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

The millimetre-wave spectrum of 2-isocyano-3-propynenitrile, NC3NC, was obs erved in the vibrational ground state (0000) and in the excited bending sta tes (upsilon(6) upsilon(7) upsilon(8) upsilon(9)) = (1000), (0100), (0010), and (0001). Pure rotational transitions J + 1 <-- J were recorded in the r anges 40-120 and 240-315 GHz. The analysis of the spectra, aided greatly by accompanying ab initio calculations, yielded precisely determined rotation al, centrifugal distortion, and I-type doubling constants. They are in exce llent agreement with the results of coupled-cluster calculations. Wavenumbe rs and infrared (IR) intensities of various stretching vibrational transiti ons were calculated variationally. Good agreement with results from high-re solution IR spectroscopy is observed for the highest fundamentals nu(1)-nu( 3), with deviations amounting to 15.4 cm(-1) (nu(1)), 1.2 cm(-1) (nu(2)), a nd 7.0 cm(-1) (nu(3)). (C) 1999 Elsevier Science B.V. All rights reserved.