BITS PILANI, HYD_Lecture 26_TC_20.10.23.pptx

Lecture slide

1. NMR Spectroscopy General Chemistry CHEM F111 Dr. Tanmay Chatterjee Lecture-26 (20.10.2023)

2. N uclear M agnetic R esonance; nuclear spin properties Nuclear spin quantum number [ I > 0 ; for odd number of either protons or neutrons ( half integer ) or both ( integer )] Spin states and their energies; Em I = - γ N ћ Bm I ( γ N = e/2m p ) or Em I = - g N μ N Bm I ( μ N = γ N ћ )  E = E β – E α = g N μ N B = γ N ћ B = h  (  = γ B /2  ) Selection rule for transitions:  m I =  1 Larmor precession frequency (  = γ B/2  ) and nuclear magnetic resonance Population and Signal Strength; N α - N β = N γ N ħB /2k B T Recap

3. (1) a magnet capable of producing a very strong static and homogeneous field (2) a stable radiofrequency generator (3) a radiofrequency receiver (4) a detector NMR INSTRUMENTATION Recap

4. Schematic diagram of a Fourier transform NMR spectrometer with a superconducting magnet Recap

5. 5 The Technique

6. Modern NMR Instruments

7. https://stanfordhealthcare.org/medical-tests/p/pet-mri-scan.html MRI ( M agnetic R esonance I maging) scanning MRI is a medical application of nuclear magnetic resonance (NMR) which can also be used for imaging in other NMR applications, such as NMR spectroscopy. Most MRI scans essentially map the location of water and fat in the body.

8. The chemist is concerned with molecules, in which the nuclei are always surrounded by electrons and other atoms. I n diamagnetic molecules the effective magnetic field B eff at the nucleus is always less than the applied field B , i . e. the nuclei are shielded. B eff = B -  B = (1- ) B ,  is shielding constant The resonance condition is now,  = γ B eff / 2  = γ (1- ) B / 2  Concept of “Chemical Shift” The precessional frequency when placed in the electronic surrounding would not be the same for the nucleus as it is in a bare nucleus .

9. Shielding constant  =  local +  neighbour +  solvent  local is due to the shielding from the electrons immediately surrounding the nucleus.  neighbour is due to the shielding from the neighbouring groups.  solvent is the contribution from solvent molecules.

10. Why TMS ? Many (12) equiv H-atom, very less amount gives large signal (ii) H-atoms are highly shielded (seldom interferes with other signals) (iii) Relatively inert (iv) Volatile ( bp 27 o C ), easily removed from sample. A reference compound, tetramethylsilane (TMS), Si(CH 3 ) 4 is placed along with the test molecule in the sample. The changes of the resonance frequencies in the test molecule with respect to the reference compound (the differences from the reference) is considered as chemical shifts values of the test molecule. Reference Compound

11. Introduction to δ scale 90 MHz 1 H NMR spectrum of a mixture of CHBr 3 , CH 2 Br 2 , CH 3 Br and TMS δ (CHBr 3 ) > δ (CH 2 Br 2 ) > δ (CH 3 Br) > δ (TMS) Chemical shifts are reported on the δ scale δ = ( ν – ν o ) x 10 6 ppm ν o ν = ν o + ( ν o /10 6 ) δ ν o is the resonance frequency of the standard δ > 0 deshielded w.r.to TMS protons δ < 0 shielded w.r.to TMS protons

12. The NMR scale ( δ , ppm ) We use a relative scale, and refer all signals in the spectrum to the signal of a particular compound. The good thing is since it is a relative scale, the δ in a 100 MHz magnet (2.35 T) is the same as that obtained for in a 600 MHz magnet (14.1 T) . The chemical shift depends on: • the atom type (NH, aliphatic CH, aromatic CH, ...) • the chemical (spatial) environment Advantages: • more compact annotations • independent on the spectrometer field In practice, the 1 H chemical shifts are in the range 0-12 ppm

13. chemical shift = d = shift in Hz spectrometer frequency in MHz = ppm parts per million Chemical shift is the difference in frequency between the sample and the standard divided by the operation frequency of the instrument The CHEMICAL SHIFT 13 Thus, the chemical shift in  unit for the protons of benzene is the same whether a 60 MHz or 300 MHz instrument is used

14. Field dependence of an NMR spectra 10 MHz 60 MHz 200 MHz 300 MHz In δ scale all the spectra are same

15. TYPES OF HYDROGENS COME IN SELECTED AREAS OF THE NMR CHART 4 2 3 6 7 9 10 12 MOST SPECTRA CAN BE INTERPRETED WITH A KNOWLEDGE OF WHAT IS SHOWN HERE Acid COOH Aldehyde CHO Benzene CH Alkene =C-H C-H where C is attached to an electronegative atom X-C-H CH on C next to pi bonds X=C-C-H Aliphatic C-H

16. NMR Correlation Chart 12 11 10 9 8 7 6 5 4 3 2 1 -OH -NH TMS CHCl 3 , CH 2 F CH 2 Cl CH 2 Br CH 2 I CH 2 O CH 2 NO 2 CH 2 Ar CH 2 NR 2 CH 2 S C C-H C=C-CH 2 CH 2 -C- O C-CH-C C C-CH 2 -C C-CH 3 RCOOH RCHO C=C H d (ppm) DOWNFIELD UPFIELD Deshielded Shielded Ranges can be defined for different general types of protons.

17. 17 Shielded and deshielded nuclei

18. deshielding moves proton resonance to high frequency Chlorine “ deshields ” the proton, that is, it takes the valence electron density away from carbon, which in turn draws more density from hydrogen and thereby deshielding the proton. electronegative element C H Cl Deshielding by an electronegative element highly shielded protons appear at up field “ deshielded ” protons appear at down field NMR CHART d - d+ d - d+

19. Electronegativity Dependence of Chemical Shift Compound CH 3 X Element X EN of X Chemical shift δ CH 3 F CH 3 OH CH 3 Cl CH 3 Br CH 3 I CH 4 (CH 3 ) 4 Si F O Cl Br I H Si 4.0 3.5 3.1 2.8 2.5 2.1 1.8 4.26 3.40 3.05 2.68 2.16 0.23 0 Dependence of the Chemical Shift of CH 3 X on the Element X deshielding increases with the electronegativity of atom X TMS Most deshielded

20. Substitution Effects on Chemical Shift C H Cl 3 C H 2 Cl 2 C H 3 Cl 7.27 5.30 3.05 ppm -C H 2 -Br -C H 2 -CH 2 Br -C H 2 -CH 2 CH 2 Br 3.30 1.69 1.25 ppm most deshielded most deshielded The effect decreases with increasing distance. The effect increases with greater numbers of electronegative atoms.

21. 21 Protons in a molecule Lower frequency higher frequency downfield upfield

22. In ethanol we have 3 different Hydrogens with varying chemical shifts, as shown we get three distinct signals for these three kinds of protons in the NMR spectrum

23. Counting Hydrogen environments – One molecule can contain many hydrogen environments. So for each different hydrogen environment, we will see a different signal in the NMR spectrum. ‘Two’ H environments, so 2 signals in NMR spectrum. Signals from different kinds of protons typically appear at different chemical shifts. Number of peaks

24. Hydrogen’s are chemically equivalent or homotopic if replacing each one in turn by the same group would lead to an identical compound. Homotopic hydrogen’s have same chemical shift. Chemically Equivalent or Homotopic Hydrogens

25. If replacement of each of two hydrogens by some group leads to enantiomers, those hydrogen atoms are said to be enantiotopic . In the absence of a chiral influence (chiral solvent) , enantiotopic hydrogens have the same chemical shift and appear as the same (single) signal. Enantiotopic Hydrogens

26. If replacement of each of two hydrogens by some group leads to diastereomers , the hydrogens are said to be diastereotopic . Diastereotopic hydrogens have different chemical shifts and will give different signals. Diastereotopic Hydrogens

27. Equivalent hydrogens: Hydrogens that have the same chemical environment. A molecule with 1 set of equivalent hydrogens gives 1 NMR signal. H 3 C C C C H 3 H 3 C C H 3 C H 3 C C H 3 C l C H 2 C H 2 C l Propanone (Acetone) 1,2-Dichloro- ethane Cyclopentane 2,3-Dimethyl-2-butene O Equivalent (homotopic) Hydrogens A molecule with 2 or more sets of equivalent hydrogens gives a different NMR signal for each set. C H 3 C H C l C l C l C C C H 3 H H O C y c l o p e n t - a n o n e ( 2 s i g n a l s ) 1 , 1 - D i c h l o r o - e t h a n e ( 2 s i g n a l s ) ( Z ) - 1 - C h l o r o - p r o p e n e ( 3 s i g n a l s ) C y c l o h e x e n e ( 3 s i g n a l s )

28. “Number of Different Types of Equivalent Protons” CH 3 O O O O 2 4 6 1 3 5 4 How many signals in 1 H NMR spectrum? 9