Microwave spectrum,conformation, intramolecular hydrogen bond,and dipole moment of pyrrole-2-carboxaldehyde

Microwave spectra of C₄H₃NH-CHO, C₄H₃ND-CHO, and C₄H₃NH-CH¹⁸O are reported. The stable form of the molecule is demonstrated to be planar with the N-H and C-O bonds in a cis conformation. Other forms of the molecule are at least 1 kcal mol^?1 less stable. The H(1) · O distance is 2.592±0.006 Å. Six vibrationally excited states were attributed to the C-C torsional mode, the symmetrical, and the antisymmetrical aldehyde group deformation vibrations. Relative intensity measurements yielded 151±11 cm^?1, for the first frequency, 210±17 cm^?1 for the second, and 270±38 cm^?1 for the last mode. The dipole moment was determined to be μ_a = 2.47 ±0.02 D, μ_b = 0.16±0.06 D, and μ_tot = 2.48 ±0.02 D, respectively.