Potential energy surfaces, Kinetic isotopic effects - Dynamics of unimolecular reactions – Lindemann-Hinshelwood – Rice Ramsperger Kassel (RRK) theory and Rice Ramsperger Kassel – Marcus (RRKM) theory. Study of fast reactions by laser, relaxation, flash Photolysis and nuclear magnetic resonance methods. LFERs -Hammett equation, Taft equation, separation of polar, resonance and steric effects.

1. Unit –II : Chemical Dynamics
Dr. R. Valliappan
Professor of Chemistry
Annamalai University
Annamalai Nagar 608 002

2. Unit –II : Chemical Dynamics
Potential energy surfaces, Kinetic isotopic effects - Dynamics of
unimolecular reactions – Lindemann-Hinshelwood – Rice
Ramsperger Kassel (RRK) theory and Rice Ramsperger Kassel –
Marcus (RRKM) theory. Study of fast reactions by laser, relaxation,
flash Photolysis and nuclear magnetic resonance methods. LFERs
-Hammett equation, Taft equation, separation of polar, resonance
and steric effects.

3. Chemical dynamics is a field in which scientists study the
rates and mechanisms of chemical reactions. It also
involves the study of how energy is transferred among
molecules as they undergo collisions in gas-phase or
condensed-phase environments.
Therefore, the experimental and theoretical tools used to
probe chemical dynamics must be capable of monitoring
the chemical identity and energy content (i.e., electronic,
vibrational, and rotational state populations) of the
reacting species.
Chemical Dynamics

4. Potential energy surfaces
The activation energies of chemical reactions are most
conveniently considered using the method of potential energy
surfaces. The application of this method involves making a plot of
energy as a function of the various interatomic distances in the
complex that is formed when the reacting species come together.
If the reaction is between two atoms, only one distance, that
between the nuclei, is involved, and one could plot potential energy
against this distance; the result would be a two-dimensional diagram,
and would be a potential energy curve for the diatomic molecule.
A PES is a conceptual tool for aiding the analysis of molecular
geometry and chemical reaction dynamics.

5. Kinetic isotopic effects
The kinetic isotope effect (KIE) is a phenomenon
associated with isotopically substituted molecules
exhibiting different reaction rates.
Isotope effects such as KIEs are invaluable tools in
both physical and biological sciences and are used to aid
in the understanding of reaction kinetics, mechanisms, and
solvent effects.

6. Kinetic Isotope Effects (KIEs) are used to determine reaction
mechanisms by determining rate limiting steps and transition states and are
commonly measured using NMR to detect isotope location or GC/MS to detect
mass changes.
In a KIE experiment an atom is replaced by its isotope and the
change in rate of the reaction is observed.
A very common isotope substitution is when hydrogen is replaced by
deuterium. This is known as a deuterium effect and is expressed by the ratio
kH/kD (as explained above).
Normal KIEs for the deuterium effect are around 1 to 7 or 8. Large
effects are seen because the percentage mass change between hydrogen and
deuterium is great. Heavy atom isotope effects involve the substitution of
carbon, oxygen, nitrogen, sulfur, and bromine, with effects that are much
smaller and are usually between 1.02 and 1.10.
The difference in KIE magnitude is directly related to the percentage
change in mass. Large effects are seen when hydrogen is replaced with
deuterium because the percentage mass change is very large (mass is being

7. UNIMOLECULAR REACTIONS
A unimolecular reaction is one in which the activated complex is
formed from single a reactant molecule. Unimolecular reactions are
of the first order under certain circumstances, but they become of
second order at low pressures.
For unimolecular (elementary processes) reactions, the rate
is proportional to the concentration of the reacting gas, i.e., the
number of molecules in a given volume, it seems unlikely that the
chemical reaction could be a direct consequence of collisions. It
appeared reasonable to identify A in the rate equation with the
vibration frequency (v)of one of the bonds of the reacting molecule
and write,
to permit the vibration to become so vigorous as to break the bond

8. LINDEMANN'S THEORY
According to this theory, reactant molecules are supposed to be
activated by collisions with one another, and at any given time a small fraction of
the molecules should have sufficient energy to pass into the final stage without
having to receive any additional energy.
In the complex reaction, the molecules which form the final product undergoes
many individual processes. The reacting reactants when making collisions with
each other minimum activation energy for products formation.

9. HINSHELWOOD'S TREATMENT
● “every energized molecule will not enter into product formation but will go into
activated molecule.
It was pointed out by Hinshelwood that this equation is only applicable to
a molecule having one degree of vibrational freedom.
A molecule having more degrees of vibrational freedom has a greater
probability of acquiring the energy E, since this energy may be now distributed
among all the degree of freedom in the molecule.

10. THE TREATMENT OF KASSEL, RICE AND RAMSPERGER
The idea inherent in the theories of Kassel, Rice and Ramsperger is that
the energized molecule becomes an activated complex when the critical
amount of energy E* finds itself in one particular normal mode of
vibration. On every vibration here is a complete reshuffling of the quanta
of energy between the normal modes. It follows from this point of view
that the more energy E that resides in the energized molecule the greater
is the chance that an amount E* (necessarily not greater than E, since
otherwise the molecule is not energized) will find itself in the particular
normal mode in question. The rate of breakdown of the energized
molecule thus increases with the energy residing in it

11. LINEAR FREE ENERGY RELATIONSHIP (LFER) CORRELATION ANALYSIS

12. where ∆G≠
is the standard free energy of activation. A relationship between
logarithms of K or k (at constant temeprature) is thus
essentially a relationship between free energies.

13. The Hammett Equation
Chemists have, for a long time, attempted to correlate reactivities of organic
compounds with the help of several empirical equations involving rate or equillibrium
constants of meta or para substituted benzene derivatives. The equation developed by
Hammett for this purpose is called Hammett equation which takes the form:
log k = log ko + ρσ … (3)
log K = log Ko + ρσ … (4)
where k or K is the rate or equillibrium constant, respectively, for a side-chain reaction of
a meta- or para substituted benzene derivative. The symbol ko or Kodenotes the
statistical quantity approximating to k or K for the unsubstituted parent compound. The
substituent constant, σ, measures the polar effect (relative to hydrogen) of a meta or
para- substituent and is, in principle, independent of the nature of the reaction. The
reaction constant, ρ, depends on the nature of the reaction (including conditions such as
solvent and temperature) and measures the susceptibility of the reaction to polar effects.
The ionization of benzoic acids in water at 25°C is chosen as a standard
process, for which ρ is defined as 1.00. The value of σ for a given substituent is log(Ka /
Kao), where Ka is the ionization constant of the substitued benzoic acid and ka is that of

14. THE TAFT EQUATION

15. Fast Reactions

16. Flow Systems

17. This method involves the measurement of relaxation time of
the reaction; this is the time that it takes for a reaction to cover
a certain fraction of its path towards equilibrium. In this method,
the reaction is first allowed to go to equilibrium. It is then
disturbed in way, so that it is no longer at equilibrium. The
speed with which it approaches its new equilibrium is then
followed. Usually using special electronic techniques, and then
the rate constants are calculated .

18. NMR TECHNIQUE IN FAST REACTIONS

19. FLASH PHOTOLYSIS