Download Conformational preferences of flexible molecules and molecular

Conformational preferences of flexible molecules and molecular complexes
studied by free-jet rotational spectroscopy: the importance of non-covalent
interactions
Camilla Calabrese, Assimo Maris, Annalisa Vigorito, Walther Caminati and
Sonia Melandri
Università degli Studi di Bologna, Dipartimento di Chimica “G. Ciamician, via Selmi, 2,
Bologna, 40126 Italy; [email protected]
Complex conformational surfaces originate from the flexibility of molecular systems
due to the presence of multiple torsional degrees of freedom and different kinds of non bonding
interactions occurring within the molecule or with the surroundings. The high number of low
energy conformations and the presence of large amplitude motions taking place through shallow
potential energy surfaces, represent a challenge for spectroscopic and computational methods.
Accurate data on the structures and conformations of complex organic molecules,
biomolecules and molecular complexes can be obtained by rotational spectroscopy performed in
the cold isolated conditions of a free jet expansion and these properties can be directly compared
to the outcome of quantum mechanical calculations obtained in the same isolated conditions.
A few chosen examples of flexible organic molecules, bioactive molecules (drugs and
their halogen substituted analogs), astrophysical targets and molecular complexes formed in
supersonic expansions and characterized by rotational spectroscopy will be discussed.
It will be shown how non-covalent interactions compete to shape the conformational
space of the molecules and the geometry of the complexes and how these interactions can be
drastically changed through substitution (in particular halogen substitution) or complexation with
solvent molecules. These effects can influence the flexibility (e.g. locking some particular
conformation) or change the nature of the interactions (such as in the perfluorination effect).