Carbon-13 NMR Spectroscopy

Spec Mapping — OCR H432 Module 6.3.2 (a) — Carbon-13 NMR spectroscopy, covering the physical principle of NMR (nuclear spin in an external magnetic field, RF absorption at the resonance frequency, chemical shift as a probe of local electronic environment); the natural-abundance ¹³C isotope (~1.1%, spin-1/2) as the NMR-active form of carbon and the role of signal averaging; the tetramethylsilane (TMS) reference at delta = 0 ppm and the right-to-left ppm scale (0 to ~220); identification of the number of chemically distinct carbon environments by molecular symmetry analysis; use of the OCR data-booklet chemical shift table to identify the type of carbon (alkyl 5-55 ppm, C-X 10-70, C-N 30-65, C-O 50-90, C=C 100-150, aromatic 110-135, C≡N 110-125, C=O ester/amide 160-185, C=O acid 170-185, C=O aldehyde/ketone 190-220); worked examples linking environment count + chemical shift to a unique structure; deuterated solvents (CDCl₃, D₂O, DMSO-d₆) and the CDCl₃ triplet at 77 ppm (refer to the official OCR H432 specification document for exact wording).

Nuclear magnetic resonance (NMR) spectroscopy is probably the single most powerful analytical tool in modern organic chemistry. It tells you not just the molecular formula (like MS) or the functional groups (like IR), but the detailed connectivity of every atom — which carbon is bonded to which, how many hydrogens are on each, and what their neighbours look like. NMR is the technique that established the structures of complex natural products such as vitamin B12 and taxol, that confirms the identity of every newly synthesised drug, that resolves the three-dimensional fold of every protein deposited in the PDB structural-biology database, and that — in its hospital-imaging form (MRI) — produces tens of millions of clinical diagnostic scans per year worldwide. The underlying physics was first observed independently in 1946 by Felix Bloch and Edward Purcell, who shared the 1952 Nobel Prize in Physics for it (paraphrase); the technique was transformed in the 1970s by Richard Ernst's invention of pulsed Fourier-transform NMR (Nobel Prize 1991 — paraphrase), which collapsed multi-hour swept-frequency scans into multi-second pulsed acquisitions; and modern multi-dimensional NMR was developed by Kurt Wüthrich for protein structure (Nobel Prize 2002 — paraphrase). For OCR H432 you study the two simplest NMR techniques: ¹³C NMR (this lesson, looking at carbon environments) and ¹H NMR (Lesson 14, looking at hydrogen environments and the n+1 splitting rule). Together these tell you the full carbon-and-hydrogen connectivity of an unknown organic compound.

Key Definition — NMR spectroscopy: an analytical technique in which a sample is placed in a strong external magnetic field; nuclei with non-zero spin (¹H, ¹³C, ¹⁹F, ³¹P and others) align with or against the field; a radio-frequency pulse flips them to the higher-energy alignment; the resonance frequency at which absorption occurs depends sensitively on the local chemical environment of each nucleus. The output is a spectrum of intensity against chemical shift (delta, ppm), with each peak corresponding to a unique chemical environment of the observed nucleus.

---

1. The Idea Behind NMR

Some nuclei — including ¹H and ¹³C — have a property called spin, which makes them behave like tiny magnets. When you place them in a strong external magnetic field (typically 1.4 T to 21 T in modern instruments), they precess at a frequency proportional to the field strength, with two slightly different precession energies depending on whether the nuclear spin points with the field (lower energy) or against it (higher energy). If you then hit them with a radio-frequency pulse of just the right frequency (the Larmor frequency), you can flip them from the lower-energy alignment to the higher-energy alignment. The energy absorbed at that flipping frequency is what the NMR instrument detects.

The clever part: the exact frequency needed for each nucleus depends on the local chemical environment — the number and types of electrons around it, and what atoms are nearby. Atoms in different environments shield the nucleus by different amounts, leading to slightly different Larmor frequencies. Each environment gives a different frequency, and each frequency appears as a distinct peak in the spectrum.

Key Definition — Chemical environment: A nucleus together with the specific arrangement of atoms and bonds attached to it. Two nuclei are in the same environment if they are chemically equivalent by molecular symmetry (i.e. you can swap them by some symmetry operation and the molecule looks identical).

1.1 Why ¹³C specifically?

Carbon has two stable isotopes: ¹²C (~98.9% natural abundance, nuclear spin = 0, NMR-invisible) and ¹³C (~1.1% natural abundance, nuclear spin = 1/2, NMR-active). Because only about 1 in 100 carbon nuclei in any sample is the active ¹³C isotope, the signal is intrinsically weak compared with ¹H NMR. To compensate, ¹³C experiments use:

• Larger sample masses (10-50 mg, vs ~1 mg for ¹H).
• Many repeated scans (often hundreds to thousands), digitally co-added in software to average down the random noise — the signal-to-noise ratio improves as the square root of the number of scans.
• Higher-field magnets for better resolution and intrinsic sensitivity.

Modern instruments handle this routinely; a typical "overnight ¹³C" might take 16 hours and give a clean spectrum from a 20 mg sample at 100 MHz ¹³C frequency. Pulsed Fourier-transform NMR (Ernst, 1970s) acquires all frequencies simultaneously, which is why this is feasible at all.

1.2 Why ¹³C is so useful

¹³C NMR has two big advantages over ¹H NMR for the first look at a new molecule:

• The chemical shift range is wider — about 0 to 220 ppm, compared with 0 to 12 ppm for ¹H. This spreads out the peaks and makes assignment easier; overlap is rare.
• You see one peak per unique carbon. Most ¹³C spectra are recorded with broad-band ¹H decoupling, which removes the splitting from neighbouring protons and collapses each carbon environment to a single sharp peak. So counting peaks directly gives the number of unique carbons.

---

2. What a ¹³C Spectrum Looks Like

A ¹³C NMR spectrum is a plot of signal intensity (y-axis) against chemical shift δ in parts per million (ppm, x-axis). Some key conventions:

• The x-axis runs backwards — higher ppm values are on the left.
• The reference peak (at δ = 0) is tetramethylsilane (TMS): (CH₃)₄Si. It gives a sharp singlet at 0 ppm.
• ¹³C chemical shifts span a wide range: 0 to 220 ppm (much wider than ¹H, which is 0–12 ppm).
• Each peak corresponds to one unique carbon environment in the molecule.

2.1 Why TMS?

TMS is the universal reference because:

• Its 12 equivalent H atoms (and 4 equivalent C atoms) give a single sharp peak, making it easy to identify.
• Silicon is less electronegative than carbon, so the C nuclei in TMS are very shielded and appear at a chemical shift lower than almost any other organic carbon. TMS therefore marks the "right edge" of the spectrum at δ = 0 ppm.
• It is inert, soluble in most deuterated NMR solvents, and volatile enough to be removed at the end of the experiment.

---

3. Counting Carbon Environments

The first step in interpreting a ¹³C spectrum is to work out how many different carbon environments the molecule has. Each unique environment gives one peak.

3.1 How to count environments

Two carbons are in the same environment if you can swap them by some symmetry operation (rotation or reflection) and the molecule looks identical. Otherwise they are in different environments.

Example 1: Ethanol, CH₃CH₂OH

• C1 of CH₃ is bonded to 3H and 1 C(OH) → one environment.
• C2 of CH₂OH is bonded to 2H, 1 C, and 1 O → a different environment.
• Total: 2 environments → 2 peaks.

Example 2: Propan-2-ol, (CH₃)₂CHOH

• The two CH₃ groups are equivalent by the plane of symmetry through C–OH → one environment.
• The CH carbon (C2) is bonded to 2 CH₃, 1 H, and 1 OH → a different environment.
• Total: 2 environments → 2 peaks.

Example 3: Butan-2-one, CH₃COCH₂CH₃

• C1 of CH₃ (left) is bonded to 3H and the C=O.
• C2 is the carbonyl C (C=O).
• C3 of CH₂ is bonded to 2H, the C=O, and the terminal CH₃.
• C4 of CH₃ (right) is bonded to 3H and CH₂.
• All four are different → 4 environments → 4 peaks.

Example 4: Benzene, C₆H₆

• All 6 carbons are equivalent by rotation around the C6 axis.
• Total: 1 environment → 1 peak.

Example 5: Methylbenzene (toluene), CH₃–C₆H₅

• The CH₃ → 1 environment.
• The aromatic ring has four unique environments: the ipso C (attached to CH₃), ortho Cs (two, equivalent), meta Cs (two, equivalent), para C (one).
• Total: 1 + 4 = 5 environments → 5 peaks.

graph TD
  A[Count carbon environments] --> B[Draw molecule]
  B --> C[Look for planes of symmetry]
  C --> D[Group equivalent C together]
  D --> E[Each group = 1 peak]

---

4. Chemical Shift — The Position of Each Peak

The chemical shift tells you what kind of environment a carbon is in. Carbon atoms near electron-withdrawing groups (–O, –Cl, C=O) are deshielded and appear at higher δ. Carbon atoms with only alkyl neighbours are shielded and appear at lower δ.

OCR provides a chemical shift chart in the data booklet. You do not need to memorise exact values, but you should recognise the ranges to identify the type of carbon from its shift.

4.1 ¹³C chemical shift table (OCR data booklet)

Carbon type	Typical δ range (ppm)
Alkyl C (R–CH₃, R–CH₂–R, R–CHR–R, R–CR₃)	5 – 55
C–Cl, C–Br (haloalkane carbon)	10 – 70
C–N (amine carbon)	30 – 65
C–O (alcohol, ether carbon)	50 – 90
C≡C alkyne	65 – 95
C=C alkene	100 – 150
Aromatic C (benzene ring)	110 – 135
C≡N nitrile	110 – 125
C=O ester, amide	160 – 185
C=O carboxylic acid	170 – 185
C=O aldehyde, ketone	190 – 220

Things to notice:

• Alkyl carbons are near the right-hand edge (low δ) because they are electron-rich.
• Carbons bonded to oxygen (alcohols, esters) are around 50–90 ppm.
• Aromatic and alkene carbons are around 110–150 ppm.
• Carbonyl carbons are at very high δ (160–220 ppm), far to the left, because they are strongly deshielded.
• Aldehyde/ketone C=O (190–220) is further left than acid/ester C=O (160–185).

4.2 Worked example — ethanol

Ethanol has 2 carbon environments: CH₃ and CH₂OH.

• CH₃ (alkyl, far from O) → around 18 ppm.
• CH₂OH (C bonded to O) → around 58 ppm.

So the spectrum shows two peaks, at ~18 and ~58 ppm. This matches the actual spectrum of ethanol.

4.3 Worked example — ethanal vs ethanoic acid

Both are 2-carbon molecules with a C=O on C1.

Ethanal CH₃CHO:

• CH₃ → ~31 ppm (alkyl).
• CHO → ~200 ppm (aldehyde carbonyl, far left).

Ethanoic acid CH₃COOH:

• CH₃ → ~21 ppm (alkyl, slightly different shift).
• COOH → ~178 ppm (acid carbonyl, slightly less deshielded than aldehyde).

The carbonyl chemical shift distinguishes the two: 200 ppm (aldehyde) vs 178 ppm (acid).

---

5. Worked Interpretations

5.1 Unknown compound — C₃H₆O with 2 peaks

Problem: An organic compound with molecular formula C₃H₆O shows a ¹³C NMR spectrum with two peaks: one at δ ≈ 205 ppm and one at δ ≈ 31 ppm. What is the compound?

Step 1 — molecular formula candidates. C₃H₆O: possible structures include propan-2-ol, propanal, propanone (acetone), methoxyethene, propene oxide.

Step 2 — number of peaks. Only two environments → the molecule has high symmetry. Propanone CH₃COCH₃ has two equivalent CH₃ groups (1 environment, related by symmetry through the C=O) plus 1 C=O carbon (a different environment) = 2 environments total ✓. Propanal CH₃CH₂CHO has three carbons, all different → 3 environments, doesn't fit. Propan-2-ol has 2 environments but no C=O peak. Methoxyethene has 3 environments. So propanone is the only candidate consistent with two peaks.

Step 3 — chemical shifts. A peak at 205 ppm is a ketone or aldehyde C=O (range 190-220 ppm); a peak at 31 ppm is an alkyl CH₃. This matches propanone perfectly.

Answer: propanone (acetone), CH₃COCH₃.

5.2 Worked example — ¹³C spectrum of ethyl ethanoate

Ethyl ethanoate (an ester) CH₃-COO-CH₂-CH₃ has four chemically distinct carbon environments:

Carbon	Position	Predicted δ / ppm
C1: -CO- ester carbonyl	bonded to -O-R, =O, and CH₃	~170-175 (acid/ester C=O range)
C2: -OCH₂-	bonded to ester O, CH₃	~60-65 (C bonded to O)
C3: terminal -CH₃ of ethyl	bonded to OCH₂	~14-18 (alkyl)
C4: CH₃ of acid	bonded to C=O	~20-25 (alkyl adjacent to C=O)