J . Phys. Chem. 1988, 92, 6548-6553
6548
Time-Dependent Mass Spectra and Breakdown Graphs. 11. Time-Resolved Ion Momentum Spectrometry of Anisole V. Aviyente,+ M. Elam, N. Ohmichi, and C. Lifshitz* Department of Physical Chemistry and The Fritz Haber Research Center for Molecular Dynamics, The Hebrew University of Jerusalem, Jerusalem 91 904, Israel (Received: March 17, 1988)
Kinetic energy release distributions (KERDs) were determined for the unimolecular elimination of formaldehyde from the molecular ion of anisole. The experimental technique involves ion trapping in an electron space charge combined with time-resolved ion momentum spectrometry. The range of ion lifetimes studied is -7-400 p.. The KERDs are bimodal, and the relative contribution of the two subdistributions is time dependent. The experimental results were modeled by RRKM-QET calculations. These calculations were performed on the basis of two competitive hydrogen-transfer mechanisms suggested by Cooks and co-workers: (1) a four-membered cyclic transition state and (2) a five-membered cyclic transition state. Agreement with experiment is obtained for the following model: (a) critical energy of activation and 1000 K activation entropy for mechanism 1, Eo(l) = 56.5 kcal/mol; AS*loooK( 1) = 5.3 eu; (b) critical energy of activation and 1000 K activation entropy for mechanism 2, E0(2) = 63.4 kcal/mol, hs*loooK(2) = 11.35 f 0.35 eu. The results are discussed in light of previous data on time-resolved kinetic energy releases.
Introduction A considerable degree of information has been obtained in recent years concerning structures and dynamics of gas-phase ions.’q2 Much of this work has centered around “metastable Le., those ions that decompose during flight in field-free regions of single- or double-focusing mass spectrometers. Typical accelerating voltages and dimensions of mass spectrometers employed in such studies define ion lifetimes of metastable ions in the range of several microseconds. However, many decompositions of plyatomic ions having critical energies, E,,, of several electronvolts have lifetimes at near threshold energies in excess of milliseconds. These dissociations remain undetected in ordinary mass spectrometers. We have developed in recent years several ion-trapping techniques designed to study long-lived ions at near threshold energies. One of these techniques involves trapping of positive ions in the negative potential well created by an electron space charge. We have combined this technique with an MS/MS technique, time-resolved ion momentum spectrometry (TRIMS).4s5 TRIMS with space charge trapping has been employed in the present study of the primary dissociation reaction of the anisole cation-radical: [C6H50CH3]*+ [C6H6]*+-k CHzO (1) This reaction was studied by us recently employing ion-trapping, using radio frequency voltages in a CIT (cylindrical ion trap).6 Time-resolved photoionization mass spectrometry in a CIT in the millisecond range has yielded6 the following activation parameters for reaction 1: critical energy of activation, Eo = 59.6 f 0.6 kcal/mol; entropy of activation, S I l o o o K = 7.25 f 2.2 eu. The study of metastable ions has demonstrated that reaction 1 proceeds by two competitive reaction channels for ions in the microsecond lifetime range.’ This is seen in the metastable peak shape, which demonstrates bimodality, Le., two kinetic energy release values. The photoionization efficiency curves of C&6’+ have not detected6 two energy thresholds. Reaction 1 has also been studied by threshold photoelectron-photoion coincidence (TPEPIC0)8 and by resonance-enhanced multiphoton ionization (REMPI).9 Modeling of the TPEPICO data8 and of the photoionization mass spectrometry data6 by statistical theories yielded conflicting activation parameters for reaction 1, but neither of the methods was sensitive enough to require the inclusion of two reaction mechanisms to fit the rate energy dependence of the microcanonical rate coefficients. In the present study we have extended the experimental study of metastable ions due to reaction
1 into the lifetime range of several hundred microseconds. The new data require two mechanisms, and we have modeled the bimodality of the kinetic energy release distribution by RRKMQET calculations.
Experimental Section The technique of trapped-ion mass spectrometry, which employs ion trapping in an electron space charge, has been described in detail.I0 A continuous electron beam of 3-5 eV and 5 - 1 0 - ~ A current, provided by thermionic emission from a rhenium filament, is used to trap the ions produced when a pulse of variable height, typically 30 V (negative with respect to the ionization chamber) and 2-ps duration is applied to the filament. At a known and variable time after the ionizing pulse, a positive voltage pulse is applied to a repeller electrode to remove ions for mass analysis. Time-resolved ion momentum spectrometry is an MS/MS method developed by Enke and c o - ~ o r k e r s . ~ ~The - ~ ~method applies superposition of ion momentum analysis by magnetic field scanning with time-of-flight analysis through ion source pulsing and time-resolved detection. This is achieved by measuring ion flight times through a modified single magnetic sector mass spectrometer. Ions with mass m l + that dissociate metastably in the first field-free region to m2+ + m3 give rise to ions with a . ions have a lower velocity nominal mass3 m* = m 2 2 / m 1 Such than stable ions that might appear at the same mass setting; their time-of-flight (TOF) is in fact equal to that of the parent ion, tm. = tml. Employing only a magnetic sector instrument, the spectra of stable ions and metastable ions are superimposed. Ion source pulsing and time-resolved detection allow separate observation of stable ions and metastable ions at a particular magnetic field
+
( 1 ) Lifshitz, C. Int. Rev. Phys. Chem. 1987, 6, 3 5 .
(2) Lifshitz, C. Mass Spectrometry, Spec. Per. Rep. 1987, 9, 1. (3) Cooks, R. G.; Beynon, J. H.; Caprioli, R. M.; Lester, G. R. Metastable Ions; Elsevier: Amsterdam, 1973. (4) Lifshitz, C.; Gefen, S.; Arakawa, R. J. Phys. Chem. 1984, 88, 4242. (5) Aviyente, V.; Shaked, M.; Feinmesser, A,; Gefen, S.;Lifshitz, C. Int. J . Mass Spectrom. Ion Processes 1986, 70, 67. (6) Ziesel, J. P.; Lifshitz, C. Chem. Phys. 1987, 117, 227. (7) Cooks, R. G.; Bertrand, M.; Beynon, J. H.; Rennekamp, M. E.; Setser, D.W. J. Am. Chem. SOC.1913, 95, 1732. (8) Das, P. R.; Gilman, J. P.; Meisels, G. G. Int. J . Mass Spectrom. Ion Processes 1986, 68, 155. (9) Chang, T. C.; Johnston, M. V. J . Phys. Chem. 1987, 91, 884. (10) Lifshitz, C. Mass Spectrom. Rev. 1982, 1 , 309. (1 1 ) Stults, J. T.; Enke, C. G.; Holland, J. F. Anal. Chem. 1983, 55, 1323. (12) Stults, J. T.; Myerholtz, C. A,; Newcome, J. H.; Enke, C. G.; Holland, J . F. Rev. Sci. Instrum. 1985, 56, 2267. (13) Stults, J. T.; Holland, J. F.; Watson, J. 7.; Enke, C. G. Int. J. Mass Specfrom. Ion Processes 1986, 71, 169.
‘Permanent address: Chemistry Department, BoBaziGi Universitesi Bebek, Istanbul, Turkey.
0022-365418812092-6548$01.50/0
b 1988 American Chemical Societv
1-
The Journal of Physical Chemistry, Vol. 92, No. 23, 1988 6549
TRIMS of Anisole
0.25 0.20
'.
I
Hall probe
(a 1
0.15 a
0.10
0
-2 ~ e i o yt i m e setting I
$
I
-0
0.05
0 .c
I
e
0.03
(b)
C
Figure 1. Schematic drawing of the experimental setup that combines MS/MS by TRIMS with ion trapping in the ion source by an electron
space charge.
n 0)
0
value (ion momentum) due to their different flight times. The daughter ions will have the flight time of their parents ( m l )and a mass determined by (mlm*)1/2.The theoretical background of TRIMS has been developed by Stults et al." The setup that combines ion trapping in an electron space charge with the T R I M S method was employed as described in previous publication^.^-^ A schematic drawing of the instrument is shown in Figure 1. The magnet selects a particular momentum while a gated window in the detection circuit selects a particular velocity (flight time). The magnetic field strength was taken from the digitized output of a Hall probe. A boxcar averager and gated integrator (Evans Associates, Berkeley, CA) were used to make time-resolved measurements. The data/control system has been described.5 A trigger from the ion repeller pulse initiates a programmable delay generator. The gated integrator, triggered by the delay generator, integrates over a window that is adjustable to 30 ns. Averaging of multiple pulses is achieved in the gated integrator. The sampled signal and the Hall probe voltage measurements are transferred to a microcomputer through high-resolution V / F converters. The magnet controller is interfaced to the computer via a stepping motor unit. The gain in ion intensity was enhancedI4 by adding a Mech-Tronic Nuclear Model 500 amplifier, thus achieving usable signals for formaldehyde loss from the parent ion of anisole, for ion storage times up to 400 fis.
An experiment is performed as follows. A certain delay time between the ionizing pulse and repeller pulse is selected. A magnetic field strength is selected and a TOF arrival distribution is obtained. The magnetic field is advanced by means of the stepping motor, and a new T O F sweep obtained. The magnetic field is scanned through the metstable peak; for each value of B, the magnetic field strength, that value and the maximum intensity of the TOF sweep are stored in the computer. Any desired number of B scans may be summed by the computer to obtain a good signal-to-noise ratio in the metastable ion peak shape. This process is repeated for the normal parent ion of the metastable decomposition under study. The whole cycle of events is again repeated for a new preselected delay time between the ionizing and repeller pulses. The kinetic energy release (KER), T, is obtained from3
where m 1is the mass of reactant ion, Vis the accelerating voltage, d is the metastable peak width in mass units (corrected for the normal peak width5), m2 is the mass of the product ion, and m3 is the mass of the product neutral. Metstable ions dissociating in the field-free region of the instrument do not possess a well-defined energy; as a result a range of rate coefficients, k , is sampled. Figure 2 represents the cal~~
.-
0
3+
0.02
a
E
0.01
4 5 (sec-I 1 Figure 2. Curves showing the fraction of ions detected as metastable ions, as a function of the rate constant for dissociation of a mass 108 ion at different ion storage times. Storage times corresponding to a decreasing order of fractional abundance are (a) 5, 10, 20, 30, and 45 ps, respectively, and (b) 60, 75, 80, 90, 120, 150, 180, 200, 250 and 300 p s , respectively. I
2
3
log k
culated fractional abundance of ions detected as metastable anisole ions of nominal mass m* = 56.33, as a function of k, the rate constant for unimolecular decomposition, at different storage times in the ion trap. These calculations were performed on the basis of the known residence times in the various regions of our mass spectrometer for a m / z 108 ion, in a fashion explained by ChupkaI5 and computed earlier in the case of phenol: for the shorter storage times available to us previously. The distribution of sampled rate coefficients shifts to lower k's with increasing storage time as is to be expected, the most probable k being 1.07 X lo5 s-l at t = 5 p s and 2.60 X lo3 s-l a t t = 400 p s . Several problems arise in ion-trapping experiments. We have considered the possible contributions of (a) collisional relaxation in the ion source and (b) collision-induced dissociation in the field-free region. Metastable peak shapes were determined for several ion-source and field-free region pressures at a constant storage time. While collisional relaxation and collision-induced dissociation cannot be entirely ruled out, the results to be presented were found to be pressure independent within the pressure range studied, namely, - 5 X 10-'-5 X 10" Torr in the field-free region Torr in the ion source. and - 5 X 10"-5 X The energy resolution of a single-focusing instrument is considerably lower than that of a double-focusing instrument, which is customarily used nowadays for KER measurements and which includes a large electric sector with narrow slits. However, for the present reaction system, the correction for the parent (main) ion peak width amounted to no more than 2-476. Thus, for a typical experiment at t = 5 ps storage time, the width at half height of the main beam is amain = 0.183 mass units, while that for the metastable peak is am.= 0.819 mass units and the corrected square width at half-height is calculated f r 0 m ~ 9 ~ do.52= - wmain2m2/ml (3) to be 0.646, while the correction itself is only 0.024. Since dL enters
~~
(14) Lifshitz, C.; Aviyente, V.;Ohmichi, N.; Elam, M. Biomed. Emiron. Mass Speclrom., in press.
(15) Chupka, W. A. J . Chem. Phys. 1959, 30, 191.
6550
Aviyente et ai.
The Journal of Physical Chemistry, Vol. 92, No. 23, 1988
Figure 3. Metastable ion peak shapes at tn* = 56.3 (=78*/108) as
function of storage time. The anisole dissociation, reaction I , takes place in the field-free region of the magnetic sector instrument shown in Figure 1: (a) 5-ps storage time; (b) 200 p. the expression for T (eq 2), the source of error due to this contribution is not serious. The most serious source of error is the incomplete T O F resolution of normal (ion source) daughter ions at the wings of the metastable peak. This is not apparent at relatively short delay times. A typical metastable peak shape for a storage time of 5 F S is reproduced in Figure 3a. The shoulder on the high-mass side of the peak has a contribution from the I3C isotopic transition of reaction 1, 109’ 79’. At long storage times, the relative abundance of the metastable peak at m* = 56.33 drops by several orders of magnitude (see calculation, Figure 2 , and following sections) and any tailing or overshoot in the T O F arrival distribution of normal (ion source) daughter ions at the TOF position characteristic of m* becomes much more apparent. This is seen in a typical experimental result at a storage time of t = 200 ps (Figure 3b), where m / z = 58 is particularly conspicuous. The analysis of metastable peak shapes relied on the left side of the peak following judicious base-line correction.
0 0
I
4.7 200
I
400
1
I
I
600
800
T (meV)
Figure 4. Two normalized MB distributions derived from the data of Figures 5 and 6 (see text and ref 21 and 22) (- - -) and the reconstructed KERD (-) compared with the experimental KERD (0),(a) at 5 pus and (b) at 200 ps.
SCHEME I
-
Results and Discussion ( a ) Experimental Kinetic Energy Release Distributions (KERDs). Metastable peak shapes were determined as a function of ion storage time for reaction 1. The metastable peak shapes demonstrate two kinetic energy release components, as observed previ~usly.~ Kinetic energy release distributions (KERDs) were obtained from the first derivatives of the metastable ion peak shapes.ib1s The derived KERDs are bimodal, as expected and as previously observed for halo an is ole^.'^ Typical results are presented for t = 5 ps and t = 200 p s in Figure 4 (open circles). The kinetic energy releases obtained from the metastable peak widths at half-height, To,,,were observed to increase with increasing ion storage time. For example, To,5at t = 5 gs is 94 meV (16) Holmes, J. L.;Osborne, A. D. Int. J . Mass Spectrom. Ion Phys. 1971,
23, 189. (17) Lifshitz, C.; Tzidony, E. Int. J . Mass Spectrom. Ion Phys. 1981, 39, 181. (18) Jarrold, M. F.; Wagner-Redeker, W.; Illies, A. J.; Kirchner, N. J.; Bowers, M.T. Int. J . Mass Spectrom. Ion Processes 1984, 58, 6 3 . (19) Reiner, E. J.; Harrison, A. G . Int. J . Mass Spectrom. Ion Processes 1984, 58, 91.
and is 171 meV at 250 ~ s .These observations indicate that formaldehyde elimination from anisole is brought about by two competitive reaction mechanisms and/or formation of different isomers of the product ion as suggested The two suggested mechanisms involve (see Scheme I) a four-membered cyclic hydrogen-transfer transition state and a five-membered cyclic transition ~ t a t e . ~ Previous .’~ experiments7 were unable to detect a variation in the relative abundance of the two competitive reaction channels with changing experimental conditions, for example, with ionizing electron energy and temperature. Even when the anisole molecular ion was generated by fragmentation from different precursor molecules, the metastable peak shape was identical with the one obtained for the ion produced by direct ionization of neutral anisole. Only substitution at various positions of the aromatic ring by different substituents had an effect on the relative contribution of the two suggested mechanism^.'.'^ All of these studies sampled a similar range of ion lifetimes, namely, microseconds. It is to be expected, and indeed is observed in the present study, that changing the lifetime of the ion should affect the relative contribution of the two mechanisms. The schematic potential energy profile suggested by Cooks et al.’ predicts different critical activation energies, Eo, and different endothermicities for the two mechanisms. Furthermore, if indeed the two mechanisms involve a four-membered and five-membered transition state, respectively, then they are also expected to have different activation entropies, AS’. This should be generally true even if these are not the exact two mechanisms. It is thus expected that their
The Journal of Physical Chemistry, Vol. 92, No. 23, 1988 6551
TRIMS of Anisole
loo1
1
I
3.0 I
1 200
I
I
400
600
800
T(meV)
Figure 5. Log plot of w / T 1 I 2vs translational energy, T. w is the experimental energy distribution for reaction 1 at a storage time of 5 ps ( 0 )and 200 ps (A), respectively.
-p
I
0
IO
100 1000 Storage time ( p s )
Figure 7. Experimental (+) and calculated (-) percent contribution of mechanism I (the high kinetic energy release component) to the meta-
stable ion intensity as a function of ion storage time (microseconds, logarithmic scale). Two of the experimental points are shown with estimated error limits. The calculated curves correspond to models a and b of Table I.
0
200
400 T (rneV)
600
Figure 6. log plot of w / T ' / vs ~ translationalenergy, T. The KERD w( 7') is assumed to consist of two Maxwell-Boltzmann (MB) distributions, N l ( T ) and N2(T) (see text and ref 20 and 21). The parameters PI,P2, B l , and Bz of the MB distributions are obtained from the slopes and intercepts of the two straight lines that make up the logarithmic plot, by assuming that the MB distribution, Nz(T),with the low average trans-
lational energy, does not contribute to the high energy points of ~ ( 7 ' ) . rate-energy dependence, i.e., the dependence of the microcanonical rate coefficient, k ( E ) ,upon internal energy, E , should differ. To test this assumption, we had to derive the relative abundance of the two mechanisms as a function of ion storage time from the experimental KERDs (section b) and to model these relative abundances through RRKM-QET calculations (section c). ( b ) Experimental Data Treatment. Kinetic Energy Release Distributions. Bimodal KERDs can be synthesized by using two three-dimensional (3-D) Maxwell-Boltzmann (MB) distributions:20s21 4 T ) = NI(T) + N2(T) =
exp(-BIT)
+ P2T'I2exp(-B2T)
(4)
Where w( T ) is the experimental distribution, N , ( T ) is the broad high-energy component, and N2(T ) is the narrow low-energy component. The parameters P , , P2, B I ,and B2 can be derived22 ~) T. The resultant logarithmic plots by plotting (In ( w / T ' / versus for t = 5 and 200 p s are shown in Figure 5 . The plot for t = (20)Lifshitz, C. Int. J . Mass Specrrom. Ion Phys. 1982, 43, 179. (21) Lifshitz, C.; Berger, P.; Tzidony, E.Chem. Phys. Lett. 1983, 95, 109. (22) Johnson, K.; Powis, I.; Danby, C. J. Chem. Phys. 1981, 63, 1.
200 ps is redrawn in Figure 6 together with the two logarithmic representations for the subdistributions. The two resultant MB distributions derived from these data and from the analogous data for t = 5 ps are included in Figure 4 as are the reconstructed KERDs, which are compared with the experimental KERDs. This type of data treatment allows one to obtain the relative abundances of the two subdistributions from the areas ( A i ) beneath their curves, Ai= Pi7r1/2/2B?/2 and to obtain their average translational energies, since ( T i ) = 3 / 2 B;l, i = 1 or 2. While there is some scatter in the data, the average energies remain constant with increasing storage time, at ( T , ) = 620 f 80 meV and ( T 2 ) = 71 f 14 meV, respectively. On the other hand, the percent relative abundance of the high kinetic energy release component demonstrates a definite rise with increasing storage time at the expense of the low kinetic energy release component (Figure 7). (c) RRKM-QET Modeling. The experimental observation that the relative contribution of the high kinetic energy release component to the metastable ion intensity is more pronounced with increasing ion storage time, Le., when the ion lifetime is longer, indicates that the corresponding reaction channel has a lower critical energy of activation than the low kinetic energy release component. This is in agreement with the mechanism suggested by Cooks et al.' (Scheme I), according to which the high kinetic energy release component involves a four-membered hydrogentransfer cyclic transition state producing the benzene cation radical while the low kinetic energy release component involves a fivemembered transition state leading to a less stable isomeric C6H6'+ cation radical. RRKM-QET modeling of the experimental results was carried out by calculating (1) the rate energy dependences for the two competing mechanisms and (2) the metastable ion breakdown curves on the basis of these k(E) dependences. Several constraints were imposed on the models chosen, in addition to trying to get agreement with the experimental data of Figure 7: (a) A microcanonical rate coefficient of k ( E ) zz IO6 s-l is appropriate for an energy transfer of 12.05 eV by the ionizing particle. This is
6552
The Journal of Physical Chemistry, Vol. 92, No. 23, 1988
Aviyente et al.
TABLE I: RRKM-QET Modeis crit activation energy Eo,
mechanism
kca I / mol
I
56.5
I1
63.4
transition-state vibr freq," cm-' C-0 bend: 100 C-0-C bend: 60 C-0 deformation: 264 C-0 bend: 40 C-0-C bend: 20 C-0 deformation: 80,b 110'
equiv 1000 K activation entropy hs', eu
reactn path degeneracy u
5.3
3
11 .7b
6
1 1.O'
OAll the other vibrational frequencies of the transition state were left equal to those of the reactant ion, which were taken to equal those of the neutral.24 bModel a. CModelb.
a
m
0 (L
a 0.015
-
0.010IO 8
/I 2
I/ 6
12 0
12 4
E N E R G Y , eV Figure 8. Calculated microcanonical rate coefficients (logarithmic scale) versus ionizing energy for the two parallel mechanisms of Scheme I; curve 1, four-centered transition state; curve 2, five-centered transition state, model (a) of Table I. The curves are calculated for 0 K. The experimental point and error bars is from the TPEPICO study of ref 8.
0.005-
$0 based on coincidence data by Das et a1.* (b) The appearance energy of C6H{+ from long-lived anisole cation radicals is6 10.85 f 0.05 eV; as a result the sum of the ionization energy of anisole,6 IE = 8.20 eV, plus the critical energy of activation for reaction 1 should not be too different from 10.85 eV. (c) The model should be able to reproduce the time-resolved photoionization efficiency curves obtained previously6 for reaction 1 to within experimental accuracy. The microcanonical rate coefficients k ( E ) were calculated as a function of energy by a RRKM p r ~ p r a mas , ~described ~ previously.6 The vibrational frequencies of the reactant ion were taken equal to those of neutral anisole.24 The phenyl C-0 stretch was taken to be the reaction coordinate. The C-0 bend (508 cm-I), C-0-C bend (300 cm-I), and C - O deformation (264 cm-I) were varied in a systematic way in the transition state together with the value of Eo-the critical energy of activation. The reaction degeneracy was taken to be u = 3 for mechanism I and n = 6 for mechanism 11. Resultant rate energy dependences for one model calculation (model a) are reproduced in Figure 8. Breakdown curves give the fractional abundance (or "probability") of an ion as a function of the internal energy in the molecular ion. Breakdown curves were calculated for the metastable ions due to the two competitive mechanisms of Scheme (23) Hase, W. L.; Bunker, D. L. A General RRKM Program, Chemistry Department, Indiana University, Bloomington, QCPE, No. 234.
I
1
I
65
70
75
I
so
I
I
I
85
90
95
E (kcal/mol)
Figure 9. Calculated metastable ion breakdown curves as a function of ion storage time; mechanism I (-), mechanism I1 (---), fractional abundance (probability)as a function of internal energy, E. The storage times are (decreasing order of probability) (a), 5 , 10, 20, 30,45, 60,and 75 and (b) 80, 120, 200, and 400 ps, respectively.
I. The following expressions, based on first-order kinetic^,'^ were employed: Pm,*(E) =
(5) where 1 and 2 stand for the four- and five-membered transition states, respectively, E is the internal energy, P is the probability (fractional abundance), t, is the storage time, t l is the time spent in the ion source in addition to t,, t2 is the time spent in the acceleration region of the electric field, and t3 is the time spent in the field-free region. The characteristic times calculated for the anisole ion at m / z = 108 are t l = 2 ws, t 2 = 0.45 ps, and t3 = 4.94 w s . The largest
The Journal of Physical Chemistry, Vol. 92, No. 23, 1988 6553
TRIMS of Anisole error ( f l ps) enters into t , , since the ionizing pulse is 2 ps long. This error is relatively more serious in calculating the ion lifetimes, for short storage times, than for long storage times. To obtain agreement with the experimental metastable ion abundances, it was necessary for the critical energies of the two-component mechanisms to differ by 0.3 eV and for the low kinetic energy release component to possess a looser transition state (an activation entropy higher by -6 eu; see Table I) than the high kinetic energy release component. This latter attribute causw the two respective rate energy dependences to cross each other in the range of microcanonical rate coefficients k ( E ) lo6 s-' as is seen in Figure 8. The higher activation entropy of mechanism I1 is consistent with the formation of a more loose five-membered transition state as compared to the tighter four-membered transition state of mechanism I. The metastable ion breakdown curves for the two mechanisms calculated on the basis of model a for different storage times, t,, are given in Figure 9. The percent contribution of the two mechanisms was derived from the relative areas beneath the corresponding curves. The calculated metastable ion abundance of mechanism I is plotted versus the storage time, in Figure 7, for comparison with the experimental data. A second set of calculated abundances is presented in Figure 7 for a slightly different model-model b. The models differ only in the activation entropy of the low kinetic energy release component. The properties of the two models are summarized in Table I. As noted earlier6 the choice of transition-state frequencies is somewhat arbitrary. However, it is the entropy of the transition state that affects the rate energy dependence, rather than the specific choices of vibrational frequencies. Inspection of Figure 8 reveals why the threshold for mechanism I1 cannot be detected by photoionization efficiency curves. At the appropriate threshold energy the parallel reaction via mechanism I which has a 0.3 eV lower threshold is about 4 orders of magnitude faster. Scheme I is not necessarily the only mechanism in agreement with the rate energy dependences of Figure 8 and with the time
=
(24) Owen, N. L.; Hester, R. E. Spectrochim. Acta, Part A 1969,25, 343.
dependence of the percentage abundances of the two components of the metastable ion dissociation (Figures 7 and 9). An alternative scheme could involve nonadiabatic dissociations from different electronic states of the anisole cation radical.2s The mechanism proposed by Scheme I has the advantage that the effects of substituents can be rationalized>19and nonadiabatic behavior need not be invoked. Scheme I is not necessarily true in its entirety; Le., details of the mechanisms would still need to be worked out. Previous studies of time dependences of KERs have normally encountered a decrease in the KER with increasing ion lifetime.5-26-29 This type of behavior can arise from either the relationship between energy partitioning and the internal energy content for a single reacting configuration2' or from the presence of more than one reacting configuration2* or more than a single mechanism. The rise of the KER with increasing ion lifetime encountered in the present study of reaction 1 must be related to the presence of more than one mechanism. It may be a more general phenomenon that could also be encountered in other systems, since the mechanism having the lower critical energy of activation can very often be the one leading to a more stable product configuration and thus to a higher reverse activation energy and also to a higher KER. Acknowledgment. This research has been supported by the Basic Research Foundation administered by The Israel Academy of Sciences and Humanities, as well as by grants by the United States-Israel Binational Science Foundation (BSF), Jerusalem. Professors M. T. Bowers and R. C. Dunbar serve as American Cooperative Investigators for the BSF grants. Registry No. Anisole, 100-66-3. (25) Lorquet, J. C., personal communication. (26) Beynon, J. H.; Hopkinson, J. A.; Lester, G. R. Int. J. Mass Spectrom. Ion Phys. 1968, I , 343. ( 2 7 ) Lifshitz, C.; Gotchiguian, P.; Roller, R. Chem. Phys. Lett. 1983, 95, 106. ( 2 8 ) Holmes, J. L.; Burgers, P. C.; Terlouw, J. K. Can. J. Chem. 1981,59, 1805. (29) Burgers, P. C.; Holmes, J. L. Inr. J . Mass Spectrom. Ion Processes 1984, 58, 15.
