Breaking the Symmetry in Methyl Radical: High resolution IR spectroscopy of CH 2 D Melanie Roberts Department 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 CH 2 D 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… CH 3 : 1 IR-active stretch  doubly-degenerate asymmetric stretch CH 2 D: 3 IR-active stretches  CH symmetric and CH antisymmetric stretch  CD stretch Hydrogen I= 1 / 2, Deuterium, I=1 CH 2 D: small dipole moment

Experimental Setup Servoloop locked optical transfer cavities for high frequency precision (~20 MHz)

Radical Source High radical densities at slit (~ /cm 3 !) Sub-Doppler molecular linewidths (~40 MHz in Ne expansion) High radical densities at slit (~ /cm 3 !) Sub-Doppler molecular linewidths (~40 MHz in Ne expansion)

Rotational Energy Levels Energy labels: N KaKc Watson Hamiltonian C B A

Nuclear Spin Statistics Ψ=Ψ el Ψ vib Ψ rot Ψ ns --++α 1/√2 (α β +β α) β --+-1/√2(α β - β α) Electronic Vibration RotationNuclear Spin

Experimental Data

Symmetric Stretch: DFT calculations: B3LYP/ G(3df,3pd) Anharmonicity benchmarked to 10 CH radicals 1 10               2 11 Predictions Data

Boltzmann Plot Para Ortho T rot = 18.9(3) K

Ground State Rotational Constants AxisFTIR Studies * (cm -1 )This Work (cm -1 ) A (89) 9.586(8 ) B (49) (1 ) C (10) (7 ) *K. Kawaguchi, 2001 Band Origin (cm -1 ) (1)

What is really happening? CH bond length increases  Ground state: (10)Å  Excited state: (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 Unsplit rotational level Spin- rotation splitting Nuclear hyperfine splitting J = N + S F 1 = J + I H 2 F 2 = F 1 +I D

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