1. 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 and Sang Kuk Lee sklee@pusan.ac.kr Department of Chemistry Pusan National University Pusan 609-735, Korea A part of this work appears in J. Chem. Phys. 2011, 135, 214305. Confirmed Assignments of Isomeric Dimethylbenzyl Radicals Generated by Corona Discharge (TI10)

2. 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 10 -6 sec), highly reactive in chemical reaction.  Cannot exist at ordinary condition. Need a special care for preservation.  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.

3. 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 show strong visible emission of the D 1 → D 0 transition.  We want to see the substituent effect on electronic energy by substitution of methyl group, fluorine, chlorine atoms into benzene ring. This talk WI12

4. 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.  Important species in atmospheric and combustion chemistry.  Seven delocalized π electrons on the molecular plane.  Shows strong emission in the visible region. H H

5. 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 ) 2 2 2 B 2 1 st Excited state: (1b 2 ) 2 (2b 2 ) 2 (1a 2 ) 1 (3b 2 ) 2 1 2 A 2 Ground state: (1b 2 ) 2 (2b 2 ) 2 (1a 2 ) 2 (3b 2 ) 1 1 2 B 2

6. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 6 Configuration Interaction D 4 (B 2 ) D 3 (A 2 ) D 2 (B 2 ) D 1 (A 2 ) D 0 (B 2 ) φ7φ7 φ4φ4 φ5φ5 φ2φ2 φ3φ3 φ1φ1 D 5 (B 2 ) Without CIWith CI φ6φ6 D 6 (B 2 )

7. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 7 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 Vibronic relaxation : Transfer of population Experimentally, D 1 → D 0 is observable in the visible region.

8. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 8 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

9. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 9 Experimental : CESE system

10. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 10 Overview of CESE system

11. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 11 Discharge in CESE Emission in CESE system Demonstration with Helium 1.2cm Electrode

12. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 12 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 electronic energy of the D 1 → D 0 transition as well as vibrational frequencies in the ground state (D 0 ) of the radicals.

13. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 13 CESE spectrum (p-fluorobenzyl) LIF- DF spectrum J. Chem. Phys. 1990, 93, 8488 * He atomic line * * * CESE spectrum Chem. Phys. Lett. 1999, 301 407 CESE spectrum is similar to the LIF-DF spectrum observed while pumping the origin band of the electronic transition.

14. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 14 CESE vs. LIF-DF (p-chlorobenzyl) Spectrum shows enough resolution as well as S/N for benzyl-type radicals.

15. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 15 Dimethylbenzyl Radicals  There exist 6 isomers with the position of methyl groups.  Easily producible from trimethylbenzenes by dissociation of methyl C-H bond.  Problem 1: Production of several isomers simultaneously according to the –CH 3 group to be dissociated. 1,2,3-trimethylbenzene → 2,3- and 2,6-isomers 1,2,4-trimethylbenzene → 2,4-, 2,5-, and 3,4-isomers 1,3,5-trimethylbenzene → 3,5-isomer only  Problem 2: Identification of each isomer in the mixed spectrum.

16. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 16 Characteristics of Benzyl chloride as Precursor Although dimethylbenzyl chloride generates only one isomer in corona discharge by dissociation of C-Cl bond, it shows weak emission intensity and produces a large amount of small fragment such as C 2 emitting strong fluorescence in the same spectral region. Nevertheless, we employed dimethylbenzyl chlorides as precursor to identify each isomer (b) (a) * Production of C 2

17. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 17 Spectrum from 1,2,4-trimethylbenzene a (a) (b) (c) (d) * * * * * * * * * * Mixed spectrum of 3 isomers a J. Chem. Phys. 2011, 135, 214305.

18. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 18 Spectrum from 2,4-dimethylbenzyl chloride (a) (b) (c) (d) * * * * * * * * * * Mixed spectrum of 3 isomers 2,4-isomer

19. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 19 Spectrum from 2,5-dimethylbenzyl chloride (a) (b) (c) (d) * * * * * * * * * * Mixed spectrum of 3 isomers 2,4-isomer 2,5-isomer

20. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 20 Spectrum from 3,4-dimethylbenzyl chloride (a) (b) (c) (d) * * * * * * * * * * Mixed spectrum of 3 isomers 2,4-isomer 2,5-isomer 3,4-isomer Cl atomic C 2 bands

21. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 21 We have found that the previous assignment based on ab initio calculation was not correct. Also, the prediction of electronic energy is not accurate.

22. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 22 Table. Vibronic assignments of isomeric dimethylbenzyl radicals generated from 1,2,4-trimethylbenzene Mode 3,4-Dimethyl2,4-Dimethyl 2,5-Dimethyl 1,2,4- Trimethyl benzene Sym Obs a Calc b Obs a Calc b Obs a Calc b Obs Origin21592 24801 21306 24880 20558 23410 9b436437436434436438439a' 6b474477466476468473472a' 6a552561546556562570555a' 1734739706716740729717a' 12768749772782784754746a' 7b924933916925930943925a' 7a125212731285126412901246a' a This work. b TDDFT and B3LYP/6-311G* calculations.

23. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 23 Spectrum from 1,2,3-trimethylbenzene Origin of 2,6-dimethylbenzyl Origin of 2,3-dimethylbenzyl

24. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 24 Spectrum from 2,6-dimethylbenzyl chloride Origin of 2,6-dimethylbenzyl C 2 bands Cl atomic The 2,3-dimethylbenzyl chloride was not commercially available.

25. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 25 3,5-Dimethylbenzyl radical a D 1 → D 0 (0) D 2 → D 0 (0) C2C2 a J. Phys. Chem. A 111, 6003 (2007).

26. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 26 Table. Transition energy of dimethylbenzyl radicals of the D 1 → D 0 transition a Molecules This workPrevious work Obs. b Calc. c Solid state d Gas phase e 3,4-dimethylbenzyl21592(1)24801(2)21044(2)21306 2,4-dimethylbenzyl21306(2)24880(1)21205(1)21600 2,5-dimethylbenzyl20558(6)23410(6)20157(6)20558 2,3-dimethylbenzyl21166(3)24177(3)20244(5) 2,6-dimethylbenzyl20614(5)23584(5)20325(4) 3,5-dimethylbenzyl20842(4)24069(4)20406(3) a Measured in vacuum (cm-1). b This work. c TDDFT calculation. d Chem. Phys. 1, 95 (1973), e BKCS 29, 2341 (2008) (previous work).

27. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 27 Substituent effect on electronic energy of substituted toluenes a a Represents the red-shift of the origin bands from parental toluene at 37474 cm -1. The shift sows increasing trend from o- to p-substitutions. Substitutions into benzene ring shift the origin band to red region in the S 1 →S 0 transition of the toluene. The shift is strongly dependent on the nature and position of substituents. Substituentso -m -p - CH 3 169510749 F-8888614 Cl6118721193

28. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 28 Substituent effect of –CH 3 on electronic energy a a Represents the red-shift of the origin bands from parental toluene and benzyl radical at 37474 and 22002 cm -1. Substitutions into benzene ring shift the origin band to red region in the S 1 →S 0 transition of the toluene and D 1 →D 0 transition of the benzyl radical. The shift is strongly dependent on the nature and position of substituents. Substituentso -m -p - Toluene169510749 Benzyl radical657517302

29. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 29 Table. The origin bands (cm -1 ) of the dimethylbenzyl radicals in D 1 →D 0 transition MoleculesObs.Shift a Calc. Empirical b Diff d TDDFT e Diff f benzyl22002000259883986 2,3-dimethyl211648381174-336241773013 2,4-dimethyl21306696959(657) c -263(39)248793573 2,5-dimethyl2055814441174270234092851 2,6-dimethyl206161386131472235842968 3,4-dimethyl21592410819(517) c -409(-107)248003208 3,5-dimethyl2084211601034126240693227 a Spacing from the origin band of benzyl radical at 22002cm -1. b Data based on the shift of mono-substitution. Summation of two mono-substitutions. c Data from mono-substitution. d Difference between the observation and the simple summation. e TDTFT calculation, f Difference from the observation. Substituento-m-p- CH 3 657517302

30. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 30 MoleculesObs.Shift a Calc. Empirical b Diff d TDDFT e Diff f 2,3,4-trimethyl2084411581476(1174) c 318(-16)238453001 2,3,5-trimethyl1953624661691775226743138 2,3,6-trimethyl1979622061831375223652569 2,4,5-trimethyl2083211701476(1174) c 306(-4)242723440 2,4,6-trimethyl2080011661616(1314) c 450(-112)239053069 3,4,5-trimethyl2083612021336(1034) c -134(132)247043904 Table. The origin bands (cm -1 ) of the trimethylbenzyl radicals in D 1 →D 0 transition a Spacing from the origin band of benzyl radical at 22002cm -1. b Data based on the shift of mono-substitution. Summation of three mono-substitutions. c Data from bi-substitution. d Difference between the observation and the simple summation. e TDTFT calculation, f Difference from the observation. Substituento-m-p- CH 3 657517302

31. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 31 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-Dimethylbenzene37305 m-Dimethylbenzene36964 p-Dimethylbenzene36725 Extension of space for delocalized π electrons reduces electronic energy.

32. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 32 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 CH 3 The position of nodal points changes with electronic states.

33. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 33 Summary  We successfully observed the vibronic emission spectra of dimethylbenzyl radicals using a technique of CESE.  We clearly identify the bands belonging to each isomer using dimethylbenzyl chlorides as a precursor.  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 observation may provides direct evidence of nodal points at a given electronic state.

34. Copyright © Professor Sang Kuk Lee, Department of Chemistry, Pusan National University. All rights reserved. 34 Acknowledgments National Research Foundation of Korea (2011-2014) Funding for Basic Sciences