Characterization by Transmission IR Spectroscopy

The IR range of combination and overtone modes hes above ca. 4000 cm . It is also extremely difficult to investigate this region by transmission IR spectroscopy because of the weak intensities of the respective bands which usually are only 3 to 5% of that of the fundamentals. In this context one has to keep in mind that the absolute transmission of zeolite wafers in the region of fundamental OH stretching modes is sometimes only 1% or even less (vide supra, e.g.. Sect. 5.4.1.2.3). 

Measured frequency (cm ) Assigned to combination vibrations Derived Computed frequency (cm ) 

Frequency of the fundamental stretching vibration of the OH group in a Bronsted center, =Si-OH-Al=, of H-ZSM-5 zeolite. 

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