Did you know? A 7 Hz proton NMR doublet stays split by 7 Hz on both 400 and 600 MHz instruments, yet it occupies less space on a ppm axis at 600 MHz. The coupling constant has not changed—the unit conversion has.
This original Hougang JC2 H3 Chemistry tutor guide applies the 2027 9813 first-order NMR outcomes to chemical shift, scalar J coupling, Hz/ppm conversions, the Δν/J ratio and why a higher-field spectrum can look simpler. All numerical spectra are explicitly original hypothetical examples.
EDUKATESG · HOUGANG · JC2 H3 NMR CHEMISTRY
15 chapters · 400/600 MHz conversion table · Independent calculations · FAQs · Official syllabus and references
Full chapter index: ppm and Hz are different measures
- 1. One doublet, two instruments, different apparent widths
- 2. Define J-coupling as an interaction between nuclear spins
- 3. Understand why chemical shifts are reported in ppm
- 4. Derive the 400 MHz conversion before memorising a shortcut
- 5. Repeat the same calculation at 600 MHz
- 6. Follow the two lines of a fictitious resonance at δ3.50
- 7. Distinguish two signal centres from one multiplet’s component lines
- 8. Explain why higher field can make first-order patterns easier to recognise
- 9. A doublet is not two separate proton environments
- 10. Apply the concept to a triplet and quartet
- 11. Extend to a doublet of doublets with two different J values
- 12. Know when field-independent coupling is an approximation
- 13. Avoid mixing ¹H and ¹³C conversion factors
- 14. Distinguish chemical-shift referencing from a physical coupling constant
- 15. Final transfer: two instruments and one unknown compound
1. One doublet, two instruments, different apparent widths
A proton NMR doublet can contain two lines separated by a coupling constant of 7 Hz. If the same ordinary spin system is observed on both a 400 MHz and a 600 MHz proton NMR instrument, its intrinsic scalar coupling remains about 7 Hz under comparable conditions. Yet the separation measured along an axis labelled in parts per million is smaller on the 600 MHz spectrum.
On a 400 MHz ¹H instrument, one ppm corresponds to 400 Hz. Therefore a 7 Hz line separation is 7/400=0.0175 ppm. On a 600 MHz instrument, one ppm corresponds to 600 Hz, so the same 7 Hz splitting occupies only 7/600≈0.0117 ppm.
The difference does not mean a stronger magnetic field has weakened the bond connecting the coupled nuclei or removed their nuclear spin interaction. The physical J-coupling in Hz remains approximately constant; the ppm scale uses a different absolute frequency conversion at each spectrometer operating frequency.
The 2027 H3 Chemistry 9813 Section 1.4 includes chemical shift and first-order spin-spin splitting. These numerical instrument comparisons are a useful extension that makes the Hz/ppm distinction explicit while respecting that detailed instrumentation is not required.
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2. Define J-coupling as an interaction between nuclear spins
Scalar spin-spin coupling arises through bonding electrons and changes the energies or resonance frequencies associated with the spin states of coupled nuclei. In a simple first-order proton spectrum, one neighbouring spin-½ proton can split an observed proton resonance into a doublet with two lines separated by the coupling constant J.
J is normally reported in hertz, an absolute frequency unit. Its magnitude reflects the coupled spin system, including the bonding path and molecular geometry. Many ordinary vicinal proton–proton coupling constants are several hertz, although actual values vary with structure.
A chemical shift δ describes the location of a resonance relative to a reference, whereas J describes fine splitting within a multiplet. The two are not the same physical quantity even if both are seen as horizontal distances on a displayed spectrum.
A tutor should first label the centre of a multiplet and the distance between its component lines. The centre relates to chemical shift; the spacing relates to J.
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3. Understand why chemical shifts are reported in ppm
The absolute proton resonance frequency depends strongly on the static magnetic field. A typical 400 MHz proton instrument has a reference ¹H resonance near 400 million cycles per second; a 600 MHz instrument has a corresponding reference near 600 million cycles per second.
Chemical environments shift these frequencies slightly relative to a reference such as tetramethylsilane, TMS. The chemical-shift definition expresses the offset as a fraction of the reference frequency multiplied by one million: δ≈10⁶(ν−νref)/νref. This gives a convenient ppm scale that is largely comparable across different field strengths.
For example, a resonance at δ3.50 ppm remains around δ3.50 on 400 and 600 MHz instruments under corresponding conditions. Its absolute frequency offset from the reference changes: approximately 1400 Hz on 400 MHz and 2100 Hz on 600 MHz.
The ppm scale therefore normalises the chemical-shift position to the instrument field. It does not claim that a 7 Hz spin coupling becomes a different intrinsic coupling when expressed on that normalised axis.
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4. Derive the 400 MHz conversion before memorising a shortcut
Because one ppm is one part in a million, a 400 MHz proton resonance frequency gives 400×10⁶ Hz×10⁻⁶=400 Hz per ppm. A frequency separation of 7 Hz can therefore be converted to ppm by dividing 7 by 400.
The result is 0.0175 ppm. The same operation applies to any first-order splitting on the 400 MHz ¹H scale. A 12 Hz coupling corresponds to 0.0300 ppm and a 5 Hz coupling to 0.0125 ppm under the same simplified conversion.
Students sometimes divide the frequency by 400 million and call the result ppm without multiplying by a million. That would produce a much smaller numerical answer in the wrong representation. The simpler Hz-per-ppm conversion avoids a factor-of-a-million mistake.
A tutor should make the units explicit: 7 Hz divided by 400 Hz/ppm yields 0.0175 ppm. Unit cancellation verifies the method.
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5. Repeat the same calculation at 600 MHz
At 600 MHz proton resonance, one ppm corresponds to 600 Hz. The identical 7 Hz J-coupling therefore occupies 7/600≈0.01167 ppm, typically rounded to 0.0117 ppm in a fictional exercise.
Compare 0.0117 ppm with 0.0175 ppm at 400 MHz. The apparent width in ppm is two-thirds of the earlier width because the instrument frequency increased by a factor of 600/400=1.5. The corresponding inverse relation is 400/600=2/3.
If a student incorrectly states that the coupling constant fell from 7 Hz to 4.67 Hz, they have multiplied a physical property by the wrong factor. What shrank was the ppm coordinate distance, not the intrinsic Hz separation.
A doublet plotted with identical centimetres-per-ppm scaling can therefore look narrower in the higher-field spectrum even though its frequency splitting is unchanged.
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6. Follow the two lines of a fictitious resonance at δ3.50
Imagine an ideal first-order doublet centred at δ3.50 ppm with J=7.0 Hz. Each component is separated from the centre by half the coupling, 3.5 Hz. On a 400 MHz proton scale, 3.5 Hz corresponds to 0.00875 ppm.
The two component positions are therefore approximately 3.49125 and 3.50875 ppm, ignoring sign conventions associated with plotting direction. Their difference is 0.01750 ppm, exactly the converted 7 Hz spacing.
On a 600 MHz scale the half-spacing is 3.5/600≈0.005833 ppm. The corresponding positions become about 3.49417 and 3.50583 ppm, separated by 0.01167 ppm. The centre remains at 3.50 ppm and J remains 7 Hz.
These are deliberately invented numerical line positions to illustrate the coordinate conversion; they are not measured resonances for a named compound.
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7. Distinguish two signal centres from one multiplet’s component lines
Suppose two different proton environments have chemical shifts δ3.50 and δ3.70 ppm, a difference of 0.20 ppm. On a 400 MHz proton instrument, the centres are separated by 0.20×400=80 Hz. On a 600 MHz instrument, they are separated by 0.20×600=120 Hz.
This is the opposite field trend from a fixed J when viewed on the ppm axis. The chemical-shift difference remains approximately 0.20 ppm under comparable conditions, but its absolute frequency separation increases with field. Meanwhile a 7 Hz coupling stays roughly 7 Hz and occupies less ppm space.
This distinction is crucial when interpreting spectral crowding. Increasing field strength often spreads chemically different resonances farther apart in hertz relative to their scalar couplings, even though the ppm chemical-shift locations look similar.
A student should never apply the same 400-to-600 MHz scaling to every spectral distance without first deciding whether it represents a chemical-shift difference or a spin-spin coupling.
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8. Explain why higher field can make first-order patterns easier to recognise
In a two-spin system with different chemical environments, the ratio of their resonance-frequency separation Δν to their coupling J affects how closely the spectrum follows a simple first-order description. If the frequency separation is large relative to the coupling, ordinary doublet and multiplet patterns are often easier to interpret.
Use a fictional pair with chemical shifts 0.20 ppm apart and J=7 Hz. At 400 MHz the centres are separated by 80 Hz, so Δν/J≈11.4. At 600 MHz the centres are 120 Hz apart, giving Δν/J≈17.1. The second ratio is larger despite the unchanged J.
Thus some second-order distortions may become less prominent at higher field, although the exact line shape depends on the spin system. It is not universally true that every overlap or complexity disappears when field strength increases.
The H3 syllabus assesses first-order splitting and multiplicity. This field-dependent ratio is optional understanding that helps students appreciate the limits of using n+1 rules without a correct spin-system context.
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9. A doublet is not two separate proton environments
An ideal proton doublet from one observed hydrogen coupled to one neighbouring spin-½ proton shows two component lines, often with 1:1 intensity. Both lines belong to one chemical environment and integrate together for the observed proton count.
When displayed at 400 or 600 MHz, the component positions in ppm may differ by the conversion just described, but the integrated area of the complete doublet still corresponds to the same relative proton population under appropriate quantitative acquisition conditions.
A student who sees two lines at different ppm and claims they must be two independent hydrogen atoms has confused multiplicity with chemical-shift environments. This is the same conceptual mistake that appears in interpretations of doublets of doublets and quartets.
Correct structure determination uses centre position, total integrated area and splitting information as three complementary types of evidence.
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10. Apply the concept to a triplet and quartet
A first-order triplet from coupling to two equivalent spin-½ neighbours contains three components with relative intensities 1:2:1. If the coupling constant is 7 Hz, adjacent components are separated by 7 Hz; the full outer-to-outer frequency span is 14 Hz.
At 400 MHz the full span is 14/400=0.0350 ppm. At 600 MHz it is 14/600≈0.0233 ppm. The triplet’s intrinsic J remains 7 Hz. A quartet with four evenly spaced lines and the same J has an outer span of 3×7=21 Hz, equivalent to 0.0525 and 0.0350 ppm respectively.
Students should distinguish adjacent-line separation from full multiplet width. A quartet has four component lines but only three intervals between its extremes; multiplying by four instead of three gives the wrong total width.
These numerical calculations are optional transfer practice that makes first-order multiplicity more meaningful; a real spectrum may have overlap or higher-order distortions.
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11. Extend to a doublet of doublets with two different J values
A first-order doublet of doublets can arise when one proton has two distinct spin-spin coupling partners, with coupling constants Jₐ and Jᵦ. Its four ideal component positions relative to the centre are ±Jₐ/2 ±Jᵦ/2, so the outer-to-outer frequency span equals Jₐ+Jᵦ when the four components are resolved.
If Jₐ=12 Hz and Jᵦ=5 Hz, the full frequency span is 17 Hz. At 400 MHz that corresponds to 0.0425 ppm, and at 600 MHz to about 0.0283 ppm. Both underlying coupling constants remain the same in hertz under comparable conditions.
This example connects to the previously published Hougang H3 doublet-of-doublets guide while adding a new question: why the same two spin interactions occupy different widths on ppm plots taken at different fields.
A student should not misidentify the middle adjacent gap of a dd as one of the coupling constants without analysing the four line positions.
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12. Know when field-independent coupling is an approximation
For ordinary scalar J coupling in a fixed molecular spin system, J is treated as independent of the applied magnetic field. This is a reliable standard result used throughout first-order proton NMR interpretation and supported by university and instrument-manufacturer references.
Real measured multiplets can nevertheless change apparent shape at different field strengths. Second-order effects, overlap, exchange dynamics, line broadening and experimental resolution may influence the visible pattern. The apparent separation of two maxima in a complicated spectrum may not equal the true underlying scalar J without more analysis.
Thus the correct statement is that the intrinsic coupling in hertz is field-independent under the ordinary model; it does not promise that every observed peak grouping will be equally easy to recognise on every instrument.
Students should avoid turning an important first-order rule into a claim that all possible nuclear interactions are completely independent of field.
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13. Avoid mixing ¹H and ¹³C conversion factors
A 400 MHz instrument label usually refers to the proton resonance frequency. Carbon-13 on the same magnetic field resonates at a lower frequency, approximately 100 MHz rather than 400 MHz. One ppm on a carbon-13 spectrum therefore corresponds to roughly 100 Hz under that field, not 400 Hz.
A common error is to read a carbon NMR spectrum, see the instrument described as 400 MHz and automatically use 400 Hz per ppm for every nucleus. A correct conversion requires the observation frequency of the nucleus actually being measured.
The 2027 H3 curriculum’s specified ¹H interpretation does not require detailed multinuclear instrumentation, so this point is optional enrichment. It reinforces careful unit reading and prevents nonsensical conversions when students encounter more advanced spectral data.
The safest exam habit is to identify the observed nucleus, chemical-shift units and any supplied spectrometer frequency before doing arithmetic.
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14. Distinguish chemical-shift referencing from a physical coupling constant
Tetramethylsilane, TMS, provides the conventional δ0 ppm reference for proton NMR. A centre at δ3.50 ppm refers to its shielding environment relative to that reference, not an absolute 3.50 Hz resonance frequency. The actual radio-frequency resonance lies near hundreds of millions of hertz in a typical instrument.
By contrast J=7 Hz is an absolute small frequency separation arising from scalar coupling between nuclei. Expressing that same interval in ppm is a convenient plotting conversion, not a change to the molecular property.
A student should report a spectrum in an interpretable convention, for example δ3.50 ppm (doublet, J=7 Hz, integral 1H) when supported by the data. The report keeps chemical shift, multiplicity, coupling constant and integration distinct.
Those components of a spectral assignment are not interchangeable, and each provides different evidence about the structure.
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15. Final transfer: two instruments and one unknown compound
An original paper exercise gives two ¹H NMR spectra of the same stable compound at 400 and 600 MHz under otherwise comparable conditions. In both, one resonance has a doublet centre δ3.50 ppm and a reported J=7 Hz. The student should calculate a 0.0175 ppm spacing on the 400 MHz spectrum and a 0.0117 ppm spacing at 600 MHz.
A second pair of resonances lies 0.20 ppm apart. Their centres differ by 80 Hz at 400 MHz and 120 Hz at 600 MHz, even though the ppm difference stays 0.20. A student who can explain both conversions has separated chemical-shift scaling from spin coupling.
The final question asks why the 600 MHz multiplets may appear more first-order. A greater Δν/J ratio can reduce the relative importance of certain strong-coupling distortions, although a real spectrum may still have overlap and other complexities.
Parents can ask one sentence: what stayed at 7 Hz and what got smaller? The answer is intrinsic scalar coupling remained seven hertz while the plotted ppm distance became smaller at higher proton resonance frequency.
Worked evidence: 400 and 600 MHz proton NMR conversions
The entire numerical table is original paper-only practice with a fictional 7 Hz spin coupling and stable chemical shifts. It illustrates the units under an ordinary ¹H NMR model; it is not measured equipment output.
| Comparison | ¹H field frequency | Frequency separation | Displayed ppm separation | Interpretation |
|---|---|---|---|---|
| Doublet, J=7 Hz | 400 MHz | 7 Hz | 0.01750 ppm | One multiplet centre unchanged in ppm |
| Doublet, J=7 Hz | 600 MHz | 7 Hz | 0.01167 ppm | Narrower ppm spacing |
| Doublet half-spacing | 400 MHz | 3.5 Hz | 0.00875 ppm | Each component offset from centre |
| Doublet half-spacing | 600 MHz | 3.5 Hz | 0.00583 ppm | Each component offset from centre |
| Two centres Δδ=0.20 ppm | 400 MHz | 80 Hz | 0.20 ppm | Shift difference in Hz grows with field |
| Two centres Δδ=0.20 ppm | 600 MHz | 120 Hz | 0.20 ppm | Shift difference in ppm stays the same |
| Triplet J=7 Hz full span | 400 MHz | 14 Hz | 0.0350 ppm | Two intervals |
| Triplet J=7 Hz full span | 600 MHz | 14 Hz | 0.0233 ppm | Two intervals |
| Quartet J=7 Hz full span | 400 MHz | 21 Hz | 0.0525 ppm | Three intervals |
| Quartet J=7 Hz full span | 600 MHz | 21 Hz | 0.0350 ppm | Three intervals |
| dd J=12 and 5 Hz full span | 400 MHz | 17 Hz | 0.0425 ppm | Two distinct couplings |
| dd J=12 and 5 Hz full span | 600 MHz | 17 Hz | 0.0283 ppm | Same physical J values |
A fixed J in hertz occupies J/(proton MHz value) ppm, while two chemical shifts separated by a fixed Δδ ppm have a frequency difference of Δδ×(proton MHz value) hertz. These opposite conversions explain the visually narrower multiplets and often improved separation of chemical environments at higher field.
Four independent spectroscopy diagnostics
- Given J=7 Hz, convert the doublet’s adjacent spacing to 0.0175 ppm at 400 MHz and 0.01167 ppm at 600 MHz.
- Given δA=3.50 and δB=3.70 ppm, calculate an 80 Hz centre difference at 400 MHz and 120 Hz at 600 MHz.
- For a triplet with J=7 Hz, compute a 14 Hz outer span and convert it to 0.0350 and 0.0233 ppm.
- Given Δδ=0.20 ppm and J=7 Hz, explain why the Δν/J ratio grows from about 11.4 to about 17.1 when the field increases from 400 to 600 MHz.
Tutor preparation checklist
- Confirm 2027 H3 Chemistry 9813 Section 1.4 and the student’s current NMR chapter.
- Define chemical shift δ relative to TMS and distinguish it from spin coupling J.
- Use the 400 and 600 Hz-per-ppm conversions for ¹H correctly.
- Keep intrinsic scalar J in Hz unchanged under comparable spin-system conditions.
- Explain why chemical-shift differences in Hz expand at higher field while ppm differences remain stable.
- Calculate adjacent multiplet spacing separately from total multiplet width.
- Distinguish doublet, triplet, quartet and doublet-of-doublets first-order patterns.
- Use Δν/J to explain why some second-order distortions diminish at higher field.
- State that field-dependent line shapes can still change even if intrinsic J does not.
- Treat detailed instrument operation and multinuclear conversions as optional enrichment.
Frequently asked questions
Does the NMR coupling constant change from 400 to 600 MHz?
For a stable ordinary scalar spin system, J stays approximately the same in hertz.
Why does a 7 Hz doublet look narrower in ppm at 600 MHz?
One ppm spans 600 Hz rather than 400 Hz, so 7 Hz is 0.0117 ppm rather than 0.0175 ppm.
Does the chemical shift centre change in ppm?
Usually not appreciably for the same structure and reference conditions; ppm normalises the field scale.
How many Hz are one ppm at 400 MHz for ¹H?
About 400 Hz per ppm.
How many Hz are one ppm at 600 MHz for ¹H?
About 600 Hz per ppm.
What happens to two resonance centres separated by 0.20 ppm?
Their Hz difference rises from 80 Hz at 400 MHz to 120 Hz at 600 MHz, while the ppm difference stays 0.20.
Why might higher-field spectra look more first-order?
Chemical-shift separations in Hz grow relative to field-independent scalar coupling, increasing Δν/J for the same spin pair.
Does a four-line quartet integrate for four hydrogens?
Not necessarily. Line multiplicity and relative integrated proton count are different pieces of information.
Is this explicitly required for H3 Chemistry 9813?
First-order spin-spin coupling and the δ scale are required; specific 400/600 MHz arithmetic is an optional transfer application.
Connected H3 NMR study guides
- JC2 H3 Chemistry | Two Couplings in a Doublet of Doublets
- JC2 H3 Chemistry | Residual Chloroform Proton at 7.26 ppm
- Hougang Chemistry Tuition and Tutor Library
Bring a 2027 H3 syllabus and a supplied ¹H NMR spectrum to a tutoring consultation. Confirm actual teacher, venue, group sizes, prices and lesson availability directly. No examination results or session places are guaranteed.
Official syllabus and NMR scientific references
- SEAB 2027 H3 Chemistry 9813, Section 1.4 Nuclear Magnetic Resonance
- Hebrew University NMR: Spin-Spin Coupling and Field Independence
- Protein NMR Basics: ppm Chemical-Shift Definition
- Chemistry LibreTexts: NMR Coupling Constants and Spectroscopy
- Immutable eduKateSG Clementi small-group tutorial reference
- Hougang Chemistry Tuition and Tutor Library
The 2027 H3 syllabus specifies ¹H chemical shift and first-order spin-spin multiplicity, not detailed spectrometer design. The 400/600 MHz conversion examples are optional conceptual applications using invented J and δ data. In real spectra, linewidth, second-order effects, exchange and referencing may alter apparent multiplet shapes. Nothing in this article represents an actual NMR experiment.