Factors Influencing Chemical shifts
Factors Influencing or Affecting Chemical Shift
Followings are the factors which affects the value of chemical shift-1. Inductive Effect (Electronegativity)
2. Vander Waal's Deshielding
3. Anisotropic Effect
4. Hydrogen Bonding
5. Hybridization Effect
1. Inductive Effect (Electronegativity)
In an organic compound, a proton is covalently bonded to carbon, nitrogen, oxygen or other atoms by sigma bond. When placed in a strong magnetic field the electrons of the sigma bond circulate to generate a small magnetic field which opposes the applied field. The chemical shift for the protons is directly related to the electronegativity of the atom or group to which it is attached. A proton is said to be deshielded if it is attached with an electronegative atom or group. Greater the electronegativity of the atom, greater would be the deshielding of proton.If the deshielding is more for a proton, its δ value will also be more (downward shift). As the distance from the electronegative atom increases, the deshielding effect due to it diminishes.
Compound-1: CH3 − CH2 − F
Compound-2: CH3 − CH2 − Cl
Two signals are expected for each of the two compounds. The blue proton in compound-1 is more deshielded than blue proton in compound-2. Red protons are comparatively less deshielded than blue protons in both the compounds.
2. Vander Waal's Deshielding
In overcrowded molecules, some protons may be occupying sterically hindered position. So, electron cloud of a bulky group will tend to repel the electron cloud surrounding the proton. Thus, such proton will be deshielded and will resonate at slightly higher δ value.3. Anisotropic Effect (Space Effect)
Anisotropy refers to the property of the molecule where a part of the molecule opposes the applied field and the other part reinforces the applied field. Chemical shifts are dependent on the orientation of neighbouring bonds in particular the π bonds. Examples of nucleus showing chemical shifts due to π bonds are aromatics, alkenes, alkynes and Carbonyl Compounds. Such anisotropic shifts are useful in characterizing the presence of aromatics or other conjugated structures in molecules.Alkene: An alkene molecule in an external magnetic field is so oriented that the plane of the double bond is at right angles to the applied field. Induced circulation of electrons generates induced magnetic field which is diamagnetic around carbon atom and paramagnetic in the region of the alkene protons. Thus the protons will feel greater field strength and hence resonance occurs at lower applied field.
Alkynes: In alkynes, electronic circulation around triple bond takes place in such a way that the protons experience diamagnetic shielding effect. When the axis of the alkyne group lies parallel to the direction of the applied field, the π-electrons are induced to circulate around the axis in such a way that the induced field opposes the applied field. Thus, protons feel smaller field strength (shielding) and consequently resonance occurs at higher applied field (low δ value).
Benzene: In case of benzene, loops of electrons are delocalised cylindrically over the aromatic ring. These loops of electrons are induced to circulate in the presence of the applied field producing ring current. The induced current is diamagnetic (opposing the applied field) in the centre of the ring and is paramagnetic outside the ring. Thus, the aromatic protons experience a magnetic field greater in magnitude than applied field. Such protons are said to be deshielded and hence, smaller applied field (higher δ value) will be required to bring them to resonance. It may be noted that the protons held above and below the plane of ring resonate at low δ value.
Carbonyl protons: Carbonyl compound when placed in an external magnetic field is so oriented that 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.
4. Hydrogen Bonding
Protons involved in hydrogen bonding shows variable absorption positions over a wide range. The greater the extent of H-bonding, greater is the deshielding of the proton and greater is the chemical shift. The extent of H-bonding is a function of temperature and concentration. Increasing the concentration of solution increases the extent of intermolecular H-bonding and increases the chemical shift.At high dilution, the extent of H-bonding is highly diminished and hydroxyl protons absorb near 0.5 to 1.0 ppm. In concentrated solutions, however, hydroxyl protons absorb around 4 to 5 ppm. In case of intramolecular hydrogen bonding, the absorption position of hydroxyl proton is independent of the concentration of solution. Thus, NMR spectroscopy can be used to distinguish intermolecular H-bonding from intra-molecular H-bonding.
5. Hybridisation Effects
Hydrogen atoms attached to purely sp3 hybridised carbon atoms have their chemical shifts in the range 0 to 2 ppm. Due to greater electronegativity of sp2 hybrid orbitals than sp3 hybrid orbitals, vinylic hydrogens have greater chemical shift (5 to 6 ppm) than aliphatic hydrogens on sp3 hybridised carbon atoms ( ≈ 1 to 2 ppm). Aromatic hydrogen atoms on sp2 hybridised carbon atoms appear in the range farther downfield (6.5 to 8.0 ppm).The downfield positions of vinyl and aromatic resonances are, however, greater than one would expect based on these hybridisation differences. This is due to the diamangetic anisotropy resulting from the circulating π-electrons in these systems. Acetylenic hydrogens at sp-hybridised carbon appear anomalously at 2 to 3 ppm owing to diamagnetic anisotropy of circulating cylindrical π-electron cloud in acetylene.
Chemical Shift
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