UV-VIS SPECTROSCOPY
X-ray:
core electron excitation
Nuclear spin states (in a magnetic feldd
Spectroscopic Techniques and Chemistry they Probe
UV-vis UV-vis region bonding electrons
Atomic Absorption UV-vis region atomic transitions (val. e-)
FT-IR IR/Microwave vibrations, rotations
Raman IR/UV vibrations
FT-NMR Radio waves nuclear spin states
X-Ray Spectroscopy X-rays inner electrons, elemental
Spectroscopic Techniques and Common Uses
UV-vis UV-vis region analysis/Beer’s LawQuantitative
Atomic Absorption UV-vis region
Quantitative analysis Beer’s Law
FT-IR IR/Microwave Functional Group Analysis
Raman IR/UV
Functional Group Analysis/quant
FT-NMR Radio waves Structure determination
X-Ray Spectroscopy X-rays Elemental Analysis
Different Spectroscopies
•
UV-vis – electronic states of valence
e/d-orbital transitions for solvated transition
metals
•
Fluorescence – emission of UV/vis by certain
molecules
•
FT-IR – vibrational transitions of molecules
•
FT-NMR – nuclear spin transitions
Why should we learn this stuf?
After all, nobody solves structures with
UV any longer!
Many organic molecules have chromophores that absorb UV
UV absorbance is about 1000 x easier to detect per mole than NMR
Still used in following reactions where the chromophore changes. Useful because timescale is so fast, and sensitivity so high. Kinetics, esp. in
biochemistry, enzymology.
Most quantitative Analytical chemistry in organic chemistry is conducted using HPLC with UV detectors
Uses for UV, continued
Knowing UV can help you know when to be skeptical of quant results. Need to calibrate response factors
Assessing purity of a major peak in HPLC is improved by “diode array” data, taking UV spectra at time points across a peak. Any diferences could suggest a unresolved component. “Peak Homogeneity” is key for purity analysis.
Sensitivity makes HPLC sensitive
e.g. validation of cleaning procedure for a production vessel
But you would need to know what compounds could and could not be detected by UV detector! (Structure!!!d
One of the best ways for identifying the presence of acidic or basic groups, due to big shifts in for a chromophore containing a phenol, carboxylic acid, etc.
The UV Absorption process
• * and * transitions: high-energy,
accessible in vacuum UV (max <150 nmd. Not usually observed in molecular UV-Vis.
• n * and * transitions: non-bonding
electrons (lone pairsd, wavelength (maxd in the 150-250 nm region.
• n * and * transitions: most common transitions observed in organic molecular UV-Vis, observed in compounds with lone pairs and multiple bonds with max = 200-600 nm.
• Any of these require that incoming photons match in energy the gap corrresponding to a transition from ground to excited state.
What are the
nature of these
absorptions?
Example: * transitions responsible for ethylene UV absorption at ~170 nm
calculated with ZINDO semi-empirical excited-states methods (Gaussian 03Wd:
HOMO u bonding molecular orbital LUMO g antibonding molecular orbital
h 170nm photon
How Do UV spectrometers work?
Two photomultiplier inputs, diferential voltage drives amplifer.
Matched quartz cuvettes
Sample in solution at ca. 10-5 M.
System protects PM tube from stray light
D2 lamp-UV
Tungsten lamp-Vis
Double Beam makes it a diference technique
Diode Array Detectors
Diode array alternative puts grating, array of photosens.
Semiconductors after the light goes through the sample.
Advantage, speed, sensitivity,
The Multiplex advantage
Disadvantage,
resolution is 1 nm, vs 0.1 nm for normal UV
Experimental details
What compounds show UV spectra?
Generally think of any unsaturated compounds as good candidates. Conjugated double bonds are strong absorbers Just heteroatoms are not enough but C=O are reliable
Most compounds have “end absorbance” at lower frequency. Unfortunately solvent cutofs preclude observation.
You will fnd molar absorbtivities in L•cm/mol, tabulated. Transition metal complexes, inorganics
Solvent must be UV grade (great sensitivity to impurities with double bondsd
An Electronic Spectrum
Wavelength, , generally in nanometers (nmd 0.0
extinction
Make solution of
concentration low enough that A≤ 1
(Ensures Linear Beer’s law behaviord
Even though a dual beam goes through a solvent blank, choose solvents that are UV transparent.
Can extract the value if conc. (Md and b (cmd are known
UV bands are much
broader than the photonic transition event. This is because vibration levels
Solvents for UV (showing high
Organic compounds (many of
themd have UV spectra
From Skoog and West et al. Ch 14 One thing is clear
Uvs can be very non-specifc Its hard to interpret except at a cursory level, and to say that the spectrum is consistent with the structure
Each band can be a superposition of many transitions
An Example--Pulegone
Frequently plotted as
log of
molar
extinction
So at 240 nm, pulegone has a molar
extinction of 7.24 x 103
Antilog of 3.86
Can we calculate UVs?
Elec tronic Spectra
Wavelength (nm) Molar Absorptivity (l/mol-cm)
220 230 240 250 260 270 280 290 300
0
Wavelengt h (nm) Molar Absorptivity (l/mol-cm)
220 230 240 250 260 270 280 290 300 0
Nacety lindol
Semi-empirical (MOPACd at AM1, then ZINDO for
The orbitals involved
Electronic Spectra
Wavelength (nm) Molar Absorptivity (l/mol-cm)
200 210 220 230 240 250 260 270 280 290 300 0
atoms whose MO’s
The Quantitative Picture
•
Transmittance:
T = P/P
0B(path through sample)
P0
(power ind P (power outd
•
Absorbance:
A = -log
10T = log
10P
0/P
•
The Beer-Lambert Law (a.k.a. Beer’s Lawd:
A =
e
bc
Where the absorbance A has no units, since A = log10 P0 / P
e is the molar absorbtivity with units of L mol-1 cm-1
b is the path length of the sample in cm
Beer-Lambert Law
Linear absorbance with increased concentration--directly proportional
Makes UV useful for quantitative analysis and in HPLC detectors
Quantitative Spectroscopy
•
Beer’s Law
A
l1= e
l1bc
e is molar absorptivity (unique for a
given compound at l
1)
Beer’s Law
•
A = -logT = log(P
0/P) = ebc
•
T = P
solution/P
solvent= P/P
0•
Works for monochromatic light
•
Compound x has a unique e at different
wavelengths
cuvette source slit
Characteristics of
Beer’s Law Plots
•
One wavelength
•
Good plots have a range of
absorbances from 0.010 to 1.000
•
Absorbances over 1.000 are not that
valid and should be avoided
Standard Practice
•
Prepare standards of known
concentration
•
Measure absorbance at
l
max
•
Plot A vs. concentration
•
Obtain slope
Typical Beer’s Law Plot
y = 0.02x
0 0.2 0.4 0.6 0.8 1 1.2
0.0 20.0 40.0 60.0
concentration (uM)
UV-Vis Spectroscopy
•
UV-
organic molecules
–
Outer electron bonding transitions
–
conjugation
•
Visible – metal/ligands in solution
–
d-orbital transitions
Characteristics of UV-Vis spectra of
Organic Molecules
•
Absorb mostly in UV unless highly
conjugated
•
Spectra are broad, usually to broad for
qualitative identification purposes
•
Excellent for quantitative Beer’s
Law-type analyses
Molecules have quantized energy levels: ex. electronic energy levels.
en Q: Where do these quantized energy levels come from?
A: The electronic confgurations associated with bonding.
Each electronic energy level (confgurationd has
Broad spectra
•
Overlapping vibrational and rotational
peaks
Molecular Orbital Theory
2s 2s
s s*
s
s*
p*
p
C C
Absorptions having max < 200 nm are difcult to observe because
everything (including quartz glass and air) absorbs in this spectral region.
C C
Example: ethylene absorbs at longer wavelengths: max = 165 nm = 10,000
=
hv
The n to pi* transition is at even lower wavelengths but is not as strong as pi to pi* transitions. It is said to be “forbidden.”
Example:
C C
HOMO LUMO
Conjugated systems:
Preferred transition is between Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO).
Note: Additional conjugation (double bondsd lowers the HOMO-LUMO energy gap:
Example:
O
O
O
Similar structures have similar UV spectra:
Lycopene:
max = 114 + 5(8d + 11*(48.0-1.7*11d = 476 nm
Polyenes, and Unsaturated
Carbonyl groups;
an Empirical triumph
R.B. Woodward, L.F. Fieser and others
Predict max for π * in extended conjugation systems to within ca. 2-3 nm.
Homoannular, base 253 nm
heteroannular, base 214 nm Acyclic, base 217 nm
Attached group increment, nm Extend conjugation +30 Addn exocyclic DB +5
Similar for Enones
215 202 227 239
Base Values, add these increments…
Extnd C=C +30
Add exocyclic C=C +5
Homoannular diene +39
alkyl +10 +12 +18 +18
Cl/Br +15/+25 +12/+30
b g d,+
Some Worked Examples
O
Base value 217 2 x
alkyl subst. 10 exo
DB 5 total
232 Obs. 237
Base value 214 3 x
alkyl subst. 15 exo
DB 5 total
234 Obs. 235
Base value 215 2 ß
alkyl subst. 24
total 239 Obs.
Distinguish Isomers!
HO
2C
HO
2C
Base value 214 4 x
alkyl subst. 20 exo
DB 5 total
239 Obs. 238
Base value 253 4 x
alkyl subst. 20 total
Generally, extending conjugation
leads to red shift
“particle in a box” QM theory; bigger box
Substituents attached to a chromophore that cause a red shift are called “auxochromes”
Strain has an efect…
Interpretation of UV-Visible Spectra
• Transition metal
complexes; d, f electrons.
• Lanthanide
complexes – sharp lines caused by
“screening” of the f electrons by other orbitals
• One advantage of this
is the use of holmium oxide flters (sharp linesd for wavelength calibration of UV
Benzenoid
aromatics
From Crewes, Rodriguez, Jaspars, Organic Structure Analysis
UV of
Benzene in heptane
Group K band () B band() R band
Alkyl 208(7800d 260(220d
---OH 211(6200d 270(1450d
-O- 236(9400d 287(2600d
-OCH3 217(6400d 269(1500d
NH2 230(8600d 280(1400d
-F 204(6200d 254(900d -Cl 210(7500d 257(170d -Br 210(7500d 257(170d
-I 207(7000d 258/285(610/180 d
-NH3+ 203(7500d 254(160d
-C=CH2 248(15000d 282(740d
-CCH 248(17000d 278(6500
-C6H6 250(14000d
-C(=OdH 242(14000d 280(1400d 328(55d
-C(=OdR 238(13000d 276(800d 320(40d
-CO2H 226(9800d 272(850d
-CO2- 224(8700d 268(800d
-CN 224(13000d 271(1000d
Substituent efects don’t really add up
Can’t tell any thing about substitution geometry
Exception to this is when adjacent substituents can interact, e.g hydrogen bonding.
E.g the secondary benzene band at 254 shifts to 303 in salicylic acid
Heterocycles
Nitrogen heterocycles are pretty similar to the benzenoid anaologs that are isoelectronic.
Quantitative
analysis
Great for non-aqueous titrations Example here gives detn of
Single clear point, can exclude intermediate state, exclude light scattering and Beer’s law applies
More Complex Electronic Processes
• Fluorescence: absorption of
radiation to an excited state, followed by emission of
radiation to a lower state of the same multiplicity
• Phosphorescence: absorption of
radiation to an excited state, followed by emission of
radiation to a lower state of diferent multiplicity
• Singlet state: spins are paired,
no net angular momentum (and no net magnetic feldd
• Triplet state: spins are unpaired,
Metal ion transitions
Degenerate D-orbitals of naked Co
D-orbitals
of hydrated Co2+
Octahedral Confguration
Co2+
H2O H2O
H2O H2O
H2O
H2O
Instrumentation
•
Fixed wavelength instruments
•
Scanning instruments
Fixed Wavelength Instrument
• LED serve as source
• Pseudo-monochromatic light source
• No monochrometer necessary/ wavelength selection
occurs by turning on the appropriate LED
• 4 LEDs to choose from
photodyode sample
beam of light
Scanning Instrument
cuvette Tungsten
Filament (vis)
slit
Photomultiplier tube
monochromator
Deuterium lamp Filament (UV)
sources
•
Tungten lamp (350-2500 nm)
•
Deuterium (200-400 nm)
Monochromator
•
Braggs law, nl = d(sin i + sin r)
•
Angular dispersion
,
d
r/
dl
= n / d(cos rd
•
Resolution, R
=
l
/
Dl
=
nN, resolution is
Sample holder
Single beam vs. double beam
Diode array Instrument
cuvette Tungsten
Filament (vis)
slit
Diode array detector 328 individual detectors
monochromator Deuterium lamp
Filament (UV)
