LIF and Microwave Spectroscopy of CH2CHS

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LIF and Microwave Spectroscopy of CH2CHS
M. Nakajima, A. Miyoshi, Y. Sumiyoshi, and Y. Endo
The University of Tokyo
61st International Symposium on Molecular
Spectroscopy June 22, 2006
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About CH2CHS
Sulfur analogue of the vinoxy radical, CH2CHO
Similar spectroscopic property
Past studies
Only one ab initio study has been reported
for spectroscopic property of CH2CHS.
Yamaguchi et al., J. Chem. Phys. 93, 4223 (1990).
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Experimental
Precursor ethyl vinyl sulfide, C2H5-S-CHCH2.
248-nm photolysis (room-temp. LIF observation)
Pulsed discharge (under the jet-cooled condition)
Spectroscopic detection
Laser-induced fluorescence spectroscopy
Fourier-transform microwave (FTMW) spectroscopy
FTMW-mmW double-resonance spectroscopy
Sumiyoshi et al., J. Chem. Phys. 123, 054324
(2005).
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Low-Lying Electronic States of CH2CHS
CASPT2/cc-pVTZ
Te(ab initio) 21142 cm-1
2382 cm-1
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LIF Excitation Spectra
Only one intense band has been observed under the
room-temperature and jet cooled conditions.
Poor vibrational structure !
Photolysis
Origin Band
_at_21820 cm-1
Discharge
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High-Resolution LIF Spectrum
Origin Band
Jet-cooled condition
Congested rotational structure!
Ground state molecular constants were determined
from MW spectroscopy to make definite rotational
assignments.
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FTMW Spectrum of CH2CHS
Under jet-cooled condition (Discharging
C2H3-S-C2H5S)
gt70 lines are observed for one
rotational transition !
Electronic spin hyperfine components
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Analysis of Pure Rotational Transitions
CH2CHS is a doublet species and has
three inequivalent H-nuclei.
Complicated hyperfine structures
Ignore hyperfine component!
The strongest hyperfine component was picked
up for each spin component in one rotational
transition.
Determined only rotational and
spin-rotation constants.
ANa2 BNb2 CNc2 eaaNaSa
ebbNbSb eccNcSc (eab eba)(NaSb
SaNb)/2
Hamiltonian
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Observed MW Transitions
FTMW spectroscopy
Double-resonance
a-type 15
N 1 3 Ka 0, 1
b-type 11
sum 26
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Result of Analysis for MW Transitions
Fitting 26 MW transitions (including spin
components)
in MHz
s 3.8 MHz
Inertial Defect 0.039 amu Å2
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High-Resolution LIF Excitation Spectrum
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Upper State Molecular Constants
Lower state constants
Fixed to the values determined by MW spectroscopy.
Inertial Defect -0.33 amu Å2
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Simulation of Origin Band Spectrum
Obs.
Sim.
Trot. 8 K
FWHM(Gaussian) 0.012 cm-1
FWHM(Lorentzian) 0.010 cm-1
No line broadening was observed.
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Ab Initio Geometrical Optimization
CASPT2(RS2C)/cc-pVTZ
CASSCF 11 electrons in 9 active orbitals
Ab initio
Exp.
at zero-vibrational levels
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Ab Initio Geometries
CASPT2/cc-pVTZ
CASSCF 11 electrons in 9 active orbitals
C-C bond twists by 20.
Planer geometry
Non-planer geometry
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Fluorescence Lifetime
Ab initio spontaneous emission lifetime
MRCI/aug-cc-pVTZ
tf (ab initio) 560 ns
Fast non-radiative processes exist even in
the zero-vibrational level of the excited
electronic state.
Causing the poor vibrational structure in the LIF
excitation spectra.
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Electronic Structures of CH2CHX (XS,O)
From the eaa" value,
ASO of CH2CHS -207 cm-1 (for the state )
50 unpaired electron density is on the terminal
carbon.
Similar estimation for CH2CHO ASO -37 cm-1
93 unpaired electron density on the terminal
C-atom
Good agreement with the value calculated from
the hyperfine constants. (slightly lt80 )
Substitution of O by S atom enhances
delocalization of unpaired electron in the
CH2-CHX conjugation system.
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Summary
The CH2CHS radical was first observed by LIF
and MW spectroscopy.
Molecular constants of CH2CHS were determined.
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SVL Dispersed Fluorescence Spectrum
Pumping R-branch head of origin band (_at_21821.6
cm-1 )
400 cm-1 intervals
n4 CH2 scis. n8 C-S str. n5 HCS bend. n9 CCS
bend. n6 C-C str. n12 C-C torsion n7 CH2 rock.
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Ab Initio Vibrational Frequencies
RCCSD(T)/cc-pVTZ
in cm-1
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Spin-Rotation Constant, eaa (Ground State)
Approximate representation of eaa
eaa" -0.61875 cm-1 (exp.)
A" 1.86183 cm-1 (exp.)
ASO(CH2CHS) -207 cm-1 (In the ground electronic
state)
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Electronic Structure (Ground State)
Magnitude of ASO in a molecule may be
approximated as a weighted average of atomic ASO
proportional to the unpaired electron density on
each atom in the molecule.
Unpaired electron is localized on the terminal C
or S.
ASO(C) 29 cm-1 ASO(S) 382.4 cm-1
S.D. spin density on each atom
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Present Study
LIF MW observation of CH2CHS.
Determination of molecular constants including
rotational constants.
Information on the excited state dynamics.
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FTMW-mmW Double-Resonance
Once FTMW spectrum was observed, FTMW-mmW double
resonance spectroscopy was used for searches of
new rotational transitions.
mmW radiation which is resonant to the c-b or c-a
transition irradiates to break the
coherence between the a and b levels.
Decrease FID signal !
Monitor b-a transition (coherence between the a
and b levels) by FTMW spectrometer.