12.5 Spectroscopy and the Electromagnetic Spectrum
3 min read•may 7, 2024
interacts with molecules in fascinating ways. Light waves can cause molecules to vibrate, rotate, or undergo . These interactions form the basis of , a powerful tool for analyzing molecular structures.
Different types of radiation excite different molecular motions. light causes bond vibrations, while UV and trigger electronic transitions. By studying how molecules absorb and emit radiation, we can deduce their structures and properties.
Electromagnetic Radiation and Spectroscopy
Electromagnetic radiation and organic molecules
Top images from around the web for Electromagnetic radiation and organic molecules
Photon Energies and the Electromagnetic Spectrum | Physics View original
Is this image relevant?
1 of 3
Electromagnetic radiation consists of oscillating electric and magnetic fields that travel through space as waves characterized by (λ), (ν), and energy (E)
Molecules can absorb electromagnetic radiation when the energy of the matches the energy difference between two molecular causes molecules to transition from a lower energy state to a higher energy state
Different types of molecular motions and transitions require different amounts of energy
Stretching and bending vibrations of bonds absorb infrared (IR) radiation ()
Electronic transitions absorb and visible light (colors)
Molecules absorb radiation at specific wavelengths corresponding to the energy required for a particular transition while wavelengths not absorbed by the molecule are transmitted through the sample (transparent materials)
Energy levels and transitions
Molecules have discrete energy levels corresponding to different states:
Ground state: lowest energy level
Excited states: higher energy levels
Transitions between energy levels can occur through:
: molecule gains energy by absorbing a photon
: molecule loses energy by emitting a photon
Electronic transitions involve changes in the distribution of electrons in molecular orbitals, typically requiring higher energies than molecular vibrations
Photon energy calculations
The relates the energy of a photon to its frequency and wavelength:
E=hν=λhc
E is the energy of the photon (in joules)
h is (6.626×10−34 J⋅s)
ν is the frequency of the electromagnetic wave (in Hz)
c is the speed of light (2.998×108 m/s)
λ is the wavelength of the electromagnetic wave (in meters)
Frequency and wavelength are inversely proportional
Higher frequency corresponds to shorter wavelength and higher energy ()
Lower frequency corresponds to longer wavelength and lower energy ()
Different regions of the electromagnetic spectrum have different wavelengths and energies:
Infrared (IR) spectroscopy measures the absorption of IR radiation by a sample typically presented as a plot of (%T) vs. (ν~) or wavelength (λ)
Wavenumber is the reciprocal of wavelength, expressed in cm−1
Different functional groups absorb IR radiation at characteristic wavelengths
O-H and vibrations: 3200-3600 cm−1 (alcohols, amines)
vibrations: 2800-3300 cm−1 (alkanes)
vibrations: 1600-1800 cm−1 (ketones, aldehydes)
vibrations: 1600-1700 cm−1 (alkenes)
%T represents the amount of IR radiation transmitted through the sample
Lower %T indicates greater absorption at a particular wavelength
Absorption peaks appear as downward-pointing peaks or dips in the spectrum
Presence of absorption peaks at characteristic wavelengths indicates the presence of specific functional groups while absence of expected absorption peaks suggests the absence of certain functional groups
Comparing the IR spectrum of an unknown compound to reference spectra can aid in structural elucidation (identifying unknown compounds)
The relates the absorption of light to the properties of the sample through which the light is traveling, allowing for quantitative analysis of solution concentrations