15-33
Recommended Approach to Solving Structure Determination Problems
If given a molecular formula and IR and/or NMR spectra use the following steps as a guide
1. Calculate the DU/HDI from the given molecular formula.
Assess possible functional group(s) with listed elements in the molecular formula and the
number of calculated DU/HDI (i.e. a molecular formula with at least two oxygens and at least 1
DU, likely involves a carboxylic acid or ester).
2. Analyze the IR spectrum.
Further assess functional group(s) present in the structure.
3. Count the number of signals in the 13C NMR spectrum.
Compare to the given molecular formula and determine if there is symmetry within the
structure.
4. If necessary, calculate the integrations in the 1H NMR spectrum for each signal.
Verify that the total number of calculated hydrogen atoms match the number of hydrogen
atoms in the molecular formula. If off by one or two hydrogen(s) consider the possibility of an
OH or NH/NH2 in the structure that does not have a signal appear due to hydrogen bonding.
5. Analyze each signal in the 1H NMR spectrum.
Write down the observed splitting pattern and number of expected neighbors associated with
the signal, as well as assess chemical shifts, and predict what the signal likely represents (i.e. a
triplet with an integration of 3, is likely a CH3 attached to a CH2) Note: The first guess is not
always correct. Be aware that signals can overlap, thus making the analysis of the corresponding
signals difficult.
6. Assess chemical shifts in the 13C NMR spectrum.
Determine if the expected types of carbons deduced from the 13C NMR spectrum correspond
with proposed types of hydrogen atoms in the 1H NMR spectrum.
7. Propose potential structure(s) based on the pieces determined in steps 5 and 6. Do NOT start this
step until every signal in step 5 has been analyzed!
If there are issues with potential structure(s), go back and reassess specific signals in step 5.
Note: The first structure proposed is not always the correct structure, and that is ok, consider
other possibilities.
8. Check your work with the 1H NMR spectrum.
Once a reasonable structure has been proposed, label the unique hydrogen atoms in the
structure, and determine if they correspond well with all the signal(s) in the 1H NMR spectrum.
9. Check your work with the 13C NMR spectrum.
Once a reasonable structure has been proposed, label the unique carbons in the structure, and
determine if they correspond well with all the signal(s) in the 13C NMR spectrum.
10. If necessary, address any significant discrepancies in the proposed structure.
If there are any major inconsistencies in steps 8 or 9, go back to step 7 (or any step prior) to
reanalyze any of the data given to determine if previous assumption(s) need to be corrected.
Once another reasonable structure has been proposed, verify with steps 8 and 9.
11. Assign signals in the 1H NMR spectrum.
Label signals in the 1H NMR spectrum with the corresponding letter in the proposed structure.
12. Assign signals in the 13C NMR spectrum.
Label signals in the 13C NMR spectrum with the corresponding letter in the proposed structure.
Figures: Copyright © 2017 John Wiley & Sons, Inc. All rights reserved.
Recommended Approach to Solving Structure Determination Problems
If given a molecular formula and IR and/or NMR spectra use the following steps as a guide
1. Calculate the DU/HDI from the given molecular formula.
Assess possible functional group(s) with listed elements in the molecular formula and the
number of calculated DU/HDI (i.e. a molecular formula with at least two oxygens and at least 1
DU, likely involves a carboxylic acid or ester).
2. Analyze the IR spectrum.
Further assess functional group(s) present in the structure.
3. Count the number of signals in the 13C NMR spectrum.
Compare to the given molecular formula and determine if there is symmetry within the
structure.
4. If necessary, calculate the integrations in the 1H NMR spectrum for each signal.
Verify that the total number of calculated hydrogen atoms match the number of hydrogen
atoms in the molecular formula. If off by one or two hydrogen(s) consider the possibility of an
OH or NH/NH2 in the structure that does not have a signal appear due to hydrogen bonding.
5. Analyze each signal in the 1H NMR spectrum.
Write down the observed splitting pattern and number of expected neighbors associated with
the signal, as well as assess chemical shifts, and predict what the signal likely represents (i.e. a
triplet with an integration of 3, is likely a CH3 attached to a CH2) Note: The first guess is not
always correct. Be aware that signals can overlap, thus making the analysis of the corresponding
signals difficult.
6. Assess chemical shifts in the 13C NMR spectrum.
Determine if the expected types of carbons deduced from the 13C NMR spectrum correspond
with proposed types of hydrogen atoms in the 1H NMR spectrum.
7. Propose potential structure(s) based on the pieces determined in steps 5 and 6. Do NOT start this
step until every signal in step 5 has been analyzed!
If there are issues with potential structure(s), go back and reassess specific signals in step 5.
Note: The first structure proposed is not always the correct structure, and that is ok, consider
other possibilities.
8. Check your work with the 1H NMR spectrum.
Once a reasonable structure has been proposed, label the unique hydrogen atoms in the
structure, and determine if they correspond well with all the signal(s) in the 1H NMR spectrum.
9. Check your work with the 13C NMR spectrum.
Once a reasonable structure has been proposed, label the unique carbons in the structure, and
determine if they correspond well with all the signal(s) in the 13C NMR spectrum.
10. If necessary, address any significant discrepancies in the proposed structure.
If there are any major inconsistencies in steps 8 or 9, go back to step 7 (or any step prior) to
reanalyze any of the data given to determine if previous assumption(s) need to be corrected.
Once another reasonable structure has been proposed, verify with steps 8 and 9.
11. Assign signals in the 1H NMR spectrum.
Label signals in the 1H NMR spectrum with the corresponding letter in the proposed structure.
12. Assign signals in the 13C NMR spectrum.
Label signals in the 13C NMR spectrum with the corresponding letter in the proposed structure.
Figures: Copyright © 2017 John Wiley & Sons, Inc. All rights reserved.
