Infrared Laser Spectroscopy of the n-Propyl and i-Propyl Radicals in Helium Droplets: Significant Bend-Stretch Coupling Revealed in the CH Stretch Region ISMS 2016, RF11, 6/23/16 Peter R. Franke,1 Daniel Tabor,2 Christopher P. Moradi,1 Gary E. Douberly,1 Jay Agarwal,3 Henry F. Schaefer III,3 and Edwin L. Sibert III2 1Department of Chemistry, University of Georgia, Athens, GA 30602, USA 2Department of Chemistry and Theoretical Chemistry Institute, University of Wisconsin Madison, Madison, Wisconsin 53706, USA 3Center for Computational Quantum Chemistry, University of Georgia, Athens, GA 30602, USA

Chain Branching in Low-Temperature Hydrocarbon Oxidation Chemistry Kinetics of alkylperoxy radical isomerizations are key!

n-propyl oxidation QOOH (hydroperoxypropyl radical) + O2 Prototype system for low-temperature hydrocarbon oxidation (ignition chemistry) O2QOOH Chain Branching PES courtesy of W. D. Allen (UGA)

Cs symmetry J. Phys. Chem. B., 2015, 119, 728-735 Vibrational wave functions for the methylene torsional motion of the n-propyl radical, superimposed on the fitted enthalpic (0 K) potential energy curve along the distinguished reaction coordinate τ. The energy levels are 33.2 cm−1 (n = 0), 109.7 cm−1 (n = 1), 161.9 cm−1 (n = 2), and 277.2 cm−1 (n = 3), where n is the vibrational quantum number. Cs symmetry J. Phys. Chem. B., 2015, 119, 728-735

Helium Nanodroplet Isolation Spectroscopy He Droplet Source Pick - up cells “pick-up cells” ~1010 molecules·cm-3 Doping Laser Excitation Droplet Production Photo-induced Ionization Cross-Section Reduction

Δ AIR VACUUM x n-butyl nitrite Metering Valve Water-cooled Precursor Cu electrodes Valve Droplet beam intersects @ 90° Ta wire coiled around quartz tube x Δ n-butyl nitrite

Mass Spectrometry 39 Δ 43 [CH3(CH2)3ONO]

n-propyl radical

n-propyl radical

Difference Mass Spectra Amplitude Modulate Laser and Scan Quadrupole. Process with Lock-in Amplification

n-propyl radical

i-propyl radical

Infrared Spectra of the n- and i-propyl radicals

CCSD(T)/ANO(1) Quartic Force Field; Normal Mode Representation n-propyl VPT2+K CCSD(T)/ANO(1) Quartic Force Field; Normal Mode Representation Resonance Criteria: Martin Test * Fermi & Darling-Dennison Resonances No Resonances *

CCSD(T)/ANO(1) Quartic Force Field; Normal Mode Representation i-propyl VPT2+K CCSD(T)/ANO(1) Quartic Force Field; Normal Mode Representation Resonance Criteria: Martin Test * Fermi & Darling-Dennison Resonances No Resonances *

Local Mode Model Hamiltonian Unscaled B3LYP Force Constant Matrix in Dimensionless Normal Coordinates Scaled Normal Mode Hamiltonian Three step description of the derivation of the zero order local mode Hamiltonian. Matrices are for n-propyl radical at the staggered Cs sym geom. In the bottom matrix the first three elements correspond to the CH3. The arrow on the bottom picture corresponds to a local mode with site frequency 2842.4 cm-1. Note matrix of fundamentals is the same as the force constant matrix in dimensionless normal coordinates. The transformation between the normal and local is an orthogonal transformation of the mass weighted Cartesian coordinates. The approximation is that we now neglect terms ai^dagger aj^dagger. This is a good approximatin. -CH3 Local Mode Hamiltonian -CH2 -CH2

Adding Anharmonicity Initial Local Mode Picture Add in select stretch-bend Fermi coupling terms, calculated ab initio, that are scaled by a single scaling factor Matrices are for n-propyl radical at the staggered Cs sym geom. Our model is the local mode plus select anharmonic couplings. Only significant local coupling is between stretches and bends on the same CH2/CH3 group. We find this coupling to be similar for all systems. Also add in bend anharmonicity, whose values are taken from perturbative studies of CH2DCH2D and CH3CD3 , to separate energies of bend overtones and combination states.

Local Mode Model Hamiltonian n-propyl Daniel Tabor Ned Sibert Univ. Wisconsin i-propyl

Summary Pyrolytic decomposition of organic precursors combined with mass spectrometry allows for the spectroscopic study of a broad range of molecular radicals and carbenes within helium droplets. Low temperature and isotropic environment allows for a detailed study of the vibrational complexity associated with hydrocarbon radicals relevant to combustion. Local mode Hamiltonian approach reveals stretch-bend Fermi coupling as source of complexity in the 2850 to 2950 cm-1 region for both n-propyl and i-propyl radicals. * * *

Acknowledgments Co-authors: Peter R. Franke, Christopher P. Moradi (UGA) Henry F. Schaefer, Jay Agarwal (UGA) Daniel Tabor, Ned Sibert (Univ. of Wisconsin) Support: U.S. Department of Energy, Office of Science (BES-GPCP)