Transcript Document 7432842
UV-VIS Molecular Spectroscopy
Chapter 13-14 From 190 to 900 nm!
Reflection and Scattering Losses
LAMBERT-BEER LAW Power of radiation after passing through the solvent
P solution solvent
A a b c
absorptivi pathlength ty concentrat ion
Power of radiation after passing through the sample solution
Beer’s law and mixtures
Each analyte present in the solution absorbs light! The magnitude of the absorption depends on its e A total = A 1 +A 2 +…+A n A total = e 1 bc 1 + e 2 bc 2 +…+ e n bc n If e 1 = e 2 = impossible e n then simultaneous determination is Need n l ’s where e ’s are different to solve the mixture
Ingle and Crouch,
Deviations from Beer’s Law
I r I
0 ( ( 2 2 1 1 ) 2 ) 2 Successful at low analyte concentrations (0.01M)!
High concentrations of other species may also affect
Consider the equilibrium: A + C AC If e is different for A and AC then the absorbance depends on the equilibrium.
[A] and [AC] depend on [A] total .
A plot of absorbance vs. [A] total will not be linear.
Instrumental deviation with polychromatic radiation
Effects of Stray Light
P S T A
P S P
T P S A A
P P kc
P P S S
Effects of Signal-to-Noise
Bad at High T 0.2
Bad at Low T 0 1 2 3 4 5 6 7 8 9 10 % RELATIVE CONCENTRATION UNCERTAINTIES 11
Components of instrumentation:
Sources Sample Containers Monochromators Detectors
Components of instrumentation:
Sources: Agron, Xenon, Deuteriun, or Tungsten lamps Sample Containers: Quartz, Borosilicate, Plastic Monochromators: Quarts prisms and all gratings Detectors: Pohotomultipliers
Deuterium and hydrogen lamps (160 – 375 nm)
D 2 + E e → D 2 * → D’ + D’’ + h
Excited deuterium molecule with fixed quantized energy Dissociated into two deuterium atoms with different kinetic energies E e = E D2* = E D’ + E D’’ + hv Ee is the electrical energy absorbed by the molecule. E D2* energy of D 2* , E D’ and E D’’ is the fixed quantized are kinetic energy of the two deuterium atoms.
Tungsten lamps (350-2500 nm) Blackbody type , temperature dependent Why add I 2 W + I 2 in the lamps?
→ WI 2 Low limit: 350 nm 1) Low density 2) Glass envelope
General Instrument Designs Single beam Requires a stabilized voltage supply
General Instrument Designs Double Beam: Space resolved Need two detectors
General Instrument Designs Double Beam: Time resolved
Double Beam Instruments
1. Compensate for all but the most short term fluctuation in radiant output of the source 2. Compensate drift in transducer and amplifier 3. Compensate for wide variations in source intensity with wavelength
e = 8.7 x 10 19 P A A: cross section of molecule in cm 2 (~10 -15 ) P: Probability of the electronic transition (0-1) P>0.1-1 allowable transitions P<0.01 forbidden transitions
M h M* M M* ( heat absorption 10 -8 sec) (relaxation process) M* M* A+B+C (photochemical decomposition) M
Visible Absorption Spectra
The absorption of UV-visible radiation generally results from excitation of bonding electrons.
can be used for quantitative and qualitative analysis
Molecular orbital is the nonlocalized fields between atoms that are occupied by bonding electrons. (when two atom orbitals combine, either a low-energy bonding molecular orbital or a high energy antibonding molecular orbital results.)
The molecular orbital associated with single bonds in organic compounds
The molecular orbital associated with parallel overlap of atomic P orbital.
No bonding electrons
Molecular Transitions for UV-Visible Absorptions
What electrons can we use for these transitions?
MO Diagram for Formaldehyde (CH
= C n = O
Singlet vs. triplet
In these diagrams, one electron has been excited (promoted) from the n to * energy levels (non-bonding to anti-bonding). One is a Singlet excited state, the other is a Triplet.
Type of Transitions
σ → σ*
High energy required, vacuum UV range CH 4 :
= 125 nm
n → σ*
Saturated compounds, CH 3 O H etc (
l n → * and → *
= 200 - 700 nm = 150 - 250 nm)
Examples of UV-Visible Absorptions LOW!
UV-Visible Absorption Chromophores
Effects of solvents
Blue shift (n *) (Hypsocromic shift) Increasing polarity of solvent better solvation of electron pairs (n level has lower E) peak shifts to the blue (more energetic) 30 nm (hydrogen bond energy) Red shift (n * and – *) (Bathochromic shift) Increasing polarity of solvent, then increase the attractive polarization forces between solvent and absorber, thus decreases the energy of the unexcited and excited states with the later greater peaks shift to the red 5 nm
UV-Visible Absorption Chromophores
Typical UV Absorption Spectra Chromophores?
Effects of Multiple Chromophores
The effects of substitution Auxochrome function group Auxochrome is a functional group that does not absorb in UV region but has the effect of shifting chromophore peaks to longer wavelength as well As increasing their intensity.
Now solvents are your “container”
They need to be transparent and do not erase the fine structure arising from the vibrational effects
Polar solvents generally tend to cause this problem Same solvent must be Used when comparing absorption spectra for identification purpose.
Summary of transitions for organic molecules
* transition in vacuum UV (single bonds)
*, l e ~ 150-250 nm ~ 100-3000 ( not strong) * requires unsaturated functional groups (eq. double bonds) most commonly used, energy good range for UV/Vis l
~ 200 - 700 nm * : e *: e ~ 10-100 ~ 1000 – 10,000
List of common chromophores and their transitions
Most organic spectra are complex Electronic and vibration transitions superimposed Absorption bands usually broad Detailed theoretical analysis not possible, but semi-quantitative or qualitative analysis of types of bonds is possible.
Effects of solvent & molecular details complicate comparison
Rule of thumb for conjugation
CH CH 3 2 CH 2 If greater then one single bond apart e are relatively additive (hyperchromic shift) l constant CH =CHCH 2 2 CH=CH CH 2 2 CH=CH 2 l max = 184 l max =185 e max = ~10,000 e max = ~20,000 H 2 If conjugated - shifts to higher l ’s ( red shift ) C=CHCH=CH 2 l max =217 e max = ~21,000
Spectral nomenclature of shifts
What about inorganics?
Common anions n
* nitrate (313 nm), carbonate (217 nm)
Most transition-metal ions absorb in the UV/Vis region.
In the lanthanide and actinide series the absorption process results from electronic transitions of 4f and 5f electrons.
For the first and second transition metal series the absorption process results from transitions of 3d and 4d electrons.
The bands are often broad.
The position of the maxima are strongly influenced by the chemical environment.
The metal forms a complex with other stuff, called ligands. The presence of the ligands splits the d-orbital energies.
Transition metal ions
A charge-transfer complex consists of an electron-donor group bonded to an electron acceptor. When this product absorbs radiation, an electron from the donor is transferred to an orbital that is largely associated with the acceptor.
1) 2) Large molar absorptivity (ε max >10,000) Many organic and inorganic complexes