High-resolution spectroscopic measurements of the ν5 bending vibration-rotation band of HCCN in its (X)over-tilde3Σ- state at 129 cm-1

The HCCN radical is known to be a quasi-linear molecule in its ground triplet state ((3)Sigma (-) in the linear configuration). Vibration-rotation transitions in the HCC bending (nu (5)) fundamental band have been detected around 129 cm(-1) by the technique of far-infrared laser magnetic resonance (FIR LMR). Four FIR laser lines were used to record a total of 530 resonances. The observed transitions can be regarded equivalently as K-a = 1 <-- 0 transitions of a bent molecule or as l(s) = 1 <-- 0 transitions of a linear molecule. The LMR data, combined with previous measurements on this molecule at microwave and submillimeter wavelengths, have been used to determine parameters of an asymmetric rotor Hamiltonian. The nu (5) band origin has been determined more accurately as 128.907 968 7 (40) cm(-1). In addition, it has been possible to characterize the electron spin splittings in the K-a = 1 levels more reliably. Both H-1 and N-14 hyperfine splittings have been observed for the K-a = 1 levels for the first time; they show that the nuclear spin-electron spin dipolar interactions are markedly noncylindrical. Values for the electron spin and rotational g-factors (g(s)(alpha alpha) and g(r)(alpha alpha) have also been determined. (C) 2001 Academic Press.