Stewart

Stewart A M and Christenson H K 1990 Use of magnetio foroes to oontrol distanoe in a surfaoe foroe apparatus Meas. Sc/. Tec/ino/12 1301-3... 

Hehre W J, Stewart R F and Popie J A 1969 Seif-consistent moiecuiar-orbitai method i. Use of Gaussian expansions of Siater-type atomic orbitais J. Chem. Phys. 51 2657-64... 

Stewart J J P 1991 Semiempirical molecular orbital methods Reviews in Computationai Chemistry vo 1, ed K B Lipkowitz and D B Boyd (New York VCH) pp 45-81... 

Stewart J J P 1989 Optimization of parameters for semlempirical wavefunctlons J. Comput. Chem. 10... 

Dewar M J S, Mealy E F and Stewart J J P 1984 Location of transition states in reaction mechanisms J. Chem. Soc. Faraday Trans. II80 227... 

Stewart R, Whitesides G M, Godfried H P and Siivera i F 1986 improved adhesion of thin oonformai organio fiims to metai surfaoes Rev.Sci. Instrum. 57 1381-3... 

Wall M C, Stewart B A and Mullin A S 1998 State resolved oollisional relaxation of highly vibrationally exoited pyridine ( ii = 38,000 om ) and CO2 influenoe of a permanent dipole moment J. Chem. Rhys. 108 6185-96... 

Stewart, J.J.P. MOPAC A Sem iem pirical Molecular Orbital Program../. Computer-Aided Molecular Design 4 1-105, 1990. 

Stewart, R.P. Small Ciaussian exptuisioris of. Slaler-Type Orbitals,/. Chi .m. Phys. S2 d.G-42S. 1970. 

The following data ( fable 1) for niolcctilcs, including hydrocarbon s, strained ring system s. molecn les with heieroatom s, radicals, and ions conies from a review by Stewart. For most organic molecules,, YM 1 reports heals of formation accurate to within a few kilocalories per rn ol. bor soni e molecules (particularly inorgari ic compoun ds wdth several halogens, such as perch loryl fluoride, even the best sem i-em pineal method fails completely. 

Hrmrs in ion i/.ation potentials are typically a few ten tli s of an electron volt, a small percentage of the total. Stewart gives tables with... 

Algebraically, the dusty gas flux relations are identical with one of the many particular cases of the Feng and Stewart model, as we shall see. However, the two differ conceptually in their approach to deriving the flux i elations. ... 

Of course, these shortcomings of the Wakao-Smith flux relations induced by the use of equations (8.7) and (8.8) can be removed by replacing these with the corresponding dusty gas model equations, whose validity is not restricted to isobaric systems. However, since the influence of a strongly bidisperse pore size distribution can now be accounted for more simply within the class of smooth field models proposed by Feng and Stewart [49], it is hardly worthwhile pursuing this."... 

The class of models formulated by Feng and Stewart conceptually... 

The flux N (a,w) is the Sum of contributions from a gaseous phase flux and a flux due to surface diffusion. The surface diffusion contribution is given by equation (7.7) or, more generally, by the corresponding relation which follows from equation (7.5). For the gaseous phase contribution Feng and Stewart assume flux relations of the dusty gas form, (5.1)- ... 

This is the most general flux relation of the Feng and Stewart class. 

Equations (8.21) still contain too many adjustable parameters to be of much value for predictive purposes, and Feng and Stewart propose three simpler special cases which may be of practical value. 

The pore size distribution function (a) appears parametrically in the flux relations of Feng and Stewart, so their models certainly cannot be completely predictive in nature unless this distribution is known. It is... 

Though a porous medium may be described adequately under non-reactive conditions by a smooth field type of diffusion model, such as one of the Feng and Stewart models, it does not necessarily follow that this will still be the case when a chemical reaction is catalysed at the solid surface. In these circumstances the smooth field assumption may not lead to appropriate expressions for concentration gradients, particularly in the smaller pores. Though the reason for this is quite simple, it appears to have been largely overlooked,... 

Equation (9.23) is to be compared with the Feng and Stewart relations (9.4) which describe the fluxes in the same system under non-reactive conditions, The factor (BA coth BA - 1) has the form sketched in Figure 9.2. From the definition of B given by equation (9.19) it is seen chat 9- 0 as and each tend to zero, their ratio remaining equal to Che... 

They then compared measured and predicted fluxes for diffusion experiments in the mixture He-N. The tests covered a range of pressures and a variety of compositions at the pellet faces but, like the model itself, they were confined to binary mixtures and isobaric conditions. Feng and Stewart [49] compared their models with isobaric flux measurements in binary mixtures and with some non-isobaric measurements in mixtures of helium and nitrogen, using data from a variety of sources. Unfortunately the information on experimental conditions provided in their paper is very sparse, so it is difficult to assess how broadly based are the conclusions they reached about the relative merits oi their different models. 

T-Jhile the stoichiometric relations have rendered the above problem tractable by permitting an explicit solution of the dusty gas model flux relations, it should be pointed out that they do not lead to equally radical simplifications with all flux models. In the case of the Feng and Stewart models [49- for example, Che total flux of species r is formed by in-... 

The nearest thing to a complete justification of equations (11.3)i for pellets of arbitrary shape, is an argument given by W. E. Stewart [74], which does not depend on any particular choice of flux relations. In Chapter 10 it was pointed out that all isothermal flux models must have the general form... 

Stewart s argument provides a prescription for constructing a solution of equations (11.61) - (11.63) provided the matrix Is nonsingular for all relevant values of jc, and provided the differential equations (11.64) and (11.65) have solutions consistent with their boundary conditions. It is possible, in principle, to check the nonsingularity of for any... 

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