Electronic Transitions of Palladium Monoboride and Platinum Monoboride

Y.W. Ng, H.F. Pang, Y. S. Wong, Yue Qian, and  A. S-C. CheungDepartment of ChemistryUniversity of Hong Kong

 June 2012

67th OSU International Symposium on Molecular Spectroscopy

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

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Contents Introduction

Experimental Setup

Results

Summary3

IntroductionInterest in transition metal monoboride

Spectroscopic Interest Molecular & electronic structure Synthesizing metal monoboride in gas phase

Pervious study Limited studies on metal boride

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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 B2H65

IntroductionPervious Works on PdB

Knight et al (J Chem. Phys. 97 2987 (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 X2Σ+ state

ro=1.608 Å

Kharat et al (Int. J Quant. Chem. 109 1103 (2009)) Studying 4d transition metal monoboride by density functional theory

(DFT) calculations

PdB X2Σ+ state

ro = 1.856 Å

ωe = 725.6cm-1

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IntroductionPervious Works on PtB

Kalamse et al (Bull. Mater. Sci. 33 233 (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:

PtB X2Σ+state

ro = 1.809 Å

ωe = 906cm-1

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No experimental observation of electronic transition of palladium monoboride and platinum monoboride

Laser ablation/reaction free jet expansion

Molecule production:

Pd (Pt) + B2H6 (0.5% in Ar) → PdB (PtB) + etc.

Ablation Laser : Nd:YAG, 10Hz, 532nm, 5mJ

Free Jet Expansion : i) backing pressure: 6 atm B2H6 (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

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ExperimentSchematic Diagram of Laser Vaporization/ LIF Experimental Setup

Pulsed Nd: YAG laser

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Metal rod

Monochromator

Fix the wavelength of the OPO laser Scan the grating in monochromator Wavelength resolved fluorescence spectrum

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v’

v”

0

012

ΔG3/2

ΔG1/2

Excitation Laser

Scanning grating

ΔG1/2 ΔG3/2

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 spectrumWithout monochromator

filtering

Filtered fluorescence spectrumWith 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

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The analysis of the[19.7]2Σ+ – X2Σ+ transitions of PdB in the spectral region between465 and 520 nm using laser induced fluorescence (LIF) spectroscopy

19000 19500 20000 20500 21000 21500 22000

[20.2]23/2

(0,1)

(0,1) (1,0)(0,0)

(2,0)(1,0)

Wavnumber (cm-1)

(0,0)

[21.2]21/2

Results (PtB)

Low resolution broad band spectrum of PtB

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The analysis of the [21.2]2Π1/2 – X2Σ+ and

[20.2]2Π3/2 – X2Σ+

transitions of PtB in the spectral region between 455 and 520 nmusing laser induced fluorescence (LIF) spectroscopy

Confirmation of PdB and PtB Signal intensity is proportional to the abundance of

the isotopes Abundance 11B : 10B ≈ 4:1 Intensity of two bands ≈ 4:1

B carrier

Five peaks with similar intensity representing the five palladium isotopes104Pd (11.14%) 105Pd (22.33%)106Pd (27.33%) 108Pd (26.46%)110Pd (11.72%)

Pd carrier

Spectra of Pt isotopic species is observed194Pt (32.9%) 195Pt (33.8%)196Pt (25.3%) 198Pt (7.2%)

Pt carrier

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20280 20300 20320

Pd10B

Wavenumber (cm-1)

Pd11B

21300 21320

Pt10B

Wavenumber (cm-1)

Pt11B

Results (PdB)

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R1, R2 branches and P1, P2 branches No Q branch 2Σ+ - 2Σ+ transition

19750 19755

4.5 1.5

P2(J)

14.5

0.5

R2(J)

4.5 1.5

P1(J)

17.5

R1(J)

0.5

Wavenumber (cm-1)

Results (PdB)

Observed vibrational transitions of PdB 17

+

+

Results (PdB)

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Molecular constants for Pd11B (cm-1)

[19.7]2+ X2+

∆G1/2 541.12 753.98

Bo 0.4741 0.5353

ro(Å) 1.847 1.738

Results (PtB)

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2 P-branches (P1 and P12) doublet state Strong R and Q branches ΔΛ= +1 Ω’=0.5 – Ω”=0.5 transition 2Π1/2 - 2Σ+ transition

0.5

1.5

0.5

1.5Ω” = 0.5

J

Ω’ = 0.5

P1(1.5)R1(0.5) Q1(0.5)

21290 21300

Wavenumber (cm-1)

8.5

0.5

0.5 5.51.510.5

1.55.5

P1(J)

Q1(J)

P12

(J)

R1(J)

Pt11B

0.5

0.5

1.5

Results (PtB)

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2 R-branches (R2 and R21) doublet state Strong R and Q branches ΔΛ= +1 Ω’=1.5 – Ω”=0.5 transition 2Π3/2 - 2Σ+ transition

1.5

0.5

1.5

2.5

Ω” = 0.5

J

Ω’ = 1.5

P2(2.5)R2(0.5) Q2(1.5)

20220 20230

Wavenumber (cm-1)

P2(J)

2.59.5

Q2(J) 1.5

15.5R

2(J)

0.5

R21

(J)

2.5 6.5

10.50.5

1.5

2.5

Vibrational bands observed for PtB

X2Σ+

[21.2]2П1/2[20.2]2П3/2

01

0

1

2

01v

v v

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Results (PtB)

Results (PtB)

2Π3/2 is lower in energy than 2Π1/2 inverted Π state

Bo value of 2Π3/2 is larger than 2Π1/2 regular Π state

[21.2] 2Π1/2 and [20.2] 2Π3/2 come from different 2 states

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Molecular constants for Pt11B (cm-1)

[21.2]2П1/2 [20.2]2П3/2 X2Σ+

ΔG1/2 613.9 636.26 903.60

Bo 0.4699 0.4995 0.5274

ro(Å) 1.856 1.800 1.751

Molecular orbital energy level diagram of PdB & PtB

Electronic Configuration

PdB(PtB)

1σ

1π

2σ

2π

3σ

Pd(Pt)

B

d

s

2p

1δσ

δ

π

σ

σ

π

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Ground State:1σ21π41δ42σ1 2Σ+

Excited State:1σ21π41δ42π1 2Π

1σ21π41δ43σ1 2Σ+

1σ21π41δ32σ12π1 2Π

Comparison of Group 10 monoboride

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Molecule NiB PdB PtB

Ground state Symmetry

2+ 2+ 2+

ro (Å) 1.698 1.738 1.751

ΔG1/2 (cm-1) 768.2 754.0 903.6

Bond length increases down the group from NiB to PtB

The larger ΔG1/2 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]2Σ+ - X2Σ+ of PdB [21.2]2Π1/2 – X2Σ+ and [20.2]2Π3/2 – X2Σ+ of PtB

Ground state of PdB and PtB: 2Σ+

Bond length at ground state of PdB, ro = 1.738Å

Bond length at ground state of PtB, ro = 1.751Å

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