Protoisomerization of Indigo and Isoindigo Dyes Confirmed by Gas-Phase Infrared Ion Spectroscopy.

Gas-phase infrared multiple-photon dissociation (IRMPD) spectra are recorded for the protonated dye molecules indigo and isoindigo by using a quadrupole ion trap (QIT) mass spectrometer coupled to the free electron laser for infrared experiments (FELIX). From their fingerprint IR spectra (600-1800 cm) and comparison with quantum-chemical calculations at the density functional level of theory (B3LYP/6-31++G(d,p)), we derive their structures. We focus particularly on the question of whether -to- isomerization occurs upon protonation and transfer to the gas phase. The -configuration is energetically favored in the neutral forms of the dyes in solution and in the gas phase. Instead, the -isomer is lower in energy for the protonated forms of both species, but indigo is also notorious for not undergoing double-bond -to- isomerization, in contrast to many other conjugated systems. The IR spectra suggest that protoisomerization from to indeed occurs for both dyes. To estimate the extent of isomerization, on-resonance kinetics are measured on diagnostic and common vibrational frequencies to determine the ratio of -to- isomers. We find ratios of 65-70% and 30-35% for indigo versus 75-80% and 20-25% for isoindigo. Transition-state calculations for the isomerization reactions have been carried out, which indeed suggest a lower barrier for protonated isoindigo, qualitatively explaining the more efficient isomerization.