Abstarct: In this thesis, the molecular structure, vibrationtional spectrum and nuclear magnetic resonance of two enantiomers of citalopram were investigated at B3LYP/6-311G** level of theory. Then, the sensitivity of this molecule toward the autoxidation mechanism was predicated by the bond dissociation energy for hydrogen abstraction (H-BDE). According to the calculated H-BDE values, it was concluded that the hydrogens of N(CH3)2 group and of its alkane chain are might be sensitive towards the autoxidation mechanism. To predict the relative stability and reactivity for the tixtle molecule, calculations of energies of frontier molecular orbitals (FMOs) in addition to the global chemical reactivity have been done. Also, local reactivity properties have been addressed through molecular electrostatic potential (MPE) and average local ionization energy (ALIE) surfaces. And then was compared to the values and surfaces of corresponding properties in The B3LYP/6-311++G** level of theory for SSRIs drugs. The results of this comparison showed less hardness and more chemical reactivity for Citalopram than the those of understudied drugs. Therefore, it might have more interactions with SERT proteins rather than other SSRIs, resulting in a higher concentration of Serotonin in the synapticcleft. The nonlinear optical property (NLO) of these SSRIs drugs were investigated and the results indicated that all of them have good NLO properties and fluvoxamine molecule (FVX) has considerable NLO properties rather than others.