Spectroscopy {infrared spectroscopy} (IR) can detect functional groups and chemical-bond types. IR is simple, is cheap, uses infrared wavelengths 1 micrometer to 20 micrometers, and has narrow absorption bands. Infrared light source is Nernst glower or nichrome wire. Infrared light detector is thermocouple, thermister, or bolometer. Sodium-chloride prisms or reflection gratings select wavelength. Double beam allows scanning.
solvent
Solids, liquids, and gases have concentration 0.1% to 10%. Water absorbs infrared light, so it cannot be solvent. Carbon disulfide solvent is for wavelengths 7.5 micrometers to 16 micrometers. Carbon tetrachloride solvent is for wavelengths 2.5 micrometers to 7.5 micrometers.
container
Sodium chloride or potassium chloride windows hold sample. Path length is 0.1 millimeters to 1.0 millimeter.
theory
Organic-molecule functional groups have chemical bonds with rotations and vibrations. Rotations and vibrations have energies in infrared-light range. IR typically measures vibrations, rather than rotations, because vibrations have higher energies and frequencies. Rotations appear superimposed on vibrational spectral lines and make wider vibrational bands.
theory: dipole moment
The greater the dipole moment, the greater the infrared frequency. The longest chemical bonds and most-polarized functional groups have highest frequencies. Chemical-bond frequencies from highest to lowest are N-H, O-H, C-H, C=O, C=C, C-O, C-C, and H-H.
theory: rotations
Molecule rotation states depend on molecular symmetries. Spherical molecules have no rotational states. Linear molecules have one state, if they are symmetric along axis. Linear molecules with asymmetry have two states. Asymmetric molecules have three states, one for each axis.
Massive molecules have long dipole and have close rotational energy levels. Molecules with large bond distance have long dipole.
theory: vibrations
Chemical bond has stretching vibration. Two same-atom chemical bonds have bending vibrations. Vibrations involve simple harmonic motion, along bond axis or around bond angle. Vibrations give information about bond rigidity and bond-breaking energies. Strong bonds and long bond distances have more energy and make close energy levels. The most-polarized bonds have longest wavelength.
Bond bending is easier than bond stretching, because forces are less, so bending-vibration frequencies are less than stretching-vibration frequencies.
One bond has one less vibrational state, because rotation around bond makes vibrations cancel, because they have all directions. Homonuclear diatomic molecules have no stretching or bending vibrations, because they have no dipole. Heteronuclear diatomic molecules have only stretching vibrations along one bond.
Molecules with three atoms can have bending vibrations. n atoms can have 3*n vibrational states {degrees of freedom, atom}: three for translations and three or less for rotations. Pi-bond twisting is vibration.
problems
Vibrational energy levels get closer together nearer bond-dissociation energy. Liquids and solids have more vibration effects, because they have bonding and van der Waals forces.
Physical Sciences>Chemistry>Analytical Chemistry>Spectroscopy
5-Chemistry-Analytical Chemistry-Spectroscopy
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Date Modified: 2022.0224