Direct measurement of the energy thresholds to conformational isomerization. II. 3-indole-propionic acid and its water-containing complex.

The methods of stimulated emission pumping-hole-filling spectroscopy (SEP-HFS) and population transfer spectroscopy (SEP-PTS) were used to place direct experimental bounds on the energetic barriers to conformational isomerization in 3-indole-propionic acid (IPA) and its water-containing complex. By contrast with tryptamine (Paper I), IPA has only two conformations with significant population in them. The structures of the two conformers are known from previous work [P. M. Felker, J. Phys. Chem. 96, 7844 (1992)]. The energy thresholds for A→B and B→A isomerizations are placed at 854 and 754cm−1, respectively. Lower bounds on the isomerization barrier in the two directions are determined from the last transition not observed in the SEP-PT spectra. These are placed at 800 and 644cm−1 for A→B and B→A, respectively. The combined results place bounds on the relative energies of the A and B minima, with E(B)−E(A)=46–210cm−1. Like the IPA monomer, the IPA-H2O complex forms two conformational isomers. Both these i...