1. 60th International Symposium on Molecular Spectroscopy
High-resolution IR spectra of Jet-Cooled Vinyl Radical : Symmetric CH2 Stretching Vibrations
JILA, CU-Boulder
Feng Dong
David J. Nesbitt
60th International Symposium on Molecular Spectroscopy
June 21, 2005
TJ09

2. Vinyl Radical Background
Simplest open-shell olefinic radical
Important reactive intermediate
Electronic spectroscopy, reactions with O2/H2 or decompositions to acetylene or vinylidene
Previous high-resolution IR study of CH2 wagging mode at 895 cm-1
Low resolution time-resolved FTIR of all fundamental vibrational frequencies
Kanamori, Endo and Hirota, JCP (1990); Letendre and Dai, JCP(2000)

3. Challenges for CH Stretch Detection
Very low infrared intensities
20 km/mol for cyclopropyl  0.2% peak absorbance
4.4 km/mol for the strongest symmetric CH stretch in vinyl  ~ 4x10-4 peak absorbance
Intensity sharing due to the tunneling
High sensitivity detection required
Difficult to generate high density of radicals
Lack of accurate experimental/theoretical fundamental frequencies
5x10-5 absorbance sensitivity
1013 ~ 1014 radicals/cm3

4. Accurate Frequency Prediction
Scaled frequency s ~ 10 cm-1
>150 cm-1 difference between FTIR and prediction
In the region of FTIR reported frequency (3260 – 3100 cm-1), no signature of vinyl
Very rich spectra between 2880 – 3080 cm-1
Preliminary assignment of symmetric CH stretch between cm-1
Vinyl
B3LYP/ G(3df,3pd)

5. Tunneling Dynamics Two tunneling transitions allowed sym → sym
3:1
1:3
asym
sym
Two tunneling transitions allowed
sym → sym
asym → asym
Small tunneling splitting due to large barrier height
DEtun” ~ 0.06 cm-1

6. Signature of Vinyl Symmetric CH stretch
Very rich spectra
Three A-type bands assigned between cm-1
Comparing with Hirota’s ground state combination difference (symmetric tunneling)
I – s = 20 MHz
II – s = 28 MHz
III – s > 1 GHz
Set I
Set II
Set III

7. Rotational Constants for the Three Bands
Set I
Set II
Set III
Hirota’s A” B”
(14)
(15)
(12) C”
(14)
(14)
(10) A’
(50)
(40)
(50) B’
(17)
(15)
(12) C’
(17)
(15)
(9) n0
(4)
(3)
(4) s
0.0007
0.0006

8. Band Origin for CH Symmetric Stretch
FTIR
Scaled
Experiment
3103
2930
2901.9
 200 cm-1  FTIR emission at v=3 → v=2

9. Vinyl from Different Precursors
Same three bands
Same intensity ratio for band I and II
~ 2.9 times more vinyl generated with VyBr than VyCl

10. Summary Two bands of the symmetric CH stretch observed
Both bands from the symmetric tunneling level
Vibrational band origin ~200 cm-1 red-shift of FTIR measurement, indicating the v = 3 → v = 2 emission observed in FTIR

11. Future Studies
Search for the asymmetric tunneling transition of symmetric CH stretch
Decode the rich spectra to extract the information of asymmetric and lone CH stretch in our spectra
Direct tunneling dynamics from lone CH stretch

12. Acknowledgement David J. Nesbitt Chandra Savage Erin S. Whitney NSF
AFOSR