1H NMR Spectroscopy: An Overview

• The presented material covers applied organic spectroscopy
• Focus on Nuclear Magnetic Resonance (NMR), specifically 1H NMR

1H NMR Spectroscopy

• 1H NMR Spectroscopy is explored
• The coupling of peaks occurs
• No longer being simple peaks, splitting occurs

Comparison With 13C NMR

• 1H is the major isotope of hydrogen, with a 99.985% natural abundance
• 13C is a minor isotope, at only 1.1%
• 1H NMR is quantitative; the area under the peak indicates the number of hydrogen nuclei
• Quantitative information may not be available in 13C NMR
• Protons interact magnetically (couple) to reveal the structure's connectivity
• Coupling between 13C nuclei is rare
• 1H NMR shifts give a more reliable indication of local chemistry than 13C spectra

Number of Signals

• Every proton in the molecule gives a signal
• Exchangeable protons (e.g., OH, NH/NH2) are an exception
• Identical (isochronous) protons give the same signal
• To test if protons are identical:
  • Consider propane as an example
  • Two protons exchange locations when the molecule rotates 180° around the indicated axis
  • Freely rotating molecules renders the three hydrogens as equivalent
  • Equivalent arrangements of propane contains six equivalent hydrogens and 2 equivalent hydrogens
• In ethanol (CH3CH2OH)
  • There shows arrangements of 3 equivalent hydrogens, 2 equivalent hydrogens, and 1 hydrogen
• For a molecule of CH3CCH2Br(CH3)2:
  • Methyl and Methylene protons are indicated

Number of Signals: Worked Examples

• Chloromethane (1 signal): All 3 H atoms bond to the same carbon atom
• Chloroethane (2 signals): CH2 is in a different environment from CH3
• 1-Chloropropane (3 signals): CH3, CH2, and CH2 are in different environments
• Methoxyethane (3 signals): CH3 and CH2 are both closer to O than CH3
• Butan-2-one (3 signals): CH3, CH2, and CH3 are in different environments
• Ethyl ethanoate (3 signals): CH3, CH2, and CH3 are in different environments
• Ethoxyethane (2 signals): The 6 H atoms in the two CH3 groups and the 4 H atoms in the two CH2 groups are equivalent.
• 2-Chloropropane (2 signals): The 6 H atoms in the two CH3 groups are equivalent.
  • 1 H is in a different environment because it bonds to a carbon with a Cl atom
• (Z)-but-2-ene (2 signals): The two CH3 groups and the two H atoms are equivalent
• 2-methylprop-1-ene (2 signals): The two CH3 groups and the two H atoms are equivalent
• Methylbenzene (4 signals): The two H atoms at equal distances from CH3 are equivalent

Alkenes and Aromatics Signals

• Examples are given with the predicted amount of signals

Exchangeable Hydrogens and Signals

• Acidic protons in a molecule readily exchange with deuterium, most frequently from the NMR solvent
• Commonly encountered acidic exchangeable protons: -OH, -SH, -NH/-NH2
• Illustrated by the reaction RCH2-OH + CDCl3 forming RCH2-OD + CHCl3
• Chloroform signal appears at δ=7.26
• Deuterated chloroform solvent occurs
• Perform quick shake with heavy water (deuterium oxide, D₂O) for suspected exchangeable proton (-OH; -SH; -NH/-NH2) test
• Deuterium oxide is shown
• Illustrative reaction: (RCH2)2-NH + D2O forms (RCH2)2-ND + HOD
• The signal appears at δ=2.78

Chemical Shift

• Refers to the δ-scale relative to Tetramethylsilane (TMS)
• Exceptions include OH and NH/NH2, which have non-definable regions due to H-bonding and exchanging
• Downfield and Upfield are indicated with the reference peak of TMS
• Low external magnetic field
• Increasing externally applied magnetic field
• Factors affecting signal position includes, but not limited to: number of substituents and electronegativity

Chemical Shift: Tables

• Tables of approximate values are available
• Factors affecting the position of a signal also includes the number of substituents and electronegativity

Chemical Shift: Regions

• Hydrogens involved from saturated systems occur within the range of 0-2 ppm
• Methyl (CH3) at ~0.9 ppm
• Methylene (CH2) at ~1.2 ppm
• Methine (CH) at ~1.7 ppm

Factors Affecting Signal Position

Inductive Effects

• Electronic effects through single bonds are one of the factors
• EWG (electron withdrawing groups) (O, N, hal) reduces electron density at neighboring H's, decreases shielding, and causes a downfield shift
• EDG (electron donating groups) (metals, Si) increases electron density at neighboring H's, increases shielding, and causes an upfield shift

Chemical Shift: Substituent Example

• CH3X is discussed
• X = Li at -1.94
• X = H at 0.23
• X = NH2 at 2.47
• X = Br at 2.69
• X = Cl at 3.06
• X = OH at 3.39
• X = F at 4.27

Number of Substituents

• CH4 at 0.23
• CH3Cl at 3.10
• CH2Cl2 at 5.5
• CHCl3 at 7.26
• The number of electronegative substituents and their proximity is a factor

Resonance Effects

• The electronic effects through DOUBLE bonds
• An illustration of HA and HB and OMe are indicated
• OMe, NO2, and the chemical structure of 7.27 are given with the indicated chemical shifts

Magnetic Anisotropy

• The effect of localized or induced magnetic fields within a compound that especially surround π-systems
• Alkenes (δ = 4.5-7)
• Aldehydes (δ = 9-10)
• Alkynes (δ = 2-3)

Magnetic Anisotropy Illustration

• The sp² carbon has high s character and withdraws electrons, deshielding the hydrogen

Chemical Shift: Other Factors

• van der Waals interactions and H-Bonding are also a factor
• Alcohols (δ = 0.5-4.5)
• Amines (δ = 1.0-5.0)
• It is advisable to check with D₂O shake

Chemical Shift: Example

• An example of a molecule is provided with each position labeled with the specific chemical shift in mind

Integration

• Importance placed on area underneath the peak, not intensity or height
• Areas are in the same ratio
• Integral over entire doublet still gives ratio of protons
• Process: simply measuring the height of each integration curve with a ruler, gives the ratio of protons represented by each peak e.g. 1:2/2:4 or 1:2:3/2:4:6

Integration: Examples

• The ethyl ethanoate and acetic acid spectrum are given with integration examples
• The ratio of signals in these examples are provided

Signal Splitting

• Makes ¹H NMR more powerful than 13C NMR
• More important than chemical shift/integration
• Spin-spin coupling allows a view of not just individual atoms, but how the C-H skeleton joins together

Hydrogens on Adjacent Carbons (3 Bonds)

• Hydrogens on adjacent carbons can "see" or "sense" each other
• The influence hydrogen on each other creates signal splitting

HA and HB

• The ¹H NMR spectrum will show as two doublets
• One for HA and one for HB
• The integration over the entire doublet still gives the ratio of protons

Signal Splitting: Summary

• Neighboring hydrogens split the signal of an adjacent hydrogen into multiple peaks
• Orientations of fields of the CH hydrogen is a factor
• When the field of H strengthens the applied field, it deshields H
• When the field of H weakens the applied field, it shields H

More Than 1 H

• Described by the n+1 rule
• Protons with n equivalent neighboring protons show n+1 peaks
• intensities are given by Pascal's triangle.
• What happens if there is more than one H on the adjacent carbon?
  • A group adjacent to CH₂ will see three different environments in the ratio 1:2:1
• Table 14.2: Multiplicity of the Signal and Relative Intensities of the Peaks in the Signal
  • The number of equivalent protons causing splitting, the Multiplicity of the signal and relative peak intensities are all given

Worked Examples of Signal Splitting

• Methyl propanoate (s,t,q): singlet is no neighboring H's, downfield as next to electronegative O
• The quartet is 3 neighboring H's, (n+1) = 3+1 = 4, downfield as next to electron-withdrawing carbonyl (C=O) group
• The triplet is 2 neighboring H's, (n+1) = 2+1 = 3 (1:2:1)
• Other worked examples of signal splitting are provided