Structural chemistry

Structure of vitamin B12, recognized with the 1954 Nobel Prize in Chemistry in 1964.

Structural chemistry is a part of chemistry and deals with the connectivity and shape of individual chemical species, which includes molecules, ions, and polymers. Structure is central to understanding the behavior of chemical species, so the topic is foundational. Structure is integrated into the major branches of chemistry, i.e. inorganic chemistry ^[2] and organic chemistry. ^[3] One indicator of the centrality of structural chemistry, many of the Nobel Prize in Chemistry have been awarded for structural insights^{[ citation needed ]} or techniques important for determining structure. ^[4]

In chemistry, "structure" takes distinct meanings depending on the length scale and time scale. With respect to length scales, on a local level, structure refers to interatomic distances and angles. On a larger length, symmetry and periodicity are important descriptors.

Sodium chloride structure showing the packing of the Na and Cl ions and visualizing the polyhedra, octahedra in this case, defined by the placement of these ions.

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. ^[4]

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. ^[4]

At the beginning of the 20th century only visible light spectroscopy gave direct structural information, but that changed rapidly. By 1915 William Bragg and son received the Nobel prize for developing early forms of X-ray crystal structure analysis and in 1922, Francis Aston was awarded the prize for creating the first mass spectrometer. The discovery of quantum mechanics in the 1920 lead to a succession of fundamental physical studies that could be applied to determining chemical structure, including the Raman effect, Mossbauer spectroscopy, and nuclear magnetic resonance (NMR). Beginning in the 1970s, electronic detectors and computer-aided analysis techniques, dramatically increased structural data and reduced the time to create and analyze it. Especially notable for structural chemistry was the development of direct methods of X-ray crystallography by Herbert A. Hauptman and Jerome Karle, leading to the 1985 Nobel prize in Chemistry. ^[4]

Inorganic vs organic structures

Broadly speaking, inorganic structural chemistry focuses on solids, especially crystalline solids as the vast majority of inorganic compounds are metals combined with non-metal elements forming solids. These structures are typically regular lattices of infinite extent in 2 or 3 dimensions. This allows the application of highly effective X-ray and neutron diffraction methods to determine chemical structure. ^{[5] : 4}

Organic structural chemistry deals with finite molecules which often exist as vapor, liquid, and solid states under different conditions. Simple finite molecules can be studied in the vapor phase with electron diffraction, but often spectroscopic techniques coupled with theory are required. ^{[5] : 4}

Methodology

Many methods have been developed to determine chemical structure. The techniques vary with the nature of the sample: gas, liquid, and solid. One of the most important techniques is X-ray crystallography, which is used to analyze solids. ^{[6] [7]} Another dominant technique is NMR spectroscopy, which is routinely used in organic chemistry but is applicable to a variety of sample types. ^[8]

In addition to static structures, structural chemistry also includes a focus on dynamics, such as fluxionality. In such cases, the time scale of the technique must be matched with the time scale of the dynamics. A classic example is dimethylformamide (DMF) where the dynamics are analyzed by dynamic NMR spectroscopy. ^[9]

See also

• Chemical structure

References

1. Dodson G (December 2002). "Dorothy Mary Crowfoot Hodgkin, 12 May 1910 – 29 July 1994". Biographical Memoirs of Fellows of the Royal Society. 48: 181–219. doi:10.1098/rsbm.2002.0011. PMID   13678070. S2CID   61764553.
2. Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, ISBN   0-12-352651-5
3. Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, ISBN   978-0-471-72091-1
4. Rankin, David W. H.; Morrison, Carole A.; Mitzel, Norbert W. (2013). Structural methods in molecular inorganic chemistry. A Wiley series of advanced textbooks. Chichester, West Sussex, United Kingdom: Wiley. ISBN   978-1-118-46288-1.
5. Wells, A. F. (1975). Structural Inorganic Chemistry. United Kingdom: Clarendon Press.
6. David W. H. Rankin, Norbert W. Mitzel, Carole A. Morrison (2013). Structural Methods in Molecular Inorganic Chemistry. Chichester: John Wiley & Sons. ISBN   978-0-470-97278-6.
7. "X-ray diffraction | Definition, Diagram, Equation, & Facts | Britannica". www.britannica.com. Retrieved 2023-12-08.
8. Horst Friebolin (2010). Basic One and Two-Dimensional NMR Spectroscopy. Wiley-VCH. ISBN   3527312331.
9. H. S. Gutowsky; C. H. Holm (1956). "Rate Processes and Nuclear Magnetic Resonance Spectra. II. Hindered Internal Rotation of Amides". J. Chem. Phys. 25 (6): 1228–1234. Bibcode:1956JChPh..25.1228G. doi:10.1063/1.1743184.