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
Pho
toel
ectro
n C
ount
s
0
200
400
600
800
1000
1200
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
toel
ectro
n C
ount
s
0
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
toel
ectro
n C
ount
s
0
100
200
300
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
toel
ectro
n C
ount
s
0
100
200
300
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
Pho
toel
ectro
n C
ount
s
0
50
100
150
200
250
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
Pho
toel
ectro
n C
ount
s
0
50
100
150
200
250
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
Pho
toel
ectro
n C
ount
s
0
100
200
300
400
500
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
Pho
toel
ectro
n C
ount
s
0
100
200
300
400
500
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
toel
ectro
n C
ount
s
0
100
200
300
400
500
“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
Pho
toel
ectro
n C
ount
s
0
100
200
300
400
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