J. Phys. Chem. 1984, 88, 3622-3627
3622
be deduced from Table 11, in which the last two configurations will always oscillate at any coupling rate (since they are very near each other) while the middle configuration will be stable for some coupling rates. The nature of the nonlinearity is very different in the various oscillators. In the SOM it is quadratic, in the Brusselator it is cubic, while in the Kumar model it is exponential. In the Lotka model the nonlinearity is quadratic but the oscillations are conservative and not of the limit cycle type as the other oscillators. It seems therefore that neither the nature of the nonlinearity nor the nature of the oscillations has any bearing on the above problem.
It would be nice to have some general mathematical results regarding this problem. Until these are available, one should like to have some experimental confirmation to the presented results. This confirmation can come, of course, from the ceriumbromate-bromide system described by the N F T model or from the BZ system described by the SOM or the FKN model, and possibly from some system adequately described by the Kumar model. The main experimental difficulty may be in finding the particular range of coupling rates for which this phenomenon occurs. From the presented calculations it seems to occur well within experimental feasibility.
Dissociation Dynamics of Energy-Selected Phenol Ions Maria L. Fraser-Monteiro,+Luis Fraser-Monteiro,f Jos de Wit, and Tomas Baer * Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 2751 4 (Received: October 7, 1983)
The dissociation dynamics of energy-selected phenol ions (C6HsOH+)have been studied by photoelectron photoion coincidence (PEPICO) spectrometry. Photoionization appearance energies, absolute dissociation rates between lo4 and 3 X 1O6 8,and the kinetic energy release distributions were measured. The absolute rates and the energy released were compared to those predicted by the statistical theory (RRKMIQET). The ionization energy of phenol was found to be 8.47 eV which is 0.1 eV higher than a previous PES value. The 0 K onset for CO loss of 11.59 eV which produces the cyclopentadiene ion (CSH6+) was found by fitting experimental dissociation rates to those predicted by the statistical theory. The 3.12-eV activation energy is considerablyhigher than the 1.29-eV endoergicity. The reverse activation energy of 1.83 eV is a result of a strained transition state. Approximately 40% of this energy is released, in a broad distribution, as kinetic energy. The calculated onset for the subsequent dissociation of C5H6+to CsH5++ H is 12.7 eV. However, because of the slow rate of dissociation at low ion energies, the observed onset is 13.9 eV.
Introduction The lowest energy dissociation channel of the phenol ion produces the two stable species C5H6+ and CO, whose heats of formation are well-known.' It is evident that this reaction does not involve just a simple bond break, and this complexity is reflected in the 0.5-eV kinetic energy release which as been measured in a number of mass-spectrometric In addition, evidence for as many as five stable C6H60+ isomers has been obtained by collision-induced-dissociation(CID) studies of several C6H60+ ions generated by ionization or fragmentation of various samples.* However, that same study revealed that the kinetic energy released in the CO loss reaction is the same for all five isomers, suggesting that they dissociate via a common transition state. The mechanism for the C O loss reaction has been of considerable interest. The formation of the keto form (structure 2) from
1
2
the enol (structure 1) has been suggested by a number of worke r ~ ,although ~ , ~ this is not a universally accepted opinion.'** Even if the keto structure is involved in the reaction, it is certainly not the transition state. In fact, it is not clear whether the transition state should occur before or after the keto form. Although this reaction has now been well studied, there is still insufficient evidence on which to propose a definitive mechanism for C O loss. In addition, calculations of a lowest energy dissociation path seem Permanent address: Faculty of Science, University of Lisbon, Portugal. *Permanent address: Faculty of Science and Technology, New University of Lisbon, Portugal.
0022-3654/84/2088-3622$01.50/0
prohibitively expensive at this time for such a complicated reaction involving seven large atoms. Part of the problem in understanding the loss of C O from the phenol cation is the poor knowledge of the activation energy. The onset of C O loss has been measured in two electron impact ionization studies and found to be 11.5* and 12.5 eV,9 respectively. The only photoionization studies reported have been the photoelectron spectra (PES) of phenol,lO,llfrom which the adiabatic ionization potential has been determined to be 8.37 eV.'O A dissociation path which may be related to the formation of C5H6+is the production of C5H5+. This ion can be formed either by the loss of HCO from phenol or from the loss of H from C5H6+. The onset, as measured by electron impact mass s p e c t r ~ m e t r y , ~ ~ , ~ ~ is about 14 eV, which is well above the dissociation limit to either of the two neutral products. (1) H. M. Rosenstock, K. Draxl, B. W. Steiner, and J. T. Herron, J. Phys. Chem. Ref. Data, Suppl. I , 6 (1977). (2) C. B. Theissling, N. M. M. Nibbering, and Th. J. De Boer, Adu. Mass Spectrom., 5, 642 (1971). (3) M. K . Hoffman, M. D. Friesen, and G. Reichmond, Org. Mass Spectrom., 12, 150 (1977). (4) C. J. Porter, A. G. Brenton, and J. H. Beynon, In?.J. Mass Spectrom. Ion Phys., 35, 353 (1980). (5) D. H. Russell, M. L. Gross, and N. M. M. Nibbering, J. Am. Chem. SOC.,100, 6133 (1978). (6) D. H. Russell, M. L. Gross, J. van der Greef, and N. M. M. Nibbering, Org. Mass Spectrom., 14, 474 (1979). (7) A. Maquestiau, R. Flammang, G. L. Glish, J. A. Laramee, and R. G. Cooks, Org. Mass Spectrom., 15, 131 (1980). (8) A. Maquestiau, Y . van Haverbeke, R. Flammang, C. De Meyer, D. G. Das, and G. S. Reddy, Org. Mass Spectrom., 12, 631 (1977). (9) J. D. Henion and D. G. I. Kingston, J. Am. Chem. SOC.,95, 8538 (1973). (10) T. P. Debies and J. W. Rabalais, J. Electron Spectrosc. Relut. Phenom., 1, 355 (1972). (11) K. Kimura, S. Katsumata, T. Ishiguro, A. Y . Kirakawa, and M. Tsuboi, Bull. Chem. SOC.Jpn., 48, 2736 (1975). (12) S.Tajima and T. Tsuchiya, Bull. Chem. SOC.Jpn., 46, 3291 (1973). (13) R. G. Gillis, G. J. Long, A. G. Moritz, and J. L. Occolowitz, Org. Mass Spectrom., 1, 526 (1968).
0 1984 American Chemical Society
Dissociation Dynamics of Energy-Selected Phenol Ions I
I
I
I
I
I
The Journal of Physical Chemistry, Vol. 88, No. 16, 1984 3623 INTERNAL ENERGY (eV)
I
35
I
I
I
i
11031b21'0l
I
I
4
Ib 2
30
I
I
Figure 1.
C,&OH+
Photoionization efficiency (PIE) spectrum of phenol: ( m / r 94); C5H6+ ( m / z 66); and C&+ ( m / z 65).
io .......................................... I
11 5
We have undertaken this photoionization and PEPICO study of the phenol ion dissociation in order to establish precise values for the activation energies. In addition it is sometimes possible to extract information about the precursor, or the transition-state structures, from a comparison of the experimentally measured rates with those predicted by the statistical theory (RRKM/ QET).14 The important parameters in calculating the RRKM/QET rates are the activation energy and the vibrational frequencies of the molecular ion and the transition state. In some cases, it is necessary to assume an activation energy which is larger than the apparent experimental activation energy (the difference between ionization and fragment ion appearance en erg^),'^^'^ Such a situation is a clear indication that the parent ion isomerizes to a more stable structure prior to dissociation. To the extent that frequencies are related to the ion structure, it is also possible to draw conclusions about parent-ion and transition-state structures. However, the sensitivity is not very great so that only large changes in parent-ion and transition-state structures are apparent. Experimental Procedure and Results The photoionization apparatus, which has been described in detail previously,'' consists of a H2 low-pressure light source, whose "many-line" continuum is dispersed by a 1-m normal incidence monochromator. The 200-pm slits provided a photon resolution which varied between 0.012 eV at 9.5-eV photon energy and 0.026 eV at 14-eV photon energy. Ions are formed in an ionization/ acceleration region pressurized to about 5 X torr, while the main chamber is maintained at below 2 X 10" torr. Collimated hole structures placed on both sides of the ionization region allow both the ions and electrons to be extracted and collected. Photoionization-Efficiency (PIE) Spectra. A PIE spectrum is a plot of the mass-analyzed ion signal vs. the photon energy. For these experiments, ions were mass analyzed by a quadrupole mass filter and collected by a channeltron electron multiplier. The ion signal was normalized to the photon intensity. The counting time per datum point was between 40 and 100 s. About 1.2 points per angstrom were collected for most of the runs. Figure 1 shows the PIE spectra for the parent phenol ion and the two fragments which are produced below 14 eV. In favorable cases it is possible to obtain precise onsets for ionization, and dissociation to various fragment ions. In this case, only the ionization potential (IP) could be extracted with some confidence. Out I P is 8.47 f 0.01 eV, which is above the 8.37 eV quoted by Debies and Rabalais'O on the basis of a photoelectron spectrum. This discrepancy, which is well beyond the experimental uncertainty of 0.01 eV of both determinations, is difficult to understand. (14) (a) R. A. Marcus and 0. K. Rice, J . Phys. Colloid Chem., 55, 894 (1951);(b) H.M. Rosenstock, M. B. Wallenstein, A. L. Warhaftig, and H. Eyring, Proc. Natl. Acad. Sci. U.S.A.,38, 667 (1952). (1 5) L.Fraser-Monteiro, M. L. Fraser-Monteiro, J. J. Butler, and T. Baer, J . Phys. Chem., 86, 747 (1982). (16) J. J. Butler, T. Baer, and S. A. Evans, J . Am. Chem. Soc., 105,3451 (1983). (17) M. L.Fraser-Monteiro, L. Fraser-Monteiro, J. J. Butler, T. Baer, and J. R. Hass, J . Phys. Chem., 86, 739 (1982).
I
I
I
. ........ ,
...
i'04
103
CALCULATED RATE CONSTANTS
PHOTON ENERGY (eV)
I
I (5')
.. ;. . ........ ;.
..':
1
I
12
I
I
I
I
I
12 5
PHOTON ENERGY (eV)
Figure 2. Expanded plot of C5H6+ PIE spectrum near the dissociation onset. The calculated phenol ion dissociation rates are shown in the upper
scale. TABLE I: Thermochemistry of Phenol Ion Dissociation AHcozss, AH'o, AE298, onset: species kJ/mol kJ/mol eV eV
C6H5OH C H OH' C:Hi: CSH5
-96.36" -85.2' 722' -733' 960" 971b 1022' 1034d
8.47 h 0.01 12 13.9
11.59 f 0.10 12.96 0.10
*
"Reference 1. 'Converted by using the vibrational frequencies in Table 11. cThis study. dReference 18. eRRKM/QET. The PES IP is based on what is believed to be a 0-0 transition to the ionic vibrational ground state. Because this is a strong peak, it ought to appear as a strong onset in the PIE scan. Our energy scale is calibrated by the position of the Lyman a emission line in the light source, while the PES spectrum is calibrated by the Ar doublet peaks. The fact that our threshold PES (not shown in Figure 1) shows the same 0-0vibrational transition at 8.48 eV, and that a more recent PES of Kimura et al." lists vertical IPS which are 0.1 eV above those of Rabalais, would appear to establish the correct adiabatic I P to be 8.47 f 0.01 eV. Figure 1 shows the spectra for the two fragment ions C5H6+ ( m / z 66) and C5H5+( m / z 65). The fragmentation onsets for neither of these two ions could be precisely determined. In fact, as the expanded version of the C5H6+spectrum of Figure 2 shows, there is no sharp onset. Rather, we have a signal which rises very gently out of the background noise at about 12 eV. The onset for the C,H,+ ion is also difficult to detect. One limiting factor here is a low signal-to-noise ratio, which results partly from the low light intensity near 14 eV. However, we can obtain an upper limit to the onset which is 13.8 eV. These onsets, as well as some additional thermochemical information, are listed in Table I. Rate Constants of Energy-Selected Zons. The ions are energy selected by measuring them in coincidencewith zero kinetic energy electrons with the PEPICO technique.19 Electrons are passed through small holes (collimated hole structure) with a lengthto-diameter ratio of 40 before being detected by a channeltron electron multiplier. The collimated hole structure serves as a filter for low-energy electrons. The electron signal provides.the start pulse for measuring the ion time of flight (TOF). The quadrupole mass filter was replaced by a simple 9 cm long drift region for the PEPICO experiments. The electron and ion signals were fed into a time to pulse height converter, the output of which went to a multichannel pulse height analyzer. The ion internal energy, Ei, is specified by the relation Ei = hv - IP Eth,where hv is the photon energy, IP is the ionization potential, and Ethis the
+
(18) R. Bombach, J. Dannacher, and J. P. Stadelmann, J. A m . Chem. SOC.,105,4205 (1983).
(19) T.Baer in "Gas Phase Ion Chemistry", M. T. Bowers, Ed., Academic Press, New York, 1979,p 256.
3624
The Journal of Physical Chemistry, Vol. 88, No. Id, 1984
Fraser-Monteiro et al.
lb
I
2lO
21
I
22
TIME OF FLIGHT &IS)
Figure 5. PEPICO TOF distribution of CSH6+from phenol ions at 13.6 eV. The solid line through the experimental points is a least-squares fit using a weighted sum of the single energy release basis functions corresponding to 0.07-, 0.28-, 0.63-, 1.12-,and 1.75-eV energy release. The TIME OF FLlGHTaJS)
Figure 3. PEPICO TOF distributions for the product CSH6+ion at three photon energies. The solid lines are calculated TOF distributions assuming the indicated phenol ion dissociation rates. ION ENERGY AT OK (eV)
CGH60 +-C5H6++
CO
Figure 4. Phenol ion decay rate, k ( E ) , as a function of ion energy. The solid line is an RRKM/QET calculation using the frequencies in Table 11, and an activation energy of 3.12 eV.
average thermal energy of the phenol molecule at room temperature. Fragment ions produced from rapidly dissociating parent ions have symmetric TOF distributions. However, fragment ions from slowly dissociating parent ions are formed while being accelerated in the 12.5 V/cm electron field of the ionization region and are therefore asymmetric. Examples of such TOF distributions are shown in Figure 3. The solid lines are calculated T O F distributions assuming the usual laws of physics and the indicated dissociation rates. A plot of the dissociation rate vs. ion internal energy obtained from a number of such asymmetric TOF distributions is shown in Figure 4. In this figure, the bottom scale is simply the photon energy, while the top scale is the 0 K energy which takes into account the initial phenol thermal energy. The two scales differ by IP-Eth. In the data analysis, we have taken into account the thermal energy simply by adding the average thermal energy to the photon energy. There had been some controversy concerning the appropriate manner in which the thermal energy should be This problem has now
basis functions are shown below the TOF distribution. been largely resolved.22 We have recently shown that when the energy is well above the dissociation threshold, the thermal energy can be simply added to the photon energy as is done in Figure 4.23324 That is, it is not necessary to average the rates over the distribution of thermal energies. However, near the dissociation threshold, this procedure breaks down and the thermal distribution must be explicitly taken into account.21 All the measured rates in Phenol are at energies more than 1 eV in excess of the dissociation threshold. Kinetic Energy Release Distribution (KERD). At photon energies sufficiently high, the dissociation rate becomes high enough for the T O F distribution to be symmetric. This symmetrization can be enhanced by reducing the electric field in the ionization region. Under these conditions, it is possible to extract information about the initial kinetic energy of the fragment ion. The larger the energy released, the broader will be the TOF distribution. Figure 5 shows the TOF distribution of C5H,+ at a phenol ion energy of 13.6 eV, obtained with an electric field of 9.5 V/cm in the acceleration region. Unfortunately, this experiment could be done only in the vicinity of 13.6 eV. Below this energy, the peak is asymmetric, which prevents its analysis for kinetic energy release, while above 13.6 eV the m / z 65 ion begins to overlap the C,H,+ TOF distribution. The KERD for the C O loss from the phenol ion is characterized by a distribution of kinetic energies rather than a single energy. We extract the KERD by simulating the experimental distribution with a small set of calculated single release energy basis functions. The solid line through the experimental points is a least-square fit of a weighted sum of the basis functions shown underneath the points. The energies of the basis functions are 0.070, 0.280, 0.630, 1.120, and 1.750 eV. The KERD is extracted from the coefficients of the basis functions by multiplying them by the appropriate Jacobian to account for the unequal energy spacing of the basis functions. Details of the procedure and reasons for the unequal energy spacing can be obtained from previously published The KERD is shown in Figure 6. The average energy calculated from this distribution is 0.74 eV, a value in excess of the release energies reported by several mass-analyzed ion kinetic energy spectroscopy (MIKES) studies which range from 0.4 to 0.6 eV.2-7 The large range of the latter values is, in part, a result (20) H. M. Rosenstock, R. Stockbauer, and A. C. Parr, J. Chem. Phys., 71, 3708 (1979). (21) H. M. Rosenstock,
R. Stockbauer, and A. C. Parr, J . Chem. Phys., 73, 773 (1980). (22) J. Dannacher, H. M. Rosenstock, R. Buff, A. C. Parr, R. Stockbauer, R. Bombach, and J. P. Stadelmann, Chem. Phys., 75, 23 (1983). (23) T. Baer and R. Kury, Chem. Phys. Lett., 92, 659 (1983). (24) W.A. Brand and T. Baer, Int. J . Mass Specirom. Ion Phys., 49, 103 (1983). (25) (a) D. M. Mintz and T. Baer, J . Chem. Phys., 65, 2407 (1976); (b) T. Baer, U. Buchler, and C. E. Klots, J . Chim. Phys., 77, 739 (1980).
Dissociation Dynamics of Energy-Selected Phenol Ions
The Journal of Physical Chemistry, Vol. 88, No. 16, 1984 3625 TABLE II: Vibrational Frequencies (cm-I) and Moments of Inertia (amu
.A2)
~~
phenol molecule (C5H50H)" 3656, 3087, 3070, 3063, 3049, 3027, 1610, 1603, 1501, 1472, 1343, 1277, 1262, 1176, 1169, 1151, 1072, 1026,999, 995, 972, 881, 823, 817, 751, 683, 619, 527, 503, 408, 403, 309, 244 moments of inertia: 89, 139, 282
phenol ion (C6H50H+)C 3656, 3087, 3070, 3062, 1395, 1343, 1277, 1210, 972, 881, 817, 815, 751, 309, 244, 196 moments of inertia: 89,
I
0
I
500
I
100
I
1 50
I
200 eV
KINETIC ENERGY RELEASE (eV)
Figure 6. Kinetic energy release distribution (KERD) for the dissociation of phenol to CSH6+t CO at a photon energy of 13.6 eV. The solid line is drawn through the experimental points, which are derived from the coefficients for the basis functions of Figure 5. The average release energy is 0.74 eV, for a total available energy of 3.8 eV.
of the various ion acceleration voltages utilized. The PEPICO and MIKES kinetic energy release values cannot be compared in a simple way, because the mass-spectrometric measurements were obtained from the dissociation of long-lived metastable parent ions, whose average internal energy (based on our rate data of Figure 4) range between 12.5 and 12.8 eV. However, this 1-eV difference in the parent ion internal energy is most likely not the cause of the discrepancy, because the major source of kinetic energy release is from the reverse activation energy, and the energy in excess of the dissociation barrier is generally thought to contribute relatively little to the total. As pointed out by Maquestiau et al.,7 the MIKES data themselves are inconsistent, a fact attributed to a varying collection efficiency for fragment ions with kinetic energy. Apart from this, the generally lower kinetic energy release values from the MIKES technique are expected because they are based on the full width at half-maximum of the mass peak, whereas the PEPICO average release energy is determined from the whole distribution. This effect can easily be noted in Figure 5. The basis function which most closely corresponds to the full width at half-maximum of the T O F peak is one with an energy release of about 0.6 eV, which is considerably lower than our calculated average energy release of 0.74 eV. The experimental discrimination against the high-energy release components reduces their effect on the average energy when only the half-width of the peak is considered.
Discussion Dissociation Rates and the Statistical Theory. By subtracting the ionization energy of 8.47 eV from the C5H6+onset of 12 eV obtained from the PIE spectrum (Figure 2), we obtain an apparent activation energy of about 3.5 eV. It is well-known that the dissociation rate of a large molecule with such a large activation energy is extremely slow at the dissociation limit. As a result, an insufficient number of product ions are formed near the dissociation limit to be experimentally observable, unless the ions are trapped a long time before product detection.26 This shift of the observed onset to higher energies is known as the kinetic shift. It is certainly a factor here, so that we must consider the true dissociation onset as a variable. However, by fitting the calculated RRKM/QET rates to the measured rates of Figure 4, one can extract the true dissociation onset. The statistical theory rate constant for unimolecular decay of an ion with an E is given byI4 &E) = O W E - E,)/[hp(E)l (1) where u is the path degeneracy, k P is the sum of the transition-state vibrational and rotational states from 0 to E - Eo, and p ( E ) is the molecular ion density of states at energy E. Vibrational (26) C . Lifshitz, M. Goldenberg, Y.Malinovich, and M. Peres, Org. Mass Spectrom., 17, 453 (1982).
-
3049, 3027, 1669, 1603, 1500, 1472, 1176, 1150, 1040, 1024, 995, 976, 686, 556, 516, 503, 403, 139, 282
C6HSOH+ C5H6++ co transition state 3656, 3087, 3070, 3063, 3049, 3027, 1669, 1603, 1500, 1472, 1395 1210, 1040, 1024, 995, 976, 972, 881, 817, 815, 751, 686, 686, 556, 516, 516, 503, 403, 309, 244, 196, 196 moments of inertia: 89, 139, 282
cyclopentadiene ion (C5H6+)d 3140, 3140, 3120, 3110, 2930, 2860, 1530, 1460, 1450, 1370, 1360, 1260, 1260, 1180, 1060, 1050, 955, 915, 905, 880, 850, 810, 740, 730, 710, 590 moments of inertia: 60, 61, 118
-
*
C5H6+ C5H5+t H transition state 3140, 3140, 3120, 3110, 2930, 2860, 1530, 1450, 1370, 1360, 1260, 1260, 1180, 1060, 1050, 955, 915, 905, 880, 850, 810, 740, 730, 710, 590 moments of inertia: 60, 61, 118
From ref 28. *From ref 29. 'Obtained by combining some ionic frequencies from ref 27 and 28. dEstimated from frequencies of C4H40 and C4H4Sobtained from ref 30. frequencies must be assumed for the transition state and the molecular ion in order to calculate the sum and density of states. A complete set of ionic vibrational frequencies is seldom available for ions as large as phenol. In H e I PES, only one or two vibrational modes are excited or resolved so that little is known about the others. However, as a result of recent PES experiments following multiphoton ionization of as many as 14 vibrational frequencies have been measured for this ion. It is particularly interesting that a number of these frequencies are lowered from those of the phenol neutral and that the ionic frequencies appear to resemble those of the neutral excited lB2 state?* We have then available to us an excellent set of frequencies for calculating the phenol ion density of states. The choice of the transition-state frequencies is more difficult to make. As a first try, we used the frequencies of the molecular ion, because the transition state is most likely a rather strained one. With this set of frequencies, the best fit to the experimental data was obtained when Eo was 3.0 eV. However, the slope of the k ( E ) vs. E curve was considerably less than the experimental one in Figure 4. We were thus forced to loosen the transition state somewhat by converting some of the higher frequencies to lower ones. The best fit was obtained with the set of frequencies shown in Table 11, and with an assumed Eo of 3.12 eV. These frequencies are not characteristic of a tight transition state. Reducing the 1277 and 1176 cm-' by half, and changing the 1151 to 200 cm-', suggests that part of the molecule is loosening up. However, we cannot conclude whether this is in the ring or whether is involves the C O or O H groups; and we certainly cannot shed any light on the possible role of the keto form (2). However, we can eliminate a ring-opened transition state because such a structure would have significantly lower vibrational frequencies. The calculated value for Eo allows us to draw the potential energy diagram of Figure 7. It is evident that the kinetic shift (27) S. Anderson, J. Durant, and R. N. Zare, private communication. (28) H. D. Bist, J. C. Brand, and D. R. Williams, J. Mol. Spectrosc., 24, 402 (1967). (29) Landolt-Bornstein, "Numerical Data and Functional Relationships in Science and Technology", new series, Group 11, Vol. 4, Springer-Verlag, West Berlin, 1967. (30) T. Shimanouchi, Nufl.Stand. Ref Data Ser. (US.Natl. Bur. Stand), NSRDS-NBS 39 (1972).
Fraser-Monteiro et al.
3626 The Journal of Physical Chemistry, Vol. 88, No. 16, 1984
AEO (13.8
ENERGETICS AT OK 13.13
13-
2
Y
-
,1265 .H
12-
0
2 >
g
11-
W
2
w
z P
situation was encountered in the dissociation of the aniline ion.34 In that case it could be demonstrated with even greater certainty that the cyclic C5H6+is formed. The reaction mechanisms and the transition states for these two reactions are probably very similar. The two neutral fragments C O and H C N are isoelectronic. Kinetic Energy Release Distribution. The KERD of Figure 6 is not one which is characteristic of a statistical distribution. This is also evident in the average energy release. A statistical distribution of energy among the vibrational, rotational, and translational degrees of freedom can be calculated by assuming that all degrees of freedom are characterized by a single temperature, T. The relation between the total available energy for distribution, E*, and the temperature is given by35
109 76-
E* = k T + (R- ')kT 2
+ khv,[exp(hv,/kT)
- 11-I
(3)
i=l
where R is the number of rotational degrees of freedom of the products, and the vI)s are the n vibrational frequencies of the products. It is evident that in this equation, the translational and rotational average energies are treated in the equipartition limit, while the quantum-mechanical average of the vibrational energy is used. Furthermore, it should be noted that two degrees of freedom have been removed to account for conservation of angular Figure 7. Potential energy surface for the dissociation of phenol ions. momentum. One is from the translational energy (the first term), while the other is from the rotational energy term. The transof the C&+ onset is approximately 0.4 eV. The calculated lational energy is just kT. The results of the calculation show dissociation rates down to threshold are shown in Figure 2. These that the statistically expected average energy for a total energy rates give an explanation for the appearance of the m / z 66 ion (3.8 eV) above the cyclic product ion ground state at 9.76 eV is at an energy of about 12 eV. It is possible to calculate the fraction 0.194 eV. This is much lower than the experimental value of 0.74 of the phenol ions which will have dissociated at a rate k s-l in eV . the time (7) prior to reaching the quadrupole mass filter. This The fact that more than the statistically predicted average fraction is given by the relation kinetic energy is released for CO loss from the phenol ion is not unexpected. Dissociations with a significant reverse activation fraction = [ l - exp(-k~)] (2) energy often exhibit such behavior. This arises from the strained In our experiment, 7 = 20 ps, so that the fraction of ions which transition state which releases its energy as the CO molecule have dissociated at the rates lo4, lo3, and lo2 s-l are 0.18, 0.02, separates from the C5H6+ion. Why this energy is released as and 0.002, respectively. Thus, the fragment ion signal disappears translational, rather than primarily rotational or vibrational energy, at around 12 eV because there are simply not enough ions to be is a difficult and important question. It is difficult because a detected above the noise. There is another, unrelated, reason why rigorous analysis would involve an ab initio calculation of the no fragment ion signal is expected when the dissociation rate multidimensional surface. It is important because the kinetic becomes less than lo2 s-l. Infrared fluorescence on that time scale energy release distribution may contain valuable information about will stabilize the parent ion, thereby quenching d i s s ~ c i a t i o n . ~ ~ the nature of the transition state and the product structures. An In a very recent study, Lifshitz and Gefen32investigated the approximate technique has been developed for the analysis of the C5H6+onset as a function of the dissociation time by trapping kinetic energy release in terms of the transition-state structure.36 the phenol ions up to 0.8 ms prior to mass analysis. They found However, even this method is difficult to apply to such a large that the onset decreased to about 11.5 eV at the longest trapping ion. times. This value compares extremely well with our 298 K onset Mechanism and Energetics of C5H5+Formation. Two questions of 11.46 eV obtained by fitting our rate data with the concerning the production of the C5H5+ions are (a) the identity RRKM/QET theory. The good agreement is, in part, certainly of the precursor which could be either C,&OH+ or C5H6+and fortuitous. If we assume a sensitivity of 2 fragment ions in 1000 (b) the dissociation onset. Related to this problem is the C5H5+ parent ions along with the ion residence time of 0.8 ms, eq 2 gives heat of formation, which is not very well established. The identity us a minimum dissociation rate of 2.5 s-l. According to Figure of the precursor to C5H5+was established to be the C5H6+ by 2, the Lifshitz and Gefen onset should therefore be about 11.8. experiments on a ZAB/2F double focusing mass spectrometer. However, this is probably within the uncertainty of the electron Precursor ion spectra were obtained by fixing both the magnetic energy determination in the Lifshitz and Gefen experiment. field of the magnetic sector and the electric sector voltage, in order The structure of the C5H6+produced in the phenol ion dissoto pass the C5H5+ions formed in the source of the mass specciation can be established with a fair degree of certainty from our trometer. The spectrum of precursor ions was obtained by results. The cyclic C5H6' structure is over 1.5 eV more stable sweeping the acceleration voltage. This showed that about 94% than any linear ones.33 These linear structures would therefore of the daughter ions were formed from C5H6+. The rest came all lie within 0.3 eV of the dissociation limit at 11.59 eV. Now from C~HSOH'. if these linear structures were formed, they could only do so with If the structures of the C5H5+and C5H6+ions are similar, the a kinetic energy release of less than 0.3 eV. It is clear from the loss of an H atom from C&+ ought to occur with a very small KERD in Figure 6 that only a very small fraction of the dissoreverse activation energy. However, our measured onset at 13.9 ciation products are formed with a kinetic energy release of less eV is considerably above the thermochemical dissociation limit than 0.3 eV. Of course, we cannot rule out the possibility that H, whether one assumes the Lossing and to C5H5++ CO the linear structures are formed at higher energies. A similar
+
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(31) (a) R. L. Wwdin and J. L. Beauchamp, Chem. Phys., 41, 1 (1979); (b) J. P. Honovich and R. C. Dunbar, J . Am. Chem. Soc., 104,6220 (1982). (32) C. Lifshitz and S. Gefen, Org. Mass Specrrom., in press. (33) J. L. Holmes, private communication.
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(34) T. Baer and T. E. Carney, J. Chem. Phys., 76, 1304 (1982). (35) C. E. Klots, J . Chem. Phys., 64, 4269 (1976). (36) (a) J. R. Christie, P. J. Derrick, and G. J. Richard, J . Chem. SOC., Faraday Truns. 2, 74, 304 (1978); (b) N. W. Cole, G. J. Richard, J. R. Christie, and P. J. Derrick, Org. Muss Spectrom., 14, 337 (1979).
J. Phys. Chem. 1984, 88, 3627-3633 TraegeP7 value for the AHfOo(CsHS+)= 1080 kJ/mol or the more recent value of Bombach et al.'* of 1038 kJ/mol. Both of these limits are shown in Figure 7. The discrepancy between our observed onset and the calculated one can again be explained by the kinetic shift. Our sensitivity for detection of this ion at 13.9 eV is considerably less than the corresponding sensitivity of the CSH6+ion at around 12 e v . If we assume that a minimum rate of lo4 s-l at an energy of 13.9 eV is required for significant signal to be observable, we can calculate the statistically expected dissociation rate by using the activation energy as a variable parameter. The calculation with the correct Eo yields a rate of lo4 s-l at an energy of 13.9 eV. This analysis is based on far less certain ground than was the calculation of the Eo for the C O loss from the phenol ion. The reason for this is that we have no dissociation rates for H loss from CsH6+so that we must use the assumed rate a t only one energy. Secondly, the CSH6+ions are not state selected. In fact, they are formed in a distribution of internal energy states. This distribution is approximately the complement of the KERD which produces the C5H6+ions from phenol. For instance, the dip in the KERD of Figure 6 at low translational energies indicates that few CSH6+ions with the maximum possible internal energy are formed. These considerations were taken into account in estimating an activation energy of 3.2 f 0.1 eV. The heat of formation of C5HS+is of considerable interest.38 There is a series of cyclic ions C,H,+, n = 3-8 ( n # 9, whose (37) F. P. Lossing and J. C. Traeger, J . Am. Chem. Soc., 97, 1579 (1975). (38) R. W. Brill, T. J. Buckley, J. R. Eyler, S. G. Lias, and P. J. Ausloos, presented at the 31st Annual Conference on Mass Spectrometry and Allied Topics, Boston, MA, 1983.
3627
energies and structures are now reasonably well-known. The one exception is the case of the five-membered ring. Until recently, the best value for the AHfo(CsHs+)was one obtained by Lossing and Traeger,37which was based on the measured onset of H atom loss from CSH6+. This is precisely the same reaction which produces the ion in the present study. Our rate calculations show that we can expect a considerable kinetic shift in the measured onset. The experiment of Lossing is also subject to this shift, so that we can consider the Lossing value only as an upper limit. A more recent study by Bombach et a1.18 is based on a PEPICO rate study of the sequential dissociation of the toluene ion: C7H8+ C7H7+ CSH5+.The analysis is somewhat similar to the one carried out in this study and is subject to some of the same sources of error resulting from the problems in dealing with sequential dissociation reactions. Our results do not lead to an improved value for the CSHS+heat of formation, except that we have uncovered at least one source of the discrepancy between the Lossing ang Bombach values. The latter is probably closer to the true one.
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Acknowledgment. M. L. Fraser-Monteiro and L. FraserMonteiro thank the Council for International Exchange of Scholars for Fulbright Fellowships. We are grateful to J. Ronald Hass of NIEHS, Research Triangle Park, for carrying out the CsH5+precursor ion study. Finally, we thank S. Anderson and J. Durant for sending us the phenol ion frequencies prior to publication. This work was supported by a grant from the Chemistry Division of the National Science Foundation. Registry NO. C6H@H+, 40932-22-7; C&+, 29661-18-5; phenol, 108-95-2.
76563-67-2; CsHs',
ArF (193 nm) Laser Photolysis of HN3, CH3NH2,and N2H,: Formation of Excited NH Radicalst H. K. Haakl and F. Stuhl* Ruhr Universitat, Physikalische Chemie I , 0-4630 Bochum, West Germany (Received: November 8, 1983)
HN,, CH3NH2,and N2H4were photolyzed with an ArF (193 nm) excimer laser. Emissions from excited N H in the (A311) and the (C'II) states and also from CN(B) and CH(A,B,C) were observed in the wavelength range 200-500 nm. The NH fluorescence was investigated in detail and in comparison with the recently observed formation of NH(A311) from ammonia. The formation processes and the rotational population of the imidogen radicals, their lifetimes, and their quenching by the parent molecules were studied.
Introduction Irradiation with light from excimer lasers has recently become a new tool for the study of electronically excited molecules.' In order to generate UV emission from fragments of stable molecules either the absorption of more than one laser photon is essential or the parent molecule must be weakly bound. Fluorescences occurring at wavelengths shorter than the photolyzing excimer laser light definitely indicate the occurrence of multiphoton processes.2 Recently, we have begun a search for vacuum-UV and UV emissions from simple molecules generated by ArF excimer laser photolysis at 193.3 nm.2-5 During these investigations we have observed strong N H (ND) ( A 3 n X32-) fluorescence when photolyzing a r n m ~ n i a .This ~ emission was found to occur in a
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t Presented in part at the International Symposium on Chemical Kinetics Related to Atmospheric Chemistry, Tsukuba, Japan, June, 1982. *Present address: Fa. Buck, Mozartstr. 2, D-8230 Bad Reichenhall, West Germany.
0022-3654/84/2088-3627$01.50/0
two-photon resonance process. Furthermore, it was shown that the excess energy of this process preferentially appears as rotational excitation of the N H (ND) (A311) fragment. We therefore became interested in the possibility of generating N H emission from different parent molecules such as HN3, CH,NH,, and N2H4. In the photolysis of each of these molecules we again observed N H emission. This paper reports the results of these studies and compares them with our previous work on NH, (ND,). Since the completion of the present experiments, reports on the ArF laser photolysis of CH3NH,6 and of N2H47have been pub(1) W. M. Jackson, J. B. Halpern, and C. S . Lin, Chem. Phys. Lett., 55, 254 (1978). (2) H. K. Haak and F. Stuhl, Chem. Phys. Left., 68, 399 (1979). (3) H. K. Haak and F. Stuhl, J. Photochem., 17,69 (1981). (4) K. Shibuya and F. Stuhl, J . Chem. Phys , 76, 1184 (1982). ( 5 ) H. K. Haak and F. Stuhl, J . Phys. Chem., 88, 2201 (1984). (6) N. Nishi, H. Shinohara, and I. Hanazaki, "The Review of Laser Engineering", Vol. 10, The Laser Society of Japan, 1982, p 394.
0 1984 American Chemical Society
