2008 International Symposium on Molecular Spectroscopy

Anion Photoelectron Spectra of CHX2- and CX2

- Properties of the Corresponding Neutrals

Scott WrenKristen Vogelhuber, *Kent Ervin, **Anne McCoy, W. Carl Lineberger

JILA, Department of ChemistryUniversity of Colorado

*University of Nevada, Reno**The Ohio State University

June 19th, 2008

Photoelectron Spectroscopy

AB- + h AB + e-

1. Electron Affinity (EA)

EA

e

2. Vibrational Frequencies, eT0

3. Neutral Excited States:

All above properties plus term energy, T0

What We Measure:

4. Geometry Change, ΔR

ΔR

When PES is Straightforward

• Small geometry change between anion and neutral

• Resolved vibrational progressions

• Few isomers of anion

• Clear origin peak

• Vibrational progressions of neutral electronic states overlap minimally

Electron Binding Energy (eV)1.52.0

Pho

toel

ectro

n C

ount

s

EA(H2CCN)

Photoelectron Spectrum of H2CCN-

Cyanomethyl Anion

When PES is More Challenging

• Large geometry changebetween anion & neutral

• Many vibrations, poorly resolved

• Overlapping vibrational progressions in two neutralstates

• Origin peak very weak(due to poor Franck-Condon overlap)

Anion

Neutral

Photoelectron Spectrum of c-C4F8-

EA(c-C4F8)

TODAY we deal only with these more challenging systems

Outline

• How does structure of anion and neutral affect photoelectron spectrum?

Halomethyl Radicals: CHX2 Halocarbenes: CCl2

• Revisit Singlet-Triplet splitting, EST

• Contamination by CHCl2

CHX2

CHX2

CCl2

Singlet CCl2

Nearly planar

Pyramidal

Triplet CCl2

Instrumentation

Photon Energy 3.408 eV 364 nm

Buildup Cavity Power ~100 W

Resolution ~10 meV

Making Halomethyl Anions

OH- + CH2Cl2 CHCl2- + H2O

Selectively produce a single anion

CDCl2- Photoelectron Spectrum

Electron Binding Energy (eV)1.52.02.53.0

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1400

CDCl2

CDCl2-

RC-Cl= - 0.23 Å

αClCCl= + 14.1°

αClCH= + 20.8° Approx EA(Calculated)

CDCl2- Cold Spectrum (~150K)

Other Halomethyl Radicals: CHBr2

Electron Binding Energy (eV)1.82.02.22.42.62.83.03.2

Pho

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200

400

600 CHBr2

CHBr2-

RC-Br= - 0.25 Å

αBrCBr= + 14.1°

αBrCH= + 21.2° Approx EA(Calculated)

CHBr2-

Electron Binding Energy (eV)1.82.02.22.42.62.83.03.2

Pho

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Other Halomethyl Radicals: CHI2

•Very similar structure and spacings to CHBr2 and CHCl2

•Preliminary spectra— analysis and calculations are underway

CHI2-

Electron Binding Energy (eV)1.82.02.22.42.62.83.03.2

Pho

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ount

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Other Halomethyl Radicals: CHI2

•Very similar structure and spacings to CHBr2 and CHCl2

•Preliminary spectra— analysis and calculations are underway

CHI2-

All three halomethyl spectra exhibit complex vibrational progressions involving the H atom,

with several strongly coupled modes

We are currently working with Anne McCoy to model the coupled vibrations in CHX2

Making Carbenes: Ion Chemistry

•O- + CH2Cl2

CHCl2- + •OH

CCl2- + H2O

•CHCl2 + -OH

Cl- + •OCH2Cl

55%

25%

13%

6%

Two main product channels!

Electron Binding Energy (eV)1.52.02.53.0

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Previous Experiments1

1. R.L. Schwartz et al., J. Phys. Chem. A 1999, 103 8213-8221

• 364 nm PES spectrum

• Reported both 1A1 and 3B1 states

• Reported Singlet-Triplet splitting

EST= 0.13(.13) eV

1A1

3B1

1999 CCl2- Spectrum

EST

1999 CCl2- Spectrum

Electron Binding Energy (eV)1.52.02.53.0

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Previous Experiments1

1. R.L. Schwartz et al., J. Phys. Chem. A 1999, 103 8213-8221

1A1

3B1

This spectrum exhibits all of the complicating features mentioned earlier, so one needs (and should have been) wary of any EST conclusion.

•All (lots!) of calculations since 1999 sharply disagree with our EST value

EST

•Experimental resolution improved

•Recent experiments suggest possibility of a contaminant ion along with CCl2-

Motivations for Reinvestigation

Electron Binding Energy (eV)1.52.02.53.0

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New “CCl2-” Photoelectron Spectrum

•O- + D2CCl2 CCl2- + CDCl2

- + D2O

3B11A1

Differences from 1999:• 351 nm• Used D2CCl2• Mass Bias for CCl2-

Electron Binding Energy (eV)1.52.02.53.0

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New “CCl2-” Photoelectron Spectrum

•O- + D2CCl2 CCl2- + CDCl2

- + D2O

3B11A1

How might CDCl2-

contamination affect CCl2

- spectrum?

Electron Binding Energy (eV)1.52.02.53.0

Pho

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“CCl2-” and Authentic CDCl2- Spectra

1A1

3B1

•O- + D2CCl2 CCl2- + CDCl2

- + D2O

Let’s subtract out thecontamination

Electron Binding Energy (eV)1.52.02.53.0

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New, Clean CCl2- Photoelectron Spectrum

1A1 originEA=1.596 eV

3B1 Calc1

origin2.5 eV

ΔEST

~0.9 eV

1Dyke et. al.ChemPhysChem 2005 6, 2046-2059And now Experimentalist AND Theorist are in agreement!!!

Conclusions

• Spectra of the halomethyl anions CHCl2, CHBr2, and CHI2 reveal extensive vibrational progressions indicating a large geometry change between the ion and the neutral

• New photoelectron spectra of the dihalocarbenes CCl2, CBr2, and CI2 show that the singlet-triplet gaps of the neutral are much larger than we had previously reported

CCl2- Comparison with Theory1

Electron Binding Energy (eV)1.5 2.0 2.5 3.0 3.5 4.0 4.5

1Dyke et. al.ChemPhysChem 2005 6, 2046-2059

Exp (eV) Calc1(eV)1A1(0-0) 1.596( ) 1.574(0.011)3B1(0-0) - 2.475(0.021)

EST > 0.5 0.900(0.011)

3B1 calculated origin

EST much larger than previously reported

Triplet origin of CCl2 cannot be experimentally observed

Singlet state of CCl2 agrees well with theoretical predictions

Why Study Carbenes?

Example: Addition of carbene into C-C double bond

Same products but different mechanism based on state of carbene

Making Carbenes- Ion Chemistry

40mm

Anion m/z

CCl2- 82.9

HCCl2- 83.9

•O- + H2CCl2

HCCl2- + •OH

CCl2- + H2O

24080

40

mm

m=1

Velocity Mass Filter Resolution

Why Study Carbenes?

• Carbene: Contains a neutral divalent carbon atom with two nonbonding electrons

• Important intermediates in organic synthesis• Singlet and triplet states of the neutral lie close in energy

anion

h2B1

Singlet

p

σ

3B1

Triplet

p

σ

1A1

neutral