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

VinylideneImportant in…

Acetylene pyrolysis

Acetylene oxidation

Interstellar medium

Unsaturated carbenes in organometallic reactions

Vinylidene / Acetylene Isomerization

Ervin, K. M.; Ho, J.; Lineberger, W. C. J. Chem. Phys. 1989, 91, 5974-5992.

Neutral lifetime = 0.04-0.2 psBarrier height= 0.9 – 2 kcal/mol

H C=C: → H-C≡C-H H

Neutral

Anionmuch larger barrier toisomerization

Barrier height= 50 kcal/mol

Vinylidene Anion Synthesis

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.

Ar + N2O

Ion Mass (m/z)

Rel

ativ

e In

ten

sity

Ar + N2O + C2H4

10 15 20 25 30 35

Experimental Set-up

MCP neutral detector

mid - IR(600-4500 cm-1)

YAG

OPO/OPA

MCP ion detector

reflectronvalve

pulsed

1 keV electron gun

ion beam

drift tube

YAG

1064 nm

photo-electrons

bi-plane

Photodetachment data

Embedded resonances in direct detachment

Below threshold detachment

~3950 cm-1

VDE

2500 3000 3500 4000 4500Photon Energy (cm-1)

Ar Predissociation Spectroscopy

Aˉ·Arm + h → Aˉ·Arn + (m-n)Ar

Anion Vibrations

D2CC–

H2CC–

1600 1800 2000 2200 2400 2600 2800

Arg

on

Lo

ss S

ign

al

Photon Energy (cm-1)

3000 3200 3400 3600 3800 4000 4200 4400

Anion Vibrations

D2CC–

H2CC–

1600 1800 2000 2200 2400 2600 2800

Arg

on

Lo

ss S

ign

al

Photon Energy (cm-1)

Fermi resonances important with CH symmetric and 2 quanta of CH2 scissors

CH(D) symmetric 2606 (1954) cm-1

CH(D) asymmetric 2664 (1979) cm-1

overtone of CH2 scissors 2749 cm-1

E

Anion Vibrations

2800

Photon Energy (cm-1)

3000 3200 3400 3600 3800 4000 4200 4400

CC stretch with CH(D) asym. and sym. stretches

1st overtone of CC stretch

CH stretch with CH2 scissors

overtone of CC or

CD stretch with CD2 scissors

1st overtone of sym & asym stretches

H2CC¯

D2CC¯

3800 3900 4000 4100 4200 4300 4400

~ 52 cm-1 splitting

~ 41cm-1 splitting

Photon Energy (cm-1)

H2C=C¯·X X = N2O

X = Ar

Different Messenger Species…

Photodetachment Data

Embedded resonances in direct detachment

Below threshold detachment

~3950 cm-1

VDE

2500 3000 3500 4000 4500Photon Energy (cm-1)

CH stretches

Combination bands of CH & CC stretches

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 GardenierTimothy GuascoGary WeddleRachael RelphBen ElliottMike KamrathChristopher Leavitt