matrix isolation and computational study of iso-difluorodibromomethane (f 2 cbr-br): a route to...
TRANSCRIPT
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