11424
J. Phys. Chem. 1995, 99, 11424- 11431
Guided Ion Beam Studies of the Reactions of Agf with C2H6, C3H8, HC(CH3)3, and c-CsH6 Yu-Min Chent and P. B. Armentrout" Department of Chemistry, University of Utah, Salt Luke City, Utah 84112 Received: February 20, 1995; In Final Form: May 1, 1995@
Reactions of Ag+ with C2H6, C3H8, HC(CH3)3, and c-C3& have been studied by using guided ion beam mass spectrometry. By using a meter-long flow tube ion source, we are able to create Ag+ ions that are in the a ' s ground electronic state and obtain corresponding state-specific reaction cross sections as a function of kinetic energy. It is found that C-C bond cleavage is the predominant process in all of these systems. Dehydrogenation and formation of AgH+ R products are not observed. The reaction mechanisms are believed to be similar to those for the analogous Cu' reactions proposed in previous studies. The energy dependence of the endothennic reaction cross sections are modeled to obtain thermodynamic information. Unlike the results for most other metal ions studied previously, the thermochemistry derived for Ag+ from several systems is inconsistent, such that the accuracies of the bond energies derived are suspect. Possible explanations for these problems are discussed, and the most likely 0 K bond dissociation energies (in eV) are found to be Do(Ag+-CH3) = 0.69 f 0.05, Do(Ag+-C2H5) = 0.68 f 0.08, Do(Ag-H) = 2.06 f 0.10, and Do(Ag-CH3) = 1.39 f 0.07 eV, and lower limits of Do(Agf-CH2) 2 1.11 f 0.04 and Do(Ag+-C&) 1 0.23 f 0.07 eV. All these values are most conservatively viewed as lower limits, a conclusion that appears to be verified by comparisons with other experimental and theoretical results from the literature.
+
Introduction Since the first observation by Allison, Freas, and Ridge in 1979 that atomic transition metal ions (M+) can activate C-H and C-C bonds,' a tremendous amount of experimental work has centered on the gas phase reactions of M+ with small hydrocarbons. We have previously used guided ion beam mass spectrometry to study the reactions of first-row transition metal ions with small hydrocarbons.2 Such studies have provided insight into the electronic requirements for activation of C-H and C-C bonds by M+,2,3elucidated periodic trends in the reactivity,2 and measured metal-hydrogen and metal-carbon bond dissociation energies (BDEs)! The thermochemistry obtained from these studies is of obvious fundamental interest and also has implications in elucidating a variety of catalytic reactions involving transition metal system^.^ In this work, we extend these studies to one of the second-row transition metal ions, Ag+ and describe its reactions with ethane, propane, 2-methylpropane, and cyclopropane. This is part of an ongoing project to use guided ion beam mass spectrometry to systematically study the activation of small hydrocarbons by the secondrow transition metal cations. Such studies are less extensive than those for the first-row transition metal cations, although there are a few other studies reported in the l i t e r a t ~ r e . ~We -~ are unaware of any previous studies concerning the reactions of Ag+ with hydrocarbons. Recently, we have studied the activation of hydrogen by Agf,Io which is directly analogous to the activation of the C-H and C-C bonds in the alkane molecules. Our results indicate that the reaction proceeds through a direct mechanism, without forming a stable insertion intermediate. This is attributed to the relatively repulsive interaction between the 4da(Ag+) electrons and the H2 molecule. We have also studied the reactions of Cu+ with the title hydrocarbonsl1%l2 and observed results that are analogous to the present study. 'Present address: Department of Chemistry, MIT, Cambridge, MA 02179 @
Abstract published in Advance ACS Abstracts, June 15, 1995.
TABLE 1: Silver-Ligand Bond Dissociation Energies (in eV) at 0 Ka literature bond Ag+-H Ag-H Ag+-CH3 Ag-CH3 Ag+-CH2 Ag+-CZH5 Agf-C2H4
experimental 2.28(0.10)' 2.19(0.09)J
theoretical 0.46: 0.09' 1.97,'2.20,g,* 1.22' 2.01: 2.27(0.04)' 0.95,* l.Ol(O.10)~ 1.64,h 1.72,' 1.68(0.09)' 1.60(0.13)," 1.93(0.18)'
1.43(0.13)"
this work 0.4 1(0.06)d 2.06(0.10) 0.69(0.05) 1.39(0.07) 2 l.ll(O.04) 0.68(0.08) >0.23(0.07)
Uncertainties in parentheses. Reference 17. Reference 16. Reference 10. e Reference 13. fReference 19. Values adjusted from D, by using information in reference 13 for AgH and reference 24 for AgCH3. g Reference 18. Reference 24. ' Reference 20. Reference 14. Values adjusted from D,by using the vibrational frequency in reference 21. kReference 22. 'Reference 23. Values are averages of CCSD(T) and B3LYP values and are adjusted from De (except in the case of AgCH2'). Reference 25. Reference 15. Adjusted to 0 K value by using the vibrational frequencies in reference 4a. a
One specific goal of the present study is to determine accurate thermochemistry for silver-hydrogen and various silver-carbon BDEs. BDEs for the cationicI0 and n e ~ t r a lsilver ~ ~ , hydrides ~~ and Agf(C2&), (x = 1 and 2)15 have been measured experimentally. Theoretical calculations have been performed on cationic and neutral silver hydride^,'^-^^ s i l ~ e r - m e t h y l s , ' ~ ~ ~ ~ ~ ~ ~ and ~ilver-methylene.~~,~~ The available information in the literature is collected in Table 1. In the present work, we attempt to obtain some of these BDEs by measuring the endothermic thresholds for reactions of Ag+ with the four hydrocarbons. This work is facilitated by the use of a dc-discharge flow tube ion source that produces Ag+ ions exclusively in the alS ground electronic state.10s26Thus, the BDE measurements have no complexities associated with the presence of excited state ions. Nevertheless, competition between various reactions appears to -grossly- influence the thresholds such that consistent thermodynamic information is not obtained from several systems.
0022-3654/95/2099-11424$09.00/0 0 1995 American Chemical Society
Reactions of Ag+ with C2H6, CsHg, HC(CH3)3, and c - C ~ H ~
J. Phys. Chem., Vol. 99,No. 29, 1995 11425
Despite these limitations, it seems appropriate to suggest the most likely values for several bond energies, because there are no other experimental measurements available for anything but the AgH and Ag+(ethene) molecules.
0.0
ENERGY (eV. Lab) 10.0 20.0
30.0
Experimental Section General Procedures. The guided ion beam instrument on which these experiments were performed has been described in detail p r e v i o ~ s l y .Ag+ ~ ~ ~ions ~ ~ are created in a flow tube source, described below. The ions are extracted from the source, accelerated, and focused into a magnetic sector momentum analyzer for mass analysis. Mass-selected ions are slowed to a desired kinetic energy and focused into an octopole ion guide that radially traps the ions.29 The octopole passes through a static gas cell containing the neutral reactant. Gas pressures in the cell are kept sufficiently low (usually less than 0.1 mTorr) that multiple ion-molecule collisions are improbable. Except where noted, all results reported here are due to single bimolecular encounters, as verified by pressure dependent 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 by a high-voltage scintillation detector. Ion intensities are converted to absolute cross sections, as described p r e v i ~ u s l y . ~ ~ Uncertainties in absolute cross sections are estimated to be f20%. Laboratory ion energies (lab) are converted to energies in the center-of-mass frame (CM) by using the formula ECM= Elabm/(m M), where M and m are the ion and neutral reactant masses, respectively. Two effects broaden the cross section data: the kinetic energy distribution of the ion and the thermal motion of the neutral reactant gas (Doppler b r ~ a d e n i n g ) .The ~~ distribution of the ion kinetic energy and absolute zero of the energy scale are determined by using the octopole beam guide as a retarding potential analyzer.27 The distribution of ion energies, which is independent of energy, is nearly Gaussian and has an average full width at half-maximum (fwhm) of -0.45 eV (lab). The Doppler broadening has a width of -0.45Ec~I'~ for the reactions of Ag+ with the four hydrocarbon^.^^ Uncertainties in the absolute energy scale are f0.05 eV (lab). Ion Source. Ag+ ions are produced in a dc-discharge flow tube (FT) source.28 The flow gases used in this experiment are -90% He and -10% Ar, maintained at a total pressure of 0.50.7 Torr and room temperature. A dc-discharge at a voltage of 2-3 kV is used to ionize argon and accelerate these ions into a cathode made of silver metal, sputtering Ag+ ions. The ions are swept down a meter-long flow tube and undergo -lo5 collisions with the He and Ar flow gasses. Trace amounts of high lying excited states of Ag+ ('5.0 eV) can survive these flow conditions, but are easily removed by introducing 0 2 to the flow tube several centimeters downstream of the source at a pressure of -2 mTorr.26 Thus, the Ag+ ions created in the FT source are exclusively in the a ' s ground state, as demonstrated elsewhere.26 Thermochemical Analysis. Endothermic reaction cross sections are modeled by using eq 1,31
+
0.0
2:o
S.'O
ENERGY (eV.
8.0
CM>
Figure 1. Cross sections for reactions of Ag' with CzHs as a function of kinetic energy in the center-of-mass frame (lower x-axis) and laboratory frame (upper x-axis). The arrow indicates Do(CH3-CH3) at 3.81 eV.
the rotational energy (E,,, = 3k~T/2= 0.039 eV), as described p r e v i ~ u s l y .The ~ ~ vibrational frequencies used here were taken from S h i m a n ~ u c hfor i ~ ~C2H6, c - C ~ Hand ~ , C3H8 and from Chen et al.34for HC(CH3)3. Before comparison with the data, eq 1 is convoluted with the kinetic energy distributions of the ion and neutral reactants.27 The 00, n, and EOparameters are then optimized by using a nonlinear least squares analysis to give the best reproduction of the data. Error limits for EO are calculated from the range of threshold values for different data sets over a range of acceptable n values and the absolute error in the energy scale.
Results Ag+ 4- C&. Results for the reaction of Ag+ with C2H6 are shown in Figure 1. We observe three ionic products formed in endothermic reactions 2-4. Ag'
+ C,H,
-
AgCH,'
+ CH,
(2)
+ + H, + AgH
C2H5+ AgH
(3)
C,H,+
(4)
The dominant reaction channel is the homolytic cleavage of the C-C bond to form AgCH3+, reaction 2. This cross section reaches a peak near 4 eV and then declines rapidly at higher energies. This is due to the decomposition of AgCH3+ to form Ag+ CH3, an overall reaction in which ethane dissociates into two methyl groups. The peak position is in good agreement with the thermodynamic threshold for this dissociation, which is equivalent to Do(CH3-CH3) = 3.812 f 0.007 eV, Table 2. The second most likely reaction channel is the heterolytic cleavage of the C-H bond to form C2Hs+ AgH, reaction 3. At an energy > -6 eV, this cross section begins to decline, largely due to decomposition of C2H5+ to form C2H3+ H2 in the overall reaction 4. This is supported by the observation that the C2H3+ cross section has an apparent onset near 5 eV and the sum of the two cross sections is a smooth function of energy. AgH+ is not observed in this system, despite a careful
+
+
where E is the relative kinetic energy of the ions, EOis the 0 K reaction threshold, a0 is an energy independent scaling factor, and n is an adjustable parameter. Equation 1 takes into account the internal energy of the neutral reactants (at 305 K) by treating the calculated cross section as a sum over vibrational states (with energies Ei and populations gi, where zgi = 1) and by including
+
Chen and Armentrout
11426 J. Phys. Chem., Vol. 99, No. 29, 1995
TABLE 2: Literature Thermochemistry at 0 K AfHn (eV)
species ~~
ENERGY
