Chemical structure
A chemical structure of a molecule is a spatial arrangement of its atoms and their chemical bonds. Its determination includes a chemist's specifying the molecular geometry and, when feasible and necessary, the electronic structure of the target molecule or other solid. Molecular geometry refers to the spatial arrangement of atoms in a molecule and the chemical bonds that hold the atoms together and can be represented using structural formulae and by molecular models; complete electronic structure descriptions include specifying the occupation of a molecule's molecular orbitals. Structure determination can be applied to a range of targets from very simple molecules to very complex ones.
History
Theories of chemical structure were first developed by August Kekulé, Archibald Scott Couper, and Aleksandr Butlerov, among others, from about 1858. Kekulé proposed the earliest ideas about valency by suggesting the elements had a prefered number of chemical bonds. Couper developed the first chemical 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.In 1883 Alexander Crum Brown deduced the crystal structure of NaCl and built a model of it using knitting needles and wool balls. He also proposed a structure for ethanoic acid that matches modern models well before experimental structural analysis techniques were developed.
Background
Concerning chemical structure, one has to distinguish between pure connectivity of the atoms within a molecule, a description of a three-dimensional arrangement and the precise determination of bond lengths, angles, and torsion angles, i.e. a full representation of the atomic coordinates.In determining structures of chemical 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.
Structural elucidation
The methods by which one can determine the structure of a molecule is called structural elucidation. These methods include:- concerning only connectivity of the atoms: spectroscopies such as nuclear magnetic resonance, and various methods of mass spectrometry. Techniques such as absorption spectroscopy and the vibrational spectroscopies, infrared, and Raman, provide, respectively, important supporting information about the numbers and adjacencies of multiple bonds, and about the types of functional groups ; further inferential studies that give insight into the contributing electronic structure of molecules include cyclic voltammetry and X-ray photoelectron spectroscopy.
- concerning precise metric three-dimensional information: can be obtained for gases by gas electron diffraction and microwave spectroscopy and for the crystalline solid state by X-ray crystallography or neutron diffraction. These technique can produce three-dimensional models at atomic-scale resolution, typically to a precision of 0.001 Å for distances and 0.1° for angles.