Jet-Cooled Chlorofluorobenzyl Radicals: Spectroscopy and Mechanisma Young Wook Yoon, Sang Kuk Lee sklee@pusan.ac.kr Department of Chemistry Pusan National University Pusan 609-735, Korea aSupported by the National Research Foundation (NRF) of Korea

Motivation In our laboratory, we have developed a technique of corona excited supersonic expansion (CESE) for production of vibronically excited but jet-cooled benzyl-type radicals by corona discharge. Substituted benzyl radicals, so called benzyl-type radicals, have been less studied due to the difficulties associated with purchasing precursors and producing radicals. We studied substituent effect on electronic transition energies by observing vibronic emission spectra of various benzyl-type radicals. Chlorofluorobenzyl radicals are very recent subject for identification of substituent effect.

Characteristics of Transient Molecules Molecular radicals, molecular ions, and highly excited molecules belong to this class. Determine reaction pathway at the transition state as reaction intermediates. Very unstable, short lifetimes (less than 10-6 sec), and highly reactive in chemical reaction. Cannot exist at ordinary condition. Need a special care for production, preservation, and observation.

Benzyl Radical A prototype of aromatic molecular radicals Seven delocalized π electronic system Strong visible emission of the D1 → D0 transition Well studied by Fukushima and Obi H C2V point group 2nd Excited state: (1b2)2 (2b2)1 (1a2)2 (3b2)2 22B2 1st Excited state: (1b2)2 (2b2)2 (1a2)1 (3b2)2 12A2 Ground state: (1b2)2 (2b2)2 (1a2)2 (3b2)1 12B2

B-type (visible region) Energy Levels of Benzyl Radical 2B2 D2 -1 Vibronic relaxation: Transfer of population 800cm A-type D1 2A2 22000cm -1 B-type (visible region) D0 2B2 Theoretically, D2 → D0 and D1 → D0 are allowed. Experimentally, only D1 → D0 is observable in the visible region.

Homo-Substituted Benzyl-type Radicals studied Fairly strong visible emission. Isomers studied up to n=3. The substituent effect of –CH3 on electronic transition energy has been identified. Fairly strong visible emission. Isomers studied up to n=2. Difficult to get precursor molecules of n>2. Weak visible emission. Isomers studied up to n=2. Difficult to get precursor molecules of n≥2. Difficult to produce radicals from precursors.

Hetero-Substituted Benzyl-type Radicals Fairly strong visible emission. Easy to produce from precursors. Recent subjects. Very weak visible emission. Difficult to get precursors. C-Cl dissociation is possible. Very recent subject.(Subject of this presentation) Commercially available precursors are limited. C-Cl dissociation is also possible.

Experimental : CESE system Resistor Box High Voltage DC Power Supply Pump Expansion Chamber Quartz Window Collecting Lens Cathode Ground Monochromator (Path length = 2.0m) Spectrometer Controller Signal HV Computer Printer Oscilloscope Sample + Buffer gas Optical Table (A) (B) (C) Nozzle Very simple scheme

Pinhole-type Glass Nozzle Rev. Sci. Instrum 57, 2274 (1986). Large soot deposits of carbon near hole, destabilizing the discharge Schematics of glass nozzle designed for corona discharge and supersonic jet expansion Useful for OH radical, but not suitable for hydrocarbons

Modification of Glass Nozzle Original glass nozzle Rev. Sci. Instrum. 57, 2274 (1986) Made by grinding one end of glass tube Flat bottom surface : Large soot deposit Short path length : Deflecting beam Useful for carbon-free precursors Modified glass nozzle Chem. Phys. Lett. 358, 110 (2002) Made a hole through one end of glass tube Round bottom surface : Reducing soot deposit Long path length : Straight beam Useful for hydrocarbon precursors

Principle of CESE System Corona Excited Supersonic Expansion Carrier gas(He) + Sample Vacuum Chamber Pv Po = 3 atm Pv = 5 Torr HV = ~2.0 kV Current = ~3 mA Supersonic Expansion (+) (-) e- Corona Discharge P0 High Press. (P0) anode cathode Emission Very simple scheme Reducing Doppler broadening up to 30K Improved S/N ratio Simplification of spectrum Laser-free technique for transient species CESE Spectra

Visible Emission in CESE System Demonstration with Helium Discharge in CESE 1.2cm Anode inside glass tube The bright visible He emission disappears with injection of precursor because of energy transfer from He* to precursor.

Mechanism for CESE Spectrum X * He* Sn − ·H D0 D1 CESE Spectrum -1 Origin band Emission Radical formation Precursor D2 Vibronic relaxation S0 Vibrational relaxation Radical The CESE spectrum provides directly Electronic energy of the D1 → D0 transition. Vibrational mode frequencies in the D0 state.

Characteristics of CESE Spectrum LIF- DF spectrum ( pumping) J. Chem. Phys. 1990, 93, 8488 * He atomic line * CESE spectrum Chem. Phys. Lett. 1999, 301 407 The CESE spectrum is similar to LIF-DF spectrum observed with excitation of origin band.

Production of Benzyl-type Radicals Two precursors can be used for benzyl-type radicals. Benzylic C-Cl bond (285 kJ/mol) dissociation Methyl C-H bond (355 kJ/mol) dissociation Removal of Cl Removal of F

Spectra Observed from Two Different Precursors Difference R-CH2Cl R-CH3 R-CH2Cl generates only one benzyl-type radicals, but R-CH3 produces two radicals.

Suggested Mechanism for Benzyl-type Radicals 519J 397J

Chlorofluorobenzyl Radicals Studied We observed only 3 isomers among 6 possible isomers due to the limitation of available precursors 2,4-disubstitutions 2,5-disubstitutions 2,6-disubstitution

2,4-disubstitutions JCP 136, 174306 (2012) Precursor: 2-chloro-4-fluorotoluene. Observed 2-chloro-4-fluorobenzyl and 4-fluorobenzyl radicals. BKCS 34, 3565 (2013) Precursor: 2-fluoro-4-chlorotoluene. Observed 2-fluoro-4-chlorobenzyl and 2-fluorobenzyl radicals.

2,5-disubstitutions CPL 612, 134 (2014) Precursor: 2-chloro-5-fluorotoluene Observed 2-chloro-5-fluorobenzyl and 3-fluorobenzyl radicals. CPL 608, 6 (2014) Precursor: 2-fluoro-5-chlorotoluene Observed 2-fluoro-5-chlorobenzyl and 2-fluorobenzyl radicals.

2,6-disubstitutions CPL 637, 148 (2015) Precursor: 2-chloro-4-fluorotoluene Observed 2-chloro-4-fluorobenzyl and 4-fluorobenzyl radicals.

Spectroscopic Data (cm-1) Determined

Substitution at the 4-position The substituent at the 4-position has a negligible contribution to the substituent effect on electronic transition energy because nodal point, zero amplitude of π electronic orbitals is located at the 4-position in the D1 state. Thus, 2-chloro-4-fluorobenzyl and 2-fluoro-4-chlorobenzyl radicals show the similar effects to the 2-chlorobenzyl and 2- fluorobenzyl radicals, respectively. Node at 4-position at D1 state D1 (A2) D0 (B2) No extension of delocalization of π electrons to the substituent at 4-position

Orientation effect The 2,5-isomers, two substituents of anti-parallel alignment, have quite large substituent effect on electronic transition energy because the plane available for delocalized π electrons changes from circular to elliptical motion of π electrons. Anti-parallel alignments : Large substituent effect Parallel alignments : Small substituent effect

For a linear translational motion, me : Mass of electron For a circular motion, R: Radius of circular motion Elliptic motion has larger transition energy than a circular one, giving a larger red-shift from benzyl radical.

Summary We successfully observed the vibronic emission spectra of chlorofluorobenzyl radicals using a technique of CESE. We determined the electronic transition energies and vibrational mode frequencies of the newly observed chlorofluorobenzyl radicals. The red-shift of the electronic transition energy of benzyl-type radicals was discussed using the model of substituents orientation and node position in Hϋckel molecular orbital theory.

Acknowledgments Financial Support for Basic Sciences from The National Research Foundation of Korea

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