Conformational properties of cis- and trans-N-Cyclopropylformamide studied by microwave spectroscopy and quantum chemical calculations.

The microwave spectra of cis- and trans-N-cyclopropylformamide, C3H5NHC(═O)H, have been investigated in the 31-123 GHz spectral region at room temperature. Rotational isomerism about the Cring-N bond is possible for both cis and trans. MP2/cc-pVTZ and CCSD/cc-pVTZ calculations indicate that there are two conformers in the case of cis, called Cis I and Cis II, while only one rotamer, denoted Trans, exists for trans-N-cyclopropylformamide. The quantum chemical methods predict that Cis I has an electronic energy that is 8-9 kJ/mol higher than the energy of Cis II. The CCSD H-Cring-N-H dihedral angle is 0.0° in Cis I, 93.0° in Cis II and 79.9° in Trans. The CCSD and MP2 calculations predict a slightly nonplanar structure for the amide moiety in both Trans and Cis II, whereas Cis I is computed to have a planar amide group bisecting the cyclopropyl ring. Surprisingly, the MP2 and CCSD methods predict practically the same energy for Trans and Cis II. The spectra of Cis II in the ground state and in two vibrationally excited states were assigned, while the spectrum of Cis I was not found presumably because of a low Boltzmann population due to a relatively large energy difference (8-9 kJ/mol). The spectra of the ground vibrational state and seven vibrationally excited states of Trans, were assigned. Vibrational frequencies of several of the excited state of both Cis II and Trans were determined by relative intensity measurements. The experimental and CCSD rotational constants are in satisfactory agreement. The MP2 values of the quartic centrifugal distortion constants of both species are in relatively poor agreement with their experimental counterparts. The MP2 vibration-rotation constants and sextic centrifugal distortion constants have little resemblance with the corresponding experimental values.