Crystallization reliability

Kurst G R, R A Stephens and R W Phippen 1990. Computer Simulation Studies of Anisotropic iystems XIX. Mesophases Formed by the Gay-Berne Model Mesogen. Liquid Crystals 8 451-464. e F J, F Has and M Orozco 1990. Comparative Study of the Molecular Electrostatic Potential Ibtained from Different Wavefunctions - Reliability of the Semi-Empirical MNDO Wavefunction. oumal of Computational Chemistry 11 416-430. 

Osazone formation. Forms a yellow osazone, m.p. 208° soluble in hot water. See p. 137 for preparation. If examined under the microscope very characteristic clusters of hedge-hog crystals will be seen (Fig. 63(c), p. 139). The difference in the crystalline appearance of lactosazonc and maltosazone should be very carefully noted, as this difference forms the chief and most reliable method of differentiating between these two sugars. 

A crystal material is excited by the force imposed on it by an internal I v mounted mass. A voltage is produced by the crystal proportional to accel eration. This voltage is then amplified by a charge amplifier type signal conditioner from whence the signal can be transmitted long distance. (1.000 feet is not uncommon) to the monitor/readout unit. It is calibrated in terms of gravitational units (g), which are proportional to force. Force is ttnc of the most reliable indicators of equipment distress. 

By way of example, Volume 26 in Group III (Crystal and Solid State Physics) is devoted to Diffusion in Solid Metals and Alloys, this volume has an editor and 14 contributors. Their task was not only to gather numerical data on such matters as self- and chemical diffusivities, pressure dependence of diffusivities, diffusion along dislocations, surface diffusion, but also to exercise their professional judgment as to the reliability of the various numerical values available. The whole volume of about 750 pages is introduced by a chapter describing diffusion mechanisms and methods of measuring diffusivities this kind of introduction is a special feature of Landolt-Bornstein . Subsequent developments in diffusion data can then be found in a specialised journal. Defect and Diffusion Forum, which is not connected with Landolt-Bdrnstein. 

Crystallization proeess systems design and operation is a eomplex matter requiring extensive data for systematie evaluation. Whilst simplified design methods and heuristies are available, the simple faet remains that the more and better the data input, the better the final design and reliability of the plant. Ideally, amongst the data required are the following ... 

The X-ray crystal analysis of 5-trimethylsilanyl-4-trimethylsilanylethynyl-l//-pyrazole-3-carboxylic acid ethyl ester was obtained only with / = 0.17 because the crystals of the molecule diffracted extremely weakly and only a very limited data set was available. This means that although the gross stereochemistry of the molecule has been determined, individual bond lengths are not reliable (88JOM247). 

Although the Langelier index is probably the most frequently quoted measure of a water s corrosivity, it is at best a not very reliable guide. All that the index can do, and all that its author claimed for it is to provide an indication of a water s thermodynamic tendency to precipitate calcium carbonate. It cannot indicate if sufficient material will be deposited to completely cover all exposed metal surfaces consequently a very soft water can have a strongly positive index but still be corrosive. Similarly the index cannot take into account if the precipitate will be in the appropriate physical form, i.e. a semi-amorphous egg-shell like deposit that spreads uniformly over all the exposed surfaces rather than forming isolated crystals at a limited number of nucleation sites. The egg-shell type of deposit has been shown to be associated with the presence of organic material which affects the growth mechanism of the calcium carbonate crystals . Where a substantial and stable deposit is produced on a metal surface, this is an effective anticorrosion barrier and forms the basis of a chemical treatment to protect water pipes . However, the conditions required for such a process are not likely to arise with any natural waters. 

Here the last expression was found by taking the arithmetic mean between the two forms, Eq. 11.45 and Eq. 11.74. Formulas of this type have actually been used in the cellular method" for treating crystals, but our own experience from work on atoms is that the orbital energies sk seem to be the quantities in the HF scheme which are most easily influenced by numerical uncertainties and errors. Even if Eqs. 11.74 and 11.75 are practically simpler to handle than Eq. 11.45, they are probably less numerically reliable. Further investigations on this point are desired. 

Pt and Pt-group metals (poly- and single crystals) have long been among the most intensively studied systems in electrochemistry6 8,io,ii,i4,25,140,186,188,206,412,752-7%. nevertheless reliable ff=0 values... 

Of the three principal classes of crystals, ionic crystals, crystals containing electron-pair bonds (covalent crystals), and metallic crystals, we feel that a good understanding of the first class has resulted from the work done in the last few years. Interionic distances can be reliably predicted with the aid of the tables of ionic radii obtained by Goldschmidt1) by the analysis of the empirical data and by Pauling2) by a treatment based on modem theories of atomic structure. The stability,... 

Preceding papers. h Preliminary values obtained through redetermination of parameters in crystals (cal-cite and sodium nitrate) by Mr. Norman Elliot. The values in parentheses are based on older parameter determinations. c L. Pauling and L. O. Brockway, Proc. Nat. Acad. Sci., 20, 336 (1934). The value 1.25 A. reported in crystals of oxalic acids and oxalates is probably less reliable. 

Revised Values of Double-Bond Covalent Radii.—This investigation has led to the value 1.34 A. for the carbon-carbon double-bond distance, 0.04 A. less than the value provided by the table of covalent radii.111 4 Five years ago, when this table was extended to multiple bonds, there were few reliable experimental data on which the selected values for double-bond and triple-bond radii could be based. The single-bond radii were obtained -from the study of a large number of interatomic distances found experimentally by crystal-structure and spectroscopic methods. The spectroscopic value of the triple-bond radius of nitrogen (in N2) was found to bear the ratio 0.79 to the single-bond radius, and this ratio was as-... 

X-Ray diffraction from single crystals is the most direct and powerful experimental tool available to determine molecular structures and intermolecular interactions at atomic resolution. Monochromatic CuKa radiation of wavelength (X) 1.5418 A is commonly used to collect the X-ray intensities diffracted by the electrons in the crystal. The structure amplitudes, whose squares are the intensities of the reflections, coupled with their appropriate phases, are the basic ingredients to locate atomic positions. Because phases cannot be experimentally recorded, the phase problem has to be resolved by one of the well-known techniques the heavy-atom method, the direct method, anomalous dispersion, and isomorphous replacement.1 Once approximate phases of some strong reflections are obtained, the electron-density maps computed by Fourier summation, which requires both amplitudes and phases, lead to a partial solution of the crystal structure. Phases based on this initial structure can be used to include previously omitted reflections so that in a couple of trials, the entire structure is traced at a high resolution. Difference Fourier maps at this stage are helpful to locate ions and solvent molecules. Subsequent refinement of the crystal structure by well-known least-squares methods ensures reliable atomic coordinates and thermal parameters. 

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