1. Rotational Spectrum of the Methyl Salicylate – Water Complex: The Missing Conformer and the Tunneling Motions
Supriya Ghosh, Javix Thomas, Yunjie Xu, and Wolfgang Jäger
Department of Chemistry, University of Alberta, Edmonton, Canada

2. 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).

3. 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

4. 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
Intraspecific
signalling
Interspecific
signalling
Auto-signalling
Victim of attack
Sang-Wook et al. Science 318, 113 (2007); Vladimir et al. Nature 385, 718 (1997).

5. Dual Fluorescence in Methyl Salicylate
Fluorescence Spectra
A. Weller, Z. Elektrochem. 60, 1144 (1956).

6. Proton-Transfer in Methyl-Salicylate
ketoB – S1
enol – S1
A. Weller, Z. Elektrochem. 60, 1144 (1956).

7. 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.
Catalán, J. Phys. Chem. Chem. Phys. 14, 8903 (2012).

8. Alternate Mechanism
Two different ground state conformers are responsible for dual fluorescence.
Catalán, J. Phys. Chem. Chem. Phys. 14, 8903 (2012).
Energy barrier:
15 kcal/mol
ketoB
ketoA
0.0 kcal/mol
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).

9. 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).

10. 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).

11. Conformers of Methyl Salicylate - Water
ΔDe: kJ/mol
ΔD0: kJ/mol
ΔD0BSSE: kJ/mol
ketoB - waterB
ketoB - waterA
ΔDe: kJ/mol
ΔD0: kJ/mol

12. Broadband Spectra of Methyl Salicylate - H2O
Conformer I
Conformer II
Intensity
Experiment * * *
Frequency / GHz

13. Splittings of Rotational TransitionsE
3,3,1-2,2,0 transition
of Conformer I
8,4,5-7,4,4 transition
of Conformer II

14. Spectroscopic Parameters
Splittings were observed for both conformers
No c-type transition were observed for conformer II (c-dipole moment component 0.9 Debye )

15. 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/ G(2d,p) level of theory

16. Different Splittings for Conformer II
8,2,6-7,2,5 transition of Conformer II
Most transitions of conformer II show such splitting.

17. H2O Rotation about O-H Bond (Conformer II)
Energy barrier (ZPE and BSSE corrected) = kJ/mol
Large amplitude motion
Consistent with absence of c-type transitions

18. 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

19. 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.