IUPAC Compendium of Chemical Terminology - The "gold book" for definitions of chemical terms. Based on the 1997 2nd edition, but regularly updated.
NIST Chemistry Webbook. Synonyms, structure and mass spectra data for mostly organic compounds.
ChemBioFinder. Free to search, but you need to register.
Sigma-Aldrich. Search under Products for compounds and other products for sale. For compounds includes structure, formula, and MSDS.
ChemView. EPA database of health and safety information for regulated compounds.
Properties of organic compounds includes spectral peaks; included in
See also NIST Webbook.
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