constructed of two connected 4-membered rings (unit D4R) is much more unambiguous, which is mainly due to the greater stiffness of the system. Compared to
single rings, they are much less deformed, and the connection of the rings with each
other stiffens the unit, which greatly facilitates the unequivocal identification of
characteristic vibrations. The interpretation scheme for this system is shown in
Fig. 10.6.
The effect of shifting most of the bands associated with particular vibrations
toward the lower wave numbers together with the increase in the substitution of Si
with Al is clearly visible. In the spectrum of aluminum-free pseudomolecules
(H 8 [Si 8 O 20 ]), the band with the highest integral intensity (at 1114 cm
−1 ) is derived
from the asymmetric stretching vibrations in Si–O–Si bridges. Substitution of one
[SiO 4 ] tetrahedron by [AlO 4 ] in a unit (H 8 [Si 7 AlO 20 ]
– spectrum) causes the band
associated with these vibrations to shift to lower wave numbers (1098 cm
−1 ). At the
same time, the bands from asymmetric stretching vibrations of Si–O–Al at about
1120 cm
−1 become clearly visible (in H 8 [Si 7 AlO 20 ]
– spectrum, it is visible only as a
slight inflection). The introduction of further aluminum tetrahedra (H 8 [Si 5 Al 3 O 20 ]
3–
pseudomolecule spectrum) causes the band associated with Si–O–Si vibrations to
change position to 1047 cm
−1 and band related to m as Si–O–Al vibrations to appear
as a separate one at 1086 cm
−1 . Finally, in the spectrum of the H 8 [Si 4 Al 4 O 20 ]
4–
unit, in which only Si–O–Al bridges exist, the bands at 1050 and 958 cm
−1 are
presented. Analysis of this type leads to the conclusion that the introduction of
aluminum in subsequent units modifies the bands associated with Si–O(T) vibrations and simultaneously causes the appearance of additional bands not present in
the spectrum of pure silicon units. This is a significant difference to the experimental spectra of aluminosilicates, in which the presence of [AlO 4 ] tetrahedral does
not cause the appearance of additional bands originating from the asymmetric
stretching vibrations m as Si–O–Al, but only modifies the position and the envelope
of bands associated with Si–O–Si bridge vibrations [51]. The shifting of the m as Si–
O–(Si,Al) band toward the lower wave numbers together with the increasing aluminum content in the material framework is attributed to the longer Al–O bond
length compared to Si–O, and hence its lower force is constant [52].
In a similar way, the influence of aluminum substitutions on other types of
vibrations can be analyzed [46]. The tendency to change the position of the bands
with an increase in aluminum content in SBU also applies to the characteristic RO
vibrations, the positions of which are marked in Fig. 10.6. Similar observations
were made by Mozgawa [46] for calculations carried out for S4R, S6R, and D6R
units.
In summary, it can be concluded that isolated molecules are fairly good models
for describing vibrational spectra of 3D aluminosilicates, including zeolites, and the
calculated spectra can be used for the interpretation of experimental ones. It was
found that it is possible to determine the number and positions of bands originating
from RO and PO characteristic vibrations and that double rings are more rigid than
single ones and to a lesser extent deformable and therefore are a better model for
identifying characteristic vibrations, and the substitution with aluminum in tetrahedral positions reduces the frequency of SBU characteristic vibrations.
10 Vibrational Spectroscopy of Zeolites …
311
single rings, they are much less deformed, and the connection of the rings with each
other stiffens the unit, which greatly facilitates the unequivocal identification of
characteristic vibrations. The interpretation scheme for this system is shown in
Fig. 10.6.
The effect of shifting most of the bands associated with particular vibrations
toward the lower wave numbers together with the increase in the substitution of Si
with Al is clearly visible. In the spectrum of aluminum-free pseudomolecules
(H 8 [Si 8 O 20 ]), the band with the highest integral intensity (at 1114 cm
−1 ) is derived
from the asymmetric stretching vibrations in Si–O–Si bridges. Substitution of one
[SiO 4 ] tetrahedron by [AlO 4 ] in a unit (H 8 [Si 7 AlO 20 ]
– spectrum) causes the band
associated with these vibrations to shift to lower wave numbers (1098 cm
−1 ). At the
same time, the bands from asymmetric stretching vibrations of Si–O–Al at about
1120 cm
−1 become clearly visible (in H 8 [Si 7 AlO 20 ]
– spectrum, it is visible only as a
slight inflection). The introduction of further aluminum tetrahedra (H 8 [Si 5 Al 3 O 20 ]
3–
pseudomolecule spectrum) causes the band associated with Si–O–Si vibrations to
change position to 1047 cm
−1 and band related to m as Si–O–Al vibrations to appear
as a separate one at 1086 cm
−1 . Finally, in the spectrum of the H 8 [Si 4 Al 4 O 20 ]
4–
unit, in which only Si–O–Al bridges exist, the bands at 1050 and 958 cm
−1 are
presented. Analysis of this type leads to the conclusion that the introduction of
aluminum in subsequent units modifies the bands associated with Si–O(T) vibrations and simultaneously causes the appearance of additional bands not present in
the spectrum of pure silicon units. This is a significant difference to the experimental spectra of aluminosilicates, in which the presence of [AlO 4 ] tetrahedral does
not cause the appearance of additional bands originating from the asymmetric
stretching vibrations m as Si–O–Al, but only modifies the position and the envelope
of bands associated with Si–O–Si bridge vibrations [51]. The shifting of the m as Si–
O–(Si,Al) band toward the lower wave numbers together with the increasing aluminum content in the material framework is attributed to the longer Al–O bond
length compared to Si–O, and hence its lower force is constant [52].
In a similar way, the influence of aluminum substitutions on other types of
vibrations can be analyzed [46]. The tendency to change the position of the bands
with an increase in aluminum content in SBU also applies to the characteristic RO
vibrations, the positions of which are marked in Fig. 10.6. Similar observations
were made by Mozgawa [46] for calculations carried out for S4R, S6R, and D6R
units.
In summary, it can be concluded that isolated molecules are fairly good models
for describing vibrational spectra of 3D aluminosilicates, including zeolites, and the
calculated spectra can be used for the interpretation of experimental ones. It was
found that it is possible to determine the number and positions of bands originating
from RO and PO characteristic vibrations and that double rings are more rigid than
single ones and to a lesser extent deformable and therefore are a better model for
identifying characteristic vibrations, and the substitution with aluminum in tetrahedral positions reduces the frequency of SBU characteristic vibrations.
10 Vibrational Spectroscopy of Zeolites …
311