Study Notes on Mass Spectrometry, Infrared Spectroscopy, and Nuclear Magnetic Resonance Spectroscopy

Chapter 14: Mass Spectrometry and Infrared Spectroscopy

  • Introduction to Structural Determination Techniques

    • Markovnikov and Zaitsef established rules for regioselectivity in alkene formation.

    • Proving structures of products used to be difficult and time-consuming.

    • Recent advances have simplified structural determination in organic compounds.

    • Overview of four techniques:

    • Mass Spectrometry

    • Infrared Spectroscopy

    • Nuclear Magnetic Resonance (NMR)

    • Ultraviolet Spectroscopy

    • Each technique yields distinct structural information:

    • Mass Spectrometry = Molecular weight and probable formula, functional groups present, structural framework.

    • Infrared Spectroscopy = Structural information through functional group identification.

    • Nuclear Magnetic Resonance = Detailed structural and stereochemical information.

    • Ultraviolet Spectroscopy = Electronic transitions and structure information through conjugated systems.

Electromagnetic Radiation and Absorption Spectroscopy

  • Electromagnetic Radiation

    • Visible light is part of a larger spectrum known as electromagnetic radiation, characterized by both particle and wave properties.

    • Key Definitions:

    • Wavelength ($BB$): The distance between crests of a wave.

    • Frequency ($
      u$): The number of crests passing a given point in one second; measured in Hertz (Hz).

    • Relationship:
      c = BB imes
      u

    • where $c$ is the speed of light ($3.0 imes 10^8$ m/s).

    • Quanta: Electromagnetic energy is transmitted in discrete packets called quanta (or photons).

    • Energy of a quantum can be calculated as:
      B5 = h
      u

    • where:

      • $B5$ = energy of one photon

      • $h = 6.626 imes 10^{-34}$ J⋅s (Planck's constant).

  • Interaction with Organic Compounds

    • Organic compounds absorb specific wavelengths of electromagnetic radiation.

    • Absorbed wavelengths indicate energy transitions, such as electron promotion or molecular vibrations (bending, stretching, or spinning).

    • An absorption spectrum can be constructed by plotting absorbed wavelengths (x-axis) against transmitted light intensity (y-axis).

Infrared Spectroscopy

  • IR Absorption

    • Organic molecules absorb light in the infrared region (2.5 x 10^{-6} m to 2.5 x 10^{-5} m or 2.5 to 25 µm).

    • Energy associated with molecular vibrations: bond stretching or bending.

    • Absorption occurs when light frequency matches molecular vibration frequency.

    • Functional groups absorb IR light at specific, characteristic wavelengths, making IR spectroscopy effective for functional group detection.

  • Wavenumber

    • Frequency of IR light absorbed is usually represented as a wavenumber ($cm^{-1}$):
      extWavenumber=rac1extwavelengthext(cm)ext{Wavenumber} = rac{1}{ ext{wavelength} ext{(cm)}}

    • Energy of light absorbed is inversely related to wavelength; thus, directly proportional to wavenumber.

  • Interpretation of IR Spectra

    • Many organic molecules show numerous stretching and bending modes in IR, leading to complex spectra.

    • Notably, the fingerprint region (1500 to 400 cm^{-1}) exists, where spectra of different compounds are unlikely to match.

    • Characteristic absorbances occur for certain functional groups and remain consistent across compounds:

    • Common Functional Groups and IR Absorptions:

      • Alkanes:

      • C-H stretch: ~ 2980 cm^{-1}

      • C-C stretch: Specifics depend on context

      • Alkenes:

      • C=C stretch: ~ 1600-1680 cm^{-1}

      • Alkynes:

      • C≡C stretch: ~ 2100-2300 cm^{-1},

      • C-H stretches vary.

      • Alcohols:

      • O-H stretch: Broad, intense ~ 3200-3600 cm^{-1}.

      • Amines:

      • N-H peaks: Sharper, less intense than O-H.

      • Aromatic Compounds:

      • Characteristic C-H and C-C stretches (approximately ~ 1600 cm^{-1}).

      • Carbonyls:

      • C=O stretch: Sharp, intense ~ 1700 cm^{-1}.

Mass Spectrometry

  • Purpose of Mass Spectrometry

    • Instrumental technique to determine molecular weights and hence approximate molecular formulas of compounds.

    • Provides structural information through spectral data.

  • Working Mechanism

    • Mass spectrometers operate by deflecting ionic sample through a magnetic field, creating a graph of mass-to-charge ratio (m/z) versus intensity (number of ions).

    • Ions, usually with a charge ($z=+1$), are detected.

    • Non-ionic compounds must be ionized for detection; common method is electron-impact ionization.

  • Spectrum Interpretation

    • Molecular Ion Peak: Peak corresponding to the radical cation's mass (original substrate), minus the mass of an electron (negligible mass).

    • Base Peak: Peak of highest intensity in the mass spectrum.

    • Isotope Adjacency Effects:

    • M + 1 peak indicates presence of 13C.

    • Halogen isotopes (Cl and Br) produce M + 2 peaks corresponding to their natural isotope ratios.

  • Fragmentation Patterns

    • High-energy radical cations can fragment into smaller ions, represented by additional peaks.

    • Example fragmentation mechanisms:

    • Hydrocarbons break through C-C and/or C-H bonds.

    • Alcohols undergo fragmentation via alpha cleavage and dehydration.

    • Carbonyl compounds undergo McLafferty rearrangements involving beta-hydrogen.

  • Degrees of Unsaturation

    • Calculated using:(2n+2−a−d+c)/2(2n+2 - a - d + c)/2 where n is the number of carbons, a is the number of hydrogens, d is the number of double bonds, and c is the number of rings.

Chapter 15: Nuclear Magnetic Resonance Spectroscopy

  • Introduction

    • NMR is critical for structural analysis of organic molecules, allowing depiction of C-H frameworks.

  • Spinning Nuclei

    • Atomic nuclei behave like bar magnets due to spin and net positive charge, creating magnetic fields.

    • Nuclei align themselves in external magnetic fields (B0) in one of two energy states; energy difference is influenced by the strength of B0.

  • NMR Absorption

    • Electromagnetic (radio wave) light has energy sufficient to flip nuclear magnetic moments.

    • Magnetic field strength varies, causing different frequencies for absorption among non-equivalent nuclei.

    • Chemical Shift: As defined by $ ext{Chemical Shift} = B0 - B{ ext{shielding}}$.

    • TMS as Standard: Tetramethylsilane (TMS) is used for reference at zero ppm.

  • Features of 1H-NMR Spectrum

    • Four key features provide structural insights:

    1. Number of Signals

    2. Position of Signals

    3. Intensity of Signals

    4. Splitting of Signals

  • Chemical Equivalence

    • Different protons resonate at varying field strengths based on environment.

    • Equity leads to signals at the same frequency (homotopic, enantiotopic, diastereotopic).

Summary

  • Understanding mass spectrometry and infrared spectroscopy provides essential techniques in organic chemistry for structural determination, revealing molecular weights, functional groups, and transformations.

  • Nuclear magnetic resonance offers a deeper understanding of molecular structure through proton environments and interactions, essential for elucidating complex organic molecules.