Structure and dynamics of cyclic amides: The rotational spectrum of 1,3-dimethyl-2-imidazolidinone

The structure and the internal dynamics of the lactam 1,3-dimethyl-2-imidazolidinone, also known as N,N′-dimethylethyleneurea, have been investigated through the analysis of its free-jet absorption rotational spectrum. One conformer has been assigned. The pure μb-type spectrum, recorded in the 59.6–74.4 GHz frequency range entails an inertial defect Δc= −16.39 uÅ2, indicating that the molecule has C2symmetry with a twisted arrangement of the ring. The methyl internal rotation barrier V3= 7.181 (3) kJ mol−1and the14N diagonal nuclear quadrupole coupling constants χaa= 2.14 (14) and (χbb-χcc) = 7.26 (6) MHz were determined from the analysis of the hyperfine structure. They are in good agreement with the ab initio MP2/6-311++G(d,p) calculations which also estimate the electric dipole moment value as 3.9 D.