24-25 Lecture 1 Introduction (Advanced Spectroscopy) PDF

Advanced Pharmaceutical Analysis - Spectroscopy Lecture 1 (PC E10 & PC E13) 1  Quantum Theory A theory of physics in which: “Energy exists only in discrete quantities, called quanta”. It was originated in 1900 by the German physicist Max Karl...

Advanced Pharmaceutical Analysis - Spectroscopy Lecture 1 (PC E10 & PC E13) 1  Quantum Theory A theory of physics in which: “Energy exists only in discrete quantities, called quanta”. It was originated in 1900 by the German physicist Max Karl Ernst Ludwig Planck (1858–1947), who suggested that “electromagnetic radiation is quantized”, i.e. it can be emitted or absorbed only in tiny packets, not continuously. ‫) هي كلمة مشتقة من الالتينية‬Quantum) ‫كلمة‬ ‫تشير إلى أصغر كمية من الطاقة أو الوحدات‬ ‫المنفصلة الصغيرة من الطاقة‬ Einstein Theory of Relativity: E = mc2, energy and mass are equivalent and transmutable. Energy (E) equals mass (m) times the speed of light (c) squared (2). 2  Each quantum of radiation, called a photon, has an energy equal to hf (hv), where h is the Planck constant and f (v) the frequency of the radiation. Planck's quantization of energy is described by the his famous equation: Quantum mechanics is the field of physics that explains how extremely small objects simultaneously have the characteristics of both particles (tiny pieces of matter) and waves (a disturbance or variation that transfers energy). Physicists call this the “wave-particle duality” Quantum Theory in Chemistry states that there are only certain allowed energy states for an electron and that these are quantized. Further, it tells us that no two electrons, in the same system, can occupy the same energy state, and that all the energy states are filled from the lowest levels to the highest levels. 3  Interaction of EMR with matter EMR “Electric Magnetic Radiation” interacts with matter only (i) when the matter has some electric and magnetic effect (ii) and are influenced by the electric and magnetic components of the EMR. The net change in the electric/magnetic dipole moment in the molecule or nuclear spin, interact with the magnetic/electrical component of the EMR by either absorption or emission of the EMR. Total energy of molecules = Translational + rotational + vibrational + electronic Absorption or emission of EMR causes a change in any of these types of energies. In molecular spectroscopy, we measure the change in these energy states. 4 Translational energy: It is due to the overall movement of the molecule. Energy levels are not quantized. Hence no spectroscopy. Rotational energy: It is due to spinning of molecules about the axis passing through the center of gravity - Rotational Levels are quantized – Rotational spectroscopy (Microwave spectroscopy) A Vibrational energy: It is due to vibrations in molecules – Vibrational Levels are Quantized – IR Spectroscopy (Vibrational spectroscopy). B Electronic energy: Consists of electronic levels which are quantized – C UV/visible spectroscopy (Electronic spectroscopy). If E is the total energy of a molecule, it can be expressed as the sum of translational, rotational, vibrational and electronic contributions. E = Etrans +Erot + Evib + Eelec 5  A B C Energy Level Diagram An energy level diagram for a molecule. The greater the distance between lines, the greater the energy of the absorbed or emitted photon and thus the shorter the photon’s wavelength. 6  Introduction for Spectroscopy Spectroscopy deals with the interaction of matter with electromagnetic radiations (EMR). The electromagnetic spectrum 7 Electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. The types of electromagnetic radiation are broadly classified into the following classes (regions, bands or types): 1. Gamma radiation 2. X-ray radiation 3. Ultraviolet radiation 4. Visible light (light that humans can see) 5. Infrared radiation 6. Microwave radiation 7. Radio waves, TV. 8 All forms of electromagnetic radiations travel at the same velocity but characteristically differ from each other in terms of frequencies and wavelength 9 Radio waves, at the low-frequency end of the spectrum, have the lowest photon energy and the longest wavelengths—thousands of kilometers, or more. They can be emitted and received by antennas, and pass through the atmosphere, foliage, and most building materials. Gamma rays, at the high-frequency end of the spectrum, have the highest photon energies and the shortest wavelengths—much smaller than an atomic nucleus. Ionizing and nonionizing radiation: Gamma rays, X-rays, and extreme ultraviolet rays are called ionizing radiation because their high photon energy is able to ionize atoms, causing chemical reactions. Visible light and radiation of longer wavelengths are nonionizing; their photons do not have sufficient energy to cause these effects. Electromagnetic waves are typically described by any of the following three physical properties: (1) Frequency f, (2) Wavelength λ, (3) Photon Energy E. 10 - The propagation of these radiations involves both electric and magnetic forces which give rise to their common class name electromagnetic radiation. - In spectroscopy, only the effects associated with electric component of electromagnetic wave are important. Therefore, the light wave traveling through space is represented by a sinusoidal trace. - In the diagram λ is the wavelength and distance A is known as the maximum amplitude of the wave. Although a wave is frequently characterized in terms of its wavelength λ, often the terms such as: Wavenumber (ν−, number of waves per centimeter), Frequency (ν), cycles per second (cps or hertz; Hz) are also used. Wave like propagation of light ( λ = wavelength , A = amplitude) 11 The unit commonly used to describe the wavelength is centimeters (cm), the different units are used to express the wavelengths in different parts of the electromagnetic spectrum. For example, in the ultraviolet and visible region, the units use are angstrom (Ǻ) and nanometer (nm). In the infrared region, the commonly used unit is wavenumber (ν−), which gives the number of waves per centimeter. Thus The four quantities wavelength, wavenumber, frequency and velocity can be related to each other by following relationships 12 The energy associated with electromagnetic radiation is defined by the following equation: where: c = 3x108 m/s is the speed of light in vacuum h = 6.62607015×10−34 J·s = 4.13566733(10)×10−15 eV·s is the Planck constant. The energy of an electromagnetic wave increases with decreasing wavelength – and vice versa 13 Absorption of different Electromagnetic radiations by Organic Molecules In absorption spectroscopy, though the mechanism of absorption of energy is different in the ultraviolet, infrared and nuclear magnetic resonance regions, the fundamental process is the absorption of a discrete amount of energy. The energy required for the transition from a state of lower energy (E1) to state of higher energy (E2) is exactly equivalent to the energy of electromagnetic radiation that causes transition. Energy transition for the absorption of any electromagnetic radiation 14 Where E is energy of electromagnetic radiation being absorbed, h is the universal Planck’s constant, 6.624 x 10-27 erg sec and ν is the frequency of incident light in cycles per second (cps or hertz, Hz), c is velocity of light 2.998 x 1010 cm s-1 and λ = wavelength (cm) Therefore, higher is the frequency, higher would be the energy and longer is the wavelength, lower would be the energy. As we move from cosmic radiations to ultraviolet region to infrared region and then radio frequencies, we are gradually moving to regions of lower energies. A molecule can only absorb a particular frequency, if there exists within the molecule an energy transition of magnitude E = h ν Although almost all parts of electromagnetic spectrum are used for understanding the matter, in organic chemistry we are mainly concerned with energy absorption from only ultraviolet and visible, infrared, microwave and radiofrequency regions. 15 Ultraviolet – visible spectroscopy (λ 200 - 800 nm) studies the changes in electronic energy levels within the molecule arising due to transfer of electrons from π- or non-bonding orbitals. It commonly provides the knowledge about π-electron systems, conjugated unsaturation, aromatic compounds and conjugated non-bonding electron systems etc. Infrared spectroscopy (ν- 400-4000 cm-1) studies the changes in the vibrational and rotation movements of the molecules. It is commonly used to show the presence or absence of functional groups which have specific vibration frequencies viz. C=O, NH2, OH, CH, C-O etc. Nuclear magnetic resonance (radiofrequency ν 60-600 MHz) provides the information about changes in magnetic properties of certain atomic nuclei. 1H and 13C are the most commonly studied nuclei for their different environments and provide different signals for magnetically non- equivalent nuclei of the same atom present in the same molecule. 16 Spectroscopy, study of the absorption and emission of light and other radiation by matter, as related to the dependence of these processes on the wavelength of the radiation Atomic spectroscopy deals with electromagnetic radiations emitted or absorbed by atoms. Molecular spectroscopy deals with electromagnetic radiations emitted or absorbed by molecules. 17