Conformational Analysis of the Jet-Cooled Peptide Mimetic Ethylacetamidoacetate from Torsion-Rotation Spectra

Published

September 1, 2003

Author(s)

R J. Lavrich, Angela R. Hight Walker, David F. Plusquellic, I Kleiner, R D. Suenram, Jon T. Hougen, Gerald T. Fraser

Abstract

Rotational spectra of two conformers of the dipeptide mimetic, ethyl acetamidoacetate, were measured in a molecular beam using a Fourier-transform microwave spectrometer. In each conformer, internal rotation of the acetyl methyl group gives rise to observable splittings in the spectrum. From analysis of the torsion-rotation interactions, the methyl groups' orientations have been determined in the principal axis frames of both conformers and unambiguously identify their conformational forms. One conformer exists in the all-trans configuration and belongs to Cs point group and the second, higher-energy conformer has C₁ symmetry. Two separate theoretical fitting procedures are applied to assess the reliability of the structural information and are shown to be essentially equivalent. For example, methyl torsional barriers are 65.1(1) cm^-1 vs 66.7 (1) cm^-1 and 65.9(1) cm^-1 vs 67.0(1) cm^-1 for the Cs and C₁ conformers, respectively and principal axis orientations of the methyl groups agree to 0.4 . The small differences in the torsional barriers and rotor axis angles for the two conformers are a result of a change in the orientation of the ethyl group on the other end of the molecule. The predicted energy ordering of these two conformers at the MP2/6-311G(d,0) level of theory is in disagreement with experimental observations

Citation

Journal of Chemical Physics

Volume

119

Issue

No. 11

Pub Type

Journals

Keywords

conformational analysis, ethyl acetamidoacetate, least-squares fit, peptide mimetic, rotational spectrum

Citation

Lavrich, R. , Hight, A. , Plusquellic, D. , Kleiner, I. , Suenram, R. , Hougen, J. and Fraser, G. (2003), Conformational Analysis of the Jet-Cooled Peptide Mimetic Ethylacetamidoacetate from Torsion-Rotation Spectra, Journal of Chemical Physics (Accessed October 6, 2024)