Structures of molecules can be determined experimentally or predicted computationally. For small molecules, such as ethanol and cyclohexane, both spectroscopic methods and rigorous quantum chemical computations can provide highly accurate molecular geometries in the vapor phase. Such highly accurate methods are not readily applicable for most drug molecules due to their larger size. However, experiments can give quite accurate structures of drugs in the crystalline state or while bound to target macromolecules. Approximate computations can give sufficiently accurate structures for drugs in the gas or liquid phase. Often, the effects of the medium on the structure are not too significant; in this case use of experimental crystal structures or computationally predicted gas phase structures is permissible for description of molecules in solution. In other cases, care must be taken to use advanced computational methods that account for effects of the environment.
Databases such as Klotho provide model structures for many common small molecules. Chemicals with Pharmaceutical Activity from University of Oxford offers access to many drug models and the Protein Data Bank offers access to experimentally determined structures of macromolecules and macromolecular complexes.
Several web sites generate 3D molecular structures from the SMILES string using the program CORINA. CORINA uses built-in tables of standard bond lengths and angles to create a reasonable model for small or rigid molecules. However, the model geometry for larger and flexible molecules is likely to be quite different than the most prevalent geometry in aqueous solution. One such site that generates 3D model structures is Online SMILES Translator by National Institutes of Health.
Most chemists are well familiar with drawing 2D molecular structures and several programs allow effortlessly draw 2D representations of three-dimensional molecules. Two of the most popular 2D chemical diagram editors for Windows and MacOS systems are ChemDraw from CambridgeSoft and MDL Draw from Elsevier MDL. Students can download a fully functional free chemical drawing program MDL Isis/Draw from Elsevier MDL website after registration. Some chemical drawing tools allow generation and export of 3D coordinates of the drawn molecule. For example, molecular structures from Isis/Draw can be pasted into Accelrys ViewerLite (used to be a free program, now a rare find on the web). The JME Molecular Editor allows to sketch simple molecules on-line and export these structures into the SMILES string.