13.4 Chemical Shifts in 1H NMR Spectroscopy

Chemical shifts in 1H NMR spectroscopy reveal the electronic environment of hydrogen atoms in molecules. By measuring these shifts relative to a reference compound, chemists can identify different types of protons and functional groups present in a sample.

Understanding chemical shift ranges helps in structure determination. Factors like electronegativity, hybridization, and magnetic anisotropy influence proton chemical shifts, providing valuable insights into molecular structure and bonding.

Chemical Shifts in 1H NMR Spectroscopy

Chemical shift interpretation for hydrogens

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• Chemical shift ($\delta$) measured in parts per million (ppm) relative to reference compound tetramethylsilane (TMS)
  • TMS assigned chemical shift of 0 ppm serves as standard reference point
• Chemical shift of proton depends on its electronic environment
  • Electron-withdrawing groups (EWGs) deshield protons causing downfield shift to higher ppm (carboxylic acids)
  • Electron-donating groups (EDGs) shield protons causing upfield shift to lower ppm (alkyl groups)
• Typical chemical shift ranges for different types of protons:
  • Aromatic protons: 6-8 ppm (benzene)
  • Alkene protons: 4.5-6.5 ppm (ethylene)
  • Protons adjacent to heteroatoms (-OH, -NH, -SH): 2.5-4.5 ppm (alcohols, amines, thiols)
  • Aliphatic protons: 0.5-2.5 ppm (alkanes)

Chemical shift ranges of functional groups

• Protons attached to sp3 hybridized carbons:
  • Methyl (-CH3): 0.8-1.2 ppm (tert-butyl group)
  • Methylene (-CH2-): 1.2-1.4 ppm (ethylene glycol)
  • Methine (-CH<): 1.4-1.7 ppm (isopropanol)
• Protons attached to sp2 hybridized carbons:
  • Alkene protons: 4.5-6.5 ppm (1-butene)
  • Aromatic protons: 6-8 ppm (toluene)
• Protons attached to heteroatoms:
  • Alcohols (-OH): 1-5 ppm variable due to hydrogen bonding (ethanol)
  • Amines (-NH2, -NH-): 1-3 ppm (aniline)
  • Thiols (-SH): 1-2 ppm (ethanethiol)
  • Carboxylic acids (-COOH): 10-13 ppm (acetic acid)
• Protons adjacent to electron-withdrawing groups (EWGs):
  • Halogens (-CH2Cl): 2-4 ppm (chloroethane)
  • Carbonyls (-CH2C=O): 2-3 ppm (acetone)

Factors affecting proton chemical shifts

• Electronegativity of neighboring atoms
  • Protons near electronegative atoms (O, N, F, Cl) experience deshielding effect resonating at higher ppm (alcohols, amines)
• Hybridization of adjacent carbons
  • Protons attached to sp3 hybridized carbons resonate at lower ppm compared to those attached to sp2 or sp hybridized carbons (alkanes vs alkenes/alkynes)
• Magnetic anisotropy
  • Protons near pi systems (alkenes, aromatic rings) experience deshielding effect due to induced magnetic field (styrene)
• Hydrogen bonding
  • Protons involved in hydrogen bonding (-OH, -NH) exhibit variable chemical shifts depending on extent of hydrogen bonding (carboxylic acids)
• Steric effects
  • Bulky substituents near proton can cause shielding resulting in upfield shift (tert-butyl group)

Nuclear Magnetic Resonance Principles

• Nuclear magnetic resonance (NMR) is the basis for 1H NMR spectroscopy, allowing for the identification of hydrogen environments in molecules
• Spin-spin coupling occurs when nearby nuclei influence each other's magnetic environment, resulting in signal splitting
• Relaxation time refers to the time required for excited nuclei to return to their ground state, affecting signal intensity and resolution