Vibrationally Resolved Photoelectron Imaging of Au3Hâ - The Journal

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Vibrationally Resolved Photoelectron Imaging of Au3H− Zhiling Liu, Zhengbo Qin, Xia Wu, Hua Xie, Ran Cong, and Zichao Tang* State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ABSTRACT: We report a combined photoelectron velocity map imaging spectroscopy and density functional theory investigation on the Au3H− anion. Transition between the anionic electronic ground state and the neutral electronic ground state is revealed. Vibrationally resolved spectra were recorded at two different photon energies, providing a wealth of spectroscopic information for the electronic ground state of the Au3H. Franck−Condon simulations of the ground-state transition are carried out to assist in the assignment of the vibrationally resolved spectra. The electron affinity and vertical detachment energy of Au3H are measured to be 2.548 ± 0.001 and 2.570 ± 0.001 eV, respectively. Three stretching vibrational modes are determined to be activated upon photodetachment, with the frequencies of 2100 ± 100, 177 ± 10, and 96 ± 10 cm−1.

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INTRODUCTION Metal hydrides are an intensively studied group of compounds due to their practical and potential usage in organic synthesis1−3 and efficient storage of hydrogen for commercial vehicular applications.4 These compounds have provided a significant prototype for understanding the hydrogen absorption on metal surfaces. Thus, the spectroscopic and structural properties of metal hydrides are of fundamental interest and have been extensively investigated from both experimental and theoretical aspects. Various kinds of experimental technologies have been employed to investigate the metal hydrides, including matrix isolation infrared spectroscopy,5−17 infrared Fourier transform emission spectroscopy,18−27 Fourier transform microwave (FTMW) spectroscopy,28 collision-induced dissociation (CID),25 photoelectron spectroscopy (PES),29−35 and photoelectron imaging spectroscopy.36−40 All of the abovementioned techniques are quite fruitful, in particular, when combined with theoretical calculations in elucidating the geometries and electronic structures of the metal hydrides. Among various metal hydrides, gold hydride compounds are of particular interest on account of their important roles in homogeneous and heterogeneous catalysis.41−43 Gold hydride is supposed to be a key intermediate in several gold-catalyzed reactions,44 such as hydrosilylation,45 hydrogenation,46 C−H bond action,47 and aerobic oxidation of alcohols.48 The research of gold cluster hydrides is very important for understanding the adsorption of hydrogen onto gold surfaces. Furthermore, gold hydrides can be used as a prototype for examining the relativistic effects of gold.49−51 Investigations on the geometrical and electronic structures of these clusters will shed light on the chemical bonding of the H−Au interaction and further understanding of the catalytic mechanism of the gold-catalyzed reactions. Numerous experimental and theoretical works have been carried out for decades, with the focus on the geometrical and electronic properties of gold hydride clusters. Lester Andrews and coworks have investigated a considerable number of gold hydrides through a combination © 2014 American Chemical Society

of matrix-isolated infrared spectroscopy and density function theory (DFT) calculations.7,11,13−15 The surprising similarity in the PES of bare Aun+1− and AunH− has demonstrated that the chemistry of gold has significant resemblance to that of the hydrogen atom.30 Our group previously reported the photoelectron imaging of AuH− and Au2H−, and the vibrational features have been abstracted from the photoelectron spectra.37 Very recently, Wang and co-workers have reported the fantastic electronic structure and chemical bonding in gold dihydride using PES and ab initio calculations.35 Theoretically, Zhao et al. have carried out a systematic DFT investigation on gold cluster hydrides involving a single hydrogen atom, with the focus on the geometric, electronic, and bonding properties of AunH (n ≤ 5).52 Zhang et al. and Baishya et al. have reported their DFT studies on the multihydrogen atom interaction with gold clusters, exploring the structures and electronic and reactivity properties of Au6Hn (n = 1−12)53 and Au7Hn (n = 1−10),54 respectively. Previous experiments in our lab have studied the photoelectron velocity imaging spectroscopy of a number of coinage metal hydrides.37−40 The variation of the vertical detachment energies (VDEs) of these clusters with interchange of Cu, Ag, Au, and H has been demonstrated to depend on the electronegativities of these elements.39,40 The vibrational velocity imaging spectroscopies of AuH and Au2H have already been reported.31,37 In the current work, we present a joint experimental and theoretical study on the electronic and geometrical structure and chemical bonding in Au3H. Vibrationally resolved photoelectron images of Au3H− have been recorded at various photon energies, yielding a wealth of electronic and spectroscopic information. The photoelectron spectra are interpreted by comparing with theoretical calculations and Franck−Condon (FC) simulations. The Received: November 27, 2013 Revised: January 19, 2014 Published: January 24, 2014 1031

dx.doi.org/10.1021/jp411639r | J. Phys. Chem. A 2014, 118, 1031−1037

The Journal of Physical Chemistry A

Article

the anion and neutral species, whereas the VDE was calculated as the difference in energy between the anion and the neutral species both computed at the optimized geometry of the anion. To confirm the peak assignments and gain more spectroscopic parameters, the PESCAL program70 was employed to carry out FC simulations based on the results of the geometry optimizations and the harmonic vibrational analysis of the DFT calculations. The FC factors are computed in the harmonic oscillator approximation including Duschinsky rotation using the Sharp−Rosenstock−Chen method.71 In conjunction with electronic structure calculations, FC simulation serves as a guide for electronic and vibrational assignments.

vibrationally resolved photoelectron spectra provide confident evidence for determining the geometrical structure of Au3H. The identical spectral features between the images of Au3H and Au4 suggest that these species possess structural and electronic similarities.

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EXPERIMENTAL DETAILS

The experiments were carried out using our collinear velocity map photoelectron imaging analyzer equipped with a laser ablation source. The apparatus has been described in detail elsewhere.55 Only a brief description of the experiments is given below. The Au3H− anions were generated by laser vaporization of pure gold target in the presence of a helium carrier gas seeded with methanol vapor. The stagnation pressure of the gas mixture was adjusted about 1−3 atm to flow into vacuum through a pulse general valve. The negative clusters were cooled and expanded into the source chamber and then entered the extraction region. The cluster anions were extracted perpendicularly by a −1.2 kV high pulse voltage and were subjected to a Wiley−McLaren time-of-flight (TOF) mass spectrometer.56 Then, a laser beam (355 nm from a Nd:YAG laser, 470 nm from a dye laser) was guided into the interaction region and detached an electron from the anion. The generated photoelectrons in the detachment region were extracted by modified velocity map imaging electrodes, based on the original design of Eppink and Parker.57 After passing through a 36 cm TOF tube, the photoelectrons were mapped onto a detector consisting of a 40 mm diameter microchannel plate (MCP) assembly and a phosphor screen. The two-dimensional (2D) images on the phosphor screen were recorded by a chargecoupled device (CCD) camera. The photoelectron kinetic energy spectra were calibrated by the known spectrum of Au−. All of the raw images were reconstructed using the basis set expansion (BASEX) inverse Abel transform method,58 and the photoelectron spectra were abstracted. Each image is accumulated with between 50 000 and 100 000 laser shots at a 10 Hz repetition rate. The energy resolution is better than 30 meV at an eKE of 1 eV.

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RESULTS AND DISCUSSION Photoelectron Imaging. Photoelectron imaging and corresponding spectra of Au3H− recorded at 355 and 470 nm are presented in Figures 1 and 2, respectively. The raw image

Figure 1. The photoelectron raw image (top on the left), reconstructed images (bottom on the left), and the photoelectron spectrum for Au3H− recorded at 355 nm. Laser polarization is vertical in the plane of the page. The blue line below the experimental spectrum indicates the FC simulation for the ground-state detachment transition of Au3H− using PESCAL. The red vertical lines are the calculated FC factors.

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COMPUTATIONAL DETAILS All theoretical calculations were carried out using the Gaussian 09 package.59 The B3LYP hybrid functional, which makes use of the Hartree−Fock exact exchange and Becke’s exchange functional and Lee−Yang−Parr correlation functional,60−62 was employed. Our calculations were performed using analytical gradients, together with the aug-cc-pVTZ-PP relativistic effective core potential and basis set for Au63 and the aug-ccpVTZ basis set for H.64 For all calculations, the ultrafine integration grid (99 590) was used to ensure the accuracy of DFT results. Four candidate structures, that is, the zigzag, the Y-shape with a dangling hydrogen, the distorted Y-shape with a hydrogen bridge, and the rhombus, were taken as initial geometries. To further investigate the reliability of the theoretical methods, the ground-state and low-lying structures of Au3H0/− were further evaluated via geometrical optimizations and frequency calculations using the second-order Møller−Plesset perturbation theory (MP2)65−69 together with above-mentioned basis sets. Single-point energy corrections with CCSD(T) were performed to confirm the ground state and energy differences for Au3H− and Au3H, using the MP2optimized geometry. The electron affinity (EA) was calculated as the difference in energy between the optimized structures of

Figure 2. The photoelectron raw image (top on the left), reconstructed images (bottom on the left), and the photoelectron spectrum for Au3H− recorded at 470 nm. Laser polarization is vertical in the plane of the page. The arrow indicates the adiabatic EA. The blue line below the experimental spectrum indicates the FC simulation for the ground-state detachment transition of Au3H− using PESCAL. The red vertical lines are the calculated FC factors.

(top on the left) collected in the experiments shows the projection of the three-dimensional (3D) laboratory frame photoelectron probability density onto the plane of the imaging detector, and the reconstructed image (bottom on the left) represents the central slice of the 3D distribution from its 2D projection. The laser polarization is vertical in the image plane (double yellow arrow). Photoelectron spectra are obtained by 1032



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Figure 3. Optimized ground-state and low-lying structures of Au3H0/− at the B3LYP level. The Au and H are shown in yellow and white, respectively. The relative energies to the anion ground state are given in square brackets in eV. The Nimag stands for the number of imaginary frequencies. Bond lengths in Å and bond angles in degrees are given above the bond. Note that the neutral rhombic structure is unstable and has been optimized into the Y-shaped structure at the B3LYP level.

Table 1. Summary of Spectroscopic Information, HOMOs, and LUMOs of Au3H and Au4

a

This work. bPrediction at the B3LYP level. cPrediction at the MP2 level. dPrediction at the CCSD(T) level. eFrom ref 80. fFrom ref 81.

of the neutral species, as bored out by our FC simulation. Note that there are many congested features on the high binding energy side of the spectra in Figure 2 due to the low signal-tonoise ratio. It is also worth mentioning that the vibrational progressions revealed in the 470 nm spectrum are much more extensive than the one in the 355 nm spectrum, suggesting that the possible structural changes involved in the low-frequency vibrational modes upon electron detachment are expected to be larger than that of the high-frequency mode. Theoretical Calculations. We have carried out theoretical calculations to elucidate the structural and electronic properties of neutral and anionic Au3H clusters. Figure 3 depicts the calculated results of Au3H0/− at the B3LYP level. The symmetries, the electronic states, and the relative energies are exhibited for each structure. The calculated EAs, VDEs, and frequencies are summarized in Table 1, where they are compared with available experimental results. The groundstate anion is estimated to be the zigzag structure (Cs, 2A′) at the B3LYP level, followed by a degenerate Y-type structure with a dangling hydrogen (C2v, 2B2) and a distorted Y-shape with a hydrogen bridge (Cs, 2A′). The rhombic structure (C2v, 2B2) is found to be higher in energy by 0.93 eV than the lowest-lying structure. However, the energy order of neutral low-lying isomers reverses. The distorted Y-shape becomes lowest-lying in energy, with the Y-type structure (C2v, 1A1) and the zigzag (Cs, 1A′) being higher in energy by 0.07 and 0.25 eV, respectively. Note that the neutral rhombic structure is unstable and has been optimized into the Y-shaped structure at the B3LYP level.

integrating the reconstructed images, considering the Jacobian factors. The photoelectron spectra are plotted versus the electron binding energy (eBE), which is obtained by subtracting the electron kinetic energy (eKE) from the respective detachment photon energies (eBE = hν − eKE). Ganteför et al. have already reported the PES of Au3H− at 266 nm using a conventional “magnetic-bottle” negative ion TOF photoelectron spectrometer.29,30 Their PES reveals two spectral bands, which correspond to the transitions from the anionic electronic ground state to the ground state and first excited state of neutral Au3H. Our 355 nm spectrum allows the ground-state transition to be observed and is significantly better resolved with vibrational structures. The ground-state transition now is split into two sub-bands with a spacing of 2100 ± 100 cm−1, which corresponds to the Au−H stretching frequency. The lower photon energies are used for photodetachment, and the higher resolution is achieved. We shifted the photon energy to 2.638 eV (470 nm from dye laser) for the obtainment of highly resolved spectra. The dominated sub-band was then well-resolved and split into many peaks. The peak indicated by the arrow in Figure 2 defines the EA of the Au3H to be 2.548 ± 0.001 eV. The VDE of Au3H− is measured from the dominant peak to be 2.570 ± 0.001 eV. Two vibrational progressions are apparently revealed in the spectra, with one much more intense than the other. The vibrational spacing of one progression is determined to be 177 ± 10 cm−1, while the other is measured to be 96 ± 10 cm−1. The relatively weak feature below the EA value in the PES is the vibrational hot-band, which is from the anion vibrational excitation state to the ground vibrational level 1033



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Figure 4. Optimized ground-state and low-lying structures of Au3H0/− at the MP2 level. The Au and H are shown in yellow and white, respectively. The relative energies to the anion ground state are given in square brackets in eV. The Nimag stands for the number of imaginary frequencies. Bond lengths in Å and bond angles in degrees are given above the bond. Note that the rhombic structures are unstable at the MP2 level. The numbers in bold in the square brackets stand for the relative energies at the CCSD(T) level.

The low-lying structures of Au3H0/− obtained at the B3LYP level were further evaluated via structure reoptimizations and frequency calculations at the MP2 level. At the MP2 level, the anionic and neutral zigzag are optimized into the Y-type and distorted Y-shape with a hydrogen bridge, respectively, whereas all others keep the same steric configurations (Figure 4). The Y-shaped structure is predicted to be the global minimum for both anionic and neutral Au3H, successively, followed by the distorted Y-shaped structure. Note that the rhombic structure is unstable and has been optimized into the Y-shaped structure for both the anion and neutral at the MP2 level. All of the energies of the MP2-optimized structures were reevaluated via single-point energy calculations at the CCSD(T) level and are shown in bold type in Figure 4. Structural and Spectral Assignments. The ground states of anionic and neutral Au3H were previously predicted at the PW91PW91 level by Zhao et al. to be Y-type52 with a dangling hydrogen and a distorted Y-shaped with a hydrogen bridge, respectively, while Ganteför et al.’s PES research suggests the Ytype structure as the ground state for Au3H.30 Our vibrationally resolved photoelectron imaging of Au3H− present here provides confident evidence for determining the ground state of Au3H. The photoelectron spectrum at 355 nm reveals two sub-bands with a spacing of 2100 cm−1 in the ground-state transition of Au3H−, implying that the hydrogen atom belongs to not a bridged hydrogen but a dangling hydrogen. Our theoretical calculations at the MP2 level reveal three isomers for both anionic and neutral Au3H, and only the Y-shaped isomer has a dangling hydrogen, suggesting that the observed spectra arise from the photodetachment of the Y-shaped anion. Furthermore, the ADE and VDE of the Y-shaped structure are predicted to be 2.55 and 2.58 eV, respectively, at the MP2 level (Table 1). Thus, the theoretical values, in excellent agreement with experimental data, further provide credible evidence for the structural assignment. FC simulation was performed to assist us in the spectral assignments. The simulation used theoretical geometries and vibrational frequencies of anionic and neutral states calculated at the B3LYP level. The individual vibronic peak contours were approximated as a Gaussian function with 30 and 7 meV fwhm (full width at half-maximum) in the 355 and 470 nm spectra,

respectively. A vibrational temperature of 120 K was assumed for the anion in the simulation. The simulation provides the relative intensities of individual vibronic transitions, which are exhibited as red sticks in Figures 1 and 2. The solid blue curve represents the simulated spectrum, while the experimental spectrum is plotted as a black line. The simulated PES in Figure 1 confirms the Au−H assignment in the 355 nm spectrum. The simulated PES in Figure 2 agrees well with the experimental data and reproduces the intensities and positions of the peaks observed in the PES, providing further proof of the assignment that the observed features in the PES arise from the groundstate transition from Y-shaped anion Au3H− to Y-shaped neutral species. In general, the vibrational excitation upon photodetachment derives from a significant structural difference between the initial anions and corresponding neutral species along the normal coordinate. Therefore, those vibrational modes (usually totally symmetric modes) that contain significant motion along the normal coordinates will be revealed in the FC profile. Analogously to Au4, there are three potentially active totally symmetric a1 vibrational modes in C2v Y-shaped Au3H, the Au− H stretching mode in the AuH subunit, the Au−Au stretching mode in the Au 2 subunit, and the stretching mode perpendicular between them. The 355 nm spectrum reflects the Au−H stretching vibration, while the higher resolved spectra at 470 nm reveal the Au−Au stretching vibration in the Au2 subunit and the stretching mode perpendicular between these two subunits. Thereinto, the dominant vibrational progression in the 470 nm spectra is attributed to the Au− Au stretching vibration in the Au2 subunit. Consistent with the experimental observation of the more extensive vibrational progressions of the Au−Au stretching mode than that of the Au−H stretching mode, our theoretical calculations at both the B3LYP and MP2 levels consistently show that the difference of Au−Au distances between the neutral and anionic Au3H is much larger than that of the Au−H bond (Figures 3 and 4, respectively). Gold−Hydrogen Analogy: A Comparison between Au4− and Au3H−. Despite the different physical and chemical properties between hydrogen and gold, the closely similar electronegativity values and similar valence orbital structures in 1034



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the form of a single valence electron possess hydrogen in gold of special relevance considering the so-called gold−hydrogen analogy pioneered by Kiran et al.72−75 The significant resemblance of H with Au in its interaction with gold,30 boron,75−77 nitrogen,78 and silicon72−74 demonstrates that the atomic hydrogen behaves analogously to the gold atom.79 Our spectroscopic and theoretical investigations of Au4− and Au3H− provide a typical instance of Au/H analogy.80 A detailed comparison of the spectroscopic parameters between Au3H and Au4 are present in Table 1. The main spectral bands at 355 nm are very similar for Au3H− and Au4−.80,81 The spectra have an analogical spectral pattern for both Au3H− and Au4−, with a sharp-rising edge and a long trailing edge on the higher-energy side. The spectral peak positions are close to each other. The slight decrease in VDEs is consistent with the decrease of their electronegativity when the terminal gold atom of the gold tetramer is replaced by a hydrogen atom.39 Furthermore, the vibrationally resolved spectra at lower photon energies reveal three vibrational progressions for both Au3H and Au4. The PES similarities between Au4− and Au3H− strongly hint that these two species are structurally similar. The structural similarity was confirmed by our theoretical calculations. Both Au4 and Au3H were predicted to possess similar C2v Y-shaped structures, which can be considered as an assembly of two subunits in a mutually perpendicular way (two Au2 subunits for Au4 and a Au2 subunit and a AuH subunit for Au3H). The similar electronic structures can be illustrated through the σ−σ* donor−acceptor interaction model established by Zanti et al.,82 in which the chemical bonding between these two subunits is interpreted as an overlap between bonding σ and antibonding σ* areas belonging to different subunits (see the MOs in Table 1). Our FC simulations demonstrated that the three observed vibrational progressions stem from the stretching of these two subunits and the stretching in perpendicular between them. In brief, the exchange of a Au atom by a H atom induces minor geometric and electronic structural changes between Au4 and Au3H.

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-411-84379365. Fax: +86411-84675584. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21103186 and 21273233), the Ministry of Science and Technology of China, and the Chinese Academy of Sciences.

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REFERENCES

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CONCLUSIONS A joint theoretical calculations and photoelectron velocity map imaging spectroscopy study was carried out to investigate the geometrical and electronic properties of ground electronic states of Au3H0/−. The vibrational spectroscopy of Au3H from high-resolution photoelectron imaging leads to a more accurate EA and provides confidently experimental evidence for the resolution of the ground-state Au3H in the gaseous phase. The adiabatic detachment energy and VDE of Au3H − are experimentally measured to be 2.548 ± 0.001 and 2.570 ± 0.001 eV, respectively. The comprehensive comparisons between the experiment and theoretical calculations suggest that the Y-shaped structure is the global minimum for both the neutral and anionic Au3H. The photoelectron imaging of Au3H− in the present current work exhibits three vibrational progressions of 2100 ± 100, 177 ± 10, and 96 ± 10 cm−1, which correspond to the Au−H stretching mode in the AuH subunit, the Au−Au stretching mode in the Au2 subunit, and the stretching mode perpendicular between these two subunits, respectively. The wealthy and accurate spectroscopic information obtained from our photoelectron imaging spectra will also be benefit the benchmark of the performance of various kinds of theoretical methods. 1035



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