CHEM 205: more on NMR: J coupling and Splitting of signals
• Why does 1 signal split?
o Described by N+1 rule where N is the number of equivalent H bonded to an
o Due to magnetic nuclei nearby that exert internal B field
• For lecture, will be focusing on vicinal coupling of freely rotating structures (sp3
• Simplest example: AX spin system
o Notice for this system:
Each signal is split into 2, since the possibility of the neighboring H is only
spin up or down. The two spins are equal and opposite, thus producing 2
split peaks of equal intensity. Hx is to the left due to the more
electronegative chlorine which causes increased shift.
Also notice that the frequency distance in Hz between the peaks in each
doublet is called the spin-spin coupling constant J. In H NMR, J is 8Hz in
• J-coupling values are constants (since magnetic fields within the
molecule are constant) depending ONLY on the molecule and
independent of B unlike larmor frequencies
o However when converted to chemical shift, it will depend.
o AX2 spin system Eg:
• Splitting is not seen when proton is rapidly exchanged (eg. H in –OH group while in
• Back to the N+1 rule and finding the relative intensities of split lines. Can use pascal’s
triangle. Note, max N=9 for CH(CH3)3
• Other notes:
o Protons don’t split if they are separated by more than 3 sigma bonds
o Labile Hs can exchange with solvent which cause line broadening and no
splitting • Take a look at notes on notability for actual examples of identifying NMR (Lec 21 slides)
• The scenario gets trickier when involving fixed, planar functional groups such as
o Eg. For toluene, the H bound to the aromatic ring have high chem shifts (as
discussed last lecture) and strong J coupling between those hydrogens. But
splitting disappears and concentrated into single tight multiplet since their chem
shifts are quite close to each other.
• When sketching NMR of a given molecule
o Determine the number of signals (how many non-equivalent H present?)
o Determine chem shift values of these Hs from the table