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 on
chirality) and the precise determination of bond lengths, angles, and torsion angles, i.e. a full representation of the (relative) 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 (or other solid).
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 (
proton and
carbon-13 NMR), and various methods of
mass spectrometry (to give overall molecular mass, as well as fragment masses). 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 (whose internal bonding gives vibrational signatures); 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 (rotational) spectroscopy (and other rotationally resolved spectroscopy) and for the crystalline solid state by
X-ray crystallography or
neutron diffraction. These techniques can produce three-dimensional models at atomic-scale
resolution, typically to a precision of 0.001 Å for distances and 0.1° for angles (in unusual cases even better). Additional sources of information are: When a molecule has an unpaired electron spin in a
functional group of its structure,
ENDOR and
electron-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 as
electron microscopy are also applicable in some cases. ==TeX for chemical structures==