1. JET-COOLED LASER-INDUCED FLUORESCENCE SPECTROSCOPY OF T-BUTOXY NEIL J. REILLY* and JINJUN LIU Department of Chemistry University of Louisville TERRY A. MILLER Laser Spectroscopy Facility Department of Chemistry The Ohio State University LAN CHENG and JOHN F. STANTON Department of Chemistry The University of Texas, Austin International Symposium on Molecular Spectroscopy University of Illinois Urbana-Champaign 06/26/15 * Current address: Department of Chemistry, Marquette University.

2. Outline  Introduction  Computational  Quantum chemical calculations  Franck-Condon factor (FCF) calculations  Experimental  Laser-induced fluorescence (LIF) spectroscopy Vibrationally resolved Rotationally and fine-structure resolved  Dispersed fluorescence (DF) spectroscopy  Results and Discussion  Summary and Future works

3. Introduction  Alkoxy radicals (RO) are key intermediates in the oxidation of hydrocarbons both in combustion and in the atmosphere.  Lowest-energy electronic states of alkoxy radicals are either degenerate or nearly degenerate. They are subject to Jahn-Teller effect (JT) or pseudo- Jahn-Teller effect (pJTE), as well as spin-orbit (SO) interaction. * “The Atmospheric Chemistry of Alkoxy Radicals”, J. J. Orlando, G. S. Tyndall, T. J. Wallington, Chem. Rev. 103, 4657 (2003). * “Quantitative Insights about Molecules Exhibiting Jahn-Teller and Related Effects”, T. Barckholtz and T. A. Miller, Int'l. Rev. of Phys. Chem. 17 435 (1998). t-butoxymethoxy ？

4. Quantum Chemical Calculations  Method and basis set: (SA-)CASSCF(7,5)/6-31+G*  Results:  Geometries.  Rotational constants.  Vibrational modes and frequencies.  A-X transition frequency.  Frank-Condon factors for both A  X and A  X transitons.† pxpx pypy pzpz pz*pz* 2s(O) * CASSCF=Complete Active Space Self-Consistent Field † Mozhayskiy, V. A.; Krylov, A. I. ezSpectrum. http://iopenshell. usc.edu/downloads BCr CO ν CO TeTe (GHz) (Å)(cm -1 ) 4.8674.5121.45936 31666 4.4714.6281.69544

5. Moderate-resolution LIF/DF Apparatus Nd:YAG Pulsed Dye Laser Computer Photolysis Laser Doubling Crystal Vacuum Chamber t-butyl nitrite/He Δν~0.1 cm -1 Nd:YAG ν x3 ν x2 Excitation Laser PMT Box- Car spectrograph iCCD Δν~30 cm -1 T~1K Photolysis Excitation Free jet expansion t-butyl nitite/He OGC PMT=photomultiplier tube; OGC=optogalvanic cell iCCD=intensified charge coupled device LIF: laser-induced fluorescence; DF: dispersed fluorescence

6. 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 t-butyl nitrite/He Δν~0.1 cm -1 Nd:YAG ν x3 ν x2 Excitation Laser PMT Box- Car spectrograph iCCD Δν~30 cm -1 OGC DF LIF LIF: laser-induced fluorescence DF: dispersed fluorescence

7. Vibrationally Resolved LIF Spectrum * Impurity T 00 =25869.3 cm -1 * v=0 1 2 3 4 ν ’ 7 : CO stretch

8. Assignment of LIF Spectrum * Impurity * a 1 modes: ν6ν6 sym. C-(CH 3 ) 3 stretch ν7ν7 CO stretch ν8ν8 C-(CH 3 ) 3 umbrella e modes: ν 21 asym. C-(CH 3 ) 3 stretch ν 22 C-(CH 3 ) 3 scissoring ν 23 C-(CH 3 ) 3 rock ν 24 methyl internal rotation State: a 2 modes: ν 12 methyl internal rotation 848 cm -1

9. e 1/2 e 3/2 Vibronic Structure and Selection Rules v"=0 1 st -order JTE 2 rd -order JTE SO a1a1 e e e 1/2 e 3/2 v" g =1 a1a1 e e v" u =1 e e a 1 +a 2 e 3/2 e 1/2 e a2a2 a1a1 |j|, j=l+½Λ 1/2 3/2 1/2 a1a1 v'=0

10. DF Spectra Pumping Origin and CO Stretch Bands v=0 1 2 ν ” 6 : CO stretch aζ e d =36 59 80 cm -1 Pumped LIF Bands: ν ’ 7 : CO stretch

11. Assignment of DF Spectrum LIF Bands Pumped: ? a 1 modes: ν6ν6 CO stretch ν7ν7 sym. C-(CH 3 ) 3 stretch ν8ν8 C-(CH 3 ) 3 umbrella e modes: ν 21 asym. C-(CH 3 ) 3 stretch ν 22 C-(CH 3 ) 3 scissoring ν 23 C-(CH 3 ) 3 rock ν 24 methyl internal rotation State: a 2 modes: ν 12 methyl internal rotation

12. DF Spectra Pumping Non-CO-stretch Bands X2EX2Ee modes:A2A1A2A1 ν 21 asym. C-(CH 3 ) 3 stretch ν 21 ν 22 C-(CH 3 ) 3 scissoring ν 22 ν 23 C-(CH 3 ) 3 rock ν 23 ν 24 methyl internal rotation ν 24 X2EX2Ea 1 modes:A2A1A2A1 ν6ν6 CO stretch ν7ν7 ν7ν7 sym. C-(CH 3 ) 3 stretch ν6ν6 ν8ν8 C-(CH 3 ) 3 umbrella ν8ν8 LIF Bands Pumped: (+779) (+1319) (+1077) X2EX2Ea 2 modes:A2A1A2A1 ν 12 methyl internal rotation ν 12

13. DF Spectra Pumping Non-CO-stretch Bands LIF Bands Pumped: (+313) (+848) (+1077) X2EX2Ee modes:A2A1A2A1 ν 21 asym. C-(CH 3 ) 3 stretch ν 21 ν 22 C-(CH 3 ) 3 scissoring ν 22 ν 23 C-(CH 3 ) 3 rock ν 23 ν 24 methyl internal rotation ν 24 X2EX2Ea 1 modes:A2A1A2A1 ν6ν6 CO stretch ν7ν7 ν7ν7 sym. C-(CH 3 ) 3 stretch ν6ν6 ν8ν8 C-(CH 3 ) 3 umbrella ν8ν8 X2EX2Ea 2 modes:A2A1A2A1 ν 12 methyl internal rotation ν 12

14. DF Spectra Pumping Non-CO-stretch Bands LIF Bands Pumped: (+768) (+1077) X2EX2Ee modes:A2A1A2A1 ν 21 asym. C-(CH 3 ) 3 stretch ν 21 ν 22 C-(CH 3 ) 3 scissoring ν 22 ν 23 C-(CH 3 ) 3 rock ν 23 ν 24 methyl internal rotation ν 24 X2EX2Ea 1 modes:A2A1A2A1 ν6ν6 CO stretch ν7ν7 ν7ν7 sym. C-(CH 3 ) 3 stretch ν6ν6 ν8ν8 C-(CH 3 ) 3 umbrella ν8ν8 X2EX2Ea 2 modes:A2A1A2A1 ν 12 methyl internal rotation ν 12

15. Spin-orbit (SO) Interaction The Jahn-Teller interaction quenches the spin-orbit interaction and vice versa, because both of them are “competing” for the orbital angular momentum. vibrational A.M. orbital A.M. spin JT SO a : atomic-like SO ζ e : electronic reduction factor, due to lower symmetry (away from the cylindrical limit) in molecule d : Ham reduction factor, due to JTE CH 3 OCD 3 Ot-butoxy aζ e d (expt.) 6155 aζ e d (expt.) 36 aζeaζe 133 aζeaζe 117 ν4ν4 32922443 ν 21 1009 d (ν 4 only) ~1 d (ν 21 only) ~1 ν5ν5 16241170 ν 22 472 d (ν 5 only) 0.550.58 d (ν 22 only) 0.51 ν6ν6 1186892 ν 23 353 d (ν 6 only) 0.900.75 d (ν 23 only) 0.75 d 0.480.42 d 0.32 aζedaζed 6457 aζedaζed 37 d ( ν 22 only)*d ( ν 23 only)=0.39 v.s. d=0.32 Bilinear JT effect CH 3 OCD 3 Ot-butoxy aζ e d (expt.) 6155 aζ e d (expt.) 36

16. Summary and Future Work  Moderate-resolution LIF and DF spectra of t-butoxy unravel the vibrational structures of its A 2 A 1 and X 2 E electronic states, respectively.  The LIF and DF spectra are dominated by transitions to vibrational modes that resemble the six modes of CH 3 O, as well as internal rotations.  Rotational constants of both states have been determined in simulating the high-resolution LIF spectrum.  The experimentally determined spin-orbit splitting of the X 2 E state is significantly smaller than that of CH 3 O. 36 v.s. 61 cm -1, which may partially be explained by the bilinear Jahn-Teller effect that involves the methyl rock and scissoring modes.  Spin-orbit splitting increases with increasing vibrational quantum number of the CO-stretch mode.  Moderate-resolution LIF and DF spectra of t-butoxy unravel the vibrational structures of its A 2 A 1 and X 2 E electronic states, respectively.  The LIF and DF spectra are dominated by transitions to vibrational modes that resemble the six modes of CH 3 O, as well as internal rotations.  Rotational constants of both states have been determined in simulating the high-resolution LIF spectrum.  The experimentally determined spin-orbit splitting of the X 2 E state is significantly smaller than that of CH 3 O. 36 v.s. 61 cm -1, which may partially be explained by the bilinear Jahn-Teller effect that involves the methyl rock and scissoring modes.  Spin-orbit splitting increases with increasing vibrational quantum number of the CO-stretch mode.  Rotational spectra of other LIF bands.  Ab initio calculations of vibronic structure of the X 2 E state.  Rotational spectra of other LIF bands.  Ab initio calculations of vibronic structure of the X 2 E state.

17. Summary  Moderate-resolution LIF and DF spectra of t-butoxy unravel the vibrational structures of its A 2 A 1 and X 2 E electronic states, respectively.  The LIF and DF spectra are dominated by transitions to vibrational modes that resemble the six modes of CH 3 O, as well as internal rotations.  The experimentally determined spin-orbit splitting of the X 2 E state is significantly smaller than that of CH 3 O. 36 v.s. 61.5 cm -1, which may partially be explained by the bilinear Jahn-Teller effect that involves the methyl rock and scissoring modes.  Spin-orbit splitting increases with increasing vibrational quantum number of the CO-stretch mode.  Moderate-resolution LIF and DF spectra of t-butoxy unravel the vibrational structures of its A 2 A 1 and X 2 E electronic states, respectively.  The LIF and DF spectra are dominated by transitions to vibrational modes that resemble the six modes of CH 3 O, as well as internal rotations.  The experimentally determined spin-orbit splitting of the X 2 E state is significantly smaller than that of CH 3 O. 36 v.s. 61.5 cm -1, which may partially be explained by the bilinear Jahn-Teller effect that involves the methyl rock and scissoring modes.  Spin-orbit splitting increases with increasing vibrational quantum number of the CO-stretch mode.

18. Acknowledgements Funding: Current Group Members: Former Members: Dr. Neil Reilly UMass Boston Dr. Bill Pandit Northwestern Collaborators: Terry A. Miller John F. Stanton Lan Cheng

19. Vibrational Frequencies: Experimental v.s. Calculated sym. description X2EX2EA2A1A2A1 methoxy notation cal’d (harmonic ) expt. (j=1/2) cal’d expt. centerSO v=0036 a1a1 sym. C-(CH 3 ) 3 stretchν7ν7 780 74035 ν6ν6 834779ν1ν1 a1a1 C-(CH 3 ) 3 umbrellaν8ν8 433 40431 ν8ν8 427432ν2ν2 a1a1 CO stretchν6ν6 936 90360 ν7ν7 544 ν3ν3 a2a2 methyl internal rotationν 12 212 218 -ν 12 207- easym. C-(CH 3 ) 3 stretchν 21 1009--ν 21 981-ν4ν4 eC-(CH 3 ) 3 scissoringν 22 472 50965 ν 22 445-ν5ν5 eC-(CH 3 ) 3 rockν 23 353 33325 ν 23 330313ν6ν6 emethyl internal rotationν 24 278 25812 ν 24 264231

20. CH 2 DO 2-butoxy Effect of Methyl Substitution on JT & pJT Effects ethoxyiso-propoxy t-butoxymethoxy in cm -1 * Quadratic JTE and SO splitting not shown in potential energy surfaces (PESs).

21. FCF Calculations descriptionMethoxy counterpart aCO stretchν3ν3 bsymmetric C-(CH 3 ) 3 stretchν1ν1 cC-(CH 3 ) 3 umbrellaν2ν2 dmethyl internal rotation easymmetric C-(CH 3 ) 3 stretchν4ν4 fC-(CH 3 ) 3 scissoringν5ν5 gC-(CH 3 ) 3 rockν6ν6

22. Ar + Laser Excimer Laser (XeCl) Pulsed Dye Amplifier CW Ring Dye Laser Computer Photolysis Laser PMT Doubling Crystal Vacuum Chamber Etalon Chopper PD Calibration System I2I2 Lock- in t-butyl nitrite/He High-resolution LIF Apparatus Δν~100 MHz Δν<1 MHz @ the Ohio State University Excimer Laser (XeF) Box- Car PMT=photomultiplier tube; PD=photodiode

23. @ Ohio State Rotationally Resolved LIF Spectra LIF Band: Effective Hamiltonian X2EX2EA2A1A2A1 fitcal’dfitcal’d C (GHz)4.491(5)4.5124.464(5)4.471 B (GHz)4.913(3)4.8674.665(2)4.628 Cζ t - cc /4 (GHz)0.742(4) ( aa+bb )/2(GHz)0fixed aζ e d (cm −1 )-36fixed T v (cm −1 )26928.8377(7)

24. Summary and Future Work  Moderate-resolution LIF and DF spectra of t-butoxy unravel the vibrational structures of its A 2 A 1 and X 2 E electronic states, respectively.  The LIF and DF spectra are dominated by transitions to vibrational modes that resemble the six modes of CH 3 O, as well as internal rotations.  The experimentally determined spin-orbit splitting of the X 2 E state is significantly smaller than that of CH 3 O. 36 v.s. 61.5 cm -1, which may partially be explained by the bilinear Jahn-Teller effect that involves the methyl rock and scissoring modes.  Spin-orbit splitting increases with increasing vibrational quantum number of the CO-stretch mode.  Moderate-resolution LIF and DF spectra of t-butoxy unravel the vibrational structures of its A 2 A 1 and X 2 E electronic states, respectively.  The LIF and DF spectra are dominated by transitions to vibrational modes that resemble the six modes of CH 3 O, as well as internal rotations.  The experimentally determined spin-orbit splitting of the X 2 E state is significantly smaller than that of CH 3 O. 36 v.s. 61.5 cm -1, which may partially be explained by the bilinear Jahn-Teller effect that involves the methyl rock and scissoring modes.  Spin-orbit splitting increases with increasing vibrational quantum number of the CO-stretch mode.  Rotational spectra of more LIF bands.  Ab initio calculations of vibronic structure of the X 2 E state.  Rotational spectra of more LIF bands.  Ab initio calculations of vibronic structure of the X 2 E state. LIF Band: