Spectroscopy is the branch of science that deals with the study of
interaction of electromagnetic radiation with matter.
It is the most powerful tool available for the study of atomic & molecular
structure and is used in the analysis of a wide range of samples. The two
main types are:
Atomic Spectroscopy; This Spectroscopy is concerned with the interaction
of electromagnetic radiation with atoms which are commonly in the lowest
energy state called as ground state.
Molecular Spectroscopy ; This Spectroscopy deals with the interaction of
electromagnetic radiation with molecule.
• Electromagnetic radiation consist of discrete packets of energy which are
called as photons.
• A photon consists of an oscillating electric field (E) & an oscillating
magnetic field (M) which are perpendicular to each other.
Wavelength (, lambda): Distance from one wave peak to the next.
Units: m, cm, m, nm
Frequency (, nu): Number of peaks that pass through a given point per
Units: Cycles/second or s-1 or Hertz (Hz)
Number of waves per cm.
1)σ → σ* transition.
σ electron from orbital is excited to corresponding anti-bonding
The energy required is large for this transition.
e.g. Methane (CH4) has C-H bond only and can undergo σ → σ*
transition and shows absorbance maxima at 125 nm.
2) π → π* transition
• π electron in a bonding orbital is excited to corresponding anti-
bonding orbital π*.
• Compounds containing multiple bonds like alkenes, alkynes,
carbonyl, nitriles, aromatic compounds, etc undergo π → π*
• e.g. Alkenes generally absorb in the region 170 to 205 nm.
3) n → σ* transition
• Saturated compounds containing atoms with lone pair of
electrons like O, N, S and halogens are capable of n → σ*
• These transitions usually requires less energy than σ → σ*
• The number of organic functional groups with n → σ* peaks
in UV region is small (150 – 250 nm).
4) n → π* transition
• An electron from non-bonding orbital is promoted to anti-
bonding π* orbital.
• Compounds containing double bond involving hetero atoms
(C=O, C≡N, N=O) undergo such transitions.
• n → π* transitions require minimum energy and show
absorption at longer wavelength around 300 nm.
5) & 6) σ → π* transition & π → σ* transition .
These electronic transitions are forbidden transitions & are only
Thus, n → π* & π → π* electronic transitions show absorption in
region above 200 nm which is accessible to UV-visible
Defined as any isolated covalently bonded group that shows a
characteristic absorption of Electromagnetic radiation in the UV or
It is a Greek word. Chroma = “color” & phoros = “bearer”
Compound containing chromophore is CHROMOGEN
Eg: C=C, C=O, NO2
TYPES OF CHROMOPHORES
1. INDEPENDENT CHROMOPHORES: If one chromophore is
required to impart colour Eg: Azo group –N=N-, Nitroso group –
2. DEPENDENT CHROMOPHORES: If more than one
chromophore is required to impart colour Eg: Acetone having one
ketone group is colorless whereas diacetyl having two ketone
groups is yellow.
Identification of a chromophore depends on a number of factors as
i) Spectrum consisting of a band near 300 mµ may contain two or
three conjugated units.
ii) Absorption bands near 270-350 mµ with very low intensity εmax
10-100 are due to n → π*transitions of the carbonyl group.
iii) Simple conjugated chromophores such as dienes or α, β-
unsaturated ketones have high εmax values, i.e., 10,000 to 20,000.
Auxochrome: A saturated/ unsaturated group with non
bonding electrons when attached to chromophore altering
both wavelength as well as intensity of absorption.
Eg: OH, NH2, NHR, COOH, CN, Cl etc..
1) Basic/positive auxochromic groups Effective in acid
Eg: OH, OR, NHR etc.
2) Acidic/negative auxochromic groups Effective in alkaline
Eg: NO, CO, CN etc.
Substituents may have any of four effects on a
i. Bathochromic shift (red shift) – a shift to longer
wavelength ; lower energy
ii. Hypsochromic shift (blue shift) – shift to shorter
wavelength ; higher energy
iii. Hyperchromic effect – an increase in intensity.
iv. Hypochromic effect – a decrease in intensity.
Bathochromic shift: - Absorption shifted towards longer
- Change of solvent/ auxochrome -Red shift/ bathochromic shift
- n to * transition for carbonyl compounds experiences
bathochromic shift when the polarity of the solvent is
Hypsochromic shift : - Shift towards shorter wavelength
-(Blue shift) - Change of solvent towards higher polarity or
removal of conjugation
- Aniline – 280 nm (conjugation of pair of electrons of nitrogen
with benzene ring) In acidic solution it will form - NH+ 3 ,
due to the removal of conjugation or removal of lone pair of
electrons, the absorption takes place at lower wavelength
Hyperchromic shift: - Shift due to increase in intensity-
εmax increase - Due to the introduction of auxochrome
Ex: Pyridine - 257 nm and εmax is 2750; 2 – methyl pyridine
262 nm and εmax is 3560
Hypochromic shift: - Inverse of hyperchromic shift
i.e., decrease of intensity - introduction of any group to the
compounds which is going to alter the molecular pattern of
ex: biphenyl absorption is at 250 nm and 19000 εmax -
Whereas 2 –methyl biphenyl has an absorption of 237 nm and
Solvent is the one in which solute is dissolved completely.
Solvent is the important factor for u.v./visible spectroscopy.
Ideal solvent :
It should be cheaper.
It should be easily available.
It should be transparent & less polar Should not possess any
kind of absorption when it is exposed to radiation.
A most suitable solvent which does not absorb the radiation.
Most commonly used solvent is 95% ethanol, it is cheap and
is transparent down to 210m μ .
Commercial ethanol is not used because it is having benzene
which absorbs strongly in u.v.region.
The position as well as the intensity of absorption maximum
get shifted for particular chromophore by change in the
polarity of solvent.
The wavelenth for the non-polar compounds is usually shifted
by change in polarity of solvent.
α, β - unsaturated carbonyl compounds show 2 different
n -π* transition, absorption band moves to shorter
wavelength. Due to H-bonding with solvent molecules occurs
to lesser extent with the carbonyl group in the excited state.
E.g. A max of acetone is 279m μ in hexane, while in water
264m μ . 13
π → π* transition, absorption band moves longer wavelength.
The dipole-dipole interactions with the solvent molecules
lower the energy of the energy of the excited state more than
that of the ground state. Value of A max in ethanol will be
greater than that in hexane.
П* orbital gets more stabilised by H-bonding with the polar
solvent like water & ethanol. Because of greater polarity of
П* orbital. Thus small energy is needed for transition and
absorption shows a red shift.