Matrix isolation and computational study of iso-difluorodibromomethane (F2CBr-Br): A route to molecular products in CF2Br2 Photolysis

Lisa George, Aimable Kalume, and Scott A. Reid, Department of Chemistry, Marquette University, Milwaukee, WI 53233; Patrick Z. El-Khoury and Alexander Tarnovsky, Department of Chemistry and center for Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403

Date: June 22, 2010

• Introduction to CF2Br2 Photolysis (The nature of the photodissociation process is discussed in light of

previous and present results)

• Experimental Techniques & Computational Methods

• Infrared, UV/Vis, Computational results and discussion

Outline of the Talk

•CF2Br2 (Halon 1202) Relevant to atmospheric chemistry, fire retardants, and potential for ozone depletion

•UV photodissociation of CF2Br2 involving C-Br fission in an initial step, followed by a second C-Br fission as a result of IVR, leads to :CF2

•Isoenergetic dissociation channels 1) a radical channel yielding CF2Br + Br, and 2) a molecular elimination channel yielding :CF2 + Br2

Photochemistry of Halons

What are the products of CF2Br2 photolysis?

CF2Br2 + hν

CF2Br + Br

CF2 + Br2

MAJOR MINOR4% in CRDS expt. ~ 188 and 226 nm .

Lin et al. J.Chem. Phys. 2000, 125, 143312

Previous Studies on the Photolysis of CF2Br2

M.E Jacox, Chemical Physics Letters, 1978, 54, 176

CF2Br2 :Ar = 1:270 (CF2Br radical C2F4 and C2F4Br2 )

Cameron and Bacskay, J.Phys.Chem. . 2000, , 104, 11212, reported iso-CF2Br2 as TS Br + CF2Br :CF2 + Br2

CASSCF/cc-pVDZ (Dalton programs)

10-6 mbar

To DP

Sample/RG

KBr window

Cold Head 5 K

hνPulsed deposition

LaserPhotolysis

Measure

Anneal

Experimental setup

(FTIR,UV/Vis)

(35K)

Inspiration for this study

Goal of this study was to observe the iso-form of CF2Br2 (F2CBr-Br) using Matrix isolation infrared and UV/Vis. Spectroscopy, supported by computation

Recent observation and characterisation of iso-CF2I2 led to this study

iso-CF2I2 spectrum

Photolysis of CF2Br2

F2CBr2

F2CBr-Br

hν

240 and 266 nm

Infrared and UV/Vis Spectra

As Deposited

240 nm

Annealed at 35 K

TDB3LYP/aug-cc-pVTZ 359nm (331nm f=0.27)536nm (528nm f=0.005)

B3LYP/aug-cc-pVTZ

ν1 1188,1203 (769)

ν7 1240, 1234 (295)

ν3 509, 493 (79)

Relaxed PES of iso-CF2Br2

B3LYP/aug-cc-pVDZ

Relaxed PES along the Br-Br stretching and C-Br-Br bending coordinates at the

B3LYP/cc-pVXZB3LYP/aug-cc-pVXZMP2/cc-pVXZMP2/aug-cc-pVXZ (X=D, T, Q)

2.5 Å

158 (140)

:CF2---Br2 complex

Relaxed PES along theC-Br stretching and C-Br-Br bending coordinates

B3LYP/aug-cc-pVDZ

……

B3LYP/aug-ccpVTZ//CCSD(T)/aug-cc-pVDZ

+

+Stationary points on the CF2Br2 PES

IRC Plot

B3LYP/aug-ccpVTZ

Reaction Sequence

CF2Br2 + hϑ CF2Br + Br (1)

CF2Br + Br iso-CF2Br2 (2)

iso-CF2Br2 :CF2 + Br2 (3)

CF2Br (+hϑ) :CF2 + Br (4)

Summary

•Wavelength dependent mechanisms of the photochemical reactions of Difluorodibromomethane (CF2Br2; Halon 1202) has been proposed

•Iso-CF2Br2, a weakly bound isomer of CF2Br2 isomer was observed in matrix infrared and UV/Visible spectra obtained following photodissociation of CF2Br2 at either 240 nm or 266 nm• Iso-CF2Br2 is an intermediate in the Br + CF2Br :

CF2 + Br2 reaction confirmed by PES

Professor Scott. A. Reid

• Aimable Kalume• Dr. Victoria Ayles• Lloyd Muzangwa National Science Foundation and Petroleum Research Fund

Acknowledgments

Experimental Procedure

• CF2Br2 is mixed with Ar , M:S ratio is 1: 5000

• Gas mixture is co-deposited on a cold substrate KBr or CaF2at 5K and Probed by IR, UV/Vis. Spectroscopy

• Low Temp. He displex (ARS Displex DE-204S)

Supersonic jet sampling, Pulse duration is 500 micro Sec., repetition rate of 5 Hz

FTIR spectrometer (Mattson, Galaxy series) equipped with a DTGS detector (1 cm-1 resolution and 128 scans ), Agilent 8453 diode array spectrophotometer

• Photolysis Sources used 266 nm and 240 nm

B3LYP/aug-cc-pVTZ , CCSD(T)/aug-cc-pVDZ (Single point)

Photolysis Condition

1)The cold window was irradiated with laser light at 266 nm, generated from the fourth harmonic of a pulsed Nd:YAG laser (Continuum Minilite)

2) 240 nm, generated from the frequency doubled output of a dye laser system (Lambda-Physik Scanmate 2E) operating on Coumarin 480 dye, pumped by the third harmonic (355 nm) of a Nd:YAG laser (Continuum NY-61).

Typical irradiation times were 1.5 h at 240 nm (10 ns pulses, 1 mJ/pulse, 10 Hz), and 5 h at 266 nm (5 ns pulses, 2.3 mJ/pulse, 5 Hz).

The gas-phase absorption cross-section of the parent at 210 K is around 30 times larger at 240 nm than at 266 nm.

CF2Br2:Ar matrix (240 nm, annealed, computation)

B3LYP/aug-cc-pVTZ

ν1 1188,1203 (769)

ν7 1240, 1234 (295)

ν3 509, 493 (79)

wavenumber (cm-1)

Photolysis at 266 nm

CF2Br2 annealing

Parent annealed