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 fro m 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 bigges t ions are involved (Rb+, Cs+), the penetration of N-2 into the latera l pockets is not possible. The ($) over bar v(N=N) frequency is found to depend linearly upon the 1/(R(x) + R(NN))(2) (R(x) = cation radius) : this is considered a clear proof of the predominant role of the elec trostatic forces. When the perturbed ($) over bar v(N=N) stretching fr equency is compared with the ($) over bar v(N=N) of N-2 perturbed by d ispersion 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 pos itive ions is always to shift the frequency toward higher values (as f ound for CO in M(+)-mordenite and M(+)-ZSM5): this result strongly sug gests an end-on interaction of N-2 with all alkali-metal extraframewor k cations, Indications that N-2 behaves as an hindered rotator (especi ally in Cs+-mordenite) were also obtained and discussed.