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The microwave spectra of propiolamide Little, Gary Bruce


The ground vibrational state microwave spectra of propiolamide, HCCCONH₂, and two isotopically-substituted species, HCCCOND₂ and DCCCOND₂, have been assigned in the frequency region 8-40 GHz and the rotational, centrifugal distortion and ¹⁴N nuclear quadrupole coupling constants have been determined. The spectra for three excited vibrational states of HCCCONH₂, the in- and out-of-plane C≡C-C bending modes and the nitrogen inversion mode, have also been analyzed; the vibrational frequencies, as determined by relative intensity measurements, are 273±75 cm⁻¹, >273 cm⁻¹ and 333±75 cm⁻¹, respectively. The molecule has been shown to be essentially planar with either no barrier to the inversion motion or only a very small one. This result is based mainly on the small change in the ground state inertial defect upon amido-group deuteration (Δ(HCCCOND₂)- Δ(HCCCONH₂)=-0.0311 amu-A²), the high frequency of the inversion motion, the absence of c-type transitions in the spectra, the similarity of the distortion constants in the ground and first excited inversion states and the small magnitude of X[sub cc]. Thus it is concluded that the planar resonance form of propiolamide, H-C≡C-C(O⁻)H⁺₂, is making the only significant contribution to the state of the molecule. Internuclear parameters of propiolamide have been determined by the substitution method and no anomalies have been found. Stark effect measurements have yielded a value of 3.67±0.02 D for the electricdipole moment (μ[sub a] =1.08+0.02 D, μ[sub b] =3.51+0.02 D) and it has been deduced to be directed between the C-C and C=0 bonds at an angle of ≃11° to the latter.

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