Abstract: Using the B3LYP method of density functional theory (DFT), the author optimized the geometry of gas phase phenylalanine (Phe) molecule at the level of 6-311+G(2df) basis set, and analyzed the characteristics of intermediate and transition state of each elementary reaction in the process of chiral transition of Phe molecular system. Under PBE0/def2-TZVP, the time-dependent density functional theory (TDDFT) was used to obtain the electronic excitation characteristics of transition state in the process of chiral transition of Phe molecular system under gas phase conditions. The results show that according to the criteria of Δr index and the hole-electron index of the electronic excitation characteristics, the obvious excitation characteristics
 can be identified effectively, and the judgment results of the excitation characteristics of the transition state molecular system in the elementary reaction are given directly.

Key words: phenylalanine (Phe), hole-electron, transition state, electronic excitation