Ultrafast Dynamics of Carbocyclic Geminal Dibromides Darya S

Ultrafast Dynamics of Carbocyclic Geminal Dibromides Darya S. Budkina, Kanykey E. Karabaeva, R. Marshall Wilson, * Alexander N. Tarnovsky* Department of Chemistry and Center for Photochemical Sciences Bowling Green State University Bowling Green, Ohio

Our Research Topics nanoparticles photodissociation and excited-state dynamics organic photochemistry transition metal photochemistry

Photodissociation and Structural Change in Solution: What Happens and Why? Dynamics of a chemical bond are at heart of chemistry Control of photoinduced processes: what and how? state-selective photochemistry, conical intersections, multidimensionality of potential energy surfaces 70% of chemistry occurs in solution: modified PES, solvation, solvent caging, vibrational relaxation Why Polyhalomethane Molecules? Although best known as synthetic toxicants, thousands of halogen compounds are, in fact, part of our natural environment Organohalogens: CH 2 I 2, CH 2 Br. I, CH 2 Cl. I, CHBr 3, etc. naturally occurring (volcanic gases, coastal macroalgae) Halogen Effects on Chemical Bonding: Routes to Designing New Potential Low-Cost Materials Eames et al. Nat. Comm. 2015, 6, 7497. HOMO-LUMO band gap tuning, stimulus of electron transfer with Cl, Br, I functionalization in LEDs, perovskites, self-assembly and recognition controlled by electrophilic halogens, etc.

Our Work on UV Photochemistry of Di- and Polyhalomethanes Parent iodides • CH 2 I 2 • CF 2 I 2 Photoproduct Reference CH 2 I–I isomer CF 2 I–I isomer PCCP, 2016, 18, 28883 JCP, 2010, 132, 124501 • CHI 3 CHI 2–I isomer Chem. Phys. Chem. 2009, 10, 1895 mixed halogens • CH 2 Br. I CH 2 Br–I isomer Ultrafast Phenomena, 2012 • CH 2 Cl. I CH 2 Cl–I isomer J. Chinese Chem. Soc. 2000, 47, 769 • CH 2 Br 2 CH 2 Br–Br isomer CPL, 2010, 493, 61 • CHBr 3 • BBr 3 • PBr 3 CHBr 2–Br isomer BBr 2–Br isomer PBr 2–Br isomer JCP, 2013, 138, 124501 Nat. Chem. 2015, 7, 562 ibid. bromides

The Isomer Species of CH 2 I 2: the journey from 200 fs to 1 ms CH 2 I 2 iso-CH I 2 2 MS-CASPT 2, 6 -311 G** basis set CH 2 XI: for a long time, isomer formation via geminate recombination of radical pair was assumed

Work on the UV (255 -nm) Photochemistry of CHBr 3 in Solution Combined study using ultrafast transient S 1 absorption and ab initio computations S 0 Pal, S. K. ; Mereshchenko, A. S. ; Butaeva, E. V. ; El-Khoury, P. Z. ; Tarnovsky, A. N. Global sampling of the photochemical reaction paths of bromoform by ultrafast deep-UV through near-IR transient absorption and ab initio multiconfigurational calculations. J. Chem. Phys. 2013, 138, 124501.

Gas-Phase Dynamics of S 1 -Excited CHBr 3 Br. HCBr-Br isomer absorption • Delayed, ~30 fs rise • Fast decay, ~90 fs • Computational assignment: Br. HCBr-Br isomer The formation of iso-CHBr 3 is a single elementary act, the recoil from a solvent cage is not necessary: direct isomerization! Mereshchenko, A. S. ; Butaeva, E. V. ; Borin, V. A. Eyzips, A. ; Tarnovsky, A. N. Roamingmediated ultrafast isomerization of geminal tri-bromides in the gas and liquid phases. Nat. Chem. 2015, 7, 562 BBr 3 and PBr 3: similar sub-100 fs isomer formation dynamics in the gas and solution phases!

CHBr 3 CASPT 2 Semiclassical Trajectory Calculations 101 39 3. 28 3. 66 3. 37 76 The Franck-Condon (0 th fs), S 1/S 0 CI, and isomeric (80 th fs) geometries along the representative trajectory.

CHBr 3 CASPT 2 Semiclassical Trajectory Calculations x, y, z coordinates of the central C atom along the trajectory t = 39 fs, isomeric species t = 29 fs, S 1/S 0 hop 29 fs, hop 59 fs, isomer t = 89 fs t = 0 fs, Franck. Condon 0 fs, FC 89 fs ‘Wandering’ or roaming’ of the central carbon atom with respect to the center of mass

Revisiting the UV Photochemistry of CH 2 I 2 in the Gas Phase Pulses with temporal duration below 100 fs are essential for (i) resolving nuclear motions in halogenated alkanes, featuring time periods in the 30 to 100 fs range, and (ii) following photochemical reaction dynamics predicted to occur on a 50 fs time scale. Excitation at 330 nm, ~45 fs apparatus function The generation of 35– 40 fs UV and UV laser pulses at the typical energies (about 1 J) requires to excite weak (200– 500 M-1 cm-1) absorbers, which is nontrivial. CH 2 I 2 S 1 Sn absorption iso-CH 2 I 2 “direct” Borin, V. A. , Matveev, S. M, Budkina, D. S. ; El-Khoury, P. Z. ; Tarnovsky, A. N. Direct photoisomerization of CH 2 I 2 vs. CHBr 3 in the gas phase: a joint 50 fs experimental and multireference resonance-theoretical study PCCP 2016, 18, 28883

Geminal Carbocyclic Dibromides Important natural products and intermediates in synthesis of complex heterocyclic molecules ? • Photochemistry: comparison to dibromomethane, bromoform • Isomer formation? Mechanism?

Dibromocycloalkanes: UV-Vis absorption Chemical Formula: C 3 H 2 Br 2 Molecular Weight: 199. 87 Chemical Formula: C 4 H 6 Br 2 Molecular Weight: 213. 90 Chemical Formula: C 5 H 8 Br 2 Molecular Weight: 227. 93

1, 1 -Dibromocyclopropane: 250 -nm 40 -fs excitation in a 100 - m jet methylcyclohexane Short-times • • • Long-times 20 fs: initial 450 -nm band 80 fs: the formation of the 400 -nm product band 1 -6 ps: rise, blue shift, and spectral narrowing of the product absorption 20 -200 ps: spectral narrowing and some decay of the product 400 -nm band 1000 ps: thermalized and metastable 395 -nm product band

methylcyclohexane C 3 H 4 Br 2 acetonitrile iso-C 3 H 4 Br 2 VET EOMCCSD 1) BP 86 B 3 LYP S 1 473 nm (f=1. 8 10 -4) 486 nm (f=1. 5 10 -4) 480 nm (f=1. 5 10 -4) S 2 431 nm (f=0. 0028) 445 nm (f=5 10 -4) 439 nm (f=5 10 -4) S 3 346 nm (f=0. 62) 376 nm (f=0. 57) 371 nm (f=0. 59) 1) EOM-CCSD(T)/cc-p. VTZ on CCSD/cc/cc-p. VDZ geometry. PCM methylcycloxane solvent CHBr 3 iso-CHBr 3 methylcyclohexane acetonitrile

1, 1 -Dibromocyclopropane: 250 -nm 40 -fs excitation in a 100 - m jet methylcyclohexane Short-times Long-times • 20 fs, 450 -nm band: excited state absorption of parent molecules • 80 fs: initial isomer product 400 -nm band, “direct” isomerization • 1 -6 ps: cage isomerization (2 nd channel of the isomer formation) and vibrational relaxation • 20 -200 ps: vibrational relaxation of the isomer (and some decay) • 1000 ps: thermalized, stable isomer product

acetonitrile methylcyclohexane 1, 1 -Dibromocyclobutane isomer acetonitrile 1, 1 -Dibromocyclopentane isomer methylcyclohexane

35 -fs “Transformer” Spectrometer Combining 200 -3600 nm Transient Absorption and FSRS

Conclusions Dibromocycloalkanes are synthesized using previously described procedures. Formation of isomer photochemical products (CH 2)n-C-Br-Br (n=1 -3), similar to the isomeric species observed in isomerization of bromoform (HBr. CBr-Br) and in isomerization of dibromomethane (H 2 C-Br-Br). methylcyclohexane acetonitrile lifetime max (nm) C 3 H 4 Br 2 > 10 ns 395 4 ns broad, 550 C 4 H 6 Br 2 > 10 ns 425 230 ps broad, 575 C 5 H 8 Br 2 2 ns 403 400 ps broad, 450 CH 2 Br 2 > 10 ns 1) - 96 ps 3) broad, 425 CHBr 3 microsecond range 2) 420 276 ps 2) broad, 530 1) Zheng, X. M. Kwok, W. M. Phillips, D. L. J. Phys. Chem. A 2000, 10464 S. K. ; Mereshchenko, A. S. ; Butaeva, E. V. ; El-Khoury, P. Z. ; Tarnovsky, A. N. J. Chem. Phys. 2013, 138, 124501 3) El-Khoury, P. Z. ; Pal, S. K. ; Mereshchenko, A. S. ; Tarnovsky, A. N. Chem. Phys. Lett. 2010, 493, 61. 2) Pal, < 80 fs isomerization in the case of 1, 1 -dibromocyclopropane, confirming that direct isomerization (the one that is faster than a 100 -200 fs time scale for solute-solvent encounters) is typical for geminal halides. Photochemistry of bromoform, including the (100 -fs) very initial epoch, where there is similarity between the gas-phase and solution-phase reaction paths, is directly extendable to larger geminal halide molecular systems.

Acknowledgements Tarnovsky group: present members Darya S. Budkina Vasily Vorobyev Firew Gemede Isabel Kovačiċ March, 2018 former members Anna Eyzips (FKQ Advertising) Christopher M. Hicks (US Army) Dr. Evgeniia V. Butaeva (USC, now KLA-Tencor)) Dr. Veniamin A. Borin (Hebrew U. Jerusalem) Dr. Sergey M. Matveev (U. Illinois, Urbana-Champaign) Prof. Andrey S. Mereshchenko (Saint Petersburg State U. ) Dr. Max S. Panov (Saint Petersburg State U. ) Prof. Suman K. Pal (Indian Institute of Technology, Mandi) Dr. Patrick Z. El-Khoury (PNNL) Dr. Igor L. Zheldakov (Eastman Chemical Company) Collaborators: Theory: Massimo Olivucci (U. Siena, BGSU) Chemistry: R. Marshall Wilson (BGSU) X-rays: Chris Milne (PSI), Majed Chergui (EPFL) May, 2016 • • Funding: BGSU OSC and SDSC CRDF NSF past and present: (CAREER, MRI)

Transient Absorption Spectroscopy Advantages: • Not limited to emissive processes • Best ultrafast time resolution ( 10 fs - a timescale of vibrational motion) Two pulses: pump (excitation) and broad or single probe (deep-UV/UV/Vis-near-IR) Acquire absorption changes (ΔA) when pump is on and off Sample Delay line Detection Probe Pump Acquisition