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Advanced Level Organic Chemistry: 15.5 UV and Visible Spectroscopy

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Doc Brown's Advanced Chemistry

PART 15.5 uv and visible spectroscopy

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses Spectroscopic methods of analysis and molecular structure determination

All my advanced A level organic chemistry notes

SPECTROSCOPY INDEXES

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This section is indexed under Doc Brown's Advanced Level Organic Chemistry

PART 15.5 Emission and absorption spectroscopy including colorimetry and flame photometry

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses uv visible spectroscopic methods of analysis molecular structure determination uv & visible spectroscopy organic and inorganic examples of uv absorption spectra and visible emission, absorption and reflectance spectra


Index of pages on ultraviolet and visible spectroscopy

15.5.1 The origin of colour, the wavelengths of visible light, our perception! (this page for simple introduction)

15.5.2 uv-visible spectroscopy theory, spectrometer, examples of absorption & reflectance spectra explained

15.5.3 uv-visible absorption spectra - index of examples: uses, applications, more on the chemistry of colour

Organic Chemistry Part 15 SPECTROSCOPY, is mainly organic uv-visible spectra, but there are sections on the emission and absorption spectroscopy of elements and inorganic examples including transition metal complexes and colorimetry

I had already written a few sections on visible absorption/emission spectroscopy before writing Part 15 e.g.

Identifying and element analysis from emission spectroscopy and flame photometry

Electron configuration of transition metal ions and colour theory

Colorimetric analysis and determining a transition metal complex ion formula


15.5.1 The origin of colour, wavelengths and colours of visible light - our perception!

Sub-index for this page 15.5.1

(a) The wavelengths and colours of the visible light spectrum

(b) Why, and how, do we see different colours?

(c) Colour wheels, complimentary colours and the wavelengths of visible light


(a) The wavelengths of the colours of the visible light spectrum

picture of the visible spectrum

Table of the colours and wavelengths of the visible light spectrum, every colour has its own range of wavelengths and corresponding frequencies.

E, the energy of a photon in J, is given by Planck's equation: E = hv

h = Planck's constant = 6.63 x 10-34 Js, v = frequency, Hz.

You need to be familiar with inter-converting wavelengths and frequencies: c = λ

c = speed of light, 3.0 x 108 ms-1, λ = wavelength, m, v = frequency, Hz

In my uv-visible spectroscopy pages, wavelengths are usually quoted in nanometres (1 nm = 1.0 x 10-9 m)

See general spectroscopy index page for examples of calculations

Below is a rough table guide to what colour is transmitted if the specified colour is absorbed.
1. Colour that we perceive 2. ~Wavelength range ( λ/nm) 3. Colour on absorption
Ultraviolet

(invisible)

<380 (end of near uv region,

start of visible region)

If column 1 colour is absorbed,

column 3 colour is transmitted

Violet 380 - 435 Yellow
Blue 435 - 500 Yellow-orange
Cyan (blue-green) 500 - 520 Orange-red
Green 520 - 565 Purple (magenta)
Yellow 565 - 590 Violet-blue
Orange 590 - 625 Blue
Red 625 - 740 Blue-green
Infrared

(invisible)

>740 (start of ir region,

end of visible region)

 

See a colorimetric exercise for a similar table of colour matching for analysis using a colorimeter.

It should be emphasised that the colour you perceive, and the reason for it, is quite complex, as you will find out in my discussions of individual spectra.

The table above, and colour wheels illustrated in (b) and (c) are only an approximation to the predicted colour and what is actually observed!

What is absorbed by, or transmitted through a material, in the uv-visible region of the electromagnetic spectrum, all depends on the energy of the electronic quantum levels of the outer bonding/non-bonding electrons of the molecule.

See The in depth theory of uv-visible absorption and reflectance spectra

diagram image of the visualisation of the wavelengths of visible light in nanometres nm

A more visual representation of the wavelengths of the colours of visible light in nanometres (1 nm = 1.0 x 10-9 m)).


(b) Why and how do we see different colours?

The colour of an object depends on which wavelengths of visible light are absorbed, transmitted or reflected.

Complementary colours

colour wheel of 8 eight complimentary colours matching colour absorbed with colour transmitted

To a good approximation you can often predict a colour or vice versa from what colours of visible light are absorbed by use of a complementary colour wheel.

Which ever segment colour is absorbed from white light, the material will display the complementary colour in the opposite segment.

e.g. something absorbing blue-violet will look yellow-green

 

The 'extremes' of 'colour'

diagram explaining why a substance is transparent and colourless or opaque black

A material that is transparent to all visible light wavelengths will appear colourless e.g. water, ethanol, pure sodium chloride crystals, glass with no pigment, perspex plastic.

A material that reflects all visible light wavelengths will appear an opaque white e.g. chalk, titanium dioxide powder or any other 'white' mineral pigment.

A material that absorbs all visible light wavelengths will appear an opaque black e.g. charcoal or soot.

These are the 'extremes' in terms of 'colours', but so-called 'coloured' materials owe their colour to what wavelengths are absorbed, transmitted or reflected.

 

Examples of coloured liquids or solids

The diagrams below illustrate resulting colours from absorption, transmission and reflectance of particular wavelengths of visible light

diagram explaining why a tranparent solution liquid is green violet-blue or orange-red

A simple colour absorption-transmission diagram of coloured liquids or solutions of materials. more labels

1A chlorophyll absorbs in the blue and red, so appears green by transmitted light.

1B absorption in the green-yellow-red region produces, in transmission, a violet-blue range of colour.

1C carotene absorbs in the green and blue and so appears as an orange colour.

For more see absorption spectra on the more advanced theory page.

 

diagram explaining why a tranparent solution liquid is green violet-blue or orange-red

A simple colour absorption-reflectance diagram of coloured solids

2A Absorption of e.g. red and blue on the surface makes the object look green from the reflected light.

2B Absorption in the red-yellow-green on the surface makes the object look a violet-blue colour.

2C Absorption on the surface of the blue-green wavelengths makes the solid look orange-red

For more see absorption reflectance spectra on the more advanced theory page.

 

diagram explaining why a solid pigment is blue and a pigment solution of a dye is blue

The diagram illustrates a comparison of a blue solid pigment that is soluble in a solvent.

Absorption by the pigment in the yellow-red region produces, by transmission, a blue coloured solution e.g. of a dye.

Absorption by the pigment surface in the yellow-red region produces, by reflectance, a blue coloured solid.

See also The uv-visible absorption spectra of the photopigments in the human eye


(c) Colour wheels, complimentary colours and the wavelengths of visible light

Diagram of colour wheels complimentary colours primary colours secondary colours ultraviolet visible light infrared wavelengths in nanometres nm

I emphasise again that the colour wheels illustrated above are only an approximation to the predicted colour and what is actually observed!

The complexity of electronic excitations from ultraviolet and visible light photons in organic molecules or inorganic positive and negative ions doesn't always produce the colour you expect, but the complementary colour discs work well enough in most situations.


SPECTROSCOPY INDEXES

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