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 as a consequence of a multitude of different types of vibrations and rotations about the bonds in the molecule. Vibrations fall into the basic categories of stretching and bending. A stretching vibration involves a continuous change in the interatomic distance along the axis of the bond between the two atoms. Bending vibrations are characterised by a change in the a ngle between two bonds and are of the four types: scissoring, rocking, wagging and twisting. Figure 1 Type of molecular vibrations. Note that + indicates motion from the page towards the reader and – indicates motion away from the reader.

2025 JC2 H3 INFRA-RED SPECTROSCOPY ©2025/ASRJC/CHEM 4 2.2 Number of vibrational modes A molecule can move freely about in one of the three ways – translation, rotation and vibration. Each of these motions involves a change in the position and orientation of the atoms in the molecule in space. To describe a motion, we make use of the Cartesi an coordinate axes (x, y and z). Hence a polyatomic molecule containing N atoms has 3N degrees of freedom when we treat each of the N atoms to be moving independently from each other. Regardless of the value of N, any molecule will have three degrees of freedom attributed to the translational motion along the three Cartesian axes. For a non-linear molecules, the other 3 degrees of freedom correspond to rotation about each axis. For a H2O molecule, its rotation is depicted as: But for a linear molecule, such as CO 2, there are only two degrees of freedom pertaining to rotation about the axes. As shown below, rotating the molecule along the x -axis, in which the atom lie, does not displace any atom from its position and hence does not count towards one degree of freedom. Hence, subtracting the translational and rotational degrees of freedome (six for a non -linear molecule and five for a linear molecule) from the total 3N degrees of freedom will give us the vibrational degrees of freedom. Type of degrees of freedom Linear Non-linear Total 3N 3N Translational 3 3 Rotational 2 3 Vibrational 3N – 5 3N – 6 Knowing the number of fundamental vibrations allow us to predict the number of absorption peaks in an IR spectrum. But the number of peaks observed may be fewer than expected. This is because in order for a particular vibrational mode to be IR active, the vibrational mode must involve a change in net dipole moment of the molecule during the vibration. In addition, the energy levels of some vibration modes may be too close to each other such that the different peaks become convoluted.

2025 JC2 H3 INFRA-RED SPECTROSCOPY ©2025/ASRJC/CHEM 5 2.3 Predicting the number of IR absorption Molecule type Example No. of vibration modes Vibrations and IR absorptions involved diatomic HCl (polar) 3(2) – 5 = 1 One IR absorption: • stretching of the H-Cl bond O=O (non polar) 1 No IR absorption since there is no change in the dipole moment. triatomic Non linear H2O 3(3) – 6 = 3 Three IR absorptions: Linear CO2 3(3) – 5 = 4 Two IR absorptions: C OO The _____________________ mode results in no change in overall dipole of the molecule. This is because the change in one of the C=O bond dipole on stretching is exactly compensated by the change in the other C=O bond dipole. Hence this vibrational mode is not IR active. O C O O C O O C O symmetrical stretching asymmetrical stretching bending _____________________ mode causes one IR absorption. O C O O C O O C O symmetrical stretching asymmetrical stretching bending There are _________________ modes. However, they have the same energy (i.e. they are degenerate) as the two bending modes are similar motions that onl y differ in the plane in which they occur. => 1 IR absorption observed.

2025 JC2 H3 INFRA-RED SPECTROSCOPY ©2025/ASRJC/CHEM 6 IR spectrum of CO2 3 INTERPRETATION OF IR SPECTRUM IR spectra is plotted as a graph of transmittance against wavenumber (cm-1). Wavenumbers is the reciprocal of the wavelength in cm, or the number of wavelengths per cm. The higher the wavenumer, the higher the energy involved. Wavenumber is related to frequency (Hz) as follows: f = c x 1/ where c = 3 x 1010 cm s-1 We can use the IR absorption frequencies to determine bond types and functional groups in an organic compound. Full interpretation of an IR spectrum is almost impossible because most organic molecules are so large resulting in many complicated absorbances. It is important not to over-interpret IR spectra. The useful IR region is from 4000 – 400 cm-1. The lower frequency part of the spectrum (1500 cm-1 to around 400 cm-1) of a molecule is called the “fingerprint region”. This part serves as a unique fingerprint of a specific compound. The following table lists various absorption peaks that can be associ