Virginia Tech 2

Continuing Molecular Structure Determination

  • Overview: This lecture continues the discussion on molecular structure determination, focusing on spectroscopy.

  • The material is divided into two parts:

    • Part One: UV-Vis, IR, and mass spectrometry (MS)

    • Part Two: NMR spectroscopy (to be covered later)

  • Focus on Data Interpretation: The goal is to interpret data rather than memorize it.

Spectroscopy and Data Interpretation

  • Interpretation Emphasis:

    • Understanding how to use spectral data to determine functional groups and connectivity in molecules.

    • Use tables (provided) for reference, not memorization.

    • Visualize data interpretation as assembling a puzzle.

  • Key Considerations:

    • Identify functional groups from peaks.

    • Understand the type of bonds present (single, double, triple).

    • Assess stereochemistry if relevant.

Mass Spectrometry (MS) Overview

  • Definition: Mass spectrometry involves analyzing and weighing ions.

  • Ionization Methods:

    • Electron Impact (EI): Introduces energy via electrons (radical cations or anions).

      • Focus for this course.

    • Electrospray Ionization: Softer technique that primarily focuses on proton loss or gain (not covered in detail).

  • Fragmentation: Molecules will fragment upon ionization.

Isotopic Patterns in MS

  • Peaks represent isotopic abundance of atoms in the molecule.

  • Molecular Ion Peaks: Represent the molecular ion (m+) and isotopic peaks (m+1).

  • Carbon Isotopes: 1.1% natural abundance of C-13.

    • Presence of multiple carbon atoms (e.g., 6 Carbons leads to ~6.6% m+1 peaks).

  • Chlorine and Bromine Isotopes:

    • Chlorine (Cl): ~25% for Cl-37 leads to a 1:3 ratio in m to m+2.

    • Bromine (Br): 1:1 ratio for m to m+2.

  • Nitrogen Rule: Odd mass indicates an odd number of nitrogen atoms. Even mass suggests even numbers (including zero).

Infrared (IR) Spectroscopy Overview

  • Energy and Wavelength: Relationship explained; longer wavelengths correspond to lower energy, and vice versa.

  • Regions of Spectrum:

    • Gamma rays (high energy) to radio waves (low energy).

  • IR Absorption Mechanism:

    • Molecules absorb IR light causing vibrations (single bond stretches).

    • Peaks on an IR spectrum correlate to specific bond types.

  • IR Spectral Regions:

    • Fingerprint region identifies distinctive molecular patterns.

    • CH stretching (>2500 cm-1), OH and NH peaks, CC stretching.

Analyzing Different Structures Using Infrared Absorption

  • Monosubstituted vs. Disubstituted Benzene: Differences in peak patterns allow identification (e.g., ortho, meta, para substitution).

  • Characterizing Substitution Patterns: Peaks in the range of 700-800 cm-1 can distinguish between substituents on benzene.

  • Triple Bonds: Validated by specific stretching peaks (e.g., terminal alkyne vs nitrile).

  • Influence of Hydrogen Bonding: Generally leads to broader and more intense O-H and N-H peaks.

UV-Vis Spectroscopy Summary

  • Experiment Setup: Light source disperses wavelengths through a prism to analyze samples. Absorbance correlates with properties of the structure.

  • Conjugated Systems: More π bonds lead to longer absorption wavelengths (e.g., β-carotene).

  • Absorption Wavelength Implications: Compounds without double bonds or lone pairs do not exhibit UV-Vis absorbance.

Conclusion: Understanding these spectroscopic methods and interpretations is crucial for determining molecular structures and understanding organic chemistry principles.

Note: Remember to utilize provided tables and data rather than memorizing; reasoning through structures is key.