During your Chemistry lab, you are asked to identify the molecule that you have isolated in your sample. Is it pentanol? Ethanol? 1,4-dichloropropanol? How can we tell? Well, that is where spectroscopy comes into play. Spectroscopy is used to determine the identity of molecules that are unobservable by the naked eye.
Like a detective, you can use a couple of different spectroscopy techniques to find out different information about a molecular sample of interest to determine its identity.
Now that we know the basic function and use for spectroscopy, we can dive a little further into looking at key terms, definitions, and topics that will be important for the MCAT
Let’s get started!
Spectroscopy on the MCAT: What You Need to Know
Organic chemistry accounts for 15% of the Chemical and Physical Foundations of Biological Systems Section of the MCAT.
Questions pertaining to spectroscopy will most likely be asked within passage-based questions, more so than fundamental discrete questions. It is important to practice using information from a spectroscopy graph/analysis to determine the molecular structure of a molecule.
Luckily for you, this review will give you a concise yet comprehensive review of spectroscopy that you are required to understand.
Let’s get started!
Important Sub-Topics - Spectroscopy
Fundamentals of Chemical Spectroscopy
Spectroscopy is the study of the interaction between electromagnetic radiation with matter.
The electromagnetic spectrum ranges from gamma rays to radio frequencies. Depending on the spectroscopic technique, different frequencies of radiation interact with matter; allowing us to elucidate the molecular structures of matter, and verify the chemical identities of chemical products or reactants.
(Coming Soon!) Full Study Notes : Fundamentals of Chemical Spectroscopy
For more in-depth content review on fundamentals of chemical spectroscopy, check out these detailed lesson notes created by top MCAT scorers.
NMR Spectroscopy
NMR stands for nuclear magnetic resonance which is one of the most widely used spectroscopic techniques to characterize organic compounds. NMR functions on the principle that nuclei have protons that spin in the presence of an external magnetic field. The spin of protons results in a spectral analysis that can provide more information about what atoms are close to it.
The spectral analysis includes:
The position of the signal that can tell you about the electronic structure and potential atoms that are directly around the proton.
The size/area under each signal can tell you about the number of equivalent protons (i.e. the protons that are exposed to the same chemical environment).
The multiplicity can reveal the number of neighboring non-equivalent protons.
(Coming Soon!) Full Study Notes : NMR Spectroscopy
For more in-depth content review on NMR Spectroscopy, check out these detailed lesson notes created by top MCAT scorers.
Infrared Spectroscopy
Infrared spectroscopy involves the emission of infrared light which passes through a molecular sample, which causes covalent bonds within molecules to vibrate. These bonds vibrate by stretching and bending.
The spectrometer measures the percent of light that is transmitted through the molecule (percent transmittance) over a range of frequencies. The frequencies at which the light is absorbed depend on the types of functional groups and bonds in the molecular sample. This information is shown on a percent transmittance by wavenumber graph. It is important to know which functional groups are represented by the shape and location of inflection points on the graph.
(Coming Soon!) Full Study Notes : Infrared Spectroscopy
For more in-depth content review on Infrared Spectroscopy, check out these detailed lesson notes created by top MCAT scorers.
Mass Spectroscopy
This form of spectrometry involves the separation of compounds based on their mass-to-charge ratio.
First, the sample is vaporized and ionized by bombarding it with electrons. Once the sample is ionized, it is passed through an electric or magnetic field where it accelerates and deflects; separating the particle based on its mass. Faraday collectors detect the charge of the particles due to the current generated by the ions, which will indicate relative abundance.
(Coming Soon!) Full Study Notes : Mass Spectroscopy
For more in-depth content review on Mass Spectroscopy, check out these detailed lesson notes created by top MCAT scorers.
Important Definitions and Key Terms - Spectroscopy
Key Term | Definition |
|---|---|
Electromagnetic radiation | Radiation has both electric and magnetic fields that travel perpendicular to one another. |
NMR Resonance | When the frequency of radio waves matches the energy required to flip the spin of proton nuclei. This is detected by the NMR spectrometer and turns it into signals in an NMR output. |
Equivalence | Chemically equivalent protons which are in the same chemical environment. |
Shielding | When proton nuclei are surrounded by a less electronegative environment, they become shielded, resulting in a chemical shift upfield (to the right of the spectrum). |
Deshielding | When proton nuclei are surrounded by a more electronegative environment, they become deshielded, resulting in a chemical shift downfield (to the left). |
Multiplicity | The number of peaks in one NMR signal, which indicates the number of neighboring protons that are non-equivalent. |
Chemical Shift | The resonant frequency of a proton nucleus relative to a standard in a magnetic field. |
Upfield/Downfield | Describes the location of the peaks within the NMR spectroscopy graph. |
Wavenumber | Present on the X-axis of an IR spectrum (1/wavelength). |
Fingerprint region | The region between 400 cm-1 and 1500 cm-1 in an IR spectrum that is unique and can provide information about the functional groups present within a molecular sample. |
Ionization | The addition or removal of electrons or ions to produce a charged particle. |
Mass-to-charge-ratio | The division of the weight of an ion by its charge. |
Faraday collectors | A metal design to catch charged particles in a vacuum. |
Additional Reading Links (Coming Soon!) – Study Notes for Spectroscopy on the MCAT
Additional Reading -- Organic Chemistry Topics:
- Aldehydes and Ketones on the MCAT
- Bonding on the MCAT
- Carboxylic Acids and Derivatives on the MCAT
- Isomers on the MCAT
- Laboratory Techniques and Separations on the MCAT
- Nitrogen Containing Compounds on the MCAT
- Phosphorus Containing Compounds on the MCAT
- Organic Chemistry Nomenclature on the MCAT
- Nucleophiles and Electrophiles on the MCAT
- Alcohols and Ethers on the MCAT
- Redox Reactions Organic Chemistry on the MCAT
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