Erich Hückel
Encyclopedia
Berlin
Berlin is the capital city of Germany and is one of the 16 states of Germany. With a population of 3.45 million people, Berlin is Germany's largest city. It is the second most populous city proper and the seventh most populous urban area in the European Union...
- February 16, 1980, Marburg
Marburg
Marburg is a city in the state of Hesse, Germany, on the River Lahn. It is the main town of the Marburg-Biedenkopf district and its population, as of March 2010, was 79,911.- Founding and early history :...
) was a German
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
physicist
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
and physical chemist
Physical chemistry
Physical chemistry is the study of macroscopic, atomic, subatomic, and particulate phenomena in chemical systems in terms of physical laws and concepts...
. He is known for two major contributions:
- The Debye–Hückel theory of electrolyticElectrolyteIn chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
solutionSolutionIn chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
s - The Hückel methodHückel methodThe Hückel method or Hückel molecular orbital method proposed by Erich Hückel in 1930, is a very simple linear combination of atomic orbitals molecular orbitals method for the determination of energies of molecular orbitals of pi electrons in conjugated hydrocarbon systems, such as ethene,...
of approximate molecular orbitalMolecular orbitalIn chemistry, a molecular orbital is a mathematical function describing the wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region. The term "orbital" was first...
(MO) calculations on π electron systems.
Hückel was born in the Charlottenburg
Charlottenburg
Charlottenburg is a locality of Berlin within the borough of Charlottenburg-Wilmersdorf, named after Queen consort Sophia Charlotte...
suburb of Berlin
Berlin
Berlin is the capital city of Germany and is one of the 16 states of Germany. With a population of 3.45 million people, Berlin is Germany's largest city. It is the second most populous city proper and the seventh most populous urban area in the European Union...
. He studied physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
and mathematics
Mathematics
Mathematics is the study of quantity, space, structure, and change. Mathematicians seek out patterns and formulate new conjectures. Mathematicians resolve the truth or falsity of conjectures by mathematical proofs, which are arguments sufficient to convince other mathematicians of their validity...
from 1914 to 1921 at the University of Göttingen.
On receiving his doctorate
Doctor of Philosophy
Doctor of Philosophy, abbreviated as Ph.D., PhD, D.Phil., or DPhil , in English-speaking countries, is a postgraduate academic degree awarded by universities...
, he became an assistant at Göttingen, but soon became an assistant to Peter Debye
Peter Debye
Peter Joseph William Debye FRS was a Dutch physicist and physical chemist, and Nobel laureate in Chemistry.-Early life:...
at Zürich
Zürich
Zurich is the largest city in Switzerland and the capital of the canton of Zurich. It is located in central Switzerland at the northwestern tip of Lake Zurich...
. It was there that he and Debye developed their theory (the Debye–Hückel theory, in 1923) of electrolytic solutions, elucidating the behavior of strong electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
s by considering interionic forces, in order to account for their electrical conductivity
Conductivity (electrolytic)
The conductivity of an electrolyte solution is a measure of its ability to conduct electricity. The SI unit of conductivity is siemens per meter ....
and their thermodynamic activity coefficient
Activity coefficient
An activity coefficient is a factor used in thermodynamics to account for deviations from ideal behaviour in a mixture of chemical substances. In an ideal mixture, the interactions between each pair of chemical species are the same and, as a result, properties of the mixtures can be expressed...
s.
After spending 1928 and 1929 in England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
and Denmark
Denmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...
, working briefly with Niels Bohr
Niels Bohr
Niels Henrik David Bohr was a Danish physicist who made foundational contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. Bohr mentored and collaborated with many of the top physicists of the century at his institute in...
, Hückel joined the faculty of the Technische Hochschule
University of Stuttgart
The University of Stuttgart is a university located in Stuttgart, Germany. It was founded in 1829 and is organized in 10 faculties....
in Stuttgart
Stuttgart
Stuttgart is the capital of the state of Baden-Württemberg in southern Germany. The sixth-largest city in Germany, Stuttgart has a population of 600,038 while the metropolitan area has a population of 5.3 million ....
. In 1935, he moved to Phillips University in Marburg
Marburg
Marburg is a city in the state of Hesse, Germany, on the River Lahn. It is the main town of the Marburg-Biedenkopf district and its population, as of March 2010, was 79,911.- Founding and early history :...
, where he finally was named Full Professor a year before his retirement 1961. He was a member of the International Academy of Quantum Molecular Science
International Academy of Quantum Molecular Science
The International Academy of Quantum Molecular Science is an international scientific learned society covering all applications of quantum theory to chemistry and chemical physics. It was created in Menton in 1967. The founding members were Raymond Daudel, Per-Olov Löwdin, Robert G. Parr, John...
.
Theories of unsaturated organic molecules
Hückel is most famous for developing simplified quantum mechanicsQuantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
methods to deal with planar unsaturated organic
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s. In 1930 he proposed a σ/π separation theory to explain the restricted rotation of alkenes (compounds containing a C=C double bond
Double bond
A double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...
). This model extended a 1929 interpretation of the bonding in triplet oxygen
Triplet oxygen
Triplet oxygen is the ground state of the oxygen molecule. The electron configuration of the molecule has two unpaired electrons occupying two degenerate molecular orbitals...
by Lennard-Jones. According to Hückel, only the ethene σ bond is axially symmetric about the C-C axis, but the π bond is not; this restricts rotation. In 1931 he generalized his analysis by formulating both valence bond
Valence bond theory
In chemistry, valence bond theory is one of two basic theories, along with molecular orbital theory, that were developed to use the methods of quantum mechanics to explain chemical bonding. It focuses on how the atomic orbitals of the dissociated atoms combine to give individual chemical bonds...
(VB) and molecular orbital
Molecular orbital
In chemistry, a molecular orbital is a mathematical function describing the wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region. The term "orbital" was first...
(MO) descriptions of benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
and other cycloconjugated hydrocarbons. Although undeniably a cornerstone of organic chemistry, Hückel's concepts were undeservedly unrecognized for two decades. His lack of communication skills contributed. The famous Hückel 4n+2 rule
Hückel's rule
In organic chemistry, Hückel's rule estimates whether a planar ring molecule will have aromatic properties. The quantum mechanical basis for its formulation was first worked out by physical chemist Erich Hückel in 1931...
for determining whether ring molecules composed of C=C bonds would show aromatic properties was first stated clearly by Doering
William von Eggers Doering
William von Eggers Doering was a Professor Emeritus at Harvard University and the former Chair of its Chemistry Department...
in a 1951 article on tropolone
Tropolone
Tropolone is a derivative of tropone with a hydroxyl group in the 2-position.Two methods for the synthesis of tropolone are by bromination of 1,2-cycloheptanedione with N-bromosuccinimide followed by dehydrohalogenation at elevated temperatures and by acyloin condensation of the ethyl ester of...
. Tropolone had been recognised as an aromatic molecule by Dewar in 1945.
In 1936, Hückel developed the theory of π-conjugated biradicals (non-Kekulé molecules). The first example, known as the Schlenk-Brauns hydrocarbon, had been discovered in the same year. The credit for explaining such biradicals is usually given to Christopher Longuet-Higgins
H. Christopher Longuet-Higgins
Hugh Christopher Longuet-Higgins FRS was both a theoretical chemist and a cognitive scientist. He was born on April 11, 1923 in Kent, England and died on March 27, 2004....
in 1950.
In 1937 Hückel refined his MO theory of pi electrons in unsaturated organic molecules. This is still used occasionally as an approximation, though the more precise PPP Pariser–Parr–Pople method
Pariser–Parr–Pople method
In molecular physics, the Pariser–Parr–Pople method applies semi-empirical quantum mechanical methods to the quantitative prediction of electronic structures and spectra, in molecules of interest in the field of organic chemistry...
succeeded it in 1953. "Extended Hückel MO theory" (EHT
Extended Huckel method
The extended Hückel method is a semiempirical quantum chemistry method, developed by Roald Hoffmann since 1963. It is based on the Hückel method but, while the original Hückel method only considers pi orbitals, the extended method also includes the sigma orbitals.The extended Hückel method can be...
) applies to both sigma and pi electrons, and has its origins in work by William Lipscomb
William Lipscomb
William Nunn Lipscomb, Jr. was a Nobel Prize-winning American inorganic and organic chemist working in nuclear magnetic resonance, theoretical chemistry, boron chemistry, and biochemistry.-Overview:...
and Roald Hoffmann
Roald Hoffmann
Roald Hoffmann is an American theoretical chemist who won the 1981 Nobel Prize in Chemistry. He currently teaches at Cornell University in Ithaca, New York.-Escape from the Holocaust:...
for nonplanar molecules in 1962.
Further reading
- E. Hückel, 'Zur Quantentheorie der Doppelbindung.' ('Quantum theory of double linkings.') Z. Physik, 1930, 60, 423.
- E. Hückel, 'Quantum-theoretical contributions to the benzene problem. I. The electron configuration of benzene and related compounds' Z. Physik, 1931, 70, 204-86 and 'Quantum theoretical contributions to the problem of aromatic and non-saturated compounds' 1932, 76, 628
- E. Hückel, The theory of unsaturated and aromatic compounds, Z. Elektrochem.Angew. Physik. Chem., 1937, 42, 752 and 827.
- E. Hückel, 'Theory of the magnetism of so-called biradicals', Z. Physik. Chem., 1936, B34, 339.
- R. Pariser, R. G. Parr, 'A semi-empirical theory of the electronic spectra and electronic structure of complex unsaturated molecules.', J. Chem. Phys., 1953, 21, 466-71; J. A. Pople, 'Electron interaction in unsaturated hydrocarbons.' Trans. Faraday Soc., 1953, 49, 1375-85.
- R. Hoffmann, W. N. Lipscomb, 'Theory of polyhedral molecules. I. Physical factorizations of the secular equation.', J. Chem. Phys. 1962, 36 2179-89.
- E. Hückel, Ein Gelehrtenleben: Ernst u. Satire (1975 ISBN 3527256369).
- A. Karachalios, Erich Hückel (1896 –1980): From Physics to Quantum Chemistry (Springer, 2010 ISBN 9789048135592).