CSO 202A. Atoms Molecules and Photons

Transcription

1 CSO 202A Atoms Molecules and Photons Piazza sign-up link: piazza.com/iitk.ac.in/secondsemester2017/cso202 Piazza class link: piazza.com/iitk.ac.in/secondsemester2017/cso202/home Instructors: Dr. Manabendra Chandra and Dr. Madhav Ranganathan Lec: M,W,Th: 12:00-12:50 L3

2 Tutors: Sourav Karmakar Navin Subba Aqeel Malani Tut: Friday: 17:00-17:50 T Grading scheme: Quiz-1 (45 mins): 15% (module 1) 3rd Feb Quiz-2 (45 mins): 15% (module 3) 31 st Mar MidSem (2 hrs): 35% (module 1 + module 2) EndSem (2 hrs) : 35% (module 3 + module 4)

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

4 Course plan Module 2 Module 1 CSO 202A Module 3 Module 4 Module 1: Chemical Reaction Dynamics with Molecular Beams Module 2: Ultrafast Chemical Reaction Dynamics with Ultrashort- Pulsed Lasers Module 3: Catalytic Reactions on Solid Surfaces Module 4: Chemistry with Supercomputers

5 Module 1 : Chemical Reaction Dynamics with Molecular Beams

6 Consider the reaction: This way of writing down chemical reactions, using arrows and symbols, is incomplete 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?

7 Answers can be found by doing experiments using crossed molecular beam technique 2Na + O2 Na2O + O

8 The Nobel Prize in Chemistry 1986 Dudley R. Herschbach Yuan T Lee John C Polanyi "for their contributions concerning the dynamics of chemical elementary processes"

9 Module 2 : Ultrafast Chemical Reaction Dynamics with Ultrashort- Pulsed Lasers

10 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.

11 Question How can one study transition state(s) in real time? Answer Need ultrafast probe and detection technique

12 Consider a simple dissociation reaction ICN I + CN J. Chem. Phys., 1987, 87(4), 2395

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

14 The Nobel Prize in Chemistry 1999 Ahmed Zewail for his studies of the transition states of chemical reactions using femtosecond spectroscopy"

15 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).

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

17 Module 3: Catalytic Reactions on Solid Surfaces

18 Fritz-Haber synthesis of Ammoia Consider the reaction N2 + 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?

19 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

20 Nitrogen cycle Chile Saltpeter NaNO3 : Natural source of Nitrates

21 Gerhard Ertl's Mechanism The Physical Chemistry elements involved - Gas laws and transport processes, reaction thermodynamics, kinetics, catalysis, surface chemistry, experiments, Ultra-High vaccuum

22 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)

23 Nobel Prize in Chemistry 2007 Gerhard Ertl for his studies of chemical processes on solid surfaces

24 Module 4: Chemistry with Supercomputers 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 and write Schrodinger equation Solve for lowest energy state and obtain E0(R) Vary R and find Req corresponding to minimum E0

25 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

26 Nobel Prize in Chemistry 1998 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"

27 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)

28 Nobel Prize in Chemistry 2013 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".