Nuclear magnetic resonance spectroscopy

Welcome to Nuclear Magnetic Resonance (NMR) Spectroscopy

Welcome to one of the most powerful detective tools in organic chemistry! If you have ever wondered how chemists determine the exact 3D architecture of complex molecules, Nuclear Magnetic Resonance (NMR) Spectroscopy is the answer. While techniques like Infrared (IR) spectroscopy reveal functional groups and Mass Spectrometry gives molecular mass and fragments, NMR maps the carbon and hydrogen skeleton of a molecule with incredible precision.

Don't worry if this topic feels a bit intimidating at first! By breaking spectra down into simple rules—counting peaks, checking peak areas, and analyzing splitting patterns—you will quickly become confident at solving any NMR puzzle the CCEA examiner throws your way.

1. Fundamental Principles of NMR

Nuclear Spin and Magnetic Fields

Just like electrons have spin, certain atomic nuclei have an intrinsic property called nuclear spin. In CCEA A2 Chemistry, we focus on two specific nuclei:

• Proton / Hydrogen-1: \(^1\text{H}\)
• Carbon-13: \(^{13}\text{C}\)

Key Rule: Any nucleus that contains an odd number of nucleons (odd number of protons and/or neutrons) possesses nuclear spin. Because a spinning nucleus is a moving electrical charge, it generates a tiny magnetic field around itself.

What is Resonance?

Under normal conditions, these tiny nuclear magnets point in completely random directions. However, when placed into a powerful external magnetic field (\(B_0\)), they are forced to align in one of two ways:

1. Low-energy state: Aligned with the external magnetic field (spin \(+\frac{1}{2}\)).
2. High-energy state: Aligned against the external magnetic field (spin \(-\frac{1}{2}\)).

The difference in energy between these two states is called \(\Delta E\). When we irradiate the sample with electromagnetic radiation in the radiofrequency (RF) region of the spectrum, the nuclei absorb energy and "flip" from the lower energy state to the higher energy state. This absorption process is known as resonance.

Key Takeaway

Nuclei with odd nucleon numbers (\(^1\text{H}\) and \(^{13}\text{C}\)) act like tiny magnets. When placed in an external magnetic field, they absorb radiofrequency radiation to flip between energy states.

2. The Reference Standard and Solvents

Tetramethylsilane (TMS): The Benchmark Standard

To measure where peaks appear, we need a universal reference point. The standard reference substance used in both \(^1\text{H}\) and \(^{13}\text{C}\) NMR is tetramethylsilane (TMS), which has the formula \(\text{Si(CH}_3)_4\). Its chemical shift is defined as exactly \(\delta = 0\text{ ppm}\).

CCEA examiners frequently ask why TMS is chosen. You must know these four key properties:

• Single, sharp, intense peak: All 12 hydrogen atoms (and all 4 carbon atoms) are chemically equivalent, producing one strong, sharp reference signal even when added in tiny amounts.
• Highly shielded: Silicon is less electronegative than carbon, which pushes electron density onto the methyl groups. This causes the TMS signal to appear far to the right (upfield), well away from almost all organic analyte signals.
• Chemically inert: It does not react with the sample being analyzed.
• Volatile and non-toxic: It has a low boiling point (\(26.5^\circ\text{C}\)), meaning it can easily be evaporated away to recover the sample intact after testing.

Choosing the Right Solvent

To analyze a solid or viscous liquid, it must be dissolved in a solvent. However, if we used normal water (\(\text{H}_2\text{O}\)) or trichloromethane (\(\text{CHCl}_3\)), the massive number of solvent hydrogen atoms would drown out the signals of our sample!

To prevent solvent interference, we use solvents that contain no \(^1\text{H}\) atoms:

1. Deuterated solvents: Such as deuterochloroform / deuterated trichloromethane (\(\text{CDCl}_3\)). Deuterium (\(^2\text{H}\) or \(\text{D}\)) has an even mass-to-charge spin state and does not resonate in the \(^1\text{H}\) frequency window.
2. Non-protic solvents: Such as tetrachloromethane (\(\text{CCl}_4\)), which contains zero hydrogen atoms.

Key Takeaway

TMS (\(\text{Si(CH}_3)_4\)) provides the reference line at \(\delta = 0\text{ ppm}\) because it is inert, volatile, highly shielded, and gives a single intense peak. Solvents like \(\text{CDCl}_3\) or \(\text{CCl}_4\) avoid solvent signal interference.

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

Low-Resolution vs. High-Resolution Spectra

In low-resolution \(^1\text{H}\) NMR, spectra appear as single lines. In high-resolution \(^1\text{H}\) NMR, higher instrument resolution reveals fine splitting patterns.

Low-Resolution \(^1\text{H}\) NMR provides TWO pieces of information:

• Number of peaks: Tells you the number of chemically non-equivalent proton environments in the molecule.
• Peak area / Integration trace: The area under each peak is directly proportional to the relative number of protons in that particular chemical environment.

High-Resolution \(^1\text{H}\) NMR provides a THIRD vital piece of information:

• Spin-spin splitting pattern: Arises from the magnetic coupling between non-equivalent protons located on adjacent carbon atoms.

The \((n+1)\) Splitting Rule

When a proton is sitting on a carbon atom, its magnetic environment is influenced by the spin states of hydrogens on adjacent (neighbouring) carbons. This splitting obeys the \((n+1)\) Rule:

If a proton environment has \(n\) non-equivalent protons on directly adjacent carbon atoms, its NMR peak splits into \((n+1)\) sub-peaks.

• \(n = 0\) neighbours \(\rightarrow 0 + 1 = 1 \rightarrow\) Singlet (Relative peak intensities: \(1\))
• \(n = 1\) neighbour \(\rightarrow 1 + 1 = 2 \rightarrow\) Doublet (Relative peak intensities: \(1:1\))
• \(n = 2\) neighbours \(\rightarrow 2 + 1 = 3 \rightarrow\) Triplet (Relative peak intensities: \(1:2:1\))
• \(n = 3\) neighbours \(\rightarrow 3 + 1 = 4 \rightarrow\) Quartet (Relative peak intensities: \(1:3:3:1\))
• \(n \ge 4\) neighbours \(\rightarrow\) Multiplet (such as quintet, sextet, septet; following Pascal's triangle for relative peak heights)

Classic Example: The Ethyl Group (\(\text{-CH}_2\text{-CH}_3\))

An isolated ethyl group attached to an electronegative group (e.g. in bromoethane, \(\text{CH}_3\text{CH}_2\text{Br}\)) shows a textbook pattern:

• The \(-\text{CH}_3\) protons have \(2\) neighbours on the adjacent \(-\text{CH}_2-\) group. Using \((n+1)\), \(2 + 1 = 3\), so the methyl signal is a triplet with an integration of \(3\text{H}\).
• The \(-\text{CH}_2-\) protons have \(3\) neighbours on the adjacent \(-\text{CH}_3\) group. Using \((n+1)\), \(3 + 1 = 4\), so the methylene signal is a quartet with an integration of \(2\text{H}\).
• Rule of Thumb: A triplet and quartet pair is the signature fingerprint of an ethyl group!

Key Takeaway

Peak count = number of environments; Integration = relative number of protons in that environment; Splitting \((n+1)\) = number of protons on neighbouring carbons plus 1.

4. Labile Protons and the \(\text{D}_2\text{O}\) Shake

Why Are \(\text{-OH}\) and \(\text{-NH-}\) Signals Tricky?

Protons directly attached to electronegative heteroatoms (such as the \(\text{-OH}\) of alcohols and carboxylic acids, or the \(\text{-NH}_2\) / \(\text{-NH-}\) of amines and amides) are called labile protons. They behave differently from carbon-bound protons:

• Their chemical shifts vary widely over broad ranges (e.g., \(\text{ROH}\) appears at \(\delta = 0.5\text{--}5.0\text{ ppm}\), \(\text{R-NH}_2\) at \(\delta = 1.0\text{--}4.5\text{ ppm}\), and \(\text{R-COOH}\) at \(\delta = 10.0\text{--}12.0\text{ ppm}\)).
• Their peaks are often broad.
• They undergo rapid proton exchange with one another in solution. Because this exchange is faster than the NMR timescale, labile protons do not participate in spin-spin splitting under standard conditions and usually appear as singlets.

The \(\text{D}_2\text{O}\) Shake Technique

Because \(\text{-OH}\) and \(\text{-NH}_2\) peaks can pop up in overlapping regions, how do we prove a peak belongs to one of them? We perform a \(\text{D}_2\text{O}\) shake:

1. Record the standard \(^1\text{H}\) NMR spectrum.
2. Add a few drops of deuterium oxide (\(\text{D}_2\text{O}\)) to the NMR tube and shake thoroughly.
3. The labile hydrogen atoms rapidly exchange with deuterium:

\(\text{R-OH} + \text{D}_2\text{O} \rightleftharpoons \text{R-OD} + \text{HOD}\)
\(\text{R-NH}_2 + 2\text{D}_2\text{O} \rightleftharpoons \text{R-ND}_2 + 2\text{HOD}\)

4. Because deuterium (\(\text{D}\)) does not produce a signal in the \(^1\text{H}\) spectrum, the original peak corresponding to the \(\text{-OH}\) or \(\text{-NH}_2\) group completely disappears, confirming its identity.

Key Takeaway

Adding \(\text{D}_2\text{O}\) replaces labile \(\text{-OH}\) or \(\text{-NH}_2\) protons with deuterium, causing their NMR peaks to vanish.

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

Features of \(^{13}\text{C}\) NMR

Carbon-12 (\(^{12}\text{C}\)) has an even number of nucleons (\(6\) protons, \(6\) neutrons) and has no nuclear spin. However, approximately \(1.1\%\) of naturally occurring carbon is Carbon-13 (\(^{13}\text{C}\)), which has an odd mass number and is NMR active.

Four key characteristics of \(^{13}\text{C}\) NMR spectra:

• Number of peaks = Number of carbon environments: Each unique carbon environment in the molecule gives rise to exactly one peak.
• Much wider chemical shift scale: The scale ranges from \(\delta = 0\text{ to } 220\text{ ppm}\) (compared to \(0\text{ to }12\text{ ppm}\) for \(^1\text{H}\) NMR).
• Proton-decoupled (No splitting): Routine \(^{13}\text{C}\) spectra are run in a decoupled mode that removes carbon-hydrogen coupling, so every peak appears as a single sharp singlet.
• Peak heights are NOT quantitative: Unlike \(^1\text{H}\) NMR, the peak heights or areas in routine \(^{13}\text{C}\) NMR are not directly proportional to the number of carbons in that environment.

Symmetry in \(^{13}\text{C}\) NMR

Always watch for molecular symmetry when counting carbon environments!

• Propan-2-one (\(\text{CH}_3\text{COCH}_3\)): Contains \(3\) carbon atoms, but because the two methyl groups are chemically equivalent, it shows only \(2\) peaks (\(1\) carbonyl carbon and \(1\) methyl carbon environment).
• Pentan-3-one (\(\text{CH}_3\text{CH}_2\text{COCH}_2\text{CH}_3\)): Contains \(5\) carbon atoms, but due to symmetry, shows only \(3\) peaks (\(1\) carbonyl, \(1\) methylene \(\text{-CH}_2\text{-}\), and \(1\) methyl \(\text{-CH}_3\)).

Key Takeaway

\(^{13}\text{C}\) NMR spectra consist of singlets across \(0\text{--}220\text{ ppm}\). The number of peaks equals the number of distinct carbon environments (accounting for symmetry). Peak areas do not tell you the number of carbon atoms.

6. Working with the CCEA Data Leaflet

In your ACH22 exam, you are provided with standard tables. Always quote chemical shift ranges directly from the Data Leaflet when justifying your answers.

\(^1\text{H}\) NMR Chemical Shift Values (Table 2)

• \(\text{ROH}\): \(\delta = 0.5\text{--}5.0\text{ ppm}\)
• \(\text{R-CH}_3\): \(\delta = 0.7\text{--}1.2\text{ ppm}\)
• \(\text{R-NH}_2\): \(\delta = 1.0\text{--}4.5\text{ ppm}\)
• \(\text{R}_2\text{CH}_2\): \(\delta = 1.2\text{--}1.4\text{ ppm}\)
• \(\text{R}_3\text{CH}\): \(\delta = 1.4\text{--}1.6\text{ ppm}\)
• \(\text{R-CO-CH}_3 / \text{R-CO-CH}_2\text{R}\) (\(\text{H}\) on C adjacent to \(\text{C=O}\)): \(\delta = 2.1\text{--}2.6\text{ ppm}\)
• \(\text{R-CH}_2\text{Cl}\) or \(\text{Br}\): \(\delta = 3.1\text{--}4.2\text{ ppm}\)
• \(\text{R-O-CH}_3 / \text{R-O-CH}_2\text{R}\) (\(\text{H}\) on C adjacent to \(\text{-O-}\)): \(\delta = 3.1\text{--}3.9\text{ ppm}\)
• \(\text{R-COO-CH}_3 / \text{R-COO-CH}_2\text{R}\) (ester alcohol side): \(\delta = 3.7\text{--}4.1\text{ ppm}\)
• \(\text{R}_2\text{C=CH-R}\) (alkene protons): \(\delta = 4.5\text{--}6.0\text{ ppm}\)
• \(\text{R-CHO}\) (aldehyde proton): \(\delta = 9.0\text{--}10.0\text{ ppm}\)
• \(\text{R-COOH}\) (carboxylic acid proton): \(\delta = 10.0\text{--}12.0\text{ ppm}\)

\(^{13}\text{C}\) NMR Chemical Shift Values (Table 3)

• \(\text{C-C}\) (alkane carbons): \(\delta = 5\text{--}40\text{ ppm}\)
• \(\text{C-Cl}\) or \(\text{C-Br}\): \(\delta = 10\text{--}70\text{ ppm}\)
• \(\text{C-N}\): \(\delta = 25\text{--}60\text{ ppm}\)
• \(\text{C-O}\) (alcohols, ethers, esters): \(\delta = 50\text{--}90\text{ ppm}\)
• \(\text{C=C}\) (alkenes/aromatics): \(\delta = 90\text{--}150\text{ ppm}\)
• \(\text{C}\equiv\text{N}\) (nitriles): \(\delta = 110\text{--}125\text{ ppm}\)
• Aromatic carbons: \(\delta = 110\text{--}160\text{ ppm}\)
• \(\text{R-COO-}\) (esters, carboxylic acids): \(\delta = 160\text{--}185\text{ ppm}\)
• \(\text{R-CHO} / \text{R-CO-R}\) (aldehydes, ketones): \(\delta = 190\text{--}220\text{ ppm}\)

7. Step-by-Step Structural Deduction & Common Pitfalls

Step-by-Step Strategy for \(^1\text{H}\) NMR Interpretation

Step 1: Ignore the TMS peak at \(\delta = 0\text{ ppm}\).
Step 2: Count the number of signals to determine the number of proton environments.
Step 3: Check the integration ratio to find the relative number of hydrogens producing each signal.
Step 4: Use the splitting pattern \((n+1)\) to determine how many hydrogens are on neighbouring carbon atoms (\(\text{neighbours} = \text{number of lines} - 1\)).
Step 5: Match chemical shifts (\(\delta\)) to the CCEA Data Leaflet to identify adjacent functional groups (e.g. carbonyls, electronegative halogens, ester oxygens).
Step 6: Assemble the structural fragments like jigsaw pieces and verify that all coupling partners match!

Top Examiner Pitfalls to Avoid

• Confusing Protons vs. Neighbouring Protons: Remember that integration gives the number of protons giving rise to that signal, while the splitting pattern tells you the number of protons on the adjacent carbon.
• Splitting Equivalent Protons: Protons in the same environment do not split each other. For example, the three protons in an isolated \(-\text{CH}_3\) group do not split one another into a quartet—they appear as a singlet unless there is a neighbouring carbon with hydrogens.
• Splitting Across Oxygen or Nitrogen: Under standard conditions, protons on an \(\text{-OH}\) or \(\text{-NH}_2\) group do not couple with protons on adjacent carbon atoms.
• Vague Shift Quotes: Do not write "around 2 ppm". Write: "peak at \(\delta = 2.1\text{--}2.6\text{ ppm}\) indicates a \(\text{-CH}_3\) adjacent to a \(\text{C=O}\) group".
• Treating \(^{13}\text{C}\) Peak Heights as Integrations: Carbon-13 peak heights do not correlate to carbon counts—only count the number of peaks!

Quick Revision Summary Box

• Reference: TMS (\(\text{Si(CH}_3)_4\)) at \(\delta = 0\text{ ppm}\); inert, volatile, highly shielded, 12 equivalent protons.
• Solvent: \(\text{CDCl}_3\) or \(\text{CCl}_4\) (contains no \(^1\text{H}\)).
• \(^1\text{H}\) NMR: Number of peaks = environments; Area = proton ratio; Splitting = \((n+1)\) rule.
• \(\text{D}_2\text{O}\) Shake: Labile \(\text{-OH}\) and \(\text{-NH}_2\) peaks disappear via exchange.
• \(^{13}\text{C}\) NMR: Singlet peaks across \(0\text{--}220\text{ ppm}\); number of peaks = carbon environments (check for symmetry!).