1959
J . Phys. Chem. 1989, 93, 1959-1963 et al.14 saw no evidence to suggest this reaction.
concentration of CH300H in various conditions, Le., polluted and clean troposphere and the stratosphere, can be calculated. Our measured absorption cross sections for CH300H are lower than those of Molina and ArguelloI6 by a factor of -30% while our value of k , is -90% lower than that of Niki et al.Is at 298 K. However, we obtain a negative activation energy which yields a cm3 molecule-' s-' at the average trovalue of k , N 6 X pospheric temperature of 260 K. With the changes in the cross sections and k , , the relative CH300H loss due to photolysis and reaction with OH remains approximately the same. The current major uncertainty in the laboratory data dealing with CH300H is the rate coefficient for the CH302 H 0 2 reaction. On the basis of our measured value for kl, it might be expected that higher organic peroxides would also have a rate coefficient greater than or equal to k , . The two different pathways for reaction 1 have different effects on tropospheric chemistry related to methane oxidation. If reaction l a takes place, it is equivalent to chain termination by OH + H 0 2 reaction:
Summary The reaction of hydroxyl radical with C H 3 0 0 H is fast and proceeds via abstraction of H from the peroxy and methyl groups of the peroxide: CH300H + OH 5 CH302 + H2O
(la)
CH300H + OH -k, CH,OOH
+ HzO LCH20 + OH
+
(1b)
(21)
C H 2 0 0 H formed in reaction l b decomposes to give OH and CH20with a lifetime shorter than 20 ps (at 205 K). Measuring OH loss rates in the reaction CH300H OH, therefore, yielded the rate coefficient for channel l a only, which was determined exp((220 f 21)/T) cm3 to be kl, = (1.78 f 0.25) X molecule-I s-!. For measurement of the overall rate coefficient, k , = k,, klb for the reaction, isotopically labeled hydroxyl radicals, OD and 180Hwere used. The measured rate coefficients using OD and 180Hwere, within experimental errors, the same and yielded k l = (2.93 f 0.30) X exp((l90 f 14)/7') cm3 molecule-' s-l. Further evidence for the two-channel mechanism was obtained by direct observation of OH production in the H abstraction reaction, CH300H OD C H 2 0 0 H + HOD followed by CH200H CH20 OH. The possibility of OH production via the exchange reaction CH300H + OD CH3OOD + OH is unlikely since it was not observed in the analogous reaction between H202 and OD. The reaction of OD with CH300D was half as fast as that between OH and CH300H. Both reaction 1 and channel l a show negative activation energies in the temperature range 203-423 K, and the possibility of a complex reaction pathway with an attractive part to the CH300H-OH interaction rather than a simple H atom abstraction mechanism cannot be excluded.
must be of the same order of magnitude in the atmosphere to cm3 molcompete with the decomposition, Le., k211 1 X ecule-l s-l at atmospheric temperatures. If the reaction is that fast, then depending on the products of reaction 21, CHOOH or C H 2 0 0 can be formed if O2 abstracts an H atom. We saw no evidence for an increase in k l , in the presence of O2 This suggests that either reaction 21 is slow, it regenerates OH, or both. Niki
Acknowledgment. We thank J. B. Burkholder and J. J. Orlando for performing the infrared analyses and V. Sankar Iyer for the 'H NMR analyses. This work was supported by NOAA as part of the National Acid Precipitation Assessment Program. Registry No. OH, 3352-57-6; CH300H, 3031-73-0; CH200H, 74087-87-9; '*OH, 65553-37-9; H20, 7732-1 8-5; H2, 1333-74-0; H, 12385-13-6; D2,7782-39-0.
CH302 OH net
+ HO2
+
+ CH300H OH
+ H02
CH300H
+
-+
+
+
+02
CH302 + H2O H20
+0 2
-
On the other hand, reaction l b leads to formation of CH20, which is a net HO, producer in the atmosphere. In either case, formation of C H 3 0 0 H and its subsequent reaction with OH is a net HO, sink in the atmosphere. The fate of CH200H formed in reaction l b under atmospheric conditions of O2composition may depend on whether it reacts with O2 or not. We have estimated that C H 2 0 0 H should decompose to CH20 and OH within -20 ps at 205 K, and the decomposition is likely to be faster at higher temperatures. Therefore the reaction of CH200H with 02:
C H 2 0 0 H + O2
k21
products
+ +
-
+
Photochemistry of Mo(CO), in the Gas Phase Jane A. Ganske and Robert N. Rosenfeld* Department of Chemistry, University of California, Davis, California 9561 6 (Received: August 10, 1988)
We report a study of the photochemistry of Mo(CO)~in the gas phase. Time-resolved infrared laser absorption spectroscopy is used to monitor the.vibrationa1spectroscopy and lifetimes of the coordinatively unsaturated species formed upon photolyses at 351, 248, and 193 nm. The infrared spectra observed indicate that MO(CO)~ has C , symmetry, Mo(CO), has C , symmetry, and Mo(CO), has C, symmetry. All three unsaturated species undergo rapid association reactions with Mo(CO)~and with CO. MO(CO)~ recombines with CO with a high-pressure limiting rate constant of 2.0 (*0.2) X lo6 Torr-l s-l. The corresponding rate constants for Mo(CO)~and Mo(CO), are 7.5 (&1.5) X lo6 and 1.8 (fl.O) X lo7 Torr-ls-', respectively.
Mo, etc.), have been reported to catalyze reactions such as the ( 1 ) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry: University Science Books: Mill Valley, CA, 1987.
0022-3654/89/2093-1959$01.50/0
(2) Masters, C. Homogeneous Transition Metal Catalysis; Chapman and Hall: London, 1981.
0 1989 American Chemical Society
1960 The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
geometric isomerization and the hydrogenation of olefins. Such catalytic activity has been observed in the gas phase3 as well as in so1ution.l Flash photolytic methods have proven to be an effective means for generating coordinatively unsaturated organometallic c~mplexes.~ Solution-phase studies have shown that coordinatively unsaturated species can bind solvent molecules on a picosecond time scale.5 This suggests that attempts to characterize the reactivity of species, M(CO),, may yield data only on the corresponding solvated species, M(CO),-S. Cryogenic matrix isolation spectroscopy may suffer a similar difficulty (Le., matrix molecules may bind at open coordination sites). Nevertheless, such experiments6 have provided a wealth of data on the vibrational spectra of many species, M(CO),. Kinetic data on the association reactions of unsaturated s p i e s with various ligands are not available from experiments in low-temperature matrices. Studies of the spectroscopy and reactivity of unsaturated metal centers in the gas phase offer the opportunity to characterize the chemistry of species unperturbed by interaction with solvent and thus obtain data on the intrinsic reactivity of these important intermediate^.^ Additionally, metal centers with more than one vacant coordination site can be easily generated in the gas phase.7 This has not been found to be the case in s o l ~ t i o n . ~ * ~ Time-resolved infrared absorption spectroscopy has found extensive use in studies of the spectroscopy and kinetics of unsaturated transition-metal complexes both in solution10and in the gas p h a ~ e . ~ ? Here, ~ ' - ~ ~we report the application of this method in studying the photochemistry of Mo(CO), in the gas phase and the vibrational spectra and reactivity of the unsaturated species, Mo(CO), ( n = 3-5). We consider our results in light of analogous data on other metal carbonyls that have been reported recently.
Experimental Section The instrumentation used in our experiments has been previously described in detailI4 and so is discussed only briefly, here. Mo(CO)~,either neat or mixed with one or more additional gases, is contained in a 100-cm Pyrex cell fit with CaF2 windows. The sample is photoactivated with light from a rare gas halide excimer laser operating at 1-2 Hz on the ArF*, KrF*, or XeF* transitions (corresponding to 193, 248, or 351 nm, respectively). The laser pulse width is ca. 15-20 ns and the laser fluence is typically held in the range 1-5 mJ/cm2. The output of a continuous-wavecarbon monoxide laser is directed through the absorption cell colinearly with respect to the UV laser. The CO laser is grating tuned and is operated on selected rovibrational transitions in the range 1850-2 100 cm-I for the experiments reported here. The UV and IR laser beams are co- or counterpropagated through the absorption cell and are combined and separated by using dichroic optics. Transient changes in the intensity of the transmitted IR beam following the UV laser pulse are monitored with an InSb detector having a measured rise time of ca. 200 ns. Signals from
Ganske and Rosenfeld 200 I
5
150
1
I
1880
1900
1920
1940
1960
1980
2000
Frequency (cm - l ) Figure 1. Infrared spectrum observed following the 35 I-nm photolysis of M o ( C O ) ~ . [Mo(CO),] = 0.030 Torr, [He]= 40 Torr. UV laser fluence 2 mJ/cm2. Delay time (Le., interval after firing of UV laser) 868 ns. 300 I
1
'.- ; i l
; I -2E
200
1
0
1860 1880 1900 1920 1940 1960 1980 2000 Frequency (cm - l )
Figure 2. Infrared spectrum observed following the 248-nm photolysis of M o ( C O ) ~ . [Mo(CO),] = 0.030 Torr, [ A r ] = 50 Torr. UV laser fluence 4 mJ/cm2. Delay time 465 ns.
- 400
1
c L
: 200 5
-
a, U
3 E 100 -
L
0 '
1860 1880 1900 1920 1940 1960 1980 2000
(3) (a) Whetten, R. L.; Fu, K. J.; Grant, E. R. J . Chem. Phys. 1982, 77, 3769. (b) Fu, K. J.; Grant, E. R. J . Am. Chem. SOC.1982, 104, 4270. (4) (a) Breckenridge, W. H.; Sinai, N. J. Phys. Chem. 1981,85, 3557. (b) Breckenridge, W. H.; Stewart, G. M. J . Am. Chem. SOC.1986, 108, 364. (5) Welch, J. A.; Peters, K. S.; Vaida, V. J . Phys. Chem. 1982, 86, 1941. (6) (a) Perutz, R. N.; Turner, J. J. J . Am. Chem. Soc. 1975,97,4800. (b) Burdett, J. K.; Graham, M. A.; Perutz, R. N.; Poliakoff, M.; Rest, A. J.; Turner, J. J.; Turner, F. J . Am. Chem. SOC.1975, 97, 4805. (c) Perutz, R. N; Turner, R. J . Am. Chem. SOC.1975, 97,4791. (7) Poliakoff, M.; Weitz, E. Adu. Organomet. Chem. 1986, 25, 277. (8) Geoffroy, G. L.; Wrighton, M S. Organometallic Photochemistry; Academic: New York, 1979. (9) Wrighton, M. S.Chem. Reu. 1974, 74, 401. . ( I O ) (a) Hermann, H.; Grevels, F.; Henne, A,; Schaffner, K. J . Phys. Chem. 1982,86, 5151. (b) Schaffner, K.; Grevels, F. W. J . Mol. Struct. 1988, 173, 51. ( I 1) Rayner, D . M.; Nazram, A. S.; Drouin, M.; Hackett, P. A. J . Phys. Chem. 1986, 90, 2882. (12) (a) Weiller, B. H.; Miller, M. E.; Grant, E. R. J . Am. Chem. SOC. 1987, 109, 352. (b) Weiller, B. H.; Grant, E. R. J. Am. Chem. Soc. 1987, 109, 1051. (13) Fletcher, T. R.; Rosenfeld, R. N. High Energy Processes in Organometallic Chemistry; Suslick, K. S . , Ed. ACS Symp. Ser. 333; American Chemical Society: Washington, DC, 1987; p 100. (14) Fletcher, T. R.; Rosenfeld, R. N. J . Am. Chem. Soc. 1985, 107, 2203.
Frequency (cm - ' ) Figure 3. Infrared spectrum observed following the 193-nmphotolysis of M o ( C O ) ~ . [Mo(CO),] = 0.030 Torr, [CO] = 0.300 Torr, [Ar] = 50 Torr. UV laser fluence 4 mJ/cm2. Delay time 713 ns. C O was added to the gas mixture in this case to reduce consumption of Mo(CO), and to reduce the formation of clustering products which tended to deposit on the cell windows.
this detector are amplified, digitized at 32 MHz, and then stored in a microcomputer for subsequent averaging and data analysis. Normally, 5&100 transients are averaged to obtain signal to noise ratios 1 5 . Measurements of the spectra of coordinatively unsaturated photolysis products are obtained by accumulating a set of (averaged) transients for each CO laser line in the range 1850-2100 cm-I. The amplitudes of each transient at a selected time following photolysis are then measured and plotted. The resolution of the resulting spectra is determined by the spacing between adjacent CO laser lines, Le., ca. 4 cm-'. Kinetic measurements are made by tuning the CO laser to the (previously assigned) frequency corresponding to a desired species and recording the amplitude
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 1961
Photochemistry of Mo(CO), of the signal vs time. In studies of the recombination of coordinatively unsaturated species with CO, gas mixtures consisting of 0.007-0.040 Torr of Mo(CO),, 0.150-0.400 Torr of CO, and 10-200 Torr of buffer gas (He, Ar, or SF,) are used. All gases are obtained from commercial vendors and are used as supplied. Mo(CO), is sublimed and stored under vacuum prior to use. Partial pressures of the various species in the absorption cell are measured by using capacitance manometers.
Results and Discussion Infrared spectra, recorded following the photolysis of Mo(CO),, are shown in Figures 1-3. These spectra were taken at times following photolysis indicated in the figure captions. The times were selected to maximize the extent of rovibrational relaxation of the hot nascent fragments, while also minimizing the extent of secondary, bimolecular reactions of the fragments. At earlier times, the observed spectra are broadened and red-shifted relative to those shown in the figures. At later times, bimolecular association reactions deplete the concentrations of the primary products. In general, the temporal evolution of the bands observed depends on the partial pressures of Mo(CO),, CO, and buffer gas. Increasing [ Mo(CO),] promotes clustering reactions, leading to the formation of binuclear complexes; increasing [CO] promotes recombination; increasing the buffer gas concentration accelerates rovibrational cooling. Spectra have been assigned on the basis of both comparisons with matrix isolation and other gas-phase studies, as well as measurements of reaction rates with CO. Recombination kinetics were determined by photolytically generating the species, Mo(CO),, and monitoring its decay rate in the presence of C O and buffer gas. Additionally, the kinetics of the formation of the recombination product, MO(CO),+~,were also monitored. Both types of measurements gave identical results, within experimental error. Rate constants were determined both by dividing absorption signal decay rates by the measured C O pressure and by plotting the decay rate vs [CO]. Both methods gave identical results, within experimental error. All rate constants reported here were determined in the presence of sufficient buffer gas to ensure high-pressure limiting condition^,'^ unless otherwise noted. The extent of Mo(CO), fragmentation following UV photolysis depends on the energy of the incident photon and the bond dissociation energies for the species, Mo(CO),, where n 16. The first bond dissociation energies (BDE's) for a variety of metal carbonyls have been but reliable data on subsequent BDE's is limited. Average bond energies, Le., the value of A H o / n for ( l ) , have been used as a guide. Recent work's*'s M(CO),
-
M
+ nCO
(1)
has, however, shown such average bond energies to be a poor measure of the individual BDE's. The first BDE of Mo(CO), has been measured by Lewis et a1.I6 using the "very low pressure photolysis" method. The find DHO [(CO)5Mo-CO] = 40 kcal/ mol. No data on the BDE's of MO(CO)~or Mo(CO)~have been reported. Such thermochemical quantities can be determined on the basis of RRKM fits to recombination kinetic datal5 and will be reported in a subsequent article. The species formed upon the UV photolysis of Mo(CO), include C O and coordinatively unsaturated species, Mo(CO),. We consider our results on each of these species, below. Carbon Monoxide. Following UV photolysis, transient absorption is observed in the region 2100-2010 cm-', due to carbon monoxide. This assignment follows both from the magnitude of the absorptions (ai= 1 cm-' Torr-') and the pressure dependence (15) Fletcher, T. R.; Rosenfeld, R. N. J . Am. Chem. Soc. 1988, 110, 2097. (16) Lewis, K. E.; Golden, D. M.; Smith, G. P. J. Am. Chem. SOC.1984, 106, 3905. (17) Bernstein, M.; Simon, J. D.; Peters, K. S. Chem. Phys. Lett. 1983, 100, 241. (18) (a) Engelking, P. C.; Lineberger, W. C. J . Am. Chem. SOC.1979,101, 5569. (b) Stevens, A. E.; Feigerle, C. S.; Lineberger, W. C. J. Am. Chem. SOC.1982, 104, 5026.
TABLE I: Infrared Absorption Frequencies (in Units of c d ) for Unsaturated Molybdenum Carbonyls'
matrix Mo(CO),
a, e
Mo(CO)~
a, a, b,
a, Mo(CO)~
b, a, e
Mo(CO)*
2,
gas phase
CH4
Ar
ref 19
this work
2093 1967 1926 2057 1945 1927 1887 1981 1862 1911
2098 1973 1933
1985 1942
1983 1940
1951
1972
1970
1895
1911
1869
1885
1914 1990 1888
Matrix data from ref 6a. of their decay rates. The 351-nm photolysis of Mo(CO),, eq 2, has an exoergicity, hu - DHO = 41 kcal/mol. This energy is
sufficient that, if the majority is partitioned to Mo(CO), internal degrees of freedom, secondary dissociation, yielding M o ( C O ) ~ CO, may occur. In fact, we find that the CO product carries away relatively little vibrational energy (ca. 99% formed in u = 0) and M o ( C O ) ~is observed following the 351-nm photolysis of Mo(CO),. The fact that both MO(CO)~and M o ( C O ) ~are observed immediately after photolysis (vide infra) indicates that the reaction exoergicity is distributed among the internal modes of the nascent M O ( C O ) ~product as well as the relative rotational and translational degrees of freedom of the primary products. Molybdenum Pentacarbonyl. Vibrational spectroscopy of matrix-isolated MO(CO)~ and some of its 13C-labeledisotopomers suggests that this coordinatively unsaturated species has a square pyramidal, C4,, geometry.6a We have assigned absorptions observed here at 1940 and 1983 cm-' to the vco(al) and vco(e) modes of Mo(CO),, respectively. These assignments are consistent with the M O ( C O ) frequencies ~ observed in other worklg (see Table I). Additional confirmation comes from kinetic studies of the recombination of M O ( C O )with ~ CO, eq 3. The rate constant for
+
(3) this reaction, k3, increases with buffer gas pressure, [MI, and attains a high-pressure limiting value, k3,- = 2.0 (f0.2) X lo6 Torr-' s-l for [He] 20 Torr. Our assignment of the 1940- and 1983-cm-' bands to M O ( C O ) ~ is confirmed by the following observations: 1. The rise time for the transients observed at these frequencies is detector limited for [He] L 20 Torr. 2. Both bands decay with the same rate constant in the presence of co. 3. In the presence of CO, Mo(CO), is regenerated with the same rate constant which describes the decay of the 1940- and 1983-cm-I bands. 4. The measured rate constant, k,,,, is independent of photolysis wavelength. We find that Mo(CO), is a primary product of the 351- and 248-nm photolyses of Mo(CO),. It is not observed following 193-nm photolyses unless CO is added to the gas mixture. In this case, Mo(CO), grows in with a rate constant equal to that for M o ( C O ) ~decay (vide infra). This indicates that the M O ( C O ) product ~ of the 193-nm photodissociation of Mo(CO),contains sufficient internal energy that it undergoes secondary decomposition on a time scale comparable to, or faster than, our detection system time constant. The M o ( C O ) ~formed following the 351- and 248-nm photolyses of Mo(CO), is, initially, rovibrationally excited. It undergoes vibrational relaxation, upon collisions with helium, with a rate constant of ca. lo5 Torr-' s-I. This is similar in magnitude to the rate constants reported for the (19) Ishikawa, Y.; Hackett, P. A.; Rayner, D. M. J. Mol. Struct. 1988, 174, 113.
1962 The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
vibrational relaxation of other metal carbonyls. If Mo(CO), is photolyzed at 351 nm, in the absence of any added CO, the Mo(CO), product is observed to decay at a rate dependent on the partial pressure of Mo(CO),. This decay is attributable to a clustering reaction, resulting in the formation of M O ~ ( C O ) and ,~, has a rate constant of ca. 5 X 10, Torr-' s-I. As noted above, matrix isolation studies of the infrared spectroscopy of Mo(CO), apparently indicate the species exists principally in the square pyramidal geometry, I, rather than the D3htrigonal bipyramidal geometry, 11. Symmetry considerationsm
co
CO
I
I1
suggest that both isomers should have two infrared-active CO stretching vibrations with an intensity ratio vco(al)/vco(e) = 1:3 and 3:2 for the C4, and D3h isomers, respectively. Our data are consistent with the conclusions reached by other workers68 that the minimum energy geometry corresponds to the C4, structure, I. Molybdenum Tetracarbonyl. Thermochemical considerations suggest that the production of M o ( C O ) ~is possible following the photolysis of Mo(CO), at any wavelength 5351 nm. It has previously been shown that the 351-nm photolysis of Cr(C0)6 yields the corresponding penta- and tetracarbonyls?' whereas only W(CO)5 is observed22upon the 351-nm photolysis of W(CO)6. We find that Mo(CO)~is formed upon the photolysis of Mo(CO), at both 351 and 248 nm. Absorption bands observed at 1914 and 1970 cm-' are assigned to the vco(bl) and vCO(b2)modes of Mo(CO)~.These assignments are in accord with both the matrix isolation studies of Turner and co-workers68and the gas-phase results of Ishikawa et al.19 Such findings indicate that M o ( C O ) ~ adopts a C2, geometry (111). A D4hstructure ( I v ) is predicted,
I11
IV
on the basis of symmetry considerations,20 to have only a single IR-allowed vco(e) band. M o ( C O ) ~is not formed in appreciable yield following the photolysis of Mo(CO), at 193 nm. In this case, however, the addition of C O results in the formation of this species, with time, as a result of recombination with M o ( C O ) ~ . The kinetics of the recombination of M o ( C O ) ~with CO, eq 4, can be studied in analogy to the corresponding reaction of Mo(CO),
+ CO
ki
MO(CO)~
(4)
Mo(CO),, eq 3. Reaction 4 is found to have a high-pressure limiting rate constant, k4,- = 1.6 (f0.4) X lo7 Torr-' s-'. This rate constant was observed for [Ar] = 40 Torr. A systematic study of the pressure dependence of k4 will be reported later.23 The measured value of k4,- indicates that Mo(CO), reacts with C O on nearly every gas kinetic collision. This high reactivity is similar to that previously observedl5 for the reaction of Cr(C0)4 with CO, where the high-pressure limiting rate constant was found to be 7.5 (*1.5) X 10, Torr-' s-I. The M o ( C O ) ~formed upon the 248-nm photolysis of Mo(CO), is initially found to be rovibrationally excited. This is evidenced by absorption bands which narrow and blue shift with increasing
Ganske and Rosenfeld numbers of collisions. The rate constant for vibrational relaxation of [Mo(CO),]* is found to be approximately 4 X lo5 Torr-' s-I for collisions with argon. The absorption features assigned to Mo(CO)~are observed to decay even in the absence of added CO. We ascribe this decay to a clustering reaction with Mo(CO),, leading to the formation of a binuclear complex. (5). The rate
2M o ~ ( C O ) I O
M o ( C O ) ~+ M o ( C O ) ~
constant for the clustering reaction is found to be, k5 = 2 ( f l ) X lo7 Torr-' s-I. Thus, M o ( C O ) ~reacts with Mo(CO), on essentially every gas kinetic collision, Le., with no appreciable barrier. This is analogous to behavior previously observedI4 for Cr(C0)4, which was found to cluster with Cr(C0)6 with a rate constant of 1.8 (f0.3) X lo7 Torr-' s-l. The vibrational spectrum of the adduct, Mo2(CO),,,, could not be unambiguously assigned due to overlap with other spectral features. Molybdenum Tricarbonyl. The most intense infrared absorption for M o ( C O ) ~is expected to be the vco(e) transition, near 1885 cm-I. In fact, we observe an absorption band centered at 1888 cm-I in spectra recorded following the photolysis of Mo(CO)~ at both 248 and 193 nm. Matrix isolation studies have been reported6awhich indicate that M o ( C O ) ~has two IR-active bands, corresponding to vco(al) and vco(e). The a l band is observed in CH,, but not Ar, matrices.6a The observation of two vco bands is consistent with a C3,geometry (V), but inconsistent with a D3h geometry (VI), which symmetry considerations require to have
V I
V
only a single allowed vco mode. We assign the band observed here at 1888 cm-' to the vco(e) mode of Mo(CO)~.An additional band is observed at 1990 cm-l, which we tentatively assign to the vco(al) mode. This assignment is regarded as tentative because the band overlaps other absorption features, which prevents us from confirming the assignment via kinetic measurements (Le., of recombination rates with CO). In the recently reported results of Ishikawa et al.,I9 a high-frequency vco(al) band was not reported. The kinetics of Mo(CO), recombination with CO, eq 6, were examined in order to confirm the assignment of the 1888-cm-' band. We find the high-pressure limiting rate constant is k6,M o ( C O ) ~+ CO
k6
Mo(CO)~
(6)
= 1.8 (fl.O) X lo7 Torr-' s-I. This rate constant was measured with [M] = [SF,] = 20 Torr so the onset of the high-pressure limit is 5 2 0 Torr, in this case. Again, the value observed for k6 indicates a negligible barrier for the recombination of M o ( C O ) ~ with CO. This is similar to observations reported for the recombination reactions of other coordinatively unsaturated metal carbonyls. For example M(CO)3 recombines with C O with a rate constant of 9.6 X lo5, 7.0 X lo5, and 1.3 X lo6 Torr-' s-' for M = Cr, Fe, and V, respectively (see ref 24-26). When Mo(CO), is generated in the absence of CO, the absorption signal corresponding to the tricarbonyl is found to decay at a rate proportional to the pressure of M o ( C O ) ~ .In analogy to (5), this decay is due to a clustering reaction, (7), which occurs on every gas kinetic collision; i.e., we find k7 = 2 (*1) X lo7 Torr-' SKI. Again we conclude there is essentially no barrier to the
MO(CO), (20) Cotton, F. A. Chemical Applications of Group Theory, 2nd ed.; Wiley-Interscience: New York, 197 1 . (21) Seder, T. A,; Church, S. P.; Weitz, E. J . Am. Chem. SOC.1986, 108, 4721. (22) Ishikawa, Y . ;Hackett, P. A,; Rayner, D. M. Chem. Phys. Lett. 1988, 145, 429. (23) Ganske, J. A.; Rosenfeld, R. N., manuscript in preparation.
(5)
+ Mo(C0)6 -% Mo,(C0)9
(7)
(24) Seder, T. A.; Church, S . P.; Weitz, E. J . Am. Chem. Soc. 1986, 108, 4721. (25) Seder, T. A.; Ouderkirk, A. J.; Weitz, E. J . Chem. Phys. 1986, 85, 1977. (26) Ishikawa, Y . ;Hackett, P. A,; Rayner, D. M. J . Am. Chem. Soc. 1987, 109, 6644.
J . Phys. Chem. 1989, 93, 1963-1969 clustering reaction. Spectral features due to the binuclear cluster, M O ~ ( C O )were ~ , not readily discerned because of overlap with other absorption bands. In the 193-nm photolysis of Mo(CO),, the available energy is likely sufficient that the formation of M O ( C O )is~ thermochemically possible. Low-temperature matrix studies suggest that M O ( C O )has ~ a uc0 band near 1911 cm-I. We observe no strong absorption features in the region about 1920 cm-' (Le., allowing for a blue shift on going from a cryogenic matrix to the gas phase) that can be assigned to MO(CO)~, although it is possible that such features, if present, would be obscured by nearby M o ( C O ) ~bands. However, we note that the addition of CO accelerates the rise time of the M o ( C O ) ~band at 1888 cm-' [as a result of rovibrational relaxation of hot, nascent M o ( C O ) ~ ]but does not increase the amplitude of the absorption feature. This suggests that the recombination of M O ( C O ) with ~ C O is not occurring to any significant extent and we thus infer that no appreciable amount of M O ( C O ) is ~ formed upon the 193-nm photolysis of Mo(CO),.
Conclusions Time-resolved infrared laser absorption spectra have been obtained for the coordinatively unsaturated molybdenum carbonyls, Mo(CO)~,Mo(CO),, and Mo(CO)~.These reactive intermediates were generated by UV laser photolysis of the parent hexacarbonyl. We find that M O ( C O ) and ~ M o ( C O ) ~are produced upon the 351and 248-nm photolyses of Mo(CO),, whereas M o ( C O ) ~and Mo(CO)~are formed following 248- and 193-nm photolyses. The infrared spectra observed support the following assignments for , Mothe Mo(CO), geometries: M O ( C O ) ~C4"; , M o ( C O ) ~ C2";
Absence of Spin-Orbit Effects on V+(a3F,)
1963
(CO),, C3".These assignments are in accord with those made on the basis of low-temperature matrix isolation spectra. The kinetics of the reactions of unsaturated molybdenum carbonyls with C O have been determined. We find the highpressure limiting rate constants for recombination with C O are M O ( C O ) ~2.0 , (f0.2) X lo6 Torr-' s-'; Mo(CO)~,1.6 (f0.4) X lo7 Torr-' SKI; and M O ( C O ) ~1.8 , (fl.O) X lo7 Torr-I s-'. All three recombinations occur near the gas kinetic collision rate, indicating a negligible barrier for these association reactions. These findings are in accord with other work on the recombination kinetics of coordinatively unsaturated metal carbonyls which indicate that such reactions are fast so long as they occur with spin conservation. We have also reported evidence that unsaturated molybdenum carbonyls react with Mo(CO),, forming binuclear clusters. These association reactions are also found to occur on essentially every gas kinetic collision. The results reported here demonstrate the utility of infrared laser absorption methods in characterizing the chemistry of metal carbonyls in the gas phase. Both spectroscopic and kinetic data may be obtained on highly reactive species which, in some cases, cannot be generated in solution. In a subsequent paper, we will report on the use of kinetic data, such as that reported here, in evaluating metal-carbon bond strengths in coordinatively unsaturated molybdenum carbonyls.
Acknowledgment is made to the National Science Foundation for support of this work. Registry No. M o ( C O ) ~ , 13939-06-5; C O , 630-08-0; M O ( C O ) ~ , 32312-17-7; Mo(C0)4, 44780-98-5; Mo(CO)S, 55979-29-8.
+ CPHBReaction Cross Section at 0.2 eV
Scott D. Hanton, Lary Sanders, and James C. Weisshaar* Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706 (Received: October 10, 1988)
We have used resonant two-photon ionization and angle-resolved photoelectron spectroscopy to create and characterize V+ ion beams in a variety of electronic-state distributions. For seven different ionization wavelengths, branching fractions have been measured for five different resolved V+ states: the ground a5D and first excited a5F terms (with spin-orbit levels J unresolved) and the J = 2, 3, and 4 levels of the second excited a3F term. In a crossed-beam configuration using pulsed time-of-flight mass spectrometric detection of reactant and product ions, we measure relative total V+ + CzH6 reaction cross sections at 0.2 eV of kinetic energy, averaged over each of the seven state distributions. A least-squares fitting procedure then yields state-specific relative reaction cross sections. The a5D and a5F cross sections are negligible (l0.005) compared with the a3FJ cross sections. Within the substantial error limits, the three a3FJ cross sections are indistinguishable. The ratio of cross sections u ( J = 2 ) / o ( J = 4 ) is 1.07 i 0.35 (95% confidence limits). The J = 3 cross section is poorly determined by the data. The result suggests that nonadiabatic transitions between potential energy surfaces correlating with J = 2, 3, and 4 of a3F are rapid on the time scale of reagent approach. At least for the Vf(a3F) + C2H6 reaction, the absence of severe spin-orbit effects validates an important conceptual simplification commonly assumed in previous work, namely, that transition-metal cation reactivity depends on the initial electronic term (configuration, L, and S ) , but not on the spin-orbit level J . We present a comprehensive model of the V+ + CzH6 reaction, postulating C-H bond insertion as the rate-determining step.
Introduction The gas-phase chemistry of transition-metal cations M+ has been explored extensively in the past 10 years by using an arsenal of sophisticated ion cyclotron resonance,' ion beam,2 fast-flow
r e a ~ t o r tandem ,~ mass ~pectrometry,~ and cros~ed-beam~ techniques. Reactions of M+ species with H2, 02,alkanes, alkenes, and other species have been studied at varying levels of detail. Certain first transition series metal cations such as Sc+ and Fe+ are chemically noteworthy6 for their ability to activate both C-C
(1) See, for example: Allison, J.; Freas, R. B.; Ridge, D. P. J. Am. Chem. SOC.1979, 101, 1332. Jones, R. W.; Staley, R. H. J. Am. Chem. SOC.1982, 104, 1235. Byrd, G. D.; Burnier, R. C.; Freiser, B. S. J. Am. Chem. Soc. 1982, 104, 3565. (2) See, for example: Armentrout, P. B.; Beauchamp, J. L. J. Am. Chem. SOC.1981, 103, 784. Elkind, J. L.; Armentrout, P. B. J. Phys. Chem. 1985, 89, 5626. Elkind, J. L.; Armentrout, P. B. J. Phys. Chem. 1987, 91, 2037.
(3) (a) Tonkyn, R.; Ronan, M.; Weisshaar, J. C. J. Phys. Chem. 1988,92, 92. (b) Tonkyn, R.; Weisshaar, J. C. J. Phys. Chem. 1986, 90, 2305. (c) Tonkyn, R.; Weisshaar, J. C. J. Am. Chem. SOC.1986, 108, 712. (4) Larson, B. S.; Ridge, D. P. J. Am. Chem. SOC.1984, 106, 1912. (5) Sanders, L.; Hanton, S.; Weisshaar, J. C. J. Phys. Chem. 1987, 91,
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5145.
0 1989 American Chemical Society
