UV-VIS SpectroscopyUV-VIS Spectroscopy

An obvious difference between certain compounds is their color.

The human eye is functioning as a spectrometer analyzing the light reflected from the surface of a solid or passing through a liquid.

Although we see sunlight (or white light) as uniform or homogeneous in color, it is actually composed of a broad range of radiation wavelengths in the ultraviolet (UV), visible and infrared (IR) portions of the spectrum.

• The component colors of the visible portion can be separated by passing sunlight through a prism, which acts to bend the light in differing degrees according to wavelength.

• Visible wavelengths cover a range from approximately 400 to 800 nm. The longest visible wavelength is red and the shortest is violet.

• The wavelengths of what we perceive as particular colors in the visible portion of the spectrum are displayed

Violet:   400 - 420 nmViolet:   400 - 420 nm Indigo:   420 - 440 nmIndigo:   420 - 440 nm Blue:   440 - 490 nmBlue:   440 - 490 nm Green:   490 - 570 nmGreen:   490 - 570 nm Yellow:   570 - 585 nmYellow:   570 - 585 nm Orange:   585 - 620 nmOrange:   585 - 620 nm Red:   620 - 780 nmRed:   620 - 780 nm

Color wheel: complementary colors are diametrically opposite each other. Thus, absorption of 420-430 nm light renders a substance yellow, and absorption of 500-520 nm light makes it red. Green is unique in that it can be created by absoption close to 400 nm as well as absorption near 800 nm.

When white light passes through or is reflected by a colored substance, a characteristic portion of the mixed wavelengths is absorbed. The remaining light will then assume the complementary color to the wavelengthsabsorbed.

• Electromagnetic radiation such as visible light is commonly treated as a wave phenomenon, characterized by a wavelength or frequency.

• Wavelength is the distance between adjacent peaks, and may be designated in meters, centimeters or nanometers (10-9 meters).

• Frequency is the number of wave cycles that travel past a fixed point per unit of time, and is usually given in cycles per second, or hertz (Hz).

The visible spectrum constitutes but a small part of the total radiation spectrum.

This electromagnetic spectrum ranges from very short wavelengths (including gamma and x-rays) to very long wavelengths (including microwaves and broadcast radio waves). The following chart displays many of the important regions of this spectrum, and demonstrates the inverse relationship between wavelength and frequency.

The Electromagnetic SpectrumThe Electromagnetic Spectrum

The energy associated with a given segment of the spectrum is proportional to its frequency.

* = c

E = h = frequency

= wavelength

C = velocity of light ( 3 x 1010 cm / sec )

E = energy

h = Planck’s constant (6.626068 × 10-34 m2 kg / s )

Wavelength * frequency =velocity

Interaction between light and matter:Interaction between light and matter:

When radiation interacts with matter a number of processes can occur including:

reflection scattering absorbance fluorescence/phosphorescence (absorption and re-

emission)

photochemical reaction

When measuring UV-Visible spectra we want only the absorbance process to occur. Any of the other processes will adversely affect the precision of our measurements.

Electronic SpectraElectronic Spectra

Absorption occurs due to the fact that all molecules

possess electrons which can be exited or raised to

higher energy level. Many can be excited by radiation of

UV or visible wavelengths but others are only excited by

vacuum UV.

Electronic SpectraElectronic Spectra

The amount of energy a molecule can have in each form is not a continuum but a series of levels or states.

The photons of UV and Visible light have sufficient energy to cause transitions between the different electronic energy levels of bonds (double bonds) in molecules, whereas photons of infra-red radiation have sufficient energy to cause transitions between vibration energy levels.

The Energy difference between the electronic energy levels is given by the equation:

E = h The absorption of energy excites the electron from the ground state to an electronic excited state

Any change in electronic energy is accompanied by vibration and rotational energy levels.

Chemical Structure and UV Chemical Structure and UV AbsorbanceAbsorbance

The absorbance energy depends on the nature of the bonds within a molecule. Electrons in organic molecules may be in :Strong bond ,weaker bond or non bonding n. When energy is absorbed all these types of electrons can be elevated to excited anti-bonding states , represented with an asterisk:

Ene

rgy

n

Conjugation promotes λmax.

Example of Ethylene and Butadiene

Comments on possible Sigma transitionsComments on possible Pi-transitionsComments on n-transitions

                                                                                                 

            

Basic TerminologiesBatho-chromic Shift shifting to a longer wavelengthHypo-chromic Shift shifting to a shorter wavelengthHyper-chromic effect increase in intensity of absorptionHypochromic effect decrease in absorption intensity

Some Chromophores and the wavelengths of their Some Chromophores and the wavelengths of their absorbance maxima.absorbance maxima.

Chromophore Formula Example (nm)Carbonyl (ketone) RR'C=O Acetone 271Carbonyl (aldehyde) RHC=O Acetaldehyde 293Carboxyl RCOOH Acetic Acid 204Amide RCONH2 Acetamide 208Ethylene RCH=CHR Ethylene 193Acetylene RC=CR Acetylene 173Nitrile RC=N Acetonitrile <160Nitro RNO2 Nitromethane 271

*The position of the absorbance maximum of a chromophore is not fixed, as it is to some extent dependent upon the molecular environment of the chromophore.

Spectroscopy is the study of the absorbance and emission of electromagnetic radiation (light) by matter.

The collection of frequencies absorbed by a sample is its absorption spectrum.

SpectroscopySpectroscopy

AbsorbanceAbsorbance

I0 Incident radiation I Transmitted radiation

Absorbance = log (I0 / I)

path-length

sample

Transmittance = 100* ( I/ I 0 )

Absorbance may be presented as

transmittance (T = I/I0) or

absorbance (A= log I0/I).

If the sample compound does not absorb light at a specific wavelength, I() = I0()

I=I0 T = 1 and A = 0

If the sample compound absorbed light, I()< I0 ()

I<I0 T< 1 and A > 0

Beer's law :Beer's law :

The amount of light absorbed is proportional to the number of absorbing molecules through which the light passes.

The result of plotting absorbance against concentration is shown below.

A= A= b c b c

The extinction coefficientThe extinction coefficientIs a characteristic of a given substance at a specific wavelength.

A= absorbance, C = sample concentration in moles/literb = length of light path through the cuvette in cm

A= A= b c b c==b Cb C

Limitations to Beer’s Law

Fundamental dilute concentration refractive index

Chemical Equilibrium

Instrumental Stray radiation

UV-Visible InstrumentationUV-Visible Instrumentation• Several types of Spectrometer

Single Beam Spectrophotometer

Double Beam Spectrophotometer

InstrumentationInstrumentation Light source

– Deuterium and hydrogen lamps

– W filament lamp– Xe-arc lamps

Sample containers

– Cuvettes Plastic Glass Quartz

Filter:

– the monochromator select a narrow portion of the spectrum of a given source

Detector

– Photovoltaic– Phototube– Photomultiplier

Components of a SpectrophotometerComponents of a SpectrophotometerLight SourceLight Source

Deuterium Lamps － a truly continuous spectrum in the ultraviolet region is produced by electrical excitation of deuterium at low pressure. (160nm~375nm)

Tungsten Filament Lamps － the most common source of visible and near infrared radiation.

Prepare the StandardsPrepare the Standards

The concentration and volume of the stock solution added should be chosen to increase the concentration of the unknown by about 30% in each succeeding flask.

Application of UV-Visible Application of UV-Visible SpectroscopySpectroscopy

In structure determination : UV-VIS spectroscopy is used to detect the presence of chromophores like dienes, aromatics, polyenes, and conjugated ketones, etc.

Number and location of substituents Qualitative analysis and purification control in dye stuff Calculation of dissociation constant Study kinetics of fast reactions. Structure elucidation in vitamins