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