Microwave Spectra, Planarity, and Conformational Preferences of <i>cis</i>- and <i>trans</i>-<i>N</i>‑Vinylformamide

The microwave spectra of a mixture of <i>cis</i>- and <i>trans</i>-HNCO forms of <i>N</i>-vinylformamide, (H₂CCHNHC­(O)­H), have been measured at room temperature in the 18–75 GHz spectral range. The spectra of two forms were assigned. The first of these forms has a cis arrangement for the HNCO chain of atoms, whereas the second form has a trans arrangement. The C–C–N–C chain of atoms is <i>antiperiplanar</i> (180°) in both forms. The inertial defect of the ground vibrational state of cis is −0.142(5) × 10^–20 u m², whereas this parameter is −0.087098(26) × 10^–20 u m² for trans. It is concluded that the equilibrium structures of both cis and trans are <i>completely</i> planar. The dipole moment determined from Stark effect measurements is μ_<i>a</i> = 9.96(8), μ_<i>b</i> = 2.22(3), μ_<i>c</i> = 0 (by symmetry), and μ_tot = 10.20(8) × 10^–30 C m [3.06(2) D], for cis, and μ_<i>a</i> = 7.64(16), μ_<i>b</i> = 9.24(10), μ_<i>c</i> = 0 (by symmetry), and μ_tot = 12.0(2) × 10^–30 C m [3.59(5) D] for trans. The spectrum of one vibrationally excited state, presumably the first excited state of the torsion about the C–N bond of cis, was assigned and the frequency of this state was determined to be 76(15) cm^–1 by relative intensity measurements. The spectra of two vibrationally excited states of trans were assigned. These states are assumed to be the first excited state of the torsion about the CN bond, and a low bending vibration. Relative intensity measurements yielded 101(20) and ca. 300 cm^–1, respectively, for the frequencies of these normal vibrations. Accurate values of the quartic centrifugal distortion constants, the dipole moments, and the vibration–rotation constants have been obtained for both cis and trans. The experimental work has been augmented by high-level quantum chemical calculations at the B3LYP/cc-pVTZ and CCSD­(T)/cc-pVTZ levels of theory. The theoretical calculation performed without symmetry restrictions correctly predict that cis and trans are both planar. The CCSD­(T) rotational constants are in excellent agreement with their experimental counterparts, whereas the B3LYP quartic centrifugal distortion constants and the vibration–rotation constants are in fairly good agreement with experiments. The CCSD­(T) dipole moments deviate more than expected from the experimental dipole moments. It is estimated that further conformers of cis and trans must be at least 4 kJ/mol higher in energy