Knowledge Bank

A high-resolution spectroscopic study of the $\nu 5$ bending fundamental of HCCN has been carried out using far-infrared laser magnetic resonance (FIR LMR) spectroscopic techniques. This study has provided an accurate determination of the $\nu 5$ separation $3,864,563.78(11)$ MHz $(128.9079721(37)cm-1)$, a more accurate determination of the spin-spin parameters, an improved set of molecular g-factors, and the first observation of hyperline splittings in the $\nu 5=1$ levels. The analysis of the HCNN radical was done using an `A' reduced asymmetric rotor Hamiltonian and included the FIR LMR data (this work), microwave $dataa$, and millimeter-wave $datab$. A fit was performed including $\Delta K=2$ matrix elements and no improvement was observed compared with the fit using $\Delta K=0$. Thus, in this analysis there is no support for a slightly bent structure. We can now accurately predict zero field transitions which will be valuable for conducting searches for the $\nu 5$ fundamental transition in stellar and interstellar sources.