MCAT Organic Chemistry > Spectroscopy
Absorption Spectroscopy is the measurement of the absorption of radiation during the interaction of this radiation with a sample. There are a few major types of spectroscopy and other means of chemical analysis that are tested on the MCAT.
- Infrared Spectroscopy
- Visible Spectrum
- UV Spectrum
- Proton (H1) NMR Spectroscopy
- Mass Spectroscopy
- Chemical Indicators
Absorption of light in the visible region by a molecule yields the complementary color. When a molecule absorbs a specific color, it leads to the absence of that color and produces instead the complementary color. To find the complement of a color, just find its partner across the color wheel.
All of the rest of the colors are made from a combination of these three colors.
The absorption of visible light causes certain structural changes to occur in molecules.
Pi-electron and Non-bonding Electron Transitions
Whenever a bond between two atoms occurs, their electrons and atomic orbitals must conjoin and form a cohesive unit. Every time a bonding orbital forms, three types of orbitals form:
- Molecular Orbitals
- Non-Bonding Orbitals
- Anti-Bonding Orbitals
For the most part, electrons like to stay in bonding orbitals because it is most stable, and remember everything in nature always wants to be in the most stable conformation. If the bonding orbitals are already filled by electrons then the spares will migrate to the non-bonding or anti-bonding orbitals.
If given enough energy such as through UV radiation absorption electrons will move between the various orbitals and rotate throughout the molecule. However, if given more energy these electrons will escape from the bonding orbitals and jump to the anti-bonding orbitals thus breaking the bond.
For this topic on the MCAT you should only be concerned with knowing how the pi-electrons of the double bond behave when zapped with ultra-violet radiation. Double bonds absorb UV radiation because the pi-electrons of the double bond can easily transition from the bonding and non-bonding orbitals into the anti-bonding orbitals.
When electromagnetic energy in the ultraviolet and visible spectra pass through a compound, a portion of the radiation is usually absorbed by the compound. The rate of absorption is directly dependent on two factors: the wavelength of the radiation, and the structure of the compound. This absorption of radiation is caused by the transfer of energy from the radiation beam to electrons that can be excited to higher energy orbitals.
Conjugated systems decrease the amount of electromagnetic radiation that is absorbed and the more conjugated double bonds there are, the longer the wavelengths of absorbed radiations. if there are enough conjugated double bonds then the molecule can be quantified in the visible spectrum.
The electron is generally excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). Alkenes and non-conjugated dienes have absorptions below 200 nm because the energy difference between the HOMO and LUMO is large. In conjugated dienes these energy levels are much closer together and the wavelengths of absorption are longer than 200 nm, and are discussed in the following subsection.
When ultraviolet radiation hits a substance, it is absorbed only at double bonds in the molecule. A UV-Vis spectrum is typically measured from 200-800 nm, spanning the near UV and visible regions. The wavelength of maximum absorption (lmax) is reported in units of nanometers (nm). Molar absorptivity (e), or how much ultraviolet radiation is absorbed by one mole of a compound, is also reported.
The main point to remember for conjugated systems is that, the longer the conjugated system is, the smaller the energy difference will be between the HOMO and the LUMO. A smaller energy gap results in longer lmax in the ultraviolet-visible spectrum.
β-Carotene, found in carrots, has 11 conjugated double bonds and an absorbance maximum at 497 nm which is in the blue-green region of the visible spectrum. β-Carotene is perceived as red-orange, the complementary color of blue-green.