Learning Outcomes
By the end of this lesson, students should be able to:
i. Explain the fundamental principles of infrared (IR) absorption, including the concept of molecular vibrations and the change in dipole moment during vibration.
ii. Describe the types of molecular vibrations, such as stretching, bending, and rocking motions, and their corresponding IR absorption frequencies.
iii. Relate the strength of IR absorption to the polarity of the bond, the change in dipole moment during vibration, and the mass of the atoms involved.
iv. Identify the characteristic IR absorption frequencies of common functional groups, such as alkanes, alkenes, alkynes, carbonyl compounds, and alcohols.
v. Analyze IR spectra of simple molecules and assign the observed absorption bands to specific functional groups.
Introduction
Infrared (IR) spectroscopy, a powerful analytical technique, provides valuable information about the structure and functional groups of molecules by analyzing their absorption of infrared radiation. This lesson delves into the origin of IR absorption, exploring the types of molecular vibrations, factors influencing absorption strength, and the characteristic absorption frequencies of common functional groups.
i. Molecular Vibrations: The Musical Language of IR
When molecules absorb IR radiation, they undergo vibrational transitions, where the atoms within the molecule move relative to each other. These vibrations can be visualized as stretching, bending, and rocking motions of bonds, similar to how a spring oscillates when stretched and released.
Types of Vibrational Modes: IR spectroscopy can detect various vibrational modes, including stretching (increase or decrease in bond length), bending (change in bond angle), and rocking (back-and-forth rocking motion).
Change in Dipole Moment: A Key Factor
IR absorption occurs when the vibration of a molecule leads to a change in its dipole moment. If the dipole moment changes during vibration, the molecule can interact with the oscillating electric field of the IR radiation, causing absorption.
ii. Strength of IR Absorption: A Symphony of Factors
The strength of IR absorption depends on several factors:
Polarity of the Bond: Stronger IR absorption is observed for bonds with larger dipole moments.
Change in Dipole Moment: Larger changes in dipole moment during vibration lead to stronger IR absorption.
Mass of Atoms: Heavier atoms involved in the vibration generally result in weaker IR absorption.
iii. Characteristic IR Frequencies: The Fingerprint of Functional Groups
Each functional group exhibits a unique set of characteristic IR absorption frequencies, providing a molecular fingerprint:
Alkanes: Strong C-H stretching absorption near 3000 cm-1 and weaker C-H bending absorption near 1400 cm-1.
Alkenes: Characteristic C=C stretching absorption near 1650 cm-1.
Alkynes: Characteristic C≡C stretching absorption near 2200 cm-1.
Carbonyl Compounds: Strong C=O stretching absorption in the range of 1700-1750 cm-1.
Alcohols: Broad O-H stretching absorption near 3300 cm-1 and C-O stretching absorption near 1050 cm-1.
iv. Analyzing IR Spectra: Unveiling the Molecular Story
By analyzing IR spectra and identifying the characteristic absorption bands, chemists can determine the presence of specific functional groups and gain insights into the structure of molecules.
IR spectroscopy, through its exploration of molecular vibrations and the change in dipole moment, provides a powerful tool for identifying functional groups and understanding the structure of molecules. By recognizing the characteristic absorption frequencies of common functional groups and analyzing IR spectra, chemists can unravel the molecular stories hidden within the vibrational symphony.
