ROTATIONAL SPECTROSCOPY: A SPECIFIC TOOL TO INVESTIGATE THE CONFORMATIONAL PREFERENCES OF MOLECULES

Rotational spectroscopy is an important tool to evaluate the conformational attitude of molecules, to define their structure and to characterize the large amplitude vibrational modes that can be present. Conformationals studies are interesting for example to understand the biological activity of small organic molecules, such as drugs and pollutantes and to verify “lock and key” principle effect. Moreover the experimental results may be used as benchmarks for computational data since they are obtained in the same conditions: the isolated molecule. Rotational spectroscopy is performed in gas-phase in high vacuum conditions and so are the quanto-mechanic ab initio calculations, which are used to describe the PES which governs the geometrical arrangements of conformational isomers and to obtain the spectroscopic constants which allow the research and the assignment of rotational transitions. Finally it should be noted that high vacuum conditions reproduce those of the interstellar medium and this makes these kind of experiments very useful to guide astrophysical studies that aim at findig life precursors. In our laboratory the experimental conditions are achieved in supersonic expansions using Molecular Beam Fourier Transform Microwave Spectroscopy (MBFTMW) [1] or Free Jet Absorption Microwave Spectrocopy (FJAMW) [2] [3] techiniques. To support all these considerations we show some examples of different kind of molecules studied with this technique. At first a study which compares the conformers of Benzylamine (BA) and 2-Fluorobenzylamine (2-FBA). Because of the importance of the organic fluoride compounds, we want to evaluate the changes that substitution of H atoms by fluorine causes to these molecules. The rotational spectra of 2-FBA show the presence of two of the four stable conformers predicted with quantum chemical calculations: the global minimum is stabilized by an intramolecular hydrogen bond between the fluorine atom and one hydrogen of the aminic group, whereas the other conformer is characterized by a complex tunnelling motion of the aminic hydrogen atom. Compare of to BA we predicted a new conformer and we found a different splitting of the same tunneling motion, due to the new geometry caused by the presence of the fluorine. The second example is 2-Methylaminoethanol (MAE) that is relevant both for astrophysical research and also in biological chemistry. Because of the high flexibility of its chain, MAE presents an elaborate conformational surface, which is explored by B3LYP and MP2 quanto-mechanic methods. The conformational preferences of MAE are dominated by the intramolecular hydrogen bond between the OH and NH2 groups, that gives rise to 24 different conformers and two of them were observed. The search for higher energy conformers has been undertaken. References [1]Caminati W.; Millemaggi A.; Alonso J.L.; Lesarri A.; Lopez J.C.; Mata S., «CHEMICAL PHYSICS LETTERS», 2004, 392, pp. 1 - 6 [2] S. Melandri, W. Caminati, L. B. Favero, A. Millemaggi, P. G. Favero, J. Mol. Struct. (THEOCHEM) 1995, 352/353, 253 ± 258. [3] S. Melandri, G. Maccaferri, A. Maris, A. Millemaggi, W. Caminati, P. G. Favero, Chem. Phys. Lett. 1996, 261, 267 ± 271