CHE202A - Chapter 12

CHE202A - Chapter 12 - 1 Chapter 12 Infrared Spectroscopy...

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Unformatted text preview: 1 Chapter 12 Infrared Spectroscopy & Mass Spectrometry 1 2 Spectroscopy is an analytical technique which helps determine structure. 12-1 Introduction 2 It involves the use of the absorption , emission , or scattering of electromagnetic radiation by matter to qualitatively or quantitatively study the matter or to study physical processes. It destroys little or no sample. The amount of light absorbed by the sample is measured as wavelength is varied. 3 Infrared (IR) spectroscopy measures the bond vibration frequencies in a molecule and is used to determine the functional group. Mass spectrometry (MS) fragments the molecule and measures the masses. Nuclear magnetic resonance (NMR) spectroscopy detects signals from hydrogen atoms and can be used to distinguish isomers. Ultraviolet (UV) spectroscopy uses electron transitions to determine bonding patterns. Types of Spectroscopy 3 4 12-2 The Electromagnetic Spectrum 4 Examples: gamma rays, X rays, microwaves, radio waves, visible light, IR, and UV. Frequency and wavelength are inversely proportional. c = , or = c/ , where c is the speed of light. Energy per photon = h , where h is Plancks constant. 5 Visible light and gamma rays and microwaves are really the same things. They are all electromagnetic radiation; they just differ in their wavelengths. 5 6 Electromagnetic Radiation Electromagnetic radiation consists of discrete packets of energy called photons . A photon consists of an oscillating electric Feld component, E, and an oscillating magnetic Feld component, M. The electric and magnetic Felds are orthogonal (perpendicular) to each other, and they are orthogonal to the direction of propogation of the photon. The electric and magnetic Felds of a photon ip direction as the photon travels. The number of ips, or oscillations, that occur in one second is called the frequency , . [units of oscillations per second, or s-1 ( Hertz )]. The physical distance in the direction of propogation a photon makes one complete oscillation is called the wavelength , , of the electromagnetic radiation. 6 7 The equations summarizing the relationships between energy ( E ), frequency ( v ), and wavelength ( l ): E = hv v = c / E = hc/ ( h = 6.63 x 10-27 erg sec; c = 2.998 x 10 10 cm sec-1 ) 7 8 12-3 The Infrared Region The IR region covers 7.8 x 10-7 m (just above the visible region) to 2.0 x 10-3 m (just below the microwave region) Organic chemistry uses mainly the vibrational IR, which covers 2.5 x 10-6 m (2.5 mm) to 2.5 x 10-5 m (25 mm) Absorption of IR radiation in this region causes bonds to change from a lower vibrational energy level to a higher one 8 9 The frequency of IR radiation is commonly expressed in wavenumbers Wavenumber : the number of waves per centimeter, cm-1 (read reciprocal centimeters) Expressed in wavenumbers, the vibrational IR extends from 4000 cm-1 to 400 cm -1 2.50 m 10,000 m cm-1 = 4000 cm-1 25.0 m 10,000 m cm-1 = 400 cm-1 9 10...
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This note was uploaded on 10/10/2010 for the course CHE 202 taught by Professor Richard during the Spring '05 term at SUNY Buffalo.

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CHE202A - Chapter 12 - 1 Chapter 12 Infrared Spectroscopy...

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