S3.2.12 Structural analysis (Higher Level Only)

Techniques Used for Structural Analysis: Integrating Mass Spectrometry, IR Spectroscopy, and ¹H NMR Spectroscopy

1. Imagine you’ve been handed a mystery compound.
2. 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:

1. Molecular Ion Peak (M⁺): Indicates the molecular mass of the compound.
2. Fragmentation Patterns: Offer insights into the molecule's structure by showing how it breaks apart.

How Does It Work?

1. In MS, a molecule is bombarded with high-energy electrons, causing it to ionize and fragment.
2. These fragments are then separated based on their mass-to-charge ratio ($m/z$).
3. The molecular ion peak corresponds to the intact molecule minus one electron, while smaller peaks represent fragments.

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?

1. When IR radiation passes through a sample, bonds in the molecule absorb specific frequencies, causing vibrations such as stretching or bending.
2. These absorptions appear as peaks in an IR spectrum.

Key Functional Group Regions

• O–H (Alcohols/Carboxylic Acids): Broad peak around $3200–3600cm⁻¹$.
• C=O (Carbonyls): Sharp peak around $1700–1750cm⁻¹$.
• C–H (Alkanes): Peaks in the range $2800–3000cm⁻¹$.

¹H NMR Spectroscopy: Understanding Hydrogen Environments

What Does ¹H NMR Reveal?

¹H NMR spectroscopy provides detailed information about:

1. The number of hydrogen environments: Each unique environment produces a distinct signal.
2. The relative number of hydrogens in each environment: Shown by the integration trace (area under each peak).
3. Splitting patterns: Reveal the number of neighboring hydrogens (via the N + 1 rule).

How Does It Work?

1. ¹H NMR measures the interaction of hydrogen nuclei with an external magnetic field.
2. Each hydrogen atom's chemical environment affects the frequency at which it absorbs radio waves, producing distinct signals.

Key Features of ¹H NMR

• Chemical Shift (δ): Indicates the type of hydrogen environment (e.g., δ = 0.9–1.5 ppm for alkyl groups, δ = 9.0–13.0 ppm for carboxylic acids).
• Integration Trace: Proportional to the number of hydrogens in each environment.
• Splitting Patterns: Determined by the number of neighboring hydrogens (e.g., a doublet indicates 1 neighbor, a triplet indicates 2 neighbors).

Combining Data: Solving the Structural Puzzle

Determining the structure of an unknown compound often requires integrating data from multiple techniques. Let’s walk through an example step-by-step.

Identifying an Unknown Compound

You are given the following data for a compound:

1. Mass Spectrum: Molecular ion peak at $m/z=74$.
2. IR Spectrum: Sharp peak at $1700cm⁻¹$ (C=O group).
   • ¹H NMR Spectrum:
   • Signal at δ = 2.1 ppm (singlet, integration = 3).
   • Signal at δ = 9.8 ppm (singlet, integration = 1).

Step 1: Molecular Formula from MS

The molecular ion peak at $m/z=74$ suggests a molecular formula of $C₃H₆O₂$ (using relative atomic masses: C = 12, H = 1, O = 16).

Step 2: Functional Groups from IR

The sharp peak at $1700cm⁻¹$ indicates the presence of a C=O group. No broad O–H peak is observed, ruling out carboxylic acids.

Step 3: Hydrogen Environments from ¹H NMR

• The singlet at δ = 2.1 ppm corresponds to a $CH₃$ group adjacent to a carbonyl group (e.g., $CH₃C=O$).
• The singlet at δ = 9.8 ppm corresponds to an aldehyde proton (H–C=O).

Step 4: Combine the Evidence

The molecular formula ($C₃H₆O₂$), functional group (aldehyde), and NMR data suggest the structure is propanal ($CH₃CH₂CHO$).