Multiplicity: The key to putting the molecule together
The multiplicity (sometimes referred to as splitting) tells how many hydrogen atoms are immediately next door to the hydrogens producing that peak. The multiplicity is the most important piece of information, since it allows you to connect the pieces together to identify the molecule. Your textbook will describe the details of how the splitting is produced, but the idea is pretty easy: for ‘n’ identical or near-identical next-door-neighbor hydrogens, there will be ‘n+1’ peaks observed. For example a methyl group without any next-door neighbor hydrogens (such as CH3Cl) would appear as a singlet (0+1=1). If that methyl had one next-door hydrogen (CH3CHCl2) that single hydrogen would split the methyl into 1+1 = 2 peaks: it would appear as a doublet. Two next-door hydrogens (CH3CH2Cl) would split the methyl into 2+1 = 3 peaks. So a quartet (4 peaks) would be produced from three next-door neighbor hydrogens.
When there are two or more very different types of next-door neighbor hydrogens, the splitting can sometimes get more complex. Rather than the neighbor hydrogens working together to produce splitting described above, each group can produce its own splitting. For example two very different neighbor CH groups can each split the peak into a doublet to make a doublet of doublets, Figure 2. Or neighbor CH and CH3 groups can split a peak into a doublet of quartets. Sometimes the splitting is neat and we can determine each group. Other times the peaks all overlap and all that is seen is a multiplet. Multiplets make interpretation a bit more difficult, but by using the NMR Puzzle the other pieces will define the splitting for you (see Example 3)!
