Welcome to Nuclear Magnetic Resonance (NMR) Spectroscopy!

Hello! You've reached one of the most powerful detective tools in organic chemistry. If Mass Spectrometry tells us the mass of a molecule, and Infrared (IR) Spectroscopy tells us which functional groups are present, then Nuclear Magnetic Resonance (NMR) Spectroscopy tells us the precise skeletal arrangement of the atoms.

Don't worry if the name sounds complex—the core concept is all about listening to how different atoms "chime in" when placed in a strong magnetic field. By the end of this chapter, you will be able to use NMR data to solve the structure of complex organic molecules!

3.3.15.1 The Principles of NMR Spectroscopy

What is NMR?

NMR relies on the fact that certain atomic nuclei (like hydrogen-1 and carbon-13) possess a property called nuclear spin. Since these nuclei are charged particles spinning, they act like tiny magnets.

When placed in an incredibly strong external magnetic field:

  1. These tiny nuclear magnets align either with the field (lower energy state) or against the field (higher energy state).
  2. We then apply a pulse of radio waves (electromagnetic radiation).
  3. When the radio wave frequency exactly matches the energy difference between the two spin states, the nucleus absorbs the energy and "flips" its spin. This is called resonance.
  4. When the nucleus relaxes back, it emits a radio signal, which is detected and translated into an NMR spectrum.

Why the Molecular Environment Matters

The crucial insight is that the frequency (or energy) required to make a nucleus resonate is not constant. It depends heavily on the tiny magnetic fields generated by the electrons surrounding that nucleus.

Imagine you are trying to talk to the nucleus (your magnet). The electrons form a cloud around it, which acts like a shield.

  • Shielded Nucleus: If the electron density is high (the nucleus is well "covered"), it experiences less of the external magnetic field. It will require a lower frequency radio wave to resonate.
  • Deshielded Nucleus: If the electron density is low (often due to an adjacent electronegative atom like Oxygen or Chlorine pulling the electrons away), the nucleus feels the full force of the external magnetic field. It will require a higher frequency (higher energy) radio wave to resonate.

This difference in required frequency, based on the electronic environment, is measured using the chemical shift ($\delta$).

Key Term: Chemical Shift ($\delta$)

The chemical shift ($\delta$) is a measure of how far the resonance signal of a specific nucleus has shifted from a standard reference signal. It is measured in parts per million (ppm).

  • Low $\delta$ values (e.g., 0-2 ppm): Highly shielded nuclei.
  • High $\delta$ values (e.g., 5-10 ppm): Highly deshielded nuclei (often near O, N, or halogen atoms).
The Standard Reference: Tetramethylsilane (TMS)

To standardize spectra across the world, chemists use Tetramethylsilane (\(\text{TMS}\), $\text{Si}(\text{CH}_3)_4$) as a reference compound.

  • Why TMS is used:
    1. It gives a single, sharp signal because all 12 hydrogen atoms and all 4 carbon atoms are chemically equivalent (in the exact same environment).
    2. It is highly shielded, so its signal appears at the far right of the spectrum, defined as $\boldsymbol{\delta = 0\ \text{ppm}}$.
    3. It is chemically inert (doesn't react) and volatile (easily removed from the sample).

Quick Review: NMR Basics

  • NMR gives information about the position (environment) of ${}^{1}\text{H}$ and ${}^{13}\text{C}$ atoms.
  • The environment dictates the chemical shift ($\delta$).
  • Electronegative atoms cause deshielding, resulting in a higher $\delta$ value.

Carbon-13 NMR Spectroscopy (\({}^{13}\text{C}\) NMR)

The syllabus notes that ${}^{13}\text{C}$ NMR spectra are simpler than ${}^{1}\text{H}$ NMR spectra. This is because we typically only worry about two pieces of information: how many unique carbons and where they appear (chemical shift).

1. Number of Signals = Number of Environments

The simplest rule of ${}^{13}\text{C}$ NMR is determining the number of chemically equivalent carbon atoms.

  • If a molecule has 5 signals (peaks), it means it contains 5 distinct carbon environments.
  • Example: Ethanol, $\text{CH}_3\text{CH}_2\text{OH}$. The $\text{CH}_3$ carbon is different from the $\text{CH}_2$ carbon, so it shows two signals.
  • Example: Propanone, $\text{CH}_3\text{COCH}_3$. The two $\text{CH}_3$ groups are equivalent due to symmetry, so there are only two signals (one for the methyl carbons and one for the carbonyl carbon).

2. Interpreting Chemical Shift ($\delta$) Data

By comparing the chemical shift ($\delta$) values from the spectrum with tables provided in your data booklet, you can identify the type of functional group the carbon belongs to.

Generally, the more attached to highly electronegative atoms or double bonds a carbon is, the further downfield (higher $\delta$) its signal appears.

  • Alkyl carbons ($\text{C}-\text{C}$ and $\text{C}-\text{H}$): Very shielded, appearing in the region of $\delta = 0 - 50$ ppm.
  • Carbons attached to oxygen/nitrogen ($\text{C}-\text{O}$): Deshielded, appearing around $\delta = 50 - 90$ ppm.
  • Alkenes ($\text{C}=\text{C}$): Deshielded, appearing around $\delta = 100 - 150$ ppm.
  • Carbonyl carbons ($\text{C}=\text{O}$): Highly deshielded due to the strong pulling power of oxygen, appearing around $\boldsymbol{\delta = 160 - 220\ \text{ppm}}$.

Key Takeaway: ${}^{13}\text{C}$ NMR

${}^{13}\text{C}$ NMR is your quick checker: the number of signals tells you the number of unique carbon groups, and the $\delta$ value tells you what type of group they are (e.g., ketone, alcohol, alkane).

Proton NMR Spectroscopy (\({}^{1}\text{H}\) NMR)

${}^{1}\text{H}$ NMR is the more detailed technique and is essential for working out connectivity in a molecule. It gives us three key pieces of information:

  1. Chemical Shift ($\delta$): The environment of the proton.
  2. Integration: The number of protons in that environment.
  3. Spin-Spin Coupling ($n+1$ rule): The number of non-equivalent protons on adjacent atoms (connectivity).

1. Chemical Shift ($\delta$) and Environments

Just like ${}^{13}\text{C}$ NMR, the $\delta$ value tells you about the electronic environment of the hydrogen atoms (protons). Protons in the same environment are called equivalent protons.

If two protons can be swapped without changing the molecule, they are equivalent and give a single signal.

  • Example: In propane ($\text{CH}_3\text{CH}_2\text{CH}_3$), the two end $\text{CH}_3$ groups are equivalent, so there are two signals: one for the $\text{CH}_3$ protons and one for the $\text{CH}_2$ protons.

You must use your data booklet to identify the general type of proton (e.g., aldehyde proton, aromatic proton, alcoholic proton).

2. Integration Data: How Many Protons?

The integration refers to the area under each signal peak. In ${}^{1}\text{H}$ NMR, the area is directly proportional to the relative number of hydrogen atoms in that specific environment.

Integrated spectra provide a stepped line above the peaks. The height of the step corresponds to the relative number of protons.

Step-by-step interpretation:

  1. Measure the height of the vertical jump for each peak group (this is the integral value).
  2. Find the simplest whole-number ratio of these heights.
  3. This ratio tells you the relative number of protons in each environment.

Example: If the integral ratio is 2:3, it means there are two protons in one environment and three protons in the other. If the molecular formula is $\text{C}_2\text{H}_5\text{Br}$, this 2:3 ratio accounts for all 5 hydrogens ($\text{CH}_2\text{CH}_3$).

3. Spin-Spin Splitting (Coupling): The \(n+1\) Rule

This is the trickiest, but most useful, part of ${}^{1}\text{H}$ NMR! The pattern of peaks (or splitting) tells you about the number of non-equivalent hydrogen atoms on the adjacent carbon atom(s). This shows connectivity—like shaking hands with your neighbour.

The rule used for determining the number of peaks (the multiplicity) is the $\boldsymbol{(n+1)}$ rule.

Number of peaks = \(n+1\)

Where \(n\) is the number of non-equivalent adjacent protons.

Important Accessibility Note: The syllabus limits this splitting analysis to doublet, triplet, and quartet patterns in aliphatic compounds (straight or branched chains, not rings).

Common Splitting Patterns:
  • $n=0$: Singlet (1 peak)

    Meaning: The proton environment has zero non-equivalent neighbours. (e.g., $\text{R}-\text{OH}$ or $\text{R}-\text{C}(\text{O})-\text{CH}_3$).

  • $n=1$: Doublet (2 peaks)

    Meaning: The proton environment has one non-equivalent neighbour.

  • $n=2$: Triplet (3 peaks)

    Meaning: The proton environment has two non-equivalent neighbours. (Classic example: a $\text{CH}_3$ group next to a $\text{CH}_2$ group, like in ethanol).

  • $n=3$: Quartet (4 peaks)

    Meaning: The proton environment has three non-equivalent neighbours. (Classic example: a $\text{CH}_2$ group next to a $\text{CH}_3$ group).

Don't worry if this seems tricky at first. Practice makes perfect! Remember the mnemonic: The number of neighbours ($n$) is always one less than the number of peaks you see ($n+1$).

A Crucial Caution: Protons that DON'T Split

You should not see splitting (coupling) between:

  1. Protons that are equivalent (they don't split each other).
  2. Protons attached to oxygen ($\text{O}-\text{H}$) or nitrogen ($\text{N}-\text{H}$) groups. These protons exchange rapidly, which averages out the coupling signal, usually resulting in a singlet.

Practical Considerations in NMR

Solvents for NMR Samples

Since most organic compounds contain hydrogen, running a sample in a normal hydrogen-containing solvent (like water or chloroform, $\text{CHCl}_3$) would result in a huge signal for the solvent, drowning out the actual sample peaks.

Therefore, samples are dissolved in deuterated solvents (or solvents without hydrogen, like $\text{CCl}_4$).

  • Deuterium (D or ${}^{2}\text{H}$): Deuterium nuclei do not resonate in the frequency range used for ${}^{1}\text{H}$ NMR.
  • Common Deuterated Solvents: The most common is deuterated chloroform, $\text{CDCl}_3$, where the hydrogen atom in $\text{CHCl}_3$ has been replaced by deuterium.

Deducing Structure: The Synthesis of Data

The real power of NMR is revealed when you combine all the data points—$\delta$, integration, and splitting—to work out the structure piece by piece.

  • Step 1 (Molecular Formula): Determined by Mass Spectrometry.
  • Step 2 (Functional Groups): Determined by IR and $\delta$ values in ${}^{13}\text{C}$ and ${}^{1}\text{H}$ NMR.
  • Step 3 (Building Blocks): Use the ${}^{13}\text{C}$ data to find the number of carbon environments.
  • Step 4 (Connectivity): Use ${}^{1}\text{H}$ integration (number of H's) and splitting ($n+1$ rule) to figure out which carbon groups are next to which.

Did you know? The technology behind NMR is also used widely in medicine. When applied to human tissue, it is known as Magnetic Resonance Imaging (MRI), providing detailed pictures of internal organs without using harmful radiation!

Key Takeaway: Solving the Structure

NMR provides three vital clues:

  1. Position ($\delta$): What functional group the atom is part of (e.g., C=O, C-O).
  2. Integration (${}^{1}\text{H}$ only): How many protons are in that environment.
  3. Splitting (${}^{1}\text{H}$ only): What its immediate neighbours are (connectivity).

Use these clues systematically to build your molecule!