Conformational Flexibility, UV-Induced Decarbonylation and FT-IR Spectra of 1-Phenyl-1,2 Propanedione in Solid Xenon and in the Low Temperature Amorphous Phase

SUSY LOPES, GOMEZ ZAVAGLIA, ANDREA, LESZEK LAPINSKI Y RUI FAUSTO.

Funchal, Madeira

Congreso; Matrix-Isolation Spectroscopy 2005 (MATRIX 2005); 2005

Grupo de Espectroscopia a Bajas Temperaturas, Universidad de Coimbra

P3.13- Conformational Flexibility, UV-Induced Decarbonylation and FT-IR Spectra of 1-Phenyl-1,2-Propanedione In Solid Xenon and in the Low Temperature Amorphous Phase S. Lopes, A. Gómez-Zavaglia, L. Lapinski and R. Fausto Department of Chemistry - University of Coimbra - Portugal -,  Faculdad de Farmacia y Bioquímica - Universidad de Buenos Aires - Argentina -,  and Institute of Physics of the Polish Academy of Sciences - Warsaw - Poland    1-phenyl-1,2-propanedione has been isolated in low temperature xenon matrices and studied by FTIR spectroscopy, supported by DFT(B3LYP)/6-311++G(d,p) calculations. In good agreement with previous electron diffraction data [Shen, Q., and Hagen, K. (1993) J. Phys. Chem. 97, 985], the calculations predicted the existence of only one stable conformation for the compound, in which the O=C-C=O dihedral angle is 135.6º. On the other hand, the experimental data clearly reveals that, in the as-deposited xenon matrices (T= 20 K), there is a distribution of molecules with different O=C-C=O dihedral angles around the equilibrium value. This distribution results from the efficient trapping of the conformational distribution existing in the gas phase, prior to deposition, which is determined by the low frequency, large amplitude torsional vibration around the C-C central bond. Upon annealing to higher temperatures (T~ 45 K), the initially trapped conformational distribution can be modified in a certain degree, favoring more polar structures (corresponding to smaller O=C-C=O dihedral angles), as a result of the interactions with the matrix media. Irradiation of the matrix with UV light (l > 235 nm) led to decarbonylation of the compound, with generation of acetophenone and carbon monoxide, with an almost complete consumption of the reagent after 1100 minutes of irradiation (k= 2.8x10-2 min.-1). Aggregation of the compound resulting from the matrix warming was also investigated, providing useful information for interpretation of the spectroscopic data obtained for the low temperature amorphous state of the neat compound.