Spectroscopy

Table Of Contents
1 Infrared Spectroscopy
2 Intramolecular Vibrations and Rotations
3 Characteristic Absorptions
4 Ultraviolet Spectroscopy
5 Electron Transitions
6 Conjugated Systems
7 Nuclear Magnetic Resonance Spectroscopy
8 Proton NMR (1H-NMR)

Infrared Spectroscopy

  • Measures molecular vibrations which can be seen as bond stretching, bending or combinations of different vibrational modes.

Intramolecular Vibrations and Rotations

  • Infrared light range has a wavelength range of: 700 nm to 1 mm
    • Useful absorptions occur at wavelengths between 2500 to 25,000 nm.
  • Wavenumber: like the frequency but in units of cm-1.
    • Wavenumber = 1/λ
  • When light with wavenumbers between 4000 to 400 cm-1 is absorbed, the molecules enter an excited vibrational states.
    • Four types can occur: Symmetric bend, asymmetric bend, symmetric stretch, asymmetric stretch
  • Fingerprint Region: wavenumbers in range of 1500 to 400 cm-1 where complex vibration patters occur. Patterns are associated with the motion of the molecule as a whole.
  • For the absorption to be recorded, the vibration must result in a change in the bond dipole moment.
    • Molecules with same electronegativity (O2 & Br2) are not useful, neither are symmetric molecules like C2H2.

Characteristic Absorptions

  • Can be used to identify the functional groups. Most important peaks to know are:
    • Hydroxyl (O-H): absorbs wide peak at 3300 cm-1 for alcohols and 3000 cm-1 for carboxylic acids.
    • Carbonyl (C double bonded to O): sharp peak at 1700 cm-1
    • N-H Bonds: have a sharp peak at 3300 cm-1
  • Bond between any atom and hydrogen always has a high absorption frequency
  • As more bonds are added between carbon atoms, the absorption frequency increases
  • All frequencies in the fingerprint region are out of scope.
  • Transmittance: amount of light that passes through the sample and reaches the detector
  • IR spectra is plotted as Transmittance vs wavenumber

Ultraviolet Spectroscopy

  • UV spectra are obtained by passing ultraviolet light through a sample that is usually dissolved in an inert, non-absorbing solvent, and absorbance is caused by the electron transitions between orbitals.
  • Most important information gathered: wavelength of the maximum absorbance
    • Can tell us the extent of conjugation within the system
    • As conjugation increases, the energy lowers and thus the wavelength increases.

Electron Transitions

  • UV spectroscopy works because molecules with pi-bonds or nonbonding electrons can be excited by UV light to high energy orbitals
  • Molecules with a lower gap between the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO) are more easily excited and can thus absorb longer wavelengths.

Conjugated Systems

  • Conjugated molecules are molecules with unhybridized p-orbitals
  • These can be excited by UV light
  • Conjugation causes a shift in the absorption spectrum that results in higher maximum wavelengths.

Nuclear Magnetic Resonance Spectroscopy

  • NMR: the most important spectroscopic technique on the MCAT
    • Certain atomic nuclei have magnetic moments that are oriented at random, but when placed in a magnetic field, the moments of these nuclei tend to align with the field or opposite to the field.
      • Nuclei with magnetic moments that are aligned with the field are in the alpha state
    • From the alpha state, radiofrequency pulses can be radiated onto the nuclei to excite the lower energy nuclei into the Beta-state.
    • Absorption of radiation leads to excitation at different frequencies
  • MRI: a non-invasive diagnostic tool that uses proton NMR
  • NMR Spectrum: plot of frequency vs absorption of energy
    • Standardized method uses a chemical shift (d) with units of ppm of spectrometer frequency.
    • This is plotted on the x-axis and increases to the left (downfield)
    • Tetramethylsilane (TMS) is used as a calibration standard to mark the location of 0 ppm
  • Can be conducted on any atom which possesses a nuclear spin (odd mass number or odd atomic number or both)

Proton NMR (1H-NMR)

  • Most hydrogens come into resonance from 0 to 10 ppm downfield from TMS
  • Protons that are chemically equivalent will have the same magnetic environment and will thus correspond to the same peak.
  • Height of each peak is proportional to the number of protons.
  • Deshielding: the more a protons electron density is pulled away (by more electronegative elements), the less it can shield itself from the applied magnetic field.
  • Spin-Spin Coupling (splitting): when two protons are in close proximity but are not magnetically identical.
    • Results in a doublet being formed: two peaks of identical intensity, equally spaced around the true chemical shift of a proton.
    • n+1 Rule: if a proton has n protons that are three bonds away, it will split into n+1 peaks
    • Do not include protons attached to oxygen or nitrogen
    • Magnitude of splitting is called the coupling constant.
  • Chemical Shift ranges:
    • Deshielded aldehyde: 9-10 ppm
    • Carboxylic acid:10.5-12 ppm
    • Hydrogen on Aromatic Ring: 6.0-8.5 ppm
    • Sp3 Hybridized carbons: 0-3 ppm
    • Sp2 Hybridized carbons: 4.6-6.0 ppm
    • Sp Hybridized carbons: 2.0-3.0 ppm
    • When electronegative groups are present, they pull electron density away from the proton and further deshield it.