Conformation of ethylene glycol and phase change in silica sodalite

Richardson, J. W.; Pluth, J. J.; Smith, J. V.; Dytrych, William J.; Bibby, D.M.

doi:10.1021/j100312a052

Cited by 117 publications

(64 citation statements)

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“…X-ray diffraction studies of pure silica sodalites that contain ethylene glycol show that the framework adopts a density in the range of 17.0-17.4 T-atoms nm −3 , with Si-O-Si angles of 159.7-155.2 • , in agreement with values we predict for the low-density end of the ideal framework [28,33]. We were unable to locate a structure refinement of pure-silica sodalite free of extra-framework inclusions.…”

Section: Resultssupporting

confidence: 87%

Flexibility mechanisms in ideal zeolite frameworks

Treacy

Kapko

Rivin

2014

Phil. Trans. R. Soc. A.

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Zeolites are microporous crystalline aluminosilicate materials whose atomic structures can be usefully modelled in purely mechanical terms as stress-free periodic trusses constructed from rigid corner-connected SiO 4 and AlO 4 tetrahedra. When modelled this way, all of the known synthesized zeolite frameworks exhibit a range of densities, known as the flexibility window, over which they satisfy the framework mechanical constraints. Within the flexibility window internal stresses are accommodated by force-free coordinated rotations of the tetrahedra about their apices (oxygen atoms). We use rigidity theory to explore the folding mechanisms within the flexibility window, and derive an expression for the configurational entropic density throughout the flexibility window. By comparison with the structures of pure silica zeolite materials, we conclude that configurational entropy associated with the flexibility modes is not a dominant thermodynamic term in most bulk zeolite crystals. Nevertheless, the presence of a flexibility window in an idealized hypothetical tetrahedral framework may be thermodynamically important at the nucleation stage of zeolite formation, suggesting that flexibility is a strong indicator that the topology is realizable as a zeolite. Only a small fraction of the vast number of hypothetical zeolites that are known exhibit flexibility. The absence of a flexibility window may explain why so few hypothetical frameworks are realized in nature.

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Section: Resultssupporting

confidence: 87%

Flexibility mechanisms in ideal zeolite frameworks

Treacy

Kapko

Rivin

2014

Phil. Trans. R. Soc. A.

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“…Defects have been introduced into unit cells of pure siliceous sodalite and chabazite materials (each of which has 36 atoms in the unit cell). The lattice parameter, a, of the cubic sodalite has been optimized in our previous work (8.878 , compare with 8.830 from experiment [44] for the single crystals that contained ethylene glycol); the lattice parameters, a and a, of the monoclinic chabazite have been taken from experiment (9.421 and 94.2). All lattice parameters have been fixed, whilst we have undertaken full geometry optimization with respect to all atomic coordinates.…”

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confidence: 99%

Computational Investigation into the Origins of Lewis Acidity in Zeolites

et al. 2000

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The catalytic abilities of zeolites are attributed largely to their acidic properties, in both the Brønsted (proton donating) and Lewis (electron accepting) senses, although the origin and mechanism of the Lewis acidity remain unclear. Here is presented a computational approach to identifying Lewis acid sites in zeolite frameworks by analyzing the major candidate structures, such as the Al hydroperoxide shown in the Figure.

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“…NMR is potentially more sensitive than X-ray diffraction to local environment (22,23), and is the analytical tool of choice for amorphous or disordered solids. Thus correlations of the chemical shift with easily calculated structuredependent functions are becoming increasingly important.…”

Section: Resultsmentioning

confidence: 99%

Long-range shielding and NMR chemical shifts in silicon carbide polytypes

Guo¹,

Hartman²,

Richardson³

1992

Can. J. Chem.

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This paper is dedicated to the memory of Professor Thomas BirchallDEQI GUO, J. STEPHEN HARTMAN, and MARY FRANCES RICHARDSON. Can. J. Chem. 70, 700 (1992). Despite an apparently successful empirical correlation, it is incorrect to ascribe variations in ' 9~i and "C chemical shifts in silicon carbide polytypes to diamagnetic anisotropy effects. Silicon carbide, an important industrial material ( l ) , is known for its remarkable display of polytypism. There are more than 200 known polytypes, which are characterized by having identical Si-C double layers arranged in different stacking sequences (2, 3). Since our initial work on the distinctive 2 9~i and I3c Magic Angle Spinning (MAS) NMR spectra of the 6H, 3C, and 15R polytypes (4, 5), NMR has become an important characterization tool for silicon carbide (6). All polytypes so far studied other than 3C and 2H, several peaks in their MAS NMR spectra (4, 5, 7-9) despite the fact that the immediate-neighbour tetrahedron in S i c is essentially identical for all sites (10). We (4, 5) and others (7,11) showed that the number of peaks could be correlated with the number and geometrical arrangement of higher neighbours. However, peaks could not be readily assigned to particular atoms in the structures. We undertook the present work as a possible approach to S i c peak assignments. McConnell (12) expressed the relationship between the chemical shielding increment Au and the molar diamagnetic anisotropy AX of distant (cylindrically symmetrical) bonds as: Calculationswhere AX is the difference between the susceptibility parallel to the bond and the susceptibility perpendicular to it, R is the distance between the nucleus of interest and the electrical center of gravity of the distant bond, and 0 is the angle between the vector R and the bond vector (Fig. 1).Expansion of eq.[I] to include all contributions within a certain distance from a central atom gives the shielding increment for that atom:The chemical shift, in ppm to low field of a reference com-'~u t h o r s to whom correspondence may be addressed. pound, is related to the shielding by 6,,,,, = u,, , , , , urn,,,, and can be divided into local and long-range parts, so that where 6,,,, is the overall chemical shift of atom X(j) in the structure, 6; is the "intrinsic" chemical shift, i.e., the shift that atom X would have if it were bonded only to its four neighbours, and (Ax)(G,) represents the chemical shift due to long-range shielding because of the diamagnetic susceptibility of chemical bonds. G,, the geometric factor, is a term dependent only upon the crystal structure and the location of the electrical centre of gravity for the bonds, initially taken at the midpoint. The sum is over all bonds n out to a certain radius. Equation [4] allows a value of AX to be calculated explicitly and compared to experimental values.Bonds to nearest neighbours were eliminated from the sum, both because the point dipole approximation is not valid at such short ranges (12) and because the nearest-neighbour effects are assumed to be a...

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Conformation of ethylene glycol and phase change in silica sodalite

Cited by 117 publications

References 0 publications

Flexibility mechanisms in ideal zeolite frameworks

Flexibility mechanisms in ideal zeolite frameworks

Computational Investigation into the Origins of Lewis Acidity in Zeolites

Long-range shielding and NMR chemical shifts in silicon carbide polytypes

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