Photoinduced reaction in the ion-molecule complex Mg+-C6H5OCF3

The metal cation-molecule complex of Mg+ cation with C6H5OCF3 molecule was produced in a laser-ablation supersonic expansion nozzle source. The intra-complex reaction in Mg+-C6H5OCF3 by photons in the spectral region of 230-440 nm has been studied. The formation of Mg+ was found to be the predominant dissociation path way, and at the same time, the other products MgF+, C6H5+, and C6H5OCF3+ were also observed. We monitored all of the photodissociation products as a function of the excitation wavelength in the 230-440 nm spectral region. The action spectra of the all products except the dissociative charge-transfer product C6H5OCF3+ are very similar. They consist of two pronounced bands, one of which is on the red side and the other is on the blue side of the Mg+ 32P ← 32S atomic transition. The two peaks of these products are all located at about 243 nm and 332 nm, respectively. The calculated results at the B3LYP/6-31+G∗∗ level show that there are three isomers: one is with Mg+ attached to the O atom, while it also partially interacts with one of F atoms (4), the next is with Mg+ simultaneously to the two F atoms (4′), and the last is with Mg+ sitting above the aromatic ring (4″). The isomer 4 is more stable than 4′ and 4″ by ∼3-5 kcal/mol. Although the three isomers are comparable in energy, on the basis of the formation channels of the products and their action spectra, the photoreaction products in our experiment are deduced to originate from 4. The branching fraction ratios for the all product except C6H5OCF3+ were nearly constant. A photoreaction mechanism is proposed that the conical intersection opens an efficient pathway for the nonadiabatic transition from the excited state to the ground state, producing an intermediate, which yields the photoproducts with a constant branching ratio. The charge-transfer product ion C6H5OCF3+ is ascribed to a predissociative charge transfer process.