Spectroscopy
“seeing the unseeable”
Color
400
500
600
l, nm
Violet
400-420
Indigo
420-440
Blue
440-490
Green
490-570
Yellow
570-585
Orange
585-620
Red
620-780
700
800
“color wheel”
b-carotene
b-carotene, lmax = 455 nm
lmax is at 455 – in the far blue region of the
spectrum – this is absorbed
The remaining light has the complementary
color of orange
electromagnetic relationships:
λυ = c
λ 1/υ
E = hυ
E υ
E = hc/λ
E 1/λ
λ = wave length
υ = frequency
c = speed of light
E = kinetic energy
h = Planck’s constant
λ
c
Two oscillators will strongly interact when their energies
are equal.
E1 = E2
λ1 = λ2
υ1 = υ2
If the energies are different, they will not strongly interact!
We can use electromagnetic radiation to probe atoms and
molecules to find what energies they contain.
Spectroscopy
The higher energy ultraviolet and visible
wavelengths affect the energy levels of the outer
electrons.
Infrared radiation is absorbed by matter resulting
in rotation and/or vibration of molecules.
Radio waves are used in nuclear magnetic
Resonance and affect the spin of nuclei in a
magnetic field.
Energy Level Diagram for an Atom of Sodium
E0
Ground State
330 nm
E1
590 nm
E2
Energy Level Diagram for a Simple Molecule
E2
Relaxation from the E2
energy state to E0 may
go to different vibrational
energy states, emitting
different wavelengths.
E1
Excitation to the next
electronic energy
level caused by
adsorption of
specific wavelengths
E0
Ground State
e4
e3
e2
e1
Vibrational Energy
Levels
Molecular Orbital's
Orbitals
Stable Energy to Excited Energy
LUMO: Lowest Unoccupied Molecular Orbital
HOMO: Highest Occupied Molecular Orbital
UV/VIS SPECTROSCOPY
Visible
(380-780 nanometers)
Ultraviolet (UV) (10 – 380 nanometers).
Below about 200 nm, air absorbs the UV light
and instruments must be operated under a vacuum
The Spectroscopic Process




Instrumentation
monochromator/
beam splitter optics
I0
I0
I
log(I0/I) = A
detector
UV-VIS sources
sample
Deuterium lamp – covers the UV – 200-330
Tungsten lamp – covers 330-700
reference
•
•
I0
200
l, nm
700
UV Spectroscopy_DAD
A recent improvement is the diode-array spectrophotometer here a prism (dispersion device) breaks apart the full spectrum
transmitted through the sample
Each individual band of UV is detected by a individual diodes on a
silicon wafer simultaneously – the obvious limitation is the size
of the diode, so some loss of resolution over traditional
instruments is observed
Diode array
sample
UV-VIS sources
Polychromator
– entrance slit and dispersion device
UV Spectroscopy
1. Only quartz is transparent in the full 200-700 nm
range; plastic and glass are only suitable for visible
spectra
2. Concentration is empirically determined
A typical sample cell (commonly called a cuvette):
Wavelengths Absorbed by Functional
Groups
Wavelengths Absorbed by Functional
Groups
Example of a Method to Determine the
Absorption Spectra of an Organic
Compound
Woodward’s Rules For Conjugated Carbonyl
Compounds
Aldehyde:
208 nm
Extended conjugation:
30 nm
Homodiene component:
39 nm
a-Alkyl groups or ring residues: 10 nm
d-Alkyl groups or ring residues: 18 nm
Calculated:
304 nm
OTHER CONCEPTS IMPORTANT
TO UV/VIS SPECTROSCOPY
 UV/Vis
spectra can be used to some extent for
compound identification, however, many compounds
have similar spectra.
 Solvents can cause a shift in the absorbed
wavelengths. Therefore, the same solvent must be
used when comparing absorbance spectra for
identification purposes.
 Many inorganic species also absorb energy in the
UV/Vis region of the spectrum.
UV Spectroscopy
Solvents must be transparent in the region to be observed; the
wavelength where a solvent is no longer transparent is
referred to as the cutoff
Since spectra are only obtained up to 200 nm, solvents typically
only need to lack conjugated  systems or carbonyls
Common solvents and cutoffs:
acetonitrile 190
chloroform 240
cyclohexane 195
1,4-dioxane 215
95% ethanol 205
n-hexane
201
methanol
205
isooctane
195
water
190
UV Spectroscopy
Aromatic Compounds
When the number of fused aromatic rings increases, the l
for the primary and secondary bands also increase
For heteroaromatic systems spectra become complex with
the addition of the n  * transition and ring size
effects and are unique to each case
259nm 278nm 279nm
Infrared Spectra
Infrared radiation
λ = 2.5 to 17 μm
υ = 4000 to 600 cm-1
These frequencies match the frequencies of covalent bond
stretching and bending vibrations. Infrared spectroscopy
can be used to find out about covalent bonds in molecules.
IR is used to tell:
1. what type of bonds are present
2. some structural information
Absorption Frequencies of Functional
Groups
See Appendix B (2 tables) and Table 2
Some characteristic infrared absorption frequencies
BOND
COMPOUND TYPE
FREQUENCY RANGE, cm-1
C-H
alkanes
2850-2960 and 1350-1470
alkenes
3020-3080 (m) and
RCH=CH2
910-920 and 990-1000
R2C=CH2
880-900
cis-RCH=CHR
675-730 (v)
trans-RCH=CHR
aromatic rings
monosubst.
965-975
3000-3100 (m) and
690-710 and 730-770
ortho-disubst.
735-770
meta-disubst.
690-710 and 750-810 (m)
para-disubst.
810-840 (m)
alkynes
3300
O-H
alcohols or phenols
3200-3640 (b)
C=C
alkenes
1640-1680 (v)
aromatic rings
1500 and 1600 (v)
C≡C
alkynes
2100-2260 (v)
C-O
primary alcohols
1050 (b)
secondary alcohols
1100 (b)
tertiary alcohols
1150 (b)
phenols
1230 (b)
alkyl ethers
1060-1150
aryl ethers
1200-1275(b) and 1020-1075 (m)
all abs. strong unless marked: m, moderate; v, variable; b, broad
1
C5H10O
3400-3200 cm-1:
no OH or NH present
3100 cm-1: no peak to suggest unsaturated CH
2900 cm-1: strong peak indicating saturated CH
2200 cm-1: no unsymmetrical triple bonds
Structure:
IUPAC Name: 3-pentanone
1710 cm-1: strong carbonyl absorbance
1610 cm-1: no absorbance to suggest carbon-carbon double bonds
2
C8H8O
3400-3200 cm-1:
no OH or NH present
3100 cm-1: moderate peak suggesting unsaturated CH
2900 cm-1: weak peak indicating possible saturated CH
Structure:
2200 cm-1: no unsymmetrical triple bonds
IUPAC Name: acetophenone
1690 cm-1: strong carbonyl absorbance
1610 cm-1: weak absorbance bands consistent with carbon-carbon double bonds
3
C7H8O
3400-3200 cm-1: strong peak indicating OH is present
3100 cm-1: weak peak suggesting possible unsaturated CH
2900 cm-1: weak peak indicating possible saturated
CH 2200 cm-1: no unsymmetrical triple bonds
1720 cm-1: no carbonyl absorbance
1450-1500 cm-1: moderate absorbance bands
consistent with aromatic carbon-carbon double bonds
Structure:
IUPAC Name: benzyl alcohol
Raman Spectroscopy
• When radiation passes through a transparent
medium, the species present scatter a fraction of
the beam in all directions.
• In 1928, the Indian physicist C. V. Raman
discovered that the visible wavelength of a
small fraction of the radiation scattered by
certain molecules differs from that of the
incident beam and furthermore that the shifts in
wavelength depend upon the chemical structure
of the molecules responsible for the scattering.
Definition of NMR Spectroscopy
Nuclear magnetic resonance spectroscopy:
commonly referred to as NMR, is a technique
which exploits the magnetic properties of
certain nuclei to study physical, chemical, and
biological properties of matter
Compared to mass spectrometry, larger
amounts of sample are needed, but nondestructive
Energy Differentiation
In the presence of an external magnetic field (B0), two spin states
exist, +1/2 and -1/2 (For I=1/2).
The magnetic moment of the lower energy +1/2 state is aligned with
the external field, and that of the higher energy -1/2 spin state is
opposed to the external field.
Aligned against
the applied field
Aligned with
the applied field
g- Values for some nuclei
Isotope
Net Spin
g / MHz T-1
Abundance / %
1H
1/2
42.58
99.98
2H
1
6.54
0.015
3H
1/2
45.41
0.0
31P
1/2
17.25
100.0
23Na
3/2
11.27
100.0
14N
1
3.08
99.63
15N
1/2
4.31
0.37
13C
1/2
10.71
1.108
19F
1/2
40.08
100.0
Schematic NMR Spectrometer
Fourier transformation and the
NMR spectrum
RF Pulse
The NMR spectrum
Fourier
transform
The Fourier transform
(FT) is a
computational method
for analyzing the
frequencies present in
an oscillating signal
1H
1H
NMR and 13C NMR Spectrum
NMR spectra
d ppm
Down field
13C
NMR spectra
d ppm
High field
Chemical Shift-d
When an atom is placed in a magnetic field, its electrons
circulate about the direction of the applied magnetic
field. This circulation causes a small magnetic field at
the nucleus which opposes the externally applied field
The magnetic field at the nucleus (the effective field)
is therefore generally less than the applied field by a
fraction :
B = B0(1-s),So u =gB0
(1-s) / 2
Standard for Chemical Shift
In NMR spectroscopy, the standard is often
tetramethylsilane, Si(CH3)4, abbreviated TMS.
Tetramethyl silane (TMS) is used as reference because it is
soluble in most organic solvents, is inert, volatile, and has
12 equivalent 1H and 4 equivalent 13C. TMS signal is set to 0