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Analysis and Fit of the Fourier-Transform Microwave Spectrum of the Two-Top Molecule N-methylacetamide



N Ohashi, Jon T. Hougen, R D. Suenram, Francis J. Lovas, Y Kawashima, M Fujitake (Japan), J Pyka


The jet-cooled Fourier-transform microwave spectrum of N-methylacetamide (CH3-NH-C(=O)-CH3), a molecule containing two methyl tops with relatively low barriers to internal rotation, has been recorded and fit to nearly experimental measurement uncertainty. Measurements were carried out between 10 GHz and 26 GHz, with the nitrogen quadrupole splittings resolved for many transitions. The permutation-inversion group for this molecule is G18 (not isomorphic to any point group), with irreducible representations A1, A2, E1, E2, E3, and E4. One of these symmetry species and the usual three asymmetric rotor quantum numbers JKaKc were assigned to each torsion-rotation level involved in the observed transitions. F values were assigned to hyperfine components, where F = J + IN. Transitions involving levels of A1 and A2 species could be fit to an asymmetric rotor Hamiltonian. The other transitions were first fit separately for each symmetry species using a Pickett-like effective rotational Hamiltonian. Constants from these fits show a number of additive properties which can be correlated with sums and differences of effects involving the two tops. A final global fit of 839 hyperfine components to 48 molecular parameters was carried out using a newly written two-top computer program, giving a root-mean-square deviation of observed-minus-calculated residuals of 4 kHz. This program was written in the principal axis system of the molecule and uses a free-rotor basis set for each top, a symmetric-top basis set for the rotational functions, and a single-step diagonalization procedure. Such an approach requires quite long computation times, but it is much less prone to subtle programming errors (a consideration felt to be important since precise fits of low-barrier two-top molecules are still rather uncommon in the literature). The two internal rotation angles in this molecule correspond to the Ramachandran angles and often defined to describe polypeptide folding. Barriers to internal rotation about these two angles were found to be 79 cm-1 and 73 cm-1, respectively.
Journal of Molecular Spectroscopy
No 1


internal rotation, microwave spectrum, N-methylacetamide, Ramachandran angles


Ohashi, N. , Hougen, J. , Suenram, R. , Lovas, F. , Kawashima, Y. , Fujitake (Japan), M. and Pyka, J. (2004), Analysis and Fit of the Fourier-Transform Microwave Spectrum of the Two-Top Molecule N-methylacetamide, Journal of Molecular Spectroscopy (Accessed June 9, 2023)
Created August 31, 2004, Updated October 12, 2021