There is a continuum of wavelength (or frequency) of EMR. Different wavelengths will interact with different physical properties of matter. This results in a number of different Spectroscopy methods.
EMR has both wave-like properties and particle-like properties.Wave-like properties:Depends on the wavelength (λ), frequency (ν) and are related by the speed of light(c): c = λ ν or λ = c / ν or ν = c / λParticle-like properties:
(c is the speed of light: 3 x 108 m/s)Depend on the quantized energy of a photon where the energy is given by: E = h ν = h c / λ
(h = Plank's constant 6.626 x 10-34 Js)
Note that the energy is directly proportional to the frequency and inversely proportional to the wavelength.
Excited States
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All spectroscopy methods rely on the molecule absorbing energy and moving from a ground energy state to an excited energy state. We can measure the amount and frequency of energy absorbed which will correspond to specific physical features in the molecules. For example, the absorption of energy by an electron in the ground state will result in an excited electronic state (see image right). |
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- Radio wavelengths interaction with the nuclear spin on some atoms and results in Nuclear Magnetic Resonance (NMR) spectroscopy.
- Microwave wavelengths interact with the rotational motion of a molecule and results in Raman and IR spectroscopy.
- Infrared (IR) wavelengths interact with the vibrational motion of the atoms within a molecule and results in IR spectroscopy.
- Visible-Ultraviolet (UV-Vis) wavelengths interact with the electronic state of a molecule (promoting electrons to a higher energy state) and results in UV-Vis spectroscopy.
- X-rays do not interact with the atoms in a molecules, but are deflected by the nucleus and can be used to determine the 3D spatial arrangement of atoms in a molecule. This is X-ray or powder diffraction crystallography.
| IR Spectroscopy | NMR Spectroscopy | Mass Spectrometry (MS) | UV-Vis Spectroscopy |
| 1H-NMR 13C-NMR | |||
| Geting Structures from Spectra | |||