ALCHEMI

An interesting development of analytical electron microscopy, and a potentially very useful one for mineralogical research, has been made by Spence and Tafto (1982,1983). The technique, known as ALCHEMI (atom location by channeling enhanced microanalysis), is the electron analogue of an x-ray technique originally used by Batterman (1969). The theoretical basis of the technique was discussed in Chapter 4, but it is appropriate to summarize that discussion before considering the ALCHEMI technique in detail. 

Consider a set of alternating parallel planes (A and B) in a crystal containing atoms A and B, respectively, and impurity atoms X that are either on plane A or plane B. We now measure the x-ray counts corresponding to atoms A, B, and X for two orientations of the crystal near the exact Bragg angle, one orientation with s 0 and the other with s 0. 

However, this very simple procedure is not applicable if C is not exactly unity or zero. To determine intermediate values of C, we must again record x-ray counts from A, B, and X atoms for only two orientations. One of these orientations must be near the exact Bragg angle for a first-order reflection, and the other must be an orientation that avoids the excitation of any strong low-order Bragg beams. For this second orientation, a standing wave is not established in the crystal the electron intensity is uniform across the unit cell and no channeling takes place. Thus, 

The absence of superscripts on Aa, Ab, and Ax indicates the orientation for which no Bragg beams are excited. 

For an orientation near the exact Bragg angle, Eq. (7.11) becomes 

---

Perturbation theory is also used to calculate free energy differences between distinct systems by computer simulation. This computational alchemy is accomplished by the use of a switching parameter X, ranging from zero to one, that transfonns tire Hamiltonian of one system to the other. The linear relation... 

Inductive learning has been the major process of acquiring chemical knowledge from the very beginnings of chemistry - or, to make the point, alchemy. Chemists have done experiments, have made measurements on the properties of their compounds, have treated them with other compounds to study their reactions, and have run reactions to make new compounds. Systematic variations in the structure of compounds, or in reaction conditions, provided results that were ordered by developing models. These models then allowed predictions to be made. 

Nomenclature is the compilation of descriptions of things and technical terms in a special field of knowledge, the vocabulary ofa technical language. In the history of chemistry, a systematic nomenclature became significant only rather late. In the early times of alchemy, the properties of the substance or its appearance played a major role in giving a compound a name. Libavius was the first person who tried to fix some kind of nomenclature in Alckeinia in 1,597. In essence, he gave names to chemical equipment and processes (methods, names that are often still valid in our times. 

Name/ version Alchemy 2000 V2.0 ACD/ Chems-ketch 5.0 Chem- Drauf 7.0.1 Chemsite 3.01 Chemistry 4D Draw 6.0 Chem Window 6.0 Isis/Drow Z4 CACTVS 3.176 ACD/SDA 1.30 jehem- Paint7 V1.Zpre JME Mervin 2.10.5 ... 

The investigation of molecular structures and of their properties is one of the most fascinating topics in chemistry. Chemistry has a language of its own for molecular structures which has been developed from the first alchemy experiments to modem times. With the improvement of computational methods for chemical information processing, several descriptors for the handling of molecular information have been developed and used in a wide range of applications. 

It was in the great school of Alexandria that these separate paths came together and eventually led to the alchemy and iatrochemistry of future generations and, eventually, the chemistry of modern science. 

This is not the place to discuss the frequently reviewed historical and philosophical aspects of alchemy, but it is worthwhile to recall some rather late adherence to the precepts of alchemy by giants of the human intellectual endeavor. Johann Wolfgang Goethe is best known for his poetry and literature as the author of Faust. Tie himself, however, considered some of his major achievements to be in science. His interests were varied but also related to chemistry. He developed an early interest in alchemy, which, however, he overcame in later life. Goethe s classic character Faust reflects his fascination with the alchemist s effort to produce gold but eventually recognizes its futility and failure. 

In the earlier part of the sixteenth century Paracelsus gave a new direction to alchemy by declaring that its true object was not the making of gold but the preparation of medicines. This union of chemistry with medieine was one characteristic goal of iatrochemists, of whom he was the predeeessor. The search for the elixir of life had usually... 

Tripos is a force field created at Tripos Inc. for inclusion in the Alchemy and SYBYL programs. It is sometimes called the SYBYL force field. Tripos is... 

Alchemy 2000 (we tested Version 2.05) is a graphic interface for running molecular mechanics and semiempirical calculations. Calculations can be done with the built-in Tripos force field or by calling the MM3 or MOPAC programs, which are included with the package. Alchemy is designed by Tripos and sold by SciVision. 

In the days of alchemy and the phlogiston theory, no system of nomenclature that would be considered logical ia the 1990s was possible. Names were not based on composition, but on historical association, eg, Glauber s salt for sodium sulfate decahydrate and Epsom salt for magnesium sulfate physical characteristics, eg, spirit of wiae for ethanol, oil of vitriol for sulfuric acid, butter of antimony for antimony trichloride, Hver of sulfur for potassium sulfide, and cream of tartar for potassium hydrogen tartrate or physiological behavior, eg, caustic soda for sodium hydroxide. Some of these common or trivial names persist, especially ia the nonchemical Hterature. Such names were a necessity at the time they were iatroduced because the concept of molecular stmcture had not been developed, and even elemental composition was incomplete or iadeterminate for many substances. 

G. T. Seaborg, "Transuranium Elements, Products of Modem Alchemy," Benchmark Papers in Physical Chemisty and Chemical Physics, Vol. 1, Dowden, Hutchison Ross, Stroudsburg, Pa., 1978. 

The word alcohol, like alchemy, has its origins in the Middle East. The Arabs are said to have made cosmetic paints by heating and vaporizing a mixture of compounds. The residue was used to paint eyeUds and called "kohl." When they later heated wines, they gave the product the same name as the cosmetic "kohl" or "al kohl." The word whiskey is said to be derived from the Celtic "uisge baugh" or "water of life."... 

Sir Isaac Newton spent much of his life pursuing an elusive dream, the transmutation of base materials into gold. Though he was not successful during his lifetime, he did manage to discover the equations of motion that, tliree centuries later, make alchemy possible on a computer. To perfonn this feat, Newton s equations need only be supplemented by the modem technology of free energy simulations. 

TP Straatsma, JA McCammon. Computational alchemy. Annu Rev Phys Chem 43 407-430,... 

Dalai, S., Balasubramanian, S., Regan, L. Protein alchemy changing p sheet into a helix. Nature Struct. Biol. 

Phone -K 800 ALCHEMY E-mail nationaI aIphachisigma.org Web site www.alphachisigma.org... 

Molecular mechanics simulations use the laws of classical physics to predict the structures and properties of molecules. Molecular mechanics methods are available in many computer programs, including MM3, HyperChem, Quanta, Sybyl, and Alchemy. There are many different molecular mechanics methods. Each one is characterized by its particular/orce eW. A force field has these components ... 

G. T. Seaborg (ed.), Transuranium Elements Products of Modem Alchemy, Dowden, Hutchinson Ross, Stroudsburg, 1978. This reproduces, in their original form, 122 key papers in the story of man-made elements. 

Hermesfinger, m. (Pharm.) hermodactyl. hermetisch, a. hermetic. — hermetische Wis-senschaft, hermetic science, alchemy, hemach, adv. afterwards, hereafter, then. 

---