Electronic Structures And Fine Electronic Spectrum Of Fluorobenzenes

In this paper, ab initio methods are employed to study the electronic structures and the spectral properties of difluorobenzene and1,3,5-trifluorobenzene. The results show that the calculated results agree well with the experimental ones. In other words, these computational methods and theoretical levels are appropriate to study these systems.1. Electronic structures and spectral properties of difluorobenzeneThe equilibrium geometries of the ground states (S0) of these three DFB molecules in gas phase are first optimized at the B3LYP and MP2levels with the6-311++G**basis set. In order to determine a correct displacement between ground and excited states, the optimization of So states is repeated using the multi-reference CASSCF method with the same basis set, by which the equilibrium geometries of the first excited singlet states (S1) of three DFBs are also optimized. In this work, the fluorescence excitation (FEX) spectra of the first excited singlet states for three DFBs molecules (para-, meta-and ortho-difluorobenzene) are simulated by the Franck-Condon calculations with the displaced harmonic oscillator approximation plus the distorted correction. The results agree well with the experimental assignments. The calculated results indicate that the spectral profiles of three DFBs are primarily described by the Franck-Condon progression of their totally symmetric vibrational modes. Some inferred and unassigned vibrational transitions in experiment are confirmed according to the present calculated results. In addition, in the simulated fluorescence spectra we tentatively assign several combination bands with relative moderate intensity and weak vibrational lines which appear in the experimental observations but the corresponding assignments are not given. More importantly, the present study demonstrates that the combination of ab initio electronic-structure theory and displaced harmonic oscillator approximation plus distorted effect is a useful tool to simulate the FEX spectra of dihalogenated benzene molecules.2. Theoretical study on the electronic structures and spectral properties of1,3,5-trifluorobenzeneIn this section, the equilibrium geometries of the ground state (S0) of1,3,5-trifluorobenzene in gas phase are first optimized at the B3LYP, BHandHLYP, PBE1PBE levels with6-311++G**and aug-cc-pVDZ basis sets. In order to determine a correct displacement between ground and excited states, the optimization of So state is repeated using MP2method with the same basis sets. The equilibrium geometries of the first excited singlet state (S1) of1,3,5-trifluorobenzene are optimized at the TD-B3LYP, TD-BHandHLYP, TD-PBE1PBE methods with6-311++G**and aug-cc-pVDZ basis sets. We simulate1,3,5-trifluorobenzene’s absorption spectrum using Franck-Condon approximation. The present work reproduces satisfactorily the experimental absorption spectrum of1,3,5-trifluorobenzene.