1. 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

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

3. 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)

4. 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), cm−1 (n = 1), 161.9
cm−1 (n = 2), and cm−1 (n = 3), where n is the vibrational
quantum number.
Cs symmetry
J. Phys. Chem. B., 2015, 119,

6. 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

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

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

9. n-propyl radical

10. n-propyl radical

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

12. n-propyl radical

13. i-propyl radical

14. Infrared Spectra of the n- and i-propyl radicals

15. 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 *

16. 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 *

17. 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 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

18. 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.

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

20. 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. * * *

21. 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)