1. 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)

2. 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.

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

4. 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.

5. 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.

6. 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

7. Optimal Scaling Parameters for Alkane Chains
0.9610
G(d,p) basis

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

9. Potential Contributions to Select Fermi Couplings in cm-1.

10. 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

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

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

14. Electronic Excitation Spectrum of Ethyl-Benzyl Radical

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

16. 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

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

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

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

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

23. Flip colors

24. Ground State Excited State2 f 1 3
Ground State Excited State

26. 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.

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

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

30. Experimentally we find a shift of 25o

31. 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.

32. 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

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

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

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

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

38. Ph-CD2-CH3 Ph-CH2-CH3 Ethylbenzene Fits Wavenumber (cm-1)
B3LYP/ 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
CH2 Str
CH3 Str
0.975 CH2 Bend
0.975 CH3 Bend
Ethyl B3LYP New Combination Coupling
Wavenumber (cm-1)
CH2 and CH3 spectral features overlap.