ASR IR Spectroscopy Notes

2025 JC2 H3 INFRA-RED SPECTROSCOPY ©2025/ASRJC/CHEM 2 ANDERSON SERANGOON JUNIOR COLLEGE H3 CHEMISTRY INFRA-RED (IR) SPECTROSCOPY CONTENT 1 Introduction 2 Molecular Vibrations 3 Interpretation of IR spectrum 4 Characteristic IR absorption frequencies LEARNING OUTCOMES Students should be able to: (a) explain the origin of IR spectroscopy in simple molecules in terms of (i) stretching vibrations (ii) bending vibrations [detailed knowledge of instrumentation is not required] (b) predict the number of IR absorptions for a given simple molecule (e.g. CO2 or SO2), and identify the molecular vibrations which give rise to them (c) identify characteristic IR absorptions in the IR spectrum of a compound which may contain different functional groups [Absorptions of common functional groups will be provided in the Data Booklet.] (d) suggest structures for a compound from its IR spectrum (e) predict the characteristic IR absorptions that will be present in the IR spectrum of a compound, given its structure 1. INTRODUCTION Infra-Red (IR) spectroscopy covers the IR radiation of the electromagnetic spectrum that has a longer wavelength and lower energy than visible light. The energy available is insufficient to cause electronic transitions, but it is enough to cause molecular vibrations and rotations. Molecules are not motionless entities with the rigid bond lengths and angles that we are used to seeing in bo oks. The atoms in a molecule act ually display periodic motions known as vibrations, which arise from the bending or stretching of bonds. 1.1 Basic Principle The absorption of IR is, like other absorption processes, a quantized process. A molecule absorbs only selected frequencies (energies) of IR radiation. The absorption of IR corresponds to energy changes in the order of 8 to 40 kJ/mol. Radiation in this energy range corresponds to the range encompassing the stretching and bending vibrational frequencies of the bonds in most covalent molecule. In other words, the absorption of IR radiation is associated with small energy differences in the possible vibrational states / energy levels (i.e. stretching and bending of bonds). If the frequency of the radiation matches the energy gap between vibrationa l energy levels of the molecule, radiation will be absorbed, causing a change in the amplitude of molecular vibration.

2025 JC2 H3 INFRA-RED SPECTROSCOPY ©2025/ASRJC/CHEM 3 Note, a molecule will absorb IR radiation corresponding to the frequency of the bond’s natural vibration only if that particular mode of vibration causes the dipole moment of the molecule to change. Using information on the absorptions, IR spectroscopy can be used to identify specific functional groups present in a molecule. 2 MOLECULAR VIBRATIONS 2.1 Types of Molecular Vibrations The relative positions of atoms in a molecule are not fixed but instead fluctuate continuously