Draw diagrams to represent the normal modes of vibration of CS2 and OCS. Indicate which modes wi

step 1 So vibrating molecules, when they absorb infrared radiation, have six types of normal modes of v

Draw diagrams to represent the normal modes of vibration of...

Key Concepts

Normal Modes of Vibration

In molecular vibrations, normal modes refer to the independent patterns in which all parts of a molecule move sinusoidally with the same frequency. In a linear molecule, these include stretches (where bond lengths change) and bends (where the bond angles change). Each mode is associated with a specific frequency and pattern of atomic displacement, and collective motions are analyzed under the assumption that each vibrational mode is independent of the others.

Infrared Activity and Selection Rules

Infrared (IR) spectroscopy detects vibrational motions that involve a change in the dipole moment of a molecule. For a vibrational mode to be IR active, the vibration must lead to a change in the dipole moment, which is determined by the symmetry properties of the mode. In symmetric molecules, such as CS?, some modes like the symmetric stretch may be inactive in the IR spectrum because they do not produce a changing dipole, while other modes (e.g., the antisymmetric stretch or bending modes) can be IR active.

Molecular Symmetry in Vibrational Analysis

The symmetry of a molecule plays a crucial role in determining its vibrational modes and their physical properties. For example, in molecules like CS?, which are linear and symmetric, the center of symmetry (inversion center) means that certain vibrational motions (e.g., symmetric stretching) do not change the dipole moment, rendering them IR inactive. In contrast, molecules lacking such high symmetry, like OCS, may display vibrational modes that are all IR active because any movement tends to alter the dipole moment.

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