Pore-opening vibrations

Among pore-opening vibrations of zeolite structures, those resulting in fluctuations of window aperture are of great importance for the majority of zeolite applications, because the window openings dictate the molecular sieving... 

In their combined FTIR /INS investigation of H-Y, Jacobs et al. [253] found framework T-0 vibrations bands at 1130,765,565,319 cm and assigned them to the asymmetric stretching, v s, the symmetric stretching v, the D6R vibration (shoulder) and the pore opening vibration, respectively, essentially in line with... 

Figure 14 shows the atomic displacements and compares the vibrational frequencies of the 4-R pore-opening mode computed in Refs. 97, 98, and 153. 

The thermal decomposition of ammonium rho-zeolites- has also been followed by inelastic and quasielastic neutron scattering [112]. As the deammoniation temperature was increased, the assigned vibrational features of the bound ammonium ion at 1452 cm (bending modes), 306-363 cm" (ammonium-coupled pore-opening modes of the zeolite framework), and 80-113 cm" (hindered rotational modes) were foimd to diminish with the emergence of scattering features due to H-rho. 

Detailed consideration has been given to the nature of the vibrational motion giving rise to the external linkage frequencies related to double rings and pore openings. The isolation of these building units in the most open zeolite frameworks may lead to uncoupling of their vibrations... 

In this case, the critical diameters of the starting material molecules are equal to or shghtly larger than the pore openings of the zeolite, but as a result of molecular vibrations imder the reaction conditions, are able to enter the zeohte pores, where they react. Here the reactions are largely diffusion controlled. 

Figure 13. Far infrared spectrum of dehydrated NaY. P denotes a pore opening mode and F an oxygen framework vibration. The vibration of cations in the various sites are referred to by the site number (1,1, II, III) (from 82)...

From another standpoint, it may be said that the mobility of the guest species within the channels is considerably lower than that in the bulk fluid that is, the occluded molecules have sustained significant decreases in translational, rotational, and vibrational degrees of freedom (entropy effects), compared with their vapors (12). Similar effects would be expected in relatively small pore ion-exchange gels, such as Dowex 50 or Amberlite I.R. 120, with aqueous or polar solvents systems (42). Thus, zeolitic water may be viewed as intermediate in mobility between the bulk liquid and ice (1). Further, clusters of molecules may exist in zeolite cavities. In faujasite, due to its openness, these clusters are not merely isolated, but form continuous filaments of dense zeolitic fluid (12). 

E. M. Flanigen You have raised a difficult point in our interpretation which we have considered and discussed in the paper. Our present feeling is that the internal tetrahedron vibrations are highly coupled, and that it is by reason of the large pore volume which isolates polyhedral units, such as the double four rings in zeolite A, that they can be seen as distinct entities. We propose that that is why, in the less open structures with higher densities and lower pore volumes, you do not see the same kind of general features in the infrared pattern as you do in the most open frameworks. 

Water in well-characterized pores is a system of general interest because it serves as model system for the non-bulk or inhomogeneous water that is ubiquitous in biological and geological systems, as well as in nano-sfructured materials. Often confined or interfacial water is highly relevant to the properties and functions of entire systems, e.g., those in ion channels and clay minerals. X-ray diffraction studies show that water can fill the inner space of open-ended single-walled carbon nanotubes (SWCNTs) under ambient conditions and freezes into crystalline solids. These are often referred to as ice nanotubes . Ice stmctures in confined systems are characterized as stacked n-membered rings or equivalently as a rolled square-net sheet. The formation of the ice nanotubes in CNTs has also been observed by NMR, neutron diffraction, and vibrational spectroscopy studies. 

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