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Breaking the Symmetry in Methyl Radical:
High resolution IR spectroscopy of CH2D
Melanie RobertsDepartment of Chemistry and Biochemistry, JILA
University of Colorado at Boulder National Institute of Standards and Technology
Chandra Savage, Feng Dong, David Nesbitt
Previous Work on CH2D
M. E. Jacox, 1977 Matrix isolation spectroscopy
J. L. Brum et al., 1993 Gas phase, REMPI Electronic structure
K. Kawaguchi, 2001 Gas phase, FTIR Out-of-plane bending mode Q, R branches only
The Broken Symmetry…
CH3: 1 IR-active stretch doubly-degenerate asymmetric stretch
CH2D: 3 IR-active stretches CH symmetric and CH antisymmetric stretch CD stretch
Hydrogen I=1/2, Deuterium, I=1 CH2D: small dipole moment
Experimental Setup
•Servoloop locked optical transfer cavities for high frequency precision (~20 MHz)•Servoloop locked optical transfer cavities for high frequency precision (~20 MHz)
Radical Source
High radical densities at slit (~1014 -1016 /cm3!)
Sub-Doppler molecular linewidths (~40 MHz in Ne expansion)
High radical densities at slit (~1014 -1016 /cm3!)
Sub-Doppler molecular linewidths (~40 MHz in Ne expansion)
Rotational Energy Levels
220
221
211
212
202
110
111
101
000
Ene
rgy
labels: NKaKc
Watson Hamiltonian
C
B
A
Nuclear Spin Statistics
Ψ = Ψel Ψvib Ψrot Ψns
- - + + α α
1/√2 (α β +β α)
β β
- - + - 1/√2(α β - β α)
Electronic Vibration Rotation Nuclear Spin
Experimental Data
Symmetric Stretch: DFT calculations:
B3LYP/6-311++G(3df,3pd)
Anharmonicity benchmarked to 10 CH radicals
1 10
21
1 1 01
20
21 1
0
21
1 0 00
10
1
1 11
11
0
1 10
11
1
2 11
21
21 0
1
00
0
2 12
11
12 0
2
10
1
2 11
11
0
3 13
21
2
3 03
20
2
3 12
21
1
Predictions
Data
Boltzmann Plot
Para
Ortho
Trot = 18.9(3) K
Ground State Rotational Constants
Axis FTIR Studies* (cm-1) This Work (cm-1)
A 9.596969(89) 9.586(8 )
B 5.90686(49) 5.9067(1 )
C 3.6164(10) 3.6195(7 )
*K. Kawaguchi, 2001
Band Origin (cm-1)
3065.64(1)
What is really happening?
CH bond length increases Ground state: 1.0788(10)Å Excited state: 1.0854(23)Å
Rotational Constant changes (excited state – ground state) ΔA = 0.16 cm-1
ΔB = 0.02 cm-1
ΔC = 0.03 cm-1
Fine and Hyperfine Terms
ITSH
ITSH
SIaH
SNSNH
eQeQ
DDDD
ff
sr
)(21
Unsplit rotational
level Spin-rotation splitting Nuclear
hyperfine splitting
J = N + S
F1 = J + IH2
F2 = F1 +ID
Fine and Hyperfine Structure
Summary CH2D Symmetric Stretch Band
Rotational Structure Fine Structure
Hyperfine Structure
Antisymmetric Stretch
Future Work Oxygenated Radicals
Acknowledgements•Chandra Savage•Feng Dong•Scott Davis•Erin Whitney•David Nesbitt•OSEP•NSF/IGERT