Spectoscopy and Structure Help
Spectral properties are used to determine the structure of molecules and ions. Of special importance are ultraviolet (UV), infrared (IR), nuclear magnetic resonance (NMR), and mass spectra (MS).
Different types of electromagnetic radiation correspond to different molecular transformations depending on the energy separation between the two states of the molecule involved. The molecule can be raised from its lowest energy state (ground state) to a higher energy state (excited state) by a photon (quantum of energy) of electromagnetic radiation of the correct wavelength.
|Region of Electromagnetic Spectrum||Type of Excitation|
|Infrared (IR)||Molecular vibration|
|Radio||Spin (electronic or nuclear)|
Wavelengths (λ) for UV-visible spectra are typically expressed in nanometers (1 nm = 10–9 m), while for infrared spectra, wavenumbers (reciprocal centimeters, or cm–1) are used.
In a typical spectrophotometer, a dissolved compound is exposed to electromagnetic radiation over a specified wavelength. The radiation passing through the sample is recorded as a function of the wavelength or wavenumber.
Ultraviolet or visible light cause an electron to be excited from a lower-energy orbital to a high-energy orbital. Compounds that absorb this type of radiation are unsaturated, and conjugated double bonds absorb lower energy (longer wavelengths in the visible region) than isolated double bonds do. Species that absorb in the visible region are colored, and black is observed when all visible light is absorbed.
Infrared radiation causes excitation of molecular vibration states. Diatomic molecules such as H–H and H–Cl vibrate in only one way; the atoms move, as though attached by a coiled spring, toward and away from each other. This mode is called bond stretching. Triatomic molecules, such as CO2 (O=C=O), possess two different stretching modes. In the symmetrical stretch mode, each O moves away from the C at the same time. In the antisymmetrical stretch, one O moves toward the C while the other O moves away.
An observed absorption band at a specific wavelength indicates the presence of a particular bond or group of bonds in a molecule. Although absorption is affected only slightly by the molecular environment of the bond or group, it is possible to determine from small variations in the band frequencies such factors as the size of the ring containing a C=O group or whether C=O is part of a ketone, acid chloride, or an acid. Conversely, the absence of a certain band in the spectrum usually rules out the presence of the bond that would produce it.
Between 1400 and 800 cm–1, there are many peaks which are difficult to interpret. This range, called the fingerprint region, is useful for determining whether two samples are composed of the same compound. No two compounds have exactly same IR spectrum.
The C–H stretch absorption, the C=O stretch absorption, and the fingerprint region are all visible in the IR spectrum of ethyl acetate, shown below.
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