NC3NC: a combined millimetre-wave spectroscopic and ab initio investigation

The millimetre-wave spectrum of 2-isocyano-3-propynenitrile, NC3NC, was observed in the vibrational ground state (0000) and in the excited bending states (v6 v7 v8 v9)=(1000), (0100), (0010), and (0001). Pure rotational transitions J+1←J were recorded in the ranges 40–120 and 240–315 GHz. The analysis of the spectra, aided greatly by accompanying ab initio calculations, yielded precisely determined rotational, centrifugal distortion, and l-type doubling constants. They are in excellent agreement with the results of coupled-cluster calculations. Wavenumbers and infrared (IR) intensities of various stretching vibrational transitions were calculated variationally. Good agreement with results from high-resolution IR spectroscopy is observed for the highest fundamentals ν1–ν3, with deviations amounting to 15.4 cm−1 (ν1), 1.2 cm−1 (ν2), and 7.0 cm−1 (ν3).