Chem 342: Organic Chemistry II - Infrared Spectroscopy and Mass Spectrometry

CHEM 342: Organic Chemistry II - Infrared Spectroscopy and Mass Spectrometry

Mass Spectrometry Overview

  • Mass Spectrometry (MS) Definition

    • MS is NOT a spectroscopic method.

    • It involves the generation, separation, and characterization of ions in the gas phase based on their mass-to-charge ratio.

  • Key Requirements for MS:

    • Molecules must be ionized.

    • Ions must be separated.

    • Ions must be detected.

Information Provided by Mass Spectrometry

  • Data Retrieved:

    • Molecular Mass of the analyte.

    • Information about the molecular formula.

    • Structural information through fragments.

    • Isotopic information.

  • Spectrum Peaks:

    • Peaks show the abundance of corresponding ions.

    • Most stable ions are generally more abundant; they withstand high-energy conditions better.

  • Base Peak:

    • The most abundant peak in a mass spectrum is termed the BASE PEAK.

Principles of Mass Spectrometry

  • Historical Context:

    • In the late 19th century, J.J. Thomson accelerated ions in a vacuum tube, using a magnetic field for separation based on mass, which was recorded on a photographic plate.

    • In 1913, mass spectrometry demonstrated the existence of isotopes (e.g., separation of Ne22 from Ne20).

  • Ionization Technique: Electron Impact Ionization

    • Operates at 25-80 eV.

    • Ejects one electron from the molecule, resulting in a radical cation.

    • About 15 eV is necessary to ionize, and excess energy (~50 eV) typically causes fragmentation.

    • Often, the molecular ion is not seen due to fragmentation.

  • Molecular Ion:

    • The mass of a radical cation equals that of the parent molecule since the electron is negligible.

  • Fragmentation Process:

    • Parent ions fragment into a radical and a cation.

    • Neutral fragments are not detected.

Types of Mass Spectrometry Instruments

  • Standard MS Instruments:

    • Components:

    • Heated filament, sample inlet, ionizing electron beam, slit, magnet, detector.

    • Ions are deflected based on their mass-to-charge (m/z) ratio.

  • Quadrupole MS:

    • Ions are accelerated and separated by their velocities based on mass. Lighter ions reach the detector first.

  • Time-of-Flight (TOF) MS:

    • Operates similarly to quadrupole; ions are accelerated and drift based on mass, with lighter ions arriving before heavier ions.

Mass Spectrum Analysis

  • Base Peak Measurement:

    • The tallest peak in the mass spectrum signifies the base peak, normalized to 100% abundance.

  • Fragment Peaks:

    • Peaks below the M+• peak represent fragments of the molecular ion.

Case Studies: Specific Compounds

  • Propane:

    • M+• peak observed at m/z = 44.

    • Base peak observed at m/z = 29.

  • Hexane:

    • M+• peak at m/z = 86.

    • Observed fragments: m/z = 71, 57, and 43 (corresponding to stable structures).

  • Dimethylpropane:

    • Shows fragmentation, with stable fragments primarily detected.

Distinguishing Isomers through Mass Spectra

  • Isomer Analysis:

    • Various peaks signify different structural variations; analyzing these helps distinguish isomers.

Mass Spectrometry of Aromatic Compounds

  • Benzene:

    • Stable molecular ions lead to minimal fragmentation.

  • Cyclopentane:

    • Requires multiple fragmentation events to significantly reduce mass.

Interpretation of Molecular Ion Peaks

  • Odd/Even M+• Peaks:

    • An odd M+• peak likely indicates molecules with an odd number of nitrogen atoms, while an even M• suggest an even number of nitrogen atoms.

  • (M+1)+ Peak Analysis:

    • Methane demonstrates an M+1 peak due to the presence of 13C (abundance ~1.1% higher than the molecular ion).

    • More carbons increase the M+1 peak's relative abundance, e.g., decane (C10H22) has an M+1 peak at 11%.

Isotope Ratio and Halogen Analysis

  • Ratios for Bromine and Chlorine:

    • Bromine isotopes (79Br and 81Br) exist in a ~100:98 ratio (~1:1).

    • Chlorine isotopes (35Cl and 37Cl) in a ~100:32 ratio (~3:1).

  • 3-Bromopropionic Acid Example:

    • Peaks observed at m/z = 152/154 for isotopes.

  • 3-Chloropropanol Example:

    • Peaks at m/z = 93/95.

Fragmentation Pathways for Alcohols

  • Fragmentation Pathways:

    • Main pathways: alpha cleavage and dehydration.

    • Example shown: R'COH + R• results in fragment ions.

  • Alpha Cleavage with Amines:

    • Amines can also undergo alpha cleavage, demonstrating the need for understanding structural sensitivity.

Carbonyl Compound Fragmentation

  • McLafferty Rearrangement:

    • Carbonyl compounds can undergo rearrangement if a six-membered ring transition state can form.

  • Example of McLafferty Detection:

    • Carbonyl compound (e.g., C10H20O2) demonstrated with m/z = 88 post-rearrangement encounters.

Summary of Fragmentation Characteristics

  • Fragment losses correlate to specific radicals:

    • M - 15: Loss of methyl radical (•CH3).

    • M - 29: Loss of ethyl radical (•CH2CH3).

    • M - 43: Loss of propyl radical (•CH2CH2CH3).

    • M - 57: Loss of butyl radical (•CH2CH2CH2CH3).

    • M - 18: Loss of water from alcohols (H2O).

    • M - X for McLafferty Rearrangement.

High-Resolution Mass Spectrometry

  • Purpose:

    • High resolution enables detection of subtle mass differences, important for distinguishing isomeric compounds.

  • Resolution Capabilities:

    • Measures m/z to an accuracy of four decimal places.

  • Atomic Mass Context:

    • Fundamental about atomic weights, comparison with real mass values.

Isotope Mass Data

  • Table of Isotope Relative Atomic Mass (amu) & Natural Abundance:

    • 1H: 1.0078 amu (99.99%)

    • 2H: 2.0141 amu (0.01%)

    • 12C: 12.0000 amu (98.93%)

    • 13C: 13.0034 amu (1.07%)

    • Other isotopes included for context of various reactions and stability characterizations.

Conclusion

  • Emphasis on understanding fragmentation patterns, spectroscopic qualities, and resolution capabilities are essential for effective qualitative and quantitative analyses in mass spectrometry studies.