LETTER pubs.acs.org/JPCL
Gas-Phase Synthesis and Vibronic Action Spectroscopy of Ag2Hþ R. Mitric,*,† J. Petersen,†,‡ A. Kulesza,†,‡ M. I. S. R€ohr,‡ V. Bonacic-Koutecky ,‡,§ C. Brunet,# R. Antoine,# P. Dugourd,# M. Broyer,# and R. A. J. O’Hair^ †
Fachbereich Physik, Freie Universit€at Berlin, Arnimallee 14, D-14195 Berlin, Germany Institut f€ur Chemie, Humboldt-Universit€at zu Berlin, Brook-Taylor-Strasse 2, D-12489 Berlin, Germany § Interdisciplinary Center for Advanced Science and Technology, University of Split, Mestrovicevo Setaliste bb., HR-21000 Split, Croatia # Laboratoire de spectrometrie ionique et moleculaire UMR 5579, Universite Lyon 1 and CNRS, 43 Bd du 11 Novembre 1918, F-69622 Villeurbanne cedex, France ^ School of Chemistry, Bio21 Institute of Molecular Science and Biotechnology, ARC Centre of Excellence in Free Radical Chemistry and Biotechnology, The University of Melbourne, Melbourne, Victoria, 3010, Australia ‡
bS Supporting Information ABSTRACT: We present the first vibrationally resolved electronic spectrum for the isolated silver hydride cation Ag2Hþ, which has been synthesized by multistage mass spectrometry in a quadrupole ion trap. The experimental photofragmentation spectrum has been obtained by action spectroscopy. High-level ab initio simulations reproduce experimental findings and provide precise assignment of structural properties of ground and excited electronic states of Ag2Hþ, which serves as an intermediate in important chemical reactions such as carbon-carbon coupling. SECTION: Dynamics, Clusters, Excited States
of both experimental12,13 and theoretical14-16 investigations. Concerning its structural properties, most theoretical studies predict that the ground state has a C2v geometry with a Ag—Ag bond distance between ∼2.816,17 and ∼3.0 Å,14 in contrast to the D¥h geometry proposed earlier.15 The experimental studies have been focused on the unimolecular fragmentation and ionmolecule reactions with organic substrates in a quadrupole ion trap. In particular, the gas-phase metathesis reaction mediated by Ag2Hþ has been recently reported.12,13 Thus, due to the importance of Ag2Hþ as a model reaction intermediate, the precise characterization of its gas-phase ground- and excited-state electronic and structural properties is highly desirable. Action spectroscopy performed in different ion storage devices has proven to be an efficient technique to probe structural properties of gas-phase ions.18-21 In this Letter, we used a quadrupole ion trap to combine gas-phase synthesis by multistage mass
N
oble metal clusters have been widely used as models for the chemistry that occurs on catalytic surfaces and nanoparticles1-10 due to the ability to systematically control their size, shape, charge state, and composition. In this context, gas-phase studies of model systems using mass-spectrometrybased approaches as well as theoretical chemistry4 provide valuable insights into the chemistry observed in real applications. Moreover, new metal-organic compounds in the gas phase can be efficiently produced experimentally using a combination of electrospray ionization and gas-phase ion chemistry. This opens the route for the preparation of reaction intermediates in the gas phase and for the systematic investigation of their fundamental reactivity. Remarkably, it has been recently shown that small silver and silver hydride cations such as Ag5þ and Ag4Hþ can be easily assembled from gas-phase precursor ions and undergo a series of ion-molecule reactions with alkylhalogenides, ultimately leading to C—C bond coupling.11 Ag2Hþ represents the smallest reactive prototype which still contains the metal-metal bond in the ground electronic state. It has already been the subject r 2011 American Chemical Society
Received: December 25, 2010 Accepted: February 10, 2011 Published: February 21, 2011 548
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Table 1. (a) Fragmentation Channels and Yields Observed by UV Photofragmentation and CID; (b) Energies of Ag2Hþ Fragmentation Channels in the S0 State Calculated with the CCSD Method (a) transition
fragments
S0 f S1 (260-280 nm)
Agþ þ AgH
47.9%
AgHþ þ Ag
46.6%
Ag2þ þ H S0 f S2 (235-240 nm)
Agþ þ AgH AgHþ þ Ag Ag2þ þ H Agþ þ AgH
S0 (CID)
yield
5.4% 60.0% 30.7% 9.2% 100.0%
(b) fragments þ
AgH þ Ag Ag2þ þ H
Efrag (eV) 2.29 3.17
AgHþ þ Ag
4.19
Ag2 þ Hþ
9.64
The vertical excitation energy to the S1 state has a value of 4.43 eV, while the S2 state is located at 5.17 eV. In the S1 state, there are two local minima, both exhibiting linear geometries but differing in the position of the hydrogen atom (cf. middle panel of Figure 1a). In the isomer I (C¥v, R(Ag—Ag) = 3.52 Å), the hydrogen atom is bound peripherally, while in the second isomer II (D¥h), which lies 0.39 eV higher in energy, the hydrogen atom is located centrally. Similar to S0, the S2 state also has two equivalent minima with C2v symmetry (R(Ag—Ag) = 2.91 Å, R(Ag—H—Ag) = 107.3), as shown in the upper part of Figure 1a. For the main relaxation coordinate perpendicular to the center of the Ag—Ag bond, 1D cuts of the PES are presented in Figure 1b, illustrating large differences between geometries of the S0 and S1 states as well as similar geometries between the S0 and S2 in the Franck-Condon region. Thus, it can be expected that the vibronic progressions in the S1 and S2 states will exhibit different envelope shapes. In the following, we first address the experimental fragmentation channels and then present the theoretical vibrationally resolved absorption spectrum and its comparison with the experimental data, which allows for the precise assignment of all measured features. The experimental spectrum of Ag2Hþ has been recorded by monitoring the total fragmentation yield as a function of the laser wavelength. The UV excitation of Ag2Hþ results in the formation of Ag2þ, AgHþ, and Agþ fragment ions with the branching ratios for the different fragmentation channels given in Table 1. Theoretical calculations show that the energetically most favorable fragmentation channel in the S0 state (2.29 eV) involves the formation of Agþ þ AgH (cf. Table 1), in agreement with the experimental CID results reported previously.14 The fragmentation resulting from an electronic excitation to the S1 state leads also to AgHþ þ Ag with a similar branching ratio. This means that the phofragmentation in S1 does not follow a statistical law. In fact, the S1 electronic state involves the excitation from the S-type bonding molecular orbital into a P-type orbital which is antibonding along the Ag—Ag bond, as shown in Figure 2a.
Figure 1. (a) CAS-PT2 two-dimensional potential energy surfaces of Ag2Hþ for the S0 (1A1) ground and the two lowest excited singlet states S1 (1B2) and S2 (21A1) for fixed distance between the silver atoms. In the S1 state, two local minima corresponding to isomers I and II with C¥v and D¥h symmetry are present. Blue and red colors label low and high values of the potential energy. (b) One-dimensional cuts of the 2D-PES from (a) along the main relaxation coordinate involving the motion of the H atom perpendicular to the center of the Ag—Ag bond.
spectrometry with vibrationally resolved spectroscopy by photodissociation experiments in order to produce and to fully characterize both the ground as well as the two lowest excited electronic states of Ag2Hþ. The multistage mass spectrometry approach used for the formation of this species has been described previously.12,13 The combination of ion trap gas-phase optical spectroscopy with high-level ab initio calculations fully reveals the vibrational and electronic structure of Ag2Hþ. Ag2Hþ was “synthesized” by gas-phase chemistry involving collision-induced dissociation (CID) of a silver-amino acid precursor.14 The photofragmentation of Ag2Hþ was then investigated in the range between 220 and 290 nm. According to the theoretical calculations, two optically allowed electronic singlet states of 1B2 and 1A1 symmetry in the C2v point group (labeled S1 and S2 in the following) are present in this spectral range. Due to the large difference in mass between silver and hydrogen, the vibrational structure of the electronic spectrum will be mainly determined by the modes involving the motion of the hydrogen atom. Thus, in the first approximation, the Ag atoms can be fixed at their equilibrium position, and only the two-dimensional quantum mechanical motion of the hydrogen atom in the x-y molecular plane is considered. The 2D cuts from the potential energy surfaces (PES) along the x-y directions for the ground (S0) and first two excited states (S1 and S2) of Ag2Hþ together with the structures corresponding to the local minima are shown in Figure 1a. The ground state exhibits two equivalent minima corresponding to a structure with C2v symmetry (R(Ag—Ag) = 3.01 Å, R(Ag —H—Ag) = 121.4), as depicted in the lower panel of Figure 1a. 549
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Consequently, the excitation to the S1 electronic state leads to the breaking of the Ag—Ag bond and the formation of Agþ or AgHþ as dominant charged products. The S2 state corresponds to the excitation from the bonding S-type orbital into a P-type orbital which is antibonding along the AgH bonds (cf. Figure 2a). Thus, the direct fragmentation in the S2 state involves the loss of a neutral hydrogen atom and explains the increase in the observed branching ratio corresponding to this channel after excitation to S2 (cf. Table 1). Simultaneously, the coupling with the S1 and S0 states leads again to the formation of Agþ and AgHþ fragments. Thus, the analysis of the character of the excited states is in agreement with the observed fragmentation channels. The experimental vibrationally resolved photofragmentation spectrum as well as the corresponding theoretical absorption spectrum of Ag2Hþ is presented in Figure 2. The Franck-
Condon progression of the S1 state shown in Figure 2b exhibits a clear vibrational structure. Because the hydrogen is at the peripheral position for isomer I of the S1 state, in contrast to the S0 state, no vibronic transitions involving this isomer occur. Instead, the observed transitions correspond to the excited-state minimum with D¥h symmetry (isomer II), in which the hydrogen atom is located symmetrically between the two Ag atoms. The main peaks of the vibronic progression are due to the excitation of higher vibrational states of the normal mode corresponding to the motion of the hydrogen atom from the initially bent ground-state structure toward isomer II of the S1 state (cf. Table 2) along the symmetry axis perpendicular to the Ag—Ag bond (y-direction; cf. Figure 1b). The wave functions for this motion in the xy molecular plane are presented as 3D plots in Figure 3. The most intense vibronic lines from Figure 2b, denoted as (a-d) correspond to transitions from the vibrational ground state (00) of the S0 PES to the (03), (04), (05), and (06) vibrational states of the second minimum of S1 (isomer II), as shown in the lower and middle parts of Figure 3 as well as in Table 2. The labeling of the vibrational states reflects the number of nodes (cf. also Supporting Information, Figure S1) that the corresponding wave functions exhibit in the x and y directions of the molecular plane. As can be seen from Figure 2b, in the low-energy part of the spectrum, the theoretical absorption intensities are slightly higher than the experimental photofragmentation yield, indicating a lower fragmentation efficiency. Theoretical and experimental intensities are comparable and closely match at higher energies. Overall, the agreement between the theory and the experiment is very good.
Figure 2. (a) Leading excitations between the orbitals involved in the vertical transitions to S1 and S2 states. (b, c) Experimental photofragmentation spectrum (red) and theoretical vibronic spectrum (black sticks and blue line) of Ag2Hþ for the vibronic progression of the S0 (1A1) f S1 (1B2) (b) and S0 (1A1) f S2 (21A1) transition (c). The transitions referred to in Table 2 are marked as (a-d). All intensities have been normalized with respect to the highest experimental peak, and the theoretical lines have been broadened by a Lorentzian function with a width of 2 (b) or 1 nm (c), respectively. Notice that the theoretical transition wavelengths have been uniformly shifted by -9 nm for (b) and -3.2 nm for (c).
Figure 3. Vibrational eigenstates corresponding to nuclear wave functions for the motion of the H atom on the two-dimensional PES (gray grid) for the S0, S1 (isomer II), and S2 states of Figure 1. Positive values of the wave functions are colored in red, and negative ones are in blue. The labeling of vibrational eigenstates refers to the number of nodes in the x and y directions (cf. also Table 2 and Figure S1, Supporting Information, showing the projection of the wave functions on the molecular plane).
Table 2. Intense Vibronic Transitions Depicted in Figure 2 electronic transition S0 ( A1) f S1 ( B2) 1
1
S0 (1A1) f S2 (21A1)
transition energy (nm)
intensity (arb. units)
vibrational transition 00 f 03
a
276.6
0.523
b
272.5
0.773
00 f 04
c
268.9
0.872
00 f 05
d
265.0
0.698
00 f 06
a
239.4
0.282
00 f 00
b c
233.4 231.2
0.078 0.008
00 f 01 00 f 21
d
228.5
0.016
00 f 41
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The Journal of Physical Chemistry Letters The other vibronic progression, corresponding to the S2 state, has a considerably simpler shape due to the very similar geometries of the S0 and the S2 states, as shown in Figure 2c. Therefore, the most intense peak labeled by (a) corresponds to the S0(00) f S2(00) vibronic transition (cf. Table 2 and lower and upper part of Figure 3). The less intense lines (b-d) correspond to transitions from the S0(00) to the S2(01), S2(21) and S2(41) vibronic states (cf. Figure 3). The calculated vibronic progression is in excellent agreement with the measured one and allows us to fully assign all observed transitions. In summary, by coupling gas-phase ion chemistry with optical action spectroscopy in an ion trap, we have obtained for the first time the vibrationally resolved spectroscopic data for the isolated Ag2Hþ ion. Moreover, the excellent agreement between experiment and high-level ab initio simulations allowed us to precisely assign the structural properties of ground and excited electronic states of Ag2Hþ which are not accessible by other techniques. Our results open a new route for photoformation and precise characterization of small gas-phase ions which serve as intermediates in important chemical reactions such as C—C coupling.
’ EXPERIMENTAL AND COMPUTATIONAL SECTION The experimental setup consists of a quadrupole linear ion trap mass spectrometer coupled to a visible/UV tunable optical parameter oscillator (OPO) laser.22,23 The laser is injected on the axis of the linear trap. It is then possible to excite the ions by laser and/or collision. Ag2Hþ was synthesized in the gas phase by collision-induced dissociation (CID) of a silver-amino acid precursor, as already described in detail elsewhere.14 Briefly, [(M þ Ag—H) þ Ag]þ (M = glycine, H2N—CH2—COOH) was formed by electrospraying a 99.5:0.5:0.01 methanol/water/ acetic acid solution containing the reactants mixed in a ∼1:3 ratio (AgNO3/glycine). Ag2Hþ was formed in a MS2 sequence in which [(Gly þ Ag—H) þ Ag]þ was subjected to CID, and the main fragmentation channel involved the loss of (Gly—2H) and formation of Ag2Hþ. Ag2Hþ was then isolated in the ion trap and irradiated by the laser. The resulting mass spectrum (MS3 sequence) shows Ag2þ, AgHþ, and Agþ fragment ions. To record action spectra, the laser wavelength was changed by performing an automatic scan from 220 to 290 nm with a step of 0.01 nm. Mass spectra were recorded continuously during the scan (the duration of the entire scan was 62 min). The yield of fragmentation at each laser wavelength is given by σ = ln((parent þ ∑ frag)/parent)/φ, where φ is the laser fluence, parent is the intensity of the parent peak and ∑ frag represents the total intensity of the photofragment peaks. The structural properties and fragmentation channels of Ag2Hþ have been determined using the ab initio coupled cluster method (CCSD) accounting for electronic correlation.24 For silver, the 19 electron relativistic effective core potential of the Stuttgart group together with the corresponding (8s7p6d2f)/[6s5p3d2f] AO basis set25 was used, whereas for hydrogen, the aug-cc-pVTZ basis set26 was employed. For Ag2Hþ, two-dimensional potential energy surfaces (PES) for fixed Ag—Ag distance were calculated for the electronic ground state S0 as well as for the two lowest excited singlet states S1 and S2, employing the Ag—Ag distance corresponding to the respective minimum-energy structure for each electronic state. The electronic energies as well as the electronic transition dipole moments μIJ between the electronic states I and J were calculated
LETTER
on a 2D Cartesian grid using the ab initio complete active space second-order perturbation theory method (CAS-PT2)27-31 in the frame of the MOLPRO program package.32 For this purpose, we employed an active space of 22 electrons distributed in 14 orbitals. Single and double excitations were allowed only within the six highest orbitals. The PES were calculated in the region between -4.23 (-8 au) and 4.23 Å (8 au) with a grid spacing of 0.105 Å (0.2 au) for each coordinate. Subsequently, the vibrational eigenfunctions were determined numerically for each PES by diagonalizing the Hamiltonian matrix represented on the coordinate grid. The vibronic spectrum was obtained from the matrix elements of the electronic transition dipole moments μIJ with the vibrational eigenfunctions χmI and χnJ. In order to account for temperature effects, the contribution of each initial vibrational state has been weighted with the Boltzmann factor for T = 300 K.
’ ASSOCIATED CONTENT
bS
Supporting Information. Figure S1 showing projections of the vibrational eigenfunctions from Figure 3 onto the molecular plane as well as Table S1 containing the vibrational frequencies of the relevant vibrational states in the S0, S1, and S2 electronic states are provided. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT R.A.J.O thanks the Australian Research Council for financial support through the CoE program, the Universite de Lyon for a visiting Professorship, and the School of Chemistry at the University of Melbourne for a short-term study leave. V.B.-K. and P.D. thank the Hubert Curien Program for bilateral funding (Procope n 50068937). Financial support by the Deutsche Forschungsgemeinschaft is acknowledged in the frame of the Emmy Noether program, MI-1236 (R.M. and A.K.) as well as of the Research Unit FOR 1282 (R.M. and J.P.). J.P. also thanks the Fonds der Chemischen Industrie for financial support. ’ REFERENCES (1) Wachs, I. E. Extending Surface Science Studies to Industrial Reaction Conditions: Mechanism and Kinetics of Methanol Oxidation over Silver Surfaces. Surf. Sci. 2003, 544, 1–4. (2) Lambert, R. M.; Williams, F. J.; Cropley, R. L.; Palermo, A. Heterogeneous Alkene Epoxidation: Past, Present and Future. J. Mol. Catal. A: Chem. 2005, 228, 27–33. (3) Yang, X.; Wang, A.; Wang, X.; Zhang, T.; Han, K.; Li, J. Combined Experimental and Theoretical Investigation on the Selectivities of Ag, Au, and Pt Catalysts for Hydrogenation of Crotonaldehyde. J. Phys. Chem. C 2009, 113, 20918–20926. (4) Lim, K. H.; Mohammad, A. B.; Yudanov, I. V.; Neyman, K. M.; Bron, M.; Claus, P.; R€osch, N. Mechanism of Selective Hydrogenation of R,β-Unsaturated Aldehydes on Silver Catalysts: A Density Functional Study. J. Phys. Chem. C 2009, 113, 13231–13240. (5) Kung, H.; Wu, S.-M.; Wu, Y.-J.; Yang, Y.-W.; Chiang, C.-M. Tracking the Chemistry of Unsaturated C3H3 Groups Adsorbed on a Silver Surface: Propargyl-Allenyl-Acetylide Triple Bond Migration, 551
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