Electronic Transitions of Palladium Monoboride and Platinum Monoboride Y.W. Ng, H.F. Pang, Y. S. Wong, Yue Qian, and A. S-C. Cheung Department of Chemistry University of Hong Kong June th OSU International Symposium on Molecular Spectroscopy 1

Acknowledgments The work described here was supported by grants from the Research Grants Council of the Hong Kong SAR, China. (Project numbers P). 2

Contents  Introduction  Experimental Setup  Results  Summary 3

Introduction Interest in transition metal monoboride  Spectroscopic Interest  Molecular & electronic structure  Synthesizing metal monoboride in gas phase  Pervious study  Limited studies on metal boride 4

Introduction  Pd and Pt are elements from same Group (Group 10)  Same outermost shell electronic configuration  Likely to have same ground state symmetry on PdB & PtB  Similar chemical properties  Catalysts for hydrogenation, dehydrogenation, reductive alkylation, hydrogenation of carbonyl and selective hydrogenation of nitro compound  Likely to have similar reaction towards B 2 H 6 5

Introduction Pervious Works on PdB  Knight et al (J Chem. Phys (1992))  Studying PdB by electron spin resonance (ESR) spectroscopy  Performing ab initio calculations on PdB using unrestriced Hartree-Fock method and limited STO-3G basis set PdB X 2 Σ + state r o =1.608 Å  Kharat et al (Int. J Quant. Chem (2009))  Studying 4d transition metal monoboride by density functional theory (DFT) calculations PdB X 2 Σ + state r o = Å ω e = 725.6cm -1 6

Introduction Pervious Works on PtB  Kalamse et al (Bull. Mater. Sci (2010))  Using DFT calculated the ground state symmetry, bond length and vibrational frequency of 5d transition metal mononitrides and monoborides ranging from La to Hg: PtBX 2 Σ + state r o = Å ω e = 906cm -1 7 No experimental observation of electronic transition of palladium monoboride and platinum monoboride

Laser ablation/reaction free jet expansion Molecule production: Pd (Pt) + B 2 H 6 (0.5% in Ar) → PdB (PtB) + etc. Ablation Laser : Nd:YAG, 10Hz, 532nm, 5mJ Free Jet Expansion : i) backing pressure: 6 atm B 2 H 6 (0.5% in Ar) ii) background pressure: 1x10 -5 Torr LIF spectrum in the visible region Laser system: Optical Parametric Oscillator laser Gas-Phase PdB (PtB ) Production Method 8

Experiment Schematic Diagram of Laser Vaporization/ LIF Experimental Setup Pulsed Nd: YAG laser 9 Metal rod

Monochromator  Fix the wavelength of the OPO laser  Scan the grating in monochromator  Wavelength resolved fluorescence spectrum 10 v’ v” Δ G 3/2 Δ G 1/2 Excitation Laser Scanning grating Δ G 1/2 Δ G 3/2 Wavelength resolved fluorescence spectrum

Monochromator  Serve as an optical filter  Set the grating at a particular wavelength  Small spectral region is detected by PMT 11 Total fluorescence spectrum Without monochromator filtering Filtered fluorescence spectrum With monochromator filtering

Experiment  The pulsed valve, ablation laser, excitation laser and oscilloscope are synchronized appropriately by a delay generator 12 Pulsed Nd: YAG laser

Results (PdB) Low-resolution broad band spectrum of PdB 13 The analysis of the [19.7] 2 Σ + – X 2 Σ + transitions of PdB in the spectral region between 465 and 520 nm using laser induced fluorescence (LIF) spectroscopy

Results (PtB) Low resolution broad band spectrum of PtB 14 The analysis of the [21.2] 2 Π 1/2 – X 2 Σ + and [20.2] 2 Π 3/2 – X 2 Σ + transitions of PtB in the spectral region between 455 and 520 nm using laser induced fluorescence (LIF) spectroscopy

Confirmation of PdB and PtB  Signal intensity is proportional to the abundance of the isotopes  Abundance 11 B : 10 B ≈ 4:1  Intensity of two bands ≈ 4:1  B carrier  Five peaks with similar intensity representing the five palladium isotopes 104 Pd (11.14%) 105 Pd (22.33%) 106 Pd (27.33%) 108 Pd (26.46%) 110 Pd (11.72%)  Pd carrier  Spectra of Pt isotopic species is observed 194 Pt (32.9%) 195 Pt (33.8%) 196 Pt (25.3%) 198 Pt (7.2%)  Pt carrier 15

Results (PdB) 16  R 1, R 2 branches and P 1, P 2 branches  No Q branch  2 Σ Σ + transition

Results (PdB) Observed vibrational transitions of PdB

Results (PdB) 18 Molecular constants for Pd 11 B (cm -1 ) [19.7] 2  + X2+X2+ ∆G 1/ BoBo r o (Å)

Results (PtB) 19  2 P-branches (P 1 and P 12 )  doublet state  Strong R and Q branches  ΔΛ= +1  Ω’=0.5 – Ω”=0.5 transition  2 Π 1/2 - 2 Σ + transition Ω” = 0.5 J Ω’ = 0.5 P 1 (1.5)R 1 (0.5) Q 1 (0.5)

Results (PtB) 20  2 R-branches (R 2 and R 21 )  doublet state  Strong R and Q branches  ΔΛ= +1  Ω’=1.5 – Ω”=0.5 transition  2 Π 3/2 - 2 Σ + transition Ω” = 0.5 J Ω’ = 1.5 P 2 (2.5)R 2 (0.5)Q 2 (1.5)

Vibrational bands observed for PtB X2Σ+X2Σ+ [21.2] 2 П 1/2 [20.2] 2 П 3/ v v v 21 Results (PtB)

 2 Π 3/2 is lower in energy than 2 Π 1/2  inverted Π state  B o value of 2 Π 3/2 is larger than 2 Π 1/2  regular Π state  [21.2] 2 Π 1/2 and [20.2] 2 Π 3/2 come from different 2  states 22 Molecular constants for Pt 11 B (cm -1 ) [21.2] 2 П 1/2 [20.2] 2 П 3/2 X2Σ+X2Σ+ ΔG 1/ BoBo r o (Å)

Molecular orbital energy level diagram of PdB & PtB Electronic Configuration PdB (PtB) 1σ1σ 1π1π 2σ2σ 2π2π 3σ3σ Pd (Pt) B d s 2p 1δ1δ σ δ π σ σ π 23 Ground State: 1σ 2 1π 4 1δ 4 2σ 1  2 Σ + Excited State: 1σ 2 1π 4 1δ 4 2π 1  2 Π 1σ 2 1π 4 1δ 4 3σ 1  2 Σ + 1σ 2 1π 4 1δ 3 2σ 1 2π 1  2 Π

Comparison of Group 10 monoboride 24 MoleculeNiBPdBPtB Ground state Symmetry 2+2+2+2+2+2+ r o (Å) ΔG 1/2 (cm -1 )  Bond length increases down the group from NiB to PtB  The larger ΔG 1/2 of PtB indicates a stronger bonding between Pt and B atoms

Summary  First experimental observation of electronic transition of the PdB and PtB molecule  [19.7] 2 Σ + - X 2 Σ + of PdB  [21.2] 2 Π 1/2 – X 2 Σ + and [20.2] 2 Π 3/2 – X 2 Σ + of PtB  Ground state of PdB and PtB: 2 Σ +  Bond length at ground state of PdB, r o = 1.738Å  Bond length at ground state of PtB, r o = 1.751Å 25

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