1. Conformational Study of Benzyl Ether
TB09
Conformational Study of Benzyl Ether
A. O. Hernandez-Castillo, Daniel M. Hewett, Chamara Abeysekera, and Timothy S. Zwier
Department of Chemistry, Purdue University

2. 𝐌𝐨𝐭𝐢𝐯𝐚𝐭𝐢𝐨𝐧 𝜋-stacking in flexible bichromophores
Understand the associated physics and chemistry involved in incipient stages of soot formation.
Linker group.
𝜋-stacking in flexible bichromophores A B
Propyl linkage will be present in smaller abundance than ethyl in combustion environment… once form it will lead to pi-stacked structures.
Important applications, including light- harvesting complexes, chromophore arrays, conjugated polymers, and fluorescence resonance energy transfer (FRET).
1,3 diphenylpropane
𝜃 𝑑 1 =58.9°
𝜃 𝑑 2 =−79.8°
Benzylether
𝜃 𝑑 1 =62.4°
𝜃 𝑑 2 =−89.3°
Several modes involved in the coupling…. Complicated but interesting spectroscopy…. Levels are mixed
*make bigger bold
Schematic representation of the process of soot formation. (M.R. Kholghy et al. Carbon (2016))
3 atom linkage
O atom changes conformational preferences.
1,3-diphenylpropane

3. Relative Energies 1,3-diphenylpropane 𝜃 𝑑 1 =180° 𝜃 𝑑 2 =180°
𝜃 𝑑 1 =180°
𝜃 𝑑 2 =180°
𝜃 𝑑 1 =69.2°
𝜃 𝑑 2 =124.3°
Pi-stacked excimer red shifted emission
The structures were calculated at B2PLYP-D3BJ/aug-cc-pVTZ level of theory.
The energies were calculated at B3LYP/def2tzvp with Grimme’s D3BJ dispersion.
tg: 1.96 kJ/mol
sta: 3.1 kJ/mol
**Change figure BE
𝜃 𝑑 1 = 𝜃 𝑑 2 =60.9°
𝜃 𝑑 2
𝜃 𝑑 1
𝜃 𝑑 1 =62.4°
𝜃 𝑑 2 =−89.3° 2

4. Experimental Setup (Fluorescence Chamber)
Molecules are interrogated 5mm from nozzle
The fluorescence is collected with -20% efficiency
4-inch spherical mirrors
Fluorescence imaged onto a UV coated plano-convex focusing lens
emission is turned by a UV-coated 3-inch mirror and imaged onto a fast photomultiplier tube
photomultiplier tube converts the detected light into an electrical current which is digitized by a fast oscilloscope
digitized signal is sent to LabView software.
Laser Induced Fluorescence (LIF)

5. Single conformation IR and UV Spectroscopy
Fluorescence-dip infrared spectroscopy
(FDIRS)
IR-UV Holeburning Sn Sn
FDIRS relies on wavelength selectivity
IR (10 Hz) and UV (20 Hz)
IR-UV useful: spectrum overly congested, minor conformer
Difference spectrum: gated integrator operating in active baseline subtraction mode. S0 S0 4

6. UV and IR spectra: S0-S1 origin region
LIF
FDIRS
IR-UV HB
Temperature: 124 C
Concentration: 1.3 %
Pressure: 40 sccm
Pulse Valve: 500 μm orifice (Parker General Valve Series 9)
Contaminants: Benzaldehyde, benzyl alcohol, toluene
Toluene stretches: 2925 (alkyl), 3099, 3068, 3032 (aromatic)
Leakage from A
2 conformers 5

7. Ground state IR tentative assignments6

8. Taking Fermi resonance mixing into account CH2 scissor combination
Model is based on a local mode representation.
CH oscillators couple to themselves and to nearly resonant states.
𝑯= − −
CH stretch states gg
CH2 scissor overtones
CH2 scissor combination
Fermi Coupling
NOT symmetric and antisymmetric stretch
Parameters of the Hamiltonian are initially calculated using density functional theory methods (model Hamiltonian requires the computation of the Cartesian force constant matrix and linear dipole derivatives)
Overtones of CH bends: cm-1
Anharmonic couplings cm-1
(Ned Sibert// UV-Madison)
CH2 scissor combination
Symmetric stretch (low frequency end) and antisymmetric strech (high frequency) mode
Interaction with O atom …. Partially localizes the modes
2941 cm-1 3
1 and 4 modes are relatively uncoupled. 4 1
2 and 3 mix with scissor overtones. 2
Significant Fermi coupling occurs between stretches and scissor overtones of the individual CH2 groups. 7

9. stacked Taking Fermi resonance mixing into account
Large Amplitude Motions A B A B
CH stretch torsional progression
Twice the bands
Splitting: 19/9.5/17/////5.7 8

10. Excited state IR
33 cm-1 9

11. Franck Condon quenching
Future Work
Where is S0-S2? Spectroscopic consequences?
Experimental Approach
Dispersed Fluorescence
Isotopically label benzyl ether
Franck Condon quenching
Analysis
Multimode vibronic coupling model.
Mention coclutions
Include multiple vibrational modes and can describe vibronic states in more complex molecular aggregates such as bichromophores
Vertical splitting/ calc oscillator strength
Dispersed Fluprescence: detect S2 experimentally
(Nebgen, Emmert, and Slipchenko, J. Chem. Phys. 137, (2012))
Benzyl Ether
S0-S1 (nm) f1
S0-S2 (nm) f2
∆𝑬 (cm-1) gg
235.96
0.0026
233.15
0.0003
510.78
stacked
243.14
0.0001
236.30
0.0191 tg
234.38
0.0007
233.33
0.0006
192.00 tt
234.79
0.0011
233.29
0.0000
273.85 10

12. The Zwier Research Group: Dr. Sibert (UV-Madison)
Acknowledgements
The Zwier Research Group:
Dr. Zwier
Daniel P. Tabor
Dr. Sibert (UV-Madison) 11

13. Large Amplitude Motions

14. sta tg gg tt