The SMILES specification includes elaborations on the @ symbol to indicate stereochemistry around more complex chiral centers, such as trigonal bipyramidal molecular geometry. Isotopes Isotopes are specified with a number equal to the integer isotopic mass preceding the atomic symbol. Benzene in which one atom is carbon-14 is written as [14c]1ccccc1 and deuterochloroform is [2H]C(Cl)(Cl)Cl. Examples To illustrate a molecule with more than 9 rings, consider cephalostatin-1, a steroidic 13-ringed pyrazine with the empirical formula C54H74N2O10 isolated from the Indian Ocean hemichordate Cephalodiscus gilchristi: Starting with the left-most methyl group in the figure: CC(C)(O1)C[C@@H](O)[C@@]1(O2)[C@@H](C)[C@@H]3CC=C4[C@]3(C2)C(=O)C[C@H]5[C@H]4CC[C@@H](C6)[C@]5(C)Cc(n7)c6nc(C[C@@]89(C))c7C[C@@H]8CC[C@@H]%10[C@@H]9C[C@@H](O)[C@@]%11(C)C%10=C[C@H](O%12)[C@]%11(O)[C@H](C)[C@]%12(O%13)[C@H](O)C[C@@]%13(C)CO Note that % appears in front of the index of ring closure labels above 9; see above. Other examples of SMILES The SMILES notation is described extensively in the SMILES theory manual provided by Daylight Chemical Information Systems and a number of illustrative examples are presented. Daylight's depict utility provides users with the means to check their own examples of SMILES and is a valuable educational tool. Extensions SMARTS is a line notation for specification of substructural patterns in molecules. While it uses many of the same symbols as SMILES, it also allows specification of wildcard atoms and bonds, which can be used to define substructural queries for chemical database searching. One common misconception is that SMARTS-based substructural searching involves matching of SMILES and SMARTS strings. In fact, both SMILES and SMARTS strings are first converted to internal graph representations which are searched for subgraph isomorphism. SMIRKS, a superset of "reaction SMILES" and a subset of "reaction SMARTS", is a line notation for specifying reaction transforms. The general syntax for the reaction extensions is REACTANT>AGENT>PRODUCT (without spaces), where any of the fields can either be left blank or filled with multiple molecules deliminated with a dot (.), and other descriptions dependent on the base language. Atoms can additionally be identified with a number (e.g. [C:1]) for mapping, for example in . SMILES corresponds to discrete molecular structures. However many materials are macromolecules, which are too large (and often stochastic) to conveniently generate SMILES for. BigSMILES is an extension of SMILES that aims to provide an efficient representation system for macromolecules. Conversion SMILES can be converted back to two-dimensional representations using structure diagram generation (SDG) algorithms. This conversion is not always unambiguous. Conversion to three-dimensional representation is achieved by energy-minimization approaches. There are many downloadable and web-based conversion utilities. See also SMILES arbitrary target specification (SMARTS), an