CSO 202A Atoms Molecules and Photons - IIT Kanpurhome.iitk.ac.in/~madhavr/cso202A.pdf · CSO 202A Atoms Molecules and Photons ... transformed the N-cycle Nitrogen compounds are also

CSO 202A

Atoms Molecules and Photons

Instructors: Dr. Manabendra Chandra and Dr. Madhav Ranganathan

Lec: M,W,Th: 12:00-12:50 L3

Piazza sign-up link: piazza.com/iitk.ac.in/secondsemester2017/cso202Piazza class link: piazza.com/iitk.ac.in/secondsemester2017/cso202/home

Tutors: Sourav Karmakar ([email protected]) Navin Subba ([email protected]) Aqeel Malani ([email protected])

Tut: Friday: 17:00-17:50 T207-209

CSO 202A : Atoms Molecules and Photons

Grading scheme:

Quiz-1 (45 mins): 15% (module 1) 3rd Feb. 2017Quiz-2 (45 mins): 15% (module 3) 31 st Mar. 2017

MidSem (2 hrs): 35% (module 1 + module 2) EndSem (2 hrs) : 35% (module 3 + module 4)


2007: Gerhard Ertl "for his studies of chemical processes on solid surfaces"2013: Martin Karplus, Michael Levitt and Arieh Warshel "for the development of multiscale models for complex chemical systems".2014: Eric Betzig, Stefan W. Hell and William E. Moerner "for the development of super-resolved fluorescence microscopy".

Path-breaking research works in the field of chemical reaction dynamics

Module 2 Module 1

Module 3Module 4

CSO 202A


Course plan

Module 1: Chemical Reaction Dynamics with Molecular BeamsModule 2: Ultrafast Chemical Reaction Dynamics with Ultrashort- Pulsed LasersModule 3: Catalytic Reactions on Solid SurfacesModule 4: Chemistry with Supercomputers


Module 1 :

Chemical Reaction Dynamics with Molecular Beams


This way of writing down chemical reactions, using arrows and symbols, is ‘incomplete’

Consider the reaction:

Do rates depend on internal quantum states of the reactants? What are the internal quantum states of the products? What is the dependence of chemical reactivity on molecular

orientation? Dependence on the impact of collision? What are the nature of reaction intermediates and their

subsequent decay dynamics in case of complex polyatomic molecular reactants?


Answers can be found by doing experiments using crossed molecular beam technique

2Na + O2 → Na2O + O


Dudley R. Herschbach

Yuan T Lee John C Polanyi

The Nobel Prize in Chemistry 1986

"for their contributions concerning the dynamics of chemical elementary processes"


Module 2 :

Ultrafast Chemical Reaction Dynamics with Ultrashort- Pulsed Lasers


Consider a chemical transformation

CH3I + Na → CH3---I----Na → CH3 + NaI

Transition state

In any chemical reaction the motions of the electrons and nuclei of atoms determine how the molecules interact, and those interactions in turn create the forces that govern the reaction's dynamics.

If one can determine how molecular motions change during the critical transition phase, we can understand how new chemical bonds form and old ones disappear.


QuestionHow can one study transition state(s) in real time?

Answer

Need ultrafast probe and detection technique


Consider a simple dissociation reaction

ICN → I + CN

J. Chem. Phys., 1987, 87(4), 2395


J. Chem. Phys., 1987, 87(4), 2395


Ahmed Zewail

The Nobel Prize in Chemistry 1999

“for his studies of the transition states of chemical reactions using femtosecond spectroscopy"


Molecular structures for a reaction in progress involving two molecules (bimolecular). The diatomic iodine molecule (I2, top) is split by exchange of an electron with the ring molecule benzene (C6H6).


Molecular structures for a reaction in progress involving two molecules (bimolecular).


Module 3:

Catalytic Reactions on Solid Surfaces


Fritz-Haber synthesis of Ammoia

Consider the reactionN2 + 3 H2 ====> 2NH3

● Simple looking reaction, Exothermic reaction

● Requires 700 K, 200 Bar pressure

● Negligible rate without a catalyst – Fe

● Fritz Haber: Nobel Prize 1918 (Discovery)

● Carl Bosch : Nobel Prize 1931 (Commercialized the Haber process)● Elucidation of the mechanism : Gerhard Ertl: Nobel Prize 2007

Why is it such a big deal ?


Greatest discovery of the last century ??● Ammonia production is the first stage of fertilizer preparation and the most difficult

Fritz-Haber process replicated that

of N-fixing bacteria and transformed the N-cycle Nitrogen compounds are also used in explosives – World War II


Nitrogen cycle

Chile Saltpeter NaNO3 : Natural source of Nitrates


Gerhard Ertl's Mechanism

The Physical Chemistry elements involved - Gas laws andtransport processes, reaction thermodynamics, kinetics, catalysis, surface chemistry, experiments, Ultra-High vaccuum


Why is this reaction so difficult ?N2 + 3 H2 ====> 2NH3

● N – N triple bond● One of the strongest known bonds● The inertness of N2 is well known● Liquid Nitrogen is frequently used to cool● Nitrogen is easily available in the atmosphere but fixing it artificially is very hard● Fe surface provides an easier way to break N-N bond N2 (g) ===> N2 (ads) ====> 2 N (ads)

Nobel Prize in Chemistry 2007

Gerhard Ertl

for his studies of chemical processes on solid surfaces


● Consider the H2 molecule containing 2 electrons and 2 protons● How does one calculate the equlibrium bond length of H2 ?● Use the Born-Oppenheimer Approximation and fix protons andwrite Schrodinger equation

● Solve for lowest energy state and obtain E0(R)● Vary R and find Req corresponding to minimum E0

Module 4: Chemistry with Supercomputers


Chemistry with Supercomputers

But the above equation cannot be solved exactly !!!

● Numerical methods implemented on a computer● Req = 74.1 pm Bond dissociation energy = 4.52 eV

For large molecules, several bond lengths, bond angles have to be optimized !!!

● Need for advanced mathematical and numerical techniques



Walter Kohn John A Pople

The Nobel Prize in Chemistry 1998 was divided equally between Walter Kohn "for his development of the density-functional theory" and John A. Pople "for his development of computational methods in quantum chemistry"


OK – That’s the structure of a single molecule, but what about dynamics ? What about behavior of a large number of molecules ?

One popular method is Molecular Dynamics Simulations using effective potential energies and classical dynamics

B. Alder and C. Wainwright (1955) and Aneesur Rahman (1959)



The Nobel Prize in Chemistry 2013 was awarded jointly to Martin Karplus, Michael Levitt and Arieh Warshel "for the development of multiscale models for complex chemical systems".

Documents

CSO 202A Atoms Molecules and Photons - IIT Kanpurhome.iitk.ac.in/~madhavr/cso202A.pdf · CSO 202A Atoms Molecules and Photons ... transformed the N-cycle Nitrogen compounds are also