Ch.8 - Covalent Compounds: Bonding Theories and Molecular Structure

McMurry8th EditionChemistryISBN: 9781292336145Not the one you use?Change textbook

McMurry 8th Edition

Ch.8 - Covalent Compounds: Bonding Theories and Molecular Structure

Problem 126

Chapter 8, Problem 126

Bond vibrations for the symmetric and asymmetric stretch in methane are illustrated below. Decide whether each vibration will result in the absorption of IR radiation. Arrows indicate the movement of atoms during the vibration.

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Identify the type of bond vibrations shown in the image: symmetric stretch and asymmetric stretch.

Understand that for a vibration to result in the absorption of IR radiation, there must be a change in the dipole moment of the molecule.

Analyze the symmetric stretch: all bonds are stretching and compressing in unison, which typically does not change the dipole moment significantly.

Analyze the asymmetric stretch: bonds are stretching and compressing in an unsynchronized manner, which usually results in a change in the dipole moment.

Conclude that the asymmetric stretch will result in the absorption of IR radiation, while the symmetric stretch will not.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Infrared (IR) Spectroscopy

Infrared spectroscopy is a technique used to identify molecular vibrations that occur when molecules absorb infrared light. Molecules can absorb IR radiation if the vibration results in a change in the dipole moment. This property is crucial for determining which molecular vibrations will lead to IR absorption.

Molecular Vibrations

Molecular vibrations refer to the oscillations of atoms within a molecule around their equilibrium positions. These vibrations can be classified into different types, such as stretching (change in bond length) and bending (change in bond angles). Understanding these vibrations helps predict how molecules interact with IR radiation.

Symmetric vs. Asymmetric Stretching

Symmetric stretching occurs when atoms in a molecule move in and out together, maintaining the center of mass, while asymmetric stretching involves atoms moving in opposite directions. In the context of IR absorption, symmetric stretches typically do not result in a change in dipole moment, whereas asymmetric stretches often do, making them more likely to absorb IR radiation.

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