OSU 67 th International Symposium on Molecular Spectroscopy

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

Introduction • 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) 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, N 0 C 9, 20899 -20908 4: Quack, B. et al. , Global Biogeochem. Cycles 2003, 17, Art. No. 1023 OSU 67 th International Symposium on Molecular Spectroscopy 2

Background: our previous work on iso-bromoform 1 Relative energy in k. J/mol CHBr + Br 2 363 CHBr 2 + Br 267 CHBr 3(TS) 251 252 CBr 2 + HBr 204 iso-CHBr 3 • 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 1: L. George et al. , J. Chem. Phys. 135, 124503 (2011) OSU 67 th International Symposium on Molecular Spectroscopy 3

Optimized structures of iso-CXBr 3 • Structures optimized at the MP 2/aug-ccp. VTZ • General consistency with M 06/aug-cc-p. VTZ and B 3 LYP/aug-ccp. VTZ • Some discrepancies between MP 2 and DFT OSU 67 th International Symposium on Molecular Spectroscopy 4

iso-CFBr 3 : experimental results (IR and UV/vis) 421 852 • Photolysis at 230 nm followed by annealing to 30 K [9 K] • In comparison with computational predictions (B 3 LYP/augcc-p. VTZ and TDM 06/augcc-p. VTZ) 594 432 288 1161 • CFBr 3: Ar[Ne] (1: 500) OSU 67 th International Symposium on Molecular Spectroscopy 5

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

iso-CCl. Br 3 : experimental results (IR) • CCl. Br 3: Ar (1: 500) • Photolysis at 220 nm (544 k. J/mol) followed by annealing to 30 K OSU 67 th 779 816 809 878 • In comparison with computational predictions (M 06/aug-cc-p. VTZ) • Bond cleavage threshold: Ø C-Cl: 293 k. J/mol Ø C-Br: 232 k. J/mol International Symposium on Molecular Spectroscopy 7

iso-CCl. Br 3 : experimental results (UV/vis) • CCl. Br 3: Ar (1: 500) • Photolysis at 220 nm (544 k. J/mol) followed by annealing to 30 K • In comparison with computational predictions (TD-M 06/aug-cc-p. VTZ) OSU 67 th International Symposium on Molecular Spectroscopy 8

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

iso-CBr 4 : experimental results (IR) 842 • CBr 4: Ar (1: 500) 781 • Photolysis at 266 nm followed by annealing to 30 K • In comparison with computational predictions (M 06/aug-cc-p. VTZ) OSU 67 th International Symposium on Molecular Spectroscopy 10

iso-CBr 4 : experimental results (UV/Vis) 458 • CBr 4: Ar (1: 500) • Photolysis at 266 nm followed by annealing to 30 K 645 337 • In comparison with computational predictions (TD-M 06/aug-cc-p. VTZ) OSU 67 th International Symposium on Molecular Spectroscopy 11

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

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

Calculated resonance structure weights along IRC path • 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. OSU 67 th International Symposium on Molecular Spectroscopy 14

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

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

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

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

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