54
J. Phys. Chem. 1996, 100, 54-62
Reactions of Y+, Zr+, Nb+, and Mo+ with H2, HD, and D2 Michael R. Sievers, Yu-Min Chen,† J. L. Elkind,‡ and P. B. Armentrout* Department of Chemistry, UniVersity of Utah, Salt Lake City, Utah 84112 ReceiVed: August 3, 1995X
Guided ion beam mass spectrometry is used to examine the kinetic energy dependence of reactions of the second row transition metal cations, Y+, Zr+, Nb+, and Mo+, with molecular hydrogen and its isotopologues. By using a meter long flow tube ion source, we are able to create Zr+, Nb+, and Mo+ ions that are believed to be in their electronic ground state terms and primarily in the lowest spin-orbit levels and Y+ mostly in its ground state. Corresponding state-specific reaction cross sections are obtained. Analysis of the cross section data yields 0 K bond dissociation energies of D0(Y+-H) ) 2.65 ( 0.08 eV, D0(Zr+-H) ) 2.26 ( 0.08 eV, D0(Nb+-H) ) 2.28 ( 0.07 eV, and D0(Mo+-H) ) 1.72 ( 0.06 eV. This thermochemistry is compared with theoretical calculations and previous experimental measurements. Results for the HD reactions indicate that Y+(3D) reacts via a statistical mechanism, Zr+(4F), Nb+(5D), and Mo+(6S) react via largely statistical mechanisms, and Y+(1S) shows complex behavior. The reaction mechanisms and reactivity differences among these ions are explained by using simple molecular orbital concepts and by referring to potential energy surfaces calculated by Das and Balasubramanian.
Introduction In our laboratory, we have made extensive studies of the gasphase reactions of transition metal ions (M+) with molecular hydrogen, reaction 1.1,2
M+ + H2 f MH+ + H
(1)
Such studies are a useful starting point for a detailed understanding of transition metal mediated activation of covalent single bonds and can provide bond dissociation energies (BDEs) of metal hydrides by measuring the energy threshold of reaction 1. This thermochemistry is of obvious fundamental interest and also has implications in understanding a variety of catalytic reactions involving transition metal systems.3 Previously, we have performed such studies on reaction 1 and the analogous processes with D2 and HD for all first row transition metal ions.1,2 Similar research for second row transition metal ions is much less extensive. Mandich, Halle, and Beauchamp (MHB)4 have used ion beam methods to study reactions of Ru+, Rh+, and Pd+ with D2. Elkind and Armentrout (EA)5,6 have used guided ion beam techniques to study reaction 1 for most of the second row metals, but this work only reports metal hydride ion BDEs and does not provide any detailed results for reaction 1 or the D2 and HD analogues. The only detailed experimental results for reactions of second row transition metal ions with H2, D2, and HD that have been published are those for Y+.7 Recently, we began a comprehensive study of reaction 1 and its D2 and HD analogues with the second row transition metal ions. Here we examine the early metal ions, Y+, Zr+, Nb+, and Mo+. Results for the late metal ions, Ru+, Rh+, Pd+, and Ag+, which exhibit reactivity distinct from the systems in the present work, are provided elsewhere.8 The primary impetus for this renewed interest is our recently acquired ability to generate intense beams of these ions under conditions that are likely to generate ground state † Present address: Department of Chemistry, MIT, Cambridge, MA 02139. ‡ Present address: Texas Instruments, Inc., 12201 Southwest Freeway, MS 621, Stafford, TX 77477. X Abstract published in AdVance ACS Abstracts, December 1, 1995.
0022-3654/96/20100-0054$12.00/0
species,9 as detailed below. Thus, the interpretation of these results to determine M+-H BDEs has fewer complexities associated with the presence of excited state ions than previous studies. In addition, we examine the reactivity of these metal ions (except Y+) with HD for the first time. Previous work on first row transition metal cations indicates that this reaction is very sensitive to the mechanism for the reaction with dihydrogen. Several broad categories of reactivity have been established and depend on the electronic structure of the atomic ion.1,2,10 It is useful to establish whether these same reactivity “rules” can be extended to explain the reactivity and mechanisms of the second row transition metal ions. Experimental Section General Procedures. The guided ion beam instrument on which these experiments were performed has been described in detail previously.9,11 Ions are created in surface ionization (SI) and flow tube sources, described below. The ions are extracted from the source, accelerated, and focused into a magnetic sector momentum analyzer for mass analysis. Massselected ions are slowed to a desired kinetic energy and focused into an octopole ion guide that radially traps the ions.12 The octopole passes through a static gas cell containing the neutral reactant. Gas pressures in the cell are kept low (usually less than 0.3 mTorr) so that multiple ion-molecule collisions are improbable. All results reported here are due to single bimolecular encounters, as verified by pressure dependence studies. Product and unreacted beam ions are contained in the guide until they drift out of the gas cell where they are focused into a quadrupole mass filter for mass analysis and then detected. Ion intensities are converted to absolute cross sections as described previously.11 Uncertainties in cross sections are estimated to be (20%. Laboratory ion energies (lab) are converted to energies in the center-of-mass frame (CM) by using the formula ECM ) Elab m/(m + M), where M and m are the ion and neutral reactant masses, respectively. Two effects broaden the cross section data: the ion energy spread and thermal motion of the neutral reactant gas (Doppler broadening).13 The distribution of the © 1996 American Chemical Society
Reactions of Y+, Zr+, Nb+, and Mo+, with H2, HD, and D2
J. Phys. Chem., Vol. 100, No. 1, 1996 55
TABLE 1: Electronic States of Second Row Transition Metal Cationsa state
electron config
Zr+
a 1S a3D a 1D a3F a 4F
5s2 5s4d 5s4d 4d2 5s4d2
Nb+
b 4F a2D a2P a2F a 5D
4d3 5s4d2 5s4d2 5s4d2 4d4
a5F a3P a3F a3H a 6S a6D a4G
5s4d3 4d4 5s4d3 4d4 4d5 5s4d4 4d5
ion Y+
Mo+
Jb
energy (eV)
population, % 300 K 2000 Kc
0.0 avg 2.0 avg 3/2 5/2 7/2 9/2 avg avg avg avg 0 1 2 3 4 avg avg avg avg 2.5 avg avg
0.000 0.148 0.409 1.045 0.000 0.039 0.095 0.164 0.406 0.546 0.742 0.764 0.000 0.020 0.054 0.099 0.154 0.421 0.833 0.990 1.225 0.000 1.587 1.906
91.49 8.51 5) or low-spin coupled 5s14dn-1 configurations, the reaction occurs efficiently via a direct mechanism. These processes are characterized by a product branching ratio in the HD system of 3 to 4, consistent with arguments concerning conservation of angular momentum. (3) If either the 5s or 4dσ orbital is occupied, and the M+ state is high-spin (the highest spin it can possibly have), such as for high-spin coupled 5s14dn-1 configurations, the reaction is inefficient and tends to react impulsively. These processes are characterized by a product branching ratio in the HD system that favors MD+ + H by a large factor and that exhibits shifts in the thresholds for the H2 and D2 systems vs the HD system. Because the thresholds for the H2, D2, and HD systems are consistent for all metal systems studied here, none behave according to this latter rule. Yttrium. In ESA’s study7 of the reaction of Y+ + HD, the results indicated that the first excited state, 3D(5s14d1), reacts via an insertion mechanism. Based on the rules cited above, this configuration is expected to react impulsively, “rule 3”; however, ESA explained that this state may mix with the a3F(4d2) state. Because the latter state has a 4dn configuration with an empty 5s orbital, it falls under “rule 1”, such that the 3D(5s14d1) state can react adiabatically by an insertion mechanism. The present state-specific data show that the 3D state-specific cross section has a σ(MH+ + D)/σ(MD+ + H) branching ratio of 1.26 ( 0.06 (Figure 10) from threshold to ∼4.5 eV, which is clearly statistical behavior. [At the lowest energies, below ∼3 eV, the branching ratio for Y+(3D) declines slightly (Figure 10), behavior that is due to the differences in zero-point energy for the two channels. At energies above 4.5 eV, the branching ratio rises more rapidly because of the differences in the dynamics of dissociation of the MH+ and MD+ products.] However, DB30 find only one low-lying state of MH2+, the 1A1 state, which is likely to account for this statistical behavior. (The first excited states of YH2+ are the near degenerate pair, 3Σu+ and 1Σu+, which are calculated to lie above the YH+ + H product asymptote.30) The PESs calculated by DB (which do not include surfaces correlating to the a3F state) show that the surfaces evolving from the Y+(3D) + H2 asymptote cross with the 1A1 surface evolving from the Y+(1S) + H2 asymptote. (The
Reactions of Y+, Zr+, Nb+, and Mo+, with H2, HD, and D2 calculations show that two out of four triplet surfaces calculated cross the 1A1 surface; however, there must be five surfaces evolving from the 3D asymptote and another seven from the 3F asymptote.) Although DB find that spin-orbit coupling between surfaces of different spin is negligible, as noted above, such coupling seems to be the only plausible means of obtaining statistical behavior in the HD reaction. The efficiency of this coupling of PESs may help explain why the reactivity of this state of Y+ is less than that for the late transition metal ions where no coupling between diabatic surfaces is involved.8 The mechanism for the Y+(1S,5s2) ground state reaction is more difficult to characterize. ESA7 speculated that surfaces evolving from the 1S ground state may mix with those from the 1D(5s14d1) excited state to react adiabatically via a direct mechanism. The present state-specific cross section produces an energy-dependent σ(YH+ + D)/σ(YD+ + H) branching ratio (Figure 10) that is more complex than that for a direct mechanism as exemplified by the late transition metal ions. (In these systems, the branching ratios have a value of about 2 at threshold and then gradually increase to about 4 by 4 eV, before increasing rapidly at higher energies because of the dissociation dynamics mentioned above.) The σ(YH+ + D)/σ(YD+ + H) branching ratio for Y+(1S) is 4 ( 0.5 at threshold and decreases to 2.6 between 4 and 5.6 eV. This is the first system where we have observed such an energy dependence in the σ(MH+ + D)/σ(MD+ + H) branching ratio. This is unlikely to be diabatic reaction behavior of the Y+1 ( S,5s2) ground state. The double occupation of the valence 5s orbital should lead to a strong repulsive interaction between M+ and H2 (consistent with the character of the surfaces calculated by DB at small H-M-H angles, i.e., long metalH2 distances). Such a repulsive interaction can lead to impulsively reactivity, “rule 3”, which should yield more MD+ than MH+ in the HD system. This idea is confirmed by results for the reaction of Lu+(1S, 6s2) ground state with HD.7 A σ(LuH+ + D)/σ(LuD+ +H) branching ratio of 0.25 is observed in the threshold region. In contrast, the large value for the σ(YH+ + D)/σ(YD+ + H) branching ratio suggests that the Y+(1S,5s2) state surface mixes with an excited state surface and reacts adiabatically to produce more YH+ than YD+, although it is unclear why the σ(YH+ + D)/σ(YD+ + H) branching ratio at the threshold is so high. The rapid decline in the branching ratio as the energy is increased appears to indicate that there is coupling, which becomes more efficient with increasing energy, to surfaces evolving from excited states that react by an insertion mechanism. This is consistent with the PESs calculated by DB which show a barrier along the lowest energy 1A1 surface that is due to such a surface crossing. The asymptotic state of Y+ + H2 that this corresponds to is not determined by DB, but the a12b22 configuration of YH2+(1A1) correlates with Y+ in a 4dπ2 singlet coupled configuration. This can be shown to correspond to 57% Y+(1G) and 43% Y+(1D) excited states. Zirconium, Niobium, and Molybdenum. The branching ratios between the MH+ and MD+ products in the HD systems are fairly similar for reaction of Zr+, Nb+, and Mo+, (Figures 7-9) and fall in between the behavior found for Y+(3D) and Y+(1S) as shown by the example of Nb+ in Figure 10. In all cases, the branching ratios differ appreciably from those observed for the late metal ions, Ru+, Rh+, Pd+, and Ag+, as described above.8 These latter systems exhibit branching ratios that are clearly associated with direct reactivity, “rule 2”. In general, it appears that the results for Zr+, Nb+, and Mo+ are associated with reactions that occur largely by statistical mechanisms although there appear to be constraints that favor production of MH+ + D over MD+ + H.
J. Phys. Chem., Vol. 100, No. 1, 1996 61 This qualitative conclusion is consistent with the character of the PESs calculated by DB.301,31 As noted above, the highspin ground states of Zr+, Nb+, and Mo+ have PESs with barriers and intermediates calculated to be in excess of the experimentally observed thresholds. Even if the calculated barriers and intermediate energies were lower, it seems unlikely that near statistical behavior could result. Thus, the reactions probably occur by coupling these high-spin surfaces evolving from the ground state M+ + H2 reactants to those leading to the low-spin MH2+ intermediates. This mechanism is consistent with a branching ratio that is characterized largely by the statistical behavior of the ground state MH2+ intermediate, with the dynamics of the surface coupling explaining the constraints favoring the MH+ + D channel. The need for this coupling of potential energy surfaces may also explain why the reactivity of these early transition metal ions is less than that for the late transition metal ions where no coupling between two diabatic surfaces is involved.8 As noted above, DB find that spin-orbit coupling between surfaces of different spin is negligible, but such coupling seems to be the only plausible means of obtaining the near-statistical behavior observed in the HD reactions. A further indication that this is a reasonable hypothesis comes from a comparison of the branching ratios in the HD system obtained for the FT and SI data in the Zr+ and Nb+ systems (not shown in the figures). We find the branching ratios for the SI data in both systems are closer to unity, more statistical behavior. This is consistent with small populations of the lowspin excited states (Table 1) that react via an insertion mechanism without the need to couple surfaces. Conclusion In this study, by using a flow tube (FT) ion source, we are able to create Zr+, Nb+, and Mo+ ions in their ground state terms and Y+ mostly in its ground state. Thus, we are able to obtain corresponding state-specific reaction cross sections. Analyses of these data yield bond energies for the four metal hydrides, listed in Table 3, that are in good agreement with theory and previous experimental work. Simple promotion energy concepts can be used to rationalize the periodic variations in the second row transition metal hydride ion BDEs studied here and suggest an intrinsic M+(5s)-H(1s) bond energy of about 2.6 eV. The branching ratios observed in the M+ + HD reactions indicate that the Y+(3D) state exhibits statistical behavior, the Zr+(4F), Nb+(5D), and Mo+(6S) ground states react by largely statistical mechanisms, and the Y+(1S) ground state exhibits behavior that is not easily characterized. The reactivities and mechanisms in these systems can be explained by using the molecular orbital concepts generalized from previous studies on the reactions of the first row transition metal ions with dihydrogen and by referring to the potential energy surfaces for MH2+ species calculated by Das and balasubramanian.30,31 The experimental results are most easily interpreted if these reactions involve coupling between surfaces of high and low spin. Acknowledgment. We thank Dale Steele for preliminary work performed on the zirconium system. This work was supported by the National Science Foundation under Grant CHE-9221241. References and Notes (1) (2) 9, 115. Davies,
Elkind, J. L.; Armentrout, P. B. J. Phys. Chem. 1987, 91, 2037. For reviews, see: Armentrout, P. B. Int. ReV. Phys. Chem. 1990, In SelectiVe Hydrocarbon ActiVation: Principles and Progress, J. A., Watson, P. L., Greenberg, A., Liebman, J. F., Eds.; VCH:
62 J. Phys. Chem., Vol. 100, No. 1, 1996 New York, 1990; p 467. In Gas Phase Inorganic Chemistry; Russell, D. H., Ed.; Plenum: New York, 1989; p 1. (3) Crabtree, R. H. Chem. ReV. 1985, 85, 245. (4) Mandich, M. L.; Halle, L. F.; Beauchamp, J. L. J. Am. Chem. Soc. 1984, 106, 4403. (5) Elkind, J. L.; Armentrout, P. B. Inorg. Chem. 1986, 25, 1078. (6) Bond energies for PdH+ and RhH+ taken from unpublished work of Elkind and Armentrout are cited in: Armentrout, P. B.; Georgiadis, R. Polyhedron 1988, 7, 1573. (7) Elkind, J. L.; Sunderlin, L. S.; Armentrout, P. B. J. Phys. Chem. 1989, 93, 3151. (8) Chen, Y.-M.; Elkind, J. L.; Armentrout, P. B. J. Phys. Chem. 1995, 99, 10438. (9) Schultz, R. H.; Armentrout, P. B. Int. J. Mass Spectrom. Ion Processes 1991, 107, 29. (10) For a recent review, see: Armentrout, P. B. ACS Symp. Ser. 1992, No. 502, 194. (11) Ervin, K. M.; Armentrout, P. B. J. Chem. Phys. 1985, 83, 166. (12) Teloy, E.; Gerlich, D. Chem. Phys. 1974, 4, 417. Gerlich, D. Diplomarbeit, University of Freiburg, Federal Republic of Germany, 1971. Gerlich, D. In State-Selected and State-to-State Ion-Molecule Reaction Dynamics. Part 1. Experiment; Ng, C.-Y., Baer, M., Eds.; AdV. Chem. Phys. 1992, 82, 1. (13) Chantry, P. J. J. Chem. Phys. 1971, 55, 2746. (14) Sunderlin, L. S.; Armentrout, P. B. J. Phys. Chem. 1988, 92, 1209. (15) van Koppen, P. A. M.; Kemper, P. R.; Bowers, M. T. J. Am. Chem. Soc. 1992, 114, 10941. (16) Sievers, M. R.; Armentrout, P. B. Work in progress. (17) Kickel, B. L.; Armentrout, P. B. J. Am. Chem. Soc. 1995, 117, 4057. (18) Clemmer, D. E.; Chen, Y.-M.; Khan, F. A.; Armentrout, P. B. J. Phys. Chem. 1994, 98, 6522. (19) Haynes, C. L.; Armentrout, P. B. Organometallics 1994, 13, 3480. (20) Kickel, B. L.; Armentrout, P. B. J. Am. Chem. Soc. 1995, 117, 764. (21) Armentrout, P. B. In AdVances in Gas Phase Ion Chemistry; Adams, N. G., Babcock, L. M., Eds.; JAI: Greenwich, 1992; Vol. 1, p 83. (22) Weber, M. E.; Elkind, J. L.; Armentrout, P. B. J. Chem. Phys. 1986, 84, 1521. (23) Calculated from heats of formation given by: Gurvich, L. V.; Veyts, I. V.; Alcock, C. B. Thermodynamic Properties of IndiVidual Substances, 4th ed.; Hemisphere: New York, 1989; Vol. 1, Part 2. (24) Aristov, N.; Armentrout, P. B. J. Am. Chem. Soc. 1986, 108, 1806. (25) Sunderlin, L. S.; Armentrout, P. B. J. Phys. Chem. 1988, 92, 1209.
Sievers et al. (26) Clemmer, D. E.; Sunderlin, L. S.; Armentrout, P. B. J. Phys. Chem. 1990, 94, 208. (27) Clemmer, D. E.; Sunderlin, L. S.; Armentrout, P. B. J. Phys. Chem. 1990, 94, 3008. (28) Aristov, N.; Armentrout, P. B. J. Phys. Chem. 1987, 91, 6178. (29) Armentrout, P. B. Annu. ReV. Phys. Chem. 1990, 41, 313; Science 1991, 251, 175. (30) Das, K. K.; Balasubramanian, K. J. Chem. Phys. 1989, 91, 2433. (31) Das, K. K.; Balasubramanian, K. J. Chem. Phys. 1989, 91, 6254. (32) Elkind, J. L.; Armentrout, P. B. J. Phys. Chem. 1985, 89, 5626. (33) Armentrout, P. B. In Structure/ReactiVity and Thermochemistry of Ions; Ausloos, P., Lias, S. G., Eds.; Reidel: Dordrecht, 1987; p 97. (34) The zero-point energy differences between MD+ and MH+ are 0.029, 0.030, 0.032, and 0.033 eV for M ) Y, Zr, Nb, and Mo, respectively. These are calculated from the differences in the MD+ and MH+ vibrational frequencies, which are taken from ref 36 for MH+ and estimated for MD+ by using a Morse potential to scale the MH+ frequencies. (35) Elkind, J. L.; Armentrout, P. B. J. Phys. Chem. 1985, 89, 5626. Elkind, J. L.; Armentrout, P. B. J. Chem. Phys. 1986, 84, 4862. Elkind, J. L.; Armentrout, P. B. J. Am. Chem. Soc. 1986, 108, 2765. (36) Pettersson, L. G. M.; Bauschlicher, Jr., C. W.; Langhoff, S. R.; Partridge, H. J. Chem. Phys. 1987, 87, 481. (37) Schilling, J. B.; Goddard III, W. A.; Beauchamp, J. L. J. Am. Chem. Soc. 1987, 109, 5565. (38) For reviews, see: Armentrout, P. B.; Kickel, B. L. In Organometallic Ion Chemistry, Freiser, B. S., Ed.; Kluwer: Dordrecht, 1995; pp 1-45. Armentrout, P. B.; Clemmer, D. E. In Energetics of Organometallic Species; Simoes, J. A. M., Beauchamp, J. L., Eds.; Kluwer: Dordrecht, 1992; p 321. (39) Ep(5s14dn-1) is easily calculated as the mean energy of the lowest electronic states that have high-spin and low-spin 5s14dn-1 configurations. Calculation of Ep(4dn) from spectroscopic data is more complicated and described in detail in ref 4. Values obtained in this fashion are provided in ref 5. In many cases, it is more convenient and nearly as accurate to use theoretical values listed in ref 40. (40) Carter, E. A.; Goddard III, W. A. J. Phys. Chem. 1988, 92, 5679. (41) Ohanessian, G.; Goddard III, W. A. Acc. Chem. Res. 1990, 23, 386. (42) This number is taken from Table 1 and Figure 1 of ref 30 but disagrees with a value of 1.7 eV given in the text. (43) Balasubramanian, K. Personal communication.
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