键参数——定义、顺序、角度、长度

几个键参数，例如键长、键角、键序和键能，可用于表征共价键（也称为键焓）。这些键参数提供了有关化合物稳定性以及将其原子结合在一起的化学键强度的信息。

什么是债券？

Covalent bonds are classified into two types: sigma bonds and pi bonds. The number of shared bonds varies depending on the number of electrons. When two, four, or six electrons are shared, the number of bonds formed is one, two, or three, depending on the number of electrons shared. Covalent bonds are thus classified as sigma bonds or pi bonds based on the type of overlapping.

债券种类：

• Sigma Bond：沿核间轴的键合轨道的端到端（正面）重叠产生这种类型的共价键。这被称为正面或轴向重叠。这可以由下面列出的任何原子轨道组合形成。
  • ss 重叠：在这种情况下，两个半满的 s 轨道沿核间轴重叠。这种类型的重叠可以在 H ₂分子的产生中看到。
  • sp 重叠：沿核间轴，一个原子的半满 s 轨道和另一个原子的半满 p 轨道有重叠。甲烷、氨和水的形成表现出这种类型的重叠。
  • pp 重叠：这种类型的重叠发生在一个半填充的 p 轨道和另一个半填充的 p 轨道之间的核间轴上。当氟原子结合形成F ₂分子时，可以看到这种类型的重叠。
• Pi (π) 键：在 Pi 键的创建过程中，原子轨道以这样的方式重叠，即它们的轴保持相互平行并垂直于核间轴。原子轨道在键形成过程中横向重叠，在核间轴上方和下方形成碟形带电云。

Strength of Sigma and Pi Bond

1. The extent to which atomic orbitals overlap determines the strength of a bond.
2. The sigma bond, which overlaps along the internuclear axis, is more powerful than the pi bond, which overlaps sideways.
3. Pi bonds have a smaller area of overlap than sigma bonds. This is why the pi-bond breaks first, followed by the sigma bond.
4. During multiple bond formation, a pi bond is formed in addition to a sigma bond.

键参数

为了变得稳定，各种原子必须结合在一起。这种结合是由于键的形成而发生的。Ionic或电价键、共价键和配位键是三种键。结果，每一个债券都有一个与之相关的特征

Bond parameters are a set of different features or characteristics that may be observed in bonds. 

定义共价键的参数如下：

1. 债券令
2. 键角
3. 键长
4. 键焓或能量

债券令

共价键的键序是分子中两个原子之间共价键合电子对的总数。它可以通过绘制分子的路易斯结构并计算相关原子之间的电子对总数来计算。例如，C ₂ H ₂ (乙炔/乙炔)中的碳-氢键的键序为1，碳-碳键的键序为3。O2分子中的氧-氧键的键序为2. 一氧化碳分子中的碳-氧键的键序为 3。

• 单一债券的债券顺序为 1。
• 双键的键阶为 2。
• 三重键的键阶为 3。

Bond Order according to the Molecular Orbital Theory: The bond order of a covalent bond, according to molecular orbital theory, is equal to half of the difference between the number of bonding and antibonding electrons, as represented by the following formula:

Bond Order = (½) × (Total number of bonding electrons – Total number of antibonding electrons)

键角

键角定义为由同一原子形成的两个共价键之间形成的角度。键角是由任何两个相邻的共价键形成的几何角度。此键参数提供有关化合物分子几何形状的信息。

键长

分子的键长是两个化学键合原子核之间的距离。它接近两个键合原子的共价半径之和。对于共价键，键长与键序成反比；更大的键阶导致更强的键，伴随着更强的吸引力，将原子结合在一起。这些强大的吸引力导致了短键。旋转光谱、X 射线衍射和中子衍射都可用于通过实验确定该键参数。键合原子从周围吸收热能并不断振动。由于这种振动，键长会发生变化。因此，了解共价键的键长表示所涉及原子核之间的平均距离至关重要。

Periodic Trends in Bond Length: Bond lengths are proportional to the atomic radii of the atoms involved. The periodic trends that can be observed in element bond lengths are similar to the periodic trends that can be observed in element atomic radii (decreases across the period, increases down the group).

键能或键焓

键能是化学键强度的量度。它被定义为破坏一摩尔化合物（处于气态）中特定类型的所有共价键所需的能量。

Factors Affecting Bond Energy

The amount of energy required to break a chemical bond is directly proportional to its strength. As a result, bond energy is:

1. Bond energies are inversely proportional to bond length, so longer bonds have lower bond energies.
2. Bond energies are directly proportional to bond order, implying that multiple bonds have high bond energies.
3. The atomic radii of the atoms involved in the bond are inversely proportional (since the atomic radius is directly proportional to bond length).

示例问题

问题1：什么是键角？

回答：

The angle created by three atoms spanning at least two bonds is known as a bond angle. The angle of torsion for four atoms bonded together in a chain is the angle formed by the first three atoms and the plane formed by the last three atoms.

问题2：什么是更强的双键或更强的三键？

回答：

Triple bonds are stronger than double bonds because they contain two pi bonds rather than one. Each carbon atom has two hybrid sp-orbitals, one of which overlaps to form a sp-sp sigma bond with the corresponding one from the other carbon atom.

问题3：键解离焓是多少？

回答：

Bond enthalpy, also known as bond-dissociation enthalpy, average bond energy, or bond strength, is the amount of energy contained in a bond between atoms in a molecule. In fact, it is the energy that must be applied to the homolytic or symmetrical cleavage of a bond in the gas phase.

问题4：π键是如何形成的？

回答：

A pi bond is formed when a bond between two atoms is broken when one of the atoms rotates around the bond axis. A pi bond is not the same as an axial bond. Pi bonds are formed when parallel p orbitals on adjacent atoms overlap sideways. They are not made up of hybrid orbitals.

问题 5：为什么没有围绕双键的旋转？

回答：

Only alkanes allow for free rotation. Because it will break during rotation, it is restricted in both alkenes and alkynes.