coupling constant
Summary
The coupling constant J is the spacing, in hertz, between adjacent peaks within an NMR multiplet. It measures spin–spin splitting between hydrogens on neighboring carbons. J is set by the molecule, not the spectrometer, so it stays the same in Hz at any field strength.
Detail
Meaning: In ¹H NMR, nonequivalent hydrogens on adjacent carbons split each other's signals (n neighbors give n + 1 peaks). The coupling constant J is the distance between neighboring peaks in a multiplet, measured in Hz. It reflects how the spin states of two hydrogens influence each other through the bonding electrons. Because it arises from the molecule's bonds, J does not change with spectrometer field, while the separation between different signals in Hz grows with field.
Equation: Δν = Δδ × ν₀, where Δν is a frequency separation in Hz (J when it is the spacing within one multiplet), Δδ is the separation in ppm (dimensionless), and ν₀ is the spectrometer frequency in MHz (1 MHz = 10⁶ Hz).
On the MCAT: Passages give a spectrum or splitting description and ask you to assign a structure from the doublets, triplets and quartets plus integration. Two coupled hydrogens share the same J, so matching spacings show which signals are partners. A common reasoning question is why a higher-field instrument gives less overlap: signals separate more in Hz while J stays fixed. Trans alkene hydrogens typically show a larger J than cis.
Common confusion: J is not the chemical shift and not the number of peaks. It is a spacing in Hz, and in ppm it shrinks as field rises, though in Hz it stays constant.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Klein)
- Spectrometric Identification of Organic Compounds (Silverstein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.