Organic Chemistry Fourth Edition David Klein Chapter 14 Infrared Spectroscopy and Mass Spectrometry This deck contains equations authored in Math Type For the full experience please download the Math Type software plug in Copyright 2021 John Wiley Sons Inc 14 1 Introduction to Spectroscopy Overview Spectroscopy involves an interaction between matter and light electromagnetic radiation Light can be thought of as waves of energy or packets particles of energy called photons Properties of light waves include wavelength and frequency Wavelength is inversely proportional to energy Frequency is directly proportional to energy Copyright 2021 John Wiley Sons Inc 2 14 1 Introduction to Spectroscopy Light Electromagnetic spectrum The range of possible frequencies of light Copyright 2021 John Wiley Sons Inc 3 14 1 Introduction to Spectroscopy Classification Different regions of the electromagnetic spectrum are used to probe for different aspects of molecular structure Table 14 1 Some common forms of spectroscopy and their uses Type of Spectroscopy Nuclear magnetic resonance N M R spectroscopy IR spectroscopy UV VIS spectroscopy Information Obtained Region of Electromagnetic Spectrum Radio waves The specific arrangement of all carbon and hydrogen atoms in the compound Infrared Visible and ultraviolet The functional groups present in the compound Any conjugated system present in the compound Copyright 2021 John Wiley Sons Inc 4 14 1 Introduction to Spectroscopy Quantum Behavior Matter exhibits particle like properties On the macroscopic scale matter appears to exhibit continuous behavior rather than quantum behavior o Consider the example of an engine powering the rotation of a tire The tire should be able to rotate at nearly any rate Matter also exhibits wave like properties Matter on the molecular scale exhibits quantum behavior o A molecule will only rotate or vibrate at certain rates energies Copyright 2021 John Wiley Sons Inc 5 14 1 Introduction to Spectroscopy Vibrational Excitation For the electrons in covalent bonds vibrational energy levels are separated by gaps quantized If a photon of light strikes the molecule with the exact amount of energy needed the light is absorbed and vibrational excitation will occur Infrared IR light generally causes molecular vibration Different types of bonds absorb different IR energies Eventually the absorbed energy is released from the molecule as heat Copyright 2021 John Wiley Sons Inc 6 14 2 IR Spectroscopy Bond Stretching and Bending Molecular bonds can vibrate by stretching or by bending in a number of ways This chapter will focus mainly on stretching frequencies Copyright 2021 John Wiley Sons Inc 7 14 2 IR Spectroscopy Common Uses Some night vision goggles can detect IR light that is emitted IR or thermal imaging is also used to detect breast cancer Copyright 2021 John Wiley Sons Inc 8 14 2 IR Spectroscopy Necessary Energy The energy necessary to cause vibration depends on the type of bond Copyright 2021 John Wiley Sons Inc 9 14 2 IR Spectroscopy Functional Groups An IR spectrophotometer irradiates a sample with all frequencies of IR light The frequencies absorbed by the sample tell us the types of bonds functional groups that are present Most commonly samples are deposited on a salt NaCl plate Alternatively the compound may be dissolved in a solvent or embedded in a KBr pellet Copyright 2021 John Wiley Sons Inc 10 14 2 IR Spectroscopy Absorption Bands An IR absorption spectrum plots the transmittance as a function of frequency The peaks are called absorption bands Copyright 2021 John Wiley Sons Inc 11 14 2 IR Spectroscopy Wavenumbers Units of frequency in IR are called wavenumbers The values range from 400 to 4000 cm 1 Copyright 2021 John Wiley Sons Inc 12 vvc 14 2 IR Spectroscopy Peak Characteristics A signal peak on the IR spectrum has three important characteristics wavenumber intensity and shape Copyright 2021 John Wiley Sons Inc 13 14 3 Signal Characteristics Wavenumber What This Indicates The frequency wavenumber for a stretching vibration depends on 1 bond strength and 2 mass difference of the atoms bonded together Stronger bond higher stretching frequency Larger mass difference higher stretching frequency Copyright 2021 John Wiley Sons Inc 14 14 3 Signal Characteristics Wavenumber Numerical Examples The trends in stretching frequency of given bonds can be rationalized based on bond strength and mass difference Copyright 2021 John Wiley Sons Inc 15 14 3 Signal Characteristics Wavenumber Types of Bonds The wavenumber formula and empirical observations allow us to designate regions as representing specific types of bonds Copyright 2021 John Wiley Sons Inc 16 14 3 Signal Characteristics Wavenumber Diagnostic and Fingerprint Regions The region above 1500 cm 1 is called the diagnostic region The peaks in this region provide clear information The region below 1500 cm 1 is called the fingerprint There are typical many signals here and it is difficult to region analyze Copyright 2021 John Wiley Sons Inc 17 14 3 Signal Characteristics Wavenumber Challenges Because they have the same types of covalent bonds the IR spectra for 2 butanol and 2 propanol are virtually indistinguishable Copyright 2021 John Wiley Sons Inc 18 14 3 Signal Characteristics Wavenumber C H Bonds Compare the IR stretching frequencies for each C H bond below The higher the s character of the carbon the stronger the C H bond and so the higher the stretching frequency of the C H bond Copyright 2021 John Wiley Sons Inc 19 14 3 Signal Characteristics Wavenumber C H Bond Spectra Alkyl C H bonds come just under 3000 cm 1 while alkenyl and alkynyl C H bonds are over 3000 cm 1 Copyright 2021 John Wiley Sons Inc 20 14 3 Signal Characteristics Wavenumber Terminal Alkenes and Alkynes It is possible that an alkene or alkyne could give an IR spectra without any signals above 3000 cm 1 Copyright 2021 John Wiley Sons Inc 21 14 3 Signal Characteristics Wavenumber Resonance Resonance delocalization of electrons affects the strength of a covalent bond and thus the wavenumber of a stretching signal Consider the C O stretching value for the following compounds Copyright 2021 John Wiley Sons Inc 22 14 3 Signal Characteristics Wavenumber Resonance Examples The more delocalized the p electrons the weaker the p bond and the lower the stretching frequency Copyright 2021 John Wiley Sons Inc 23 14 3 Signal Characteristics Wavenumber Conjugation Conjugated carbonyls have lower stretching frequencies