UV-Visible spectroscopy: Basics

Ultraviolet Spectroscopy

In this anaytical technique, ultraviolet radiations are used.

Wavelength ranges of radiations:

• Visible light : 400 - 800 nm
• Near UV radiations : 200 - 400 nm
• Far/ Vacuum UV : below 200 nm

Generally, we carry out UV analysis in near UV region of radiations.

PRINCIPLE : Electronic transitions

• When a molecule absorbs UV radiations, the electronic excitation occur where electrons go from lower to higher energy state. 
• Electronic excitation causes the electron to go from electron bonding orbital to antibonding orbital.
• After some time electron returns to its original state (stable). While returning to its stable form (into bonding orbital) it releases excess energy. That energy is measured by the detector to access the wavelength (λmax) /absorption pattern.

TRANSITIONS:

σ → σ* (126-135 nm)

n → σ* (180-200 nm)

π → π*

n → π*

ALLOWED TRANSITION: π → π*

FORBIDDEN TRANSITIONS:  n → π*

ENERGY OF VARIOUS TRANSITIONS:

σ → σ* >> n → σ* > π → π* > n → π*

• Alkanes like methane, ethane show σ → σ* transitions (126-135 nm).
• Aliphatic alcohols and alkyl halides are generally used as solvents because they start absorbing at 260 nm.
• If our compound shows absorption in 200-260 nm range, then aliphatic alcohols (methanol, ethanol) cannot be used.
• Unsaturated compounds (olefins) undergo π → π* transitions.
• When such unsaturated compounds undergo alkyl substitution, they show absorption at longer wavelength.

IMPORTANT TERMS:

CHROMOPHORE: Any compound/ group which exhibits absorption of electromagnetic radiations.

AUXOCHROME: It is the group which when attached to chromophore shifts absorption maxima towards longer wavelength and increases the intensity of absorption.

BATHOCHROMIC (red) SHIFT: Absorption maximum shifts towards longer wavelength  (due to conjugation in molecules).

HYPOSOCHROMIC (blue) SHIFT: Absorption maximum shifts towards shorter wavelength (due to removal of conjugation).

HYPERCHROMIC SHIFT: Increase in the intensity of absorption (due to introduction of auxochrome).

HYPOCHROMIC: Decrease in the intensity of absorption (due to distortion of geometry).

• EXAMPLES OF AUXOCHROMES: -OH, -NH2, -OR, -NHR, -NR2.
• All auxochromes are non-bonding pair of electrons.
• When attached to chromophores, they extends conjugation and causes Bathochromic (red shift.

INSTRUMENTATION:

1. UV radiation source:

• Tungsten lamp: for visible region : 400-800 nm.
• Deuterium lamp: for UV region: 190-400 nm.

1. Sample cell is made up of Quartz.
2. Solvents generally used : 95% ethanol, 1,4-dioxane.
3. Beer's-Lambarts law: A= εbc.
4. Graph is plotted between concentration vs absorbance (optical density)
5. Detector : Photomultiplier tubes.

Solvent cut-offs in UV spectroscopy:

Solvent	Wavelength	Solvent	Wavelength
Acetonitrile	190	n-Hexane	201
Chloroform	240	Methanol	205
Cyclohexane	195	Isooctane	195
1,4-Dioxane	215	Water	190
95 % Ethanol	205	Trimethyl phosphate	210