MW, X-RAY AND QUANTO-CHEMICAL STUDIES OF ISOLATED MOLECULES

It is an established practice to combine experimental spectroscopic results and quantum chemical data in order to achieve information on the internal motions and structure of isolated molecules. Quantum mechanical calculations support the assignment of experimental data providing trial spectroscopic parameters that guide the initial research. At the same time, experimental results can be used as benchmarks to validate and test the accuracy of the theoretical methods. Quantum chemical calculations also allow for an overall depiction of the potential energy surfaces that drive the intra-molecular motions. Adjusting such estimated surfaces on the basis of the observed findings allows the characterization the molecular behaviour. We used this approach to study the conformational and tautomeric equilibria of several isolated molecules, from their microwave and X-ray spectra: (i) the interpretation of "composite" PES and NEXAFS data was attempted on the basis of quantum chemical calculations, (ii) rotational characterization of the molecular structures was combined with the quantitative information achieved by XPS to determine the identity and the abundances present; in the case of XPS where the sample is at thermal equilibrium the latter piece of information can be used to estimate the energy differences among the species in the sample.