How To Interpret 1H NMR Spectra

Interpreting NMR spectra can be a very challenging task; you must admit that an NMR spectrum looks nothing like the molecule that generates it.  Nonetheless, interpretation of NMR spectra doesn't have to be terribly difficult.  You just have to make sure you use all the information available, and be organized.  Here are some pointers that will be helpful as you solve any 1H NMR spectrum.  
While not necessary, the NMR Mosaic is a tool that can really help you solve these problems by letting you visualize how everything fits together.  It can also help you recognize any mistakes you might make.

The first thing to remember is that NMR interpretation is like solving a puzzle.  Just like you can't put every puzzle piece you pick up into its correct place, you cannot expect to look at an NMR spectrum and immediately write down the structure.  You have to work at it, trying first one idea, then another.  And just like you can tell when the picture puzzle is finished because all the pieces match with their neighbors, similarly the NMR spectrum is solved when all the molecular pieces from the NMR match their neighbors.

Guidelines for solving 1H NMR spectra.
  1. Each peak in the spectrum corresponds to a fragment of the molecule.
  2. Each peak contains THREE different kind of information about the molecular fragment: Integration, Chemical Shift, and Multiplicity (or splitting).  You CANNOT solve the spectrum without using all three pieces of information.
  3. Once you have connected all the fragments, it does NOT mean you have successfully solved the structure.  You must make sure that each connection is consistent with the information in the spectrum.  CHECK IT!  (The NMR Mosaic is excellent at helping you make sure the pieces truly match.)

Roadmap for solving the structure.

  1. Use the molecular formula (if given) to figure out the degree of unsaturation (the number of rings and/or multiple bonds).
  2. Use the Infrared spectrum (if given) to figure out the functional groups present.
  3. Look for any recognizable groups in the NMR such as aromatic rings, aldehydes, carboxyllic acids, or alcohols.
  4. Use the integration to determine the number of hydrogens contributing to each peak.
    1. OFTEN (but not always) the smallest integration is 1 hydrogen.
    2. OFTEN (but not always) peaks around 0.7-1.1d are methyl groups (3 hydrogens).
    3. If you know the molecular formula, the integral per hydrogen equals the total integration of all peaks divided by the total number of hydrogens.
  5. Then make the molecular fragment each peak corresponds to (remember, each peak corresponds to one fragment of the molecule, often a but now always a CHx) :
    1. Use the integration to calculate the number of hydrogens contributing to each peak (a CH, CH2, CH3, etc.).
    2. Use the chemical shift to determine which (if any) functional group(s) is attached to that fragment.
    3. Use the multiplicity to determine how many hydrogens are adjacent to the fragment.  
  6. Put the fragments together.  You have two kinds of information telling you how to connect the fragments: 
  7. If the pieces don't all match, something is not right.  Find the piece(s) that fit the worst and check them first; they are frequently the ones interpreted incorrectly.  Is the splitting correct?  Can the chemical shifts be for another functional group?  Can a CH really be an OH?  

The NMR Mosaic can help you avoid problems that have plagued students for years.