The Study of Distortion of High Symmetry Configurations of Si2F4(2+), Si2F4 and Si2F4(2-)2-

In chemistry, thermodynamic stability occurs when a system is at its lowest state of energyor in the chemical equilibrium with its surroundings [1]. The study of the origin of the equilibriumgeometric of systems has a long history. TD- DFT and DFT calculations were carried out forSi2F4(2+), Si2F4 and Si2F4(2-) at CAM-B3LYP/def2- TZVPP [2] level of theory to optimize allstructures. The planar form of Si2F4(2+) with D2h symmetry was stable, whereas the planar forms ofSi2F4 and Si2F4(2-) were not stable. Si2F4 and Si2F4(2-) have an imaginary frequency of 318.9 nm and134.6 nm, respectively. These two molecules became stable in the form of non- planar with C2hand C2v symmetry, respectively. The stability of various Si2F4 forms has already beeninvestigated [3, 4], but no reports were found for Si2F4(2+) and Si2F4(2-). In fact, Pseudo Jahn- Tellereffect (PJTE) is causing the symmetry breaking in these structures. The PJT, the Jahn- Teller (fordegenerate states) and the Renner- Teller (for linear molecules) are three properties of Jahn-Teller effect. The PJTE occurs for any systems [5]. Mixing the Ag ground state and the excitedstate leads to the distortions of high symmetry (D2h) configurations to low symmetry (C2h or C2v)configurations. The excited states were B2g and B3u for Si2F4 and Si2F4(2-) molecules, respectively.The results of the mixing were PJT (A g + B 2g) b 2g and (A g + B 3u) b 3u for Si2F4 and Si2F4(2-),respectively. The energy gap (Δ) between references states increased from Si2F4(2+) to to Si2F4(2-)(0,2.92, 4.44 eV). With increasing of the energy gap, the contribution of vibronic coupling of thereference states decreased. CAM-B3LYP/def2- TZVPP calculated results showed total energydifferences increased from Si2F4(2+) to Si2F4(2-). Also the variations of the chemical hardness (η)differences conformed to the observed trend of total energy differences. According to theseresults, the PJT stabilization energy decreases from Si2F4(2+) to Si2F4(2-). Eventually it can beresulted that PJTE of vibronic coupling to all the excited states is just caused destabilize or softenof the ground state in the direction nuclear displacements (Q). So the decreasing of energy gapdue to increasing of the vibronic coupling [5].

کلیدواژه ها:

Pseudo Jahn- Teller Effect ، Vibronic Coupling ، Energy Gap ، Chemical Hardness