Techniques Used for Structural Analysis: Integrating Mass Spectrometry, IR Spectroscopy, and ¹H NMR Spectroscopy
- Consider you’ve been handed a mystery compound.
- Your mission? To uncover its structure. How would you solve this molecular puzzle?
Chemists rely on powerful analytical tools like Mass Spectrometry (MS), Infrared (IR) Spectroscopy, and Proton Nuclear Magnetic Resonance (¹H NMR) Spectroscopy.
Mass Spectrometry (MS): Determining Molecular Mass and Fragmentation Patterns
What Does MS Reveal?
Mass spectrometry provides two critical types of information:
- Molecular Ion Peak (M⁺): Indicates the molecular mass of the compound.
- Fragmentation Patterns: Offer insights into the molecule's structure by showing how it breaks apart.
How Does It Work?
- In MS, a molecule is bombarded with high-energy electrons, causing it to ionize and fragment.
- These fragments are then separated based on their mass-to-charge ratio ($m/z$).
- The molecular ion peak corresponds to the intact molecule minus one electron, while smaller peaks represent fragments.
Propan-1-ol
The mass spectrum of propan-1-ol reveals:
- A molecular ion peak at $m/z = 60$, indicating a molecular mass of 60 g/mol.
- Fragmentation peaks at $m/z = 31$ ($CH₂OH⁺$) and $m/z = 29$ ($CH₃CH₂⁺$), corresponding to structural fragments.
- The mass spectrum of an unknown compound shows a molecular ion peak at $m/z = 88$.
- Using the relative atomic masses (C = 12, H = 1, O = 16), deduce its possible molecular formula.
- Divide the molecular mass by the approximate atomic masses of C, H, and O to estimate the number of each atom.
- Consider combinations that fit the total mass.
Infrared (IR) Spectroscopy: Identifying Functional Groups
What Does IR Reveal?
- IR spectroscopy identifies functional groups in a molecule by detecting vibrations in chemical bonds.
- Each bond absorbs IR radiation at characteristic wavenumbers (measured in cm⁻¹).
How Does It Work?
- When IR radiation passes through a sample, bonds in the molecule absorb specific frequencies, causing vibrations such as stretching or bending.
- These absorptions appear as peaks in an IR spectrum.
Key Functional Group Regions
- O–H (Alcohols/Carboxylic Acids): Broad peak around $3200–3600 \text{ cm}^{-1}$.
- C=O (Carbonyls): Sharp peak around $1700–1750 \text{ cm}^{-1}$.
- C–H (Alkanes): Peaks in the range $2800–3000 \text{ cm}^{-1}$.
Butanoic Acid
The IR spectrum of butanoic acid shows:
- A broad O–H peak around $2500–3000 \text{ cm}^{-1}$.
- A sharp C=O peak near $1700 \text{ cm}^{-1}$.
- Students often confuse the broad $O–H$ peak of carboxylic acids with the narrower $O–H$ peak of alcohols.
- Always look for the accompanying $C=O$ peak to confirm a carboxylic acid.
¹H NMR Spectroscopy: Understanding Hydrogen Environments
What Does ¹H NMR Reveal?
¹H NMR spectroscopy provides detailed information about:
- The number of hydrogen environments: Each unique environment produces a distinct signal.
