CONFORMATIONAL EXPLOSION: UNDERSTANDING THE COMPLEXITY OF THE PARA-DIALKYLBENZENE POTENTIAL ENERGY SURFACES Piyush Mishra, Daniel M. Hewett, and Timothy S. Zwier Department of Chemistry, Purdue University Source of funding: DOE Basic Energy Sciences Combustion Program

Outline Motivation Conformer Specific Spectroscopy Systems of Study Spectra Potential Energy Surfaces (PES) Isomerization Pathways Conclusion

Motivation and Goals Common components of coal, aviation fuel & vehicle exhaust. Extension of a previous collaborative work with the Sibert group (UW-Madison): Short-chain alkylbenzenes (single chain). Interaction between the two alkyl chains (as opposed to single chain conformations). Conformational complexity through PES. Originally wanted to find the folded structure in larger molecules, but now lets see where it takes us… Dan took experimental alkylbenzene spectra here. Tabor, D. P.; Hewett, D. M.; Bocklitz, S.; Korn, J. A.; Tomaine, A. J.; Ghosh, A. K.; Zwier, T. S.; Sibert, E. L. III J.Chem.Phys.2016, 144, 224310

Experimental set-up: LIF chamber 7/20/2018 Experimental set-up: LIF chamber Czerney-Turner mount

UV excitation: Collection of the total fluorescence 7/20/2018 UV excitation: Collection of the total fluorescence S0 S1 S1S0 region total fluorescence collected. A* Total fluorescence Czerney-Turner mount A UV spectrum obtained constitutes excitation due to all the conformers.

Conformer specific spectroscopy #1: Dispersed fluorescence (DFL) 7/20/2018 Conformer specific spectroscopy #1: Dispersed fluorescence (DFL) UV excitation spectra Czerney-Turner mount Study individual UV bands corresponding to unique conformers. Excitation spectra is a result of all the conformers present. Dispersed fluorescence

Conformer specific spectroscopy #1: Dispersed fluorescence (DFL) 7/20/2018 Conformer specific spectroscopy #1: Dispersed fluorescence (DFL) S0 S1 A* Single vibrational level fluorescence (SVLF) Czerney-Turner mount A Symmetry governed selection rules, ground state low energy vibrational modes. Dispersed fluorescence

Conformer specific spectroscopy #2: S0 Fluorescence-dip IR spectroscopy (S0-FDIRS) II 20 Hz 10 Hz S1 S1 Fluorescence Dip in fluorescence Fix UV laser Fix UV laser UV only Scan IR Laser UV + IR S0 S0 Difference of the 2 spectra: UV – (UV + IR) = -IR Dip occurs only when the IR frequency is in resonance with vibration of the conformer.

S0-FDIRS as applied to our experiment UV excitation (cm-1) Fix UV laser frequency IR spectrum corresponding to UV peaks Conformer A Conformer B UV bands corresponding to the similar conformers will have similar IR spectra.

Nomenclature θ‘ θ Flexible bonds (decide nomenclature) Syn (S) Anti (A)

First dihedral on the chain Nomenclature ϕ1’ ϕ1 Flexible bonds (decide nomenclature) First dihedral on the chain 1800 600 t-S-t t-S-g

Focus on first chain dihedral Nomenclature ϕ2’ ϕ2 Flexible bonds (decide nomenclature) Focus on first chain dihedral 600 -600 tt-A-tt tg-S-g’t

Computational Energies (4) (12) (12) (2) (8) (8) (8) Molecule

UV excitation spectra TD-B3LYP…….? Computations: B3LYP-GD3BJ/Def2TZVP (TD-DFT) TD-B3LYP…….? Comp Expt Comp Expt Computations are scaled wrt highest cm-1 (Anti all-trans structure). Comp Expt

Demarcation into regions is right? Iii: -g-X-g- Ii: -t-X-g- I: -t-X-t- Comp Expt * * Comp Expt * * * * * * * Alkylbenzenes: ~50cm-1 shift with addition of every g1. FDIRS done for conformation. Comp Expt * * * * * * * * * * * * * * Computations: B3LYP-GD3BJ/Def2TZVP

Vibrational frequency calculations Stretch-bend Fermi-Resonance problem. Anharmonic model of Hamiltonian developed by the Sibert group for the IR spectra of single chain alkylbenzenes were used. Structures with suitable chain configuration were added to generate IR spectra of para-dialkylbenzenes in this work. Assumption: Chains are independent. These were carried out at B3LYP/6-311++G(d,p). Tabor, D. P.; Hewett, D. M.; Bocklitz, S.; Korn, J. A.; Tomaine, A. J.; Ghosh, A. K.; Zwier, T. S.; Sibert, E. L. III J.Chem.Phys.2016, 144, 224310

Examples of frequency calculation Computations: B3LYP/6-311++G(d,p) p-DPropB p-DButB Generate: Generate: = = + + t + g = t-X-g tt + tg = tt-X-tg

? p-DEthB FDIRS Label Description d+ FR CH2 symetric stretch in FR d- CH2 antisymmetric stretch r+ FR CH3 symmetric stretch in FR ra- CH3 in-plane antisymmetric stretch rb- CH3 out-of plane antisymmetric stretch Computations: B3LYP/6-311++G(d,p) Comp Expt or Here, Ethylbenzene and p-DEthB both have similar spectra in the CH3 region implying that the chains are fairly independent of each other (as expected). Cant assign (S) and (A) using IR spectra, Therefore we did DFL. ? (c) (d) R. A. MacPhail, H. L. Strauss, R. G. Snyder, and C. A. Ellinger, J.Phys.Chem. 88, 334 (1984)

p-DEthB DFL C2v 36998 cm-1 (c) C2h 37002 cm-1 (d) Low energy ground state vibrations can be studies in DFL. Only the totally symmetric and the even overtones of non-totally symmetric vibronic modes show up. For same vib, soo only FC overlap. For vibronic activity, see initial, final and transition moment symmetry. C2h 37002 cm-1 (d)

p-DPropB FDIRS I: t-X-t (d) g-X-g III: g-X-g (b) t-X-t + (f) t-X-g Computations: B3LYP/ 6-311++G(d,p) I: t-X-t (d) g-X-g III: g-X-g (b) t-X-t (a) + (c) + (f) t-X-g II: t-X-g III II I Red:comp Black:Expt

Black: Para-dipropylbenzene Experimental evidence of non-interacting chains Single chain vs Double chain t-X-t Trans g-X-g Gauche t-X-g Trans + Gauche + = IR-UV holeburning done to find out about the vibronic bands around the origins. Only this molecule could be done. Black: Para-dipropylbenzene Red: Propylbenzene

p-DButB FDIRS III: xg-X-gx xg-X-gx II: xg-X-tx xt-X-tx xg-X-tx Computations: B3LYP/6-311++G(d,p) III: xg-X-gx xg-X-gx II: xg-X-tx xt-X-tx xg-X-tx I: xt-X-tx III II I

p-DButB FDIRS III: xg-X-gx III Ratio: ~ 1 xg-X-gx II: xg-X-tx Computations: B3LYP/6-311++G(d,p) III: xg-X-gx tg-X-gt III Ratio: ~ 1 CH2 symmetric FR xg-X-gx II: xg-X-tx tg-X-gg I: xt-X-tx Red:comp Black:Expt

p-DButB FDIRS III: xg-X-gx II Ratio: ~ 0.3 xg-X-tx II: xg-X-tx Computations: B3LYP/6-311++G(d,p) III: xg-X-gx tt-X-gg tt-X-gt II Ratio: ~ 0.3 CH2 symmetric FR xg-X-tx II: xg-X-tx gt-X-gt I: xt-X-tx Red:comp Black:Expt

p-DButB FDIRS III: xg-X-gx II: xg-X-tx I Ratio: ~ 0 xt-X-tx I: xt-X-tx Computations: B3LYP/6-311++G(d,p) III: xg-X-gx tt-X-tg I Ratio: ~ 0 CH2 symmetric FR xt-X-tx II: xg-X-tx tt-X-tt I: xt-X-tx Red:comp Black:Expt

p-DButB FDIRS Added butylbenzene FDIR spectra & compare with p-DButB (like p-DPropB). As expected: Comparison matched, characteristic feature being 2858 cm-1. Bottom line: 1) Chains are independent. 2) Demarcation holds.

PES Why do it? Better understand chain interdependence and complexity due to flexible bonds. How do we generate it? Changing suitable dihedral angles under relaxed optimization conditions. What is expected? Molecule is symmetric, PES is expected to be symmetric. Conformations and the barriers of isomerization have similar energies, ‘egg carton’ shaped PES. (A) ↔ (S) has lower barrier than torsions. Upon changing the same set of dihedrals, we should obtain very similar looking PES.

PES: p-DEthB Computations: B3LYP-GD3BJ/Def2TZVP Similar barrier for p-DPropB and p-DButB for A↔S with the first dihedral on the chain being trans. Intermediate θ‘ Anti Syn θ

PES: p-DPropB Computations: B3LYP-GD3BJ/Def2TZVP x-A-x ≡ x-S-x ϕ1 ϕ1’

PES: p-DPropB A↔S transitions Gauche on the first dihedral 14 kJ Mol-1 Computations: B3LYP-GD3BJ/Def2TZVP A↔S transitions Gauche on the first dihedral 14 kJ Mol-1 Trans on the first dihedral 6 kJ Mol-1. This is also true for p-DButB. Syn Anti

PES: p-DButB Bump! Similar to pdpb PES ϕ1 tt-A-xx ≡ tg-A-xx Computations: B3LYP-GD3BJ/Def2TZVP Bump! Studying ϕ1 and ϕ2 instead of ϕ1 and ϕ1’ is because tx-S-xt ≡ xAx as tx- chain points away from ring. Similar to pdpb PES ϕ1 tt-A-xx ≡ tg-A-xx g’g-X-xx not studied as its very high in energy Full PES would have about 10 of the above shown plot. ϕ2 J. Phys. Chem. B 2005, 109, 5300-5311

Isomerization pathway: p-DPropB Computations: B3LYP-GD3BJ/Def2TZVP ϕ1’ ϕ1

Isomerization pathway: p-DPropB Computations: B3LYP-GD3BJ/Def2TZVP

Isomerization pathway p-DButB showed a very similar isomerization pathway. Bottom line: In both the molecules, all those torsions that are closer to the ring have higher energy TS. Hence the lowest energy pathways between two conformers do not involve such TS.

Conclusion Conformational explosion with chain size (combination of various flexible bonds) 2719 (expt). Demarcation of UV spectra into the 3 regions (based on first dihedral configuration). Chains of the conformers seen experimentally are non-interacting and largely independent of each other. The rich PES looked like an egg-tray as expected. Governing force behind the conformational preferences: Steric effect.

Future Works p-DEthB IVR for one of the conformers (density of states). Long chains p-DAlkB (fold over the ring?). Diarylalkanes to study stacking efficiency (soot formation).

Acknowledgements The Zwier Group Department of Energy Dr. Tim Zwier Dr. Sibert, Daniel Tabor (UW-Madison)

/\ Thank you questions?

Symmetry of low energy ground state vibrations of p-DEthB anti pDEthB(C2h) cm-1 symmetry syn pDEthB(C2v) cm-1 24.6491 Au 48 32.5788 A2 34 46.2745 Bg 33 35.4424 B1 75.1177 Bu 66 71.9095 A1 19 157.7286 Ag 172.5383 B2 204.1211 47 204.208 227.287 32 227.16 297.8372 65 267.9362 18 315.4385 46 315.7174 390.2662 394.3112 395.7608 31 417.0189 415.9781 45 442.0563 514.3904 17 463.4609 568.5766 64 588.5621 16 661.3931 30 661.4306 711.3086 63 705.4823

Conformer specific spectroscopy #3: IR-UV holeburning (IR-UVHB) P-DpropB is the only molecule in this series to which this could be applied. Technique employed to see which vibronic bands belong to which set of origin bands. UV excitation spectrum t-X-t +t-X-g g-X-g + t-X-g

Conformers seen experimentally Computations: B3LYP-GD3BJ/Def2TZVP Iii: -g-X-g- Ii: -t-X-g- I: -t-X-t- (S) (A) t-A-g, t-S-g g-A-g, g-S-g’, g-A-g’, g-S-g t-S-t, t-A-t tg-S-gt, tg-S-gg, tg-A-g’t, tg-A-g’g’ tg-S-g’t, tg-S-g’t, tg-A-gt, tg-A-gg tg’-S-tg, gt-S-gt, g’t-A-g’t, g’t-A-gt tt-S-gt, tt-S-gg, tt-A-gt, tt-A-gg tt-S-tg’, g’t-A-tt, tt-S-tt, tt-A-tt

Reason to study PES We had seen experimentally and computationally that the two chains are independent of each other, meaning that there were no new single-chain conformations induced or removed by the presence of a second chain. We also saw a conformational explosion, meaning dramatic increase in the number of conformers as the chain size increased. Looking at the PES might tell us more about the interactions between the chains and isomerization between the conformers.

Isomerization pathway: p-DButB Computations: B3LYP-GD3BJ/Def2TZVP

SVLF of p-DEthB

Disconnectivity Diagram