Questions: Nuclear Magnetic Resonance Spectroscopy for Structure Determination

Question 1

A ¹H NMR signal at 3.5 ppm integrates for 2 protons and appears as a triplet. What do these three pieces of information collectively indicate?

• A. Two protons that are adjacent to one neighboring proton
• B. Two protons near an electronegative atom that are coupled to two neighboring protons
• C. Three protons that are coupled to two neighboring protons
• D. Two protons in an aromatic ring with two equivalent neighbors

Answer: (B)

Integration of 2 means there are 2 protons in this environment. A triplet means n+1 = 3, so n = 2 neighboring protons are coupling to this signal. The chemical shift of ~3.5 ppm suggests proximity to an electronegative atom (O, N, halogen), which deshields protons and shifts them downfield from the alkyl region (~1 ppm). All three pieces of information are independent and must be read separately.

Question 2

In ¹H NMR, a proton with a larger chemical shift (more ppm) is more shielded by surrounding electrons.

• T. True
• F. False

Answer: False

Larger chemical shift (more ppm, downfield) means the proton is more DESHIELDED — it has less electron density around it and resonates at higher frequency relative to TMS. Electron-withdrawing groups pull electron density away from nearby protons, reducing shielding and increasing their chemical shift. Protons on carbons attached to O, N, or halogens, as well as aromatic and vinyl protons, appear at higher ppm precisely because they are deshielded.

Question 3

The ¹H NMR spectrum of ethanol (CH₃CH₂OH) shows a triplet for CH₃ and a quartet for CH₂. Explain why these multiplicities arise and what structural information they provide.

Model answer: The CH₃ group has 2 neighboring protons (on CH₂), so n+1 = 3 lines (triplet). The CH₂ group has 3 neighboring protons (on CH₃), so n+1 = 4 lines (quartet). These coupling patterns confirm that the CH₃ and CH₂ groups are on adjacent carbons — the multiplicity directly encodes which carbons are bonded to each other.

The n+1 rule (first-order approximation) predicts the multiplicity of a proton signal based on how many equivalent neighboring protons couple to it. J-coupling occurs through bonds, typically up to 3 bonds. Reading the multiplicity pattern across a spectrum lets you reconstruct adjacencies: if signal A is split by signal B and vice versa, A and B are on neighboring carbons. This makes multiplicity one of NMR's most powerful connectivity-mapping tools.