1. Ultrafast photoisomerization of carbon, boron and phosphorus tribromides in solution Andrey S. Mereshchenko 1, Kanykey E. Karabaeva 1, Patrick Z. El-Khoury 2, Suman K. Pal 3 and Alexander N. Tarnovsky 1, 1 Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403; 2 Department of Chemistry, University of California Irvine, Irvine, CA 92697 3 School of Basic Sciences IIT Mandi, Vallabh Degree College Campus, Mandi 175001, India. 1

2. Previous Work 1. CF 2 I 2 iso-CF 2 -I-I Ultrafast time-resolved IR : iso-CF 2 -I-I observed at 400 fs after excitation 2. CH 2 Br 2 iso-CH 2 -Br-Br h =350 nm h =255 nm In cyclohexane: isomer stable In CH 3 CN: isomer formation in ~8.5 ps. Isomer decays (~100 ps) back to the parent. Couple cluster calculations: < 1 kcal mol -1 energy barrier for ground-state isomerization Patrick El-Khoury et al. Chem. Phys. Lett. 2010, 493, 61 CH 2 Br 2 Iso-CH 2 Br 2 TS Patrick El-Khoury et al. J. Chem. Phys. 2010, 132, 124501

3. Objects CHBr 3, BBr 3, and PBr 3 in solution 3 Methods  Ultrafast Time-Resolved Absorption Spectroscopy  DFT and TDFT calculations Goals Do all this molecules form the isomer with Br-Br bond? Whether DFT methods are good enough to predict photochemistry mechanism?

4. Steady state absorption spectra 4 λ exc = 255 nm n Br σ* X-Br 4 λ exc = 310 nm λ exc = 264 nm

5. Gas phase mechanisms 5 CHBr 3 CHBr 2 + Br CHBr + Br 2 h McGivern, W.S.; Sorkhabi, O.; Suits, A.G.; Derecskei-Kovacs, A; North, S.W. J. Phys Chem. A 2000, 104, 10085. Zou, P.; Shu, J; Sears, T.J.; Hall, G.E.; North, S.W. J. Phys. Chem. A 2004, 108, 1482. Romazin,C.; Boye-Peronne, S.; Gauyacq, D..; Benilan, Y.; Gazeau, M.C; Douin, S. J. Chem. Phys. 2006, 125, 114312. Huang, H.-Y.; Chuang W.-T.; Sharma, R.C.; Hsu, C.-Y.; Lin, K.-C.; Hu, C.-H. J. Chem. Phys. 2004, 121, 5253. PBr 3 PBr 2 + Br PBr + Br 2 h BBr 3 BBr 2 + Br BBr + Br 2 h Briggs, A. G.; Simmons, R.E. Naturwissenschaften 1980, 67, 402. Moroz, A.; Sweany, L. Inorg. Chem 1992, 31, 5236-5242. Bowie, R.A.; Musgrave, O.C. J. Chem. Soc. 1966, C, 566-571. Ogata, Y.; Izawa, Y.; Tomioka, H.; Ukigai, T. Tetrahedron 1969, 25, 1817-1824. Kennedy, T.; Sinclair, R.S.; Sinclair, T.J. J. Inorg. Nucl. Chem 1971, 33, 2369-2376. Jan-Khan, M.; Samuel, R. Proc. Phys. Soc. 1936, 48 626-641. Bramwell, M. J.; Jaeger, S. E.; Whitehead, J. C. Chem. Phys. Lett. 1992, 196, 547. Bramwell, M. J.; Hughes, C.; Jaeger, S. E.; Whitehead, J. C. Chem. Phys. 1994, 183, 127-134. Latifzadeh, L.; Balasubramanian, K. Chem. Phys. Lett. 1996, 258, 393-399. Briggs, A.G. Spectrochimica Acta A, 1981, 37, 457-458.

6. Transient Absorption Spectra: CHBr 3 in MeCH The  A signal < 0.2 ps is due to solvent At 0.2-0.3 ps, the band maxima: ~260, 355, and 395 nm The 355 nm band decays with the build up of the 395 nm band The 395 nm band narrows and red-shifts to 415 nm Final 255 and 415 nm bands Pump: 255-nm, <1.5  J pulse -1

7. Absorption of CHBr 2 · + iso-CHBr 3 Absorption of iso-CHBr 3 Br · MCH CT absorption

8. Transient Absorption Spectra: CHBr 3 in Acetonitrile Early times: 255 nm band The 500 nm band emerges after 2 ps, 8 ps rise time 255-nm Pump: <1.5  J pulse -1 Long times: the 255 nm band slowly decays The 500 nm band: decays completely, 270 ps lifetime

9. CHBr 2 + Iso-CHBr 3 Br-ACN CT complex CHBr 2 Isomer decays completely

10. Bromoform: Proposed Scheme

11. BBr 3 in Methylcyclohexane 11 Early times: The 360 nm band rises within our time resolution and decays fast. The 430 band rises and sharpens, 9 ps rise time 264-nm Pump: 4  J pulse -1 Long times: The 430 nm band decays, 210 ps lifetime

12. Absorption of iso-BBr 3 Br-MCH CT absorption Iso-BBr 3 /MCH Experimental430 nm M062x/PCM402 nm B3LYP/PCM429 nm CAM-B3LYP/PCM440 nm PBE1PBE/PCM400 nm

13. PBr 3 in Methylcyclohexane 13 Early times: The broad 370-400 nm feature rises ≤ 0.3 ps. This follows by narrowing /decay. The 560 nm band rises and blue-shifts, 11 ps rise time 310-nm Pump: 4  J pulse -1 Long times: The 380 nm band decays concurrently with the decay of the 560 nm band, 420 ps lifetime

14. Absorption of iso-PBr 3 Br-MCH CT complex and PBr 2 Iso-PBr 3 / MCH PBr 2 / MCH Experimental560, 390 nm390 nm B3LYP/PCM550, 374 nm391, 325 nm PBE1PBE/PCM487, 348 nm377, 302 nm Absorption of iso-PBr 3

15. Proposed Photochemistry of CHBr 3, BBr 3, and PBr 3 Molecule/ Solvent  decay CHBr 3 /MCH>>1 ns CHBr 3 /ACN270 ps BBr 3 /MCH220 ps PBr 3 /MCH440 ps X=CH, B, P

16. 16 Activation energy vs Reaction rate Molecule/ Solvent  decay E TS -E iso-XBr3, Kcal/mol CHBr 3 /MCH>>1 ns9.0 CHBr 3 /ACN270 ps5.7 BBr 3 /MCH220 ps5.5 PBr 3 /MCH440 ps6.1 X=CH, B, P

17. Conclusions Bromoform, tribromides of boron and phosphorus all afford isomers with Br-Br bond. Possible photochemical mechanism: through the geminate combination of radical fragments Ab-initio DFT/TDFT calculations accurately predict the UV/vis spectra of intermediates (better than 0.3 eV) as well as decay rates of isomers. 17

18. Funding: Ohio Supercomputer Center BGSU NSF (CAREER, MRI) Tarnovsky group: Veniamin Borin Max Panov Butaeva Evgeniia Acknowledgements former members of Tarnovsky group: Dr. Patrick El-Khoury (U. California, Irvine) Dr. Suman Pal (Maldi, India) Igor Zheldakov (University of Kansas, Lawrence) Nathan Crozier (Ohio State University, Columbus)

19. Bromoform in MeCH

20. MeCH ACN

21. Isomer lifetime: iso-CH 2 I 2, iso-CHI 3 ~5  s Previous assignment of the 415 nm band to CHBr 3 -Br CT is erroneous Diffusion-controlled reaction times: 10 mM ~ 10 -8 sec 90 mM ~ 10 -9 sec No major bimolecular chemistry ! No Major Bimolecular Chemistry