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 School of Basic Sciences IIT Mandi, Vallabh Degree College Campus, Mandi , India. 1

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,

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?

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

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, Zou, P.; Shu, J; Sears, T.J.; Hall, G.E.; North, S.W. J. Phys. Chem. A 2004, 108, Romazin,C.; Boye-Peronne, S.; Gauyacq, D..; Benilan, Y.; Gazeau, M.C; Douin, S. J. Chem. Phys. 2006, 125, Huang, H.-Y.; Chuang W.-T.; Sharma, R.C.; Hsu, C.-Y.; Lin, K.-C.; Hu, C.-H. J. Chem. Phys. 2004, 121, 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, Bowie, R.A.; Musgrave, O.C. J. Chem. Soc. 1966, C, Ogata, Y.; Izawa, Y.; Tomioka, H.; Ukigai, T. Tetrahedron 1969, 25, Kennedy, T.; Sinclair, R.S.; Sinclair, T.J. J. Inorg. Nucl. Chem 1971, 33, Jan-Khan, M.; Samuel, R. Proc. Phys. Soc. 1936, 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, Latifzadeh, L.; Balasubramanian, K. Chem. Phys. Lett. 1996, 258, Briggs, A.G. Spectrochimica Acta A, 1981, 37,

Transient Absorption Spectra: CHBr 3 in MeCH The  A signal < 0.2 ps is due to solvent At 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

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

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

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

Bromoform: Proposed Scheme

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

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

PBr 3 in Methylcyclohexane 13 Early times: The broad 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

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

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 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

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

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)

Bromoform in MeCH

MeCH ACN

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 ~ sec 90 mM ~ sec No major bimolecular chemistry ! No Major Bimolecular Chemistry