FSc Part 2 Chemistry Chapter 30 Complete Guide Book — Spectroscopy-2 (NMR) | PunjabTest.com

Nuclear Magnetic Resonance (NMR) lets us count proton and carbon environments and work out structures. This unit covers the principle of proton NMR, the TMS standard, chemical shift, peak areas, the n+1 splitting rule, deuterated solvents, the D₂O shake test, and carbon-13 NMR — ending with full structure deductions from spectra.

NMR relies on the magnetic properties of nuclei known as nuclear spin. Nuclei such as ¹H and ¹³C behave like tiny magnets. In a strong magnetic field they align with or against the field, and radio-frequency radiation flips them between these states. The exact frequency needed depends on the nucleus's chemical environment — that is the basis of NMR.

TMS (tetramethylsilane, Si(CH₃)₄) is the standard for ¹H NMR for two reasons: (1) its 12 protons are all equivalent, giving a single sharp peak; (2) it is chemically inert and volatile (easily removed), and its peak appears at the extreme upfield end (defined as δ = 0 ppm) without overlapping other signals. Chemical shift (δ) is measured relative to TMS and expressed in parts per million (ppm) — not in tesla, hertz or degrees Celsius.

Chemical shift (δ) tells us about a proton's chemical environment: protons near electronegative atoms or π systems are deshielded and appear at higher δ (downfield, left); protons in electron-rich environments are shielded and appear at lower δ (upfield, right). Typical ranges: alkyl C–H ~0.9–1.7 ppm, C–H next to O/N/halogen ~3–4 ppm, aldehyde H ~9–10 ppm, carboxylic-acid O–H ~11 ppm.

The primary purpose of measuring peak area in ¹H NMR is to find the relative number of protons in each environment. Example: a compound C₅H₁₀O with a peak-area ratio of 1:3:6 has 1 + 3 + 6 = 10 protons in total, so the peaks represent 1H, 3H and 6H respectively. In a low-resolution spectrum you use peak count and area ratio; in a high-resolution spectrum you also use splitting.

Splitting happens because neighbouring protons' spins slightly change the magnetic field felt by a proton. The n+1 rule: a signal splits into (n+1) peaks, where n is the number of protons on the adjacent carbon atoms (not the whole molecule, not the carbon being analysed, not the signal's area).

Examples: a proton with 2 neighbouring protons (n=2) gives a triplet. The –CH₂– signal in ethanol (next to –CH₃, 3 protons) splits into a quartet. A –CH₂– group lying between a –CH₃ and a –CH< group has 3 + 1 = 4 neighbours → a quintet (5 peaks). Splitting is typically not observed for O–H or N–H protons because of rapid chemical exchange: the proton swaps between molecules so fast that the coupling is averaged out, and the peak appears as a singlet.

Deuterated solvents (e.g. CDCl₃) are used in ¹H NMR to avoid solvent signal interference — ordinary solvents contain protons that would swamp the spectrum. Deuterium (D) does not appear in a ¹H NMR spectrum because it is a different nucleus with a different resonance frequency; the spectrometer is tuned to ¹H only.

D₂O shake test: add a few drops of D₂O to the sample and re-run the spectrum. Exchangeable protons (O–H, N–H) swap with deuterium: R–OH + D₂O ⇌ R–OD + HOD. Because deuterium resonates at a different frequency, the O–H/N–H peak disappears from the ¹H NMR spectrum. This is how you confirm a sample contains an O–H group: the peak that vanishes after the shake is the exchangeable proton.

In ¹³C NMR, the number of peaks equals the number of chemically non-equivalent carbons (not the total carbon atoms, not the hydrogens). The chemical shift of a ¹³C peak identifies the functional-group environment (C=C, C=O, C–O, etc.). ¹³C spectra are usually proton-decoupled, so each carbon gives a singlet.

Examples: propane (CH₃CH₂CH₃) shows 2 peaks (the two equivalent CH₃ carbons, and the central CH₂). Pentane shows 3 peaks (two equivalent terminal CH₃, two equivalent CH₂, one central CH₂). 2,2-dimethylpropane shows 2 peaks (four equivalent CH₃ carbons, and the central quaternary carbon).

Structure deduction from ¹H NMR: butanoic acid (CH₃CH₂CH₂COOH) shows: CH₃ triplet (3H, ~0.9 ppm), CH₂ sextet (2H, ~1.6 ppm, 5 neighbours), CH₂ triplet (2H, ~2.3 ppm, next to C=O), and COOH singlet (1H, ~11 ppm, disappears on D₂O shake). A spectrum with only a 3H triplet (~1.2 ppm) and a 2H quartet (~3.7 ppm) fits chloroethane, CH₃CH₂Cl. A compound C₄H₈O that gives a positive 2,4-DNPH test (carbonyl), a positive iodoform test (CH₃CO–) and a negative Fehling's test (not an aldehyde) is butan-2-one (CH₃COCH₂CH₃: 3H singlet, 2H quartet, 3H triplet).