Download Document

Spectroscopy Workshop
School of Chemistry
The Queen’s University of Belfast
School of
Chemistry
Workshop Content
Spectroscopy overview
Ultra-violet/visible (UV-vis)
Infra-Red (IR)
Nuclear Magnetic Resonance (NMR)
Mass Spectrometry
School of
Chemistry
Spectroscopy
In spectroscopy, transitions between
different energy levels within atoms and
molecules are recorded and then used to
give information on chemical structure.
School of
Chemistry
The range of energies that can be used
for spectroscopy is very large and spans a
large proportion of the electromagnetic
spectrum.
Visible
X-Rays
UV
Gamma
Rays
10- 11
10- 9
Microwave
IR
Radio
10- 7
10- 5
10- 3
Wavelength (cm)
School of
Chemistry
10- 1
10
10 3
Energy
Energy
In a typical experiment, the molecules or atoms
start at lower energy and go to a higher energy
level upon absorption of radiation of appropriate
wavelength.
After
School of
Chemistry
Before
DE
Absorption can only occur when the energy of
the radiation (calculated from the frequency or
wavelength) matches the energy gap.
If there are several different upper levels (and
there usually are) then several transitions will be
observed.
After
After
Energy
After
School of
Chemistry
Before
For current purposes we look only at:
UV/visible ( highest energy)
Infra red (intermediate)
Radio frequency (lowest energy).
But in all cases :
School of
Chemistry
To record a spectrum, sweep through the
appropriate range of energies and look for
absorption at particular values.
School of
Chemistry
Absorption gives peaks, when these have been
measured this gives the energy gaps within the
sample. These can then be related to structure.
Interpretation depends on the energy
range investigated.
School of
Chemistry
UV/visible Spectroscopy
Chemical compounds are coloured because
they absorb visible light.
In general, even organic compounds that are
colourless will absorb UV light.
School of
Chemistry
Absorption of visible light
Where has the
energy that was
within the photons
gone to ?
School of
Chemistry
In UV/visible spectroscopy the energy of the
absorbed photon is used is used to drive the
molecule into an excited electronic state.
Energy
Energy
In the excitation the energy of the whole molecule
increases.
After
School of
Chemistry
Before
DE
This overall change is typically due to
promotion of a single electron from a lower to
higher energy orbital. The energy of the
transition depends on the gap between the
two orbitals.
In organic compounds which have only single
bonds between the atoms the excitation
energy is very high- lies in deep UV.
School of
Chemistry
Even if have a simple p bond, the excitation
from highest occupied to lowest unoccupied
orbitals still lies in the UV.
This excitation gives a
dramatic decrease in bond
order due to excitation from
a bonding to an anti-bonding orbital.
e.g. ethene
H
H
H
H
School of
Chemistry
If we have a highly conjugated molecule
the energy separation between the
orbitals is smaller.
Excitation of the electron thus has a
proportionately smaller effect and
requires less energy- energy gap may
lie in the visible region.
School of
Chemistry
Again note that
lowest energy
transition may
lie in visible.
Anti-bonding
Bonding
Energy
Orbitals of Butadiene
H
H
H
H
School of
Chemistry
H
H
But we can also excite to
higher orbitals with sufficiently
energetic (UV) photons.
With increasing conjugation, the decreasing
energy gap is reflected by absorption at longer
wavelengths.
School of
Chemistry
The structures of many coloured compounds
show they are very extensively conjugated.
beta-Carotene
O
COOH
HOOC
H
H
N
N
trans-Crocetin
H2N
N
O
O
School of
Chemistry
N
Xanthopterin
OMe
OMe
16,17-DimethoxyViolanthrone
O
Substituents added to the compound may alter the energy of
the orbitals which e- is excited from or to.
Auxochromes: substituents that alter the wavelength or
intensity of the absorption due to the chromophore
O
NH2
O
NH2
PURPLE
ORANGE
O
O
O
NHCH3
BLUE
School of
Chemistry
O
NHCH3
OH
Changes in chemical composition can give rise to
pronounced colour changes since this can
dramatically alter the energies of the orbitals involved
in the transitions e.g. pH indicators.
Phenolphthalein
O
O
O
-2H+
O
O
OH
HO
School of
Chemistry
O-
colourless
pink
Methyl orange
H3C
H3C
N
red
N N
SO3-
H+
H3C
H3C
N
H
+
orange-yellow
N N
SO3School of
Chemistry
Summary
Absorption of UV-vis radiation occurs via excitation of
electrons from filled to unfilled orbitals i.e. they are
electronic transitions.
Molecules have characteristic absorption spectra.
The absorption can lead to coloured materials.
pH Indicators use the change in colour between the
acid and alkali forms of the molecules.
School of
Chemistry
IR Spectroscopy
Origin of the absorption
The spectrometer
The spectra
Organic compounds
Example problem
School of
Chemistry
Origin of IR absorptions
CO2
Atoms within a molecule are
never still. They vibrate in a
variety of ways (modes).
symmetric
stretch
Atoms may be considered as
weights connected by springs.
asymmetric
stretch
Each vibrational mode has its
own resonant frequency.
School of
Chemistry
bending
If the vibrational mode
involves a change in
molecular dipole
moment, the vibration
can be induced by
absorption of a photon
- it is ‘IR-active’
no dipole
no dipole
symmetric
stretch
asymmetric
stretch
change in dipole - IR active
Appropriate energy for
this is infra-red
bending
change in dipole - IR active
School of
Chemistry
The IR spectrometer
School of
Chemistry
CO2 IR spectra
The bigger the
change in dipole,
the more intense
the absorption
Transmittance /%
100
0
2800
2400
2000
1600
1200
800
400
Wavenumber /cm-1
Stretching higher energy than bending
School of
Chemistry
The symmetric stretch is not IR active
(no change in dipole)
IR spectra of organic compounds
More complex:
Ethyl ethanoate
(CH3COOCH2CH3)
C=O bond
C-O stretch
School of
Chemistry
Wavenumber/cm-1
4000
3000
2000
1500
1000
500
But functional groups have
characteristic frequencies
Wavenumber / cm- 1
4000
3000
C
2000
H
C
C
O
School of
Chemistry
H
C
N
1000
650
C
C
C
H
(all types)
N
1500
C
C
O
O
Cl
Four regions in the spectrum:
4000 3500 3000 2500 2000
O-H
N-H
C-H
stretching
C C
C O
C N
N O
stretching
N-H
bending
C C
C N
X Y Z
stretching
School of
Chemistry
Wavenumber / cm-1
1500
1000
other stretching,
bending and
combination
bands:
fingerprint
region
Example problem
Identify two main functional groups present in the
compound which gave this spectrum
Explain why infrared radiation is absorbed by molecule HCl but not by molecules H2 and Cl2.
Explain what occurs in the HCl molecule when infrared radiation is absorbed.
The simplified infrared spectrum below is that of an organic compound.
Transmittance / %
(a)
(b)
(c)
100
90
80
70
60
50
40
30
20
10
4000
3600
3200
2800
2400
2000
1900
1800
1700
1600
Wavenumber / cm-1
(i) Identify two main functional groups on the spectrum.
(ii) This compound has composition by mass C, 67.9%; H, 5.7%; N, 26.4%, and Mr of 53.
Suggest a structural formula for the compound.
C-H
School of
Chemistry
CC
CN?
C=C
C=O?
C=N?
Combine this information with the following data
to deduce its structure
C
H
N
Mr
67.9%
5.7%
26.4%
53
So, formula = C3H3N
Likely structure:
H
H
C
N
H
Cyanoethene
School of
Chemistry
Summary
Absorption of IR can occur if a vibrational mode is
associated with a change in dipole.
Functional groups have characteristic absorption
frequencies.
In combination with other analytical data, the structure
of an organic compound can often be deduced.
School of
Chemistry
NMR Spectroscopy
The Basis of NMR
Spectroscopy
The Spectrometer
Chemical Shifts
Signal Intensity and Integration
Coupling Constants
Example Spectra
School of
Chemistry
The Basis of NMR Spectroscopy
Atomic nuclei behave like small bar
magnets as a result of their charge
and spin.
In the presence of an applied magnetic
field the spin states have different energy
and the magnetic moment can align with
or against the applied field.
School of
Chemistry
The difference in energy between the two spin states is
dependent on the external magnetic field strength.
Irradiation of a sample with radio frequency energy
corresponding to the spin state separation (DE) will excite
nuclei in the +½ state to the higher energy –½ state.
School of
Chemistry
The 1H NMR Experiment
For example, consider a water sample in a 2.3487 T
external magnetic field irradiated by 100 MHz radiation.
If the magnetic field is increase to 2.3488 T the water
protons will at some point absorb rf energy (DE) and a
resonance signal will appear,
School of
Chemistry
The Chemical Shift
Not all protons give resonance signals at the same field
frequency. Electrons move in response to the applied field
and generate a secondary magnetic field which opposes the
applied field. The secondary field shields the nucleus from
the applied field and nuclei in different environments resonate
at different frequencies.
The difference in resonance frequency is measured as
a chemical shift, units d
School of
Chemistry
Proton Chemical Shift Ranges
School of
Chemistry
Signal Intensity
The relative area of the absorption signals can provide
valuable structural information.
The area under a peak is proportional to the number of
a given type of nuclei in the molecule.
O
H 3C
CH3
H
H
MEK
School of
Chemistry
The keto-enol equilibrium ratio of 2,4-pentandione can
determined by 1H NMR spectroscopy
School of
Chemistry
Spin-Spin Coupling
The applied magnetic field experienced by a proton Ha will be
modified by the local field produced by its neighbouring Hb
Ha modifies the field at Hb by aligning with or against the
applied field and and gives 2 resonant frequencies for Hb (doublet)
Similarly Hb modifies the field at Ha in 3 different ways (triplet)
School of
Chemistry
Splitting pattern can provide valuable structural information
Chemically equivalent protons act as a group and a peak
due to n adjacent protons is split into n+1 lines, with a
coupling constant J
School of
Chemistry
1H
School of
Chemistry
NMR Spectrum of
Ethyl Acetate
1H
NMR Spectrum of
1,3-Dichloropropane
School of
Chemistry
Example Problem
Given the formula, deduce what you can about the structure
Integration corresponds to 2H : 2H : 3H
A triplet must correspond to 2 near neighbour protons
A sextet corresponds to 5 near neighbour protons
Therefore CH2, CH2 and CH3 groups are present
School of
Chemistry
Solution
H
Connectivity can be deduced to be
H
H
H
H
School of
Chemistry
H
H
NO2
Summary
NMR spectroscopy involves irradiating a sample with
radio frequency radiation
Protons in different chemical environments have different
chemical shifts d
Protons in different environments can couple to each
other with a coupling constant J
The combination of chemical shifts and coupling
constants provides valuable structural information
School of
Chemistry
Mass Spectrometry
The basic principles
Applications
School of
Chemistry
What is a mass spectrometer ?
A mass spectrometer is an instrument which
produces charged particles (ions) from
chemical substances under analysis.
It then uses magnetic and/or electric fields to
separate those ions and to measure their
mass.
School of
Chemistry
Mass Spectrometer Schematic
Sample
Introduction
Data
Output
Inlet
Ion
Source
School of
Chemistry
Data
System
Mass
Analyzer
Vacuum
Pumps
Ion
Detector
Ion Generation
~70 Volts
Electron Collector (Trap)
Neutral
Molecules
+
Repeller
+
Electrons
Inlet _
_
+ ++ +
+ +
e- e- e_
Filament
School of
Chemistry
Positive Ions
Extraction
Plate
To Analyzer
The magnetic field exerts a force on these fastmoving ions and causes them to move in a
circular path, the radius of which is
dependent upon their mass to charge ratio
(m/z) and speed.
School of
Chemistry
Magnetic Mass Separation
ion not detected
m/z too small
Correct m/z ratio
ion detected
S
Ion
Source
Detector
N
Electromagnet
School of
Chemistry
ion not detected
m/z too large
Applications
Mass spectrometers are used for all kinds of
chemical analyses:
- Chemical analysis (Chemical Research)
- Environmental analysis
- Analysis of petroleum products
- Trace metals
- Biological materials
School of
Chemistry
How is mass spectral information used?
Let us use water (H2O) as an example.
If a beam of electrons is directed through water
vapour in the source of a mass spectrometer,
some of the electrons will hit water molecules
and knock off an electron, producing charged
ions from the water:
H2O + 1 (fast) electron  [H2O]+ + 2 electrons
School of
Chemistry
Electron impact on a water molecule
Some of the collisions between water molecules and
electrons will be so hard that the water molecules will
be broken into fragments. For water, those fragments
will be [OH]+, O+, and H+ with the following masses:
1 = H+
16 = O+
17 = [OH]+
18 = [H2O]+
School of
Chemistry
Mass Spectrum of Water
[H2O]+
18
Relative
Abundance
17
1
H+
O+
[OH]+
16
Mass
(mass-to-charge ratio)
School of
Chemistry
Examples
Alcohols
Pentan-3-ol
OH
CH3CH2
CH2CH3
H
School of
Chemistry
An alcohol's molecular ion is small or non-existent.
Cleavage of the C-C bond next to the oxygen usually
occurs.
A loss of H2O may occur as in the spectra below.
OH
CH3CH2
59
CH2CH3
H
m/z(parent ion) = 88
School of
Chemistry
Alkanes
Hexane
H3C C C C C CH3
H2 H2 H2 H2
School of
Chemistry
Molecular ion peaks are present, possibly with low intensity.
The fragmentation pattern contains clusters of peaks 14 mass
units apart (which represent loss of (CH2)n CH3).
43
15
School of
Chemistry
71
m/z(parent ion) = 86
H3C C C C C CH3
H2 H2 H2 H2
29
57
Aromatics
Naphthalene
School of
Chemistry
relative abundance
Molecular ion peaks are strong due to the stable structure.
128
100
80
60
40
20
0
0
20
40 60 80 100 120 140
mass / charge (m/z)
m/z(parent ion) = 128
School of
Chemistry
Esters
Ethylethanoate
O
H3C
School of
Chemistry
O CH2CH3
relative abundance
Fragments appear due to bond cleavage next to C=O
(alkoxy group loss, -OR) and hydrogen rearrangements.
100
80
60
40
20
0
-OCH2CH3
43
-C2H3
45
0
61
88
20
40
60
80
mass / charge (m/z)
100
61
H
O
H3C
School of
Chemistry
H
O CH2CH3
43
45
m/z(parent ion) = 88
Halo-organics
Chloroethene
Cl
H
H
H
School of
Chemistry
Isotopes are shown by mass spectrometry
The natural abundance of each isotope gives characteristic
fragmentation
e.g. 35Cl:37Cl is in a 3:1 ratio therefore the peaks containing
Cl are in a 3:1 ratio and separated by 2 mass units
100
H2C = CH-Cl
80
H
62
60
H
Cl
40
26
64
20
H
27
School of
Chemistry
27
0
35
30
37
40
50
m/z(parent ion) = 62/64
60
Summary
Mass spectrometry involves the ionisation of molecules and
atoms.
The mass spectrometer measures the mass to charge ratio.
On ionisation the molecule can break up giving fragments
of different m/z ratios .
Each molecule has a characteristic fragmentation pattern
which can be used to identify the molecule.
School of
Chemistry