“Matris-İzole Moleküllerin Konformerlerinin Seçiminde Stratejiler”
Dr. Rui Fausto (Coimbra Uni., Portekiz)
Tarih: 02 Ekim 2014 Perşembe, Saat 14:00
Yer: İKÜ Fizik Bölümü Ataköy Kampüsü 3. Kat
Prof.Dr. Erdal İnönü Seminer Salonu
Not: Seminerde konuşma dili İngilizce olacaktır.
Molecular conformation is a central concept in molecular physics, chemistry and biochemistry. Conformations of molecules determine the physical and chemical properties of substances and their biological functions. Many studies on conformers of small molecules have been performed, and these studies aim to constitute a basis for our understanding of the conformational behavior of more complex chemical systems, including biological macromolecules. The molecular conformations of small molecules determine not only the physical properties directly connected with the molecular structure (e.g., dipole moment and spectroscopic characteristics) but also properties influenced by intermolecular interactions. Different conformers of a given molecule may show distinct aggregation abilities, with some of them showing greater tendency to interact with other species  or promoting specific arrangements of aggregates, which in turn may lead, for example, to polymorphism [2,3]. Conformations are also important in determining photochemical reactivity .
Along the last 10 years we have been using matrix isolation, together with infrared spectroscopy, to promote conformational selection of many different types of organic molecules, thus allowing for the study of these unique forms at leisure. The techniques developed to select a given conformer among all the possible conformers of a molecule include temperature-dependent experimental procedures (e.g., annealing of the matrix of the compound or selection of the appropriate temperature of the substrate during matrix deposition), or experiments where light-induced conformational rearrangements are promoted. These latter techniques take advantage of two main characteristics of the molecular systems: (1) UV-induced conformational selection relies on the fact that the potential energy surfaces of the ground electronic state and of the stimulated excited states are different, so that after promotion to the excited state conformational relaxation can take place leading to a different conformer after return to the ground state; (2) IR-induced conformational selection may occur after selective excitation of a suitable vibrational mode of a given conformer followed by vibrational relaxation to another form, and is a consequence of the intricate processes of vibrational energy redistribution. In many cases, the produced conformers can later relax by tunneling to another form. Combinations of all these procedures led, for example, to the first simultaneous experimental observation of the 5 most stable isomeric forms of cytosine .
In this talk, general strategies for conformational selection in cryogenic matrices will be presented and a series of examples provided. These will include the selection of a single conformer in highly conformationally flexible systems exhibiting a large number of conformers in the gas phase (e.g., butanediols), controlling of the relative populations of different conformational states by UV-wavelength selection (as in p-anisaldehyde), production of high-energy conformers either by UV or IR selective irradiation of lower energy conformers (e.g., as in hydroxyacetone), creation of novel dimers of simple carboxylic acids containing higher energy units by in situ IR excitation of the most stable conformer followed by thermal mobilization (as for acetic acid), among other illustrative examples..