Conformational Analysis of the Jet-Cooled Peptide Mimetic Ethylacetamidoacetate from Torsion-Rotation Spectra
R J. Lavrich, Angela R. Hight Walker, David F. Plusquellic, I Kleiner, R D. Suenram, Jon T. Hougen, Gerald T. Fraser
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
, Hight, A.
, Plusquellic, D.
, Kleiner, I.
, Suenram, R.
, Hougen, J.
and Fraser, G.
Conformational Analysis of the Jet-Cooled Peptide Mimetic Ethylacetamidoacetate from Torsion-Rotation Spectra, Journal of Chemical Physics
(Accessed October 3, 2023)