The Six Clues to Help Solve a Structure

I like to think of solving spectroscopy problems as analogous to a detective in the dark with a flashlight, searching for whodunit. Methodical clue gathering is the best way to determine the culprit (those mischievous compounds in our case) rather than blindly waving the flashlight (or pencil) about, hoping for some sort of chance epiphany (which invariably comes not during an exam but immediately following).

So let's talk about how to systematically find those clues! It's a good idea to write down all of the clues on a piece of scratch paper as you come across them, so they are in front of you when you go to solve the structure.

Clue 1: Determine the degrees of unsaturation from the molecular formula or mass spec. From this you can determine if double bonds, triple bonds, or rings may be present on the unknown compound, and how many. Click here for a tutorial on how to determine degrees of unsaturation.

Clue 2: Look at the IR (if applicable) to determine the major functional groups present in the compound, confirming (if possible) their presence in the NMR spectrum (easy ones to confirm include aldehydes, carboxylic acids, and benzene rings).

Clue 3: Find out how many hydrogens each set of peaks on an NMR represents by comparing the ratio of the integration to the molecular formula.

Clue 4: Break the NMR peaks into fragments using the integration found in clue 3 and write each fragment down on a piece of scratch paper. For a helpful list of common fragments found with certain hydrogen integrations, click here.

Clue 5: Combine all of the fragments in a way that makes sense with the NMR peak splitting. This is the hardest part, and sometimes the best way is simply guessing and checking. Put together a possible structure and check to see if it makes sense in the NMR. If not, try another one until it does.

Clue 6: Recheck your structure with the NMR and the IR to make sure it is an exact match.

 Continue Tutorial :: An example problem