Millimeter-wave spectroscopy and coupled cluster calculations for NCCP

The rotational spectrum of the unstable NCCP molecule has been investigated in the millimeter-wave region. The ground-state spectra of the most abundant isotopomer and of the C13 and N15 containing species were studied, and lines in the v2, v3, v4, and v5 vibrationally excited states were detected for the normal isotopomer. Electric quadrupole and magnetic spin–rotation coupling constants of the nitrogen nucleus were also determined. The experimental work was assisted by high level coupled-cluster single double triple [CCSD(T)] calculations, performed using the cc-pVQZ basis, which provided accurate predictions for the αr vibration–rotation coupling constants and the ground-state rotational constants of the less abundant isotopic species. r0 and rs molecular structures of NCCP were derived directly from the experimental ground-state rotational constants of four different isotopomers, and an accurate equilibrium structure could be evaluated by combining theoretically computed vibration–rotation coupling constants with experimental ground-state rotational constants.