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

Abstract

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 ₄ and ₅ and of the low-energy stretching mode ₃ . Transitions up to J = 141 have been included in the fit for 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 k₃₄₅, k₃₄₄, k₃₅₅₅₅ and k₃₄₅₅₅. 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 α₃ vibration–rotation coupling constant, and the x_L(44), x_L(55) and x_L(45) anharmonic constants.