A chemist moves from a 300 MHz NMR spectrometer to a 600 MHz instrument (doubling B₀). Which of the following changes?
AThe J-coupling constant between two protons (measured in Hz)
BThe chemical shift of a proton (measured in ppm)
CThe Larmor frequency at which a given nucleus resonates
DThe number of chemically distinct proton environments in the molecule
The Larmor frequency ν = γB₀/(2π) scales directly with B₀, so doubling B₀ doubles the resonance frequency in Hz. J-coupling constants are mediated through bonds and are independent of B₀ — they do not change. Chemical shifts in ppm are defined as a dimensionless ratio relative to a reference, so they also remain constant across field strengths (though the Hz difference between peaks increases). The number of distinct environments is a molecular property, not a field property.
Question 2 True / False
J-coupling constants between two protons increase when the NMR experiment is performed at a higher magnetic field strength.
TTrue
FFalse
Answer: False
J-coupling arises from an indirect, through-bond interaction mediated by bonding electrons — it is a property of the molecular electronic structure, not of the external field. Moving to a higher-field instrument changes the Larmor frequency and improves spectral resolution (because chemical shift differences in Hz grow with B₀) but leaves J-coupling constants in Hz unchanged. This is precisely what allows chemists to distinguish coupling from chemical shift effects: couplings are field-independent, shifts are not.
Question 3 Short Answer
Why is chemical shift reported in parts per million (ppm) rather than in absolute frequency units (Hz)?
Think about your answer, then reveal below.
Model answer: Chemical shift in ppm is a dimensionless ratio of the resonance frequency offset to the spectrometer frequency, making it field-independent and allowing direct comparison of spectra acquired at different field strengths.
If chemical shifts were reported in Hz, a peak at 300 Hz offset on a 300 MHz instrument would appear at 600 Hz on a 600 MHz instrument — making spectra incomparable across instruments. Dividing by the spectrometer frequency (and multiplying by 10⁶ for convenient numbers) gives a ratio that is the same regardless of B₀. This is why δ values in ppm are tabulated as molecular constants in reference databases and can be reliably compared between labs using different instruments.