Kinetics and Spectroscopy of the Gas Phase (CH 3 ) 2 S – Br Adduct

1. Kinetics and Spectroscopy of the Gas Phase (CH3)2S–Br Adduct V. Dookwah-Roberts1, R.J.H. Lee2, J.M. Nicovich2, and P.H. Wine1,2 1 School of Earth & Atmospheric Sciences 2 School of Chemistry & Biochemistry Georgia Institute of Technology 4th EAS Graduate Student Symposium, Atlanta GA, November 10th 2006

2. MOTIVATION ●DMS (CH3SCH3) accounts for 10-30 % of the total sulfur flux to the atmosphere. ●DMS oxidation impacts cloud formation over oceans => climate effects. ● BrOx radicals represent one important DMS oxidant in the marine environment.

3. Literature Summary: Br + DMS ↔ Br-DMS ●Wine et al., in NATO ASI Ser., Vol I7, Ed. by H. Niki & K.H. Becker, 1993, 385. ● Ingham et al., J. Phys. Chem. A 1999, 103, 7199 ● Nakano et al., J. Phys. Chem. A 2001, 105, 11045

4. Goals of this Study ●1st study of adduct kinetics ● Map spectrum in region 300 – 500nm. Compare results with Ingham et al. and Nakano et al. ● Assess reactivity trends in reactions of weakly-bound halogen atom adducts with atmospheric gases.

5. Experimental Approach ●Laser flash photolysis − time−resolved UV−vis absorption spectroscopy ●T = 265 K; P = 200 Torr N2/O2 ●Source of Br in all experiments: CF2Br2 + hν(248nm) →Br + CF2Br

6. Time−resolved absorption data265 K, 200 TorrN2, λ = 365 nm The observed absorption is coming from a product of Br + CH3-S-CH3 (almost certainly the CH3S(Br)CH3 adduct).

7. Absorption Spectrum • at 338 nm is much smaller than found by Nakano et al. but spectrum agrees well with Ingham et al.

8. Radical − Radical Reaction Kinetics Average of 26 expts. like the one shown on the right gives 2k = (3.0 ± 0.3) x 10−10 cm3molec−1 s−1 (uncertainty is 2σ, precision only). T = 265 K, P = 200 Torr N2 [Br-DMS] / [Br] ~ 178 Slope (red line) gives 2k = (3.29 ± 0.04) x 10−10 cm3molec−1s−1 (Br-DMS + Br-DMS → products)

9. Adduct + O2 Reaction Kinetics T = 265 K; P = 200 Torr O2; [DMS] = 1.6 x 1015 per cm3; [Br]0 ≈ 1 x 1013 per cm3. Simulations suggest that upward curvature in the [Br-DMS]−1 vs. time plot would be observable if the Br-DMS + O2 rate coefficient was > 1 x 10−17 cm3molec−1s−1.

10. Adduct + NO/NO2Reaction Kinetics Experimental conditions of low T (265 K), P (200 Torr N2), low [Br]0, and high [DMS] are employed to minimize interferences from radical − radical reactions and Br + NOx reactions.

11. Summary of Reaction Kinetics of Halogen Adducts with O2, NO and NO2 a. Kleissas, K.M.; Nicovich, J.M.; Wine, P.H. J. Photochem. In press. b. Dookwah-Roberts, V.; Soller, R.; Nicovich, J.M.; Wine, P.H. J. Photochem. 2005, 176, 114-123. c. Urbanski, S.P.; Wine, P.H. J. Phys. Chem. A 1999, 103, 10935-10944.

12. SUMMARY ●smax and lmax obtained agree well with Ingham et al. but s at 338 nm is a factor of 7 smaller than obtained by Nakano et al. ●Kinetic data obtained for 1st time. ●Rate constants similar to other adducts studied eg. SCS-Cl, DMS-Cl, DMSO-Cl.

13. Future Directions ●Conduct pressure dependent study of BrO + DMS reaction kinetics.

14. Acknowledgments Support $$$ NSF & NASA Research Group Members (non-authors) Zhijun Zhao Dow Huskey Andrew Mudd Patrice Bell Katie Olsen

15. Comparative Spectroscopy ________________________________________________________________________________________________________________________________ Adduct λmax (nm) FWHM (nm) σmax (10−18 cm2) a _____________________________________________________________________________________________________________________ SCS−Cl b 365 23 23 480 85 3.9 (CH3)2S−Cl c,d 340 75 35 (CH3)2(O)S−Cl e 390 100 21 (CH3)2S−Br f 365 75 27 ________________________________________________________________________________________________________________________________ a Uncertainties are typically 30−40%. b Dookwah-Roberts, Soller, Nicovich, and Wine, J. Photochem. Photobiol. A: Chem., submitted (this work). c Urbanski & Wine, J. Phys. Chem. A 1999, 103, 10935. d Enami, Nakano, Hashimoto, Kawasaki, Aloisio, & Francisco, J. Phys. Chem. A, 2004, 108, 7785. e Wine, Nicovich, McKee, Kleissas, Parthasarathy, Pope, &Pegus, 18th Intl. Symp. on Gas Kinetics, Bristol, UK, 2004, to be published. f Ingham, Bauer, Sander, Crutzen, & Crowley, J. Phys. Chem. A 1999, 103, 7199.

16. TO PUMP CS2 MONITOR PMT Zn LAMP TP BPF TP MONOCHROMATOR COOLANT in EXCIMER LASER HR 248 HV REACTION CELL PMT HR 248 Cl2CO MONITOR PG Cl2CO/N2 PMT FM PMT Zn LAMP N2 or O2 FM BPF MIXING CELL ARC LAMP CS2 FM COOLANT out FM NO/NO2/ N2 OSCILLOSCOPE COMPUTER LFP−TRUVVAS Apparatus

17. Comparative Kinetics _________________________________________________________________________________________________________________ Adduct kO2akNOa kNO2a _________________________________________________________________________________________________________________ SCS−Cl b < 5 x 10-5 220 130 (CH3)2S−Cl c < 4 x 10−5 120 270 (CH3)2(O)S−Cl d < 1 x 10−5 150 190 SCS−OH 0.29 e,f,g 7.3 g 420 g (CH3)2S−OH 9.6 h,i _________________________________________________________________________________________________________________ a Units are 10−13 cm3 molec−1 s−1. b Dookwah-Roberts, Soller, Nicovich & Wine, J. Photochem. Photobiol. A: Chem, submitted (this work). c Urbanski & Wine, J. Phys. Chem. A 1999, 103, 10935. d Wine, Nicovich, McKee, Kleissas, Parthasarathy, Pope, & Pegus, 18th Intl. Symp. on Gas Kinetics, Bristol, UK, 2004, to be published. e Hynes, Wine, & Nicovich, J. Phys. Chem. 1988, 92, 3846. f Murrells, Lovejoy, & Ravishankara, J. Phys. Chem. 1999, 2381. g Diau & Lee, J. Phys. Chem. 1991, 95, 7726. h Hynes, Stoker, Pounds, McKay, Bradshaw, Nicovich, & Wine, J. Phys. Chem.1995, 99, 16967. i Barone, Turnipseed, and Ravishankara, J. Phys. Chem. 1996, 100, 14694.