JET-COOLED LASER-INDUCED FLUORESCENCE SPECTROSCOPY OF LARGE SECONDARY ALKOXY RADICALS 06/21/10 JINJUN LIU, MING-WEI CHEN, AND TERRY A. MILLER Laser Spectroscopy Facility Department of Chemistry The Ohio State University

Motivation I: Hydrocarbon oxidation * J. J. Orlando, G. S. Tyndall, T. J. Wallington, Chem. Rev. 103, 4657 (2003)

methoxy CH 3 O CD 3 O CH 2 DO CHD 2 O ethoxy iso-propoxy 1-propoxy 2-butoxy …… Motivation II: Jahn-Teller and pseudo-Jahn-Teller effects * J. Liu et al., JCP, 130, (2009). * D. Melnik et al., JCP, submitted * X. Q. Tan et al., JCP, 97, 9311 (1993) * J. Liu et al., 65th MSS, TG14 (2010). * S. Gopalakrishnan et al., JCP, 118, 4954 (2003). * V. Stakhursky et al., JCP, 125, (2006)

Motivation III: Conformers of alkoxy radicals Φ 1 =O-C 1 -C 2 -C 3 ; Φ 2 =C 1 -C 2 -C 3 -C 4 ; Φ 3 =C 2 -C 3 -C 4 -C 5 … N=3 N=9 N=27 For secondary alkoxy radicals, N=3 (n-3) N: number of conformers; n: number of carbon atoms. G + = gauche clockwise (Φ ~ 60 o ) T= trans (Φ ~ 180 o ) G - = gauche counterclockwise (Φ ~ -60 o )

Vacuum Chamber Experimental apparatus I: Low-resolution LIF RONO +He/Ne PhotolysisExcitation Δν~3 GHz Doubling Crystal (SHG) PMT T~1K

Frequency (cm -1 ) LIF spectra of primary alkoxy radicals G T Tco Gco Tcco 1-propoxy 1-butoxy 1-pentoxy 1-hexoxy 1-heptoxy 1-octoxy 1-nonoxy 1-decoxy G1T2G1T2 T1T2T1T2 T 1 T 2 co G1T2T3G1T2T3 T1T2T3T1T2T3 T 1 T 2 T 3 co T1G2T1G2 T 1 T 2 cco T 1 G 2 cco T1T2G3T1T2G3 T1G2T3T1G2T3 T 1 T 2 T 3 cco G 1 T 2 T 3 co T 1 G 2 T 3 cco T 1 T 2 G 3 co T 1 T 2 T 3 co+cco G1T2T3T4T5G1T2T3T4T5 G1T2T3T4G1T2T3T4 T1T2T3T4T1T2T3T4 T1T2T3T4T5T1T2T3T4T5 T 1 T 2 T 3 T 4 T 5 co T 1 T 2 T 3 T 4 co G1T2T3T4T5T6G1T2T3T4T5T6 G1T2T3T4T5T6G1T2T3T4T5T6 G1T2T3T4T5T6T7T8G1T2T3T4T5T6T7T8 T1T2T3T4T5T6T1T2T3T4T5T6 T1T2T3T4T5T6T7T1T2T3T4T5T6T7 T1T2T3T4T5T6T7T8T1T2T3T4T5T6T7T8 T 1 T 2 T 3 T 4 T 5 T 6 co T 1 T 2 T 3 T 4 T 5 T 6 T 7 co T 1 T 2 T 3 T 4 T 5 T 6 T 7 T 8 co G 1 T 2 co G 1 T 2 T 3CO

Low-resolution LIF spectra

Excimer Laser (XeCl) Pulse Dye Amplifier Ar + Laser CW Ring Dye Laser Computer XeF Photolysis Laser PMT Doubling Crystal Box- Car Etalon PD I2I2 Lock- in RONO+He Δν~100 MHz Experimental apparatus II: High-resolution LIF Δν~1 MHz

High-resolution LIF spectra of 2-pentoxy Type I: A, C 1, C 2 Type II: B, C 3, D 1 D 2, D 3 Type II (overlapping): D 4,5 D 6,7

High-resolution LIF spectra of 2-hexoxy Type I: A, C 1 Type II: B, D Type I (overlapping): C 2,3 C 4,5

High-resolution LIF spectra Type I Type II 2-pentoxy 2-hexoxy

ground state (X)excited state (B) Hamiltonian H = H Rot + H SR H = H Rot H Rot = AN a 2 + BN b 2 + CN c 2 H SR = ½  ab (N a S b + S b N a ) molecular constants rotational constants A", B", C" A', B', C' spin-rotation constants*  aa ",  bb ",  cc " ½(  ab " +  ba " ) transition types* ½(  ac " +  ca " ) a:b:c ½(  bc " +  cb " ) * Predicted based on the experimental results of iso-propoxy.      Spectroscopic model 

Quantum chemistry calculations 2-pentoxy 2-hexoxy A and X states: B3LYP/6-31+G(d) B state: CIS/6-31+G(d) * Scaled by [G. Tarczay, S. Gopalakrishnan, and T. A. Miller, J. Mol. Spectrosc. 220, 276 (2003).] ~ ~ ~

y z x y z x b a c b a c URUR USUS Spin-rotation tensor ethoxy (R) iso-propoxy (S) R: reference; S: substituted “orbital-fixed coordinate system”principal axis system

in cm -1 ethoxyiso-propoxy Calc. w/o ZPE w/o ΔE PJT (C s -C s ) w ΔE PJT (C 1 -C s ) w/ ZPE Expt. † +355(10)-68(10) Iso-propoxy v.s. ethoxy PJT: pseudo-Jahn-Teller ΔE PJT =98 cm -1 † " Dispersed Fluorescence Spectroscopy of Primary and Secondary Alkoxy Radicals," J. Jin, I. Sioutis, G. Tarczay, S. Gopalakrishnan, A. Bezant, T. A. Miller, J. Chem. Phys. 121, (2004). iso-propoxy ethoxy

Simulation with calculated/predicted parameters 2-pentoxy

Simulation using evolutionary algorithm (EA)* 2-pentoxy * W. L. Meerts and M. Schmitt Int. Rev. Phys. Chem., 25(3), 353–406 (2006)

Evolutionary algorithm (EA)* * W. L. Meerts and M. Schmitt Int. Rev. Phys. Chem., 25(3), 353–406 (2006)

Fit parameters 2-pentoxy G + T: from global fit of Bands A, C 1 and C 2 (Type I); TT: from global fit of Bands B and C 3 (Type II). [ ]: discrepancy between cal. and expt.; ( ): one standard deviation.

Simulation of overlapping bands 2-pentoxy, Band D 4,5 (CO stretch band of TT conformer) Frequency interval: δν~0.03 cm -1 D3D3

Origin CO stretch Low-resolution LIF spectra B D A B C D G + T G + CO T CO G + T TT G + T CO TT CO G + TTT TTTT G + TTT CO TTT CO G + TTTT TTTTT G + TTTT CO TTTT CO TTTTTT TTTTT CO G + TTTTT G + TTTTT CO TTTTTTT TTTTTT CO G + TTTTTT G + TTTTTT CO G + TT TTT G + TT CO TTT CO

Summary and Future work High-resolution rotationally resolved spectra of 2-pentoxy and 2-hexoxy have been obtained and simulated. Evolutionary algorithm was used for global fitting successfully. These spectra were assigned to all-Trans conformers or conformers with a G + OCCC dihedral angle. 2-pentoxy: G + T, TT; 2-hexoxy: G + TT,TTT. Good agreement between experimentally determined and calculated/predicted parameters has been found. Quantitative understanding of the pseudo-Jahn-Teller effect in secondary alkoxy radicals; Study of cyclic alkoxy radicals.

Prof. Dr. W. Leo Meerts Radboud University Nijmegen The Netherlands Prof. Dr. W. Leo Meerts Radboud University Nijmegen The Netherlands Thank you all! Thank you all! Miller Group Acknowledgements