1. Spectroscopic and computational studies of matrix- isolated iso-CXBr 3 (X = F, Cl,Br): Structure, properties, and photochemistry of substituted iso- tribromomethanes OSU 67 th International Symposium on Molecular Spectroscopy Columbus, June 18 th -22 th, 2012 Aimable Kalume a, Lisa George a, Scott A. Reid a, Brian J. Esselman b, Robert J.McMahon b a: Marquette University b: University of Wisconsin-Madison

2. OSU 67 th International Symposium on Molecular Spectroscopy Introduction 1: Wayne, R. P. Chemistry of Atmosphere, 3 rd ed.,Oxford University Press: Oxford, U.K., 2000 2:Molina et al., Nature, 1974, 249, 810-812 3: Moore R. M. et al, J. Geophys. Res.-Oceans, 1996, 101, N0C9, 20899-20908 4: Quack, B. et al., Global Biogeochem. Cycles 2003, 17, Art. No. 1023 2 Several polyhalomethanes have been detected in natural environment and are an important source of halogen atoms; involved in the “ozone depletion” phenomenon. 1-3 Bromoform and its derivatives are major producers of Br atom in the troposphere and mid-latitude lower stratosphere. 1,4 In condensed phase, the photolysis of bromoform led to facile production of the iso-bromoform In comparison with bromoform, less is known about the substituted tribromomethanes iso-CXBr 3 (X = F, Cl, Br, I)

3. OSU 67 th International Symposium on Molecular Spectroscopy Background: our previous work on iso-bromoform 1 1: L. George et al., J. Chem. Phys. 135, 124503 (2011 ) 3 CHBr 3 CHBr 3 (TS) 251 204 iso-CHBr 3 CHBr + Br 2 363 CHBr 2 + Br 267 252 CBr 2 + HBr Other iso-polyhalomethanes species that have been studied in our lab: iso-CF 2 I-I, iso-CF 2 Br-Br, iso-CH 2 Cl-I Relative energy in kJ/mol

4. OSU 67 th International Symposium on Molecular Spectroscopy Optimized structures of iso-CXBr 3 4 Structures optimized at the MP2/aug-cc- pVTZ General consistency with M06/aug-cc- pVTZ and B3LYP/aug- cc-pVTZ Some discrepancies between MP2 and DFT

5. OSU 67 th International Symposium on Molecular Spectroscopy iso-CFBr 3 : experimental results (IR and UV/vis) CFBr 3 :Ar[Ne] (1:500) Photolysis at 230nm followed by annealing to 30K [9K] In comparison with computational predictions (B3LYP/aug- cc-pVTZ and TDM06/aug- cc-pVTZ) 5 421 852 1161 288 432 594

6. OSU 67 th International Symposium on Molecular Spectroscopy Calculated stationary points on the CFBr 3 PES. 6 CFBr 2 + Br 255 CFBr 3 (TS) 261 202 iso-CFBr 3 250 CFBr + Br 2 405 CBr 2 + FBr CFBr 3 Relative energy in kJ/mol

7. OSU 67 th International Symposium on Molecular Spectroscopy iso-CClBr 3 : experimental results (IR) 7 CClBr 3 :Ar (1:500) Photolysis at 220nm (544kJ/mol) followed by annealing to 30K In comparison with computational predictions (M06/aug-cc-pVTZ) Bond cleavage threshold:  C-Cl: 293 kJ/mol  C-Br: 232 kJ/mol 878779 809816

8. OSU 67 th International Symposium on Molecular Spectroscopy iso-CClBr 3 : experimental results (UV/vis) 8 CClBr 3 :Ar (1:500) Photolysis at 220nm (544kJ/mol) followed by annealing to 30K In comparison with computational predictions (TD-M06/aug-cc-pVTZ)

9. 9 CClBr 3 (0) iso-TS (229) Calculated stationary points on the CClBr 3 PES. iso-ClBrCBr-Br (180) iso-Br 2 CCl-Br (199) iso-Br 2 CBr-Cl (200) CClBr 2 + Br (246) CBr 3 + Cl (277) CClBr + Br 2 (272) OSU 67 th International Symposium on Molecular Spectroscopy CCSD(T)//MP2/aug-cc-pVTZ The iso-TS depicted in this figure is the transition state to the most stable iso- CClBr 3 isomer.

10. OSU 67 th International Symposium on Molecular Spectroscopy iso-CBr 4 : experimental results (IR) 10 CBr 4 :Ar (1:500) Photolysis at 266nm followed by annealing to 30K In comparison with computational predictions (M06/aug-cc-pVTZ) 842 781

11. OSU 67 th International Symposium on Molecular Spectroscopy iso-CBr 4 : experimental results (UV/Vis) 11 CBr 4 :Ar (1:500) Photolysis at 266nm followed by annealing to 30K In comparison with computational predictions (TD-M06/aug-cc-pVTZ) 645 458 337

12. OSU 67 th International Symposium on Molecular Spectroscopy Photoisomerization of iso-CXBr 3 12 CBr 4 :Ar (1:500) Irradiation into the strongest S 0 →S 3 band at 440nm The photolysis of the iso- CXBr 3 results in the reformation of the parent isomer The exact mechanism has not been determined yet

13. OSU 67 th International Symposium on Molecular Spectroscopy Calculated stationary points on the CBr 4 PES. text 13

14. OSU 67 th International Symposium on Molecular Spectroscopy Calculated resonance structure weights along IRC path 14 First, calculate the IRC for the isomerization path Second, analysis of each IRC point using Natural Resonance Theory The TS near point 40 represent the cross-over from covalent to ion-pair binding.

15. OSU 67 th International Symposium on Molecular Spectroscopy Conclusions 15 We have examined the structure, photochemistry and isomerization pathways of substituted iso-tribromomethanes The electronic spectra were recorded for the first time Stationary points on the PES were characterized computationally The iso-species are relatively well bound with respect to the dissociation threshold There is always a TS connecting both isomers and this was confirmed by IRC calculations In condensed phases, the ion-pair structure is further stabilized with respect to the dissociation threshold The photolysis of the iso-species results in the reformation of the parent CXBr 3 isomer

16. OSU 67 th International Symposium on Molecular Spectroscopy 16 Acknowledgments Dr. Scott A. Reid Dr. Lisa George Dr. Rajendra Rathore Khushabu Thakur Dr. Robert J. McMahon Brian J. Esselman National Science Foundation Petroleum Research Fund of the ACS

17. OSU 67 th International Symposium on Molecular Spectroscopy Calculated resonance structure weights along IRC path 17

18. OSU 67 th International Symposium on Molecular Spectroscopy Calculated resonance structure weights along IRC path 18

19. OSU 67 th International Symposium on Molecular Spectroscopy Calculated stationary points on the CClBr 3 PES. text 19 The iso-TS depicted in this figure is the transition state to the most stable iso-CClBr 3 isomer.