Comparison is made of the interaction of NO2 with Cu+ ions in the gas phase and inside zeolites using density functional theory (B3LYP functional). The zeolite is represented by a tritetrahedra model embedded in the periodic structure of zeolite ZSM-5 and by a free space cluster model. Both models yield virtually the same results. Cu+ is coordinated to two oxygen atoms of the zeolite framework only. For the complexes with NO2 several minima and transition structures on the potential energy surfaces are localized. The naked Cu+ ion preferentially binds NO2 in the 2η1-O trans mode, while in zeolites the Cu+ site binds NO2 in 2η-O,O coordination. For the 2η-O,O structure the binding is three to four times stronger in the zeolite (43 kcal/mol) than in the gas phase which is due to a three-body zeolite framework - Cu+ ion-NO2 interaction. d10-s1d9 promotion leads to a more favorable orbital interaction between Cu+ and NO2 in the 2A'' state and, due to reduced repulsion, to a stronger electrostatic interaction between Cu+ and the zeolite framework.