Theoretical Study On The Symmetry Breaking Mechanism Induced By Pseudo Jahn-Teller Effect

Symmetry is an inherent property of molecules and crystals,and it always accompanies with symmetry breaking.The study of the spontaneous symmetry breaking mechanism plays a key role in understanding and manipulating the molecular or crystal configuration,as well as revealing the special properties caused by the configuration changes.It is a bridge to construct the relationship between structures and properties.The pseudo Jahn?Teller effect?PJTE?is a powerful theoretical tool to deal with the symmetry problem of molecules or crystals.In this thesis,combined high-level quantum chemistry calculations with the PJTE theory,the micro-mechanism of the geometric symmetry breaking and the resulting sudden polarization was investigated for several types of organic molecules.The PJTE origin of symmetry breaking of group-IV-based triphosphides two-dimensional crystal material was revealed,and the relationship of buckling configuration and sodium storage capacity was determined.These findings are expected to provide insight into organic photochemical synthesis and guide development of new materials for the anodes of sodium-ion batteries.The main findings are as follows:The PJTE origin of the symmetry breaking in high-symmetry panar structure and the formation of out-of-plane bending geometrical configuration was revealed for halogenabenzenes?CH?₅X?X = F,Cl,Br,I?and C₆H₇^-,and effective strategies were proposed to restore their planar configuration.It was found that the vibronic coupling of ¹A₁ ground state and ¹B₁ excited states with C_2v planar structure along the out-of-plane b₁ distortion triggers the symmetry breaking of planar configuration to form a stable non-planar buckling structure with C_s symmetry.The difference in PJTE stabilization energies of these molecules can be explained by analyzing numerically the energy gaps of coupled electronic states and PJTE vibronic coupling parameters.By tracking the origin of restored planar?CH?₅X²⁺ cation,as well as their isoelectronic species?CH?₅O⁺ and?CH?₅N,several effective strategies were proposed to stabilize the planar configuration for halogenabenzenes,such as removing two ? electrons from?CH?₅X or adding the substitute group with strong electron-withdrawing to replace hydrogen atoms.This study could find potential applications in controllable electronic devices or sensors.To sovle the complicated the PJTE multimode problem involving many polar modes,the “zwitterion mode” was proposed.It was revealed that the symmetry breaking of geometrical configuration and sudden polarization of octofluorostilbene was induced by PJTE.The results indicated that the PJTE coupling of lowest S₁?1¹B_u?and S₂?2¹A_g?excited states in planar configuration with C_2h symmetry along b_u vibronic mode caused the instability of planar strcture and the formation of zwitterion with C_s symmetry.The polarization in other octohalogenated stilbene was predicted and possible manipulations of the sudden polarization in Franck-Condon region by substitution strong of group with strong electron-withdrawing are also discussed.It is revealed that the sudden polarization in the photoinduced process of ethylene and butadiene are consequences of the PJTE.The reason for the disappearance of the sudden polarization of hexadiene and stilbene was analyzed.Comparing stilbene and octohalogenated stilbene,it provides a theoretical framework for the experimental study of organic photochemical reactions and sudden polarization.The crystal structure of unreported ?-MP₃?M = C,Ge,Sn?was predicted.Based on the proper simplified molecular models M₂P₆H₈,the PJTE origin of symmetry breaking of planar structure and the formation of buckling configuration of ?/?-MP₃ was revealed.It is illustrated that the vibronic coupling of 1¹A_1g ground state and ¹B_2g excited states in planar D_2h configuration of M₂P₆H₈ along 1b_2g and 2b_2g distortion triggers a preferred ?-type buckling structure,and the PJTE coupling between 1¹A_1g ground state and ¹B_2g excited states along 1b_2g distortion with 1¹A_1g state and ¹B_1g states along b_1g distortion leads to the ?-type buckled configuration.The difference in buckling height of MP₃ strongly relates to the PJTE intensity and the energy gap of the coupled excited states.Upon the Na adsorption,the buckling height of ?-GeP₃ and ?-SnP₃ decreases,thus facilitating the storage capacity of Na as anode materials of sodium-ion batteries.This process can be explained by the suppressed role of PJTE.The structure-activity relationship provides theoretical basis for the preliminary screening of anode materials for sodium-ion batteries.