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 abunda nt isotopomer and of the C-13 and N-15 containing species were studied, and lines in the upsilon(2), upsilon(3), upsilon(4), and upsilon(5) vibrationa lly excited states were detected for the normal isotopomer. Electric quadru pole and magnetic spin-rotation coupling constants of the nitrogen nucleus were also determined. The experimental work was assisted by high level coup led-cluster single double triple [CCSD(T)] calculations, performed using th e cc-pVQZ basis, which provided accurate predictions for the alpha(r) vibra tion-rotation coupling constants and the ground-state rotational constants of the less abundant isotopic species. r(0) and r(s) molecular structures o f NCCP were derived directly from the experimental ground-state rotational constants of four different isotopomers, and an accurate equilibrium struct ure could be evaluated by combining theoretically computed vibration-rotati on coupling constants with experimental ground-state rotational constants. (C) 2000 American Institute of Physics. [S0021-9606(00)00928-4].