Molecular frameworks

The introduction of phosphate into the framework has led to an extraordinary expansion of the possible pore diameters in framework molecular sieves. The discovery of VPI-5, the first 16-membered-ring aluminophosphate framework, was a landmark (8). Although it is a neutral framework, it... 

Applications to microporous solids such as molecular sieves included examples of site blocking by cations [10] and a first example of intersite exchange in mordenites [2], In open framework molecular sieves, in order to avoid the contribution of Xe-Xe interactions to the... 

Obviously the pore size determines which molecules can access the acidic sites inside the zeolite framework (molecular sieving effect) and is responsible for the shape selectivity observed with these materials (see later). The catalytic activity is also influenced by the acid strength of these sites which is determined by the Si/Al ratio (see above). The latter can be increased by post-synthesis removal of A1 atoms. Dealumination can be achieved by treatment with a... 

Zeolites and their frameworks The contents of neutral zeolite frameworks The contents of charged zeolite frameworks Molecular and ionic sieving Sorption of atoms and molecules... 

In 1982 Jones and Jacovac mechanically constructed a cubic section model using Framework Molecular Models." The cubic section model was conceived because... 

The fundamental difference between CN and Nj is simply that one molecule is centrosymmetric while the other is not. The lowest-energy UHF wave function in both cases suffers from unphysical spin localization, and it is illusory to believe that, 2 is the easier of the two molecules to calculate. The low spin-contamination solution to the UHF equations exists simply because of the molecular symmetry, while in CN the lower symmetry of the molecule allows the equations to converge to the lowest energy solution. This is a somewhat unappreciated difficulty in calculations on open-shell molecules. If one has appropriate elements of symmetry, then unphysical solutions can be avoided by enforcing the constraints on the wave function. Even if the constraints are not enforced, problems with the reference function are easily identifiable nonzero dipole moments along directions where the exact value must vanish by symmetry, unsymmetric spin densities, and so on. However, the issue is more diabolical in lower-symmetry species where localization does not break the framework molecular symmetry. In these cases, UHF and ROHF... 

Perego G, Millini R and Bellussi G 1998 Synthesis and characterization of molecular sieves containing transition metals in the framework Molecular Sieves Science and Technology vo 1, ed H G Karge and J Weitkamp (Berlin ... 

The orbitals and electrons of any particular boron atom are allocated to satisfy first the requirements of the external B-H single bonds and the bridge B-H-B bonds. The remaining orbitals and electrons are allocated to framework molecular orbitals. 

E.M. Flanigen, H. Khatami, H.A. Szymansky, in IR Spectral Studies of Zeolite Frameworks, Molecular Sieve Zeolites I., Advances in Chemistry Series, American Chemical Society, Washington DC, 1971. 

Another approach to the introduction of molecular mechanics into the curriculum is described in a paper by Biali. The stereochemical exercise discussed here involves the calculation of relative energies of the six possible isomers/conformers of [3.3]metacyclophanes. Even in this case, however, the input coordinates are calculated laboriously from framework molecular models and the calculations performed on a VAX 11/750 computer. An important development, however, described in this paper, is the transfer of coordinate data files to a visualization program (Ball and Stick) on the Macintosh microcomputer for simple visualization. This is one of the first descriptions of the use of file transfer on simple microcomputer programs to produce visually attractive structural representations. 

A distinction should also be drawn between framework molecular sieves and the wealth of crystalline solids prepared in the presence of an organic species that becomes incorporated in voids within the structure but cannot be removed, thermally or otherwise, without structural collapse. These are better described as open framework solids. In practice the distinction is blurred and, although only materials that are thermally and hydrothermally robust are likely to be commercially exploited in traditional technologies, open framework solids containing organic species can possess properties that may make them suitable for more specialised use. 

In this chapter I will show how the structural chemistry of framework molecular sieves has been elueidated by diffraction-based techniques and spectroscopies. Computer simulation of structures will be discussed separately in Chapter 4 as an additional technique. 

Brumlik Framework Molecular Model Kit (ISBN 0-13-330076-5) Models constructed with this kit allow students to see the relationship between atoms in organic molecules, including precise interatomic distances and bond angles. The flexible bonds can form strained systems, with the amount of bend in the bonds giving a qualitative idea of the amount of strain. 

Brumlik Uuiversal Molecular Model Kit (ISBN 0-13-931700-7) This scientifically accurate molecular model set demonstrates the framework of a molecule, the space filling capacity of a molecule, and molecular orbitals. This kit features color-coded atomic valence spheres and connectors. Its parts are fully interchangeable with the Brumlik Framework Molecular Model Kit... 

Examination of framework molecular models shows that, while the anti isomers 9 and 10 can only undergo polymerisation, the syn isomers 7 and 8 can, in principle at least, dimerise and cyclise the synjexo-R isomer 8 might conceivably be capable of cyclisation, i.e. it could undergo an intramolecular Diels-Alder reaction. At... 

Nicholas JB, Winans RE, Harrison RJ, Iton LE, Curtiss LA, Hopfinger AJ (1992) Ab initio molecular-orbital study of the effects of basis set size on the calculated stracture and acidity of hydroxyl-groups in framework molecular-sieves. J Phys Chem 96 10247-10257 NortierP, Borosy AP, Allavena M (1997) 6 initio Hartree-Fock smdy of Bronsted acidity at the surface of... 

Carboxylate Lanthanide Complexes with Multi-dentate Ligands Lanthanides Luminescence Applications Lanthanides in Living Systems Luminescence Luminescent Bioprobes Metal-Organic Frameworks Molecular Magnetic Materials Near-Infrared Materials. 

Fig. 36. Compound 309 Scheme of the topological arrangement of the bonds for the canonical forms, sp (left) and sp + cjp (right) together with those in the experimentally determined molecule (middle). The angles between the exocyclic SiMej substituents are shown likewise and were derived from an analysis of framework molecular models...

Robinson and Gillespie showed that simple framework molecular models enable surprisingly precise estimations to be made on the topological shortening of double bonds, triple bonds and even bonds existing in strained systems (bent bonds) [197]. This applies to the existing problem in two ways ... 

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