UV/Vis absorption by organic compounds requires that the energy absorbed corresponds to a jump from a populated orbital to an unpopulated one.

UV/Vis absorption by organic compounds requires that the energy absorbed corresponds to a jump from a populated orbital to an unpopulated one.

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bonding bonding non-bonding anti-bonding anti-bonding

N N N N

Vapor phase spectrum

Here we have no solvent and minimal interaction between the molecules

Vapor phase

In hexane

In water

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 $_max Cl^- 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 is often limited so the major type of interaction is vibrational.

symmetrical

stretching asymmetrical

stretching scissoring

Due to the large number of vibrational states, IR spectra can be very complex.

2 4 6 8 10 12 14 wavelength, µm

C-H

stretch C=O stretch

C-H bend

CH₂ rock

The real strength of IR is its ability to identify functional groups.

Functional wavenumber wavelength Group (cm^-1) (µm) 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 7.6-8.2 850-890 13.2-14

R₂NC S S^- Na⁺

n + Mⁿ⁺ R₂NC S

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 variations in wavelength.

The relationship between concentration and absorption must be established.

Your method should only be used in this range.

We’ve 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.

Experimental setup The absorbance of a solution

can be monitored during a titration.

In this example, stirring will cause the solution to circulate in the cuvette and the response can be measured.

You could also use a pump or simply transfer a sample at known intervals.

spectrophotometer stirbar

The type of titration curve you obtain is dependent on the reaction involved.

In this example

- the sample does not absorb at the measured wavelength - the titrant will absorb

equivalence point

Determine what would cause these types of titration curves.

Normal relaxation process

absorption relaxation

Large jumps are called internal conversion.

The process in well understood but is temperature dependent

h%"

h%"

Fluorescence process

absorption fluorescence

Just because a molecule demonstrates fluorescence does not mean that there is no internal 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 relaxation total relaxation rate

Fluorescence spectra tend to look like overlapping, mirror images of the original absorption spectra.

The energy range is always lower because some energy is always lost to vibrational modes.

emission excitation

300 350 400

Remember that the top spectra is absorption and the bottom is emission.

Now we not only need to deal with the $max for absorption but the emission $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 limited vibrational 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

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