J . Phys. Chem. 1985, 89, 5666-5670
5666
Chemical Reactions and Collislonal Quenching of the Chromium Atomic Ion in a Metastable Excited State William D. Reents, Jr., AT& T Bell Laboratories, Murray Hill, New Jersey 07974
F. Strobel, R. B. Freas, 111, John Wronka, and D. P. Ridge* Department of Chemistry and Center for Catalytic Science and Technology, University of Delaware, Newark, Delaware 1971 6 (Received: September 17, 1984)
The kinetics of reaction between Cr' produced by 70-eV electron impact on Cr(C0)6 are described. Approximately 26% of the Cr+ ions react at a rate 4.4 times slower than the remaining Cr+ ions. This is interpreted in terms of an excited state that is long lived on the time scale of the experiment (2 s). The variation of ion abundance with time in a mixture of Cr(CO), and CH4 following a pulse of 70-eV electrons is analyzed. The analysis indicates that the excited state reacts with CHI to form CrCH2+but that the primary result of collision between the excited Cr+ and CH4 is quenching of the excited state. If the quenching process produces ground-state Cr+ rather than another excited state, it is spin forbidden. Collision-induced decomposition of CrCH4+supports the involvement of an excited state in the reaction which produces CrCH2+. The CrCH2+ ion was not observed to react with CO. The possibility that Fe+ formed by electron impact on Fe(CO), includes ions in metastable states is discussed.
Introduction Recently we described a study of the reactions of first-row transition-metal atomic ions with butanes in the gas phase.' Cr+ and Mn+ generated by electron impact ionization on Cr(C0)6 and Mnz(CO)lowere found to differ in their reactivity from the group 8 atomic metal ions. Fe+, Co+, and Ni+ generated by electron impact on Fe(CO)5, Co2(CO),, and Ni(CO), react with the butanes to form various metal-olefin Mn+ undergoes no observable reactions.' Cr+ reacted with the butanes to form metakolefin complexes, but it does not react with isobutane to form a metal-propene complex.' A metal-propene complex is the dominant product in the reactions of the group 8 metal ions with isobutane.' It is somewhat surprising that Cr+ with its very stable d5 electronic configuration should react at all. In fact, collision-induced decomposition studies of CrC,H complexes suggest a very weak interaction between Cr+ and the butane.] The observed reactivity of Cr+ led to studies to determine whether an excited state of Cr+ might be involved. Several conclusions of these studies were added to the report of the study of the atomic metal ion chemistry with butane as a "note added in proof'.' A full description of these studies is the purpose of the present report. First, the kinetics of reactions of Cr+ with Cr(C0)6 are considered. Next, the reaction of Cr+ with CHI and the collision-induced decomposition of CrCH4+are discussed. The reaction of excited Cr+ with CH, produces CrCH2+. Since this reaction is endothermic for ground-state species, it is particularly useful for characterizing the excited state. Evidence that the excited state of Cr+ is collisionally quenched is presented and discussed. The possibility that excited states play a role in the observed chemistry of other metal ions formed by electron impact on metal carbonyls is considered. The reaction with CH4 of Cr+ produced by electron impact on Cr(C0)6 has been examined in an ion beam experiment.* Cross (1) Freas, R. B.; Ridge, D. P. J. Am. Chem. SOC.1980,I02,7129-7131. (2) Allison, J.; Freas, R. B.; Ridge, D. P. J . Am. Chem. SOC.1979, 101, 1332. (3) Larsen, B. S.; Ridge, D. P. J. Am. Chem. SOC.1984,106, 1912-1922. ( 4 ) Armentrout, P. B.; Beauchamp, J. L. J . Am. Chem. SOC. 1981, 103, 784-79 1. ( 5 ) Houriet, R.; Halle, L. F.; Beauchamp, J. L. J. Am. Chem. SOC.1983, 105. 1818-1829. (6) Byrd, G. D.; Burnier, R. C.; Freiser, B. S . J. Am. Chem. SOC.1982, 104, 3565-3569. (7) Jacobsen, D. B.; Freiser, B. S . J . Am. Chem. SOC.1983, 105, 51 97-5206. (8) Armentrout, P. B.; Halle, L. F.; Beauchamp, J. L. J . Am. Chem. SOC. 1981, 103, 6501-6502.
0022-3654/S5/2089-5666$01.50/0
SCHEME I 70 ev electron
C r ( C 0)6
Cr(C0)6 +
[Cr(c~~+]**
Cr+o Cr+*
k0
'
, Products
Cr(C0)6 k*
sections were determined at several translational energies and appearance potential measurements were made. These results will be discussed in the context of the present results.
Experimental Section The ion cyclotron resonance spectrometer used in these studies is of conventional design and can be used in either the drift mode or the trapped-drift modesg The timing for the trapped-drift experiment is done with a PAR CW-I boxcar averager. The rectangular cell is 2.54 cm X 2.54 cm in cross section and has a source region 2.54 cm long and an analyzer region 7.62 cm long. The ion collector is 2.54 cm long. Pressures are measured by an ionization gauge. The Fourier transform mass spectrometer used was a Nicolet lT/MS-lOOO which has been previously described.I0 The collision-induced decomposition experiments described here were done on a ZAB-2F reverse geometry mass spectrometer at the duPont Experimental Station and on the MS5O-TA at the Midwest Center for Mass Spectrometry at the University of Nebraska in Lincoln, Nebraska." In both cases, the collision gas was helium, the accelerating voltage 6 kV, and chemical ionization (tight) sources were used. Cr(C0)6 was introduced into the ion source by a solid sample probe. Other gas and liquid samples were introduced through either the reagent gas inlet or through the batch inlet system. The total pressure in the source was not measured directly, but the source housing pressure was in the to IO4 torr range, suggesting source pressures of several tenths of a torr. Most of the source pressure was hydrocarbon gas. The lesser part (from one-tenth to one-third) was metal compound. Ionization was by electron impact at either 280 or 70 eV. Either electron energy gave the same results. All compounds used were commercially obtained and their purity was confirmed by mass spectrometry. (9) For reviews of ion cyclotron resonance techniques, see: Beauchamp, J. L. Annu. Rev. Phys. Chem. 1971, 22, 527-560. Lehman, T. A,; Bursey, M. M. "Ion Cyclotron Resonance Spectrometry"; Wiley: New York, 1976.
(10) Reents, Jr., W. D.; Mujsce, A. M. Int. J . Muss Spectrom. Ion Processes 1984, 59, 65. (11) Gross, M. L.; Chess, E. K.; Lyon, P. A,; Crow, F. W.; Evans, S.; Tudge, H. Int. J. Muss Spectrom. Ion Phys. 1982, 42, 243-254.
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 26, 1985 5667
Cr+ in a Metastable Excited State TABLE I: Kinetic Parameters for Cr(C0)6Reactions' press.,btorr ko/k* 2.2 x 10-8 0.264 0.215 1.08 x 10-7 1.01 x 10" 0.214 av
0.225 f 0.019
f
k(CrCO+)/k'
0.750 0.785 0.709 0.743 f 0.039
0.857 0.859 0.964 0.893 f 0.061
k*,b cm3 molecule-' 7.67 x 1O-Io
s-I
8.37 X 8.40 X (8.15 f 0.41) x 1O-Io
k* and ko are rate constants for reactions of excited- and ground-state Cr+ with CrC06, respectively. k(Cr(CO)*) is the rate constant for reaction of CrCO+ with Cr(C0)6. f i s the fraction of Cr+ reacting with rate constant k*. bThe pressures listed and used to calculate k* are nominal ionization gauge pressures. As a result, the actual pressures are probably lower and the actual rate constants higher than those listed. TABLE 11: Variation of Abundance of Cr+ with Time relative abundance time. s obsd" calcdb diff
0.5
-
w 0 0.4-
z a
n
z
2
0.3-
a
i5 02 -
0.I 0
1.oooo 0.9531 0.8994 0.8090 0.7298 0.6554 0.5978 0.5403 0.4900 0.4492 0.4103 0.3743 0.3181 0.2343 0.1863 0.1456 0.0832 0.0522 0.0339 0.0175
0 0.1 0.25 0.50 0.75 1.oo 1.25 1.50 1.75 2.00 2.25 2.50 3.00 4.00 5 .OO 6.00 9.00 12.00 15.00 20.00
40
80
120
160
200
1.oooo 0.9573 0.8974 0.8070 0.7275 0.6572 0.5952 0.5403 0.4918 0.4487 0.4105 0.3765 0.3192 0.2366 0.1821 0.1448 0.0831 0.0527 0.0345 0.0173
-0.0042 0.0020 0.0020 0.0022 -0.00 19 0.0026 0.0000 -0.0017 0.0005 -0.0001 -0.0022 -0.001 1 -0.002 3 0.0042 0.0008 0.0001 -0.0005 -0.0006 0.0002
'Observed abundance of m / z 52, CrCin Cr(C0)6 at indicated times following ionizing pulse at a nominal pressure of 2.2 x IO-* torr. Calculated time = 0 abundance was set at 1. bCalculatedm / z 52, Cr+ abundances at indicated times from eq 2 using the parameters in Table
240
TIME (MSEC)
Figure 1. Variation of ion abundance with time following a 5-ms pulse of 70-eV electrons in Cr(CO)+ The solid line through the Cr+ data was calculated by using the kinetic parameters given the text. The line through the Cr(CO)+ data is the best fit to the data. The nominal torr. pressure was 4.0 X
Results and Discussion Reaction of Cr(CO),. The fragment ions produced by electron impact on Cr(CO), react with Cr(C0)6 according toi2J3
I.
cording to Scheme I the relative intensity of the m / z 52 chromium atomic ion, 152, should be given by
1971,
where (Z5& is Z52 at t = 0 and n is the Cr(C0)6 number density. Cr+* represents the excited state of Cr+ and Cr* the ground state. from eq 1 with The solid line in Figure 1 is a plot of In (152/(Z52)o) f = 0.704 and P/k* = 0.204. These values are within a standard deviation of the average of values obtained at three different pressures by using the FTMS- 1000. The parameters obtained from a simplexi4nonlinear least-squares fitting routine at the three pressures are given in Table I. The reproducibility of the rate constant determinations are evident from the data in Table I. The quality of the fit of eq 2 to time vs. relative intensity data at one of the pressures is evident in Table 11. Note, in Table I that the ratio of k* to the rate constant for disappearance of Cr(CO)+ is quite reproducible. The value of that ratio obtained from the line in Figure 1 is 0.959 indicating further that the ICR and FTMS data are quite consistent. The rate constant ratios in Table I are independent of pressure measurements. From the nominal pressures determined by a conventional ionization gauge, the absolute values of k* in Table I are determined. The values are approximately constant with pressure. This eliminates the possibility that radiative or collisional relaxation play a role in the kinetics. The radiative lifetime of the excited state can be limited as greater than or equal to 2.0 s from the rate constant vs. pressure data. To further test the reliability of the data the analogous disappearance of Fe+ in Fe(CO)S was monitored. A linear In (IFe+)
1978. (13) A preliminary account of the results of this section are in Ridge, D. P. "Lecture Notes in Chemistry", Vol. 31, Hartmann, H., Wanczek, K. P., Eds.; Springer: West Berlin, 1982; p 140.
(14) Noggle, J. H. "Physical Chemistry on a Microcomputer"; Little Brown: New York, 1985; p 148.
Cr(CO),+
+ Cr(CO),
-
+
Cr2(C0)6+n-m+ mCO
(1)
where n = 0-6 and m is usually 1-3. Using the trap-drift ICR technique it is possible to monitor the resulting disappearance of the fragment ions with time following a burst of ionizing electrons. The results of such an experiment are shown in Figure 1. Shown are the variation with time of Cr+ and CrCO+ formed in a 5-ms burst of the ionizing beam. The reaction time is the time between the ionizing pulse and ion detection. Bimolecular reactions such as reaction 1 should obey pseudo-first-order kinetics since the neutral reactant is present in large excess. From Figure 1 it is apparent that while Cr(CO)+ follows pseudo-first-order kinetics, Cr+ does not. Reaction Scheme I accounts for the peculiar kinetic behavior of the Cr+ ion. The parent ion is formed in a highly excited state by electron impact and rapidly loses 6 C O molecules. A fraction, of the resulting Cr+ is formed in a metastable excited state. The remaining fraction, 1 -J is formed in the ground state. The two states react with different rate constanti to form products. Ac(12) (a) Kraihanzel, C. S.;Conville, J. J.; Sturm, J. E. Chem. Commun. 159-161. (b) Allison, J. Ph.D. dissertation, University of Delaware,
~~
~
5668
Reents et al.
The Journal of Physical Chemistry, Vol. 89, No. 26, 1985 h
X'03
m/z '/. E
Figure 2. The collision-induced decomposition spectrum of CrCD4+ ( m / z 72) resulting from reaction of CrCO+ and CD,. The CrCD4+ ions accelerated to 6-kV collided with He. Shown is a mass spectrum of the collision fragments.
vs. time plot was obtained, in good agreement with the one already published by Foster and B e a ~ c h a m p . ' ~ The conclusion suggested by this data is that Cr+ is formed by electron impact on Cr(C0)6 in at least two long-lived states. One is presumably the ground state and the other, a long-lived excited state, which reacts with Cr(CO), much more rapidly than does the ground state. Other states may be formed initially, but evidently are not sufficiently long lived to effect the observed kinetics. It is also possible that each of the kinetically distinct populations contain ions in more than one state that happen to react at the same rate constant. Reaction with CH, and CID of CrCD,'. It i s apparently the excited state of Cr+ which accounts for reaction 3 Cr+* + CH4
-
CrCH2+ + H 2
(3)
which is identified by double resonance. The reaction is endothermic for ground-state species.8 The excited state may also account for the reaction of Cr+ with butanes.' Halle, Armentrout, and Beauchamp have described an appearance potential study which shows that an excited state of Cr+ is involved in reaction 3.8
Collision-induced decomposition (CID) of CrCH4+also suggests the participation of an excited state of Cr+ in reaction 3. Reaction 4 is quite general for a wide variety of molecules, A, reacting with
MCO+ + A
-
MA+ + co
(4)
MCO+ fragments produced by electron impact on metal carbonyls. The metal complexes thus formed can be subjected to collisioninduced decomposition in an appropriate mass spectrometer.'^^ Fragments produced by the collision are mass analyzed and conclusions can then be drawn about the structure of the parent ion. It is generally found that products of direct reaction of M+ with A correspond to fragments produced by collision-induced decomposition of MA+. This is illustrated for M = Fe and A = i-C4DI0in eq 5 and 6. This is to be expected since the decomFe+ t /-C4D10
&-
F e C d t /-C4D10
FeC4Dlo
85x
FeC,D:
t
CD4
FeC,D:
t
D,
15) 58% CID/-
FeC3DQ t CD4 FeC4D:
k%
t D2
(6)
other f r a g m e n t s
position of MA+ and the reaction between M+ and A occur on the same potential surface. The collision process samples a wider range of energies than does the bimolecular reaction. The lower energies typical of the bimolecular reaction, however, are substantially represented in the energies deposited in the complex in the collision process. This is suggested by the results in e q 5 and 6 and by other related result^.^ Figure 2 is the collision-induced decomposition spectrum of CrCD,'. The ion was formed by reaction between CD4 and (15) Foster, M. S.; Beauchamp, J . L. J . Am. Chem. SOC.1975, 97, 4808-14.
0.2
40
80 120 Tirne/lo-'s
160
200
Figure 3. Variation of ion abundance with time following a 5-ms pulse of 70-eV electrons in a Cr(C0)6 mixture with CH,. The nominal Cr(CO), pressure was 7.0 X lo-' torr and the nominal CH, pressure was 3.0 X 10" torr. The solid lines were calculated from kinetic parameters in the text.
CrCO' formed by electron impact on Cr(CO), (eq 4). Entirely absent is a CrCD2+ fragment, even though Cr' reacts with CD, to produce CrCD2+(eq 3). The best explanation for this is that the collision-induced decomposition of CrCD4+ and the reaction between Cr+ and CD4 occur on different potential surfaces. The collision-induceddecomposition occurs on the ground-state surface and the reaction occurs on an excited-state surface. This is consistent with the evidence described above that Cr+ is produced by electron impact in two states which may be recognized by the rates of their reactions with Cr(C0)6. It is consistent with Halle, Armentrout, and Beauchamp's measurement of the heat of formation of CrCH2+which indicates that reaction 3 is 1.9-eV endothermic for ground-state species8 It is also consistent with their measurements of the appearance potential of CrCH2+which is -2.5 eV above that for Cr+.8 Finally, it is consistent with simple notion that ground-state Cr+ with its very stable d5 electronic configuration should be less reactive than other ground-state atomic transition-metal ions. It is not surprising that CrCD,+ is formed in the ground-state manifold. The precursor ion, CrCO', is a triatomic species with the possibility of vibronic coupling between ground and excited electronic states. It is, therefore, less likely that CrCO' should be formed by electron impact on Cr(C0)6 in a metastable excited electronic state than the atomic Cr+ ion. Furthermore, should electronically excited CrCO' survive to collide with CH4 and form CrCH4+,the electronic excitation might well be quenched in the reactive collision. Quenching of Excited Cr+. Another aspect of the interaction of the excited Cr+ with CH4 is revealed by analysis of the time dependence of Cr+ and CrCH2+in the mixture of Cr(C0)6 and CHI. The decay of Cr+ followed by ion cyclotron resonance and the increase and subsequent decay of CrCH2+which follow an ionizing pulse of 70-eV electrons are shown in figure 3. The curves through the data points were determined from the kinetic scheme outlined in Scheme I and reactions 7-9.
The Journal of Physical Chemistry, Vol. 89, No. 26, 1985 5669
Cr+ in a Metastable Excited State TABLE III: Variation of Abundance of Cr+ and CrCH2+with Time
time, ms 0 5 10 25 35 45 60 70 80 90 100 110 120 130 150 200 225 275
Cr' abundance obsd" calcdb diff
CrCH2+abundance obsd" calcdb diff
1.0000 0.9241 0.8844 0.7056 0.6331 0.5690 0.5144 0.4627 0.4250 0.3905 0.3621 0.3350 0.3073 0.2854 0.2536 0.1799 0.1615 0.1296
0 0.0071 0.0081 0.0126 0.0138 0.0136 0.0122 0.0110 0.0094 0.0084 0.0074 0.0065 0.0053 0.0041 0.0035 0.0016 0.0012 0.0006
1.0000 0.9304 0.8683 0.7177 0.6401 0.5761 0.4988 0.4568 0.4204 0.3886 0.3605 0.3354 0.3128 0.2925 0.2563 0.1874 0.1609 0.1189
0.0064 -0.0161 0.0121 0.0070 0.0071 -0.0156 -0.0059 -0.0046 -0.0019 -0.0016 0.0004 0.0055 0.0069 0.0027 0.0075 -0.0006 -0.0106
0 0.0047 0.0082 0.0136 0.0144 0.0140 0.0124 0.0109 0.0095 0.0081 0.0068 0.0057 0.0047 0.0039 0.0026 0.0009 0.0005 0.0001
-0.0023 0.0001 0.0009 0.0006 0.0005 0.0001 0.0001 0.0001 -0.0003 -0.0006 -0.0008 -0.0006 -0.0002 -0.0010 -0.0007 -0.0007 -0.0005
"Observed abundances of m / z 52 (Cr') and m / z 66 (CrCH2+)in a mixture of Cr(C0)6(6.0 X lO-'torr) and CH4 (1.0 X lod torr) at the indicated times following an ionizing 70-eV electron beam pulse. Calculated abundances of Cr+ at t = 0 was set at l . bAbundancesof Cr+ and CrCH2+calculated from parameters given in the text. The rate constants which give a good fit to the data in Figure 3 vary over a substantial range. The ICR data are noisy because the CrCH2+peak is very weak under all conditions. It is necessary to have a large concentration of ions to see it at all. The resulting space charge effectively increases noise in the ion cyclotron resonance signal. The constants used to calculate the lines in Figure cm3 s-I, ko = 3.1 X cm3 3 a r e f = 0.75, k* = 8.4 X s-', k7 = 0.6 X lo-'' cm3 s-l, k8 = 3.4 X lo-'' cm3 s-l, and k9 = 5.8 X cm3 s-l. Note that nominal ion gauge pressures were used in all the calculations so that absolute magnitudes of the rate constants are less significant than relative values. We discuss uncertainties in these numbers more fully below. The important feature of these results is that if k8 is set at zero, the data cannot be satisfactorily fit. The best fits that the simplex finds with k8 = 0, with k* in the range (6-10) X lO-I0cm3 s-l and f in the range 0.50-0.85, give standard deviations larger by a factor of 4 or more than that for the fit in Figure 3. The lines pass between the error limits of only a few points. This supports the occurrence of the quenching process shown in reaction 8. A similar conclusion is suggested by the data in Table 111. Given there are observed intensities of Cr+ and CrCH2+ as a function of time following ionization of a mixture of Cr(C0)6 and CHI by a pulse of a 70-eV electron gun in the FT instrument. The signal to noise is not as good as it is in the data shown in Table I1 because of problems with the weak CrCH2+signal similar to those encountered in the ICR. Nevertheless, the signal to noise is better than the ICR experiment. As a result, the rate constants which give a good fit to the data are somewhat more precisely determined. The simplex nonlinear routine was used to search for best fit and f and all of the rate constants were allowed to vary. No restriction was placed on the search to ensure that f and k*, for example, matched the values in Table I. Nevertheless, the values of k* andfobtained from the search, 8.37 X 1O-Io cm3 s-' and 0.746, agree reasonably well with the values in Table I. The other values obtained are ko = 3.09 X cm3 s-l, k7 = 0.45 X cm3 s-', k8 = 3.44 X cm3 s-l, and k9 = 1.37 X cm3 s-l. Once again setting k8 = 0 makes it impossible to find a satisfactory fit. Only if k* is increased by a factor of 2.5 and f is set at 0.22 is a fit approaching that shown in Table I11 obtained. This again supports the importance of reaction 8 in the overall kinetic scheme. Analyzing the response of the standard deviation of calculated ion abundances from observed abundances ("sensitivity analysis") and the reproducibility of constants derived from several data sets suggests the following set of constants for the reactions in the mixture of Cr(CO)6 and CH,: f = 0.70 f 0.06, k* = (8.4 f 2.1) X cm3 SKI,k" = (3.1
SCHEME I1 Crtx
+
CH4-
CH-Cr-Ht*+
3
I
C H 2 Cr-H+*--+
Crt t CH?
CrCH2f + H2
H
TABLE I V Some Electronic States of Cr+
configuration 3d5
3d5
state a 6S a 6D a 4D a 4G a 4P b 4D
3d4 4s
b 4P
3d5
a 'I
3d4 4s 3d44s 3dS 3d5
eV
kcal mol-'
0 1.483 2.421 2.543 2.706 3.104 3.714 3.738
0 34.20 55.84 58.65 62.40 71.58 85.64 86.19
"Sugar, J.; Corliss, C. J . Phys. Chem. Ref Data 1977, 6, 317. f 0.9) X lo-'' cm3 s-I, k7 = (0.46 f 0.15) X cm3 s-', k8 = (6.1 f 2.7) X cm3 s-l, and k9 = (9.7 f 4.0) X 1O-Io cm3 s-l. These constants are in satisfactory agreement with those in Table I. The large uncertainties in several of them reflect the difficulty in monitoring the weak CrCH,' signal. The numbers were determined by using nominal ion gauge pressures and probably underestimate the actual absolute rates. The published sensitivity factor of 1.4 for methane,16 for example, would require that k7 and k8 be multiplied by 1.4. This would give (0.64 f 0.21) X cm3 s-l for k7 compared to a value of 1.25 X cm3 s-l obtained from the cross section for reaction 7 measured as a function of translational energy.* This number is obtained by assuming the cross section of 13 AZis constant over a Boltzmann distribution of relative velocities at 300 K and that the same relative population of excited state is produced in both experiments. Also assumed, of course, is that the pressures assigned in both experiments is correct. Given these assumptions, the agreement between the beam result and the kinetic analysis of the ICR and FTMS results is acceptable, Mechanism of CH, Interaction. A mechanism for both the quenching of the excited state of Cr+ and the formation of CrCH2+ is given in Scheme 11. The initial metal insertion followed by reductive elimination of excited methane gives the quenching process. If the metal insertion is followed by migration of a second H atom to the metal center, then reductive elimination of H2 gives the CrCH,+ product. The state of the Cr+ formed by electron impact on Cr(C0)6 has not been unambiguously identified, but as indicated in Table IV several quartet states about 2.5 eV above the ground state are probable candidates. Transitions from these states to the sextet ground state would be spin forbidden so the states should be metastable. If the Cr' goes from a quartet to a sextet, then the product CH4* might be expected to be in a triplet state to conserve spin. That is unlikely, however, since triplet states of CH, are almost certainly energetically inaccessible. Reactions which do not conserve spin are unusual, but not unknown. Numerous facile nonspin-conserving reactions of transition-metal complexes are known.17 One mechanism is to convert orbital angular momentum to spin. If Cr' in the 4P state 2.70 eV above the ground state were to convert a quantum of orbital angular momentum into a quantum of spin angular momentum, it would become the 6S ground state. It may be that the quenching mechanism in Scheme I1 facilitates such a conversion. A variety of mechanisms are possible and further experiments need to be done to evaluate them. Excited Fe'from Fe(CO),. As mentioned above, Fe+ formed by electron impact on Fe(CO), reacts with Fe(CO), with a single rate constant. Nevertheless, identification of excited Cr+ from
-
(16) Summers, R. L. NASA Technical Note TN D-5285, National Aeronautics and Space Administration, Washington, DC, June, 1969. (1 7) Pearson, R. G. "Symmetry Rules for Chemical Reactions"; Wiley: New York, 1976; p 124 ff.
5670 The Journal of Physical Chemistry, Vol. 89, No. 26, 1985
electron impact of Cr(C0)6 raises the question of whether other metal ions might be prepared in long-lived excited states in a similar way. We will briefly address that question with regard to Fe+ from Fe(CO), here. Although the kinetics of the reaction of Fe+ with Fe(CO)5 do not reveal an excited state, a reaction which is endothermic for ground-state Fe+ has been reported. Reaction 10 occurs between C H 3 0 H and Fe+ produced by electron Fe+
+ CH30H
-
FeOH+
+ CH,
(10) impact on Fe(C0)5.'8 This reaction is 0.70 f 0.20 eV endothermic for ground-state species.19 We measure the rate constant as 0.8 X 10-Io cm3 s-l which corresponds to an apparent efficiency of approximately 3%. That is, reaction occurs on 3% of collisions between Fe+ and CH30H. This result could be explained if as little as 3% of the Fe+ formed by electron impact on Fe(CO)5 is in a metastable state at least 0.50 eV above the ground state. This excited state (or states) does not evince itself very dramatically. Product distributions of reactions between Fe+ and several organic molecules seem to be almost independent of the source of Fe+. Ions produced by electron impact on Fe(CO)5 give product distributions in good quantitative agreement with ions produced by laser desorption from a metal ~ u r f a c e .The ~ product distributions are also in qualitative agreement with product distributions observed for Fe+ ions produced thermionically and accelerated to 0.5 eV in the center-of-mass frame.3 It may be that reaction of the excited state occurs on the ground-state surface. The efficient quenching observed for the excited state of Cr+ in a collision with CH4 suggests that moving from one surface to another is possible for these atomic metal ion systems. The fact that C H 3 0 H displaces CH3 from Fe(CH,OH)+ l 8 suggests that a ground-state complex of Fe+ with C H 3 0 H readily assumes a CH3-Fe-OH+ structure. If the complex on that ground-state surface has enough vibrational energy, FeOH+ will be formed. If Fe+ in an electronically excited state forms a complex with CH30Hand crosses over to the ground-state surface, it will still have considerable internal energy only it will be vibrational instead of electronic. The loss of CH3and formation of FeOH+ might then result. Thus, a reaction initially involving electronically excited Fe+ might actually occur on the ground-state surface of Fe(CH30H)+. This is one possible reason that the (18) (a) Allison, J.; Ridge, D. P. J. Am. Chem. SOC.1976,98,7445-7446. (b) Allison, J.: Ridge, D. P. J. Am. Chem. SOC.1979, 101, 4998-5009. (19) Using D(Fe*-OH) = 3.3 f 0.2 eV from Murad, E. J . Chem. Phys. 1980,73, 1381-1385, and D(CH,OH) = 92.3 f 1 kcal/mole from McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982, 33,493-532.
Reents et al. presence of excited states of Fe+ may not change either the products or even the product distribution very dramatically. Since reaction can occur on the ground-state surface, the excited state probably reacts efficiently. That is, unlike Cr+ where crossing from the excited state to the ground state results in an unreactive collision, Fe+ in an excited state probably reacts on every collision. That, in turn, leads to the conclusion that only -3% of the Fe+ ions formed by electron impact on Fe(CO), are in a long-lived excited state. Any reaction of Fe+ produced in this manner which proceeds with an efficiency significantly greater than 3% is probably exothermic for ground-state species. Reactions of CrCH,'. Preliminary investigations of the reactivity of CrCH2+ have been made. These are hampered somewhat by the fact that the relative abundance of CrCH,' in the ion cyclotron resonance cell is always small. Nevertheless, addition of ethylene to Cr(C0)6 and CH4 distinctly diminishes the CrCH2+peak. No double resonance is observed corresponding to reaction 11, but double resonance is difficult when the product
-
CrCH2+ + C2H4
Cr+
+ C,H6
(11)
-
is an abundant electron impact produced ion. It appears that C2H4 quenches the Cr+ excited state with a rate constant cm3 s-l. Reaction 11 could be occurring with a smaller rate constant. This and related systems are receiving further examination. In contrast, C O does not change the CrCH2+concentration significantly. Thus, even though reaction 12 is exothermic, it does CrCH2+ + C O
-
Cr+
+ CH2C0
-
(12)
not proceed efficiently. The upper limit on this rate is lo-'' cm3s-'. Nor does C O quench excited Cr+. Note that a quenching mechanism analogous to that suggested in Scheme I1 might be accessible to C2H4,but not to CO. It is also true that C2H4has a triplet state to which energy might be transferred from an excited quartet state of Cr+ to give the 6Sground state of Cr+. The failure of (12) to proceed efficiently may be the result of the inability of the metal center in CrCH2+to interact strongly enough with CO to activate it.
Acknowledgment. The N S F provided partial support of this work under grant C H E 81 10516. M. L. Gross and the staff at the Midwest Center for Mass Spectrometry and C. E. McEwen and M. Rudat at duPont provided invaluable assistance in obtaining the collision-induced decomposition spectra. Registry No. Cr', 14067-03-9; Cr(C0)6, 13007-92-6; CH4, 74-82-8; CrCH2', 82961-44-2; CrCH.,', 98875-14-0.
