Stretching vibration modes frameworks

Some vibrational modes of zeolites are sensitive to the amount of aluminum in the framework [93]. The substitution of aluminum for silicon atoms in the zeolite framework changes the T-O-T bond angles (where T is a tetrahedral atom that can be either Si or Al). This is primarily due to the smaller size and different charge density of the aluminum atoms compared to silicon. This results in a shift in frequency for vibrational modes in the zeolite due to external linkages. The T-O-T asymmetric (1100-980 cm ) and symmetric (800-600 cm ) stretching modes as well as structural unit vibrations Mke double four- and double six-rings exhibit a shift to lower frequencies as the aluminum content of the framework is increased. Figure 4.19 shows this relationship for a faujasite-type framework. 

For pure Si-MCM-41. this band has been assigned to the Si-O stretching vibrations and the presence of this band in the pure siliceous is due to the great amount of silanol groups present. A characteristic absorption band at about 970 cm 1 has been observed in all the framework IR spectra of titanium-silicalites. It was also reported that the intensity of 970 cm 1 band increased as a function of titanium in the lattice[17] and this absorption band is attributed to an asymmetric stretching mode of tetrahetral Si-O-Ti linkages [18] in the zeolitic framework. The increase in intensity of this peak with the Ti content has been taken as a proof of incorporation of titanium into the framework. 

Thus 28 IR active modes are expected to fall in the regions of the vibrations of the orthosilicate anions. Of these, we can expect five modes associated with V3 (asymmetric stretching) and two modes associated with Vi (symmetric stretching), three modes associated with the symmetric deformation (V2) and five with the asymmetric deformation V4, four hindered rotations, four hindered translations, and, finally, five modes associated with Al—O tetrahedra. We actually observe at least 10 components for framework vibrations. Additionally, the low-frequency modes of Na ions are expected to fall in the FIR region [68], where several bands are indeed observed. 

The presence of iron in the framework site in clinoptilolite was also studied by IR [08R1]. A shift of the external tetrahedral asyrrrmetric stretching 1062 cm band in the NZ sample to 1077 cm in the OPAZ sample was explained as a consequence of the A1 arrd Fe extraction from the fiamework and the irrtroduction of P atoms. The irrtroduction of Fe species in Fe -OPAZ arrd Fe -OPAZ samples produced a new shift of this vibration mode to higher frequency (for sample compositions see Fig. 22 caption). 

The Si02 framework vibrations that occur at about 430, 800, 1070, and 1180cm in the fused silica spectrum (discussed in Section 1.4.2) can be explained by a vibrational calculation on a continuous random network (CRN) [36,37]. The 1070 and 1180 cm bands are assigned to the TO and LO modes of the Si-O asymmetric stretching vibration, respectively. Due to the selection rules, Raman-active modes involve symmetric vibrations, which... 

By the symmetry of a normal mode of vibration, we mean tbe symmetry of the nuclear framework under the distortion introduced by the vibration. Pictorially, the symmetry of the normal mode is equal to the symmetry of the pattern of arrows drawn to indicate the directions of the nuclear displacements. The normal modes of vibration of water are the symmetric and antisymmetric stretches, and the angle bend, shown in Figure 6-1. 

The 0-H stretching mode is a pure normal mode, with relatively strong an-harmonicity, which can be deduced from the knowledge of the 0-H potential curve at larger distances, obtained from the total energies computed at different 0-H distances at the optimized structure of the total system. Note that, in view of the high frequency of the vibration, one should freeze the framework geometry, which is not supposed to be re-optimized for the various 0-H separations. 

The INS spectra of hydrated minerals and those containing hydroxyls, are dominated by the librational modes. The complementarity of INS to infrared and Raman spectroscopies is apparent, the optical spectra below 2000 cm are dominated by the X-0 (X = e.g. C, S, Al, Si) stretch and bend vibrations of the framework. Gypsum, CaS04.2H20 [11,12], provides a classic example. 

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