The integrated intensity A of infrared forbidden bands is related to the field E which induces the band. We present the relations between A and E in the case of rotating and oriented molecules, and we analyse the induced spectrum of N2 adsorbed in NaA. The field value obtained by means of the band intensity is compared with theoretical values.

Infrared spectra of CO, physisorbed on NaA zeolites have been recorded at various temperatures between 178 and 423 K for a constant level of adsorption of ca. 0.5 molecule per cavity. The v3 band profile shows a superposition of two or three bands whose intensities vary with respect to temperature. Preadsorption of water offers better conditions for the observation of bands 2 and 3 owing to the fact that band 1, which is the most intense in the absence of water, is suppressed. We assign the components of the v3 band to different types of adsorption.

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Sir: In the range of infrared spectra investigations of molecules physically adsorbed on zeolites,1 we have studied the spectra of methane adsorbed on NaA, CaA, and NaX. After Hirschfelder, the collision diameter is 3.8 Á, but calculations from C-H bond lengths and van der Waals radii for C and H atoms actually yield approximately 4.6 Á. We know the effective diameter of the apertures of NaA (4.2 Á), CaA (5 A), and NaX (8 Á) zeolite cavities;2 therefore, it seems likely that adsorption of methane by the two latter molecular sieves occurs, but in the case of the NaA zeolite, this adsorption is expected to be restricted, due to the small pore size impeding diffusion.The spectra of the three molecular sieves previously desorbed at 400°below 5 X 10~5 Torr1 and then exposed to CH4 at pressures greater than 500 Torr show no absorption bands when the zeolite pellet is at the same temperature as the beam of the spectrophotometer, i.e., 313°K. At 173°K, however, under gas pressures of approximately 20 Torr, we have observed important intensities of CH4 absorption bands in the case of NaX (Figure 1), slightly weaker intensities in the case of CaA, and very weak intensities in the case of NaA. CH4 therefore penetrates the NaA cavities, but the adsorbed amount is very small.There are four fundamental vibration frequencies for CH4: in the gaseous phase, zq and zq are ir active, while zq and z-2 are only Raman active, although zq has never been observed (likely its intensity is too weak). In the adsorbed state, besides the two allowed bands, the forbidden band zq appears with approximately the same intensity as zq and zq, due to the dipole moment induced by the field existing in-side the cavity. The intensity of zq is proportional to (am/ aq)2, i.e., E2(aa/aq)2, so it is proportional to the square of the field, and to the intensity of the Raman band (which accounts for the fact that the zq band is not apparent in the adsorbed phase). The frequencies of CH4 in the adsorbed and gas phases are given in Table I.

Two theoretical models are considered. The first is a model of energetically different sites, the second a model of the distribution of interacting molecules on energetically homogeneous cavities. The temperature dependence of distributions for adsorbed amounts com9rising between 0.2 and 0.7 molecules per cavity are compared with the results of Part 1. The first model seems more adequate and an interaction calculation confirms the heterogeneity of the cavity. On the other hand, it does not permit precise determination of the multiplicity of the sites. We have thus been led to analyse other experimental data. Isotherms and adsorption heat curves deduced from infrared results show a good fit with results obtained by other authors for up to 4 molecules per cavity with the assumption-that there are 1 and 6 sites of each type. The energy would be 57 and 50 kJ mol-', respectively, and the mean frequency of external vibration ca. 50 m-l.