Engineering Chemistry S Datta

550 ENGINEERING CHEMISTRY
A transition of electrons from the bonding to the antibonding orbital causes a triple
bond to become double, a double bond to become single and a single bond to disrupt. It also
leads to increase in bond distances by about 15% and also large changes in bond angles.
Formaldehyde molecule has a planar geometry in the ground state. On excitation an
electron on O-atom is transferred to antibonding π MO and hence the molecule can twist along
the C—O bond. Similarly for conjugated systems like ethylene or butadiene, free rotation in
the excited state leads to cis-trans isomerization. For e.g. acetylene, which is linear in ground
state has a trans geometry with sp 2 carbons in (π, π*) state.
H C C
H
CO 2
, CS 2
, HCN are linear in their ground state but bent in excited states, whereas
radicals like NH 2
, HCO and NO 2
molecules are bent in their ground state but linear in excited
states.
Excited phenol is more acidic and also ortho-para directing towards substitution in
benzene ring. The protolytic equilibrium constant pK a
for the reaction is 10 and 5.7 in the
ground and excited states. In contrast to this, aromatic carboxylic acids exhibit a decrease in
acidity in excited state relative to ground state. In general, phenols, thiols and aromatic amines
become stronger acids on excitation whereas carboxylic acids, aldehydes, ketones and
heterocyclics become stronger bases.
The dyes like methylene blue and thionine show striking difference in redox potentials
between ground and excited states.
In the excited state, the dipole moment not only has different magnitude, but different
direction also. Photochemical behaviour of a molecule varies with the change in solvent for the
above reason.
Highlights:
Laws of Photochemistry:
• Grotthurs-Drapper Law (First law):
“Only that light which is absorbed by a system can cause chemical change”.
• Stark-Einstein Law (2nd law):
“One quantum of light is absorbed per molecule of absorbing and reacting substance
that disappears”.
• Wigner’s spin conservation rule:
In any allowed electronic energy transfer process, the overall spin angular
momentum of the system should not change.
Photolysis
The direct dissociation of a molecule on absorption of a quantum of radiational energy
becomes probable when the energy absorbed is equal to or more than the bond dissociation
energy and this dissociation from an excited state is called photolysis. Three situations can
lead to cleavage of a molecule.
(1) The promotion may bring the molecule to a very high vibrational level which lies
above the right hand portion of the curve E 2
in Fig. 25.2 and hence the molecule cleaves at its
first vibration.