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John E. McMurry Paul D. Adams University of Arkansas Chapter 12 Structure Determination: Mass Spectrometry and Infrared.

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Presentation on theme: "John E. McMurry Paul D. Adams University of Arkansas Chapter 12 Structure Determination: Mass Spectrometry and Infrared."— Presentation transcript:

1 John E. McMurry Paul D. Adams University of Arkansas Chapter 12 Structure Determination: Mass Spectrometry and Infrared Spectroscopy

2  Finding structures of new molecules synthesized is critical  To get a good idea of the range of structural techniques available and how they should be used Why this Chapter?

3  Measures molecular weight  Sample vaporized and subjected to bombardment by electrons that remove an electron  Creates a cation radical  Bonds in cation radicals begin to break (fragment)  Charge to mass ratio is measured 12.1 Mass Spectrometry of Small Molecules: Magnetic-Sector Instruments

4  Plot mass of ions (m/z) (x-axis) versus the intensity of the signal (roughly corresponding to the number of ions) (y- axis)  Tallest peak is base peak (100%)  Other peaks listed as the % of that peak  Peak that corresponds to the unfragmented radical cation is parent peak or molecular ion (M + ) The Mass Spectrum

5  If parent ion not present due to electron bombardment causing breakdown, “softer” methods such as chemical ionization are used  Peaks above the molecular weight appear as a result of naturally occurring heavier isotopes in the sample  (M+1) from 13 C that is randomly present Other Mass Spectral Features

6  The way molecular ions break down can produce characteristic fragments that help in identification  Serves as a “fingerprint” for comparison with known materials in analysis (used in forensics)  Positive charge goes to fragments that best can stabilize it Interpreting Mass-Spectral Fragmentation Patterns

7  Hexane (m/z = 86 for parent) has peaks at m/z = 71, 57, 43, 29 Mass Spectral Fragmentation of Hexane

8 Alcohols:  Alcohols undergo  -cleavage (at the bond next to the C- OH) as well as loss of H-OH to give C=C 12.3 Mass Spectrometry of Some Common Functional Groups

9  Amines undergo  -cleavage, generating radicals Mass Spectral Cleavage of Amines

10  A C-H that is three atoms away leads to an internal transfer of a proton to the C=O, called the McLafferty rearrangement  Carbonyl compounds can also undergo  cleavage Fragmentation of Carbonyl Compounds

11  Radiant energy is proportional to its frequency (cycles/s = Hz) as a wave (Amplitude is its height)  Different types are classified by frequency or wavelength ranges 12.5 Spectroscopy and the Electromagnetic Spectrum

12  An organic compound exposed to electromagnetic radiation can absorb energy of only certain wavelengths (unit of energy)  Transmits energy of other wavelengths.  Changing wavelengths to determine which are absorbed and which are transmitted produces an absorption spectrum Absorption Spectra

13  IR region lower energy than visible light (below red – produces heating as with a heat lamp)  IR energy in a spectrum is usually measured as wavenumber (cm -1 ), the inverse of wavelength and proportional to frequency  Specific IR absorbed by an organic molecule is related to its structure 12.6 Infrared Spectroscopy

14  IR energy absorption corresponds to specific modes, corresponding to combinations of atomic movements, such as bending and stretching of bonds between groups of atoms called “normal modes”  Corresponds to vibrations and rotations Infrared Energy Modes

15  Most functional groups absorb at about the same energy and intensity independent of the molecule they are in  IR spectrum has lower energy region characteristic of molecule as a whole (“fingerprint” region) 12.7 Interpreting Infrared Spectra

16 Figure 12.14

17  cm -1 N-H, C-H, O-H (stretching)  N-H, O-H  3000 C-H  cm -1 C  C and  C  N (stretching)  cm -1 double bonds (stretching)  C=O  C=C cm -1  Below 1500 cm -1 “fingerprint” region Regions of the Infrared Spectrum

18  Bond stretching dominates higher energy modes  Light objects connected to heavy objects vibrate fastest: C–H, N–H, O–H  For two heavy atoms, stronger bond requires more energy: C  C, C  N > C=C, C=O, C=N > C–C, C–O, C–N, C–halogen Differences in Infrared Absorptions

19 Alkanes, Alkenes, Alkynes  C-H, C-C, C=C, C  C have characteristic peaks  absence helps rule out C=C or C  C 12.8 Infrared Spectra of Some Common Functional Groups

20 Alkynes 12.8 Infrared Spectra of Some Common Functional Groups

21  Weak C–H stretch at 3030 cm  1  Weak absorptions cm  1 range  Medium-intensity absorptions 1450 to 1600 cm  1  See spectrum of phenylacetylene, Figure IR: Aromatic Compounds


23  O–H 3400 to 3650 cm  1  Usually broad and intense  N–H 3300 to 3500 cm  1  Sharper and less intense than an O–H IR: Alcohols and Amines

24  Strong, sharp C=O peak 1670 to 1780 cm  1  Exact absorption characteristic of type of carbonyl compound  1730 cm  1 in saturated aldehydes  1705 cm  1 in aldehydes next to double bond or aromatic ring IR: Carbonyl Compounds


26  1715 cm  1 in six-membered ring and acyclic ketones  1750 cm  1 in 5-membered ring ketones  1690 cm  1 in ketones next to a double bond or an aromatic ring C=O in Esters  1735 cm  1 in saturated esters  1715 cm  1 in esters next to aromatic ring or a double bond C=O in Ketones

27 Let’s Work a Problem Propose structures for a compound that fits the following data: It is an alcohol with M + = 88 and fragments at m/z = 73, m/z = 70, and m/z = 59

28 Answer Answer: We must first decide on the the formula of an alcohol that could undergo this type of fragmentation via mass spectrometry. We know that an alcohol possesses an O atom (MW=16), so that leads us to the formula C 5 H 12 O for an alcohol with M + = 88, with a structure of: One fragmentation peak at 70 is due to the loss of water, and alpha cleavage can result in m/z of 73 and 59.

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