John E. McMurry

www.cengage.com/chemistry/mcmurry

Paul D. Adams • University of Arkansas

Chapter 12Structure Determination: Mass

Spectrometry and Infrared Spectroscopy

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?

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

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

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 13C that is randomly present

Other Mass Spectral Features

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

Hexane (m/z = 86 for parent) has peaks at m/z = 71, 57, 43, 29

Mass Spectral Fragmentation of Hexane

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

Amines undergo -cleavage, generating radicals

Mass Spectral Cleavage of Amines

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

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

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

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

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

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

Figure 12.14

4000-2500 cm-1 N-H, C-H, O-H (stretching) 3300-3600 N-H, O-H 3000 C-H

2500-2000 cm-1 CC and C N (stretching)

2000-1500 cm-1 double bonds (stretching) C=O 1680-1750 C=C 1640-1680 cm-1

Below 1500 cm-1 “fingerprint” region

Regions of the Infrared Spectrum

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

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

Alkynes

12.8 Infrared Spectra of Some Common Functional Groups

Weak C–H stretch at 3030 cm1

Weak absorptions 1660 - 2000 cm1 range Medium-intensity absorptions 1450 to 1600 cm1 See spectrum of phenylacetylene, Figure 12.15

IR: Aromatic Compounds

IR: Aromatic Compounds

O–H 3400 to 3650 cm1 Usually broad and intense

N–H 3300 to 3500 cm1

Sharper and less intense than an O–H

IR: Alcohols and Amines

Strong, sharp C=O peak 1670 to 1780 cm1

Exact absorption characteristic of type of carbonyl compound 1730 cm1 in saturated aldehydes 1705 cm1 in aldehydes next to double bond or

aromatic ring

IR: Carbonyl Compounds

IR: Carbonyl Compounds

1715 cm1 in six-membered ring and acyclic ketones 1750 cm1 in 5-membered ring ketones 1690 cm1 in ketones next to a double bond or an aromatic ring

C=O in Esters 1735 cm1 in saturated esters 1715 cm1 in esters next to aromatic ring or a double bond

C=O in Ketones

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

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 C5H12O 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.