Infrared Spectrum and Internal-Rotation Barrier of Nitroethylene

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Abstract
The infrared spectra of nitroethylene‐d 0 and 1‐nitroethylene‐d 1 have been obtained in the vapor phase over the range 4000–70 cm−1. All fundamental frequencies have been observed and the NO2 torsion was located at 103 and 95 cm−1 for the d 0 and d 1isotopes, respectively. The barrier to internal rotation has been calculated to be 6510±280 cal/mole for H2CCHNO2 and 5960±280 cal/mole for H2CCDNO2 using the ``semirigid'' model with a V 2(1‐cos2φ) potential. A normal‐coordinate analysis has been carried out in order to clarify the nature of the fundamental vibrations. It shows that there is considerable mixing between the symmetry coordinates in the normal modes. The existence of this interaction indicates that the common assumption that the torsional mode is decoupled from the other modes of the same symmetry, the semirigid model, does not appear to be valid here. The effect of this on the barrier height is discussed.