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{{Short description|Quantum and structural description of molecular geometry and bonding}}
{{Quantum matter backlink|Molecules}}

'''Quantum molecular structure''' describes the arrangement of atoms within molecules and the quantum-mechanical principles that determine molecular geometry, chemical bonding, molecular orbitals, and the structural properties of matter at the molecular scale. Molecular structure emerges from the interactions between atomic nuclei and [[Physics:Quantum atoms/electron|electrons]], governed by the laws of [[Physics:Quantum mechanics|quantum mechanics]] and [[Physics:Quantum chemistry|quantum chemistry]].

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[[File:Double Helix-y.png]]
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Diagrammatic representation of the structural features of the DNA double helix. Molecular geometry and hydrogen bonding determine the organization and replication properties of DNA.
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== Overview ==

Molecular structure concerns the spatial arrangement of atoms and the distribution of electrons within molecules. These structures determine many observable physical and chemical properties, including stability, reactivity, optical behavior, electrical properties, and biological function.<ref>{{cite book |last=Pauling |first=Linus |title=The Nature of the Chemical Bond |publisher=Cornell University Press |year=1960}}</ref>

Quantum mechanics explains molecular structure through the behavior of electrons occupying quantized [[Physics:Quantum atoms/orbital|molecular orbitals]]. Electrons interact through electromagnetic forces and occupy allowed energy states determined by the [[Schrödinger equation]].<ref>{{cite book |last=Atkins |first=Peter |title=Physical Chemistry |publisher=Oxford University Press |year=2018}}</ref>

The geometry of molecules is influenced by:
* electron configuration
* orbital hybridization
* electrostatic interactions
* molecular symmetry
* quantum exchange effects
* hydrogen bonding
* intermolecular forces

== Chemical bonding ==

Chemical bonds arise from electromagnetic interactions between atoms and the quantum-mechanical sharing or transfer of electrons.<ref>{{cite book |last=Levine |first=Ira N. |title=Quantum Chemistry |publisher=Pearson |year=2014}}</ref>

Major bonding types include:
* [[Physics:Quantum chemical bond|covalent bonds]]
* ionic bonds
* metallic bonding
* van der Waals interactions
* hydrogen bonding

Quantum mechanics explains why electrons occupy discrete orbitals and why certain molecular configurations are energetically favorable.<ref>{{cite journal |last=Pauling |first=Linus |title=The Nature of the Chemical Bond |journal=Journal of the American Chemical Society |volume=53 |issue=4 |year=1931 |pages=1367–1400 |doi=10.1021/ja01355a027}}</ref>

== Molecular orbitals ==

In molecular orbital theory, atomic orbitals combine to form molecular orbitals extending over the entire molecule.<ref>{{cite journal |last1=Mulliken |first1=Robert S. |title=Electronic Structures of Polyatomic Molecules and Valence |journal=Physical Review |volume=41 |issue=1 |year=1932 |pages=49–71 |doi=10.1103/PhysRev.41.49}}</ref>

Electrons occupy bonding, antibonding, or nonbonding orbitals depending on energy and symmetry considerations. Molecular orbital theory explains:
* molecular stability
* bond order
* electronic transitions
* spectroscopy
* conductivity
* magnetic properties

== Molecular geometry ==

Molecular geometry describes the three-dimensional arrangement of atoms. The shape of molecules depends on the distribution of electrons and the minimization of energy.<ref>{{cite book |last=Housecroft |first=Catherine |title=Inorganic Chemistry |publisher=Pearson |year=2018}}</ref>

Common molecular geometries include:
* linear
* trigonal planar
* tetrahedral
* trigonal bipyramidal
* octahedral

These structures strongly influence molecular interactions and physical properties.

== Spectroscopy and diffraction ==

Experimental methods used to determine molecular structure include:
* [[Physics:Quantum molecular spectroscopy|spectroscopy]]
* X-ray diffraction
* neutron diffraction
* electron diffraction
* nuclear magnetic resonance
* infrared spectroscopy
* Raman spectroscopy

X-ray crystallography became one of the most important methods for determining complex molecular structures.<ref>{{cite journal |last=Bragg |first=William Lawrence |title=The Diffraction of Short Electromagnetic Waves by a Crystal |journal=Proceedings of the Cambridge Philosophical Society |volume=17 |year=1914 |pages=43–57}}</ref>

== DNA double helix ==

One of the most important discoveries in molecular structure was the determination of the [[DNA]] double helix.<ref>{{cite journal |vauthors=Watson JD, Crick FH |title=Molecular structure of nucleic acids; a structure for deoxyribose nucleic acid |journal=Nature |volume=171 |issue=4356 |pages=737–738 |date=April 1953 |pmid=13054692 |doi=10.1038/171737a0 |bibcode=1953Natur.171..737W |s2cid=4253007 |url=https://www.nature.com/articles/171737a0}}</ref>

The structure was determined using X-ray diffraction experiments together with quantum and chemical models of molecular bonding.<ref>{{cite journal |vauthors=Franklin R, Gosling RG |title=Molecular configuration in sodium thymonucleate |journal=Nature |date=1953-04-25 |volume=171 |issue=4356 |pages=740–741 |url=http://www.nature.com/nature/dna50/franklingosling.pdf |pmid=13054694 |doi=10.1038/171740a0 |bibcode=1953Natur.171..740F |s2cid=4268222 }}</ref><ref>{{cite journal |vauthors=Wilkins MH, Stokes AR, Wilson HR |title=Molecular structure of deoxypentose nucleic acids |journal=Nature |date=25 April 1953 |volume=171 |issue=4356 |pages=738–740 |url=http://www.nature.com/nature/dna50/wilkins.pdf |pmid=13054693 |doi=10.1038/171738a0 |bibcode=1953Natur.171..738W |s2cid=4280080 }}</ref>

The DNA molecule consists of two complementary strands held together by hydrogen bonds between nucleotide base pairs. The helical structure explains how genetic information can be stored and replicated.<ref>{{cite journal |last=Perutz |first=MF |title=DNA helix |journal=Science |volume=164 |issue=3887 |date=June 1969 |pages=1537–1539 |doi=10.1126/science.164.3887.1537 |pmid=5796048 |bibcode=1969Sci...164.1537W |doi-access=free }}</ref>

The discovery of DNA structure transformed molecular biology and demonstrated how quantum-scale interactions could produce highly organized biological systems.

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[[File:Template from Crick and Watson’s DNA molecular model, 1953. (9660573227).jpg|300px]]
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Physical molecular templates used by Watson and Crick during construction of the DNA double-helix model.
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== Molecular biology and quantum science ==

The development of molecular structure theory contributed directly to:
* molecular biology
* biochemistry
* nanotechnology
* biotechnology
* materials science
* pharmaceutical chemistry
* quantum chemistry

Understanding molecular structure made possible the interpretation of the [[genetic code]], protein folding, enzyme function, and many biological processes.<ref>{{cite book |last=Judson |first=Horace Freeland |title=The Eighth Day of Creation: Makers of the Revolution in Biology |publisher=Simon & Schuster |year=1979 |isbn=9780671254100}}</ref>

== See also ==
{{#invoke:PhysicsQC|tocHeadingAndList|Physics:Quantum basics/See also/Matter}}

=References=
{{reflist|3}}

{{Author|Harold Foppele}}

{{Sourceattribution|Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid|1}}

Physics:Quantum molecular structure - Revision history

imported>WikiHarold: Replace raw Quantum Collection backlink with BM backlink template

imported>WikiHarold: Replace raw Quantum Collection backlink with BM backlink template