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

J. Chem. Phys. 122, 214312 (2005); doi:10.1063/1.1924455

Published 7 June 2005

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Jasper R. Clarkson, Esteban Baquero, and Timothy S. Zwier
Department of Chemistry, 560 Oval Drive, Purdue University, West Lafayette, Indiana 47907-2084
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 Ebarrier(A-->B)=771–830  cm–1 and Ebarrier(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. ©2005 American Institute of Physics
History: Received 7 March 2005; accepted 1 April 2005; published 7 June 2005
Permalink: http://link.aip.org/link/?JCPSA6/122/214312/1
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KEYWORDS and PACS

Keywords
PACS
  • 82.30.Qt
    Isomerization and rearrangement in chemical reactions
  • 82.80.Gk
    Chemical analytical methods involving vibrational spectroscopy
  • YEAR: 2005

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ISSN:
0021-9606 (print)   1089-7690 (online)
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REFERENCES (20)
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