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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
Topic 3: Molecular Spectroscopy
Topic 3: Molecular Spectroscopy:
Text: Ch. 9,10, 11 Rouessac (~1 weeks)
•  Introduction to UV/Visible Molecular Spectroscopy
Beers law and limitations
MO Theory and Absorption
etc
•  Luminescence
•  IR Techniques
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
Molecular Absorption Spectroscopy:
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
Molecular Absorption Spectroscopy:
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
Beer’s Law: A = -log T = log P0/Pt = εbc
However, this never realized as scattering and other losses also reduce
beam.
Losses can be accounted for by using solvent, ie.,
Beer’s Law:
For multiple absorbing species
A = A1 + A2 +A3.... = ε1bc1 + ε1bc1 + ε1bc1 + ...
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
Limitations of Beer’s Law:
Real Limitations
High Concentrations (>0.01 M) Analytes intact and alter properties
Results in non-linear calibrations
Salts and other electrolytes also a factor
ε is dependant on the refractive index of the medium and the refractive index
is dependant on the solution composition. Not a significant problem for
concentrations less than 0.01M
Apparent Limitations
Apparent Chemical Limitations
When analyte chemically reacts (or associates) with solvent
Apparent instrument limitations
Beer’s law applies to monochromatic radiation
Molecular species produce “broad” bands
Stray Light can also cause a deviation from linear Beer’s law
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
Limitations of Beer’s Law:
Stray Radiation Causes a similar effect to Polychromatic
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
Review of Basic MO Theory:
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
Review of Basic MO Theory:
Winter 2010
Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
Review of Basic MO Theory:
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
Transition Types and Properties:
σ → σ* Transitions:
• These require significant energy.
• CH4: σ C-H → σ* C-H; λ = 125 nm
• C2H6: σ C-H → σ* C-H; λ = 135 nm
• Spectral range difficult to observe.
• ΔE =hν significant
n → σ* Transitions:
• λ range 150 nm to 250 nm
• ε small 100 to 3000 L cm-1 mol-1
• strongly effected by solvent (shorter λ)
• Most effected by bond type
n → π* Transitions:
•  with π → π* Transitions represent the most applicable to organic molecules
•  spectrally convenient range 200 to 700 nm
•  Required unsaturated functional group referred to as chromophore
•  ε small 10 to 100 L cm-1 mol-1
•  effected by solvent (shorter λ - a blue shift or hypochromic shift)
•  hypochromic shift due to solvation stabilization of n e- (up to 30nm shift)
Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
Transition Types and Properties:
π → π* Transitions:
•  Most commonly utilized transitions (200 to 700 nm)
•  ε large 1000 to 20,000 L cm-1 mol-1
•  solvent can produce a longer λ - a red shift or bathochromic shift (~5nm)
•  bathochromic shift due to a stabilization of both π and π* (excited more)
Absorption by Inorganic Species:
•  Many inorganic anions contain n electrons and/or π bonds, thus, n→ π* and π → π*
transitions are also important
•  with Metals transitions involving d (and f) electrons important
•  d orbital transitions common and important Crystal-Field theory (Ligand field)
•  Charge-transfer absorptions also very important (very large ε)
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
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Topic 3: Molecular Spectroscopy
Crystal-Field Theory (Ligand-Field Theory): d - orbital orientation
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
Crystal-Field Theory (Ligand-Field Theory):
Ligand effect on energy
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
Additional Considerations:
Solvents: Can absorb radiation to produce overlapping peaks (spectral window), shift λ maxim by
interaction with the analyte. In general polar solvents have a more significant effect and tend to
“smooth” fine structure.
Nonabsorbing Analytes: can be measured by addition of a reagent that produces a complex or
other chromatophoric species.
λ selection: Normally chosen near maximum where curve is as flat as possible. This region is also
less sensitive to slight λ deviations.
Absorption is effected by solvent, pH, temperature, [electrolyte], and interfering substances.
Therefore, these must be known and/or reproducible. Beer’s law should never be assumed and
multipoint calibration usually required.
Standard Addition: method of choice for systems with a complex matrix.
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Topic 3: Molecular Spectroscopy
Molecular Luminescence:
Involves the measurement of emission from electronically excited species,
normally produced by adsorption of radiation (Photoluminescence) or chemical
reaction (Chemical Luminescence).
Three main types:
Fluorescence: Refers to emission occurring immediately after absorption of
excitation wavelength. Normally does not involve a change in electron spin. The
observed λ is always equal (resonance fluorescence) or longer than the
absorbed λ.
Phosphorescence: Refers to emission occurring a set time after absorption of
excitation wavelength. Involve a change in electron spin. The observed λ is
always longer than the absorbed λ.
Chemical Luminescence: The measurement of emission resulting from a
chemical reaction. The λ observed is not normally produced by the analyte but
instead by a reaction product.
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
Theory of Photoluminescence:
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
Rates of Absorption and Emission:
Absorption rate 10-14 to 10-15 sec
Fluorescent rates inversely proportional to molar absorptivities. Since ε is an indication
of absorption probability, emission probability linked.
ε = 103 to 105
→ excited state lifetime 10-7 to 10-9 sec
ε = 10 to 103
→ excited state lifetime 10-6 to 10-5 sec
ε for singlet → triplet very weak → 10-4 to 10 sec: Phosphorescence
Relaxation / Deactivation Processes, rates vary by type
Vibrational relaxation 10-12 or less. Energy transferred by collision. Significantly
enhanced in condensed phases such as liquids. In solution, fluorescence is often
from lowest vibrational state of excited state.
Internal Conversion: Transfer from initial excited state to a lower energy excited
state. These occur without the emission of radiation.
External Conversion: Transfer of energy to solvent. Also called, Collisional
Quenching reduced by low T and high viscosity
Intersystem Crossing: Conversion with a change of multiplicity
Phosphorescence: emission after intersystem crossing
Dissociation: Electron transferred to unstable vibrational state
Predisposition: Intersystem crossing to unstable vibrational state
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Topic 3: Molecular Spectroscopy
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Topic 3: Molecular Spectroscopy
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
Key Variables effecting Fluorescence:
Quantum Yield (Quantum Efficiency, φ): the ratio of molecules that luminesce to the
total number of excited molecules.
where:
kf = rate of fluorescence; ki = rate of Intersystem crossing
kec = rate of external conversion; kic = rate of internal conversion
kpd = rate of predissociation; kd = rate of dissociation
Quantum Efficiency and transition type:
σ * → σ fluorescence transitions rare as at 200 nm (140 kcal/mol) these are energetic enough
for kpd and kd to dominate
π* → n and π* → π most common, with π* → π having greatest φ
Since π* → π has larger ε ∴ kf is larger for π* → π than π* → n
Also energy difference between π* and triplet states large so that ki is much smaller
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
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Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
Inorganic Species:
Direct fluorescence - form chelate
For species that quench, monitor decrease
Transition metals have greater deactivation
mechanisms that limit fluorescence
∴ most methods apply to non-transition metals
Non-transition metals usually form colorless
chelates.
Chemistry 311: Instrumentation Analysis
Topic 3: Molecular Spectroscopy
Typical Examples for Inorganic Analysis:
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Topic 3: Molecular Spectroscopy
Chemiluminescence: Produced when a chemical reaction yields an electronically excited
species that emits light as it relaxes to it’s ground state.
A + B → C* + D
C* → C + hν
ICL = φCL (dC/dt) = φEX φEM (dC/dt)
Where: ICL = Intensity of Chemical Luminescence; φCL = quantum yield of CL
dC/dt = rate of C formation; φEX = states per C reacted; φEM = photons per C
This technique is extremely sensitive as little or no background. Very specific, very
simple apparatus required (vial and photomultiplier).
However, not a very common occurrence
Bioluminescence: Chemiluminescence from biological systems (fire fly)
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