Download Infrared Spectroscopy

Introduction to
spectroscopy
1-infrared spectroscopy
Spectroscopy
At this point in the course, we have learned many organic
reactions that can be used to synthesize organic molecules.
How do you determine whether the product that you have
obtained in an organic reaction is the product that you had
expected? Also, how would you determine the structure of
unknown organic compounds that are isolated from natural
sources?
Definition
• Spectroscopy:
It is an analytical technique concerned
with studying of the interaction of
electromagnetic radiation and matter
in order to determine the structure.
Types of Spectroscopy
• Infrared spectroscopy (IR)
– measures the bond vibration frequencies in a molecule
and is used to determine the functional group
• Mass spectrometry (MS)
– fragments the molecule and measures the masses
• Nuclear magnetic resonance spectroscopy (NMR)
– detects signals from hydrogen atoms (H NMR), or
carbon atoms ( C NMR) and can be used to distinguish
isomers
– Ultraviolet spectroscopy (UV) : next semester
electromagnetic relationships:
λ
c
Wavelength (λ): Crest-to-crest distance between
waves
Frequency (v): Number of cycles per second
number of cycles/sec or cycles. s-1
or Hertz (Hz)
electromagnetic relationships:
1.
Electromagnetic radiation displays the properties of both particles and
waves
2.
The particle component is called a photon
3.
The energy (E) component of a photon is proportional to the frequency.
Where h is Planck’s constant and n, is the frequency in Hertz (cycles
per second)
E = hn
The speed of light, c, is constant, the frequency, n, is
inversely proportional to how long the oscillation is, or wavelength:
c = 3 x 1010 cm/s
c
___
n=
l
 E = hn =
hc
___
l
High frequency (u)
Short wavelength (l)
= High energy
7
Low frequency (u)
Long wavelength (l)
= Low energy
http://www.rsc.org/learnchemistry/collections/spectroscopy/Content/FileRepository/Spectr
ascopy/Electromagnetic_radiation_1.swf
Electromagnetic (EM) Spectrum
Frequency, n in Hz
~1019
~1017
~1015
~1013
~1010
~105
0.01 cm
100 m
~10-4
~10-6
Wavelength, l
~.0001 nm
~0.01 nm
10 nm
1000 nm
Energy (kcal/mol)
> 300
g-rays
nuclear
excitation
(PET)
X-rays
core
electron
excitation
(X-ray
cryst.)
300-30
300-30
UV
electronic
excitation
(p to p*)
IR
molecular
vibration
Visible
Microwave
Radio
molecular
rotation
Nuclear Magnetic
Resonance NMR
(MRI)
Absorption Spectra
• Organic compound exposed to electromagnetic radiation, can
absorb energy of certain wavelengths.
• Changing wavelengths to determine which are absorbed and
which are transmitted produces an absorption spectrum
• Energy absorbed is shown as dips in spectrum
Infrared Absorption of Ethyl Alcohol CH3CH2OH
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Infrared Spectroscopy
Infrared radiation
IR is used to tell:
1. what type of bonds are present
2. some structural information
Infrared Spectroscopy
When a beam of an electromagnetic radiation is
passed through a substance , the radiation can be
either absorbed or transmitted depending on its
frequency and structure
Infrared spectroscopy is the measurement of the
wavelength and intensity of the absorption of infrared
light by a sample. Infrared is energetic enough to
excite molecular vibrations to higher energy levels.
12
Infrared Spectroscopy
• The wavelength of infrared absorption bands
is characteristic of specific types of chemical
bonds, and infrared spectroscopy finds its
greatest utility for identification of organic
and organometallic molecules. The high
selectivity of the method makes the
estimation of an analyte in a complex matrix
possible.
The IR Spectroscopic Process
As a covalent bond oscillates – due to the
oscillation of the dipole of the molecule – a
varying electromagnetic field is produced
The greater the dipole moment change through the
vibration, the more intense the EM field that is
generated
The IR Spectroscopic Process
When a wave of infrared light encounters this
oscillating EM field generated by the oscillating dipole
of the same frequency, the two waves couple, and IR
light is absorbed
The coupled wave now vibrates with twice the
amplitude
IR beam from spectrometer
“coupled” wave
EM oscillating wave
from bond vibration
http://www.youtube.com/watch?feature=player_embedded&v=S8R30EdcIT4#t=12
Dipole Moment (µ)
In order to absorb infrared radiation, a molecule
must undergo a net change in dipole moment as
a consequence of its vibrational or rotational
motion.
The dipole moments is determined by the
magnitude of the charge difference and
the distance between the two occurs during the
vibration
Dipole Changes During
Vibrations
• No net change in dipole moment in
homonuclear species such as O2, N2, or
Cl2, such compounds cannot absorb in
the infrared
• A polar bond is usually IR-active
• A nonpolar bond in a symmetrical
molecule will absorb weakly or not at all
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• Types of Molecular Vibrations:
Vibrations fall into the basic categories of
stretching and bending. A stretching vibration
involves a continuous change in the interatomic
distance along the axis of the bond between two
atoms. Bending vibrations are characterized by
a change in the angle between two bonds and are
of four types: scissoring, rocking, wagging, and
twisting.
Different Types of
Vibration
Symmetric Stretch
Scissoring
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Rocking
Asymmetric Stretch
Wagging
Twisting
Stretch – Vibration or oscillation along the line of the bond
H
H
C
symmetric
C
asymmetric
H
•
H
Bend – Vibration or oscillation not along the line of the bond
H
C
H
H
rock
scissor
in plane
C
C
C
H
H
H
H
H
twist
out of plane
wag
What is the Infrared?
• Just below red in the visible region
• Wavelengths usually 2.5-25  m
• More common units are wavenumbers, or cm-1,
the reciprocal of the wavelength in centimeters
(4000-400 cm-1)
• Wavenumbers are proportional to frequency and
energy
• Energy required to distort covalent bonds
(stretching, compression, wagging, etc.)
• Light that is absorbed shows up as a peak in
absorbance or a dip in transmittance.
• Absorption of Infrared light in the wavelength
range of 2.5x10-6 m to 25x 10-6 m (2.5x10-4 cm
to 25x 10-4 cm)=(2.5 m to 25 m) causes bond
vibrations
• Specific wavelengths (or frequencies) can be
correlated with specific functional groups
Wave Number => n (cm-1) => proportional to energy
Remember
Energy is inversely proportional to
wavelength but proportional to
wavenumber n
Vibrational Modes
Number of possible modes:
(1) If Linear molecule: 3N – 5
For a diatomic molecule, N = 2 so the number of modes is 3×2−5=1 . For it is
3×3−5=4
Example: Carbon dioxide is a a triatomic linear molecule (CO2), and thus has
3 x 3 – 5 = 4 modes
(2) If Nonlinear molecule: 3N – 6
Example : Triatomic molecule N=3 such as water, sulfur dioxide, or nitrogen
dioxide have 3 x 3 – 6 = 3 vibrational modes
triatomic nonlinear molecule (H2O): it is 3×3−6=3
Example 1: Water
Example 2: Carbon Dioxide
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Infrared Instruments
• An infrared spectrophotometer is;
an instrument that passes infrared light through an
organic molecule and produces a spectrum that
contains a plot of the amount of light transmitted on
the vertical axis against the wavelength of infrared
radiation on the horizontal axis.
* In infrared spectra the absorption peaks point
downward because the vertical axis is the percentage
transmittance of the radiation through the sample.
• Absorption of radiation lowers the percentage
transmittance value. Since all bonds in an organic
molecule interact with infrared radiation, IR spectra
provide a considerable amount of structural data.
How does it work?
• A molecule is
exposed to IR.
• It starts to undergo
distortions based on
the types of bonds
present.
• The spectrum
represents the
presence of chemical
groups (Functional
groups).
The IR Spectrum
Each stretching and bending vibration occurs with a characteristic
frequency as the atoms and charges involved are different for
different bonds
The y-axis on an IR
spectrum is in units of
% transmittance
In regions where the
EM field of an osc.
bond interacts with IR
light of the same n –
transmittance is low
(light is absorbed)
In regions
where no osc.
bond is
interacting with
IR light,
transmittance
nears 100%
The x-axis of the IR spectrum is in units of wavenumbers, n,
which is the number of waves per centimeter in units of
cm-1 (Remember E = hn or E = hc/l)
How are these interpreted?
• The spectrum is divided
into two distinct
sections.
– 4000cm-1 to 1300cm-1 is
the functional group
region.
– 1300cm-1 – 400cm-1 is
the fingerprint region.
• Bands in these areas
can be used as evidence
of certain functional
groups.
• Fingerprint region tends
to have small bands,
that are difficult to
individually resolve.
Summary of IR Absorptions
Bonds to H
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Triple bonds Double bonds
Single Bonds
The IR Spectrum – The detection of different
bonds
1-Lighter atoms will allow the oscillation to be
faster – higher energy. This is especially true
of bonds to hydrogen – C-H, N-H and O-H
2-Stronger bonds will have higher energy
oscillations
Triple bonds > double bonds > single bonds in
energy
C-C
1200 cm-1
C=C
1660 cm-1
CC
2200 cm-1 (weak or absent if internal)
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3-As opposed to chromatography or other
spectroscopic methods, the area of a IR band (or
peak) is not directly proportional to
concentration of the functional group producing
the peak
4-The intensity of an IR band is affected by
two primary factors:
a.Whether the vibration is one of
stretching or bending
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b. Electronegativity difference of the
atoms involved in the bond
• For both effects, the greater the change in
dipole moment in a given vibration or bend, the
larger the peak.
The greater the difference in electronegativity
between the atoms involved in bonding, the larger
the dipole moment, Hence polar bonds such as OH and C=O give more intense absorptions than
non-polar C-H, C-C and C=C bonds.
•
• Typically, stretching will change dipole moment
more than bending
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Infrared Group Analysis
1. Since most “types” of bonds in covalent
molecules have roughly the same
energy, i.e., C=C and C=O bonds, C-H
and N-H bonds they show up in similar
regions of the IR spectrum
2. Remember all organic functional groups
are made of multiple bonds and
therefore show up as multiple IR bands
(peaks)
How to Analyze an IR Spectrum;
Look for what’s there and what’s not there
• C-H absorption
– The wavenumber will tell you sp3(C-C), sp2(C=C), sp (C≡C) and
perhaps aldehyde.
• Carbonyl (C=O) absorption
– Its presence means the compound is an aldehyde, ketone,
carboxylic acid, ester, amide, anhydride or acyl halide.
– Its absence means the compound cannot be any of the
carbonyl-containing compounds.
• O-H or N-H absorption
– This indicates either an alcohol, N-H containing amine or
amide, or carboxylic acid.
• C≡C and C≡N absorptions
– Be careful: internal triple bonds often do not show up in IR
spectra.
• C=C absorption
– Can indicate whether compound is alkene or aromatic.
EXAMPLE
2-aminopentane
Amines - Primary
• Shows the –N-H stretch for NH2 as a
doublet between 3200-3500 cm-1
(symmetric and anti-symmetric modes)
EXAMPLE
pyrrolidine
Amines – Secondary
• N-H band for R2N-H occurs at 32003500 cm-1 as a single sharp peak
weaker than –O-H
Tertiary amines (R3N) have no N-H bond and will
not have a band in this region
Examples of Infrared Spectra
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O-H Stretch of a Carboxylic Acid
This O-H absorbs broadly, 2500-3500 cm-1,
due to strong hydrogen bonding
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Carbonyl Stretching
• Exact absorption characteristic of type of
carbonyl compound
• The C=O bond of simple ketones, aldehydes,
and carboxylic acids absorb around 1710 cm-1
• Usually, it’s the strongest IR signal
• Carboxylic acids will have O-H also
• Aldehydes have two C-H signals around 2700
and 2800 cm-1
Practice
Q: Identify the chemical species represented in
this IR spectra?
Types of peaks
Broad U-shape peak
-O—H bond
Sharp & strong
peak C=O
at1710 cm-1
V-shape peak
-N—H bond for 2o
amine (R2N—H)
W-shape peak
-N—H bond for 1o
amine (RNH2)
Strengths and Limitations
• IR alone cannot determine a structure
• Some signals may be ambiguous
• The functional group is usually indicated
• The absence of a signal is definite proof that
the functional group is absent
• Correspondence with a known sample’s IR
spectrum confirms the identity of the
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compound
Web References
http://www.udel.edu/chem/fox/Chem333/Fall20
13/Chem333Fall2013/Welcome_files/IR%20hando
ut.pdf
http://infrared.als.lbl.gov/BLManual/IR_Interpret
ation.pdf
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