Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 1 The 67 th International Symposium on Molecular Spectroscopy June 18-22, 2012 Young Wook Yoon, Chang Soon Huh and Sang Kuk Lee Department of Chemistry Pusan National University Pusan , Korea A part of this work appears in J. Chem. Phys. 136, (2012). Spectroscopic identification of benzyl-type radicals generated by corona discharge of precursors of mixed substituents (WI12)

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 2 Characteristics of transient molecules  Molecular radicals, molecular ions, and highly excited molecules.  Very short lifetime (less than sec), highly reactive in chemical reaction.  Cannot exist at ordinary condition. Need a special care for preservation, especially for benzyl-type radicals of mixed substituents.  Determine reaction pathway at the transition state as reaction intermediates.  Less than 2,000 transient species among more than 18,000,000 molecules in American Chemical Society Database.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 3 Motivation  We have developed a technique of Corona Excited Supersonic Expansion (CESE) which is a laser-free spectroscopic tool for observation of vibronic emission spectra of transient species.  Benzyl-type radicals are excellent candidates for CESE system because they emit visible fluorescence of the D 1 → D 0 transition.  We want to identify the isomeric benzyl-type radicals using substituent effect on electronic energy by substitution of methyl group, fluorine, chlorine atoms into benzene ring because the effect depends on the nature and position of substituents. This talk TI10 : yesterday

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 4 Benzyl radical  The simplest and prototypical aromatic free radical.  Reaction intermediate of aromatic chain reaction.  Seven delocalized π electrons on the molecular plane.  Shows visible emission the D 1 → D 0 transition.  Substitution into benzene ring reduces the electronic transition energy of delocalized π electrons due to the extended space for delocalized π electrons. H H XnXn

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 5 φ1φ1 φ2φ2 φ3φ3 φ4φ4 φ5φ5 φ6φ6 φ7φ7 b2b2 b2b2 a2a2 b2b2 a2a2 b2b2 b2b2 Molecular orbitals of benzyl radical 2 nd Excited state: (1b 2 ) 2 (2b 2 ) 1 (1a 2 ) 2 (3b 2 ) B 2 1 st Excited state: (1b 2 ) 2 (2b 2 ) 2 (1a 2 ) 1 (3b 2 ) A 2 Ground state: (1b 2 ) 2 (2b 2 ) 2 (1a 2 ) 2 (3b 2 ) B 2 The position of nodal points depends on the electronic states. At the 1 st excited state, it locates at 1 and 4 position.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 6 D1D1 D0D0 D2D2 2B22B2 2B22B2 2A22A2 B-type (visible region) A-type Theoretically, D 2 → D 0 and D 1 → D 0 are allowed. 800cm 22000cm Energy levels of benzyl radical Transfer of population via vibronic coupling Experimentally, only D 1 → D 0 is observable in the visible region.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 7 CESE Spectra Carrier gas(He) + Sample Advantage of CESE - Very simple scheme - Reducing Doppler broadening - Improved S/N ratio - Simplification of spectrum - Powerful laser-free technique for transient species P o = 3 atm P v = 5 Torr HV = ~2.0 kV Current = ~3 mA P0P0 Vacuum Chamber Supersonic Expansion PvPv Principle of CESE system Corona Excited Supersonic Expansion (+) (-) e-e- Corona Discharge High Press. (P 0 ) Emission anodecathode

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 8 Mechanism X * X X He* S0S0 SnSn − ·H− ·H D0D0 Collisional relaxation D1D1 CESE Spectrum Origin band Emission * X Radical formation The CESE spectrum provides vibrational frequencies in the ground state (D 0 ) as well as electronic energy in the D 1 → D 0 transition of the radicals.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 9 CESE spectrum (p-fluorobenzyl) LIF- DF spectrum J. Chem. Phys. 1990, 93, 8488 * He atomic line * * * CESE spectrum Chem. Phys. Lett. 1999, CESE spectrum is similar to the LIF-DF spectrum observed while pumping the origin band of the electronic transition. Thus, the CESE spectrum always shows the origin band of the electronic transition at highest wavenumber with strongest intensity. Origin band

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 10 Characteristics of CESE spectrum of benzyl-type radical  Rotational temperature of 40K due to weak backing pressure of jet expansion and DC corona discharge.  Very cold vibrational temperature through vibrational relaxation, showing the vibronic transition originating from the vibrationless state (v=0) of the D 1 state.  Show origin the band of the D 1 →D 0 transition at the highest wavenumber with strongest intensity, from which the electronic transition energy is determined.  The spacing of vibronic bands from the origin band represents the vibrational mode frequencies in the D 0 state.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 11 Benzyl-type radicals of mixed substituents H H X Y Two different substituents, X and Y, X and Y = -CH 3, F, and Cl There are many possible combinations of X and Y. 2-chloro-4-fluorobenzyl2-methyl-3-fluorobenzyl 2-chloro-5-fluorobenzyl2-methyl-6-fluorobenzyl 2-fluoro-4-chlorobenzyl2-methyl-4-fluorobenzyl 2-fluoro-5-chlorobenzyl2-methyl-5-fluorobenzyl 2-fluoro-3-methylbenzyl (WI11, previous talk) 2-chloro-3-methylbenzyl (WI11, previous talk) Yesterday, we presented benzyl-type radicals of X,Y = -CH 3. (TI10)

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 12 Precursors generating one product 2-chloro-4-fluorobenzyl 2-chloro-5-fluorobenzyl 2-fluoro-4-chlorobenzyl 2-fluoro-5-chlorobenzyl There is only one methyl group.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 13 Precursors generating one products 2-fluoro-3-methylbenzyl (WI11) 2-chloro-3-methylbenzyl (WI11) Precursors produce only one product because both methyl groups are equivalent. There are two equivalent methyl groups.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 14 Precursors generating two isomers 2-methyl-3-fluorobenzyl2-methyl-6-fluorobenzyl 2-methyl-4-fluorobenzyl2-methyl-5-fluorobenzyl Precursors produce 2 isomers because two methyl groups are different. There are two non-equivalent methyl groups.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 15 Spectrum of 2-chloro-5-fluorobenzyl radical Origin Typical vibronic emission spectrum of benzyl-type radicals

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 16 Elimination of Cl atom at higher voltage Production of 3-fluorobenzyl radical by elimination of Cl atom at higher discharging voltage

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 17 Spectrum from 2-fluoro-4-chlorotoluene 0(1) 0(2) * * 1(2) 1(1) * 6b(1) 7a(1) 8a(1) 14(1) 12(1) C2C2 7a(2) Produce two different products by dissociation of C-Cl bond.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 18 Origin Spectrum from 2-methyl-3-fluorotoluene Produce two isomers

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 19 Spectrum from 2-methyl-4-fluorotoluene

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 20 Assignment procedure  The assignments of vibronic bands belonging to benzyl- type radical were obtained by comparing with those of precursor as well as those of ab initio calculation.  The vibrational mode frequencies obtained by DFT/6- 311G* are accurate within ±3% of the observation.  The prediction of electronic energy obtained by TDDFT is accurate within ±20% of the observation.  However, the assignments of isomeric benzyl-type radicals generated from the same precursor is obtained by the shift of the origin band based on the mono- substituted benzyl radicals.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 21 Mode This work (D 0 ) Ab initio B3LYP/6-311G* (D 0 ) 2-Chloro-4- Fluorotoluene (S 0 ) Symmetry (C s ) Origin b a' 6a a' 6b a' a' TABLE III. Vibrational frequencies of 2-chloro-4-fluorobenzyl radical a a Precursor produces only one kind of benzyl-type radical.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 22 Substituent effect of mono-substitution a a Represents the red-shift of the origin bands from parental benzyl radicals at 22,002 cm -1. Substituento -m -p - CH F Cl CN Substitution into benzene ring shifts the origin band to red region of the D 1 →D 0 transition of the radicals. The shift depends on the kind and position of substituents.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 23 MoleculesOrigin bandShift Benzyl Methylbenzyl ,4-Dimethylbenzyl Fluorobenzyl ,4-Difluorobenzyl Chlorobenzyl Chloro-4-fluorobenzyl ,4-Dichlorobenzyl TABLE. Origin bands in the D 1 → D 0 transition of benzyl-type radicals Substituento-m-p- F Cl CH

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 24 Table 1. The origin bands (cm -1 ) of difluorobenzyl radicals in D 1 →D 0 transition MoleculesObs.Shift Calc. TDDFTDiffEmpiricalDiff 2,3-difluoro ,4-difluoro (78) a 78 2,5-difluoro ,6-difluoro ,4-difluoro (311) a ,5-difluoro Substituento-m-p- F Cl CH a Without substituent at 4-position

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 25 The origin bands (cm -1 ) of dichlorobenzyl radicals in D 1 →D 0 transition MoleculesObs.Shift Calc. TDDFTDiffEmpiricalDiff f 2,4-dichloro (962) a -297(60) a 2,5-dichloro ,6-dichloro ,4-dichloro (808) a -261(96) a Substituento-m-p- F Cl CH a Without substituent at 4-position

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 26 The origin bands (cm -1 ) of the benzyl-type radicals in D 1 →D 0 transition MoleculesObs.Shift Calc. TDDFTDiffEmpiricalDiff 2-chloro-4-fluoro (962) a 26 2-chloro-5-fluoro fluoro-4-chloro (78) a fluoro-5-chloro methyl-3-fluoro methyl-6-fluoro methyl-4-fluoro (657) a methyl-5-fluoro methyl-2-fluoro methyl-2-chloro Substituento-m-p- F Cl CH a Without substituent at 4-position

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 27 Shift of electronic energy Conjugated organic moleculesTransition (λ, nm) H-C=C-H170 H-C=C-C=C-H220 H-C=C-C=C-C=C-H260 For 1-D delocalization For 2-D delocalization Benzene38461 (cm -1 ) Toluene37474 o-Chlorotoluene36863 m-Chlorotoluene36602 p-Chlorotoluene36281 Extension of space for delocalized π electrons reduces electronic transition energy. Substitution into benzene ring increases the delocalizing space.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 28 Nodal points in Hückel MO Φ2Φ2 Φ3Φ3 Φ1Φ1 Φ4Φ4 Φ5Φ5 Φ6Φ6 Φ7Φ7 b2b2 a2a2 a2a2 b2b2 b2b2 b2b2 b2b2 D 0 (b 2 ) D 2 (b 2 ) D 1 (a 2 ) Nodal point at 1 and 4- positions No nodal point The position of nodal points changes with electronic states. At D 1 state, it is located at 1 and 4 positions.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 29 Possible benzyl-type radicals from precursor [1] Product of cleavage of methyl C-H bond and C-Cl dissociation [2] Product of cleavage of methyl C-H bond [3] Product of cleavage of methyl C-H bond and C-F dissociation

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 30 Spectrum observed from corona discharge of 2-chloro-4-fluorotoluene 6b (2) * * 0 (1) 0 (2) 6a (1) 1 (1) 6a (2) 7a (1) 1 (2) 9b (2) Spectrum shows only two products can be obtained from the corona discharge of 2-chloro-4-fluorotoluene

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 31 Proposed dissociation process C-Cl Cleavage C-H Cleavage 519kJ/mol 398kJ/mol 356kJ/mol We could not observe the product of cleavage of C-F bond.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 32 Summary  We successfully observed the vibronic emission spectra of 2-chloro-4fluorobenzyl radical using a technique of CESE.  We explain the smaller red-shift of the origin bands of benzyl-type radicals with substituent at 4-position using Hückel MO theory for the first time. The smaller shift is observed from other multi-substituted benzyl radicals.  This explanation provides direct evidence of nodal points at a given electronic state.  We found the dissociation products agree with the bond dissociation energy.

Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 33 Acknowledgments National Research Foundation of Korea ( ) Funding for Basic Sciences