The Equilibrium Structure Of Small Radicals: The Semi-experimental Approach At Work

A new data set of accurate semi-experimental equilibrium strctures (r$_{\mathrm{SE}}$) for small open-shell species will be presented. In the semi-experimental approach, equilibrim rotational constants ($B_e$) of different isotopologues are used in a least-square-fit to retrieve the equilibirum parameters, thus implying the validity of the Born-Oppenhaimer approximation. The required $B_e$ are obtained by correcting experimental rotational constants of the vibrational ground state ($B_0$) for the computed vibrational ($\Delta B_0$) and electronic ($\Delta B_g$) contributions. The latter have to be computed using \textit{ab initio} methods and this imposes several constrains on the theoretical methodologies that can be employed. The $\Delta B_g$'s have been retrived from HF/aug-cc-pVTZ computations of the \textit{\textbf{g}}-tensor and the $\Delta B_0$'s have been obtained at the either UCCSD(T)/cc-pVTZ or the UB2PLYP-D3/cc-pVTZ level of theory, according to the level of spin-contamination. The new data set includes several radicals for which experimental rotational constants of different isotopologues have already been reported in the literature, such as CCS, C$_n$H with $n$ = 1-7, NH$_2$, CCP, CH$_2$, PH$_2$, H$_2$CCN, CP, CN and more. This collection of equilibrium structures has three ultimate purposes: (i) to serve as benchmark-reference for electronic structure methods, (ii) give new insights, based on experimental data, on the molecular structure of unstable species and on how these structures changes across a group of the periodic table and (iii) to predict the rotational spectrum of less abundant isotopologues. The semi-experimental approach leads to r$_{\mathrm{SE}}$ that can have an accuracy below 0.1 m\AA\ for bond lenghts and $0.01^{\circ}$ for angles. For example, in the case of CCS radical, the C-S bond is predicted to be 1.31349(2) \AA\ while the C-C bond lenght is found to be 1.56379(1) \AA. For angles, the NH$_2$ molecule can be used as reference, with N-H bond distance of 1.023618(2) \AA\ and an HNH angle of 103.1488(3)$^{\circ}$