Methyl torsional state analysis of the jet-cooled microwave spectrum of N-acetylglycine

The microwave spectrum of N-acetylglycine was obtained using a NIST Fourier-transform microwave spectrometer equipped with a heated, pulsed-nozzle source. One conformer has been identified and its spectrum assigned. The conformer has C_S point group symmetry and an intramolecular hydrogen bond between the carbonyl and amide groups of the 5-membered glycine unit. Internal rotation of the methyl rotor group leads to splitting of the rotational lines into A and E symmetry tunneling states. The ¹⁴N nuclear-quadrupole hyperfine structure verifies the rotational and internal-rotor state assignments. The V₃ barrier of 57.5(1) cm⁻¹ and the angles between the C₃ axis of the methyl rotor and the principal inertial axes are in best agreement with the calculated values for the lowest energy conformer of the four conformers predicted at the MP2/6-311++G(d,p) level of theory.