Determining the structure of an unknown molecule through spectroscopic means is much like working on a puzzle. Each method provides different pieces of information about the molecule. Write down each piece of information, as you obtain it. The strategic steps provided here show one organized way of approaching the problem. You may not have all of the information discussed and you will need to make the most of what you have. Creativity is sometimes required but haphazard problem-solving techniques are rarely beneficial. Most of the problems given to you in the classroom setting will have a correct structure which will be the only structure that fits all of the data. In the "real world," there may be more than one structure that fits all of the data; thus, the process of solving the problem is as important as the answer itself.

This description assumes that you will have information from a number of instrumental methods. More detailed descriptions of the most common methods are described separately: MS, IR, ¹³C NMR, ¹H NMR

1. Determine the formula
   (if this is already given, skip to the next step)
2. • Try to infer the formula from MS data
     • Molecular ion, M_⁺, tells the presence of N and gives the formula wt.
     • M+1, and M+2 relative intensities tell the number of C atoms, and presence of Br, Cl, S, or Si
   • Come up with one or two possible formulas and revisit the mass data as you get more information to confirm the actual formula.
   • ¹³C NMR tells the minimum number of C atoms and the splitting of each peak tells the number of H and can be used to compute the minimum number of H atoms.
   • ¹H NMR integration values can also be used to help determine the number of H atoms.
3. Once you have the formula, determine the index of hydrogen deficiency, W = [2C+2-H-X+N)]/2
   C = # carbons, H = # hydrogens, X = # halogens, N = # nitrogens
4. Ascertain the presence of functional groups
   • First, decide which functional groups are possible with your formula and W
   • Look for evidence for the presence or absence of each possible functional group mainly in the IR, but also in NMR and UV-vis.
5. Determine the pieces
   • If the ¹³C NMR has any information regarding the number of attached protons (splitting, DEPT, APT, etc.), use it to generate all of the pieces (and by inference the number of hydrogens left over on heteroatoms).
   • Use the ¹³C NMR chemical shift information to determine the chemical environment of each piece.
   • Use the ¹H NMR chemical shift information and intensity to determine the pieces.
6. Propose a reasonable connectivity of the pieces
   • Use ¹H NMR splitting to determine which pieces are connected to each other and propose a reasonable structure.
   • Sometimes there is only one way to connect the pieces based on the symmetry of the molecule. (E.g. C₁₂H₁₈, ¹³C NMR: d 136 s, 9 q. The structure must be hexamethylbenzene.)
7. Evaluate the proposed structure
   • Count up all the atoms to make sure it fits the formula that you were given (or mass data).
   • Make sure that the proposed functional groups agree with the IR spectrum.
   • Assign all of the ¹³C and ¹H peaks and make sure that they all agree.
     • Pay specific attention to equivalent groups and symmetry.
   • If any signal is not completely reasonable then modify your structure accordingly. Some common modifications are shown below.