Study Notes for Mass Spectrometry and Spectroscopy

Chapter 12: Mass Spectrometry, Infrared Spectroscopy, Ultraviolet/Visible Spectroscopy

Instrumental Techniques

• Mass Spectrometry

  • Determines molecular mass and molecular formula of compounds.

• Infrared (IR) Spectroscopy

  • Identifies functional groups present in the compound.

• Ultraviolet/Visible (UV/Vis) Spectroscopy

  • Provides information on compounds that contain conjugated double bonds.

• Nuclear Magnetic Resonance (NMR) Spectroscopy

  • Offers information regarding the carbon–carbon (C–C) and carbon–hydrogen (C–H) framework of compounds.

Mass Spectrometry

• Formation of Molecular Ion

  • An electron is ejected from the compound, resulting in the creation of a molecular ion.

• Mass Spectrometer Functionality

  • Only positively charged species are detected by the recorder.

The Mass Spectrum of Pentane

• m/z Ratio

  • m/z refers to the mass-to-charge ratio of the fragment; for this context, $z = 1$.

The Molecular Ion of Pentane

• Pentane forms a molecular ion with the mass-to-charge ratio (m/z) of 72.

Fragmentation of the Molecular Ion

• Stability of Fragments

  • The abundance of fragments relates to their stability; more stable fragments are more abundant.

• Fragmentation Example

  • C-2–C-3 fragmentation occurs, resulting in more stable fragments.

Loss of Hydrogen from a Carbocation

• A peak is commonly observed at an m/z value two units below that of the carbocation upon hydrogen loss.

Stability in Fragments Across Isopentane vs. Pentane

• The peak at m/z 57 is observed to be more abundant in isopentane than in pentane due to the greater stability of the secondary carbocation compared to the primary carbocation.

• Fragmentation in Isopentane

  • Isopentane is more inclined to lose a methyl group, forming a secondary carbocation.

Natural Abundance of Isotopes in Organic Compounds

High-Resolution Mass Spectrometry

• Capability

  • Can distinguish between compounds with identical molecular masses.

Fragments from Carbon-Bromine and Carbon-Chlorine Bonds

• Bond Break Types

  • Carbon-bromine ($C–Br$) bonds typically break heterolytically, which is the splitting in a polar manner.

  • Carbon-chlorine ($C–Cl$) bonds also break heterolytically, while carbon–carbon bonds break homolytically, meaning bonds are broken equally between the two atoms involved.

a-Cleavage in Alkyl Halides and Related Concept

• Weakness and Stability of Bonds

  • Bonds that are weaker tend to break easily and lead to formation of stable fragments.

• a-Cleavage Definition

  • Describes the homolytic cleavage of the carbon–carbon bond.

Mass Spectrum of 2-Chloropentane

• Presents peaks that correspond to different fragments based on the fragmentation pattern.

a-Cleavage Behavior in Alcohols and Ketones

• Involves loss of water from alcohols if there is a hydrogen attached to a g-carbon, where two bonds break typically.

Summary of a-Cleavage Patterns

• Common Fragmentation Behavior

  1. Bonds involving more electronegative atoms break heterolytically.

  2. Bonds between similar electronegativities break homolytically.

  3. Weak bonds that lead to stable cation formation are the most prone to breaking.

Gas Chromatography–Mass Spectrometry (GC-MS)

• Analyzing Mixtures

  1. Allows simultaneous analysis of compounds through gas chromatography and mass spectrometry.

  2. Sample injected travels through the column based on boiling points.

  3. Mass spectrometer captures the mass spectrum for each component of the mixture.

The Electromagnetic Spectrum

• Frequencies and Wavelengths

  • Higher frequencies = shorter wavelengths

  • Ranges from gamma rays to radio waves with distinct behaviors in UV and IR.

Energy and Frequency Relation

• Energy and Wavelength Correlation

  • The greater the energy, the greater the frequency and the shorter the wavelength.

Vibrational Behavior of Bonds

• Types of Vibrations

  • Stretching: A vibration along the line of a bond.

  • Bending: A vibration that alters the bond angle.

Specific Stretching and Bending Patterns

• Characteristics

  • Stretching vs bending vibrations occur at different frequencies and are influenced by several factors including bond strength and atomic mass.

Functional Group Regions vs Fingerprint Regions in IR

• Functional vs Fingerprint Regions

  • The functional group regions vary between alcohols, but fingerprint regions are unique to each specific compound type.

IR Stretching Frequencies of Various Bond Types

Factors Affecting IR Spectroscopy

• Polar Bonds and Absorption

  • More polar a bond is, more intense its absorption in IR spectroscopy.

• Hooke’s Law

  • Lighter atoms exhibit absorption bands at larger wavenumbers.

• Bond Order Influence

  • Higher bond orders correspond to absorption bands at larger wavenumbers.