J. Phys. Chem. 1995, 99, 1754-1759
1754
Time-Resolved Photodissociation of Gas Phase Nickelocene Cation: Determination of Bond Strength and Radiative Relaxation Rate Chuan-Yuan Lin and Robert C. Dunbar* Chemistry Department, Case Westem Reserve University, Cleveland, Ohio 44104 Received: July 29, 1994; In Final Form: October 21, 1994@
Time-resolved photodissociation (TRPD) rate measurements on nickelocene cation were performed at two different W wavelengths in the ion cyclotron resonance (ICR) mass spectrometer. For completely thermalized ions, one-photon absorption at 266 nm gave a dissociation rate of 168 s-', and two-photon absorption at 355 nm gave a dissociation rate of (1.7 & 0.3) x lo6 s-l, which is the fastest dissociation rate yet measured in our lab. The TRPD results were modeled by RRKM calculations. The critical energy and the activation entropy at 1000 K were determined to be EO = 3.24 eV and A$looo~ = 6.34 eu. From the RRKM fit the CpNi+-Cp bond strength was assigned as 313 kJ/mol, which is significantly weaker than for ferrocene ion (357 kJImo1). The dissociation rate was used as a thermometric probe to measure the internal energies and cooling rates of initially hot ions. The radiative cooling rate in the internal energy range 1.0-0.3 eV was 0.48 s-'. The radiative cooling rate constant has the same order of magnitude as previously found for comparable ions. It is about twice that of ferrocene cation (0.28 s-').
Introduction A recent study of ferrocene ion using trapped-ion techniques in the ion cyclotron resonance (ICR) mass spectrometer gave the first good bond-cleavage thermochemistry for this ion and also characterized it as the polyatomic ion having the slowest known infrared-radiative cooling.' Even less is known about these properties for nickelocene ion than was known for ferrocene, and applying these techniques to the nickelocene case will certainly be valuable in clarifying the thermochemistry and cooling kinetics of this system. Even more interesting is the chance for direct comparisons of these fundamental thermochemical and dynamical properties between two very similar organometallic systems. In some respects the nickelocene ion turned out to be even more favorable to study than ferrocene ion, and our hope of making useful comparisons was well fulfilled. Trapped-ion ICR techniques offer convenient methods to study the kinetics and dynamics of gas phase ions. Historically, organic and simple inorganic ions have been extensively studied and characterized. More recently, study of the gas-phase ion chemistry of organometallic ions has increased with the advent of new experiments and instruments, as reviewed and illustrated in a recent book.2 Among recent studies, in addition to the TRPD work of ref 1, we can note the use of photodissociation spectroscopy in the UVlvis region to acquire spectroscopic and energetic information about organometallic ions;3 the use of collision-induced dissociation (CID) in drift-tube type instruments to measure bond dissociation energies and heats of formation of fragment ions;4 and the octopole ion beam guide work of Armentrout's group which has been fruitful in determining the bond dissociation energies and characterizing the electronic states of organmetallic ions5 The combination of time-resolved photodissociation (TRPD) data with RRKM modeling has proved to be a powerful way to extract the thermochemistry of gas-phase ions and characterize the dissociation For ions with more than 12 atoms, traditional fragmentation appearance energy measure-
ments are virtually useless for determining dissociation thermochemistry, because of large and unquantifiablekinetic shifts. However, the strategy of determining the energy dependence of the fragmentation rate and extrapolating to zero rate via RRKM modeling has proven to be highly effective in circumventing this fundamental difficulty. The energy-resolved fragmentation rate data for this purpose can be measured by the TRPD approach used in the present work,'s6 as well as by photoelectron-photoion coincidence (PEPICO)? time-resolved photoionization mass spectrometry (TPIMS),* resonanceenhanced multiphoton photoionization (REMPI)? and fast-beam TRPD.'O The metallocene system is one of the classic types of organometallic complex and has been widely studied. A tremendous amount of thermochemistry of neutral metallocene systems has been established, but much remains to be learned about the corresponding ionic species. It is particularly interesting when a direct comparison of a metallocene with its corresponding ionic species can be made. In the previous ferrocene study,' several aspects of the chemistry of ferrocene cation were studied by photodissociationtechniques. The bond cleavage between the iron ion and one of the cyclopentadienyl rings was observed. Through RRKM modeling, the bond strength of the metal-ring bond was determined to be 3.7 & 0.3 eV; a positive activation entropy at 1000 K ( A S t l m ~ was ) estimated and interpreted as characterizing a loose transition state. In the low-energy regime, the radiative cooling rate constant of ferrocene cation was determined to be 0.28 s-l, which is somewhat slower than that of the typical hydrocarbon n-butylbenzene ion (0.5 s-l).ll The present work continues our systematic study of the photodissociation of metallocene cations M(Cp)2+, where M is the metal ion and Cp is cyclopentadienyl. Nickelocene cation was chosen as our next target, since it is a case for which the neutral compound has been extensively characterized, but the ion has been little studied. The first goal in this study was to determine the critical energy for cleaving off one Cp ring, i.e., the energy needed to promote reaction 1.
* To whom correspondence should be addressed. @
Abstract published in Advance ACS Abstracts, January 15, 1995.
0022-365419512099-1754$09.0010
0 1995 American Chemical Society
Photodissociation of Gas Phase Nickelocene Cation While an average metal-ring bond strength can be readily derived from the heats of formation of NiCpi+, Ni+, and Cp', the individual bond strengths in a metal complex may be quite different from the average bond strength. An illustrative example is Fe(C0)5.'* From the heats of formation, the average bond strength of Fe-CO was determined to be 28 kcdmol. Yet the first Fe-CO bond strength, D[(CO)se-CO], is 41 k c d mol, and the last Fe-CO bond strength, D[Fe-CO], is only 10 kcavmol, which are quite different from the average value. A similar situation occurs for ferrocene ion.' The second goal was to measure the radiative cooling rate constant. As illustrated in a recent review of the kinetics of radiative cooling, l 3 substantial progress in characterizing the radiative behavior of organic cations and some simple inorganic ions has been made, but only a little work has been done on the radiative cooling of organometallic ions. ',14 For mediumsized or large ions (12 or more atoms), the high heat capacity is expected to make infrared radiative emission s10w.l~ This expectation has been borne out for various organic ions and even more strikingly for ferrocene ion, whose radiative cooling rate constant (0.28 s-l) is the slowest yet measured.' Although ferrocene cation is quite unlike typical hydrocarbon ions, it is notable that its radiative cooling rate is not very different from that of similiar size organic ions. For a better understanding of radiative cooling behavior of organometallic cations, a systematic comparison is needed. Nickelocene cation, with the same number of atoms and ion structure as ferrocene cation, is a good candidate for comparative study of the radiative cooling behavior of organometallic ions.
11. Experimental Section All of the experiments used an ion cyclotron resonance (ICR) mass spectrometer equipped with phase-sensitive detection. Parent ions were generated by an electron beam pulse of about 100 ms duration at a nominal electron energy of 12 eV. Before being photoexcited by the laser pulse, parent ions were allowed to cool using various cooling times at several pressures of neutral nickelocene gas. Photoproduct detection was achieved by applying an excite pulse of 20 ps duration with 50 V peak-topeak amplitude at the frequency of NiCp+, followed by a 3 ms ICR transient acquisition. Data acquisition mode using lightoflight-off cycles was used to give baseline stabilization and to subtract out the signal contribution from NiCp+ ions produced by electron impact. The pulse sequence and data acquisition were automated under microcomputer control. Using the normal continuously running hot filament, the effective cell temperature has been estimated as 375 K. The internal energy of the thermalized parent ions at this temperature (0.3 eV) led to an inconveniently fast rate of photodissociation in the 355 nm experiments, so a pulsed filament mode was used in which the filament was turned on only during the ion production period. Based on the filament current duty cycle, the effective cell temperature was estimated to be 325 f 10 K in pulsed filament mode versus 375 f 10 K in continuous filament mode. Additional details of the experimental apparatus have been reported in our previous publications.' Nickelocene solid (Aldrich) was used without further purification except for degassing with several freeze-pump-thaw cycles. Nickelocene pressure was measured using an ionization gauge, which was expected to give reliable relative pressures. A pressure correction factor of 7.1 (which was used for the previous study of ferrocene cation') was used to obtain the absolute pressure. As noted previously,' the absolute pressure assignment should be regarded as somewhat uncertain. The tripled (355 nm) and quadrupled (266 nm) outputs of the Lumonics HY 1200 Nd:YAG laser were used to photoexcite
J. Phys. Chem., Vol. 99, No. 6,1995 1755 the parent ions. Because of the stringent timing requirements at 355 nm, the laser was double triggered, the fiist pulse firing the flash lamp and the second pulse opening the Pockels cell. With this triggering mode, timing was precise within several hundred nanoseconds. Typical pulse energies were 8 mJ at 266 nm and 20 mJ at 355 nm.
111. Results and Discussion The loss of one Cp ring from nickelocene cation (eq 1) was the only photoproduct at these UV wavelengths. Ni+ was not observed at either 266 or 355 nm. Some examples of timeresolved photodissociation data at different cooling times are displayed in Figure 1. As shown in these plots, two-photon dissociation gave large zero-time y intercepts in the 266 nm curves. Such nonzero intercepts have been observed and discussed in other TRPD studies.16 The signal strength of the fragment ion was too small to allow elimination of this two-photon contribution by reducing the laser intensity. It was not surprising that the photodissociation intensity was small: the optical absorption spectrum of nickelocenium ion is known in solution,'' and the absorption intensity is rather weak at both of the wavelengths used in the present study (-lo3 L mol-' cm-'). A. Dissociation Rates. At 266 nm the dissociation proceeded by a one-photon mechanism which gave dissociation rates in the range 168-4440 s-l, depending on the excess internal energy of the ion. The first goal was to determine the dissociation rate of ions of specified internal energy. An advantage of the ion-trap approach is that ions can be completely thermalized at the temperature of the surroundings, so that their internal energy is well-known. This complete thermalization was achieved in the results shown in Figure IC. In this case, a cooling time of 4.3 s at a pressure 4 x lo-* Torr gave about six ion-neutral collisions before the laser pulse. As is justified by the ion cooling results described below, this is ample for removal of any superthermal internal energy by collisions and by radiative cooling. After absorbing one photon, the total internal energy of 4.97 eV (plus thermal energy at 375 K) gave the slowest dissociation rate 168 s-', At 355 nm the energy of two photons plus preexisting thermal energy at 325 K gave total internal energy of 7.2 eV. Although the dissociation rate at this internal energy is so fast as to be near the limit of the technique, it was possible to obtain a TRPD rate measurement as shown in Figure Id, using the double triggered laser operating mode. The best fit of this data to the ICR-TRPD signal equation'* gave a dissociation rate of 1.7 x 106 s-1. Detailed data analysis of the TRPD results has to take into account the effects of the TRPD signal equation, the rate energy curve, and the thermal distribution of parent ion internal energies. The fitting and deconvolution procedure was carried out as in ref 18. The ion population has a spread of internal energies and thus a spread of dissociation rates /?(E). In this modeling, a Boltzmann distribution of ion internal energies was used at 375 K for 266 nm and 325 K for 355 nm. The radiative relaxation rate was so slow compared to the fragmentation rate that it was not necessary to include the effect of radiative cooling in the fitting procedure. The curves drawn in Figure 1 represent the best fits using this convolution procedure. B. Rate-Energy Curve. The two measured dissociation rate constant values were used to assign the rate-energy curve by fitting to RRKh4 theory. Since there is no information about the vibrational frequencies of the nickelocene cation and its transition state, the neutral nickelocene vibrational frequencies19 were used. The symmetric metal-ring stretching mode of 207
1756 J, Phys. Chem., Vol. 99, No. 6, I995
Lin and Dunbar
Nickelocene Ion TRPD Curves I
0
0
0
1
266 nm
.->
6)
4.0 x IOm8 torr
-2 c
c3
40-
( c ) 4300 ms Cooling Time
.-C0
( a ) 425 ms Cooling Time
c
2
20-
El c
0
5000
IOOOO
20000
15000
25000
0
30000
5000
10000
100
-
A
.-
25000
20000
30000
Delay Time (ps)
Delay Time (ps)
e
15000
0
-
v,
C
0
.-
80-
m
(d
..-v
60-
2
40-
-A
c
-
.
(d
$
.-0 )
v
3
2
c
( b ) 1500 ms .
.-0
Cooling Time
40-
ICR Signal Equation
c
-
3
sc.
20*
20-
k=1.70ps-'; A=lps;C=IOO (aave = 0.90: wp = 0.71 ;6 = 0.38) rad ps-'
07
5000
10000
15000
20000
25000
I
I
I
30000
Delay Time (ps) Figure 1. TRPD plots, along with fitted model curves using the TRPD signal equation and convolution of the thermal distribution. (a-c) Results at 266 nm for progressively longer cooling times. (d) Results at 355 nm (two-photon dissociation) for highly thermalized ions (cooling time of 3 s at 3 x lo-' Torr).
cm-I was chosen as the reaction coordinate and was excluded from the frequency set of the transition state. Although there are a number of variables involved in the RRKM calculation, as pointed out in ref 20, the rate-energy dependence can be determined by two parameters, the critical energy of activation, Eo, and the activation entropy, A P . The increment or decrement of A 9 (representingchanges in looseness of the transition state) was achieved by varying the transition state frequencies. This reduces the multiparameter problem to a two-parameter one, which is completely determined by the two experimental points. The calculated rate-energy curve was required to pass through the two experimental points, as shown in Figure 2. The kinetic parameters used to construct the rate-energy curve were Eo = 3.24 eV and A 9 = 6.34 eu. C. Cooling of Nickelocene Cation. The rate of fragmentation was shown to be a sensitive function of the internal energy of the dissociating parent ions. Measurement of fragmentation rate can be used in a thermometric way to probe the internal energy of ions at the moment of photon The ion internal energy is derived from the measured fragmentation rate using the RRKM rate-energy curve. Parent ions as produced by electron impact contain excess internal energy, which relaxes to lower values at later times by collisional and radiative cooling processes. Using this thermometric method, the cooling of initially hot parent ions can be monitored directly.
Nickelocene Ion Rate-Energy Curve
0
I
5
266 nm (1 -photon) 355 nm (2-photon) E0=3.24 eV. ~ S = 6 . 3 4eu.
1
I
6
7
Energy (eV) Figure 2. Rate-energy points derived from the thermal ion result! t 266 and 355 nm, with the calculated RRKM rate-energy curve constrained to pass through the points.
Cooling rate measurements by this thermometric technique were made for three neutral pressures p (0.9 x lo-*, 1.4 x lo-*, 4 x lo-* Torr) and for cooling times ranging from 0.1 to 4.3 s. Figure 1 shows the measured TRPD curves at the end of several different cooling time periods at 4.0 x IO-x Torr. As can be seen in Figure 1, the TRPD curves changed
J. Phys. Chem., Vol. 99, No. 6, 1995 1757
Photodissociation of Gas Phase Nickelocene Cation 4.2,
1
(a)
Cooling Curve
1.0-
torr
0.9 x 0.8-
TABLE 1: Experimental Cooling Constant Data neutral pressure Torr) cooling constant km, (s-1) 0.9 0.63 f 0.05" 1.4 0.80 f 0.05b 4.0 1.30 zk 0.08" a
~
Ellutlal = 1.00 eV; Eth = 0.294 eV. Elmtld = 0.97 eV; Eth = 0.294
h
2
0.0-
v
2 . 0.4
1
I
I
I
I
1
2
3
4
I
-
-k, 0
1000
2000
= 0.63s.'
4000
3000
5000
Cooling Time (ms) . ._
0'41
(b)
Cooling Curve
1.0-
1.4x108torr
0.2
-
0.8 h
%
v
I w
0
0.6-
Pressure (IO-*torr)
-
Figure 4. Pressure plot of the cooling rate constants derived from the fits in Figure 3.
0.4-
o'2
(or exponential in time).13 A cooling rate constant kCmlis defined as
1 I
I
,
I
I
0
1000
2000
3000
4000
0.0
5000
Cooling Time (ms)
giving
Eint= (Einitid- E,h)e-kc"''
(c) Cooling Curve
1.0-
where Eintis the internal energy of parent ion (in excess of the thermal equilibrium energy), Einitid is the initial total internal energy from electron impact, Efi is the thermal equilibrium energy at the cell temperature (375 K), and t is the time interval between the electron beam pulse and the laser pulse. The fistorder cooling hypothesis is equivalent to saying that kcoolis a true constant independent of ,??int. Based on the functional form of eq 3, Figure 3 shows the simulated exponential cooling curves as solid lines. The cooling constants obtained from these simulations for each pressure are summarized in Table 1. The cooling of ion internal energy has contributions from both collisional and radiative processes, i.e.
4.0 x 108torr
-
0.8 h
2
v
0.6-
B w
. 0.4
-
-
0
0.2
0
0
__ ksOoting= 1.30 s.' .
1
1000
-
,
2000
'
!
3000
'
,
4000
.
(3)
I
5000
Cooling Time (ms)
Figure 3. Ion cooling data measured by the TRF'D thermometric technique at three different pressures, along with fitted exponential cooling curves.
significantly as a function of cooling time, with the shortest cooling time corresponding to the hottest parent ions and the fastest fragmentation rate. The conversion of the measured fragmentation rates into ion internal energies was done using the rate-energy curve shown in Figure 2. The actual cooling population has a spread of intemal energies, but it was assumed that assigning a single, average energy Ebt at each time is a satisfactory approximation. Figure 3 shows the decay of ion intemal energy with cooling time at these three neutral pressures. The working hypothesis is often made that the decrease of internal energy is first-order
where k, and kr are the rate constants for collisional and radiative relaxation processes, respectively, and P is the pressure of neutral nickelocene. Following eq 4, a plot of kcoolvs pressure should give a straight line, as shown in Figure 4, whose slope and y intercept are equal to k, and kr, respectively. These derived values are summarized in Table 2. A comparison of the radiative cooling kinetics of ferrocene and nickelocene cations is of interest. They have the same number of degrees of freedom and practically the same heat capacity, so the factor-of-2 difference in cooling rates should largely reflect differences in frequencies and radiative strengths of the IR-emitting vibrational modes. Since the rings are bound less tightly to the nickel ion than to the iron ion, the ring(metal ion)-ring vibrational modes for nickelocene ion occur
1758 J. Phys. Chem., Vol. 99, No. 6, 1995
Lin and Dunbar
TABLE 2: Comparative Properties of Nickelocene and Ferrocene Ions Fe(Cph+a Ni(Cp)z+ EO (lst), eV 3.7 f 0.3 3.24 f 0.07 AP, eu 3-7 6.3 f 1.6 BDE (1st): M mol-' 357 f 29 313 f 7 BDE (2nd): kJ mol-' 415 f 29 382 f 7 k,, s-' 0.28 f 0.08 0.48 f 0.04 k,s-' molecule-' cm3 (9 f 3) x (8 f 1)10-'O Reference 1. Present work. Taken as equal to EO. Derived by subtraction of the first BDE from the total BDE, as discussed in connection with Table 4. TABLE 3: Thermochemical Values heat of formation species (Mmol-' at 298 K) species Ni(Cp)z
Ni(Cp)z+ Ni
Nif
357a 955" 43ob
CP NiCp+
heat of formation (Mmol-' at 298 K) 1167b 24lC 1026 f 7d
'Reference 22. Reference 23. Reference 24. Present result. TABLE 4: Thermochemistry of Metallocene Ring Dissociations (kJ mol-')
of the ligands is stronger for the ions than for the neutrals, by 69 kJ/mol for nickelocene ion and by 55 kJ/mol for ferrocene ion (referring to the average BDE per ligand); and (3) the binding is stronger in the ferrocene case than in the nickelocene case for both neutrals and ions, by 40 or 50 kJ/mol per ligand. All three of these observations are reasonable, as we discuss below. 1. First BDE versus Second BDE. The total BDE values are derived from the following reactions, all of whose components have well-established heats of formation:
-
+ 2Cp -.Ni' + 2Cp
Ni(Cp), Ni(Cp),f
Ni
AH = 555 kJ/mol
AH = 694 Id/mol
(5)
(6)
Dividing these numbers by two gives the average BDE, 278 kJ/mol for neutral nickelocene and 347 kJ/mol for the cation. The average BDE gives a useful first estimate of the metalring bond strengths for the neutral and cation of nickelocene. Subtracting the first BDE from the total BDE gives the second BDE, that is the enthalpy of the reaction M(Cp)+
-M+ +
Cp (AH = second BDE)
(7)
~
average BDE" BDE (1st dissociation) BDE (2nd dissociation)
278
347 313 f 7b 382 f 7d
328
383 357 f 29' 415 f 2 9
Derived from thermochemistry in Table 3. Present result. Reference 1. By subtraction of the first BDE from the total BDE. at lower frequencies than those of ferrocene ion.lg8 To the extent that the radiative cooling rate is governed by emission from these modes, we would expect cooling to be slower for the nickelocene case than for the ferrocene case. However, as addressed in ref 13, these low metal-ring vibrational modes are probably too low in frequency (125-480 cm-l) to make important contributions to cooling, and the major contribution to hadis expected to come from vibrational modes in the 7001400 cm-' range. Unfortunately, the infrared intensities of these higher-frequency modes are unknown, and little more can be said about this question without measurements or calculations of these intensities. D. Thermochemistry and Metal-Ring Bond Strengths. As was the case for ferrocene ion, the mechanism of dissociating one Cp ring from nickelocene cation can be reasonably taken to be a single-step dissociation without a significant potential barrier.' This presumption is supported by the positive dissociation entropy of activation, which implies a loose transition state. It is sensible to take the critical energy of activation, EO, as a good approximation to the dissociation endothermicity (at 0 K) for such a barrierless potential surface. Once the dissociation endothermicity is assigned from the RRKM value of Eo (313 kJ mol-I), the heat of formation of NiCp+ can be determined by energy balance since the heats of formation of NiCp2+ and Cp are well established. This thermochemistry, including the value of AHdNiCp+) derived in this work, is summarized in Table 3. It is interesting to discuss our value for the dissociation energy (which we will designate as the first bond dissociation energy, BDE) with the average BDE and also with the dissociation thermochemistry of neutral nickelocene. Several useful comparisons are possible using known heat-of-formation data, which are assembled in Table 4 for both nickelocene and ferrocene. Three points can be made from Table 4: (1) the first BDE for the ion is less than the second BDE, by 69 kJ/mol for nickelocene ion and by 58 kJ/mol for ferrocene ion; (2) binding
For both nickelocene and ferrocene ions, the f i s t BDE is significantly less than the second BDE. Other things being equal, the fist BDE is expected to be less than the average BDE for ligands clustering to an atomic ion because of the electrostatic solvation effects noted in ref 1. According to this argument, the first ligand is most strongly bound to the bare metal ion, and subsequent ligands are progressively less strongly bound. This argument accounts nicely for the observed trend for both ferrocene and nickelocene ions, but should probably not be expected to be quantitative because of the likelihood of major chemical bonding effects in these systems in addition to the purely electrostatic effects. 2. Ion versus Neutral. Other things being equal, attachment of ligands to a gas-phase atomic ion is much stronger than to the corresponding neutral atom, because of the electrostatic solvation energy (charge-multipole and charge-induced-multipole interactions) in the ion case. While chemical bonding effects may well be important in these cases as well, we can understand the ion versus neutral difference in terms of a purely electrostatic picture without looking for a further explanation. 3. Nickelocene versus Ferrocene. On going from ferrocene to nickelocene, the two additional electrons in nickelocene occupying the antibonding orbital (2e1,) weaken the metalring bond strength.25 As expected from this argument, Table 4 shows the metal-ring bond strength of neutral ferrocene to be stronger than that of nickelocene by 50 kJ/mol. The ions show a similar difference, with both first BDE's and average BDE's for ferrocene ion being greater than for nickelocene ion, by about 40 kJ/mol. There are still two more electrons in nickelocene cation than in ferrocene cation. One is in the nonbonding 2a1, orbital, and one is in the antibonding 2e1, orbital. It is expected that the nonbonding electron would not greatly affect the bond strength but that the electron in the 2elg orbital would significantly weaken the bond strength for nickelocene ion compared with ferrocene ion. Our results confirm this prediction.
IV. Conclusions Table 2 provides a summary of the final results of the present work, in comparison with results from ref 1. The present study is the first which has determined a confident value of the activation entropy for dissociation of a gas-phase organometallic
Photodissociation of Gas Phase Nickelocene Cation
J. Phys. Chem., Vol. 99, No. 6, 1995 1759
TABLE 5: Radiative Cooling Rates and Degrees of Freedom
(2) Russell, D. H., Ed. Gas Phase Inorganic Chemistry; Plenum: New York, 1989. (3) Hettich, R. L.; Jackson, T. C.; Stanko, E. M.; Freiser, B. S. J. Am. Chem. Soc. 1986,108,5086. Faulk, J. D.; Dunbar, R. C. J. Am. SOC.Mass Spectrom. 1991,2, 97. (4) Sunderlin, L. S.; Wang, D.; Squires, R. R. J. Am. Chem. SOC. 1993, 115, 12060. (5) Niles, S.; Prinslow, D. A.; Wight, C. A.; Armentrout, P. B. J. Chem. Phys. 1992,97,3115. (6) See the following recent papers and references therein: Lin, C. Y.; Dunbar, R. C. J. Chem. Phys. 1994,98,1369. Klippenstein, S.J.; Faulk J. D.; Dunbar, R. C. J. Chem. Phys. 1993,98,243. Gotkis, Y.; Naor, M.; Laskin, J.; Lifshitz, C.; Faulk J. D.; Dunbar, R. C. J. Am. Chem. Soc. 1993, 115, 7402. Ho,Y.-P.; Dunbar, R. C. J. Phys. Chem. 1993, 97,11474. (7) Baer, T. Adv. Chem. Phys. 1986,64,111. (8) Gotkis, Y.; Oleneikova, M.; Naor, M.; Lifshitz, C. J. Phys. Chem. 1993,97, 12282. (9) Syage, J. A.; Wessel, J. E. Appl. Specrroc. Rev. 1988,49,1. Neusser, H. J. J. Phys. Chem. 1989,93,3897. (10) Yim, Y. H.; Kim, M. S. J. Phys. Chem. 1993,97, 12122. (11) Uechi, G. T.; Dunbar, R. C. J. Chem. Phys. 1990,93,1626. (12) Lewis, K. E.; Golden, D. M.; Smith, G. P. J. Am. Chem. Soc. 1984, 106,3905. (13) Dunbar, R. C. Mass Spectrom. Rev. 1992,11, 309. (14) Copper, B. T.; Wilson, K. L.; Buckner, S. W. In Proceedings of the 40th ASMS Conference on Mass Spectrometry and Allied Topics, May 31-June 5, 1992, Washington, DC; p 1745. Copper, B. T.; Buckner, S. W. In Proceedings of the 41th ASMS Conference on Mass Spectrometry and Allied Topics, May 31-June 5, 1993, San Francisco, CA; p 1003a. (15) Dunbar, R. C. J. Chem. Phys. 1989,90,7369. (16) Dunbar, R. C.; Lifshitz, C. J. J. Chem. Phys. 1991,11, 3542. (17) Borrell, P.; Henderson, E. Inorg. Chim. Acta 1975,12, 215. (18) Dunbar, R. C. J. Chem. Phys. 1989,91,6080. (19) (a) Lippincott, E. R.; Nelson, R. D. J. Am. Chem. Soc. 1956,77, 4990; Specrrochim. Acta 1958, 10, 307. (b) Brunvoll, J.; Cyvin, S. J.; Ewbank, J. D.; Schaffer, L. Acta Chem. S c a d . 1972, 26, 2161. (c) Kimel'fel'd, Ya. M.; Smirnova, E. M.; Aleksanyan, V. T. J. Mol. Srmct. 1973,19,329. (20) Lifshitz, C. Adv. Mass Specrrom. 1989,11, 713. (21) Ho, Y. P.; Dunbar, R. C. J. Chem. Phys. 1993,97,11474. (22) Pedley; J. B.; Rylance, J. Sussex-N. P. L. Computer Analysed Thermochemical Data: Organic and Organometallic Compounds, University of Sussex, 1977. (23) (a)Wagman, D. D.; Evans, W. H.; Parker; V. B.; Schumm, R. H.; Halow; I. S.; Bailey, M.; Chumey, K. L.; Nuttall, R. L. J. Phys. Chem. Ref:Data 1982,Suppl. 1. (b) Dyke, J. M.; Gravenor, B. W. J.; Lewis, R. A.; Moms, A. J. Phys. B: At. Mol. Phys. 1982,15, 4523. (24) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982, 33,493. (25) Haaland, A. Acc. Chem. Res. 1979,12, 415.
radiative cooling rate constant (s-l) CiaHi4' FeCpz+ NiCpz+
0.50 0.28 f 0.08 0.48 f 0.04
degrees of freedom
66 57 57
ion. (In the ferrocene study,' because only one point on the rate-energy curve was measured, it was only possible to assign A 9 within a range corresponding to a family of possible rateenergy curves.) As shown in Figure 3, the rate-energy curve in the present study was determined by two experimental points separated in internal energy by 2.2 eV, which gives a confident value for the slope and thus for A P . The positive activation entropy, indicating a loose transition state, supports a simple bond-breaking mechanism for reaction 1. It was noted in ref 1 that large size (i.e., many degrees of freedom) in a polyatomic ion gives a high heat capacity and low internal temperature which is unfavorable to infrared cooling, and this was considered to be a reasonable explanation for the very slow cooling observed for ferrocene cation. Cooling of three polyatomic ions of about this size has now been characterized, as shown in Table 5, for the 0.3-1 eV internal energy regime. The cooling behavior is similar, with kcool in the range 0.25-0.5 SKI.For the two metallocene cations, it is obvious that their radiative cooling behavior is not very different from organic ions of similar size in this near-thermal energy regime. The dissociation thermochemistry of ferrocene and nickelocene ions seems very well established by this study and that of ref 1. The discussion of Table 4 shows that this thermochemical picture is entirely sensible, with the values and comparisons being well explained in chemically reasonable terms.
Acknowledgment. Support for this research is acknowledged from the National Science Foundation and the donors of the Petroleum Research Fund, administered by the American Chemistry Society. References and Notes (1) Faulk, J. D.; Dunbar, R. C. J. Am. Chem. Soc. 1993,114, 8596.
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