Solvent-assisted conformational isomerization (SACI) is believed to drive the population into the more stable isomer. While both conformers are found in the jet-cooled spectra of 3-aminophenol (3AP), only the trans isomer was found in the REMPI spectrum of the 3AP(NH3)1 cluster. It was suggested that the cis conformer of the cluster isomerizes to the more stable trans conformer in the ground state during supersonic expansion. SACI also occurs for the 3AP monomer, reducing 50% of the cis/trans ratio when the ammonia concentration in the expansion is higher than 0.1%. Depending on the expansion condition, the cis conformer can be completely depleted. In order to establish this theory, SACI in meta-substituted phenols with substituents of varying electron donating/withdrawing properties (m-anisidine, m-chlorophenol, and m-cyanophenol) was also studied in the gas phase. By changing the substituent, we could tune relative stability of conformers, isomerization barrier between them, and intermolecular vibrational energy relaxation rate. SACI of m-anisidine (m-methoxyaniline) occurs when the binding energy of solvent is higher that the isomerization barrier, which is very similar to that of 3-aminophenol. When two conformers have similar relative energy as in m-chlorophenol, SACI does occur because forward and backward isomerization rates are similar. Inter- and intra-molecular vibrational energy redistribution rate plays an important role in addition to the binding energy. Acetaminophen that is widely used as a pain reliever and a fever reducer was also investigated to apply SACI in a real system. SACI also changes the population of the less stable conformer of acetaminophen into the more stable one by water, and the barrier between the two conformers was inferred from the binding energy of the solvents. It suggests a mechanism that acetaminophen isomerizes into a single conformer in vivo, which is need for molecular recognition and drug activity.
