19 Molecular Spectroscopy

7/29/2019 19 Molecular Spectroscopy

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UV/Vis absorption by organic compounds

requires that the energy absorbed correspondsto a jump from a populated orbital to an

unpopulated one.

!"

#"

n

#*

!*

bondingbonding

non-bonding

anti-bonding

anti-bonding


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N

N

N

N

Vapor phase spectrum

Here we have no

solvent and minimal

interaction betweenthe molecules

Vapor phase

In hexane

In water


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octahedral tetrahedral square planar

The type and degree of splitting depends on the

ligand and geometry of the complex that is formed.

This splitting of the d orbitals results in a d->d

transition that is in the UV/Vis range.

Chromium(III) examples

Ligand $maxCl- 736

H2O 573

NH3 462

CN- 380

N

N

3 + Fe2+ Fe2+

N

N

3

1,10-phenanthroline ferroin

IR radiation is of too low an energy to excite

electronic transitions.

Absorption is limited to vibrational and

rotational levels.

For liquids and solids, molecular rotation isoften limited so the major type of interaction is

vibrational.


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symmetricalstretching

asymmetricalstretching

scissoring

Due to the large number of vibrational states, IR

spectra can be very complex.

2 4 6 8 10 12 14

wavelength, mC-H

stretchC=O

stretch

C-Hbend

CH2rock

The real strength of IR is its ability to identify

functional groups.

Functional wavenumber wavelength

Group (cm-1) (m)

C-H, aliphatic 3000-2850 3.3-3.5

C-H, aromatic 3150-3000 3.2-3.3

O-H 3600-3000 2.8-3.3

C=O, aldehyde/ketone 1740-1660 5.7-6.0

-CH2Cl 1300-1200 7.6-8.2850-890 13.2-14


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R2NCS

S- Na+n + M

n+ R2NCS

S

M

n

dithiocarbamate

You want to make your measurement at a $max to

minimize errors and achieve maximum sensitivity.

Small error

Large error

identical variationsin wavelength.

The relationship between concentration and

absorption must be established.

Your methodshould only beused in thisrange.

Weve already covered the basics of absorbance

calculations.

Lets look at another type of absorbance

application - using variations in absorbance

during a titration.

This is an alternate to using an indicator and

especially useful for complexometric titrations.


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Experimental

setup

The absorbance of a solutioncan be monitored during atitration.

In this example, stirring will

cause the solution to circulatein the cuvette and theresponse can be measured.

You could also use a pumpor simply transfer a sample atknown intervals.

spectrophotometer

stirbar

The type of titration curve you obtain is

dependent on the reaction involved.

In this example

- the sample does not absorbat the measured wavelength

- the titrant will absorb

equivalence point

Determine what would cause these types oftitration curves.

Normal relaxation process

absorption relaxation

Large jumps are

called internal

conversion.

The process in

well understoodbut is temperature

dependenth%"

h%"

Fluorescence process

absorption fluorescence


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Just because a molecule demonstrates

fluorescence does not mean that there is nointernal conversion. Both processes occur.

The % of molecules that fluoresce is expressed

as the quantum yield.

Q =

We can consider all species can fluoresce but

for most, Q is very small.

rate of fluorescent relaxationtotal relaxation rate

Fluorescence spectra tend to look like

overlapping, mirror images of theoriginal absorption spectra.

The energy range is always lowerbecause some energy is always lost

to vibrational modes.

emission

excitation

300 350 400

Remember that the top

spectra is absorption

and the bottom isemission.

Now we not only need to

deal with the $max for

absorption but theemission $max.

Not many compounds undergo significant

fluorescence.

Aromatic fused ring structures are best.

Highly conjugated double bonds will also show

the effect to a smaller degree.

Basically, we need a rigid structure with limitedvibrational modes of relaxation.

Being an emission technique, it can be very

sensitive and have low detection limits.

Unfortunately, not many species undergo

fluorescence on their own.

We can extend the method by forming complexes

using fluorescent ligands or adding fluorescent

groups to other organic molecules.

OH

N C|

8-hydroxyquinoline

fluorene


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These are emission methods.

Fluorescent intensity = K c

It best to produce a calibration curve.

Must consider both the excitation and emission

$max values.

Look for the most sensitive combination.

intensity

concentration

Calibration curves are often linear over a 106 range.

Self-quenching

region

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19 Molecular Spectroscopy