Vibrational spectroscopy and molecular orbital calculations of N,N-dimethylacrylamide and N,N-dimethylpropionamide - Conformational equilibrium in the liquid state -

Molecular vibrations of N,N-dimethylacrylamide (DMAA) and N,N-dimethylpropionamide (DMPA) in the liquid state were studied by means of Raman and IR spectra over the wide frequency range. It has been established in our previous work that DMPA involves two rotational isomers within its propionyl CH3-CH2-C(O)- group. In the present work, it is revealed that, like DMPA, DMAA also involves two rotational isomers, the planar cis and nonplanar staggered ones, within its acryl CH2CH-C(O)- group. The terminal methylene carbon of the acryl group locates cis to the O atom within the amide OC-N plane (the planar cis conformer) or above the amide plane (the nonplanar staggered conformer). This is well supported by theoretical MO calculations. Torsion potential energy surfaces (PES) for the C-C-C-O dihedral angle θ of the acryl group show that a global minimum appears at ca. θ = 0° (the planar cis conformer) and a local minimum at ca. θ = 130° (the nonplanar staggered conformer). The geometry optimization followed by normal frequency analyses showed that theoretical Raman and IR bands for the conformers satisfactorily reproduce the observed ones. The enthalpy for the conformational change from the planar cis conformer to the nonplanar staggered one was experimentally evaluated by analyzing the integrated intensity changes of the bands in the range 700-800 cm− 1. The value thus obtained for the acryl group of DMAA is ca. 9 kJ mol− 1, which is significantly higher than the corresponding value for the propionyl group of DMPA (5 kJ mol− 1).