Electronic Spectroscopy Ultraviolet and visible

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Electronic SpectroscopyUltraviolet and visible
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Where in the spectrum are these transitions?
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Why should we learn this stuff?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 One wavelength may not be
the best for all compound in a mixture. Affects
quantitative interpretation of HPLC peak heights
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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
differences 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!!!) 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.
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The UV Absorption process

• ? ? ? and ? ? ? transitions high-energy,
  accessible in vacuum UV (?max lt150 nm). Not
  usually observed in molecular UV-Vis.
• n ? ? and ? ? ? transitions non-bonding
  electrons (lone pairs), wavelength (?max) 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.
• Energies correspond to a 1-photon of 300 nm light
  are ca. 95 kcal/mol

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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 03W)
h? 170nm photon
LUMO ?g antibonding molecular orbital
HOMO ?u bonding molecular orbital
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How Do UV spectrometers work?
Rotates, to achieve scan
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 difference technique
Two photomultiplier inputs, differential voltage
drives amplifier.
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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
Model from Agilent literature. Imagine replacing
cell with a microflow cell for HPLC!
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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 CO are
reliable Most compounds have end absorbance at
lower frequency. Unfortunately solvent cutoffs
preclude observation. You will find molar
absorbtivities ? in Lcm/mol, tabulated. Transiti
on metal complexes, inorganics Solvent must be UV
grade (great sensitivity to impurities with
double bonds) The NIST databases have UV spectra
for many compounds
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An Electronic Spectrum
Make solution of concentration low enough that A
1 (Ensures Linear Beers law behavior) Even
though a dual beam goes through a solvent blank,
choose solvents that are UV transparent. Can
extract the ? value if conc. (M) and b (cm) are
known UV bands are much broader than the
photonic transition event. This is because
vibration levels are superimposed on UV.
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Solvents for UV (showing high energy cutoffs)
THF 220 CH2Cl2 235 CHCl3 245 CCl4 265 benzene 2
80 Acetone 300 Various buffers for HPLC, check
before using.
Water 205 CH3C?N 210 C6H12 210 Ether 210 EtOH
210 Hexane 210 MeOH 210 Dioxane 220
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Organic compounds (many of them) have UV spectra
One thing is clear Uvs can be very
non-specific 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 Generally we
dont assign the particular transitions.
From Skoog and West et al. Ch 14
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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
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Can we calculate UVs?
Semi-empirical (MOPAC) at AM1, then ZINDO for
config. interaction level 14 Bandwidth set to
3200 cm-1
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The orbitals involved
Showing atoms whose MOs contribute most to the
bands
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The Quantitative Picture

• Transmittance
• T P/P0

• Absorbance
• A -log10 T log10 P0/P

B(path through sample)

• The Beer-Lambert Law (a.k.a. Beers Law)
• A ebc
• 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
• c is the concentration of the compound in
  solution, expressed in mol L-1 (or M, molarity)

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Beer-Lambert Law
Linear absorbance with increased
concentration--directly proportional Makes UV
useful for quantitative analysis and in HPLC
detectors Above a certain concentration the
linearity curves down, loses direct
proportionality--Due to molecular associations at
higher concentrations. Must demonstrate
linearity in validating response in an analytical
procedure.
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Polyenes, and Unsaturated Carbonyl groupsan
Empirical triumph
R.B. Woodward, L.F. Fieser and others Predict
?max for p?? in extended conjugation systems to
within ca. 2-3 nm.
Attached group increment, nm Extend
conjugation 30 Addn exocyclic DB 5 Alkyl 5 O-A
cyl 0 S-alkyl 30 O-alkyl 6 NR2 60 Cl,
Br 5
Homoannular, base 253 nm
Acyclic, base 217 nm
heteroannular, base 214 nm
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Similar for Enones
227
202
239
215
Base Values, add these increments
b
g
?
d,
XH 207 XR 215 XOH 193 XOR 193
With solvent correction of.. Water
8 EtOH 0 CHCl3 -1 Dioxane
-5 Et2O -7 Hydrcrbn -11
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Some Worked Examples
Base value 217 2 x alkyl subst. 10 exo DB
5 total 232 Obs. 237
Base value 214 3 x alkyl subst. 30 exo DB
5 total 234 Obs. 235
Base value 215 2 ß alkyl subst. 24
total 239 Obs. 237
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Distinguish Isomers!
Base value 214 4 x alkyl subst. 20 exo DB
5 total 239 Obs. 238
Base value 253 4 x alkyl subst. 20 total 273
Obs. 273
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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 effect
?max 253 239 256 248
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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
  filters (sharp lines) for wavelength calibration
  of UV spectrometers.

See Shriver et al. Inorganic Chemistry, 2nd Ed.
Ch. 14
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Benzenoid aromatics
UV of Benzene in heptane
From Crewes, Rodriguez, Jaspars, Organic
Structure Analysis
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Substituent effects dont really add up Cant
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 In
p-hydroxybenzoic acid, it is at the phenol or
benzoic acid frequency
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Heterocycles
Nitrogen heterocycles are pretty similar to the
benzenoid anaologs that are isoelectronic. Can
study protonation, complex formation (charge
transfer bands)
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Quantitative analysis
Great for non-aqueous titrations Example here
gives detn of endpoint for bromcresol
green Binding studies Form I to form II
Isosbestic points Single clear point, can exclude
intermediate state, exclude light scattering and
Beers law applies
Binding of a lanthanide complex to an
oligonucleotide
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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 different multiplicity
• Singlet state spins are paired, no net angular
  momentum (and no net magnetic field)
• Triplet state spins are unpaired, net angular
  momentum (and net magnetic field)