High-Resolution Spectroscopic Measurements of the ν5 Bending Vibration–Rotation Band of HCCN in its X̃3Σ− State at 129 cm−1

Abstract

The HCCN radical is known to be a quasi-linear molecule in its ground triplet state (3Σ− in the linear configuration). Vibration–rotation transitions in the HCC bending (ν5) 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 Ka=1←0 transitions of a bent molecule or as l5=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 ν5 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 Ka=1 levels more reliably. Both 1H and 14N hyperfine splittings have been observed for the Ka=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 (gααs and gααr) have also been determined.