Nuclear Magnetic Resonance Spectroscopy: Structure Determination Using ¹H NMR

Nuclear Magnetic Resonance Spectroscopy

¹H NMR—Structure Determination

Nuclear Magnetic Resonance (NMR) Spectroscopy is a powerful analytical technique used in organic chemistry to determine the structure of organic compounds. The ¹H NMR (proton NMR) provides information about the number, type, and environment of hydrogen atoms in a molecule.

Step 1: Determining the Number of Different Types of Protons

• Integration of NMR signals reveals the relative number of protons responsible for each signal.

• The number of NMR signals indicates the number of different types of protons present.

• Example: For a compound C4H10O, the spectrum may show three signals, corresponding to three distinct proton environments.

Integration	Signal
3	A
2	B
3	C

Additional info: Integration values are proportional to the number of protons in each environment.

Step 2: Using Integration Data to Determine Number of H Atoms per Signal

• Sum the integration values to get the total number of protons.

• Divide each integration value by the smallest value to get the ratio of protons per signal.

• Round to the nearest whole number to assign the number of protons to each signal.

For example, if the integration values are 3, 2, and 3:

• $\frac{3}{2} = 1.5$

• $\frac{2}{2} = 1$

• $\frac{3}{2} = 1.5$

Multiply by 2 to get whole numbers: 3, 2, 3 protons.

Proton Equivalency	Typical Group
Three equivalent H's	CH3 group
Two equivalent H's	CH2 group
Three equivalent H's	CH3 group

Additional info: Equivalent protons are those in identical chemical environments.

Step 3: Analyzing Splitting Patterns (Multiplicity)

Splitting patterns (multiplets) in ¹H NMR arise due to spin-spin coupling between nonequivalent neighboring protons. The number of peaks in a multiplet follows the n+1 rule:

• n+1 Rule: A proton with n neighboring protons will split into (n+1) peaks.

• Singlet: No neighboring protons (n=0).

• Doublet: One neighboring proton (n=1).

• Triplet: Two neighboring protons (n=2).

• Quartet: Three neighboring protons (n=3).

Example: A CH3 group adjacent to a CH2 group will appear as a triplet (due to two neighboring protons), while the CH2 group will appear as a quartet (due to three neighboring protons).

Signal	Splitting Pattern	Interpretation
CH3	Triplet	Adjacent to CH2
CH2	Quartet	Adjacent to CH3

Additional info: Splitting patterns help deduce the connectivity of atoms in the molecule.

Step 4: Putting It All Together—Structure Determination

• Combine information from integration, chemical shift, and splitting patterns to deduce the structure.

• Assign each signal to a specific group in the molecule.

• Check consistency with the molecular formula and all spectral data.

Example: For C4H10O, the NMR data may suggest the structure is tert-butanol or sec-butanol, depending on the number and type of signals.

Key Equations

• n+1 Rule: $\text{Multiplicity} = n + 1$

• Integration Ratio: $\text{Number of protons per signal} = \frac{\text{Integration value}}{\text{Smallest integration value}}$