Framework symmetric stretching modes

The IR spectra of silicon oxides, in the framework region mode, is dominated by a strong absorption around 1000 cm due the anti-symmetric stretching of the Si - 0 - Si unit (Raman inactive mode) and by a less intense absorption around 800cm due the symmetric stretching of the Si-O-Si unit (Raman active mode). In the transparency window between these two modes, the IR spectra of TS-1 shows an additional absorption band located at 960 cm ... 

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. 

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. 

As can be realized from Fig. 12, the wavenumbers of, e.g., the asymmetric (v s) and symmetric (Vj) stretching modes were found to be linearly correlated to the mole fraction, n i/lnsi+nAi), of the frameworks of a homologous series of fauja-site-type zeolites. 

Fig. 12. Wavenumbers of various framework vibrations as a function of the mole fraction of aiuminum in tetrahedral (T) sites of the zeolite framework s indicates the slope, i.e., the decrease of the wavenumber (cm ) per 0.1 atom fraction of A1 ion substitution (adopted from [112]). D6R means double six-membered ring, and stand for symmetric and asymmetric stretching modes, respectively (cf. text and list of abbreviations)...

The IR spectrum of zeolite Al-ECR-18 shows the most prominent band located at 1063 cm" - Fig. 67a [05K1]. This band was assigned to symmetric T-0 stretching mode. In zeolite Ga-ECR-18, this band is located at 1043 cm. Such a redshift indicates the presence of Ga in tetrahedral framework. According to [99V3], the IR... 

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 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... 

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