2007ApJ...670.1510S

Quartic force fields for the ground electronic states of the most stable C₄ radical isomers [l-C₄(X³Σ^–_g) and r-C₄(X¹A_g)] are computed at the same level of theory. These computations are performed using interaction configuration ab initio methods and the cc-pVTZ basis set. These force fields on symmetry-adapted coordinates are derived from full six-dimensional potential energy surfaces generated close to their equilibrium geometries. For both isomers, sets of spectroscopic parameters of the most abundant isotopic varieties are determined with perturbation theory. This includes rotational constants, harmonic wavenumbers, anharmonic wavenumbers for the fundamentals, and some overtones, combination modes, and l-doubling terms. Finally, our results are compared with experimental data measured with different techniques that allow us to discuss assignments of previous astrophysical observations. Specifically, we are focusing in predicting frequencies for the low bendings allowing understanding patterns detected with far-infrared techniques.