J Coupling Calculator

Calculator

Choose an input mode and provide the required values. Large screens show three columns, smaller screens show two, and mobile shows one.

Input mode Peak separation in ppm Line separation in Hz

First-order: J equals the spacing between adjacent lines.

Spectrometer frequency (MHz)

Used to convert ppm ↔ Hz (ppm × MHz = Hz).

Equivalent neighbors (n)

Predicts lines (n+1) and Pascal intensity ratios.

Peak separation Δδ (ppm)

Use |δ1 − δ2| when peaks are resolved in ppm.

Line separation Δν (Hz)

If you already measured spacing in Hz, enter it here.

Calculate

Example data

Formula used

• Coupling from ppm spacing: J (Hz) = Δδ (ppm) × ν₀ (MHz)
• Peak separation: Δδ = |δ₁ − δ₂|
• ppm spacing from coupling: Δδ (ppm) = J (Hz) / ν₀ (MHz)
• First-order multiplicity: Lines = n + 1
• Relative intensities: binomial coefficients C(n,k) for k = 0…n

Real spectra can deviate due to second-order effects, strong coupling, overlap, or non-equivalent neighbors.

How to use

1. Select an input mode: ppm spacing or Hz spacing.
2. If using ppm, enter the spectrometer frequency in MHz.
3. Enter the measured spacing (Δδ or Δν) from adjacent lines.
4. Set the number of equivalent neighbors (n) to predict patterns.
5. Press Calculate to see J, line count, and intensity ratios.
6. Use the download buttons to save CSV or PDF reports.

Professional article

1) What J coupling represents

Scalar coupling (J) is a through-bond interaction between nuclear spins that splits an NMR resonance into a multiplet. In first-order spectra, the spacing between adjacent lines equals J, independent of the chemical shift position. This calculator focuses on extracting J from measured line or peak separations.

2) Why the result is reported in hertz

Coupling is a frequency difference, so it is naturally expressed in hertz. Chemical shift in ppm is field-normalized, but J is field-independent for a given molecule and coupling pathway. Reporting J in hertz makes values comparable between instruments, solvents, and acquisition settings, provided the same coupling is being measured.

3) Converting ppm spacing to hertz accurately

For a spectrometer frequency ν₀ in MHz, a separation Δδ in ppm corresponds to Δν in hertz through Δν = Δδ × ν₀. The MHz-to-hertz scaling is built into the ppm definition, so the product gives hertz directly. Enter the instrument frequency used for the spectrum to avoid systematic conversion error.

4) Selecting the right peaks or lines

Measure spacing between adjacent lines within the same multiplet, not between distant outer peaks that may include multiple couplings. If overlap is present, use the clearest pair of neighboring lines. When signals are broadened, take the midpoint between shoulders. For higher confidence, confirm spacing consistency across several adjacent pairs.

5) Multiplicity from equivalent neighbors

For n equivalent neighbors, simple splitting predicts n+1 lines with relative intensities given by Pascal coefficients. The calculator reports both the line count and the intensity ratio (for example 1:2:1 for a triplet). These ratios guide peak assignment, integration checks, and simulation starting parameters in spectral fitting workflows.

6) Typical coupling magnitudes

Coupling strengths depend on coupling order and bonding geometry. Many common proton–proton vicinal couplings fall in the single-digit hertz range, while geminal couplings are often larger. Long-range couplings can be small and sometimes near the linewidth. Treat “typical” values as context, not rules, and rely on your measured spacing.

7) When first-order assumptions break down

The adjacent-line spacing equals J only under first-order conditions, typically when chemical shift differences are large compared with coupling. Strong coupling, second-order effects, and AB patterns can distort apparent spacings and intensities. If the multiplet looks asymmetric or “roofed,” consider a full spin simulation rather than a single-spacing estimate.

8) Reporting, saving, and reproducibility

Good reporting includes J (Hz), the nucleus pair if known, the field strength, solvent, temperature, and how spacing was measured. The CSV export captures numeric inputs and outputs for lab notebooks or LIMS entry. The PDF report is convenient for sharing with collaborators, attaching to assignments, or archiving analysis decisions alongside spectra.

FAQs