Organic Chemistry Semester 1 LABORATORY MANUAL - Moravian ...

Nuclear Magnetic Resonance Appendix E18 Part V. Techniques and Theory
Theory of Spin-Spin Splitting in Proton NMR Spectra:
(See Also: CGWW pp. 258-268 and Padías pp. 87-90.)
a. Structural requirements for Spin-Spin Splitting
(1.) Splitting is caused by the effects of the magnetic field produced by one magnetic
nucleus (in this case a proton) on the magnetic environments of other nuclei in the molecule.
(2.) Protons are only affected by other magnetically non-equivalent protons (protons with
different δ values). Thus, compounds such as Figure 9: (1.), (3.), (5.), & (6.) in which all of
the protons are indistinguishable (equivalent) yield one peak that is not split, while
spectra from compounds such as Figure 9: (2.) & (4.) with non-equivalent groups of
protons often exhibit splitting.
(3.) Although long-range splitting is possible in some situations, the major splitting
(referred to as first order splitting) occurs only among protons that are attached to
adjacent carbon atoms. Thus, Figure 9: compound (7.) yields two singlets (unsplit peaks)
since its non-equivalent protons are separated by a CCl 2 group. Thus there are no
protons on adjacent carbon atoms. However, the spectra of very similar compounds
Figure: (2.), (4.), (8.) & (9.) show splitting because their non-equivalent protons are on
adjacent carbon atoms.
NOTE: Because protons on oxygen and nitrogen atoms tend to be acidic and are thus
transferred from one molecule to another while the NMR spectrum is being
collected, usually they do not split adjacent protons nor are they split by
adjacent protons.
b. The multiplicity of Spin-Spin Splitting. (How many peaks are produced)
In general:
Protons split by n equivalent adjacent nuclei yield (2nI + 1) peaks, where I is the nuclear
spin quantum number of the adjacent nucleus. For protons I = 1 2 . So:
For Splitting by Protons Peak Multiplicity = n + 1
Where n = # of equivalent protons on adjacent carbon atoms.
c. Relative Intensities of peaks within a multiplet are given by the coefficients of the binomial
expansion (X + Y) n , where n = the number of adjacent equivalent protons. These coefficients
can be determined using Pascal’s Triangle.
Pascal's Triangle
n 1
1 1 1
2 1 2 1
3 1 3 3 1
4 1 4 6 4 1
5 1 5 10 10 5 1