Chemistry
About Gmelin
Gmelin (1772+) is a database of organometallic and inorganic structures, such as coordinations compounds, alloys, catalysts, glasses and ceramics, and minerals. It includes chemical and physical properties and relevant references to the original handbook and the journal literature. Hundreds of chemical and physical data fields can be searched, including electric, magnetic, thermal, crystal, and physiological data.
About Beilstein
Beilstein (1771+) is a database of organic compound properties, structures, reactions, and references to the journal literature of organic chemistry. Has flexible and powerful searching by substance, property and/or reaction. Substance searching includes exact structure, substructure, CAS number, molecular formula, name, and many more choices.
About SciFinder Scholar
SciFinder Scholar (1879+) identifies articles from journals, patents, conferences, books and dissertations in chemistry, medicine and related subjects, with some links to full text via Chemport and SFX. 
