J. Phys. Chem. 1985, 89, 4422-4423
4422
resonances for 2 and 1 A1 in Figure 1 are truly absent and not lost in the background fluctuations, the substitution of AI in the 1.03 would not be strictly large single crystals with Si/AI random but would be subject to the rule that Si substitutes for only one A1 out of each pair of adjacent AI atoms sharing the same bridging Si atom. Further X-ray diffraction and N M R studies are desirable of specimens of zeolite A with Si/Al close to unity. To conclude, the present estimate of Si/AI = 1.03 f 0.01 is
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(10) Vego, A. J. ACS Symp. Ser. 1983, No. 218, 217. (11) Melchior, M. T. ACS Symp. Ser. 1983, No. 218, 243. (1 2) Jarman, R. H.; Melchior, M. T.; Vaughan, D. E. W. ACS Symp. Ser. 1983, No. 216, 267. (13) Hanson, A. L.; Jones, K. W.; Smith, J. V., to be published.
consistent with the value of 1.03 deduced by Pluth and Smith from electron microprobe analysis and X-ray diffraction data. Furthermore, it is consistent with the value of 1.04 obtained by proton probe analysis by Hanson, Jones, and Smith.13 Acknowledgment. We thank G . T. Kokotailo for the supply of crystals. J.J.P. and J.V.S. thank N S F for Grants C H E 0405167 and MRL DMR-82-16982. C.S.B. thanks E. M. Flanigen for supporting the solid-state N M R program in the Central Scientific Laboratory, Union Carbide Corp. We thank K. Seff and M. D. Mellum for stimulating us to make further measurements on the Si/Al ratio of the large crystals of zeolite A. E. M. Flanigen kindly reviewed the manuscript.
Ground-State Cr+ Ions Produced by Multiphoton Ionization of Cr(CO), S. K. Huang and Michael L. Gross* Department of Chemistry, University of Nebraska-Lincoln, (Received: June 21, 1985)
Lincoln, Nebraska 68588
Multiphoton dissociation-ionization (MPD/MPI) of volatile chromium hexacarbonyl at 266 and 353 nm generates a more nearly homogeneousor ground-state population of gas-phase metal ions than does electron ionization (EI). This was demonstrated for Cr+ by using Fourier transform mass spectrometry to measure the kinetics for the reaction of Cr' generated by either MPD/MPI or E1 with neutral Cr(C0)6. The kinetics for decay of EI-produced Cr+ ions are composite whereas the rate of disappearance of MPI-produced ions is noncomposite.
Introduction Reactions of metal ions and organic molecules now attract broad interest since metal ion-ligand interactions can be studied without solvent complications. There are various ways of producing metal ions in the gas phase: (a) thermionic (b) electron ionization of various organometallics and metal carbonyls, (c) laser desorption from a solid metal ~ u r f a c e (d) , ~ surface ionizati~n,~ and (e) glow discharge.6 However, these methods are problematic because some generate too few bare metal ions and most appear to produce electronically excited metal It has been suggested that the reactivity of certain metal ions is due to their electronic e~citation.~-~ One test of this hypothesis involves measuring the reactivity of ions produced exclusively in one electronic state, independent of the other. We wish to report here that a more nearly ground-state population of gas-phase Cr' ions can be generated by means of multiphoton dissociationionization (MPD/MPI) of Cr(C0)6 at 266 and 353 nm. Experimental Section The laser beam used for MPD/MPI was from a neodymiumyittrium aluminum garnet (ND:YAG) laser focused to 0.5 mm in diameter in the center of an ion trap of a Fourier transform mass spectrometer.1° The laser was operated at a repetition rate (1) Bewett, J. P. Phys. Rev. 1936, 50, 464. (2) Feeny, R. K.; Sayle, W. E.; Hooper, J. W. Rev. Sei. Instrum. 1976, 47, 964. (3) Smith, D. H.; Carter, J. A. In?. J . Mass Spectrom. Ion Phys. 1981, 40, 211. (4) Cody, R. B.; Burnier, R. C.; Reents, W. D., Jr.; Carlin, T. J.; McCrery, D. A,; Lengel, R. K.; Freiser, B. S. In?. J . Mass Specfrom.Ion Phys. 1980, 33, 37. (5) Armentrout, P. B.; Beauchamp, J. L. Chem. Phys. 1980, 50, 21. (6) Loving, T. J.; Harrison, W. W. Anal. Chem. 1983, 55, 1523. (7) Freas, R. B.; Ridge, D. P. J . Am. Chem. SOC.1980, 102, 7129. (8) Ridge, D. P. In "Ion Cyclotron Resonance Spectrometry 11"; Hartmann, H., Wanczek, K.-P., Eds.; Springer-Verlag: West Berlin, 1982; pp 140-1 5 1. (9) Armentrout, P. B.; Halle, L. F.; Beauchamp, J. L. J . Am. Chem. SOC. 1981, 103, 6501.
0022-3654/85/2089-4422$01.50/0
of 5 Hz, and the power was measured to be 2.4 J/cm2 per pulse when high power was utilized. The laser was triggered by the quench pulse. In the electron ionization (EI) mode, ions were generated by a 15-ms pulse of 35 or 145 nA at 70 eV. The number of ions was kept low (20 000-100 000 ions) with both methods, and the trap voltage was kept at 1.0 V to avoid ion evaporation." Pressure was read from an ion gauge situated near the FTMS cell and was set in the range of (2-5) X torr. A 5.08-cm cubic analyzer cell was employed in this work. Ions were excited by using a narrow-band radio-frequency burst (RF) to avoid inadvertent z excitation of the ions.13 Detection was over a narrow bandwidth in the heterodyne mode. An apodization technique and 32000 data points were used so that accurate peak heights could be obtained."J5 Results and Discussion In order to test the capability of producing only ground-state metal ions by using MPI/MPD, kinetics of the reaction of Cr' with neutral Cr(C0)6 were measured by following the disap(10) McCrery, D. A.; Ledford, E. B., Jr.; Gross, M. L. Anal. Chem. 1982, 54, 1437. For a review of FTMS, see: Gross, M. L.; Rempel, D. L. Science 1984, 226, 261. (11) Huang, S. K.; Rempel, D. L.; Gross, M. L. to be submitted for publication in fnr. J . Muss Spectrom. fon Phys. (12) Halle, L. F.; Armentrout, P. B.; Beauchamp, J. L. J . Am. Chem. SOC. 1981, 103, 962. (13) Huang, S. K.; Rempel, D. L. In "Proceedings of the 32nd Annual Conference on Mass Spectrometry and Allied Topics", San Antonio, TX, 1984; p 546. (14) The deconvolution was based on the assumption that there are two reactive species, A and B, and that one species reacts more rapidly than the other. For two parallel ion-molecule reactions A C and B C, we may Bo exp(-k2f)] where C, = A. Bo. By write C - C, = -[Ao exp(-k,t) assuming that k2 is greater than k,,values of A. and k , were obtained and then used to calculate B. From a plot of In B vs. f, Bo and k2 were obtained. No iterations of the calculations were made. For details see: Frost, A. A.; Pearson, R. G. 'Kinetics and Mechanism", 2nd ed.; Wiley: New York, 1961; pp 163-164. (15) (a) Giancaspro, C.; Comisarow, M. B. Appl. Specfrosc.1983.37, 153. (b) Huang, S. K.; Rempel, D. L.; Gross, M. L. In "Proceedings of the 32nd Annual Conference on Mass Spectrometry and Allied Topics", San Antonio, TX, 1984; p 596.
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0 1985 American Chemical Society
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The Journal of Physical Chemistry, Vol. 89, No. 21, 1985 4423
Letters
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Figure 1. Logarithm of the intensity (arbitrary units) of Cr+ as a function of time for reaction with Cr(C0)6 (p = 5 X lo-' torr): Cr+ formed by electron ionization (0) or multiphoton ionization (m) of Cr(CO),.The MPI data were followed to 1.4 s without loss in linearity. The two solid lines represent the two-component lines which fit the experimental data from E1 experiment.
pearance of the Cr+ ions generated with both E1 and MPI methods. The kinetic studies were done carefully to avoid problems with competitive ion relaxation and evaporation." Chromium ions were chosen as a model because it is known that electron ionization of chromium hexacarbonyl generates some excited-state The MPI mass spectrum of Cr(C0)6 at 1 X lo-' torr and 266 nm (not shown) is evidence that greater than 99% of the ions formed are Cr+. MPI at 353 nm gave substantially the same mass spectrum as did MPI at 266 nm. Electron ionization at 70 eV produced, in addition to Cr+ (30% of the total ions), Cr(CO)+ (20%), Cr(C0)2+(18%), Cr(CO)3+ (2%), Cr(C0)4+ (2%), Cr(CO),+ (2%), and Cr(C0)6+ (26%). It has been suggested that the observation of CrCH2+ in the reaction of Cr+ and methane is also indicative of the presence of excited-state Cr+,7as this reaction is endothermic'* for groundstate Cr+. The Cr+ generated by using E1 in our experiments also reacted with methane to give CrCH2+,whereas Cr+ ions generated with MPI showed no detectable reactivity. Furthermore, results of the kinetics of the reaction of neutral Cr(C0)6 and Cr+, generated by using both E1 and MPI of Cr(C0)6, are consistent with this observation (see Figure 1). It is clear from Figure 1 that the rate of disappearance of Cr+ generated by E1 is composite, which is interpreted to indicate that there are at least two different reacting species. On the other hand, the kinetics of Cr+ generated by MPI are noncomposite within the resolution of the experiment. A n a l y ~ i s of ' ~ the E1 kinetics produces the two-component lines shown in Figure 1. Two conclusions emerge from the kinetic analysis: fmtly, one species reacts 2.5 times faster than the other, and secondly, the two species are formed in approximately equal amounts. Results obtained with
MPI at 353 nm were similar to those obtained at 266 nm. Ridge et a1.* obtained a rate constant ratio of 4.8, and the more reactive state was shown to be 28% of the total population. Because of the approximate nature of the execution of the deconvolution of the kinetic resultsI4 and the underlying assumptions made in its development, the precision ascribed to the relative rate constants and mixture composition obtained here is approximately 25% relative. If the kinetic analysis and interpretation of the data presented here are correct, we would expect that the slope of the kinetic plot for Cr+ formed by MPI would be identical with the plot of the slower reacting Cr+ formed by EI. This is approximately true; the small difference may be attributed to instrumental discrepancies. For example, the number of ions formed by using MPD/MPI was difficult to control and was not equal to the number produced with EI. Moreover, the ion distribution in the cell was different as the MPD/MPI-produced ions are formed on diagonallo rather than along the z axis. It is also necessary to note that the kinetic analysis is based on the assumption of two Cr+ species, and this may be an underestimate. Other primary ions such as Fe+ from Fe(CO)5 and Mo+ from Mo(CO)~generated by using both E1 and MPI reacted with their respective neutrals and gave good linearity in the kinetic plots (not shown). These experiments were conducted under the same conditions as for Cr+, and these observations serve to rule out an instrumental origin for the composite decay of Cr+ generated with EI. It is evident that electron ionization of Cr(C0)6 generates both ground-state species (%) and some long-lived excited species (possibly 6D) and that the more reactive species is not generated by using MPI. These results serve as confirmation of the earlier proposals that E1 of Cr(C0)6 leads to production of Cr+ in an excited ~ t a t e . ~ - ~ MPI of metal carbonyls in the 400-500-nm range is thought to occur upon coherent absorption of two photons followed by dissociation and then absorption of another two photons (probably incoherently) to produce metal atoms. The atoms undergo 2 1 excitation to form metal ions.I6 Kinetic analysis of ejected electrons upon MPI and photochemical studies of Cr(C0)6 in the 400-500-nm rangeI7,l8were interpreted to show the existence of various excited states of chromium atoms. However, MPI at 266 and 353 nm appears to produce only ground-state Cr+ ions possibly because production of the first excited state of Cr+ involves a forbidden transition.
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Acknowledgment. We thank Professor D. P. Ridge for helpful discussions. This work was supported by the National Science Foundation (Grants CHE-8320388 and CHE-8018245). ~~
(16) Gedanken, A.; Robin, M. B.; Keubler, N. A. J. Phys. Chem. 1982, 86, 4096. (17) Gerrity, D. P.; Rothberg, L. J.; Viada, V. Chem. Phys. k t r . 1980, 74, 1. (18) Fisanick, G.J.; Gedanken, A.; Eichlberger,T. S., IV; Kuebler, N. A.; Robin, M. B. J . Chem. Phys. 1981, 75, 5215.
