DISPERSED FLUORESCENCE (DF) SPECTROSCOPY OF JET-COOLED METHYLCYCLOHEXOXY (MCHO) RADICALS Jahangir Alam, Md Asmaul Reza, Amy Mason, Neil Reilly and Jinjun Liu Department of Chemistry, University of Louisville. International Symposium on Molecular Spectroscopy University of Illinois Urbana-Champaign 06/26/15 1 Outline Introduction and Motivation. Conformational Studies of MCHOs. Quantum Chemical Calculations. Moderate Resolution LIF/DF Apparatus. LIF and DF Spectra of MCHOs. Franck-Condon Factor (FCF) Simulation. Summary and Future Work. 2 Introduction and Motivation Cyclohexoxy radicals and its substitutes like methylcyclohexoxy (MCHO) are important reaction intermediates in combustion and atmospheric chemistry. Analysis of the vibrational structures of nearly degenerate X and A states of MCHOs helps understand the coupling between close-lying potential energy surfaces (PESs) including the pseudo-Jahn-Teller effect (pJTE). The mechanism of interaction between these two close-lying states is of significant theoretical interests and can be used to benchmark quantum chemical calculations. Experimentally obtained spectra are prerequisite of future kinetic studies. Investigation of the intramolecular interactions and the methyl substitution effect on the vibrational structure will help understand chemical kinetics. 3 J. Lin, Q. Wu, G. Liang, L. Zu; RSC Adv. 2, 583589 (2012) Q. Wu, G. Liang, L. Zu; J. Phys. Chem. A 116, 31563162 (2012) Conformational Studies of MCHO Radicals Among possible chair and twisted-boat conformers of MCHOs, only chair structures are likely to be present under jet-cooled conditions due to their lowest energy. Theoretical studies suggest that, for 4-, 3- and 2- MCHOs, diequatorial conformers have the lowest energy. For 1-MCHO, the chair-axial and chair-equatorial conformers, defined by the orientation of the oxygen atom, can co-exist in free jet expansion. 4-, 3- and 2-MCHO 1-MCHO chair- equatorial long boat twisted boat-a 1 boat-1 twisted boat-a 2 half chair chair- axial minimum -1 barrier -1 minimum -2 barrier -2 minimum -3 barrier -3 minimum -4 chair- equatorial long boat twisted boat-a 1 boat-1 twisted boat-a 2 half chair chair-axial Quantum Chemical Calculations on MCHOs The three lowest electronic states of 4-MCHO and 1-MCHO are X 2 A, A 2 A and B 2 A. 3-MCHO and 2-MCHO also have similar electronic configuration. The B 2 A X 2 A electronic transitions corresponds to the promotion of one electron from the orbital of the C-O bond to the half-filled non-bonding orbital that is localized on the O atom. On the basis of symmetry of electronic states, both B 2 A A 2 A and B 2 A X 2 A electronic transitions are allowed. Calculated by Gaussian 09 at B3LYP/6-31+G(d) and CIS Level of Theory Parameters4-MCHO3-MCHO2-MCHO 1-MCHO (chair- axial) 1-MCHO (chair- equatorial) E (A-X) (cm -1 ) 168N/A C-O stretch (B) (cm -1 ) C-O stretch (A) (cm -1 ) 1060N/A C-O stretch (X) (cm -1 ) r C-O (B) - r C-O (X) (m) r C-CH3 (B) - r C-CH3 (X) (m) Moderate-resolution LIF/DF apparatus PMT=photomultiplier tube; OGC=optogalvanic cell iCCD=intensified charge coupled device Nd:YAG Pulsed Dye Laser Computer Photolysis Laser Doubling Crystal Vacuum Chamber Methylcyclohexyl nitrite/He ~0.1 cm -1 Nd:YAG x3 x2 Excitation Laser PMT Box- Car spectrograph iCCD ~30 cm -1 OGC DF LIF LIF: laser-induced fluorescence DF: dispersed fluorescence A X B 6 LIF/DF Spectra of 4-MCHO E=76(5) Pumped LIF band: origin band at cm -1 n= x y z x+y x x+z 2y y+z 2z 2269 n CO (A) J. Lin, Q. Wu, G. Liang, L. Zu; RSC Adv. 2, 583589 (2012) 7 DF Spectra of 4-MCHO Pumped LIF bands (with respect to origin band at cm -1 ) n= b c d e f b' n=0 1 2 n CO (A) n CO (A)+ pump (A) 3 CO (A) 2 CO (A) 1 CO (A) E=76(5) 8 f e d b c b' Proof of Shift effect for 4-MCHO 9 Duschinsky Mixing The excited state PES is (i) displaced, (ii) distorted, (iii) but not rotated relative to the ground state. The excited state PES is (i) displaced, (ii) distorted, and (iii) rotated relative to the ground state. C. W. Muller, J. J. Newby, C. Liu, C. P. Rodrigo, T. S. Zwier; Phys. Chem. Chem. Phys. 12, 23312343 (2010) Vibrational modes Q 1 and Q 2 of the ground electronic state are mixed in the excited state (Q 1 and Q 2 ). 10 LIF/DF Spectra of 3-MCHO E=62(5) Pumped LIF band: origin band at cm -1 n=013 2 n CO (X/A) J. Lin, Q. Wu, G. Liang, L. Zu; RSC Adv. 2, 583589 (2012) 11 n= b c b' c' n=0 1 2 * * * * * * * * * DF Spectra of 3-MCHO Pumped LIF bands (with respect to origin band at cm -1 ) 680 cm -1 n CO (X/A) n CO (A)+ pump (X/A) n CO (A)+ pump (X/A) E=62(5) 12 CO + * =0 CO ** r CO Energy CO + * =0 CO ** ~ ~ 3-MCHO: Vibrational Population Relaxation in State B X/A 680 cm -1 B 13 LIF/DF Spectra of 2-MCHO E=110 (5) Pumped LIF band: origin band at cm -1 n=013 2 n CO (X/A) J. Lin, Q. Wu, G. Liang, L. Zu; RSC Adv. 2, 583589 (2012) 14 n= b c d e n=0 1 2 f DF Spectra of 2-MCHO Pumped LIF bands (with respect to origin band at cm -1 ) n CO (X/A) n CO (A)+ pump (X/A) E=110 (5) 15 FCF Simulation Mozhayskiy, V. A.; Krylov, A. I. ezSpectrum.n CO (X/A) Duschinsky mixing is taken into account in the simulation 16 LIF/DF Spectra of 1-MCHO E=89 (5) Pumped LIF band: chair-axial origin band at cm -1 n= n C-CH 3 (A) n= n C-CH 3 ( X) + C-O ( X) n C-CH 3 (A) + C-O (A) n C-O (A) n C-CH 3 (A) +2 C-O (A) Origin band of Chair-axial Origin band of Chair-equatorial Q. Wu, G. Liang, L. Zu; J. Phys. Chem. A 116, 31563162 (2012) r C-O (B) - r C-O (X) (m)249 r C-CH3 (B) - r C-CH3 (X) (m)35 1 C-O stretch (X) (cm -1 ) 1 C-O stretch (A) (cm -1 ) C-CH3 stretch (X) (cm -1 ) 1 C-CH3 stretch (A) (cm -1 ) Multiple Peak Fitting C-CH 3 ( X) + C-O ( X) 3 C-CH 3 (A) 1 C-CH 3 (A) + C-O (A) AssignmentsRedshifts from pumped LIF bandFrequencies with respect to 1138, , , 1103, 1619, 2125, 2639, 3172, , 1014, 1530, 2036, 2550, 3083, , 2155, 2665, 3211, , 2066, 2576, 3122, , 2077, 2590, 3109, 3626 N/A Tentative Assignments for 1-MCHO (chair-axial origin band) 18 Summary DF spectra of MCHOs provide vibronic structures of the nearly degenerate X and A states, especially the energy separation between the vibrational ground levels of these two states. The DF spectra are dominated by- o C-O stretch progression when the origin and the CO-stretch bands of the BX LIF excitation spectra were pumped. o progressions of CO-stretch modes combined with the pumped vibrational mode when non-CO-stretch vibrational levels of the B state were pumped. The transition intensities can be explained on the basis of FCFs taking Duschinsky mixing into consideration. Excited state vibrational population relaxation has been observed in the DF spectra of 3-MCHO. Calculated FCFs well reproduce the DF spectrum of 4-MCHO but not as well for the other isomers. Future Work Future Work DF spectra of 1-MCHO. High resolution LIF spectroscopy with rotational resolution. Ab initio calculations of X/A state PESs. Theoretical investigation of pJTE in MCHOs and its effect on vibronic structure. Isomers4-MCHO3-MCHO2-MCHO1-MCHO (chair-axial) E (A-X) (cm -1 ) (Expt.) Acknowledgements Funding: Group Members: Former Member: Dr. Neil Reilly UMass Boston J. Alam, Md. A. Reza, A. Mason, N. J. Reilly, and J. Liu, J. Phys. Chem. A (2015) DOI: /acs.jpca.5b03454