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Organic Chemistry: Infrared and Mass Spectrometry Fundamentals

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  • What does infrared (IR) spectroscopy measure in a molecule?
    IR spectroscopy measures the bond vibration frequencies to identify functional groups.
  • How are frequency and wavelength related in electromagnetic radiation?
    Frequency (ν) and wavelength (λ) are inversely proportional, related by \(c=\lambda \nu\) where c is the speed of light.
  • What is the typical wavelength range for infrared radiation?
    Infrared wavelengths range from about 2.5 × 10-4 cm to 25 × 10-4 cm.
  • What determines the frequency of bond stretching vibrations in IR spectroscopy?
    Frequency depends on bond stiffness (bond energy) and atomic masses of the bonded atoms.
  • How does atomic mass affect stretching frequency?
    Heavier atoms cause lower stretching frequencies due to decreased vibration frequency.
  • How does bond energy affect stretching frequency?
    Stronger bonds have higher stretching frequencies because they are stiffer.
  • What are the three fundamental vibrational modes of water?
    Water has symmetric stretching, antisymmetric stretching, and bending (scissoring) vibrational modes.
  • What is the 'fingerprint region' in an IR spectrum?
    The fingerprint region is from 600 to 1400 cm-1 and contains complex vibrations unique to each molecule.
  • What makes a vibration IR-active?
    A vibration is IR-active if it involves a change in dipole moment, typically in polar bonds.
  • How does conjugation affect C=C stretching frequency?
    Conjugation lowers the C=C stretching frequency compared to isolated double bonds.
  • Order the C-H stretching frequencies by hybridization from lowest to highest.
    sp3 C-H: ~2800-3000 cm-1, sp2 C-H: ~3000-3100 cm-1, sp C-H: ~3300 cm-1 (sharp).
  • What IR absorption is characteristic of alcohol O-H stretching?
    A broad, intense absorption centered around 3300 cm-1 due to hydrogen bonding.
  • How can you distinguish primary, secondary, and tertiary amines in IR spectra?
    Primary amines show two sharp N-H spikes, secondary amines show one sharp spike, and tertiary amines show no N-H stretch.
  • Where do carbonyl (C=O) stretches typically appear in IR spectra?
    Around 1710 cm-1 for ketones, aldehydes, and carboxylic acids.
  • How does conjugation affect the C=O stretching frequency?
    Conjugation lowers the C=O stretch to about 1680 cm-1.
  • What is the typical IR absorption range for nitrile (C≡N) stretching?
    A strong, sharp absorption just above 2200 cm-1.
  • What is the base peak in a mass spectrum?
    The base peak is the tallest peak and is assigned 100% relative abundance.
  • What is a radical cation in mass spectrometry?
    A molecule that has lost one electron, carrying a positive charge and an unpaired electron.
  • What does high-resolution mass spectrometry (HRMS) provide?
    HRMS measures masses with high accuracy (~1 part in 20,000) to determine exact molecular formulas.
  • How do isotopes affect mass spectra?
    Isotopes cause characteristic M+1 and M+2 peaks; e.g., bromine shows nearly equal M and M+2 peaks due to 79Br and 81Br isotopes.