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Factors affecting chemical shift
- 1. FACTORS AFFECTING CHEMICAL SHIFT Dr.V. S. Tambe Professor PES Modern College of Pharmacy (For Ladies), Moshi
- 2. All different typesof protons in a molecule have a different amounts of shielding (DIFFERENT ELECTRON DENSITIES). They all respond differently to the applied magnetic field and appear at different places in the spectrum. The position of signal is dependent of the applied magnetic field and chemical environment of proton. Hence, each instrument with different magnetic field would give different positions for the same proton. So as to remove this ambiguity, chemical shift term is used. This gives a number independent of the instrument used. A particular proton in a given molecule will always have the same chemical shift (constant value). Hence, position of signal will be only dependent of the chemical environment of proton.
- 3. UPFIELD Highly shielded protons appearhere. DOWNFIELD Less shielded protons appear here. SPECTRUM TMS has assigned a value of Zero ppm. But there are protons more shielded than TMS and they are assigned negative value Chemical Shift (ppm) Rather than measure the exact resonance position of a peak, we measure how far downfield it is shifted from TMS. The variation of NMR frequencies of the same kind of nucleus,due to variations in the electron distribution is called chemical shift.
- 4. hn = Bo g 2p constants frequency field strength Strongermagnetic fields (Bo) cause the instrument to operate at higher frequencies (n). NMR Field Strength 1H Operating Frequency 60 Mhz 100 MHz 300 MHz7.05 T 2.35 T 1.41 T n = ( K) Bo
- 5. chemical shift = d = shiftin Hz spectrometer frequency in MHz = ppm This division gives a number independent of the instrument used. parts per million The shifts from TMS in Hz are bigger in higher field instruments (300 MHz, 500 MHz) than they are in the lower field instruments (100 MHz, 60 MHz). We can adjust the shift to a field-independent value, the “chemical shift” in the following way: A particular proton in a given molecule will always come at the same chemical shift (constant value). 6
- 6. It is expressedin δ (delta, PPM) as mentioned and τ (tau) scale. τ = 10 - δ
- 7. 01234567 ppm Hz Equivalent of1 ppm 1H Operating Frequency 60 MHz 60 Hz 100 MHz 100 Hz 300 MHz 300 Hz Each ppm unit represents either a 1 ppm change in Bo (magnetic field strength, Tesla) or a 1 ppm change in the precessional frequency (MHz). Value of ppm is dependent on the magnetic field strength
- 8. NMR Correlation Chart 1211 10 9 8 7 6 5 4 3 2 1 0 -OH -NH CH2F CH2Cl CH2Br CH2I CH2O CH2NO2 CH2Ar CH2NR2 CH2S C C-H C=C-CH2 CH2- C- O C-CH-C C C-CH2-C C-CH3 RCOOH RCHO C=C H TMS HCHCl3 , d (ppm) DOWNFIELD UPFIELD DESHIELDED SHIELDED
- 9. aliphatic C-H CH on C nextto pi bonds C-H where C is attached to an electronega-tive atom alkene =C-H benzene CH aldehyde CHO acid COOH 234691012 0 X-C-H X=C-C-H Chemical shift scale
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- 11. INDUCTIVE EFFECT Mesomeric Effect ANISOTROPICEFFECT HYDROGEN BONDINGVANDER WAAL DESHIELDING HYBRIDIZATION
- 12. Compound CH3X CH3FCH3OH CH3Cl CH3Br CH3I CH4 CH3)4Si Element X F O Cl Br I H Si Electronegativity of X 4.0 3.5 3.1 2.8 2.5 2.1 1.8 Chemical shift δ 4.26 3.40 3.05 2.68 2.16 0.23 0 INDUCTIVEEFFECT: •It has involvement of sigma electrons • Caused by electron withdrawing or electron donating groups attached to the 1H •Electron withdrawing groups such as NH,COO-,Cl etc. Shift the 1H chemical shift to downfield regions. Cl←C←H Chlorine “deshields” the proton, it takes electron density away from carbon, which in turn takes more density away from the proton. EXAMPLES: Dependence of the chemical shift of CH3X on the element X
- 13. Substitution Effects onChemical Shift CHCl3 CH2Cl2 CH3Cl 7.27 5.30 3.05 ppm -CH2-Br -CH2-CH2Br -CH2-CH2CH2Br 3.30 1.69 1.25 ppm most deshielded most deshielded The effect decreases with incresing distance. The effect increases with greater numbers of electronegative atoms.
- 14. Silicon is electropositive.Hence, proton in TMS is highly shielded and come to the resonance upfield. Its value is fixed to zero and the relative positions of all protons is measured. TETRA METHYL SILANE (TMS) •Added to the sample, so it is called “internal standard” •TMS has 12 equivalent proton and gives an intense single unsplit signal. •Chemically inert •Low boiling point, So it can be easily removed by evaporation after the spectrum has been recorded. TMS is the common reference compound in NMR,it is set at δ=0 ppm •Its miscible in most organic solvents
- 15. More deshielded protondue to higher electronegativity of Carbon compared to hydrogen
- 16. HYBRIDIZATION: Hybridization of thecarbon to which the proton is attached influences the electron density at 1H attached. As the proportion of ‘s’ character increases from sp3 to sp2 to sp orbitals, bonding electrons move closer to carbon and away from the protons, which become deshielded. EXAMPLE SP3 hybridization=25% sp2hybridization=33.33% S character CH4 0.23 ppm CH2=CH2 5.28ppm CH3-CH3 0.86ppm
- 17. SP Hybridization=50% Scharacter
- 18. Compound Type of Hybridization %of S character Chemical Shift (ppm) Justification ALKANE SP3 25 0.9-1.3 Less S character, more shielded proton ALKENE SP2 33.3 4-6 More S character, more deshielded proton ALKYNE SP 50 2.1 Deshielding due to hybridization But shielding due to anisotropy.
- 19. Anisotropic Effect In compoundcontaining double or triple bond circulation of pi electrons about nearby nuclei generate an induced field which can either oppose or reinforce at the location of proton or the space occupied by the proton. The occurance of shielding or deshielding can be determined by the location of proton in the space and so, this effect is known as space effect. Phenomenon (spatial variation) of shielding and deshielding depending on orientation of molecule w.r.t applied magnetic field is called Anisotropy Anisotropy is property of molecule in different orientations which show variations in physical properties along different axes of molecule
- 20. Aromatic protons cometo resonance at 7.1 ppm due to this deshielding RING CURRENT: The π electrons in a compound, when placed in a magnetic field, will move and generate their own magnetic field.
- 21. In benzene, theπ-electrons are delocalized above and below the plane of the ring forming electron cloud. When a magnetic field is applied perpendicular to the plane of the aromatic ring, the circulation of π-electrons produces a ring current. The induced magnetic field is in the opposite direction (diamagnetic effect) as the applied field at the centre of the ring, but outside the ring it is in the same direction (paramagnetic effect) of the applied field. The Aromatic protons experience larger magnetic field and resonate at higher δ values (deshielding). Protons which are present above or below to the plane of the aromatic ring resonate at low δ values (Shielded).
- 22. An alkene moleculeis oriented with the plane of the double bond perpendicular to the applied magnetic field. The circulation of π-electrons is perpendicular to the carbon- hydrogen framework of the molecule. As a result the induced magnetic field caused by the circulation of π- electrons is in the opposite direction of applied magnetic field (diamagnetic) around the carbon atoms and in the direction of applied magnetic field (paramagnetic) in the region of olefenic protons. Therefore olefenic protons experience greater field strength and consequently resonate at larger value of chemical shift. ANISOTROPIC FIELD IN AN ALKENE
- 23. An alkyne moleculewhen placed in an external magnetic field is so oriented that the plane of the triple bond lies parallel to the direction of applied magnetic field. The induced magnetic field caused by the circulation of π-electrons of the triple bond is in the opposite direction of applied field (diamagnetic) in the region of acetylenic protons. Therefore acetylenic protons are shielded and experience lesser field strength and consequently resonate at smaller value of chemical shift. ANISOTROPIC FIELD FOR AN ALKYNE
- 24. Carbonyl protons (Aldehydes) Orientedwith the plane of the carbon-oxygen double bond is perpendicular to the applied field. The circulation of π-electrons generates an induced magnetic field which is in the direction of applied magnetic field at the aldehydic protons Therefore, aldehydic protons experience greater field strength (diamagnetic effect) and consequently resonate at larger value of chemical shift (deshielding). In addition, the high electronegativity of oxygen atom also contributes to the higher δ value of aldehydic proton.
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- 26. Mesomeric effect Because ofthe higher electron density at the ortho and para positions due to mesomeric effect, the hydrogen atom appears shielded with a chemical shift value of 6.5 ppm and upfield shifted compared to the other hydrogens at the meta position, where it is de-shielded.
- 27. NH, SH &OH protons NMR signals are moved on changing to solvents of different polarity Higher temperatures reduces intermolecular hydrogen bonding means lower δ values Intramolecular hydrogen bonding is unchanged by dilution Hydrogen bonding shifts resonance signal proton to lower field ( higher frequency ) – deshields protons & lengthens –OH bond Hydrogen bonding
- 28. HYDROGEN BONDING DESHIELDSPROTONS O H R O R HHO R The chemical shift depends on how much hydrogen bonding is taking place. Alcohols vary in chemical shift from 0.5 ppm (free OH) to about 5.0 ppm (lots of H bonding). Hydrogen bonding lengthens the O-H bond and reduces the valence electron density around the proton - it is deshielded and shifted downfield in the NMR spectrum.
- 29. O C O R H H C O O R Carboxylic acids havestrong hydrogen bonding - they form dimers. With carboxylic acids the O-H absorptions are found between 10 and 12 ppm very far downfield. O O O H CH3 In methyl salicylate, which has strong internal hydrogen bonding, the NMR absortion for O-H is at about 14 ppm, way, way downfield. A 6-membered ring is formed.
- 30. Vanderwaal deshielding In arigid molecule it is possible for a proton to occupy a sterically hindered position, and in consequence the electron cloud of the hindering group will tend to repel, by electrostatic repulsion, the electron cloud surrounding the proton. The proton will be deshielded and appear at higher δ values than would be predicted in the absence of the effect. Although this influence is small (usually less than 1 ppm),it must be borne in mind when predicting the chemical shift position in overcrowded molecules such as highly substituted steroids or alkaloids.
- 31. Type of protonCompound Chemical Shift Justification Primary alkyl RCH3 0.8-1.0 Shielded due to more of protons and less of carbons Secondary alkyl R2CH2 1.2-1.4 Tertiary alkyl R3CH 1.4-1.7 Deshielded due to less of electropositive protons and more of electronegative carbons benzylic ArCH3 2.2-2.5 Deshielded due to anisotropy and electron withdrawal by benzene Alkyl fluoride RCH2F 4.0 -4.5 Deshielding due to Highest negative inductive effect of Fluorine Alkyl chloride RCH2Cl 3.6-3.8 Deshielding due to inductive effect of halogen Alkyl bromide RCH2Br 3.4-3.6 Deshielding due to inductive effect of halogen Alkyl iodide RCH2I 3.1-3.3 Least Deshielding in halogen series due to less electron withdrawing inductive effect of Iodine Ether ROCH2R 3.3-3.9 Negative inductive effect of oxygen Alcohol HOCH2R 3.3-4.0 Negative inductive effect of Oxygen Ketone RCOCH3 2.1-2.6 Negative inductive effect of Carbonyl
- 32. Type of protonCompound Chemical Shift Justification Aldehyde RCHO 9.0-10.0 Deshielding due to Anisotropy of Carbonyl and Negative inductive effect of Carbonyl Vinvlic R1C=CH2 4.6 -5.0 Deshielding due to Anisotropy of Carbonyl Aromatic ArH 6.0-9.0 Deshielding due to circulation of pie electrons and ring Current, electron withdrawal by aromatic ring Alcohol Hydroxyl ROH 0.5-6.0 Hydrogen bonding Carboxylic RCOOH 10-13 Deshielding due to Anisotropy of Carbonyl and Negative inductive effect of Carboxyl Phenolic ArOH 4.5-7.7 Hydrogen bonding and anisotropy of aromatic ring Amino RNH2 1.0-5.0 Hydrogen bonding Amide RCONHR 5.0-9.0 Hydrogen bonding