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Chapter 12 - Infrared spectroscopy and mass spectrometry. This chapter identify the reliable characteristic absorptions in an infrared spectrum, and propose which functional groups are likely to be present in the molecule; explain which functional groups cannot be present in a molecule because their characteristic peaks are absent from the IR spectrum; use a mass spectrum to determine a compound’s molecular weight, and propose which elements are likely to be present;. | Chapter 12 ©2010, Prentice Hall Organic Chemistry, 7th Edition L. G. Wade, Jr. Infrared Spectroscopy and Mass Spectrometry Chapter 12 Introduction Spectroscopy is a technique used to determine the structure of a compound. Most techniques are nondestructive (it destroys little or no sample). Absorption spectroscopy measures the amount of light absorbed by the sample as a function of wavelength. Chapter 12 Types of Spectroscopy 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 their mass. MS can give the molecular weight of the compound and functional groups. Nuclear magnetic resonance (NMR) spectroscopy analyzes the environment of the hydrogens in a compound. This gives useful clues as to the alkyl and other functional groups present. Ultraviolet (UV) spectroscopy uses electronic transitions to determine bonding patterns. . | Chapter 12 ©2010, Prentice Hall Organic Chemistry, 7th Edition L. G. Wade, Jr. Infrared Spectroscopy and Mass Spectrometry Chapter 12 Introduction Spectroscopy is a technique used to determine the structure of a compound. Most techniques are nondestructive (it destroys little or no sample). Absorption spectroscopy measures the amount of light absorbed by the sample as a function of wavelength. Chapter 12 Types of Spectroscopy 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 their mass. MS can give the molecular weight of the compound and functional groups. Nuclear magnetic resonance (NMR) spectroscopy analyzes the environment of the hydrogens in a compound. This gives useful clues as to the alkyl and other functional groups present. Ultraviolet (UV) spectroscopy uses electronic transitions to determine bonding patterns. Chapter 12 Wavelength and Frequency The frequency of a wave is the number of complete cycles that pass a fixed point in a second. Wavelength is the distance between any two peaks (or any two troughs) of the wave. Chapter 12 Electromagnetic Spectrum Frequency and wavelength are inversely proportional. c = ln l = c/n where c is the speed of light (3 x 1010 cm/sec). Energy of the photon is given by E = hn where h is Planck’s constant (6.62 x 10-37 kJ•sec). Chapter 12 The Electromagnetic Spectrum Chapter 12 The IR Region From right below the visible region to just above the highest microwave and radar frequencies . Wavelengths are usually 2.5 x 10-4 to 25 x 10-4 cm. More common units are wavenumbers, or cm-1, the reciprocal of the wavelength in centimeters. Wavenumbers are proportional to frequency and energy. Chapter 12 Molecular Vibrations If the bond is stretched, a restoring force pulls the two atoms together toward their equilibrium bond length.
