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Achemical structure of amolecule is a spatial arrangement of itsatoms and their chemical bonds. Its determination includes achemist's specifying themolecular geometry and, when feasible and necessary, theelectronic structure of the target molecule or other solid. Molecular geometry refers to the spatial arrangement ofatoms in amolecule and thechemical bonds that hold the atoms together and can be represented usingstructural formulae and bymolecular models;[1] complete electronic structure descriptions include specifying the occupation of a molecule'smolecular orbitals.[2][3] Structure determination can be applied to a range of targets from very simple molecules (e.g.,diatomicoxygen ornitrogen) to very complex ones (e.g., such asprotein orDNA).
Theories of chemical structure were first developed byAugust Kekulé,Archibald Scott Couper, andAleksandr Butlerov, among others, from about 1858. Kekulé proposed the earliest ideas aboutvalency by suggesting the elements had a preferred number of chemical bonds. Couper developed the firstchemical structure diagrams, ways of representing structure on paper. Butlerov was the first to use 'structure' in chemistry and to recognize that chemical compounds are not a random cluster of atoms and functional groups, but rather had a definite order defined by the valency of the elements composing the molecule.[4]
In 1883Alexander Crum Brown deduced the crystal structure ofNaCl and built a model of it using knitting needles and wool balls. He also proposed a structure forethanoic acid that matches modern models well before experimental structural analysis techniques were developed.[4]
Concerning chemical structure, one has to distinguish between pure connectivity of the atoms within a molecule (chemical constitution), a description of a three-dimensional arrangement (molecular configuration, includes e.g. information onchirality) and the precise determination of bond lengths, angles, and torsion angles, i.e. a full representation of the (relative) atomic coordinates.
In determining structures ofchemical compounds, one generally aims to obtain, first and minimally, the pattern and degree of bonding between all atoms in the molecule; when possible, one seeks the three-dimensional spatial coordinates of the atoms in the molecule (or other solid).[5]
The methods by which one can determine the structure of a molecule is calledstructural elucidation. These methods include:
Additional sources of information are: When a molecule has an unpaired electron spin in afunctional group of its structure,ENDOR andelectron-spin resonance spectroscopes may also be performed. These latter techniques become all the more important when the molecules contain metal atoms, and when the crystals required by crystallography or the specific atom types that are required by NMR are unavailable to exploit in the structure determination. Finally, more specialized methods such aselectron microscopy are also applicable in some cases.
Chemfig andtikz are package add-ons forLaTeX for chemical structures.[8]
