NMR Spectroscopy

1. NMR Spectroscopy Dr. PALVE ANIL M. RAYAT SHIKSHAN SANSTHA’S M.P.A.S.C.COLLEGE,PANVEL,NAVI MUMBAI. Email=palve_anil@yahoo.com

2. !!! WHAT TO DO !!!! HOW TO THINK?????

3. A.M.Palve

4. Electromagnetic radiation consists of perpendicular oscillating electric and magnetic fields   = wavelength distance

5. (The energy of photon) E = h •  = c/ • E = hc/ E = hύ • h = Planck's Constant 6.62 10-27 erg - sec • c = the speed of light in vacuum c = 3.00 x 108 m/s • The wavelength of light determines how it interacts with matter. • We use these interactions as a probe to obtain structural information about samples.

6. Matter/Energy Interactions • What happens when a sample absorbs IR energy? stretching and bending of bonds (typically covalent bonds) Evibration increases momentarily IR -O-H -O —H (3500 cm-1) opposed to field • What happens when a sample absorbs Rf energy (radio frequencies)in an NMR experiment? nuclei previously aligned in a strong external magnetic field are “flipped” against the field Rf (100’s MHz) aligned with field {B0 = external magnetic field}  B0

7. Felix Bloch Stanford Univ Edward Purcell Harvard Univ Ernst Wüthrich ETHSwitzerland Paul Lauterbur Illinois Univ, USA

8. Nuclear Magnetic Resonance Spectroscopy • Nuclei aligned in a strong magnetic field can be selectively detected by subjecting a sample to radio frequency energies. • 1H-NMR Spectroscopy  detailed structural evidence for organic samples indicated by the number and types of hydrogens detected. 1. NMR theory - nuclear spin, electron shielding 2. acquisition of data - the NMR spectrometer 3. interpretation of NMR spectral data {major emphasis!}

9. Properties of Nuclei

10. Spectrometer 300 MHz

11. 400 MHz NMR Spectrometer 400MHzAvanceSystem Unix computer electronic controls super- conducting magnet

12. NMR Sample Position (prior to release into probe) NMR sample positioned at top of probe Liquid Nitrogen -196°C (77.4K) Liquid Helium -269°C (4.2K) Superconducting magnets require continuous cooling.

14. Chemical Shift

16. Less energy to flip nucleus More energy to flip nucleus chemical shift d, ppm

17. Chemical Shift

18. ChemicalEquivalence Protons in chemically identical environments within a molecule Often exhibit same chemical shift

19. Chemical Nonequivalence Sets of protons in chemically different environments within a molecule Give rise to different chemical shifts

20. Basic Information from 1H NMR Spectra • Number of Signals – No. of different types of protons. • Chemical Shifts – Type of environment for protons. • Integration – Ratios of numbers of protons. • Signal shape – Dynamics of proton environments. • Signal splitting – No. and geometry of nearby protons.

21. Ideal Solvent • No protons deuterated solvents used • Inert solvent • Low boiling • Inexpensive • Nonviscous

22. Chemical Shifts of Deuterated Solvents

23. Internal standard TMS (tetramethylsilane) • Protons of methyl group more shielded • Gives single, sharp absorption peak • Chemically inert solvent • Symmetrical • Volatile (B.P. 27°C) • Soluble in most organic solvents

24. Magnetic Anisotropy

26. 1H-NMR - Correlation Diagram Chloroform(CHCl3) = 7.26 d  Benzene(C6H6) = 7.32 d  Methylene chloride (CH2Cl2) = 5.30 d  Acetone(CH3COCH3) = 2.16 d  Toluene(C6H6CH3) = 2.32 d TMS 0.00 d common singlets type of attachment Protons Attached to sp2 Carbon Protons Attached to sp3 Carbon RCH2OR RCH2NR2 ArCH3 RCOCH3 R3CHmethine R2CH2methylene RCH3methyl Vinyl R2C=CHR RCH2X X = F, Cl, Br, I RCHO Aromatic ArH Delta Scale (d) 10 9 8 7 6 5 4 3 2 1 0 even more deshielded deshielded shielded due to: due to: ring currents (from p bonds) inductive effect (through s bonds)