Rotational spectroscopy of C-cyanophosphaethyne, NCCP, in states of multiple vibrational excitation

The rotational spectrum of the unstable NCCP molecule has been investigated in the millimetre-wave region for all the levels of multiple vibrational excitation below 1200 cm(-1). More than 600 rotational transitions in the J range from 19 to 39 have been measured and assigned to 13 new vibrational states derived from excitation of the two bending modes v(4) and v(5) and of the low-energy stretching mode v(3). Transitions up to J = 141 have been included in the fit for v(3) = 1 yielding a precise value for the sextic centrifugal distortion constant. Vibrational and rotational l-type resonance effects have been taken into account in the analyses of the observed spectra together with the anharmonic interactions which originate from the normal coordinate force constants k(345), k(344), k(35555), and k(34555). The simultaneous analyses of the rotational spectra of the interacting levels have yielded sets of spectroscopic constants with unambiguous physical meaning, including the unperturbed value of the alpha (3) vibration-rotation coupling constant, and the x(L(44)), x(L(55)) and x(L(45)) anharmonic constants.