be used as units made up of fewer atoms than unit cells and, at the same time,
representing the whole zeolite structure in the spectroscopic image. Such an
approach appeared in the literature [36–40] and allowed quite accurate interpretation of particular fragments of zeolite spectra.
10.5.1 SBU Terminated by Protons (Dependence
of Framework Type on Characteristic Vibration
Modes)
The literature data show that units made of tetrahedra terminated by protons can be
a good model for real crystalline structures [41, 42]. Attempts to interpret the
spectra of zeolites using model units were made, inter alia, by Mozgawa et al. [43],
and these were calculations carried out for isolated pseudomolecules composed of
single and double rings (SBU analogs) , terminated with protons for charge neutralization. While this approach is relatively simple and does not disturb the initial
symmetry (bond angles) of the molecules (due to the very low mass of the proton
and the lack of unpaired electrons) [44, 45], it generates, however, quite a large
number of additional bands associated with vibrations of terminal oxygen with
“mobile” hydrogen (OH groups). As a result of the calculations, it was possible to
obtain, apart from the vibrational spectra of the units, also the animation of individual normal modes of vibrations and determine the changes of the corresponding
internal coordinates (angles and/or bonds lengths) and in consequence precise
assignment of bands to a particular type of unit vibration.
On the basis of the proposed method of interpreting the vibrational spectra, it is
possible to describe the specific structure of the zeolite framework. By modeling
pseudomolecules that are equivalents of the SBU, one can recognize the characteristic normal vibrations of these units and the corresponding bands in the spectra.
In Table 10.1, positions of PO- and RO-type bands identified in different zeolite
structures were collected.
Positions of bands associated with vibrations of individual rings are not fixed; in
different types of frameworks, individual rings are made of oxygen bridges that
differ in bonds angles, and thus the positions of individual bands in the spectra
change. It has been assumed that bands associated with vibrations of
low-multiplicity rings, such as S4R, occur at higher wave numbers, comparing to
the bands associated with vibrations of rings with higher multiplicities. It is also
worth noting that despite the occurrence of different ring systems in the structures of
zeolites, including 10- and 12-membered rings, in the literature, band assignments
to rings with multiplicity higher than 8 are not likely to be seen.
The identification of characteristic vibrations for individual structural groups has
allowed, in consequence, the use of vibrational spectra to determine the type of
zeolite framework. The verification of the use of the proposed models for the
interpretation of the vibrational spectra is the comparison of the obtained theoretical
308
M. Król et al.
representing the whole zeolite structure in the spectroscopic image. Such an
approach appeared in the literature [36–40] and allowed quite accurate interpretation of particular fragments of zeolite spectra.
10.5.1 SBU Terminated by Protons (Dependence
of Framework Type on Characteristic Vibration
Modes)
The literature data show that units made of tetrahedra terminated by protons can be
a good model for real crystalline structures [41, 42]. Attempts to interpret the
spectra of zeolites using model units were made, inter alia, by Mozgawa et al. [43],
and these were calculations carried out for isolated pseudomolecules composed of
single and double rings (SBU analogs) , terminated with protons for charge neutralization. While this approach is relatively simple and does not disturb the initial
symmetry (bond angles) of the molecules (due to the very low mass of the proton
and the lack of unpaired electrons) [44, 45], it generates, however, quite a large
number of additional bands associated with vibrations of terminal oxygen with
“mobile” hydrogen (OH groups). As a result of the calculations, it was possible to
obtain, apart from the vibrational spectra of the units, also the animation of individual normal modes of vibrations and determine the changes of the corresponding
internal coordinates (angles and/or bonds lengths) and in consequence precise
assignment of bands to a particular type of unit vibration.
On the basis of the proposed method of interpreting the vibrational spectra, it is
possible to describe the specific structure of the zeolite framework. By modeling
pseudomolecules that are equivalents of the SBU, one can recognize the characteristic normal vibrations of these units and the corresponding bands in the spectra.
In Table 10.1, positions of PO- and RO-type bands identified in different zeolite
structures were collected.
Positions of bands associated with vibrations of individual rings are not fixed; in
different types of frameworks, individual rings are made of oxygen bridges that
differ in bonds angles, and thus the positions of individual bands in the spectra
change. It has been assumed that bands associated with vibrations of
low-multiplicity rings, such as S4R, occur at higher wave numbers, comparing to
the bands associated with vibrations of rings with higher multiplicities. It is also
worth noting that despite the occurrence of different ring systems in the structures of
zeolites, including 10- and 12-membered rings, in the literature, band assignments
to rings with multiplicity higher than 8 are not likely to be seen.
The identification of characteristic vibrations for individual structural groups has
allowed, in consequence, the use of vibrational spectra to determine the type of
zeolite framework. The verification of the use of the proposed models for the
interpretation of the vibrational spectra is the comparison of the obtained theoretical
308
M. Król et al.