UV & visible spectroscopy
Ultra Violet Spectroscopy
Introduction :
• UV & visible spectroscopy is the study of electronic spectra which are found in the wavelength region 100-8000 Å of the electromagnetic spectrum.
Ultra violet spectroscopy
Principle :
• If the molecule is excited with higher energy (5 to 10 ev) then this energy is sufficient to bring about electronic, vibrational & rotational transitions.
• This results into the production of electronic spectra consisting of line due to electronic, vibrational, and rotational transitions.
• Each electronic transition produces an electronic band.
• A number of such electronic transitions will produce a band system composed of various electronic bands.
Ultra violet spectroscopy
Instrumentation :Ultraviolet Spectrophotometer Hydrogen Discharge Lamp for UV
Tungsten filament lamp for visible region Prism
Quartz for UV, Glass for visible
Compound sample+ ethanol solvent
TYPES OF ELECTRONIC TRANSITIONS
Molecular orbital theory : when a molecule is excited by the absorption of energy (UV or visible light), its electrons are promoted from a bonding to an antibonding orbital.
(i) to * transition : when (sigma) electron is promoted to anti bonding ()
orbital.
(ii) n to * transition : when a non-bonding electron (n) gets promoted to an anti bonding sigma orbital (*).
(iii) to * transition : promotion of electrons to an anti-bonding orbital.
n
*
*
n-*
-*
-*
n-* E N E R G Y n non-bonding
bonding
bonding
* anti-bonding
E N E R G Y
* anti-bonding
Transitions involved in UV Spectroscopy :
(i) * transitions : It is a high energy process since bonds are very strong. For saturated hydrocarbons (methane, propane) absorption occurs near
150 m (high energy).
This transition requires radiation of very short wavelength (high energy). The region below 200 m is commonly called vacuum ultraviolet region.
(ii) n ~ * transition : Some compounds undergoing this type of transitions are : saturated halides, alcohols, ethers, aldehydes, ketones. amines etc.
This transitions require comparatively less energy.
Transitions involved in UV Spectroscopy :
(iii) * transitions. This type of transition occurs in compounds containing double or triple bonds and also in aromatics.
The excitation of electron requires smaller energy and hence, transition of this type occurs at longer wavelength.
For example: alkenes, alkynes, carbonyl compounds, cyanides, azo compounds etc.
(iv) n* transition: This type of transition requires least amount of energy out of all the transitions discussed above and hence occurs at longer wavelengths.
Saturated aldehydes show both the types of transitions. i.e., low energy.
• n* and * occurs at around 290 m and 180 m respectively.
Some terms related to UV Spectroscopy
Chromophore :
It is defined as any isolated covalently bonded group that shows a characteristic absorption in the ultra-violet or the visible region.
Auxochrome.
It is any group which does not itself act as a chromo phore but whose presence brings about a shift of the absorption band to wards the red end of the spectrum
(longer wavelength).
Bathochromic effect.
APPLICATIONS OF ULTRA-VIOLET SPECTROSCOPY
(a) Detection of functional groups.
• The technique is applied to detect the presence or absence of the chromophore.
• The absence of a band at a particular wavelength may be regarded as an evidence for the absence of a particular group in the compound.
• If the spectrum is transparent above 200 mμ, it shows the absence of (i) conjugation
(ii) a carbonyl group (aldehydes and ketones) (iii) benzene or aromatic com pounds and also (iv) bromo or iodo atoms.
• An isolated double bond or some other atoms or groups may be present. It
APPLICATIONS OF ULTRA-VIOLET SPECTROSCOPY
(b) Extent of conjugation :
• The extent of conjugation in polyenes R-(CH=CH)n-R can be estimated.
• Addition in unsaturation with the increase in the number of double bonds (increase in the value of n) shifts the absorption to longer wavelength.
• It is found that the absorption occurs in the visible region, i.e., at about 420, mμ, if n = 8 in the above polyene.
APPLICATIONS OF ULTRA-VIOLET SPECTROSCOPY
(c) Distinction in conjugated and non-conjugated compounds :
• It also distinguishes between a conjugated and a non-conjugated compound.
• The following isomers can be readily distinguished since one is conjugated and the other is not.
• The forbidden n-* band for the carbonyl group in the compound
(i) will appear at longer wave-length compared to that for the compound (ii) The alkyl substitution in an alkene causes a bathochromic shift. The
APPLICATIONS OF ULTRA-VIOLET SPECTROSCOPY
(d) Identification of an unknown compound :
• An unknown compound can be identified by comparing its spectrum with the known spectra.
• If the two spectra coincide, the two compounds must be identical.
APPLICATIONS OF ULTRA-VIOLET SPECTROSCOPY
(e) Examination of Polynuclear hydrocarbons :
• Benzene and Polynu clear hydrocarbons have characteristic spectra in the ultra-violet and visible region.
• Thus, the identification of the polynuclear hydro-carbons can be made by comparison with the spectra of known polynuclear compounds.
APPLICATIONS OF ULTRA-VIOLET SPECTROSCOPY
(f) Elucidation of the structure of vitamins A and K :
• It is useful for the elucidation of the structures of vitamins K1 and K2 and also those of A1 and A2.
• The ultraviolet spectra of vitamins K1 and K2 are due to the presence of the same chromophore, i.e., 2, 3 dimethyl naphtha-quinone.
• The absorption maxima of this compound are 243, 249, 260, 269 and 330 mμ .
APPLICATIONS OF ULTRA-VIOLET SPECTROSCOPY
(g) Preference over two Tautomeric forms :
• If a molecule exists in two tautomeric forms, preference of one over the other can be detected by ultra-violet spectroscopy.
