N-2 ADSORPTION AT 77 K ON H-MORDENITE AND ALKALI-METAL-EXCHANGED MORDENITES - AN IR STUDY

N-2 interacts with positive M(+) centers (M(+) = H+, Li+ Na+, K+, Rb+, and Cs+) present in M(+)-mordenites, with activation in the IR of the N=N stretching vibration. From the analysis of the IR spectra and from the examination of the available space present in the main channels and in the lateral pockets, it is concluded that N-2 can interact with smallest ions located in both positions and that only when the biggest ions are involved (Rb+, Cs+), the penetration of N-2 into the lateral pockets is not possible. The ($) over bar v(N=N) frequency is found to depend linearly upon the 1/(R(x) + R(NN))(2) (R(x) = cation radius): this is considered a clear proof of the predominant role of the electrostatic forces. When the perturbed ($) over bar v(N=N) stretching frequency is compared with the ($) over bar v(N=N) of N-2 perturbed by dispersion forces only (as for N-2 in the cages of rare gas matrices), it is concluded that the effect of the perturbation induced by the positive ions is always to shift the frequency toward higher values (as found for CO in M(+)-mordenite and M(+)-ZSM5): this result strongly suggests an end-on interaction of N-2 with all alkali-metal extraframework cations, Indications that N-2 behaves as an hindered rotator (especially in Cs+-mordenite) were also obtained and discussed.