Rotational Spectra of Two Hydrogen-Bonded Methyl Salicylate Monohydrates: Relative Stability and Tunneling Motions

The mechanism for the dual fluorescence of methyl salicylate (MS) is a long-standing research topic. Two distinctly different hydrogen-bonded MS water complexes, one with the carbonyl O and the other with the hydroxy O serving as the hydrogen-bond acceptor, were detected using chirped pulse and cavity-based Fourier transform microwave spectroscopy and found to be of comparable stability. This observation provides direct experimental evidence for the previously undetected hydroxy O-bonded isomer, supporting the recently proposed new mechanism for the dual fluorescence of MS in protic solvents. An interesting interplay between the methyl internal rotation and water tunneling motions was also investigated. The existence of an unconventional O center dot center dot center dot H-C hydrogen-bond was verified using the atoms-in-molecules theory. Thorough searches for the elusive ketoA, a higher energy form of MS, were unsuccessful. This result supports recent theoretical reports that only one MS species exists in the ground state.