spectroscopic and computational studies of matrix- isolated iso-cxbr 3 (x = f, cl,br): structure,...

Spectroscopic and computational studies of matrix-isolated iso-CXBr3 (X = F, Cl,Br): Structure, properties,

and photochemistry of substituted iso-tribromomethanes

OSU 67th International Symposium on Molecular Spectroscopy

Columbus, June 18th -22th , 2012

Aimable Kalume a, Lisa George a , Scott A. Reid a, Brian J. Esselman b, Robert J.McMahon b

a: Marquette Universityb: University of Wisconsin-Madison

2OSU 67th International Symposium on Molecular Spectroscopy

Introduction

1: Wayne, R. P. Chemistry of Atmosphere, 3rd ed.,Oxford University Press: Oxford, U.K., 20002:Molina et al., Nature, 1974, 249, 810-8123: 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

• 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-CXBr3 (X = F, Cl, Br, I)


Background: our previous work on iso-bromoform 1

1: L. George et al., J. Chem. Phys. 135, 124503 (2011)

0

50

100

150

200

250

300

350

400

Rela

tive e

nerg

ie in

kJ/m

ol

CHBr3

CHBr3(TS)251

204iso-CHBr3

CHBr + Br2

363

CHBr2 + Br267

252CBr2 + HBr

• Other iso-polyhalomethanes species that have been studied in our lab: iso-CF2I-I, iso-CF2Br-Br, iso-CH2Cl-I

Rela

tive

ener

gy in

kJ/

mol


Optimized structures of iso-CXBr3

• 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


iso-CFBr3 : experimental results (IR and UV/vis)

• CFBr3: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)

421

852

1161

288

432

594


Calculated stationary points on the CFBr3 PES.

0

100

200

300

400

Rel

ativ

e en

ergie

in k

J/m

ol

CFBr2 + Br255

CFBr3 (TS)261

202iso-CFBr3

250CFBr + Br2

405CBr2 + FBr

CFBr3

Rela

tive

ener

gy in

kJ/

mol


iso-CClBr3 : experimental results (IR)

• CClBr3: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

878

77980

981

6


iso-CClBr3 : experimental results (UV/vis)

• CClBr3:Ar (1:500)

• Photolysis at 220nm (544kJ/mol) followed by annealing to 30K

• In comparison with computational predictions (TD-M06/aug-cc-pVTZ)

9

0

50

100

150

200

250

300

Rela

tive E

nerg

y in k

J/m

ol

CClBr3

(0)

iso-TS(229)

Calculated stationary points on the CClBr3 PES.

iso-ClBrCBr-Br(180)

iso-Br2CCl-Br(199)

iso-Br2CBr-Cl(200)

CClBr2 + Br(246)

CBr3 + Cl(277) CClBr + Br2

(272)

OSU 67th 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-CClBr3 isomer.


iso-CBr4 : experimental results (IR)

• CBr4:Ar (1:500)

• Photolysis at 266nm followed by annealing to 30K

• In comparison with computational predictions (M06/aug-cc-pVTZ)

842

781


iso-CBr4 : experimental results (UV/Vis)

• CBr4:Ar (1:500)

• Photolysis at 266nm followed by annealing to 30K

• In comparison with computational predictions (TD-M06/aug-cc-pVTZ)

645

458

337


Photoisomerization of iso-CXBr3

• CBr4:Ar (1:500)

• Irradiation into the strongest S0 →S3 band at 440nm

• The photolysis of the iso-CXBr3 results in the reformation of the parent isomer

• The exact mechanism has not been determined yet


Calculated stationary points on the CBr4 PES.

• text


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.


Conclusions

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 CXBr3 isomer


Acknowledgments

Dr. Scott A. ReidDr. Lisa GeorgeDr. Rajendra RathoreKhushabu Thakur

Dr. Robert J. McMahonBrian J. Esselman

National Science FoundationPetroleum Research Fund of the ACS



Cl

CBr Br

Br

Cl

CBr Br

Br

Cl

CBr

Br

Br

Cl

CBr Br

Br



Br

CBr Br

Br

Br

CBr Br

Br

Br

CBr Br

Br

Br

CBr Br

Br

Br

CBr Br

Br


Calculated stationary points on the CClBr3 PES.

• textThe iso-TS depicted in this figure is the transition state to the most stable iso-CClBr3 isomer.

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