Fundamental Principles of Spectroscopic Methods PDF
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This document explains fundamental principles of spectroscopic methods, including different types of spectroscopy such as UV, IR, and NMR. It also covers electromagnetic radiation, energy levels in atoms and molecules, and the relationship between energy, frequency and wavelength.
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W I L L I A M S F U N D A M E N TA L PRINCIPLES OF SPECTROSCOPIC METHODS 4.1. explain the nature of electromagnetic radiation; (Calculation using the equation E=hv = hc/λ are required); 4.2. state the approximate wavelength ra...
W I L L I A M S F U N D A M E N TA L PRINCIPLES OF SPECTROSCOPIC METHODS 4.1. explain the nature of electromagnetic radiation; (Calculation using the equation E=hv = hc/λ are required); 4.2. state the approximate wavelength ranges of the X-ray, OBJECTIVE UV/VIS, IR and radiofrequency S regions of the electromagnetic spectrum; 4.3. recall that the energy levels in atoms and molecules are quantized. ELECTROMAGNETIC RADIATION consist of waves that have electrical and magnetic fields of energy vibrating in particular directions (transmitted through space). Each of these types of radiation has specific ranges of wavelengths and frequencies. THE N AT U R E O F WAVELENGTH is the distance between one EMR wave peak and the next peak of waves radiation, it is measure in meters (m) FREQUENCY is the number of waves produced (passing a given point) in one second. It is measured in hertz (Hz). 1 nm is equivalent to 1.00 x 10⁻⁹ meters and 1 MHz is equivalent to 1 x 10⁶ Hz, which are essential for spectroscopic analysis. SPECTROSCOPY is defined as the measurement of electromagnetic radiation absorbed, scattered or emitted by atoms, molecules or other chemical species. Light is a form of electromagnetic radiation, but visible light is only one fraction of the electromagnetic spectrum SPECTROSCOP which consist of other types of radiation Y as gamma-rays, ultraviolet, visible, infrared, microwaves and radio waves. Approximate wavelength ranges include X-rays (0.01 to 10 nm), UV/VIS (10 nm to 750 nm), IR (750 nm to 1 mm), and radiofrequency (1 mm to 1 m). ULTRAVIOLET (UV) spectroscopy uses electron transitions to determine bonding patterns. INFRARED (IR) spectroscopy measures the bond vibration frequencies in a molecule and is used to determine the TYPES OF functional group. SPECTROSCOP MASS SPECTROMETRY (MS) fragments Y the molecule and measures the masses. NUCLEAR MAGNETIC RESONANCE (NMR) spectroscopy detects signals from hydrogen atoms and can be used to distinguish isomers. The speed of electromagnetic radiation is related to frequency and wavelength by the equation: c=fλ where : SPEED, FREQUENCY c is speed in ms-1 AND WAV E L E N GT H f is frequency in hertz, 1Hz = 1s-1 λ is the wavelength in m Note: The symbol v is often used for frequency when electrons are being considered. SPEED, FREQUENCY AND WAV E L E N G T H Since electromagnetic radiation can be treated as waves the following relationships apply: ∆E = hν ν =c /λ Where E is energy/J; h is Planck’s constant (6.63×10−34 J·s), ν is frequency/Hz, c is the speed of light (2.998 X 10 8 m s-1) and λ is wavelength of the radiation/m. Wavelength is inversely proportional to the wave frequency THE ELECTROMAGNETIC SPECTRUM. THE ELECTROMAGNETIC SPECTRUM. short wavelength, UV: long high energy, valence IR: molecular wavelength, high frequency low energy, electronic vibrations low frequency excitation E N E R G Y Q U A N TA - E X C I TAT I O N Electrons in atoms possess specific energy levels, known as quanta, which are fixed and discrete values. When atoms absorb electromagnetic radiation, electrons transition from a lower energy state (E1) to a higher energy state (E2). E N E R G Y Q U A N TA : D E - E X C I TAT I O N The transition from E1 to E2 requires energy equal to the frequency of the absorbed electromagnetic radiation. As electrons return to the ground state, they release energy in the form of radiation, which can be conceptualized as photons. A molecule can only absorb a particular frequency of electromagnetic radiation if there exist an energy transition within the molecule where: (E2 - E1) = ∆E = hv The energy difference involved is given by the equation: ENERGY Q U A N TA ∆E = hv This equation can be used to calculate the energy emitted when radiation of a particular wavelength or frequency is emitted from a previously excite atom. ENERGY LEVELS An atom or molecule has electronic energy levels, which are related to the arrangement of electrons in atomic or molecular orbitals. These electronic energy levels also include vibrational and rotational sublevels. Electronic energy levels: These are the main energy levels in a molecule and are associated with how electrons are arranged in orbitals. Vibrational energy levels: These are subdivisions of the electronic energy levels, related to the vibrations of the molecule (e.g., stretching and bending of bonds). Rotational energy levels: These are further subdivisions within vibrational levels and are related to the rotation of the molecule. The energy transition that results when chemical species absorb electromagnetic radiation depends on the energy of the radiation and therefore the type of electromagnetic radiation absorbed. Electromagnetic Energy Transitions (Electronic Transitions) These transitions occur when molecules absorb or emit ultraviolet (UV) or visible light (high-energy electromagnetic radiation). Process: An electron in a molecule is excited from a lower-energy electronic level (ground state) to a higher-energy electronic level (excited state). Spectroscopy: UV-Vis spectroscopy is used to study electronic transitions. Vibrational Energy Transitions : These transitions occur when molecules absorb infrared (IR) radiation, which causes vibrations (stretching and bending) of chemical bonds. Rotational transitions This refers to changes in the rotational energy levels of a molecule. When a molecule absorbs IR radiation, it can undergo vibrational transitions (e.g., bond stretching or bending).At the same time, rotational transitions occur within vibrational energy levels, leading to a phenomenon called rotational-vibrational coupling. The visible hydrogen emission spectrum lines in the Balmer series. H-alpha is the red line on the right. The two leftmost lines are considered to be ultraviolet as they have wavelengths less than 400nm. EMISSION SPECTRUM OF HYDROGEN WORKED EXAMPLE Calculate the energy of an electron transition which emits radiation of frequency 1.01 x 1012 Hz Plank constant = 6.63 x 10-34Js Substituting into the equation ∆E = hv = 6.63 x 10-34 x1.01 x 1012 = 6.70 x 10-22J ENERGY PER MOLE If we are asked for the value per mole of electrons, we multiply the energy obtained by the Avogadro number, 6.02 x 1023. 6.70 x 10-22 x 6.02 x 1023 = 403 J.mol-1 Electromagnetic radiation can be regarded as waves that have a characteristic frequency and wavelength. Speed = frequency x wavelength. The spectrum of electromagnetic radiation ranges from radio waves (107Hz) to gamma-rays (1019Hz). S U M M A RY Energy levels in atoms are quantized- they can only have certain energy values. The energy associated with a photon is given by E=hv is the speed of light and h is Plank constant. END OF TOPIC QUESTIONS 1. a. State the relationship between the energy, frequency and wavelength of a wave. b. Compound X absorbs light of frequency 156.8 MHz Calculate the wavelength of the light absorbed. c. Calculate the frequency of the radiation of wavelength 2. a. Differentiate between electronic, vibrational and rotational energy levels. b. Explain what is meant by energy levels in atoms and molecules are quantised. 3. What energy is associated with the following transitions: a. 928 MHz b. 740 nm c. 2 x 108 Hz Identify the part of the electromagnetic spectrum where transitions (a)-(c) occurs. 4. State the approximate wavelength ranges of the following: a. X rays b. Infrared radiation c. Radiofrequencies