Interplay of theory and experiment in rotational spectroscopy

Ab initio calculations nowadays are able to provide very accurate predictions of molecular and spectroscopic properties. The predictive capabilities of ab initio computations takes a fundamental role in the ¯eld of high-resolution spectroscopy: calculations allow to guide, support and/or challenge the experimental determinations, and even to facilitate the detection of new molecules. In particular, for predicting rotational spectra, the information required are accurate estimates of: a) rotational parameters, b) type of transitions observable (and their intensity), c) if the case, ¯ne and hyper¯ne parameters. Accurate equilibrium structure as well as harmonic and anharmonic force ¯eld computa- tions are necessary in order to ful¯ll point (a), accurate dipole moment evaluations for point (b), and, ¯nally, electric ¯eld gradient, spin-rotation and spin-spin tensor calcula- tions for point (c). Additionally, vibrational averaging of properties related to points (b) and (c) are often required. All these computations may be nowadays carried out at the CCSD(T) level of theory in conjunction with large (valence and core-valence) correlation consistent basis sets. Comparison of the computed data with experimental results allows us not only to show how ab initio computations are able to accurately predict experimental data but also to demonstrate how theoretical values can replace them when missing or not determinable. It will be demonstrated how quantum-chemical calculations can be used to assist exper- imental investigations in the ¯eld of rotational spectroscopy. High-level calculations can provide reliable values for the corresponding spectroscopic parameters (rotational con- stants, etc.), thus signi¯cantly facilitating the detection of new molecules. Theoretical predictions for the hyper¯ne parameters (quadrupole coupling constants, spin-rotation tensors, spin-spin couplings, etc.) are often essential for a detailed analysis of the hy- per¯ne structure of the measured rotational spectra. Finally, calculations can be used to provide information which enable a rigorous interpretation of the obtained spectroscopic parameters. For example, it is in this way possible to derive equilibrium geometrical pa- rameters from the experimentally determined spectroscopic parameters such as rotational constants or spin-spin coupling constants using computed vibrational corrections. A number of examples will be presented to illustrate the successful interplay of theory and experiment in the ¯eld of rotational spectroscopy. Among others, those include the detection of trans-1-chloro-2-°uoroethylene using rotational spectroscopy and the deter- mination of its equilibrium structure, the rotational spectra of bromo°uoromethane, the hyper¯ne structure of the rotational spectra of H13CN and CF2.