CH Stretch Vibrational Spectroscopy and Tunneling Dynamics in Vinyl Radical JILA F. Dong1, M. A. Roberts, R. S. Walters and D. J. Nesbitt JILA, Department of Chemistry and Biochemistry, University of Colorado 1Los Gatos Research, Inc. Mountain View, CA.

Prototype for unsaturated radical combustion chemistry Large amplitude in-plane tunneling pathway Mid-IR: H. Kanamori, Y. Endo, E. Hirota. J.Chem. Phys. 92, 197 (1990). FTIR: L. Letendre, D.-K. Liu, C. D. Pibel, J. B. Halpern, H.-L. Dai. J. Chem. Phys. 112, 9209 (2000). mm: K. Tanaka, M. Toshimitsu, K. Harada, T. Tanaka. J. Chem. Phys. 120, 3604 (2004).

Time-Resolved FTIR (Dai and coworkers) Medium resolution IR emission from the photolysis of vinyl-Br at 193 nm. Bands near 3235 cm-1 attributed to vinyl CH stretches. Provides starting point for high resolution frequency searches.

Vinyl Expectations CH Stretches - anharmonic DFT prediction vs. FTIR data…~ 200 cm-1 freq. search window!

High Resolution Slit-Jet Infrared Spectrometer Sub-Doppler molecular linewidths (~ 30 MHz) High resolution tunable IR radiation (1-2 MHz) Stabilized Transfer Cavity (~ 10 MHz) Shot-noise limited Sensitivity (~1.5 x 10-5 → 107 #/cc/qs)

Jet Cooled Hydrocarbon Radicals localized discharge Clean synthesis of radicals by dissociative attachment (RX + e- → R + X-) High radical densities at slit orifice (1013-15 #/cm3) Localized discharge (~ 105 cm/s, ~ 1 ms transit time) – can avoid radical-radical chemistry! “Simple” high resolution spectroscopy at 5-10 K

Vinyl Spectral Analysis Search for vinyl bands beginning at ~3250 cm-1…signature bands found near 2900 cm-1. 3 A-type progressions observed. Unambiguous confirmation of vinyl assignment via high precision 2-line and 4-line combination differences. Identical spectra obtained with multiple vinyl precursors.

Bending/Tunneling Potentials (CCSD(T) AVnZ, CBS) High level CCSD(T) AVnZ (n=2,3,4) surface calculations Extrapolated to complete basis set CBS limit (Dunning, Peterson et al) Corrected for ZPE in 3N-7 orthogonal coordinates Adiabatic PES for ground state as well as vibrationally excited states

Reduced Mass (m) Tunneling path (q) as psuedo 1D coordinate embedded in full 3N-6 D space Reduced mass as rigorous function of full 1D tunneling and 3N-7 D orthogonal coordinates… …calculated at each point q (Rush-Wiberg) q

Large Amplitude Tunneling Methods (Rush-Wiberg/HBJ) Based on LAM rotational Hamiltonian methods by Hougen, Bunker and Johns Matrix of inertial moments and displacements along 3N-6 tunneling path Works for arbitrary choice (e.g. angles or distances) of LAM coordinate (q)

Wave Functions/Tunneling Splittings 1640 cm-1 Modified S.E. Predicts tunneling splitting of Dn = 0.513 cm-1 Near quantitative agreement with mm wave studies of Tanaka et al (Dn = 0.543 cm-1) Improved barrier height prediction of 1640(50) cm-1

Summary First high resolution CH stretch spectra of vinyl radical. Three A-type bands observed: two tunneling (a,s) bands out of ground state and one hot band. Tunneling splittings consistent with previous studies. Agreement with ab initio anharmonic predictions for symmetric CH2 stretch. Vibrationally adiabatic tunneling PES and barrier heights.

Thanks Feng Dong, Melanie Roberts Mike Deskivich Funding:

Symmetry and Spin statistics (sym. CH2 : A-type) for 180o rot. around the a-axis b Ground State a - + Ns. Rot. tunneling Vib. Elec. c A-type: N=0, ±1; Ka = 0; Kc = ±1 Excited State - + Ns. Rot. tunneling Vib. Elec. GROUND state: Lower tunneling level (+1) : Ka=even (3); Ka=odd (1) Upper tunneling level (-1) : Ka=even (1); Ka=odd (3)

High E Discharge all Hydrogens interchange… …quickly quenched in expansion