Rotational Spectrum of the Methyl Salicylate Water Complex:
Rotational Spectrum of the Methyl Salicylate Water Complex: The Missing Conformer and the Tunneling Motions Supriya Ghosh, Javix Thomas, Yunjie Xu, and Wolfgang Jger Department of Chemistry, University of Alberta, Edmonton, Canada Methyl Salicylate (Oil of Wintergreen) Occurs in leaves of plants (abundant in Sweet Birch and Eastern Teaberry or Wintergreen)
Used in ointments to relief minor aches and pains (Ben Gay, Icy Hot). 2 Atmospheric Significance Methyl salicylate is a biogenic volatile organic compound (VOC) Forms secondary organic aerosols in both gaseous and aqueous phases Is a surfactant and enhances activity of cloud condensation nuclei
Chemical Communication Some Plants release volatile chemicals in stress conditions (cold, heat, drought, insects, disease). This triggers defensive response in themselves and in neighbouring plants. Intraspecific signalling Auto-signalling Interspecific signalling Victim of attack
http://www.howplantswork.com/2011/07/16/talking-plants-airborne-chemical-signals-part-2/ Sang-Wook et al. Science 318, 113 (2007); Vladimir et al. Nature 385, 718 (1997). 4 Dual Fluorescence in Methyl Salicylate Fluorescence Spectra A. Weller, Z. Elektrochem. 60, 1144 (1956). Proton-Transfer in Methyl-Salicylate ketoB S1
enol S1 A. Weller, Z. Elektrochem. 60, 1144 (1956). No Barrier in the Excited State! Barrier height between the excited state isomers is very low No equilibrium exists between the isomers ketoB ketoB S1 enol S1
Cannot explain the origin of the near-UV peak at 361 nm. Cataln, J. Phys. Chem. Chem. Phys. 14, 8903 (2012). 7 Alternate Mechanism Two different ground state conformers are responsible for dual fluorescence. Cataln, J. Phys. Chem. Chem. Phys. 14, 8903 (2012). Energy barrier: 15 kcal/mol
ketoB 0.0 kcal/mol ketoA 2.4 kcal/mol Caminati and coworkers found only ketoB using free-jet millimeter-wave spectroscopy. S. Melandri et al., J. Phys. Chem. A 111, 9076 (2007). 8 ESIPT in Different Solvents
Dual fluorescence is only observed in protic and in polar solvents. Intensity of near UV band (350 nm) decreases with decreasing strength of inter- molecular HB. Intensity of near UV band for alcoholic solvents are slightly larger than non H-bonded system. Solute-solvent H-bonding affects dual florescence Zhou, P.; Hoffmann, M. R.; Han, K.; He, G. J. Phys. Chem. B 119, 2125 (2015).
9 New Proposal Involves H-Bonded Solvent Molecules enol-methA-S1 -> blue band ketoB-methB-S1 -> near UV band Responsible for dual fluorescence Zhou, P.; Hoffmann, M. R.; Han, K.; He, G. J. Phys. Chem. B 119, 2125 (2015).
10 Conformers of Methyl Salicylate - Water De: D0: D0BSSE: 0.15 0.00 0.00 0.35
0.00 11.48 10.89 13.71 13.42 15.01 kJ/mol 14.03 kJ/mol Broadband Spectra of Methyl Salicylate - H2O Conformer II
Intensity Conformer I Experiment * * * Frequency / GHz
12 Splittings of Rotational Transitions A E 3,3,1-2,2,0 transition of Conformer I 8,4,5-7,4,4 transition of Conformer II
Spectroscopic Parameters Splittings were observed for both conformers No c-type transition were observed for conformer II (c-dipole moment component 0.9 Debye ) 14 Methyl Internal Rotation Conformer I Conformer II V3 barrier
ab initio: 8.63 kJ/mol Experiment: 8.15 kJ/mol V3 barrier ab initio: 5.86 kJ/mol MP2/6-311++G(2d,p) level of theory 15 Different Splittings for Conformer II 8,2,6-7,2,5 transition of Conformer II
Most transitions of conformer II show such splitting. 16 H2O Rotation about O-H Bond (Conformer II) Energy barrier (ZPE and BSSE corrected) = -0.67 kJ/mol Large amplitude motion Consistent with absence of c-type transitions 17 H2O Rotation about Lone Pair O Axis
Energy barrier (ZPE and BSSE corrected) = 4.62 kJ/mol Corresponding barrier for conformer I is 8.6 kJ/mol 18 Summary Two conformers of methyl salicylate water were identified. This supports the new proposal for the dual fluorescence signature of methyl salicylate in protic solvents. The search for the missing ground state conformer of methyl salicylate was not successful.
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