High resolution rotational spectroscopy as a source of information on the structure and magneto-electric properties of molecules

Rotational spectroscopy, thanks to its intrinsic high resolution, is a powerful tool for providing information on the structure and magneto-electric properties of molecules in the gas phase. Nowadays, the combination of experimental ground-state rotational constants with calculated vibrational corrections can be considered the best approach to determine reliable equilibrium geometries for polyatomic molecules (see for instance Ref. [1]). Due to the computational contribution, these structures are usually referred to as ''empirical'', ''mixed experimental/theoretical'', or ''semi-experimental''. The accuracy of such a joint experimental and theoretical procedure for the determination of equilibrium structures has recently been investigated by Pawłowski et al1, who concluded that errors in the determined empirical bond lengths are typically below 0.001 Å for first-row elements, as long as an electron-correlated method is used in the calculation of the vibrational corrections. It will be shown that this approach can be successfully applied to polyatomic molecules containing second-row2 or third-row3 atoms. The comparison to other types of structural determinations will be also discussed. By exploiting the Lamb-dip technique it is possible to further increase the high resolution power of rotational spectroscopy and then resolve hyperfine structures. The determination of (hyper)fine parameters, such as quadrupole coupling, spin-spin coupling, and spin-rotation constants, is one of the aims of high-resolution rotational spectroscopy. These parameters are relevant not only from a spectroscopic point of view, but also from a physical and/or chemical viewpoint, as they might provide detailed information on the chemical bond, structure, .…. . Examples will be provided for small molecules4 as well as for bromofluoromethane5, a pentatomic molecule containing a third-row element. Nevertheless, the experimental determination of hyperfine constants can be a challenge because of either the lack of reliable estimates or the complexity of the hyperfine structure itself. Therefore, it could be important to have the opportunity to rely on good predictions for such parameters. These can be provided by quantum chemistry. Examples for the successful interplay of theory and experiment will be also presented. 1. F. Pawłowski, P. Jørgensen, J. Olsen, F. Hegelund, T. Helgaker, J. Gauss, K.L. Bak, J.F. Stanton, J. Chem. Phys. 116, 6482 (2002). 2. C. Puzzarini, G. Cazzoli, A. Gambi, J. Gauss, J. Chem. Phys. 125 054307 (2006). 3. C. Puzzarini, G. Cazzoli, A. Baldacci, A. Baldan, C. Michauk, J. Gauss, J. Chem. Phys. 127, 1643021 (2007). 4. G. Cazzoli, C. Puzzarini, J. Mol. Spectrosc. 233, 280 (2005); G. Cazzoli, C. Puzzarini, J. Mol. Spectrosc. 239, 64 (2006). 5. G. Cazzoli, C. Puzzarini, J. Gauss, to be published.