利用者:4dimp/下書き/σ結合
σ結合（シグマけつごう：英Sigma bond）は，化学結合のなかでもっとも強い共有性の結合である。σ結合は結合軸方向を向いた原子軌道同士が重なることによって形成される。Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups.この方法によると（In this formal approach），σ結合は結合軸方向の回転に対して対称である。この定義から，common forms of sigma bonds are s+s, pz+pz, s+pz and dz2+dz2 (z軸は結合する方向である)。
下：s軌道同士が重なることで形成されるσ結合とp軌道同士によるσ結合とp軌道同士によるπ結合
多原子分子におけるσ結合
[編集]They are obtained by head on overlap of atomic orbitals. The concept of sigma bonding is extended, albeit loosely, 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. The σ bonding in such a polyatomic molecule is highly delocalized, which conflicts with the two-orbital, one-bond concept. Despite this complication, the concept of σ bonding is extremely powerful and therefore pervasive.
多重結合をもつ化学種におけるσ結合
[編集]Compounds that feature multiple bonds, such as ethylene and chromium(II) acetate, have sigma bonds between the multiple bonded atoms. These sigma bonds are supplemented by π-bonds, e.g. in the case of ethylene, and even δ-bonds, e.g. in the case of chromium(II) acetate.
有機化合物におけるσ結合
[編集]Organic molecules are often made up of one cyclic compound or more, 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.
Nb σ = Nb atoms + Nb rings - 1
This can easily be concluded by realizing that the creation of bonds between atoms that are not connected in a ring requires the same number of atoms minus one (such as in hydrogen gas, H2, where there is only one sigma bond, or ammonia, NH3, where there are only 3 sigma bonds), and that rings do not obey this rule (such as benzene rings, which have 6 sigma bonds within the ring for 6 carbon atoms).