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 754 cm^-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 644 cm^-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)=46–210 cm^-1. Like the IPA monomer, the IPA-H₂O complex forms two conformational isomers. Both these isomers incorporate the water molecule as a bridge between the carbonyl and OH groups of the carboxylic acid. Previous rotational coherence measurements (L. L. Connell, Ph.D. thesis, UCLA, 1991) have determined that these complexes retain the same IPA conformational structure as the monomers. SEP-PTS and SEP-HFS were carried out on the IPA-H₂O complexes. It was demonstrated that it is possible to use SEP to drive conformational isomerization between the two conformational isomers of IPA-H₂O. Bounds on the energy barriers to conformational isomerization are not effected greatly by the presence of the water molecule, with E_barrier(A→B)=771–830 cm^-1 and E_barrier(B→A)=583–750 cm^-1. This is a simple consequence of the fact that the barrier is an intramolecular barrier, and the water molecule is held fixed in the COOH pocket, where it interacts with the ring only peripherally during the isomerization process. Finally, changes in the SEP-PT spectral intensity in transitions near the top of the barrier to isomerization as a function of the position of SEP excitation relative to the pulsed valve exit provide some insight to the competition between vibrational relaxation and isomerization in a molecule the size of IPA. [ABSTRACT FROM AUTHOR]