Abstract: Using the B3LYP method of density functional theory, we optimized the geometric configuration of the gasphase Sphenylalanine (S-Phe) chiral enantiomers at the 3-21G basis set, calculated the Phe molecular orbital wave function 
and charge decomposition analysis (CDA) of fragment orbit, and gave the orbital interaction diagrams of fragments COOH,C₆H₅ and NH₂ during the formation of fragments. The results show that the maximum positive values of r_i of fragments COOH,C₆H₅ and NH₂ based on CDA are r₁₉,r₁₇,r₁₅, indicating that the formation of molecular orbits leads to the transfer of electrons from the fragment orbit to overlapping regions. The maximum negative values of ri of fragments COOH and NH₂ based on CDA are r₄₄ and r₃₅, indicating that the formation of molecular orbits results in the removal of electrons occupying fragment orbits from overlapping regions.

Key words:  , phenylalanine, fragment orbit, charge decomposition, orbital interaction