As a domain expert in the field of molecular physics and spectroscopy, I can provide a comprehensive explanation of what it means for a bond to be IR inactive.

Infrared (IR) spectroscopy is a powerful analytical technique that is used to study the molecular structure and chemical composition of various substances. It operates on the principle that molecules absorb infrared light at specific frequencies that correspond to the vibrational modes of their chemical bonds. These vibrations can be either stretching or bending motions of the bonds.

Infrared (IR) inactive refers to a particular type of bond within a molecule that does not absorb infrared radiation. This lack of absorption occurs because the bond's vibration does not result in a change in the molecule's dipole moment. The dipole moment is a measure of the separation of positive and negative charges within a molecule, and it is a vector quantity that depends on the distribution of these charges.

For a bond to be IR active, it must be a polar bond, meaning it has an uneven distribution of electron density, resulting in a permanent dipole moment. When such a bond vibrates, the distribution of electron density changes, leading to fluctuations in the dipole moment. These fluctuations can interact with the electric field component of the infrared radiation, causing the molecule to absorb the radiation at specific frequencies corresponding to the vibrational modes.

Conversely, an IR inactive bond is typically found in symmetric molecules or in parts of molecules where the electron distribution is even, resulting in no net dipole moment or only a very small one. In such cases, even when the bond vibrates, the overall dipole moment of the molecule does not change significantly. As a result, there is no or very little absorption of infrared light, and the bond appears inactive in an IR spectrum.

The distinction between IR active and IR inactive bonds is important for understanding the vibrational modes of a molecule and can provide valuable insights into its structure. It is also crucial for interpreting IR spectra, which are used for the identification and characterization of various compounds.

Now, to proceed with the next step as per your instructions, here is the translation of the explanation into Chinese:

红外（IR）光谱学是一种强大的分析技术，用于研究各种物质的分子结构和化学组成。它的工作原理是分子在特定的频率吸收红外光，这些频率对应于它们化学键的振动模式。这些振动可以是键的伸缩或弯曲运动。

红外（IR）不活跃指的是分子内部的一种特定类型的键，它不吸收红外辐射。这种不吸收发生是因为键的振动没有导致分子的偶极矩发生变化。偶极矩是分子内部正负电荷分离的度量，并且是一个取决于这些电荷分布的矢量量。

对于一个键要具有IR活性，它必须是极性键，这意味着它具有不均匀的电子密度分布，从而产生永久的偶极矩。当这样的键振动时，电子密度的分布会改变，导致偶极矩的波动。这些波动可以与红外辐射的电场分量相互作用，导致分子在特定频率吸收辐射，这些频率对应于振动模式。

相反，IR不活跃的键通常存在于对称分子中，或在分子的部分区域，其中电子分布是均匀的，结果是没有净偶极矩或只有一个非常小的偶极矩。在这种情况下，即使键振动，分子的整体偶极矩也没有显著变化。因此，对红外光的吸收没有或非常少，键在红外光谱中表现为不活跃。

区分IR活性和IR不活跃键对于理解分子的振动模式非常重要，可以为理解其结构提供有价值的见解。这对于解释红外光谱也至关重要，红外光谱用于各种化合物的识别和表征。

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An IR --active-- bond is therefore a bond that changes dipole during vibration, While an IR --inactive-- bond is a symmetric bond that doesn't change dipole during vibration. Page 7. Number of Vibrations. The number of possible vibrations for a given molecule.read more >>