The electronic structure and vibrational frequencies of CNN and SiNN from local density functional methods

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Abstract
The geometry and vibrational frequencies of CNN and SiNN in the 3Σ− state have been calculated by local density functional theory. The geometry of CNN has an NN bond (1.202 Å) that is shorter than the CN bond (1.252 Å). For SiNN, the NN bond is shorter (1.164 Å) than that for CNN and the SiN bond (1.749 Å) is like an Si–N single bond. The calculated frequencies for CNN (1597, 1273, and 377 cm− 1) can account for the lower two observed transitions but can not account for the observed transition at 2847 cm− 1 as a fundamental. The infrared intensities are all predicted to be small for CNN. The calculated frequencies for SiNN (1821, 569, and 321 cm− 1) can account for the observed spectrum and we note that ω1 is predicted to have a large infrared intensity. The local density functional results for SiNN and CNN are quite different from previously reported molecular orbital predictions which could not satisfactorily account for the experimental results.