J. Phys. Chem. 1996, 100, 655-659
655
Zero-Pressure Thermal-Radiation-Induced Dissociation of Tetraethylsilane Cation Chuan-Yuan Lin and Robert C. Dunbar* Chemistry Department, Case Western ReserVe UniVersity, CleVeland, Ohio 44106 ReceiVed: July 25, 1995; In Final Form: September 26, 1995X
Zero-pressure thermal-radiation-induced dissociation (ZTRID) of tetraethylsilane cation is reported. Dissociation of weakly bound gas-phase cluster ions by the blackbody background radiation field has been shown previously to proceed at an observable rate; the present results represent the first characterization of this process for a covalently bound molecular ion. Thermal-radiation-induced dissociation of the ions was studied at the low-pressure limit in a FT-ICR mass spectromer, and cleavage of the ethyl group from silicon was observed on a time scale of seconds, with the rate being strongly temperature dependent. The activation energy for the loss of one ethyl radical from tetraethylsilane cation was derived from the slope of the Arrhenius plot in the 340-400 K temperature range to be 0.43 ( 0.07 eV. The bond energy (0 K) was determined to be 0.70 ( 0.07 eV using thermal dissociation theory and 0.71 ( 0.07 eV using master-equation modeling. A bond energy of 0.78 eV was assigned at 298 K, and a heat of formation (298 K) of 131.7 kcal mol-1 was derived for the triethylsilyl cation.
Introduction In recent work of McMahon’s group,1,2 zero-pressure thermalradiation-induced dissociation (ZTRID) has been demonstrated to be a promising technique for studying weakly bound ions with dissociation energies in the range 0.5-1.0 eV. Under extremely low-pressure conditions, McMahon and co-workers have convincingly shown that the dissociation can be activated radiatively by exchange of infrared photons with the background blackbody radiation field. As further support of their understanding of this chemistry as a radiation-driven process, they have found isotope effects which carry a distinctive signature of radiatively induced dissociation.2 Thermal dissociation is a well-established and standard approach to determining thermochemistry and has been extensively studied for neutral molecules. However, not many studies of thermal unimolecular dissociation of gas phase ions have been reported, and this has not yet been a useful thermochemical approach for gas-phase ionic systems. Recently, in our lab the ion-trapping ion cyclotron resonance (ICR) mass spectrometer was given variable-temperature capability. It seemed interesting to use the long trapping times provided by this ion trap technique to make quantitative thermal dissociation measurements as a function of temperature with the goal of deriving accurate dissociation thermochemistry. In contrast to the cluster ion studies pioneered by McMahon’s group, the focus is on covalently bound ions. ZTRID of another covalent species, the acetophenone ion, was recently reported by Sena and Riveros.3 Among ions having a dissociation energy in the 0.5-1.0 eV region, Si(C2H5)4+ is a good candidate whose thermal dissociation is expected to be observable not far above room temperature. The thermochemistry of the tetraethylsilane system, especially the triethylsilane cation, is not very well known. An accurate value of the C-Si bond strength in tetraethylsilane cation is of particular interest, since this dissociation was singled out as a standard probe reaction in studies by the Cooks group of the internal energy distribution resulting from ion activation by different methods.4 This dissociation reaction is well behaved and may be a good choice for future use as a stan* To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, December 1, 1995.
0022-3654/96/20100-0655$12.00/0
dardized bond-breaking process of exceptionally low bond energy. These applications make it important for the bond strength thermochemistry to be put on a secure basis. In the initial work by McMahon’s group, only qualitative aspects of the kinetics were considered. Using ZTRID data along with methods of thermal dissociation theory,5 there is the opportunity for a more quantitative point of view, which recent work on the cluster ions has begun to take advantage of. Traditionally, the reaction activation energy of a thermal dissociation can be easily derived from an Arrhenius temperature plot. However, the more interesting quantity, the dissociation threshold energy, is not the same as the activation energy, and careful consideration is needed to derive the former from the latter. The application of thermal dissociation kinetic theory to achieve this was described recently by Dunbar, using a steadystate truncated Boltzmann distribution6 and a modified Tolman equation to extract the dissociation thermochemistry. These are precisely the tools needed in the present study for extracting thermochemistry from ZTRID results.5 Experiment Experiments were performed on an ICR spectrometer with a 1 in. cubic cell described in detail previously.7 In brief, ions were initially formed by electron impact during the electron beam pulse. After that, the filament was turned off and allowed to cool for the rest of the trapping time. Following the electron beam pulse, ions were thermally equilibrated during a time (up to 6 s) sufficient to give five ion-neutral collisions with precursor neutral. At the end of this thermal delay time, a series of ejection pulses were applied to isolate the precursor ion. Then the successor and precursor ion abundances were monitored as a function of reaction time. The reaction time was varied from 1 to 40 s. In order to reduce the amount of the activation and ejection of the precursor ion via the double-resonance ejection pulses, the ejection amplitude was set as low as possible. However, this did not entirely eliminate perturbation of the ion cloud by the radio-frequency field. As seen in many time-resolved plots (as in Figure 1b, for instance), the dissociation at short reaction times (typically the first second) was somewhat faster than the steady-state value. This was attributed either to hot precursor ions, activated by the ejection pulses (velocity increase followed © 1996 American Chemical Society
656 J. Phys. Chem., Vol. 100, No. 2, 1996
Lin and Dunbar
by collision with a neutral to convert translational excitation to internal energy), or possibly to ions not totally thermalized before the ejection pulses. After several hundred milliseconds to 1 s, depending on the working pressure, the dissociation rate was observed to slow down from the initially fast rate and level off. The stable steady-state dissociation rate was considered to reflect a thermally equilibrated condition and was used for data analysis. The vacuum can was surrounded by heating tapes and carefully insulated. The heating arrangement was found capable of maintaining the chamber temperature within (2 K. A coppper-constantan thermocouple located inside the chamber on the trapping plate was used to monitor the cell temperature. Careful measurements of room temperature and a systematic correction of the thermocouple readings gave the cell temperature for each data collection session. An important aspect of these experiments was the operation of the electron-beam filament in a pulsed mode,8 with current passing through and heating the filament wire only during the ionization part of the experimental cycle. There are at least two reasons for pulsed as opposed to continuous filament operation. First, the light from the white-hot filament might be absorbed by the precursor ions which could give a significant amount of dissociation. Second, heating by the filament is well known to give above-ambient temperature and temperature inhomogeneity in the ICR cell, such that the effective cell temperature is 50-75 K above room temperature and is not very well characterized. Since well-characterized and homogeneous cell temperature was essential to the present experiments, the extra heating by the filament was avoided by turning it off. Although the filament was turned off after the precursor ions were generated, gradual warming of the ICR cell might still be possible for operation over many experiment cycles if there were not sufficient cooling time between cycles. To avoid this problem, a low filament-on duty cycle (∼10%) was ensured by inserting an idle period of 8-18 s in each working cycle. Tetraethylsilane (TES) from Aldrich was used without further purification except for degassing with several freeze-pumpthaw cycles. TES pressure was measured using an ionization gauge, which was expected to give reliable relative pressures. A pressure correction factor of 6.2 was used to convert gauge pressure to absolute pressure. This was not considered a very reliable absolute pressure calibration, but the present work does not rely on a particularly accurate knowledge of pressure, since the only need for absolute pressure information is to make the extrapolations to zero pressure. The extrapolation is not dependent on an accurate absolute calibration, so for this purpose the present calibration was considered sufficiently good. The base pressure (indicated by the ionization gauge) was about 1 × 10-8 Torr. Results The loss of one ethyl radical from the precursor ion, Si(C2H5)4+, gave the successor ion, Si(C2H5)3+, accompanied by a side reaction product, Si(C2H5)3(H2O)+, which was found to come from the ion-molecule reaction of precursor ion with background water in the chamber. The dissociation and reaction processes can be expressed as kr
Si(C2H5)4+98Si(C2H5)3+ + C2H5• kw
Si(C2H5)4+ + H2O98Si(C2H5)3(H2O)+ + C2H5•
(1) (2)
where kr and kw are the rate constants of dissociation and ligand-exchange reaction, respectively.
Figure 1. Low-pressure thermal dissociation of tetraethylsilane ion, showing the ZTRID dissociation and the competing biomolecular reaction with water. Pressures were (a) 1.9 × 10-7 Torr and (b) 2.2 × 10-8 Torr. Temperature was 402 K.
Two potential complications to this reaction scheme needed to be investigated and ruled out. First, there was the possibility that Si(C2H5)3+ successor ions might react further with water to give the successor-water complex. This was ruled out by isolating the successor ion (by ejecting all other reactant ions from the cell) and observing that successor-water complex ion was not formed from this reactant ion even after 22 s reaction delay time. Hence, the successor-water complex ion is generated only from the reaction of precursor Si(C2H5)4+ ion with water. Second, it was necessary to consider the possibility of back-dissociation to give the successor ion from the successor-water complex ion. Isolating the successor-water complex ion, it was shown that no successor ion was produced by such a path during the window of reaction time. Therefore, there is only one reaction path, eq 1, leading to the successor ion. The abundances of the precursor, successor, and successorwater ions were monitored as a function of the time between the initial ejection pulse sequence and the detection sequence. Two time-resolved plots are shown in Figure 1. The apparent rate constants kr and kw, were derived from fitting experimental data to the above parallel reaction scheme (eqs 1 and 2). Comparing these two illustrative plots, it is seen that the one with higher pressure, Figure 1a, gives a faster dissociation rate than the one with lower pressure, Figure 1b. The significant increase of dissociation rate constants with the increase of neutral pressures shows that collisionally activated dissociation is important; the desired radiatively activated process becomes significant only at the lowest pressures.
Dissociation of Tetraethylsilane Cation
Figure 2. Pressure dependence of the thermal dissociation of tetraethylsilane ion. Temperature was 402 K. The linear least-squares fit gives a y intercept, corresponding to the ZTRID rate, of 0.11 s-1.
Figure 3. Temperature dependence of the thermal dissociation of tetraethylsilane ion. The linear least-squares fit (- - -) gives an activation energy of 0.43 eV. A master equation calculation with the dissociation threshold energy Et 0.71 eV is also shown (s).
A plot of dissociation rate vs neutral pressure is shown in Figure 2 for one of the temperatures studied (402 K). A nonzero intercept at zero pressure is seen in this rate-pressure plot, reflecting the contribution of the collisionless (radiative) ZTRID process to the dissociation. The plot indicates that the collisionless unimolecular dissociation process is dominant only at pressures below about 2 × 10-8 Torr. Collisionally activated dissociation quickly becomes the dominant process at pressures above 2 × 10-8 Torr, but at pressures below 1 × 10-8 Torr the dissociation process is predominantly radiative. An accurately measurable zero-pressure dissociation rate was seen for temperatures from 340 to 400 K. The radiatively induced unimolecular dissociation rate constant, kr, was taken from the zeropressure intercept of each such plot. Finally, the temperature dependence of the ZTRID process was obtained from the plot of ln(kr) vs T-1 shown in Figure 3. The best linear fit to this Arrhenius plot gave the activation energy of the dissociation process as 0.43 ( 0.07 eV. Kinetic Modeling ZTRID results can be used in two essentially independent ways to derive dissociation thermochemistry, specifically the 0 K dissociation energy Et. The absolute rate of dissociation at a particular temperature can be fitted to theory, and the assumed
J. Phys. Chem., Vol. 100, No. 2, 1996 657 value of Et can be adjusted to give agreement. Or the ZTRID temperature dependence can be analyzed to find Et. The modeling involved in the absolute-rate approach is critically dependent on accurate knowledge of the absolute infrared absorption intensities of the precursor ion. In contrast, the temperature dependence gives Et essentially without knowledge of the infrared intensities. In a recent study of cluster ion ZTRID both approaches could be applied to one case, Cl-(H2O)3, and gave concordant results.9 In the present tetraethylsilane case, the absolute-intensity approach could not be used accurately, because nothing is known about the IR intensities for this precursor ion. Accordingly, our efforts focused on analyzing the temperature dependence to estimate Et. Two approaches are available for analyzing the ZTRID temperature dependence to derive the thermochemistry of tetraethylsilane ion dissociation. One is to model the time evolution of the ion population with a master equation formalism.10 The other is the kinetic formulation based on the steadystate truncated Boltzmann population distribution.6 The formalism for both approaches has been described in detail in a recent paper.5 For both of these methods, reasonable estimates of the infrared vibrational frequencies of tetraethylsilane ion are needed to calculate the vibrational heat capacity. In addition, carrying out the master equation calculation requires that we assign infrared absorption intensities for all the IR-active modes, although in the end the temperature dependence obtained from the master equation approach is not strongly dependent on this assignment of IR radiative intensities. Neither the vibrational frequencies nor the infrared absorption intensities are known for tetraethylsilane ion. A generic “standard hydrocarbon ion” approach11 proposed by Dunbar has been successfully applied to radiative association modeling.12-14 The typical values prescribed in this approach for vibrational frequency distributions and IR radiative intensities have been quite successful in such semiquantitative modeling for a variety of polyatomic ion systems.12-14 Hence, in the present kinetic modeling, both the vibrational frequencies and the corresponding absorption intensities of tetraethylsilane cation were assigned according to this scheme. It should be stressed that the final thermochemical results are not very sensitive to these parameter assignments, so that these estimates, as long as they are not grossly inaccurate, are entirely adequate for this purpose. An important assumption made in kinetic modeling using any of these approaches is that precursor ions with internal energy above the dissociation threshold energy, Et, will rapidly dissociate into successor product ions and will be removed from the ion population (the sudden-death assumption). To test the validity of this assumption in this case, an RRKM dissociation rate calculation10 was done using standard hydrocarbon ion vibrational frequencies and using the Et value of 0.7 eV from the results described below. It was found that even close to threshold the dissociation rate constant is much faster (g103 s-1) than the other processes relevant to the thermal dissociation kinetics, namely, the radiative relaxation, the collisional relaxation, and the radiative up-pumping processes. From this RRKM result, it appears that the sudden-death assumption is excellent for the tetraethylsilane ion case. Truncated Boltzmann Distribution Method. The use of the truncated Boltzmann population distribution model is straightforward and has been described in detail for the ZTRID situation.5,6 This is based on the picture shown in Figure 4, where the dashed trace is the Boltzmann ion population, in the absence of reactive depletion, at 375 K. Precursor ions with internal energy above Et will dissociate into product ions, and a fraction of the ion population lying slightly below Et can also
658 J. Phys. Chem., Vol. 100, No. 2, 1996
Lin and Dunbar TABLE 2: C-Si Bond Strength (eV) in Tetraethylsilane Cation 0.70 ( 0.07a,b 0.71 ( 0.07a,c 0.50d (1.07)e a 0 K dissociation energy. b Present results, derived from modified Tolman theorem. c Present results, derived from master equation calculation. d Difference of (electron impact) IP and AP from ref 18. e Difference of IP (adiabatic photoelectron spectroscopic value) and AP (electron impact) from ref 19. This is considered a very unreliable number.
Figure 4. Population distribution for tetraethylsilane ion, with (s) and without (- - -) the radiation driven thermal dissociation reaction. Temperature was 375 K. The dissociation threshold energy Et was 0.71 eV (5700 cm-1).
TABLE 1a spontaneous emission
i 2.88 × 10-9I1,0 νi2
∑ne
-u
1-e stimulated emission
absorption
∞
∑ne - 1)
i 2.88 × 10-9I0,1 νi2
(1 - e-u)(eν
-nu
n)1
i 2.88 × 10-9I1,0 νi2
(1 - e-u)(eν
∞
-nu
n)1
∞
∑(n + 1)e - 1)
-nu
n)1
i i a Ii n,n-1 ) nI1,0. In,n-1 is the integrated band intensity from quantum state n to state n - 1 for mode i. νi (in cm-1) is the frequency of vibrational mode i. ν ) hcνi/kT. u ) hcνi/kBTi. Ti is the internal temperature of the ion.
dissociate to successor ions by absorption of one IR photon. The resulting population depleted by these reaction processes is shown as the solid trace. The Arrhenius activation energy of the dissociation Ea ) 0.43 ( 0.07 eV can be taken from the slope of the Arrhenius plot in Figure 3. To obtain the dissociation threshold energy, Et, a corrected Tolman equation was used:5
Et ) Ea + 〈E′〉 - 0.04 eV
(3)
where 〈E′〉 is the average energy of the depleted Boltzmann ion population. 〈E′〉 is found to be 0.31 eV by integration over the area below the solid trace in Figure 4. Equation 3 then gives an estimated value of Et of 0.70 ( 0.07 eV. Master Equation Method. A matrix formulation of the master equation was used in the present modeling.5,10 To construct the J matrix, the activation, deactivation, and dissociation processes were considered individually. The rate constants of spontaneous emission, stimulated emission, and absorption were calculated according to Einstein’s A and B coefficients.5,15,16 The light intensity is the blackbody radiation distribution, which is given by the Planck relation.5 As noted above, the dissociation rate constant for all energies above the dissociation threshold energy, Et, was assumed to be very large. The equations are collected in Table 1.
After the J matrix is established, the thermal unimolecular dissociation rate constant is the least negative eigenvalue of the matrix J.10 The thermal unimolecular dissociation rate constants were calculated for several Boltzmann populations at different temperatures. A calculated Arrhenius plot of ln(kr) versus temperature was then compared to the experimental results. The dissociation energy, Et, was considered as an adjustable parameter in this calculation. The active IR vibrational frequencies and absorption intensities were chosen according to the semiquantitative “standard hydrocarbon ion” model, as given by Table II of ref 5. It was found, however, that using the standard-hydrocarbon IR intensities gave predicted rate constants too low by a factor of 4.3. Since the IR intensities are in fact unknown, we scaled all the standard-hydrocarbon IR intensity values up by 4.3 to give agreement with the observed ZTRID rates.17 The slope of the predicted Arrhenius plot is very sensitive to the dissociation threshold energy, Et. The calculated dissociation rate constants are shown in Figure 3 as the solid trace. For comparison, a linear Arrhenius fit is also shown in Figure 3 (dashed curve), and it appears that the slight curvature of the directly modeled solid curve gives a slightly better fit to the experimental points. The dissociation threshold energy, Et, derived from the master equation calculation is 0.71 ( 0.07 eV. The dissociation threshold energy of tetraethylsilane cation obtained from the master equation calculation is gratifyingly consistent with the value from the modified Tolman theorem. Discussion From the experimental results, the activation energy of reaction 1 is about 0.43 eV in the temperature range 345-400 K. This ion provides a good illustrative case for comparing two approaches for deriving the thermochemical threshold energy Et from the slope of the rate-temperature plot. Using the truncated Boltzmann distribution and the modified Tolman’s theorem, Et was estimated to be 0.70 ( 0.04 eV, while the value derived directly from master equation fitting was 0.71 ( 0.07 eV. The close consistency of these approaches is encouraging. As has been noted before, these low-pressure thermal experiments give an extreme illustration of the fact that the Arrhenius activation energy (0.43 eV) is nothing at all like the dissociation endothermicity (0.71 eV) under low-pressure (extreme falloff region) conditions. The dissociation energy of tetraethylsilane ion has apparently previously been measured only by electron-impact appearance potential techniques of questionable reliability. Table 2 is a summary of C-Si bond strength values for tetraethylsilane cation. The value of 0.5 eV is old (1967)18 but apparently believable within the uncertainty of the technique. Given that such electron impact values are typically uncertain by several tenths of an electronvolt at best, this value is in good accord with the present value. The value of 1.07 eV is noted as being derivable from data in ref 19 but probably has little validity since it is derived by comparing precursor and successor ions which were not measured in the same instrument. The present
Dissociation of Tetraethylsilane Cation
J. Phys. Chem., Vol. 100, No. 2, 1996 659
TABLE 3: Gas-Phase Heats of Formation species Si(C2H5)4+ Si(C2H5)3+ C2H5•
∆Hf (kcal mol-1 at 298 K)f 141.7a 138.7b 125c 131.7 ( 1d 27.9e
a Reference 20. b Derived from the appearance energy of Si(C H ) + 2 5 3 from ref 19. c Derived from ref 18. d Present work. e Reference 21.f The heats of formation derived for Si(C2H5)3+ assume that reaction 1 is a barrierless process.
ZTRID value of Et seems more reliable than any previous thermochemistry, since the thermal Arrhenius activation energy was measured with confidence, and the derivation of Et from it rests on quite solid theoretical foundations. Moreover, assigning the bond dissociation energy directly from the activation energy is inherently more satisfactory than deriving it from the difference of two large, uncertain numbers (the ionization energy and the fragment ion appearance energy), as must be done with the electron impact data. Thus, the bond energy of 0.5 eV used by Cooks’ group3 as a standard value in their internal energy characterization experiments appears to be low by about 0.2 eV. The heat of formation of Si(C2H5)3+ has not been reported directly in the literature. In order to derive this heat of formation from the dissociation threshold energy, one has to make the assumption that reaction 1 is barrierless. This is generally considered to be an acceptable assumption for a simple cleavage of an alkyl radical from a molecular radical ion. Making this assumption, the 0 K dissociation energy is 0.70 eV, which can be corrected to give an enthalpy of dissociation of 0.78 eV at 298 K. Taking the molecular ionization energy from ref 20, the heat of formation of Si(C2H5)3+ can be estimated from the present result as shown in Table 3. Conclusion Thermal dissociation of tetraethylsilane cation is clearly observed in the limit of vanishing neutral-molecule pressure, showing that unimolecular dissociation of this weakly covalent bound ion is radiatively activated by the background radiation in the ion trap. Measurement of the rate constants at several different temperatures allows us to obtain the Arrhenius activation energy of this reaction. Applying thermal dissociation theory further enables us to extract the dissociation thermochemistry. The heat of formation of triethylsilane cation, Si(C2H5)3+, derived in this work is about 131.7 ( 2 kcal mol-1 at 298 K, which is believed to be a more reliable value than
that derived from the appearance energy of Si(C2H5)3+ from electron impact. This quantitative study shows that ZTRID is a useful technique for measuring dissociation thermochemistry of weakly covalently bound ions. Acknowledgment. The support of the National Science Foundation and of the donors of the Petroleum Research Fund, administered by the American Chemical Society, is gratefully acknowledged. References and Notes (1) Tho¨lmann, D.; Tonner, D. S.; McMahon, T. B. J. Phys. Chem. 1993, 98, 2002. (2) Tonner, D. S.; Tho¨lmann, D.; McMahon, T. B. Chem. Phys. lett. 1995, 233, 324. (3) Sena, M.; Riveros, J. M. Rapid Commun. Mass Spectrom. 1994, 8, 1031. (4) (a) Wysocki, V. H.; Kenttamaa, H. I.; Cooks, R. G. Int. J. Mass Spectrom. Ion Processes 1987, 75, 181. (b) Schey, K. L.; Kenttamaa, H. I.; Wysocki, V. H.; Cooks, R. G. Int. J. Mass Spectrom. Ion Processes 1989, 90, 71. (5) Dunbar, R. C. J. Phys. Chem. 1994, 98, 8705. (6) Dunbar, R. C. J. Chem. Phys. 1991, 95, 2537. (7) Lin, C.-Y.; Dunbar, R. C. J. Phys. Chem. 1994, 98, 1369. (8) Lin, C.-Y.; Dunbar, R. C. J. Phys. Chem. 1995, 99, 1754. (9) Dunbar, R. C.; McMahon, T. B.; Tho¨lmann, D.; Tonner, D. S.; Salahub, D. R.; Wei, D. J. Am. Chem. Soc., in press. (10) Gilbert, R. G.; Smith, S. C. Theory of Unimolecular and Recombination Reactions; Blackwell Scientific Publiciations: Oxford, 1990. (11) Dunbar, R. C. Int. J. Mass Spectrom. Ion Processes 1990, 100, 423. (12) Dunbar, R. C.; Faulk, J. D. Chem. Phys. Lett. 1993, 214, 5. (13) Herbst, E.; Dunbar, R. C. Mon. Not. R. Astron. Soc. 1991, 253, 341. (14) Dunbar, R. C. Ion-Molecule Radiative Association. In Current Topics in Ion Chemistry and Physics; Ng, C. Y., Baer, T., Powis, I., Eds.; Wiley: New York, 1994; Vol. II, Chapter 5. (15) Dunbar, R. C. Spectrochim. Acta 1975, A31, 797. (16) Dunbar, R. C. J. Chem. Phys. 1989, 90, 7369. (17) As indicated by this discussion, it is not possible to fit simultaneously both the slope and the absolute values of this set of temperaturedependent ZTRID points using the IR intensities estimated from the standard hydrocarbon model. Bearing in mind that the actual (unknown) IR intensity values for this particular ion might easily be several times higher than the generic estimate, we considered it most reasonable not to draw quantitative thermochemical conclusions from the absolute values of the ZTRID rates. We might, on the other hand, simply fit the absolute rates using the standard hydrocarbon intensities, ignoring the fact that the resulting fit does not agree with the experimental temperature dependence. Such a fit gives a dissociation energy Et of 0.60 eV. This lower Et estimate can only be correct if there is a severe unrecognized systematic error in our temperature dependence measurements. (18) de Ridder, J. J.; Dijkstra, G. Recl. TraV. Chim. 1967, 86, 737. (19) Potzinger, P.; Ritter, A.; Krause, J. Z. Naturforsch. A 1975, 30, 347. (20) Pedley, J. B.; Rylance, J. Sussex-N.P.L. Computer Analysed Thermochemical Data: Organic and Organometallic Compounds, University of Sussex, 1977. (21) Cao, J.-R.; Back, M. H. Int. J. Chem. Kinet. 1984, 16, 961.
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