Modes of Molecular Vibrations Infrared Spectroscopy IIT JAM CSIR NET GATE Chemistry

Molecular vibrations are broadly classified into two types: stretching and bending.
Stretching involves a change in bond length without affecting the bond angle.
Bending, on the other hand, results in a change in the bond angle.
It's important to remember that during bending vibrations, there may be slight adjustments in bond lengths to maintain the center of mass of the molecules.
Both stretching and bending vibrations are further divided into specific categories, which we will explore in more detail.

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Let’s begin by looking at the simplest type of vibration in a homonuclear diatomic molecule. It is a stretching vibration without change in the dipole moment of the molecule, making this mode IR inactive.

One should remember that for a vibration to be IR active, there must be a change in the dipole moment during the vibration. In the case of a homonuclear diatomic molecule, the dipole moment is initially zero, and since no dipole moment is altered during the stretching motion, the vibration does not interact with infrared radiation, making it IR inactive.

Let’s now consider a heteronuclear diatomic molecule, where the smaller atom is more electronegative than the larger one. As a result, the molecule has a dipole moment, with the negative end pointing toward the more electronegative, smaller atom.

When the bond undergoes stretching, this dipole moment changes. Since a change in dipole moment is necessary for a vibration to absorb infrared radiation, this stretching mode becomes IR active.

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Now, let’s consider triatomic molecules. The stretching vibrations can be either symmetric or asymmetric. Let’s start by looking at symmetric stretching.

Consider a linear triatomic molecule with two identical atoms bonded to a central atom. Each bond has its own bond moment, but since the bonds are identical and the bond moments point in opposite directions, the molecule has no overall dipole moment.

In a symmetric stretching mode, both bonds stretch by the same amount, but in opposite directions. As a result, the net dipole moment of the molecule remains unchanged during this vibration, making it IR inactive.

Let’s examine symmetric stretching in a non-linear triatomic molecule. Even though the bonds are identical, the angle between them results in a net dipole moment, represented by the green arrow.

Since this is a symmetric stretching, both bonds undergo stretching simultaneously and to the same extent. However, unlike in linear triatomic molecules, the net dipole moment does change during this vibration. Consequently, this symmetric stretching is considered IR active.

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Now let’s examine what occurs during asymmetric stretching.

In this mode, the stretching vibrations do not occur simultaneously. Consequently, there is a change in the net dipole moment of the molecule during these vibrations. As a result, this asymmetric stretching mode is classified as IR active.

This explanation is valid and holds true for both linear and non-linear molecules.

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Here is a special case of an asymmetric triatomic molecule. In this scenario, the stretching is asymmetric because the atoms connected to the central atom are different.

Consider this hypothetical molecule: the bond moments are unequal, resulting in an initial net dipole moment. During the stretching vibrations, the dipole moment changes, making this mode IR active.

It's important to note that even if both bonds are vibrating simultaneously, the magnitudes of their stretches are not the same. The bond associated with the smaller atom experiences a greater stretch than the bond with the larger atom. Therefor, this stretching mode is classified as asymmetric.

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Now, let’s explore bending vibrations.

Consider a simple bending vibration in a linear triatomic molecule.
Recall that, in bending vibrations, the bond angles are altered. Inorder to maintain the center of mass of the molecule, the bond lenghts are also slightly changed.
In this mode, the bond moment changes during the bending motion, resulting in an alteration of the dipole moment. Therefore, this bending vibration is classified as IR active.
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For other non-linear and more complex molecules, the bending vibrations can be further categorized into several interesting modes like,
Wagging
Twisting
Scissoring
Rocking
Let us walk through these vibrational modes.
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