseconds) in the flight tube of an ordinary mass spectrometer, while some of these reactions are very slow at near-threshold energies. As a result, an excess internal energy above the critical energy (the so-called “kinetic shift”) is required in order to observe them. The lowest energy dissociation channel of the phenol ion, reaction 1, is one such reaction. There are at present two experimental methods to circumvent the problem of kinetic shifts, which allow the determination of activation parameters and which have been applied to the phenol ion reaction. In the first method, the reactant ions are trapped for variable times and the appearance energy (AE) of the product ions is measured as a function of storage time.’ In the second method, the reaction dynamics of energy-selected reactant ions is studied by photoelectron photoion coincidence (1) Lifshitz, C.; Gefen, S.Org. Mass Spectrom. 1984, 19, 197.
0022-3654/86/2090-431 1$01.50/0
*
(PEPICO) spectrometry. The microcanonical rate coefficient,
k ( E ) , is determined experimentally as a function of energy over a range of energies, corresponding to ion lifetimes in the microsecond regime. An RRKM/QET model calculation is performed to fit these experimental data and this provides the activation energyS2 The measurements of time-resolved AE’s by electron impact (EI) demonstrated a large kinetic shift for reaction 1,’ indicating that the reaction is very slow at near-threshold energies. The appearance energy at the longest trapping time of 0.8 ms was 11.5 eV.’ The RRKM/QET calculation, which was in agreement with experimental dissociation rates between lo4 and 3 X lo6 s-l, predicts2 a 0 K onset of 11.59 eV and a 298 K onset of 1 1.46 eV, in excellent agreement with the E1 time-resolved AE at 0.8 ms. The agreement was considered* to be in part fortuitous due to the poor energy resolution of E1 experiments. Several additional intersting studies of the phenol ion system have appeared. These include multiphoton ionization mass spectrometry3 and multiphoton ionization photoelectron spectroscopy: the latter study resulting in vibrational frequencies for (2) Fraser-Monteiro, M. L.; Fraser-Monteiro, L.; de Wit, J.; Baer, T. J . Phys. Chem. 1984, 88, 3622.
(3) Pandolfi, R. S.;Gobeli, D. A.; Lurie, J.; El-Sayed, M. A. Laser Chem. 1983, 3, 29. Yang, J. J.; El-Sayed, M. A,; Robentrost, F. Chem. Phys. 1985, 96. 1.
0 1986 American Chemical Society
4312 The Journal of Physical Chemistry, Vol. 90,No. 18, 1986
Malinovich and Lifshitz M
L'
-&-PRINTER
Figure 1. Time-resolved photoionization mass spectrometer (TPIMS). The vacuum-UV source is a Hinteregger hydrogen discharge lamp; the monochromator is a McPherson Model 225 1-m, normal-incidence instrument, the quadrupole mass spectrometer is an EA1 QUAD 11 10, and the computer is an Industrial Micro Systems, I M S 5000, microcomputer: PMT, photomultiplier; C E M , channeltron electron multiplier; CCPL, computer control protection line; HUCC, Hebrew University computer center; PG, pulse generator. In the present experiments a pulse was applied from pulse generator PGB to the end cap nearest to the mass filter to eject ions after a predetermined storage time; the other end cap was grounded.
the phenol cation. Charge-stripping spectra have demonstrated5 that the C5H6'+ ion, formed in reaction 1 from phenol ions having lifetimes of the order of microseconds, has the cyclopentadienyl structure. Time-resolved kinetic energy releases for metastable phenol ions6 have suggested that reaction 1 does not take place on a single potential energy surface; rather a surface crossing (a geniune ?ne or a conical intefsection) takes place between the ground (X2B,) and excited (A2A2)states of the phenol cation. We have developed recently the method of time-resolved photoionization mass spectrometry (TPIMS), which combines the excellent energy fesolution of photoionization (PI) with time resolution and ion storage up to milliseconds. In the present study we have applied this method to reaction 1. W e foresaw several advantages in applying TPIMS to the phenol problem: (a) Determination of the appearance energy of C5H6'+ for long-lived phenol ions with good energy definition might be able to point out whether the agreement between the time-resolved E1 experiment' and the PEPICO experiment2 was fortuitous or not. (b) Until now our detailed studies by TPIMS involved the halobenzene ion reaction^,',^ which take place on a single potential energy surface (the ground-state one) and possess no back-activation energy and the RRKM/QET formalism with a loose (orbiting) transition state applies to them extremely well. In the halobenzene systems the TPIMS data were fitted with the same (or very similar) activation parameters (activation energies and entropies) as the rate-energy dependences, which had previously been determined by PEPICO in the microsecond regime. Reaction l possesses a large back-activation energy and presumably does not take place on a single potential energy surface. It is interesting to find out whether activation parameters that fit the rate-energy dependence in the microsecond regime can be extrapolated to threshold energies and whether an RRKM/QET formalism, without modification, applies to this system over a wide range of energies. (4) Anderson, S.L.;Goodman, L.; Krogh-Jespersen, K.; Ozkabak, A. G.; Zare, R. N.; Zheng, C.-fa. J . Chem. Phys. 1985, 82, 5329. (5) Flammang, R.; Meyrant, P.; Maquestiau, A,; Kingston, E. E.; Beynon, J. H. Org. Mass Spectrom. 1985, 20, 253. (6) Lifshitz, C.; Gefen, S.; Arakawa, R. J . Phys. Chem. 1984, 88, 4242. (7) Malinovich, Y . ;Arakawa, R.; Haase, G.; Lifshitz, C. J . Phys. Chem. 1985, 89, 2253. ( 8 ) Malinovich, Y . ;Lifshitz, C. J . Phys. Chem. 1986, 90, 2200
Figure 2. Schematic drawing of the cylindrical ion trap (CIT) and its connections to the monochromator (M), to the photomultiplier (PMT), and to the quadrupole mass filter (QUAD). E, earthed end-cap electrode; D, draw-out end-cap electrode; W, sodium salicylate coated window; L, ion lens. The gas enters the cell through the top of the cylindrical electrode. The two end-cap electrodes each have a central opening covered with wire mesh. The mesh on the draw-out end cap is electrically isolated and can be pulsed independently from the outer part. In the present experiments the pulse from PGB was applied to both parts of D (see Figure 1).
Experimental Procedures The method of time-resolved photoionization mass spectrometry (TPIMS) has been described in detail.'-" A block diagram of the experimental setup with some minor modifications is shown in Figure 1. Photoionization is induced by a pulsed vacuumultraviolet light source-a Hinteregger discharge in hydrogen producing the many-line spectrum. Photoions are trapped in a cylindrical ion trap (CIT).I2 They are ejected into the quadrupole mass filter by a draw-out pulse, following a variable delay time. Ions can be stored from microseconds to milliseconds. The characteristic dimensions of the CIT employed (Figure 2 ) are rl = 2 cm and z1 = 1.5 cm. The radio frequency of the potential applied to the cylindrical barrel electrode is 0.5 MHz (Q/2?r). The rf voltage chosen for the experiments is a t the crossing point of the parent-daughter stability curves,' ensuring equal trapping efficiency, which in the case of the C6H50H'+/ C5H6'+pair was at a peak-to-peak voltage of 720 V. In addition to the ac voltage, a 10 V dc bias was applied to the barrel electrode. Computer control of the photoionization mass spectrometer and automatic data acquisition were employed as previously described.' The data of ion and light intensity counts and photoionization efficiency (PIE) are obtained on the computer screen in real time as is the PIE curve, which is accumulated on an oscilloscope. Each computer-controlled experiment is made up of many repetitive cycles. In each cycle a series of preselected wavelengths (Le., photon energy points) is scanned for the parent and daughter ions and the time intervals are chosen so as to obtain a nearly constant signal-to-noise ratio a t each energy point. A typical cycle takes between 1500 and 2000 s, ensuring minimal errors due to slowly varying parameters (e.g., sample pressure) during a cycle. The entrance and exit slits of the monochromator were 500 and 600 pm, which gave an effective resolution of 5.0 A. This corresponds to an energy resolution near the threshold for C5H6'+ formation from phenol of -0.05 eV. Sample pressures in the CIT were kept as low as possible, in order to avoid bimolecular processes. The pressures employed (9) Lifshitz, C.; Goldenberg, M.; Malinovich, Y.; Peres, M. Org. Muss Spectrom. 1982, 17, 453.
(10) Lifshitz, C.; Goldenberg, M.;Malinovich, Y . ;Peres, M. Int. J . Muss Spectrom. Ion Phys. 1983, 46, 269. (1 1) Lifshitz, C.; Malinovich, Y. In?. J . Muss Spectrom. Ion Processes 1984, 60, 99. (12) Mather, R. E.; Waldren, R. M.; Todd, J. F. J.; March, R. E. I n r . J . Mass Spectrom. Ion Phys. 1980, 33, 201.
The Journal of Physical Chemistry, Vol. 90, No. 18, 1986 4313
Dissociative Photoionization of Phenol
40
I
2 mS
C,H,OH?
6PS
0 0
a 20
401 20
-201 8
'
I
I
I
9
IO
II
I 12
13
PHOTON ENERGY ( e V )
in experiments with short storage times, (approximately microseconds) are estimated to be mbar. Those employed at long storage times (approximately milliseconds) are estimated to be 10" mbar. The samples were introduced into the CIT directly (see Figure 2) contrary to previous experiments,8 in which they were introduced through the side line leading from the monochromator to the CIT. This change led to a considerable reduction in collisionally activated dissociation at constant pressure. When the sample is introduced through the side line, ions are formed in a relatively high-pressure region and are drawn into the CIT by the negative high-voltage phase of the rf voltage causing dissociation. Contrary to previous it was found to be unnecessary to subtract any background from the experimental PIE curves of the daughter ions. Chemicals were commercially available and employed without further purification. Their purity was checked by E1 mass spectrometry. C 6 H 5 0 Dwas from MSD Isotopes Canada with a stated isotopic purity of 98.2 atom % D.
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Results and Discussion Figure 3 represents the PIE curves for parent (C6HsOH'+) and daughter (CsH6'+) ions of reaction 1 a t a storage time of 2 ms over a wide photon energy range. This experiment was carried out at a low sample pressure of a! 10" mbar. In order to obtain adequate statistics, the experiment was run for 11 days and 332 repetitive scans were obtained. Each daughter ion point was measured for 5000 s in the energy range 11.9-13.0 eV, 9380 s in the range 11.65-11.9 eV, 17 180 s in the range 11.4-11.65 eV, between 17 180 and 13 140 s in the range 11.0-1 1.4 eV, and 13 140 s in the range 8.0-1 1.4 eV. Without expansion of the plot near threshold the daughter ion appearance energy is 11.8 eV in excellent agreement with the value predicted by Fraser-Monterio et ala2for microcanonical rate coefficients of 1 < k ( E ) < 10 s-I which are in the range of our detection limit for storage times of the order of milliseconds. Figure 4 represents the PIE curves for the daughter ion of reaction 1 at two reaction times: 6 ~s and 2 ms, respectively. The onset of the steeply rising part of the PIE curve is clearly observed to shift to higher photon energies, as the parent ion residence time is shortened and low rate coefficients can no longer be sampled. The dissociation threshold on an expanded scale at 2 ms is clearly less than 11.8 eV. We investigated next the effects of increased sample pressures on the low-energy onset of C5H6'+. The results of daughter PIE values at selected photon energy points, at a storage time of 2 ms, are represented in Figure 5. While there is some uncertainty in the estimated pressures, they scaled correctly as the parent PIE values at photon energies above the phenol ionization energy and below 11 eV. The daughter PIE values were therefore normalized in Figure 5 according to the parent PIE values below 11 eV. the
-
12.0
11.5
11.0
Figure 3. Time-resolved experimental parent and daughter PIE curves at t = 2 ms: see text for details.
12.5
13.0
PHOTON ENERGY ( e V )
Figure 4. Time-resolvedexperimental daughter (C5H;+) PIE curves; (0) 6 p s ; (0) 2 ms. The relative intensities of the expeirmental PIE curves of the parent served to scale the daughter PIESat the two storage times, without any background subtraction. The error bars indicated are es-
timated from the statistics of the ion counts.
=2mS
t
CsHg'
Pressure D e p e n d e n c e n
A
o
A *o
I
11.00
a0
11.25
11,'50
11.75
12.00
12'.25
I
12:50
PHOTON E N E R G Y ( e V )
Figure 5. Pressure dependence of the daughter PIE curve. The parent ion counts at La scaled in the following way: (A) 1; (0) 1.78; ( X ) 20.2. The sample pressure in the lowest pressure experiment (A)is estimated to be - I X 10" mbar.
Normalized C5H6'+ PIE values between 11.O and 1 1.8 eV were hardly affected by increasing the sample pressure, showing a slight increase with decreasing pressures. This increase with decreasing pressure is more pronounced at higher photon energies (Figure 5). We have come to the following conclusion: Collisionally activated dissociation is less important in the case of phenol than in the case of iodobenzene,* possibly because reaction 1 is not a simple bond cleavage. The low-energy onset (at photon energies
