CONFORMATION -SELECTIVE PHOTOCHEMISTRY: THE CASE OF ANISALDEHYDE ISOLATED IN ARGON MATRIX

1. Molecular conformation is a central concept in molecular physics, chemistry and biochemistry. The conformational structure of a molecule determines not only its physical properties (e.g., dipole moment), but also its chemical and photochemical reactivity. Although these principles are generally accepted, experimental observations of significantly different rates of photochemical transformations in different conformers of the same compound are still very scarce. In the present study, we investigated photochemical transformations for two conformers of anisaldehyde. The molecule of anisaldehyde can adopt two conformational structures (Figure 1). These forms are nearly isoenergetic (DE = 0.25 kJ mol-1). Hence, both conformers are populated in the gas phase and can be trapped in low-temperature matrices. The experimental investigation of anisaldehyde monomers isolated in an Ar matrix at 12 K demonstrated that two conformers of the compound are indeed populated in a low-temperature solid Ar environment. Infrared spectra of the two forms are very similar. The infrared bands belonging to the spectrum of one of the forms are shifted by several cm-1, with respect to the corresponding bands in the spectrum of the other form. Hence, in the infrared spectrum of matrix isolated anisaldehyde a number of bands appears as doublets. The presence of two conformers of anisaldehyde in an Ar matrix was confirmed by the results of UV-induced transformations observed for this compound. Upon UV (l > 305 nm) irradiation, the population of only one conformer decreased significantly. This process was experimentally observed as decrease of intensity of one of the components (due to the O-cis form) of doublet bands, whereas the other component (due to the O-trans form) remained much less affected. Upon irradiation with UV light of shorter wavelengths (l > 245 nm) both conformers were consumed in a photoreaction leading to a ketene photoproduct. The most intense band in the spectrum of the photoproduct was observed at 2126 cm-1 that is at a frequency characteristic for the “antisymmetric” stretching vibration of the ketene group. The interpretation of the experimental data received support from theoretical calculations carried out at the DFT(B3LYP) level of approximation, with the 6‑311++G(d,p) basis set. CONFORMATION -SELECTIVE PHOTOCHEMISTRY: THE CASE OF ANISALDEHYDE ISOLATED IN ARGON MATRIX 1,2 N. Kus, 1 A. Sharma, 1 I. Reva, 3 L. Lapinski and 1 R. Fausto 1 Department of Physics, Anadolu University, 26470 , Eskisehir, Turkey 2 Department of Chemistry, University of Coimbra, P-3004-535 Coimbra, Portugal 3 Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668, Warsaw, Poland Acknowledgements: This work was supported by Portuguese Science Foundation (FCT) Research Projects POCI/QUI/71203/2007.