Exploring the Vibrational Structure of the Vinylidene Anion using Argon Predissociation Spectroscopy Kristin Breen, Helen Gerardi, George Gardenier, Timothy Guasco, Jennifer Laaser, Gary Weddle, Eric Diken, and Mark Johnson Department of Chemistry, Yale University, New Haven, CT, 06511. Presented at the 64th International Symposium on Molecular Spectroscopy June 22-26, 2009
Outline What is vinylidene Facts / synthesis Good for vibrational autodetachment Photodetachment in vinylidene Embedded resonances in direct detachment Below threshold detachment Ar-predissociation to assign vibrations N2O as a messenger species
Vinylidene Important in… Acetylene pyrolysis Acetylene oxidation Interstellar medium Unsaturated carbenes in organometallic reactions
Vinylidene / Acetylene Isomerization Neutral H C=C: → H-C≡C-H Neutral lifetime = 0.04-0.2 ps Barrier height= 0.9 – 2 kcal/mol Anion much larger barrier to isomerization Barrier height= 50 kcal/mol Ervin, K. M.; Ho, J.; Lineberger, W. C. J. Chem. Phys. 1989, 91, 5974-5992. 4
Vinylidene Anion Synthesis Ar + N2O Ion Mass (m/z) Relative Intensity Ar + N2O + C2H4 10 15 20 25 30 35 N2O + e¯ → N2 + O· ¯ OH· + H2C=CH¯ O· ¯ + H2C=CH2 OH¯ + H2C=CH· OH¯ + H2C=CH· → H2O + H2C=C¯ 100 kJ/mol* 2 kJ/mol* *Lee, J.; Grabowski, J. J. Chem. Rev. 1992, 92, 1611-1647. 5
Experimental Set-up valve pulsed reflectron bi-plane photo- electrons ion beam drift tube MCP neutral detector 1 keV electron gun MCP ion detector YAG YAG 1064 nm mid - IR OPO/OPA (600-4500 cm-1) 6
Photodetachment data Below threshold detachment Embedded resonances in direct detachment 2500 3000 3500 4000 4500 Photon Energy (cm-1) VDE ~3950 cm-1
Ar Predissociation Spectroscopy Aˉ·Arm + hn → Aˉ·Arn + (m-n)Ar
Anion Vibrations H2CC– D2CC– Argon Loss Signal Photon Energy (cm-1) 1600 1800 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000 4200 4400 Photon Energy (cm-1)
Anion Vibrations CH(D) symmetric 2606 (1954) cm-1 CH(D) asymmetric 2664 (1979) cm-1 overtone of CH2 scissors 2749 cm-1 H2CC– Fermi resonances important with CH symmetric and 2 quanta of CH2 scissors Argon Loss Signal D2CC– E 1600 1800 2000 2200 2400 2600 2800 Photon Energy (cm-1)
Anion Vibrations H2CC¯ D2CC¯ CC stretch with CH(D) asym. and sym. stretches H2CC¯ 1st overtone of CC stretch CH stretch with CH2 scissors overtone of CC or CD stretch with CD2 scissors D2CC¯ 1st overtone of sym & asym stretches 2800 3000 3200 3400 3600 3800 4000 4200 4400 Photon Energy (cm-1)
Different Messenger Species… H2C=C¯·X X = N2O ~ 41cm-1 splitting X = Ar ~ 52 cm-1 splitting 3800 3900 4000 4100 4200 4300 4400 Photon Energy (cm-1)
Photodetachment Data Below threshold detachment CH stretches Embedded resonances in direct detachment Combination bands of CH & CC stretches 2500 3000 3500 4000 4500 Photon Energy (cm-1) VDE ~3950 cm-1
Conclusions assigned peaks to photodetachment spectrum of vinylidene by using Ar-predissociation / isotopic substitution above threshold resonances combination bands overtones below threshold detachment fundamentals fermi resonances shown messenger species effect
Acknowledgements Mark Johnson Helen Gerardi George Gardenier Timothy Guasco Gary Weddle Rachael Relph Ben Elliott Mike Kamrath Christopher Leavitt