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

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)

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

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 2880-2960 cm-1 Vinyl B3LYP/6-311++G(3df,3pd)

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

Signature of Vinyl Symmetric CH stretch Very rich spectra Three A-type bands assigned between 2880-2940 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

Rotational Constants for the Three Bands Set I Set II Set III Hirota’s A” 7.91334 B” 1.08283(14) 1.08279(15) 1.06468(12) 1.083011 C” 0.94861(14) 0.94870(14) 0.94782(10) 0.948803 A’ 7.91296(50) 7.92905(40) 7.91433(50) B’ 1.08107(17) 1.08121(15) 1.06254(12) C’ 0.94408(17) 0.94604(15) 0.94490(9) n0 2901.8581(4) 2901.9305(3) 2897.2251(4) s 0.0007 0.0006

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

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

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

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

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