ASR Basic Principles of Spectroscopy Notes

2025 JC2 H3 Chemistry ©2025ASRJC/CHEM WE ARE ASR 1 Anderson Serangoon Junior College H3 Chemistry Basic Principles of Spectroscopy Lesson Outline Note: You should follow the sequence of content below. Skipping any pages will affect your understanding of later parts. All videos can be found in MS Teams Channel “Basic Principles of Spectroscopy”. Content Page Instructions for SDL lesson Relevant videos 1 Introduction 3 Self-read 2 Electromagnetic spectrum 3 - 5 Self-read 2.1 What is electromagnetic radiation? 2.2 Generation of light from atoms 2.3 Line emission spectra (quantisation of energy & energy level transitions) 3 Molecular Orbital (MO) Theory 3.1 Atomic Orbital 6 Self-read 3.2 Molecular Orbital 7 - Self-read - Watch Video 1 for elaboration 3.3 Linear Combination of Atomic Orbitals 7 - 8 3.4 HOMO and LUMO 9 3.5 Bond order 9 3.6 Homonuclear diatomic molecules 9 – 14 - Watch Video 2a_A Brief Introduction to Molecular Orbital Theory - Watch Video 2b_Summary 3.7 Conjugated and aromatic molecules 15 – 16 - Watch Video 3_Molecular Orbitals of Conjugated Alkenes 4 Self-check exercises 17 - 19 - Attempt all Self-check exercises. - Watch Video 4_Self-check Exercise 1 and 2 5 Additional readings 20 Self-read

2025 JC2 H3 Chemistry ©2025ASRJC/CHEM WE ARE ASR 2 H3 Chemistry Basic Principles Of Spectroscopy Learning outcomes Students should be able to: (a) understand basic Molecular Orbital (MO) theory, involving (i) atomic and molecular orbitals (ii) bonding, anti–bonding and non–bonding orbitals (iii) molecular orbitals with  and  symmetry (b) understand that molecular orbitals represent discrete electronic energy levels in molecules [see also (e)(ii)] (c) apply Linear Combination of Atomic Orbitals (LCAO) principles to obtain the shape and relative energ ies of molecular orbitals in the following: (i) simple homonuclear diatomic molecules such as H2, O2, and F2 (ii) benzene and linear polyenes (molecular orbitals of  symmetry only) [quantitative treatment of LCAO is not required] (d) construct and interpret molecular orbital diagrams, and identify the Highest Occupied Molecular Orbi tal (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) for the following: (i) simple homonuclear diatomic molecules such as H2, O2, and F2 (ii) benzene and linear polyenes (molecular orbitals of  symmetry only) [knowledge of orbital mixing between orbitals of the same symmetry is not required] (e) understand the following in relation to the fundamental principles of spectroscopy: (i) properties of electromagnetic radiation  the electromagnetic spectrum (with range of wavelengths for different types of radiation used in spectroscopy)  the photon as a discrete packet (quantum) of electromagnetic energy  the relationship between wavelength, frequency and speed of light, including the use of equation, E = h f (ii) the quantisation of energy in relation to  electronic, vibrational and rotational energy levels  nuclear energy levels in applied magnetic field (iii) energy level transitions associated with the absorption and emission of photos with energy matching the energy gap References 1. Organic Chemistry (7th Edition), John McMurry, Brooks/Cole 2. Organic Chemistry (Sixth Edition), Francis A. Carey, McGraw–Hill 3. Understanding Advanced Organic Chemistry and Analytical Chemistry, Chan & Tan, World Scientific 4 Teaching the Revised H3 Chemistry, Yan Yaw Kai, NIE Questions to ponder… Other than chemical tests, have you ever wondered if chemists have other ways to confirm structures of unknown compounds? How do they work? What are the limitations? Have you ever wondered what happens to the atomic orbitals in atoms after molecules are formed?

2025 JC2 H3 Chemistry ©2025ASRJC/CHEM WE ARE ASR 3 1. Introduction Spectroscopy is the study of the interaction between matter and electromagnetic radiation . While we are unable to “see” the matter at the atomic level, spectroscopy provides a method for the structural identification of organic and inorganic compounds. There are various spectroscopic techniques involving electromagnetic radiation of different frequencies. As the substance interacts with electromagnetic radiation, it provides information on the structure and properties of the substance. Thus, spectroscopy is a valuable tool to study the structure and properties of matter . Spectroscopy is widely used in modern research and its applications span across all fields of science and society. 2. Electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. 2.1 What is electromagnetic radiation? An electromagnetic (EM) radiation has the following properties:  Travels in a straight line  Can propagate through a vacuum  Behave as waves, with frequency (f) and wavelength ()  Velocity, c = 2.998 x 108 m s–1 [Data Booklet: 3.00 x 108 m s–1]  c = f [Since c is a constant, there is an inverse relationship between f and ] The diagrams below illustrates the relationship between frequency and wavelength. lower wavelength () higher frequency (f)  more penetrating Visible light region: 400 to 700 nanometers Fig. 1: Wavelength () is the distance between successive crests of a wave. Fig. 2: Frequency (f) is the number of waves per unit time. EM radiations of shorter wavelength has higher frequency.

2025 JC2 H3 Chemistry ©2025ASRJC/CHEM WE ARE ASR 4 Max Planck, a German theoretical physicist, proposed the theory of quantisation of energy of electromagnetic radiation. The theory states that electromagnetic radiation, e.g. visible light and ultraviolet ra diation, exists as packets or units of energy called quanta or photons that have neither mass nor charge. It exhibits both the properties of waves and particles, but here in spectroscopy, we shall focus on its wave–like nature. The energy of each photon (E) is related to the frequency (f) of the radiation by the equation E = h f, where h is the Planck constant, 6.63 x 10–34 J s Since f = c  , Planck’s equation can also be written as E = h c  2.2 Generation of light from atoms One of the earliest spectroscopic studies involved the investigation of electromagnetic (EM) radiation emitted by energetically excited atoms. An element, such as hydrogen, can be excited by an electrical potential until it gives off light. The emitted EM radiation is then passed through a prism, which separates it into its constituent wavelengths. Each element gives a unique line spectrum (discrete), unlike the continuous spectrum of all wavelengths given by white light. The following image shows examples of line emission spectra of different elements, where each element gives EM radiation of a few specific wavelengths (shown by the thin lines) unique to itself. How does emission spectrum come about and why is it different for each element?

2025 JC2 H3 Chemistry ©2025ASRJC/CHEM WE ARE ASR 5 2.3 Significance of line emission spectra The emission spectrum of a n element is the spectrum of electromagnetic radiation emitted due to electrons making a transition from a high energy state to a lower energy state. The energy of the emitted photons is equal to the energy difference between the two states. As shown by the line emission spectra of atoms, energetically excited atoms can only emit light of certain specific wavelengths (not the whole spectrum) , and we can conclude that only photons of certain specific energies are emitted from atoms of each element. This shows that there are specific energy gaps within atoms, which reveals the specific shel