Seeing Molecules with Light and Magnets
Spectroscopy is the study of the interaction between matter and electromagnetic radiation. It is a powerful set of tools used by chemists to determine the structure of unknown molecules.
Infrared (IR) Spectroscopy
Principle: Molecules are not static; their bonds can stretch, bend, and vibrate. Different types of bonds vibrate at specific, characteristic frequencies. When infrared radiation is passed through a sample, the bonds will absorb the frequencies of IR light that match their natural vibrational frequencies.
Information Obtained: The presence of specific functional groups.
The Spectrum: An IR spectrum plots transmittance versus wavenumber (cm⁻¹). Large 'dips' in the graph indicate absorption.
A broad dip around 3300 cm⁻¹ suggests an O-H bond (alcohol).
A sharp, strong peak around 1700 cm⁻¹ suggests a C=O bond (ketone, aldehyde, etc.).
Mass Spectrometry (MS)
Principle: This technique does not use light. Instead, it bombards a sample with electrons, knocking an electron off the molecules to form a positive ion (the molecular ion). These ions are then accelerated through a magnetic field, which deflects them. The amount of deflection depends on the ion's mass-to-charge ratio (m/z).
Information Obtained: The molar mass of the compound and the masses of its fragments.
The Spectrum: A mass spectrum plots relative abundance versus m/z.
The peak furthest to the right is usually the molecular ion peak (M⁺), which gives the molar mass of the entire molecule.
The pattern of other peaks (fragmentation pattern) can act as a fingerprint to help identify the molecule's structure.
Nuclear Magnetic Resonance (NMR) Spectroscopy
Principle: Certain atomic nuclei (most commonly ¹H, proton NMR) have a quantum mechanical property called spin. When placed in a strong magnetic field, these nuclei can exist in different spin states. They can be made to 'flip' between states by absorbing energy from radio waves. The exact frequency needed depends on the local chemical environment of the nucleus.
Information Obtained: The carbon-hydrogen framework of a molecule.
The Spectrum (¹H NMR):
Number of signals: Indicates the number of chemically non-equivalent sets of protons.
Chemical Shift (position): Indicates the chemical environment of the protons (e.g., are they next to an oxygen?).
Integration (area): The area under each signal is proportional to the number of protons in that set.