On the low-lying CCN bending mode of the nearly linear molecule NCCNO

The low-lying CCN bending mode of cyanofulminate, NCCNO, was characterized by rotational spectroscopy in the millimeter-wave and submillimeter-wave ra nge as well as by rovibrational spectroscopy in the far-infrared range. The spectra exhibit the gross features of a linear molecule. However, a closer qualitative analysis regarding the low-lying CCN bending mode revealed sig nificant deviations from a harmonic bending mode that is normally found in a linear molecule. This result was confirmed by a quantitative analysis of the combined data with an effective Hamiltonian for a linear molecule. In l inear molecule notation, the term value of the first excited state nu (7) i s 80.524 182 (10) cm(-1), and the term values for the l(7) = 0 and l(7) = 2 levels of the second excited state 2 nu (7) are 166.118 254 (16) and 164.6 04 243 (22) cm(-1). A semirigid bender analysis of our data, including rota tional transitions of the isotopomers (NCCNO)-N-15, (NCCNO)-C-13, (NCCNO)-N -15, and (NCCNO)-O-18 observed in natural abundance, yielded a considerable quartic contribution to the effective CCN bending potential function V(rho )/cm(-1) = 747.40 (81) x (p/rad)(2) + 959.2 (24) x (rho /rad)(4).