Engineering Chemistry S Datta

PHOTOCHEMISTRY 549
transitions require much less energy and are found in I.R. region whereas rotational transitions
are found in far I.R. regions.
Singlet and Triplet States
In organic molecules the electrons in the ground state are paired and have opposite
spin. By absorbing energy when electrons are promoted to an orbital of higher energy the
electrons no longer shares an orbital and these two unpaired electrons may either have same
spin which is called a ‘triplet’ or the spins may be paired which is called ‘singlet’. The energy for
the triplet state is lower than the corresponding singlet according to Hund’s Rule. Thus a
different amount of energy i.e., different wavelength is required to promote an electron from
ground to the excited singlet or triplet state. Any type of transitions are not permitted rather
transitions between energy levels are governed by selection rules and several types of transitions
are “forbidden”. Two of the types of forbidden transitions are important:
(a) Spin forbidden transitions—transitions in which spin of an electron change is
forbidden, i.e., transitions between ideal singlet and triplet states are strictly forbidden.
(b) Symmetry-forbidden transitions—n–π* transition in formaldehyde is forbidden since
overlaps of orbitals are forbidden.
Notation for Excited States of Organic Molecules
The excited states of molecules are different from the original ground states. Hence
nomenclature of these excited states separately is important.
One common system for that is to denote the multiplicities, S for singlet and T for triplet.
The ground state is denoted by so and higher excited states as S 1
, S 2
, S 3
, T 1
, T 2
, T 3
etc.
M. Kasha has developed another system of expressing the electronic states in terms of
initial and final orbitals involved in transition. There are four types of molecules:
(i) Saturated molecules with σ Molecular orbitals (MO), e.g. paraffinic hydrocarbons.
(ii) Saturated molecules with σ and non of bonding MO, e.g. CH 3
I, H 2
O.
(iii) Unsaturated molecules with σ and π MOs, e.g., C 2
H 5
, aromatic hydrocarbons, CO 2
etc.
(iv) Unsaturated molecules with σ, π, and n MOs, e.g., aldehydes, ketones, pyridine, other
heterocyclics etc. σ MOs are formed by overlapping of s and p z
orbitals like sp, sp 2 ,
sp 3 etc.
π MOs are formed by overlapping of p x
and p y
or hybridized orbitals to form double or
triple bonds.
n MOs are pure nonbonding orbitals containing nonbonding electrons. Hence the types
of electronic orbitals are σ, π, n, π* and σ*. Photochemists designate by an arrow from lower to
higher energy state transitions as n → π * , π → π * , n → σ * , a → σ * etc.
The n → π* transitions have low probability and hence give weak absorption bands
whereas π → π* transitions are most important and give intense characteristic absorption (for
conjungated hydrocarbons).
Properties of the Excited States
A molecule in the electronically excited state may be a completely different species from
the original one. Since the charge densities are different it shows different chemistry from
that of the ground state molecule, also because it has excess energy but weaker bonds. Other
physical properties like dipole moment, pK values, redox potentials also differ.