13-1 Nuclear Magnetic Resonance Spectroscopy Part-2 Prepared By Dr. Khalid Ahmad Shadid Islamic University in Madinah Department of Chemistry

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13-3 Chemical Shifts  Chemical shift: The relative energy of resonance of a nucleus resulting from its local environment  NMR spectra show applied field strength increasing from left to right. Left part is downfield and right part is upfield  Nuclei that absorb on upfield side are strongly shielded  Chart calibrated versus a reference TMS, set as 0.00

13-4 CHEMICAL SHIFT الإزاحة الكيميائية تستعمل لتعيين الترددات التي يحدث عندها امتصاص اشعة الراديو. نقوم بقياس الفرق بين التردد المقابل لرنيين البروتون تحت الدراسة والتردد المقابل لإحداث رنيين بروتون في مرجع التترامثل سايلين

13-5  1. Electronegativity Proton signals range from  0 to  12 Different types of proton will occur at different chemical shifts The magnetic field experienced by a proton is influenced by various structural factors: CompoundCH 4 CH 3 ClCH 2 Cl 2 CHCl 3  / ppm Factors Influencing Chemical Shifts

13-6  2. Hybridization of adjacent atoms. Factors Influencing Chemical Shifts

13-7  3. Hydrogen Bonding Effects  Protons involved in H-bonding (-OH or -NH) are observed over a large range of chemical shift values (  ppm) since H-bonding effects are solvation, acidity, concentration and temperature dependent Factors Influencing Chemical Shifts

13-8 Factors Influencing Chemical Shifts  Magnetic anisotropy: "non-uniform magnetic field“  Electrons in  systems (e.g. aromatics, alkenes, alkynes, carbonyls etc.) interact with the B 0 which induces a magnetic field that causes the anisotropy  As a result, the nearby protons will experience 3 fields: the applied field, the shielding field of the valence electrons and the field due to the  system  Depending on the position of the proton in this third field, it can be either shielded (smaller  ) or deshielded (larger  ) 4. Magnetic anisotropy Effect:

13-9  A carbon-carbon triple bond shields an acetylenic hydrogen and shifts its signal to lower frequency (to the right) to a smaller  value.  A carbon-carbon double bond deshields vinylic hydrogens and shifts their signal to higher frequency (to the left) to a larger  value. Factors Influencing Chemical Shifts 4. Magnetic anisotropy Effect: or Diamagnetic effects of  bonds

13-10  Figure 13.9 A magnetic field induced in the  bonds of a carbon-carbon triple bond shields an acetylenic hydrogen and shifts its signal upfield: The  electrons in a triple bond circulate around the bond axis to produce a magnetic field directly opposing the applied magnetic field Factors Influencing Chemical Shifts

13-11  Figure A magnetic field induced in the  bond of a carbon-carbon double bond deshields vinylic hydrogens and shifts their signal downfield. Factors Influencing Chemical Shifts

13-12  The magnetic field B 0 induced by circulation of the  electrons (ring Current) in an aromatic ring deshields the hydrogens of the aromatic ring and shifts their signal downfield. B0B0 B0B0  In aromatic rings: The "ring current" generates a local magnetic field which opposes B 0  However, on the periphery of the ring, the flux lines are in the direction of B 0  Thus, protons attached to the aromatic ring "feel" a larger magnetic field than protons elsewhere in the molecule  Aromatic protons will exhibit a downfield shift (7 - 8 ppm) Factors Influencing Chemical Shifts

13-13 Chemical Shift - 1 H-NMR  Figure 13.8 Average ranges of chemical shifts of representative types of hydrogens.

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13-15 ChemicalShifts 1 H-NMR

Integration of 1 H NMR Absorptions: Proton Counting  The relative intensity of a signal (integrated area) is proportional to the number of protons causing the signal  For narrow peaks, the heights are the same as the areas and can be measured with a ruler  Example: in methyl 2,2-dimethylpropanoate integral ratio is 3:9 or 1:3

13-17  Integration is used to deduce the structure. The area under the peaks gives a ratio of the number of H for each signal  Measure the height of each trace and derive a whole number ratio Integration of 1 H NMR Absorptions: Proton Counting

13-18 Good Luck