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 eta(1)-O trans mode, while in zeolites the Cu+ site binds NO2 in a (2) eta-O,O coordination. For the 2 eta-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. d(10)-s(1)d(9) promotion leads to a more favorable orbital interaction between Cu+ and NO2 in the (2)A " state and, due to reduced repulsion, to a stronger electrostatic interaction between Cu+ and the zeolite framework.