NMR Spectrum Analysis and Molecular Structure Determination

Key Concepts:

• Molecular Formula and Proton NMR Spectrum: Used to determine the structure of a molecule.
• Hydrogen Deficiency Index (HDI): Helps infer the presence of rings or double bonds in a molecule.
• Integration Values: Relates to the number of protons contributing to each signal in the NMR.
• N+1 Rule: Used to determine the number of neighboring protons based on the number of peaks in a NMR signal.
• Chemical Shift: Indicates the environment of the protons and helps identify functional groups.

Example 1: C5 H10 O

Steps to Determine Structure:

1. Calculate HDI:

   • For 5 Carbons, maximum Hydrogens = 2N + 2 = 12.
   • Actual Hydrogens = 10.
   • Missing 2 Hydrogens → HDI = 1, indicates either one double bond or a ring.

2. Integration Values:

   • Calculate relative numbers by dividing by the smallest integration value.
   • Adjust to whole numbers to account for all hydrogens.

3. Analyzing Signals:

   • Signal 1:

     • 2 protons → CH2
     • 3 peaks → 2 neighboring protons
     • Chemical shift between 2-2.5 ppm → Next to a carbonyl

   • Signal 2:

     • 3 protons → CH3
     • 1 peak → 0 neighboring protons
     • Chemical shift similar to Signal 1 → Also next to carbonyl

   • Signal 3:

     • 2 protons → CH2
     • 6 peaks → 5 neighboring protons

   • Signal 4:

     • 3 protons → CH3
     • 3 peaks → 2 neighboring protons

4. Proton Environment:

   • Protons near carbonyl are more deshielded.
   • Using chemical shifts and integration, deduce the structure.

5. Result: Structure with 5 Carbons, 10 Hydrogens, 1 Oxygen confirmed.

Example 2: C8 H10

Steps to Determine Structure:

1. Calculate HDI:

   • For 8 Carbons, maximum Hydrogens = 18.
   • Missing 8 Hydrogens → HDI = 4, indicates a Benzene ring.

2. Integration and Signal Analysis:

   • Complex Signal:

     • 5 protons in Aromatic range (6.5-8 ppm) → Benzene ring with 5 protons

   • Signal 2:

     • 2 protons → CH2
     • 4 peaks → 3 neighboring protons

   • Signal 3:

     • 3 protons → CH3
     • 3 peaks → 2 neighboring protons

3. Ethyl Group Identification:

   • CH3 and CH2 form an Ethyl group as part of the structure.

4. Result: Ensure 8 Carbons and 10 Hydrogens are accounted for; confirms Ethylbenzene.

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Conclusion

• Correctly interpreting NMR signals and utilizing HDI are crucial for inferring molecular structures.
• Chemical shift provides context on the environment of protons, aiding in identifying functional groups.