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

1-Phenyl-1,2-propanedione has been isolated in low-temperature xenon matrixes 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.; Hagen, K. J. Phys. Chem. 1993, 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
135.6°. On the other hand, the experimental data clearly reveals that, in the as-deposited xenon matrixes
(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 (λ > 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 min
of irradiation (k = 2.8 × 10-2 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.