1H+NMR+from+Smith

Chapter 14: Structure Determination 1H Nuclear Magnetic Resonance Spectroscopy

Benefits of 1H: NMR Spectroscopy

  • 1H NMR is the most useful spectroscopic method in identifying C—H frameworks in organic compounds.

  • Common types of NMR spectroscopy:

    • 1H (Proton)

    • 13C (Carbon-13)

  • Operates using strong magnetic fields (B0) and radio frequency (RF) energy sources.

  • RF waves have sufficiently low energy to change the spin state of protons.

NMR Spectroscopy: Resonance Energy

  • Protons absorb radiation when the energy difference (ΔE) equals the frequency of the radiation (ν).

  • Resonance condition: ΔE = hν (where h is Planck’s constant).

  • When in resonance, the nucleus becomes visible in an NMR spectrum.

NMR: Resonance Frequency

  • As B0 increases, the energy difference (ΔE) between low and high energy spin states increases, requiring higher ν to create resonance peaks.

  • Frequency equation: ν α B0.

  • NMR instruments are categorized by their RF frequency.

  • The nuclear environment affects the absorbance frequency of 1H; only elements with odd atomic mass numbers are NMR active (e.g., 1H, 13C).

NMR Instrumentation

  • Key components of an NMR instrument include:

    • Sample tube

    • Radiofrequency generator

    • Detector and amplifier

    • Magnetic field generation (superconducting magnet)

  • Sample preparation: The compound is dissolved in a solvent, placed in an NMR tube, and rotated within the magnetic field, where it is exposed to a pulse of RF radiation.

Interpreting a 1H NMR Spectrum

  • NMR spectrum representation: Plots signal intensity against chemical shift (ppm) on the delta scale.

    • Resonances categorized as downfield or upfield relative to TMS (Tetramethylsilane).

  • The spectrum provides insights into:

    • Families of hydrogen atoms present in the molecule.

    • Neighborhood of hydrogen, number, and connectivity of adjacent protons.

Application of 1H-NMR Spectroscopy

  1. Identification of hydrogen environments (equivalency).

  2. Determining hydrogen neighborhood (location of atoms within the molecule).

  • Concepts: Shielded vs. Deshielded.

  1. Counting hydrogen atoms in the molecule (based on integrations).

  2. Mapping hydrogen connectivity using the “n+1” rule.

Equivalency

  • Number of peaks in the spectrum indicates distinct hydrogen types in the molecule.

  • Substitution Rule: Replace an H with a halogen and name it to determine equivalency or nonequivalence of hydrogen atoms.

Environment of H: Neighborhood

  • Electron density creates a magnetic field opposing B0.

    • Higher electron density = more shielded protons (upfield resonance).

    • Decreasing electron density leads to deshielding (downfield resonance).

Effect of Delocalized Electrons on 1H

  • In aromatic systems, circulating π-electrons create a ring current affecting proton shielding:

    • Protons in areas with induced magnetic fields become deshielded, leading to downfield absorption (6.5-8 ppm).

    • In alkenes, deshielding occurs at 4.5-6 ppm, whereas alkyne protons absorb upfield (~2.5 ppm).

Chemical Shifts

  • The chemical shift scale (ppm) shows increased deshielding to the left and shielding to the right.

  • Shielded protons resonate at lower chemical shifts, deshielded protons at higher shifts.

Chemical Shifts in 1H NMR Spectroscopy

Type of Hydrogen

Chemical Shift (δ ppm)

Type of Hydrogen

Chemical Shift (δ ppm)

Reference Si(CH3)4

0

Alcohol

2.5-5.0

Alkyl (primary) -CH3

0.7-1.3

C-0-H

Alkyl (secondary) -CH2

1.2-1.6

Alkyl (tertiary)

1.4-1.8

Alcohol, ether H

3.3-4.5

Vinylic H

4.5-6.5

Aryl Ar-H

6.5-8.0

Aldehyde

9.7-10.0

Alkynyl -C=C-H

2.5-3.0

Carboxylic acid

11.0-12.0

Integration of 1H NMR Absorptions

  • Integration of signal intensity is proportional to the number of protons causing the signal, providing structural information.

Predicting a Spectrum

  • Example Problem: Predict the number of peaks, area ratios, and expected chemical shifts in the 1H NMR spectrum of 1,4-dimethylbenzene.

Neighbor: Spin-Spin Splitting

  • Peaks can split into multiple peaks due to interactions between nonequivalent protons on adjacent carbons (spin-spin splitting).

  • Splitting pattern determined by the “n+1” rule (n = number of neighboring protons).

  • Types of splitting:

    • Singlet (s), doublets (d), triplets (t), quartets (q), multiplets (m).

Rules for Spin-Spin Splitting

  • Equivalent protons on the same carbon do not split each other.

    • Example: For (CH3)3CCH2CH3.

  • Protons on heteroatoms do not split protons on carbons (e.g., CH3OH).

  • Protons more than two carbon atoms apart do not interact.

    • Example: BrCH2CH(OH)CH2CH2CH3.