Download Chapter 12: IR Spectroscopy and Mass Spectrometry

Chapter 12:
IR Spectroscopy and Mass
Spectrometry
1
Introduction (12.1)
Spectroscopy: Techniques for analyzing
molecules based on absorption and emission of
radiation. A characteristic of any spectroscopic
technique is that it does not destroy the sample.
There are 4 major types of
spectroscopies:
Infrared
Mass Spectrometry (not a spectroscopic technique)
Nuclear Magnetic Resonance (NMR)
Ultraviolet Spectroscopy
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Infrared Spectroscopy (12.3
12.3--12.12)
This type of spectroscopy is used mostly
to identify some key functional groups in
the molecules.
Infrared range between 10000-100 cm-1
but we are interested in the range 4000400 cm-1
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Bands (not lines) are observed because
the vibrational energy changes are
accompanied by rotational energy changes
There are two types of vibrations to
consider:
Stretching: rhythmical movement along
bond axis so that inter-atomic distance is
increased or decreased.
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Bending: change in bond angle between
bonds with a common atom (group)
What we observed on the spectrum are
only those vibrations that result in a
rythmical change in the dipole moment of
a molecule
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Stretching Frequencies
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How the spectrum looks like. (Bands
are from top to bottom).
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There are two important regions in the
spectrum
◦ 4000-1400 cm-1: functional group region
◦ 1400-650 cm-1: fingerprint (specific to a given
compound)
900-650 cm-1: often used to identify aromatic
substitution pattern (mono/o/m/p)
When analyzing an IR spectrum: never
consider absorptions in the fingerprint region.
Only look in the FG region
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Characteristic Absorptions (Stretching)
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Alkanes (C(C-H stretch is on the right
side of 3000 cm-1)…C
)…C--H stretch on a
sp3 carbon
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Alkenes (left of 3000 C-H stretch on a
sp2 C and 1620
1620--1650 cm-1)
Note that Aromatic compounds because of the large number of C-H
(sp2) stretches, will have a large number of bands left of 3000
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Alkynes (3300 terminal C-H stretch on sp
carbon, 2100
2100--2260 C-C triple bond stretch)
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Alcohols (O(O-H stretch is broad 3400
3400-3650 cm-1)
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Amines (NH stretch 3300
3300--3500 cm-1). Broad
and weaker than OH stretch). Primary amines
(NH2) have two sharp spikes, secondary amine
(R2NH) have only one and tertiary amine (R3N)
have no spike and no stretch band.
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Carbonyl compounds (1670
(1670--1780 cm-1).
The position of the band depends on the substitution
pattern found around the carbonyl group. The carbonyl
band is normally the strongest one in the spectrum. The
normal carbonyl absorption is around 1710-1720 cm-1
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For example when the C=O is bonded
to alkyl groups, the C=O stretch will be
in the 1710-1720 region.
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The same is true for aldehydes, but
here you will also find two very
strong C-H stretch in the 2720 and
2820 region.
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Carboxylic acids also show a band in that
general area, plus another one for the
hydroxyl group present. Note that in this
case the hydroxyl band is very broad due
to H-bonding.
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However, when the carbonyl is conjugated
with alkenes, alkynes and aromatic, the
carbonyl has a large single bond character
and the absorption is shifted to the right,
in the direction of the C-O (sp3 carbon)
absorption.
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Heteroatoms attached to the carbonyl will
also have an effect:
◦ donation of electron by resonance for N (lower
wavenumber), shifts the absorption to the right
20
inductive withdrawal for O (ie in esters) and
halogens (higher wavenumber)
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Effect of substituents on Carbonyl
(summary)
The C=O absorption varies a lot depending on
the groups attached to the carbonyl.
N o rm a l C = O s t re c h is 1 7 1 5 c m -1
F o r R -C O -G w h e re
G
C = O (c m -1 )
Cl
1 8 1 5 -1 7 8 5
Br
~ 1812
OH
1760
OR
1 7 5 0 -1 7 3 5
NH2
1 6 9 5 -1 6 5 0
C H = C H 2 o r P h 1 6 8 5 -1 6 6 5
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Nitriles also have an easily
recognizable band in the 2200-2300
cm-1 region. This band represent
the C-N triple bond stretch.
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Summary of IR Stretching Frequencies
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Interpreting IR Spectra (12.12)
The most important factor to
remember about IR spectra is that it
is a tool to help determine key
functional groups. As such, you
should never look to the right of
1600cm-1 (fingerprint region) of an
IR spectrum.
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Practice Questions
Determine the most likely functional group
(or functional groups) in the following IR
spectra.
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27
28
Lots of sp2 C-H stretch passed 3000
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Mass Spectrometry
The mass spectrometer analyses the gas phase
of molecules after they have been ionized. Only
positively charged ions are detected by mass
spectrometry.
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The mass spectrometer
Because these ions are unstable, they break
down into smaller units and these units can be
identified and the structure of the molecule can
be determined.
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The mass spectrum itself is usually represented
by a bar graph:
On the vertical axis is the relative abundance of
each ion.
On the horizontal axis is the mass of each ion,
ie a scale of m/z (mass/charge ratio)
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The tallest peak in the spectrum is assign
an intensity of 100% and is called the:
base peak
The peak further to the right in the
spectrum often represents the parent ion
(the ion from the original molecule
before any fragmentation has occurred).
Often referred to as Molecular ion or M+
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What do you get out of it?
While the mass spectrum can give you a
lot of structural information, at this point
you should only focus at trying to identify
the M+ as this will indicate what is the
mass of your compound. Be careful since
next to M+ it is possible to observe M+ + 1
(due to the contribution of 13C)
You should also know which peak is the
base peak.
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M+ can usually be observed for the following
types of molecules (order is from the most
easily detectable to very difficult [but not
impossible] to detect)…continue on next two
slides
should be
easily
recognized
aromatic compounds
conjugated alkenes
cyclic compounds
sulfides
short n-alkanes
mercaptans
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M+ usually
smaller than
the previous
set
ketones
amines
esters
ethers
carboxylic acids/aldehydes/amides/halides
M+ often
difficult
to detect
aliphatic alcohols
nitrite (R-ONO)
nitrate (RNO)
nitro compounds
nitriles
highly branched compounds
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Due to the isotopic abundance of bromine and
chlorine, their presence is easier to determine
using mass spectrometry.
M+ and M+ + 2 ratio is usually about 50/50
when Br is present in a molecule
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The presence of chlorine is usually
determined by at 75/25 ratio of M+ and
M+ + 2
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