--- title: "Carbon-13, DEPT and Coupling Constants" description: "The carbon spectrum shows the skeleton directly, and an editing experiment says how many hydrogens sit on each carbon. Coupling constants, read carefully, then report on geometry rather than just on n" canonical: https://lightmysky.com/learn/science/carbon-13-dept-and-coupling-constants-mt_9K6GkAkqq1 source: https://lightmysky.com/learn/science/carbon-13-dept-and-coupling-constants-mt_9K6GkAkqq1.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Carbon-13, DEPT and Coupling Constants The carbon spectrum shows the skeleton directly, and an editing experiment says how many hydrogens sit on each carbon. Coupling constants, read carefully, then report on geometry rather than just on neighbours. Subject: Science · Area: Chemistry · Ages 20 to 21 Page: https://lightmysky.com/learn/science/carbon-13-dept-and-coupling-constants-mt_9K6GkAkqq1 ## Ready when they can - Explains why a routine carbon spectrum shows singlets and why the integrals are not reliable - Reads a DEPT experiment to classify each carbon by the number of attached protons - Extracts a coupling constant in hertz and uses its size to argue for cis or trans geometry - Combines carbon and proton evidence to distinguish two candidate structures ## Lesson: Reading carbon spectra and couplings The carbon spectrum shows the skeleton directly, one signal per distinct carbon. Routine spectra are run with the protons decoupled: the instrument irradiates them while it records, so C-H splitting collapses and every carbon gives a singlet. Peak heights cannot be trusted for counting. Carbons relax at different rates and get different boosts from the protons (the nuclear Overhauser effect), so a carbon with no hydrogens often gives a small peak. An editing experiment called DEPT sorts carbons by attached protons. In DEPT-135, CH and CH3 carbons point up and CH2 carbons point down. DEPT-90 shows only CH carbons. Carbons with no protons stay silent in both, which is itself a clue: compare with the full carbon spectrum to find them. **Example.** A coupling constant in hertz reports on geometry, not just neighbours. Measure the gap between two lines of a multiplet in ppm and multiply by the spectrometer frequency in MHz: a gap of 0.03 ppm at 400 MHz is 12 Hz. For two hydrogens across a C=C, trans couplings are about 12 to 18 Hz and cis couplings about 6 to 12 Hz. So given cis and trans candidates, the larger coupling fits trans. Combine both nuclei to kill one candidate. If a carbon signal or a coupling fits only one structure, the other is out. Name the exact signal that decided it. **Recap.** Carbon maps the skeleton, DEPT counts attached protons, and coupling size argues geometry. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Reading a Proton NMR Spectrum: Shift, Integration and Multiplicity](https://lightmysky.com/learn/science/reading-a-proton-nmr-spectrum-shift-integration-and-multiplicity-mt_WgxqHHf-YL) ## Opens up - [Two-Dimensional NMR and Assigning a Whole Structure](https://lightmysky.com/learn/science/two-dimensional-nmr-and-assigning-a-whole-structure-mt_UEC4dilKT9)