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

T A

 

P



P solution solvent

log

T

 

A a b c

   

abc



kc

absorptivi pathlength ty concentrat ion

P P

0  log  

Power of radiation after passing through the sample solution

P P

0  

Absorption Variables

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

Assumptions

Ingle and Crouch,

Spectrochemical Analysis

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

Chemical Equilibria

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

   stray light

P



P S P

0   log

T P S A A

   log

abc

   

P P kc

0  

P P S S

  

P S

 100

P

0

Instrument Noise

Effects of Signal-to-Noise

1 0.9

0.8

0.7

0.6

0.5

0.4

0.3

Bad at High T 0.2

0.1

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

Sources

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.

Sources

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

Multi-channel Design

Molar absorptivities

 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

Molecular Absorption

  M  h   M*  M  M* ( heat absorption 10 -8 sec) (relaxation process)   M*  M*  A+B+C (photochemical decomposition) M 

h

 ( emission)

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.)

Sigma (



) orbital

The molecular orbital associated with single bonds in organic compounds

Pi (



) orbital

The molecular orbital associated with parallel overlap of atomic P orbital.

n electrons

No bonding electrons

Molecular Transitions for UV-Visible Absorptions

 What electrons can we use for these transitions?

MO Diagram for Formaldehyde (CH

2

O)

H H



= 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 :

l

= 125 nm

n → σ*

Saturated compounds, CH 3 O H etc (

l n →  * and  →  *

Mostly used!

l

= 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

  

n

   * transition in vacuum UV (single bonds)      

n

  *,  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

n

~ 200 - 700 nm     * : e  *: e ~ 10-100 ~ 1000 – 10,000

List of common chromophores and their transitions

Organic Compounds

     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

Charge-Transfer-Absorption

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