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SPECTROSCOPY OF AND PHOTOINDUCED ELECTRON TRANSFER IN THE COMPLEXES OF C2H4 WITH I AND I2

Lisa George, Aimable Kalume, and Scott A. ReidDepartment of Chemistry, Marquette University, Milwaukee, WI 53233, USA

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Background and literature review

• The Electron- Donor-Acceptor Complexes (EDA)of halogens with π-electron donors such as Ethylene are model systems to understand the mechanism of electrophilic addition and substitution reactions.

• Stereoselectivity of photoinitiated free radical addition reactions involving iodine and bromine to ethylene has received much attention.1,2

• Ihee and co-workers obtained evidence for the formation of symmetrically bridged C2H4···I radical using time-resolved X-ray diffraction techniques.3,4

• We report the photoinduced electron transfer in the C2H4···I2 complex, leading to the formation of bridged intermediate C2H4···I•, trapped in Ne matrix and interrogated spectroscopically.1: P. S. Skell et al, J. Am. Chem. Soc. 1964, 86, 2956.2: G. Maier et al, Liebigs Annalen/Recueil, 1997, 317.3: H. Ihee et al. Science, 2005, 309, 1223.4: H. Ihee et al. J. Phys. Chem. A, 2012, 116, 2713.

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Experimental and Computational Details

• C2H4/I2//Ne (1:2:1000) at 5K (Mixture of C2H4:Ne (1:1000) passing over I2 held at 40°C)

• Pulsed deposition, 1ms pulse, 5hz, 1-2h deposition, IR and UV/Vis spectra• Photolysis of C2H4···I2 at 240nm or 450nm, using Nd:YAG (Continuum NY-61)

pumped dye laser system (Lambda Physik Scanmate 2E). • Photolysis of C2H4···I• at 355nm (Nd: YAG (Continuum Minilite)

• Geometry optimization: M062x, MP2(aug-cc-pVTZ, aug-cc-pVTZpp ) (Gaussian 09)

• Electronic absorption and oscillator strength: TD-DFT ( M06/ Sadlej-pVTZ)

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UV/Vis spectrum of C2H4···I2 complex

C2H4/I2//Ne (1:2:1000) matrix following pulsed deposition on a CaF2

window at 5 K, together with TD-DFT predicted electronic absorptions of the ethylene-I2 complex at the M06/Sadlej-pVTZ level.

247

450

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UV spectrum of C2H4···I2 complex following photolysis at 240nm

C2H4/I2//Ne (1:2:1000) matrix following pulsed deposition on a CaF2 window at 5 K followed by photolysis at 240 nm, together with TD-DFT predicted electronic absorptions at M06/Sadlej-pVTZ.

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450

Photolysis of C2H4···I• complex at 355nm

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366

247

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Infrared spectrum of C2H4···I2 complex following photolysis at 240nm

800 1000 1200 1400 1600-0.06

-0.04

-0.02

0.00

0.02

0.04

Rel

ativ

e IR

inte

nsi

ty

Wavenumber (cm-1)

1145

953134214421613

7 CHʋ 2 wag3CHʋ 2deform.12 CHʋ 2 deform.2 C-C stretchʋ

M06/Sadlej-pVTZ

Infrared spectrum of C2H4···I• complex following photolysis at 355nm

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IR spectrum of C2H4···I in Ne matrix following 450 nm Photolysis

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1145 12101237 1438

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Reaction path and mechanisms for the photolytic reaction of the C2H4…I2 and C2H4…Br2 Charge Transfer complex.

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CT state

I2 excited state

× kBET kDISS

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CT state +

Participation of bridged iodoethyl intermediate

Participation of classical bromoethyl intermediate

• 240 nm, CT state of the complex• 450 nm, the I2 Chromophore• 355 nm, CT state of the radical

• 266 nm, CT state• > 300 λ , the Br2 Chromophore

L. George, L. Wittmann, A. Kalume, and S. A. Reid, Photoinduced Electron Transfer in a prototypical Mulliken Donor-Acceptor Complex: C2H4...Br2 J. Phys. Chem. Lett. 2010, 1, 2618–2621

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11

+•

–•_ _

+• +

•

•+•

–• *

hνCTBET

hνvis

•

hνCT

Photochemistry of the C2H4…I2 and C2H4…Br2 Charge Transfer complexes.

••

•

•+

hνvis

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IRC scan from symmetric bridged TS (at UM062x/aug-cc-pVTZ-pp level ), illustrating the concerted stereospecific addition

Optimized structures of the ethylene complexes with atomic and molecular iodine and bromine (M062x/aug-cc-pVTZpp).

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Scan for the C2H4I radical

Fully relaxed scan of the C-C-I bond angle with the UM062x/aug-cc-pVTZpp method shows only one minimum which corresponds to the symmetrically bridged radical

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Scan for the C2H4Br radical

•Fully relaxed scan along the C-C-Br and H-C-C-Br with DFT and MP2 method

•classical radical is the global minimum in both the methods.

The symmetricaly bridged radical is a local minimum at MP2

No experimental evidence for the bridged radical

UB3LYP/aug-cc-pVDZ

UMP2/aug-cc-pVDZ

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Summary and Future directions

• Spectroscopic interrogation of symmetrically bridged radical complex C2H4…I by trapping in Ne matrix following photolysis of C2H4…I2 charge-transfer complex.

• Irradiations into the CT band or the I2 band, produced the C2H4···I• complex

• Photolysis of the C2H4···I• complex led to the formation of C2H4···I2 , anti- and gauche-1,2 diiodoethane

• High level calculations were performed to confirm the structures of the radical intermediates in electrophilic addition of iodine and bromine to ethylene, C2H4…I (bridged) and C2H4…Br (classical).

• Examined the reaction path and identified a symmetric transition state leading to a concerted stereospecific addition.

• Experiments to understand the mechanisms in the case of C2H4···ICl.

Professor Scott. A. Reid

• Aimable Kalume• Lloyd Muzangwa• Silver Nyambo• Brandon Uhler National Science Foundation and Petroleum Research Fund

THANK YOU

Acknowledgments

Infrared spectrum of C2H4···I2 complex following photolysis at 240nm

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Observed and predicted vertical transitions

Experimental and calculated vibrational frequencies of species

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Species Experiment

M062x/Sadlej-pVTZ

M062x-aug-cc-pVTZpp

Modes

C2H4 953 1442

1002 (98) 1476 (11)

- ʋ7 CH2 wag ʋ12 CH2 deform.

C2H4…I2 953 1342 1442 1613

1013 (165) 1355 (4) 1413 (10) 1685 (20)

1013 (181) 1383 (5) 1477 (11) 1693 (20)

ʋ7 CH2 wag ʋ3CH2 deform. ʋ12 CH2 deform. ʋ2 C-C stretch

C2H4…I 953 1335 1438 1604

1013 (136) 1353 (6) 1413 (11) 1675 (27)

1012 (136) 1381 (4) 1413 (13) 1675 (27)

ʋ7 CH2 wag ʋ3CH2 deform. ʋ12 CH2 deform. ʋ2 C-C stretch

(intensity in km/mol)