67 th OSU International Symposium on Molecular Spectroscopy

67 th OSU International Symposium on Molecular Spectroscopy 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 2012 1

Acknowledgments The work described here was supported by grants from the Research Grants Council of the Hong Kong SAR, China. (Project numbers 701008 P). 2

Contents Introduction Experimental Setup Results Summary 3

Introduction Interest in transition metal monoboride Spectroscopic Interest ³ Molecular & electronic structure ³ Synthesizing metal monoboride in gas phase Pervious study ³ Limited studies on metal boride 4

Introduction Pd and Pt are elements from same Group (Group 10) ³ ³ Same outermost shell electronic configuration ® Likely to have same ground state symmetry on Pd. B & Pt. B 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 Pd. B Knight et al (J Chem. Phys. 97 2987 (1992)) ³ Studying Pd. B by electron spin resonance (ESR) spectroscopy ³ Performing ab initio calculations on Pd. B using unrestriced Hartree-Fock method and limited STO-3 G basis set Pd. B X 2Σ+ state ro=1. 608 Å Kharat et al (Int. J Quant. Chem. 109 1103 (2009)) ³ Studying 4 d transition metal monoboride by density functional theory (DFT) calculations Pd. B X 2Σ+ state ro = 1. 856 Å ωe = 725. 6 cm-1 6

Introduction Pervious Works on Pt. B Kalamse et al (Bull. Mater. Sci. 33 233 (2010)) ³ Using DFT calculated the ground state symmetry, bond length and vibrational frequency of 5 d transition metal mononitrides and monoborides ranging from La to Hg: Pt. B X 2Σ+state ro = 1. 809 Å ωe = 906 cm-1 No experimental observation of electronic transition of palladium monoboride and platinum monoboride 7

Gas-Phase Pd. B (Pt. B ) Production Method Laser ablation/reaction free jet expansion Molecule production: Pd (Pt) + B 2 H 6 (0. 5% in Ar) → Pd. B (Pt. B) + etc. Ablation Laser : Nd: YAG, 10 Hz, 532 nm, 5 m. J Free Jet Expansion : i) backing pressure: 6 atm B 2 H 6 (0. 5% in Ar) ii) background pressure: 1 x 10 -5 Torr LIF spectrum in the visible region Laser system: Optical Parametric Oscillator laser 8

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

v’ Monochromator 0 Scannin g grating v 2 ” 1 0 ΔG 3/2 ΔG 1/2 Excitation Laser ΔG 3/2 Wavelength resolved fluorescence spectrum Fix the wavelength of the OPO laser Scan the grating in monochromator Wavelength resolved fluorescence spectrum 10

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

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

Results (Pd. B) The analysis of the [19. 7]2Σ+ – X 2Σ+ transitions of Pd. B in the spectral region between 465 and 520 nm using laser induced fluorescence (LIF) spectroscopy Low-resolution broad band spectrum of Pd. B 13

Results (Pt. B) The analysis of the [21. 2]2Π 1/2 – X 2Σ+ and [20. 2]2Π 3/2 – X 2Σ+ transitions of Pt. B in the spectral region between 455 and 520 nm using laser induced fluorescence (LIF) spectroscopy Low resolution broad band spectrum of Pt. B 14

Confirmation of Pd. B and Pt. B Signal intensity is proportional to the abundance of the isotopes 11 B : 10 B ≈ 4: 1 ³ Abundance ³ 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 (Pd. B) R 1, R 2 branches and P 1, P 2 branches No Q branch 2Σ+ - 2Σ+ transition 16

Results (Pd. B) + + Observed vibrational transitions of Pd. B 17

Results (Pd. B) Molecular constants for Pd 11 B (cm-1) [19. 7]2 + X 2 + ∆G 1/2 541. 12 753. 98 Bo 0. 4741 0. 5353 ro(Å) 1. 847 1. 738 18

Results (Pt. B) J 1. 5 0. 5 R 1(0. 5) 1. 5 Q 1(0. 5) P 1(1. 5) 0. 5 1. 5 0. 5 Ω’ = 0. 5 2 P-branches (P 1 and P 12) Strong R and Q branches Ω’=0. 5 – Ω”=0. 5 transition Ω” = 0. 5 doublet state ΔΛ= +1 2Π 1/2 - 2Σ+ transition 19

Results (Pt. B) J 1. 5 Ω’ = 1. 5 0. 5 R 2(0. 5) Q 2(1. 5) P 2(2. 5) 2. 5 1. 5 Ω” = 0. 5 2 R-branches (R 2 and R 21) Strong R and Q branches Ω’=1. 5 – Ω”=0. 5 transition doublet state ΔΛ= +1 2Π 3/2 - 2Σ+ transition 20

Results (Pt. B) [21. 2]2 П 1/2 v 1 0 v 2 1 [20. 2]2 П 3/2 0 v 1 0 X 2Σ+ Vibrational bands observed for Pt. B 21

Results (Pt. B) Molecular constants for Pt 11 B (cm-1) [21. 2]2 П 1/2 [20. 2]2 П 3/2 X 2Σ + ΔG 1/2 613. 9 636. 26 903. 60 Bo 0. 4699 0. 4995 0. 5274 ro(Å) 1. 856 1. 800 1. 751 is lower in energy than 2Π 1/2 inverted Π state 2Π Bo value of 2Π 3/2 is larger than 2Π 1/2 regular Π state [21. 2] 2Π 1/2 and [20. 2] 2Π 3/2 come from different 2 states 3/2 22

Electronic Configuration 3σ σ d π s σ δ Pd (Pt) Ground State: 2π 1σ21π41δ 42σ1 2Σ+ 2σ 1δ σ 1π 1σ π 2 p B Pd. B (Pt. B) Excited State: 1σ21π41δ 42π1 2Π 1σ21π41δ 43σ1 2Σ+ 1σ21π41δ 32σ12π1 2Π Molecular orbital energy level diagram of Pd. B & Pt. B 23

Comparison of Group 10 monoboride Molecule Ground state Symmetry ro (Å) ΔG 1/2 (cm-1) Ni. B Pd. B Pt. B 2 + 2 + 1. 698 1. 738 1. 751 768. 2 754. 0 903. 6 Bond length increases down the group from Ni. B to Pt. B The larger ΔG 1/2 of Pt. B indicates a stronger bonding between Pt and B atoms 24

Summary First experimental observation of electronic transition of the Pd. B and Pt. B molecule 2 + ³ [19. 7] Σ - X Σ of Pd. B 2 2 + ³ [21. 2] Π 1/2 – X Σ and [20. 2] Π 3/2 – X Σ of Pt. B Ground state of Pd. B and Pt. B: 2Σ+ Bond length at ground state of Pd. B, ro = 1. 738Å Bond length at ground state of Pt. B, ro = 1. 751Å 25

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