WordNet

a certificate of debt (usually interest-bearing or discounted) that is issued by a government or corporation in order to raise money; the issuer is required to pay a fixed sum annually until maturity and then a fixed sum to repay the principal (同)bond certificate
a superior quality of strong durable white writing paper; originally made for printing documents (同)bond_paper
issue bonds on
bring together in a common cause or emotion; "The death of their child had drawn them together" (同)bring together, draw together
a close personal relationship that forms between people (as between husband and wife or parent and child)
fastening firmly together (同)soldering
(dentistry) a technique for repairing a tooth; resinous material is applied to the surface of the tooth where it adheres to the tooths enamel
United States civil rights leader who was elected to the legislature in Georgia but was barred from taking his seat because he opposed the Vietnam War (born 1940) (同)Julian Bond
British secret operative 007 in novels by Ian Fleming (同)James Bond
the 18th letter of the Greek alphabet

PrepTutorEJDIC

〈C〉『縛る物』 / 《通例複数形で》『束縛』,拘束,かせ / 《しばしば複数形で》(血縁,義理などの)『きずな』,縁 / 〈C〉契約 / 〈C〉『証書』,証文;『債券』,公債 / 〈U〉(石・れんがなどの)つなぎ方,組積み / 〈U〉保税倉庫入り / 〈U〉接着,接合;接着剤,接合剤 / 〈品物〉'を'担保とする / …‘の'保証人となる / (債券などを発行して)…‘の'支払いを保証する / …'を'つなぐ,結ぶ(bind together) / つながる
シグマ(ギリシア語アルファベットの第18字Σ,σ,δ;英語のS,sに相当)
合板の / 公債(債券)で保証された;担保付きの / (輸入貨物が)保税倉庫留め置きの

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/03/05 16:41:46」(JST)

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σ bond between two atoms: localization of electron density

In chemistry, sigma bonds (σ bonds) are the strongest type of covalent chemical bond.^[1] They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is symmetrical with respect to rotation about the bond axis. By this definition, common forms of sigma bonds are s+s, p_z+p_z, s+p_z and d_z²+d_z² (where z is defined as the axis of the bond).^[2] Quantum theory also indicates that molecular orbitals (MO) of identical symmetry actually mix. As a practical consequence of this mixing of diatomic molecules, the wavefunctions s+s and p_z+p_z molecular orbitals become blended. The extent of this mixing (or blending) depends on the relative energies of the like-symmetry MO's.

1sσ* antibonding molecular orbital in H₂ with nodal plane

For homodiatomics, bonding σ orbitals have no nodal planes at which the wavefunction is zero, either between the bonded atoms or passing through the bonded atoms. The corresponding antibonding, or σ* orbital, is defined by the presence of one nodal plane between the two bonded atoms.

Sigma bonds are the strongest type of covalent bonds due to the direct overlap of orbitals, and the electrons in these bonds are sometimes referred to as sigma electrons.^[3]

The symbol σ is the Greek letter sigma. When viewed down the bond axis, a σ MO has a circular symmetry, hence resembling a similarly sounding "s" atomic orbital.

——	————————————————	—————
Atomic orbitals
Symmetric (s–s and p–p) sigma bonds between atomic orbitals		A pi bond, for comparison
——	————————————————	—————
σ_s–hybrid		σ_s–p

In polyatomic compounds[edit]

They are obtained by head-on overlapping of atomic shells. The concept of sigma bonding is extended to describe bonding interactions involving overlap of a single lobe of one orbital with a single lobe of another. For example, propane is described as consisting of ten sigma bonds, one each for the two C−C bonds and one each for the eight C−H bonds. As can be seen, the concept of σ bonding is extremely powerful and therefore pervasive.

In multiple-bonded species[edit]

Compounds that feature multiple bonds, such as the dihydrogen complex, have sigma bonds between the multiple bonded atoms. These sigma bonds can be supplemented with other bonding interactions, such as by π-back donation, e.g. in the case of W(CO)₃(PCy₃)₂(H₂), and even δ-bonds, e.g. in the case of chromium(II) acetate.^[4]

In organic molecules[edit]

Organic molecules are often cyclic compounds containing one or more rings, such as benzene, and are often made up of many sigma bonds along with pi bonds. According to the sigma bond rule, the number of sigma bonds in a molecule is equivalent to the number of atoms plus the number of rings minus one.

N_σ = N_atoms + N_rings − 1

A molecule with no rings can be represented as a tree with a number of bonds equal to the number of atoms minus one (as in dihydrogen, H₂, with only one sigma bond, or ammonia, NH₃, with 3 sigma bonds). There is no more than 1 sigma bond between any two atoms.

Molecules with rings have additional sigma bonds, such as benzene rings, which have 6 C−C sigma bonds within the ring for 6 carbon atoms. The anthracene molecule, C₁₄H₁₀, has three rings so that the rule gives the number of sigma bonds as 24 + 3 − 1 = 26. In this case there are 16 C−C sigma bonds and 10 C−H bonds.

References[edit]

^ Moore, John; Stanitski, Conrad L.; Jurs, Peter C. Principles of Chemistry: The Molecular Science.
^ Clayden, Jonathan; Greeves, Nick; Warren, Stuart (March 2012) [2002]. Organic Chemistry (2nd ed.). Oxford: OUP Oxford. pp. 101–136. ISBN 978-0199270293.
^ Keeler, James; Wothers, Peter (May 2008). Chemical Structure and Reactivity (1st ed.). Oxford: OUP Oxford. pp. 27–46. ISBN 978-0199289301.
^ Kubas, Gregory (2002). "Metal Dihydrogen and σ-Bond Complexes: Structure, Theory, and Reactivity". JCAS 14: 3799–3800. doi:10.1021/ja0153417.

External links[edit]

IUPAC-definition

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English Journal

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After the first introduction of π aromaticity in chemistry to explain the bonding, structure, and reactivity of benzene and its derivatives, this concept was further applied to many other compounds featuring other types of aromaticity (i.e., σ, δ). Thus far, there have been no reports on d-AO-bas
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Wang C1, Guan L2, Danovich D3, Shaik S3, Mo Y2.
Journal of computational chemistry.J Comput Chem.2016 Jan 5;37(1):34-45. doi: 10.1002/jcc.23946. Epub 2015 May 25.
The recent σ-hole concept emphasizes the contribution of electrostatic attraction to noncovalent bonds, and implies that the electrostatic force has an angular dependency. Here a set of clusters, which includes hydrogen bonding, halogen bonding, chalcogen bonding, and pnicogen bonding systems, is i
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A theoretical investigation of the energetics and spectroscopic properties of the gas-phase linear proton-bound cation-molecule complexes, XCH(+)-N2 (X = O, S).

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The structural features, spectroscopic properties, and interaction energies of the linear proton-bound complexes of OCH(+) and its sulfur analog SCH(+) with N2 were investigated using the high-level ab initio methods MP2 and CCSD(T) as well as density functional theory with the aug-cc-pVXZ (X = D,
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Japanese Journal

Hydrogenation Effects of Ultrananocrystalline Diamond Detected by X-ray Absorption Near Edge Structure and Raman Spectroscopy

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