Transcript Document
Dong-Sun Lee / CAT -Lab / SWU
http://mail.swu.ac.kr/~cat
2010-Fall version
Chapter 24
Spectrochemical methods
This composite image of sunspot group was collected
with the Dunn solar telescope at the Sacramento Peak
Observatory in New Mexico on Mar. 29, 2001.
The lower portion, consisting of four frames, was
collected at a wavelength of 293.4 nm.
The upper portion was collected at 430.4 nm.
The lower image represents calcium ion concentration,
with the intensity of color proportional to the amount
of calcium ion in the sunspot. The upper image shows
the presence of the CH molecule.
Richard P. Feynman (1918~1988) was one of
the most well-known and renowned scientists
of the 20th century. He was awarded the Nobel
Prize in Physics in 1965.
Spectrophotometry
Spectroscopy :
the science that deals with the interaction of electromagnetic radiation with matter.
Spectrometry :
a more restrictive term,
denotes the quantitative measurement of the intensity of electromagnetic radiation at
one or more wavelengths with photoelectric detector.
Spectrum (pl. spectra) :
a display of the intensity of radiation emitted,
Intensity
absorbed, or scattered by a sample versus a
quantity related to photon energy(E), such as
wave length() or frequency().
wave length(, nm)
or frequency(, cm–1).
Spectrum
30
25
max=524nm
20
15
10
5 ppm
UV-visible absorption spectra of cefazolin antibiotics.
Absorption spectra of Fe(III)-salicylic acid
complex.
CAT-Lab/SWU
Plane-polarized electromagnetic radiation showing the electric field, and the direction of
propagation.
Electric field component of plane-polarized
electromagnetic radiation.
Properties of light :
Electromagnetic radiation ; EM wave ; radiation ; radient ray ; ray ; light
Duality ;
1) Wave theory ------ Huygens
= c
wavelength (cm/cycle) × frequency (cycles/sec) = velocity (cm/sec)
where wavelength, , is the length per unit cycle.
Frequency, , is the number of cycles per unit time.
C = 2.99792458 × 108 m/s is speed of light
2) Particle (energy packets ; photon) theory --- Newton
E = h = hc /
where E is the energy in joules (J)
h is Plancks constant (6.62608 × 10 – 34 J s)
1 erg = 10 –7 J
1 eV = 1.6021 × 10 – 19 J
Wave number, , is the number of cycles per unit length, cm.
=
=
=
=
Ex. 400 nm
1/
cm – 1 (reciprocal centimeter ; Kayser)
/c
E / hc
x eV ?
E = h
= hc /
6.63 10 – 34 J s 3.00 108 m s – 1
=
400 10 – 9 m 1.6 10 – 19 J/eV
= 3.1 eV
Change in wavelength as radiation passes from air into a dense glass and back to air.
Note that the wavelength shortens by nearly 200 nm, or more than 30%, as it passes
into glass; a reverse change occurs as the radiation again enters air.
Regions of EM spectrum
Designation
Cosmic ray
-ray
X-ray
Vacuum UV
near UV
Visible
Near IR
Middle IR
Far IR
Microwave
Radio wave
Wavelength
range
Energy or
wave number
Transition
>2.5 105 eV
Nuclear
10 – 12 m
10 – 11 m
10
–8
m
K,L shell electron
124 eV
180 10
–9
380 10
–9
780 10
–9
2500 10
–9
50 10
–6
10
–3
m
m
m
m
m
m
0.3 m
Middle shell
7 eV
Valence electron
3.3 eV
Molecular electron
1.6 eV
4000 cm
–1
200 cm
–1
10 cm
–1
Molecular vibration
Molecular vibration
Molecular rotation
Molecular rotation
Electron, & nuclear
spin
The visible spectrum
Wavelength
(nm)
380-420
420-440
440-470
470-500
500-520
520-550
550-580
580-620
620-680
680-780
Color absorbed
Violet
Violet-blue
Blue
Blue-green
Green
Yellow-green
Yellow
Orange
Red
Purple
Color observed
(complement)
Green-yellow
Yellow
Orange
Red
Purple
Violet
Violet-blue
Blue
Blue-green
Green
ROYG RIV
Red, Orange, Yellow, Green, Blue, Indigo, Violet
The electromagnetic spectrum showing the colors of the visible spectrum.
Types of interaction between radiation and matter
1.
2.
3.
4.
Reflection & scattering
Refraction & dispersion
Absorption & transition
Luminescence & emission
Emission or
chemiluminescence
Sample
Refraction
Sample
Reflection
A
Absorption along
radiation beam
Scattering and
photoluminescence
C
B
Transmission
Sample
Types of interaction between radiation and matter.
Several spectroscopic phenomena
1) depend on transition between energy states of particular chemical species
E*
E
Eo
higher energy (excited state)
applied energy
lowest energy (ground state)
2) depend on the changes in the optical properties of EM radiation that occur when it
interacts with the sample or analyte or on photon-induced changes in chemical form
(e.g. ionization or photochemical reactions)
non-radiative
process
Radiative
process
non
radiative
Emission or
chemiluminescence
A
Absorption
Antistokes
transition
D
Stokes
transition
E
B
Common types of optical transition.
Photoluminescence
C
Combination of nonradiative
and radiative deactivation
F
Absorption methods.
Photoluminescence methods.
Emission or chemiluminescence processes.
Absorption of EM radiation
Sun
Eye
Visual center
C
P0
Source
P
b
Cuvet
Monochromator
Detector
P – dP
P
Incident
light
Emerging light
b=0
Absorption of EM radiation
db
b
b=b
Molar concentration [C]
Color of a solution. White light from a lamp or the sun strikes the solution of Fe(SCN)2+.
The fairly broad absorption spectrum shows a maximum absorbance in the 460 to 500 nm
range. The complementary red color is transmitted.
Attenuation of a beam of radiation by an
absorbing solution.
Reflection and scattering losses with
a solution contained in a typical glass
cell.
2
E2 = h2 = hc/2
Incident
radiation
0
Sample
1
Transmitted
radiation
E1 = h1 = hc/1
0
(a)
(b)
A
0
2
1
(c)
Absorption methods. Radiation of incident power 0 can be absorbed by the analyte
producing a beam of diminished transmitted power (a) if the frequency of the incident
beam, 2 corresponds to energy difference, E1 or E2 (b). The spectrum is shown in (c).
Lambert Beer’s law
Transmittance
T = P / P0
%T = (P / P0) 100
Absorbance (A, O.D., E, As)
A = log T = log P/ P0
Lambert’s law
Lambert and Bouger found that the intensity of the transmitted energy decrease exponentially
as the depth (b ; path length of the beam through the sample) increases.
dP = k P db
T
A
dP/P = k db
dP/P = k db
ln P/P0 = k b
log P/P0 = (k/2.303) b
A = log P/P0 = (/2.303) b
Path length
Path length
Effect of path length on transmittance and
absorbance of light.
Beer’s law
Beer in 1852 found that concentration (C) is a reciprocal exponential function of
transmittance and absorbance is directly proportional to the concentration.
dP = P dC
dP/P = dC
dP/P = dC
ln P/P0 = C
log P/P0 = (/2.303) C
Lambert - Beer’s law
A = log P/P0 = (/2.303) C
A = bC
where is molar absorptivity
A
log T
[C]
[C]
Effect of concentration of analyte on transmittance and absorbance of light.
Limitation Beer’s law
1. Concentration deviation ;
A = log T = log P/P0 = bC
(0.434 / T) dT = b dC
(Eq 1)
(Eq 2)
Eq 2 ÷ Eq 1
(0.434 / T) dT
dC / C =
b
÷
log T
b
= (0.434 / T log T) dT
C/C = (0.434 / T log T) T
4
C/C
A
2
1
[C]
T = 36.8 % A = 0.434
normal working range15%T(0.824A)~80%T(0.097A)
Twyman Lothian curve
2. Refractive index deviation
A = bC [ n / (n2 + 2)2]
where n is refractive index
3. Instrumental deviation ; difficult to select single wavelength beam
max
The effect of polychromatic radiation on Beer’s law.
Choosing wavelength and monochromator band width.Increasing the monochromator
bandwidth broadens the bands and decreases the apparent absorbance.
Absorbance error introduced by different
levels of stray light.
Deviation from Beer’s law caused
by various levels of stray light.
4. Chemical deviation ; dissociation or reaction with solvent
ex. Acidic form intermediate form basic form
Chemical deviation from Beer’s law for
unbuffered solution of the Indicator HIn.
5. Solvent deviation
T = tsolution / tsolvent
6. Temperature ; narrower spectrum band at below
7. Pressure ; gas phase sample
50C
Typical visible absorption spectra of
1,2,4,5-tetrazine in different solvent.
Errors in spectrophotometric measurements
due to instrumental electrical noise and cell
positioning imprecision.
CAT-Lab/SWU
Absorption spectra of KMnO4
Partial energy level diagram for
sodium, showing the transitions
resulting from absorbtion at 590,
330, and 285 nm
Energy level diagram showing some
of the energy changes that occur during
absorption of IR, VIS, UV radiation by
a molecular species.
Electronic transitions
The absorption of UV or visible radiation corresponds to the excitation of
outer electrons. There are three types of electronic transition which can
be considered;
1.Transitions involving p, s, and n electrons
2.Transitions involving charge-transfer electrons
3.Transitions involving d and f electrons (not covered in this Unit)
When an atom or molecule absorbs energy, electrons are promoted from
their ground state to an excited state. In a molecule, the atoms can rotate
and vibrate with respect to each other. These vibrations and rotations also
have discrete energy levels, which can be considered as being packed on
top of each electronic level.
Types of the electronic transition
Unoccupied levels(antibonding)
*
E
*
LUMO
n
HOMO
Frontier
orbital
non-bonding
Occupied
bonding
level
Characteristics of electronic transitions.
Transition
E
Wavelength (nm)
log
Examples
*
< 200
n *
160~260
2~3
Alkenes, alkynes, aromatics
*
200~500
~4
H2O,CH3OH, CH3Cl CH3NH2
n *
250-600
1~2
Carbonyl, nitro, nitrate, carboxyl
>3
Saturated hydrocarbon
(note) forbidden transition ; * , *
James D. Ingle, Jr., Stanley R. Crouch, Spectrochemical Analysis, Prentice-Hall, NJ,1988, p. 335.
Absorbing species containing p, s, and n electrons
Absorption of ultraviolet and visible radiation in organic molecules is
restricted to certain functional groups (chromophores) that contain valence
electrons of low excitation energy. The spectrum of a molecule containing
these chromophores is complex. This is because the superposition of
rotational and vibrational transitions on the electronic transitions gives a
combination of overlapping lines. This appears as a continuous absorption
band.
Possible electronic transitions of , , and n electrons are;
* Transitions
An electron in a bonding s orbital is excited to the corresponding antibonding
orbital. The energy required is large. For example, methane (which has only
C-H bonds, and can only undergo * transitions) shows an absorbance
maximum at 125 nm. Absorption maxima due to * transitions are not
seen in typical UV-Vis. spectra (200 - 700 nm)
n * Transitions
Saturated compounds containing atoms with lone pairs (non-bonding
electrons) are capable of n * transitions. These transitions usually need
less energy than * transitions. They can be initiated by light whose
wavelength is in the range 150 - 250 nm. The number of organic functional
groups with n * peaks in the UV region is small.
n * and * Transitions
Most absorption spectroscopy of organic compounds is based on transitions
of n or electrons to the * excited state. This is because the absorption
peaks for these transitions fall in an experimentally convenient region of the
spectrum (200 - 700 nm). These transitions need an unsaturated group in the
molecule to provide the p electrons.
Molar absorbtivities from n * transitions are relatively low, and range
from 10 to100 L mol-1 cm-1 . * transitions normally give molar
absorbtivities between 1000 and 10,000 L mol-1 cm-1 .
The solvent in which the absorbing species is dissolved also has an effect on
the spectrum of the species. Peaks resulting from n * transitions are
shifted to shorter wavelengths (blue shift) with increasing solvent polarity.
This arises from increased solvation of the lone pair, which lowers the energy
of the n orbital. Often (but not always), the reverse (i.e. red shift) is seen for
* transitions. This is caused by attractive polarisation forces between
the solvent and the absorber, which lower the energy levels of both the
excited and unexcited states. This effect is greater for the excited state, and so
the energy difference between the excited and unexcited states is slightly
reduced - resulting in a small red shift. This effect also influences n *
transitions but is overshadowed by the blue shift resulting from solvation of
lone pairs.
Charge - Transfer Absorption
Many inorganic species show charge-transfer absorption and are called
charge-transfer complexes. For a complex to demonstrate charge-transfer
behaviour, one of its components must have electron donating properties and
another component must be able to accept electrons. Absorption of radiation
then involves the transfer of an electron from the donor to an orbital
associated with the acceptor.
Molar absorbtivities from charge-transfer absorption are large (greater that
10,000 L mol-1 cm-1).
Bonding in formaldehyde.
Energy level diagram for formaldehyde.
*
*
*
2p
2p
sp2
n
sp2
1s
2H
CH
atoms fragment
C
atom
C=O
fragment
C 1 s2 2s2 2p2
O
atom
O 1s2 2s2 2p4
Energy level diagram for formaldehyde.
General guideline to the use of UV data
(nm)
<270
Number of band
Single band
Intensity(log )
2~4
<2
250~360
200~250
>200
>210
>300
<250
Visible
Single band &
no major absorption
Two bands
Bands
Two absorptions
1~2
3~4
4
low
high
Transition
n *
Amines, alcohols, ethers, thiols
n *
CN
n *
C=O, C=N, N=N, NO2,
COOR, COOH, CONH
Aromatic system
, -unsaturated ketone or diene,
polyene (cf. Woodward-Fieser rule
or Fieser-Kuhn rule)
n *
*
Simple ketones, acids, esters,
amides, other and n electrones
(cf. Woodward rule or Nielson rule)
Highly colored compounds
Long chain conjugated(4~5)
chromophores
Polycyclic aromatic chromophores
Simple nitro, azo, nitroso, -diketo,
polybromo, polyiodo, quinoid
Hyperchromic effect
(increase absorption intensity)
max
max
Hypsochromic shift
(Blue shift)
Bathochromic shift
(Red shift)
Hypochromic effect
(Decrease absorption intensity)
Red shift and blue shift.
Increasing solvent polarity
Hypsochromic shift
(Blue shift)
*
Increase energy level gap
n
ex.
C=O
H
Conjugate effect
*
O
H
Bathochromic effect (red shift)
Chromophore
C=O
COOH
Red shift
(C=C)1~n
CONH
CN
Auxochromophore
-OH
-OR
C=N
N=O N=N C=S
Red shift
-NH2
-NHR
NR2
Selected electronic transitions in organic molecules
Compound
Ethane
Water
Methanol
Ethane
Acetone
Benzene
Phenol
Electronic
transition
Absorption
maximum,
max
(nm)
Molar
absorptivity,
(1 mol1 cm 1)
*
n *
n *
*
*
n *
n *
*
*
*
*
*
135
167
183
165
150
188
279
180
200
255
210
270
7000
500
16500
1860
15
60000
8000
215
6200
1400
Common solvents used in UV and their transparencies
Solvent
Water
Cyclohexane
Hexane
Methanol
Ethanol
Dichloromethane
Chloroform
Dioxane
Minimum wavelength
for 10 nm cell
(nm)
190
195
200
200
200
220
240
190
Approximate
transparency
region (nm)
180-200
210-400
205-400
205-400
210-400
210-400
250-400
220-400
Absorption characteristics of saturated compounds with hetero atoms (n*)
transmission
Compound
Absorption maximum
Molar absorptivity
max (nm)
(1mol-1cm-1)
Solvent
Chloromethane
173
200
Hexane
Methanol
Di-n-butyl
sulphide
Trimethyl lamine
Methyl iodide
Diethyl ether
177
210
200
1200
Hexane
Ethanol
199
259
188
171
3950
400
1995
3982
Hexane
Hexane
Gas phase
Gas phase
Absorption data for conjugated alkenes ( transition)
Absorption maximum
Compound
max (nm)
Molar absorptivity
(1mol-1cm-1)
Sorvent
1,3-Butadiene
217
21000
Hexane
1,3,5-Hexatriene
253
-50000
Isooctane
263
52500
Isooctane
274
-50000
Isooctane
1,3-Cyclohexadiene
256
8000
Hexane
1,3-Cyclopentadiene
239
3400
Hexane
Absorption characteristics of individual chromophores
Absorption
Chromophoric
group
maximum
Formula
Compound
Ethylenic
RCH=CHR
Ethane
Carboxyl
RHC=O
RR1C=O
Acetaldehyde
Acetone
Azo
Nitro
Nitrito
– N=N –
– NO2
– ONO
Azomethane
Nitromethane
Amyl nitrite
Sulphoxide
S=O
Cyclohexyl
methyl
sulphoxide
Molar
absorptivity
max (nm)
(1mol-1cm-1)
Transition
Solvent
165
193
290
188
279
347
271
218.5
15000
10000
16
900
5
4.5
18.6
1120
*
*
*
*
n *
n *
n *
*
210
1500
Gas phase
Gas phase
Heptane
Hexane
Hexane
Dioxane
Ethanol
Petroleum
ether
Ethanol
*
Absorption data for carbonyl chromophore
Compound
Absorption
maximum
max (nm)
Molar
absorptivity
(1mol-1cm-1)
Solvent
Transition
Saturated aldehydes and ketones
Ethyl methyl ketone
279
16
Isooctane
n
*
Acetone
279
15
Isooctane
n
*
Acetaldehyde
293
11.8
Isooctane
n
*
Cyclopentanone
299
20
Hexane
n
*
Isobutyraldehyde
290
16
Hexane
n
*
Cyclohexanone
285
14
Hexane
n
*
Acetic acid
204
41
Ethanol
n
*
ethyl acetate
207
69
Petroleum ether
n
*
Acetyl chloride
235
53
Hexane
n
*
Acetyl anhydride
225
47
Isooctane
n
*
320
50.5
Ethanol
n
*
Carbonyl-containing compounds
Conjugated ketones and aldehydes
Methyl vinyl ketone
212.5
9294
Ethanol
Unsaturated carboxylic acids and esters
CH2=CH –COOH
200
10000
CH3–(CH =CH2)–COOH
254
25000
Ethanol
Ethanol
*
*
*
Absorption characteristics of benzene substituted with chromophores
*
transition
K-band
max
Compound
Benzene
(nm)
184
-
n
B-band
max
(1mol-1 cm-1)
60000
(nm)
204
*
transition
R-band
max
(1mol-1 cm-1)
7900
(nm)
(1mol-1 cm-1)
Solvent
256
200
Ethanol
-
Ethanol
-
255
215
-
Nitrobenzene 252
10000
280
1000
330
125
Hexane
Benzonitrile
224
13000
271
1000
-
-
Water
Benzaldehyde 244
15000
280
1500
328
20
Ethanol
Absorption of benzene substituted with auxochromes
*
transition
E-band
Compound
max (nm)
B-band
(1mol-1cm-1)
max (nm)
(1mol-1cm-1)
Solution
Benzene
204
7900
256
200
Hexane
Chlorobenzene
210
7600
265
240
Ethanol
Phenol
210.5
6200
270
1450
Water
Aniline
230
8600
280
1430
Water
Q n A
Thanks
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