The effects of methyl internal rotation and ¹⁴N quadrupole coupling in the microwave spectra of two conformers of N,N-diethylacetamide

The gas phase structures and internal dynamics of N,N-diethylacetamide were determined with very high accuracy using a combination of molecular beam Fourier-transform microwave spectroscopy and quantum chemical calculations at high levels. Conformational studies yielded five stable conformers with C₁ symmetry. The two most energetically favorable conformers, conformer I and II, could be found in the experimental spectrum. For both conformers, quadrupole hyperfine splittings of the ¹⁴N nucleus and torsional fine splittings due to the internal rotation of the acetyl methyl group occurred in the same order of magnitude and were fully assigned. The rotational constants, centrifugal distortion constants as well as the quadrupole coupling constants of the ¹⁴N nucleus were determined and fitted to experimental accuracy. The V₃ potentials were found to be 517.04(13) cm⁻¹ and 619.48(91) cm⁻¹ for conformer I and II, respectively, and compared to the V₃ potentials found in other acetamides. Highly accurate CCSD(T) and DMC calculations were carried out for calculating the barriers to internal rotation in comparison with the experimentally deduced V₃ values