Funded by NSF (ES) and DOE (TZ) CONFORMATION-SPECIFIC SPECTROSCOPY OF ALKYL BENZYL RADICALS: EFFECTS OF A RADICAL CENTER ON THE CH STRETCH IR SPECTRA OF ALKYL CHAINS JOSEPH KORN and TIM ZWIER DANIEL TABOR and NED SIBERT June 23, 2016 Funded by NSF (ES) and DOE (TZ)

Goal Figure out the information content of the alkyl CH stretch region of IR spectra so that we can use this information as a probe of local environment and structure.

Rings, double bonds, radicals Multiple conformers Metal Ions Methyl and methoxy groups Figs by JMOL

Modeling the Conformation-Specific Infrared Spectra of N-Alkylbenzenes (MJO8) Daniel Tabor et al. Where‘s the Bend? Locating the First Folded Structure in Straight Chain Alkylbenzenes in a Supersonic Expansion (MJO9) Daniel Hewett et al. Infrared Laser Spectroscopy of the n-Propyl and i-Propyl Radicals in Helium Droplets: Significant Bend-Stretch Coupling Revealed in the CH Stretch Region (RF11) Christopher Moradi et al.

The Model Ingredients Harmonic CH stretches that are bilinearly coupled Harmonic HCH scissor overtones and combination bands that are bilinearly coupled Select Fermi couplings and anharmonicites Ab Initio linear dipoles.

Same in curvilinear and rectilinear normal coordinates Q=L-1S.-1S Step 1: constructing a H that consists of bilinearly coupled harmonic local modes. (H is obtained from Hessian) CH3-CH3 H3C The quadratic contributions are very different for these systems. Same in curvilinear and rectilinear normal coordinates Q=L-1S.-1S

Optimal Scaling Parameters for Alkane Chains 0.9610 6-311++G(d,p) basis

Hamiltonian Structure CH2 Stretches CH3 Stretches Bend Overtones Bend Combination States Fermi Coupling Same in curvilinear and rectilinear normal coordinates Q=L-1S-1S

Potential Contributions to Select Fermi Couplings in cm-1.

Scaled (.72) Cubic Couplings This is the main coupling for CH2 groups 22.0 cm-1 2.5 cm-1 1.5 cm-1 6.5 cm-1

Additional Anharmonicites - CH3 Start with a zero order Hamiltonian H(1) = Fermi coupling terms

Hamiltonian Structure CH2 Stretches CH3 Stretches Bend Overtones Bend Combination States Fermi Coupling

Electronic Excitation Spectrum of Ethyl-Benzyl Radical

IR spectra for ethyl-benzyl radical CH3-CH2-CH-Ph

CH stretch site frequencies for alkyl chains Pentyl Decyl Wavenumber (cm-1) ggp calculated spectra vs. other with curves B3LYP CH2 Group Phenyl ring The phenyl ring alters local mode frequencies for about two carbon atoms The CH3 has a one-carbon effect Gauche conformations can lead to 30 cm-1 shifts

IR spectra for ethyl-benzyl radical CH3-CH2-CH-Ph

Dihedral Angle Nomenclature p – perpendicular ~90 t – trans ~180o g – gauche ~60o

Franck-Condon Factors (never look a gift horse in the mouth) UC – Davis (Wiki)

Returning to the Ethyl-Benzyl Radical Spacing between levels is about 15 cm-1 .

Flip colors

Ground State Excited State 2 f 1 3 Ground State Excited State

FC Simulations: α-Propyl I have a table of angles and relative energies for each of the excited state mimina as well. Can also make a slide like slide 8 for these. This one is still a work in progress. Need the potential energy surfaces and a good way to label the theory spectra.

Vary displacement and frequency of upper well to get a good fit. From Andrei Tokmakoff

B3LYP-D3 Excited state Energy (cm-1) B3LYP B3LYP-D3

Experimentally we find a shift of 25o

Concluding Remarks Obtained RIDIR spectra for 1 conformer of the a-EthBz radical and 2 conformers of the a-PropBz radicals. The a-EthBz radical’s theoretical and experimental spectra are in excellent agreement. Torsional effects are observed. The frequency dependencies of the CH groups next to the radical affect the low frequency of the CH stretch spectrum and this allows us to narrow the possibilities of the 2 a-PropBz radicals to the tt and the tg’ structures. The full vibronic results allowed us to assign the two a-PropBz radicals.

Acknowledgements Ned Sibert Britta Johnson Patrick Walsh Ryoji Kusaka Dan Hewett Sebastian Bocklitz Joe Korn Tim Zwier Gary Douberly Jim Lisy Funding: NSF Stock images need to be added

FC Simulations: α-Propyl Vibronic Spectrum Wavenumber (cm-1)

FC Simulations: α-Propyl Hole-burn Conformer B Vibronic Spectrum Wavenumber (cm-1)

FC Simulations: α-Propyl Hole-burn Conformer B Theory tg’ D0 Vibronic Spectrum Wavenumber (cm-1)

FC Simulations: α-Propyl Hole-burn Conformer B Theory tg’ D0 Vibronic Spectrum Theory tt D0 Wavenumber (cm-1)

Ph-CD2-CH3 Ph-CH2-CH3 Ethylbenzene Fits Wavenumber (cm-1) B3LYP/6-311++G(d,p) CH3 ss/so CH3 as Ph-CD2-CH3 CH3 ss/so Exp CH2 as CH2 ss/so CH2 ss/so Ph-CH2-CH3 Exp 0.9600 CH2 Str 0.9610 CH3 Str 0.975 CH2 Bend 0.975 CH3 Bend Ethyl B3LYP New Combination Coupling Wavenumber (cm-1) CH2 and CH3 spectral features overlap.