Atoms are like two-year-olds: always in motion. Unlike two-year-olds, however, atoms are usually bonded to other atoms. This limits the direction, extent and ways in which the atoms can move. Infrared (IR) spectroscopy uses these atomic movements to identity substances. By exposing a given substance to specific infrared frequencies, scientists can determine the frequencies at which the substance absorbs more or less infrared energy. These frequencies correspond to the movements of the bonded atoms within the substance and the specific arrangement of those atoms.
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Vibrational Freedom and Absorption
Molecules absorb amounts of energy that correspond to the vibrational movements of the atoms they contain. The vibrational movement of a molecule, or its vibrational freedom, is calculated using the formula 3n - 6, in which n represents the number of atoms in the molecule. For example, a molecule of formaldehyde, CH2O, having a carbon atom double bonded to one oxygen atom and single bonded to two hydrogen atoms, all at angles of 120 degrees, has a vibrational freedom of six: (3 x 4) - 6 = 6. The six ways in which a molecule of CH2O can vibrate determine the frequencies of infrared energy that the molecule will absorb and therefore the infrared spectrum it will produce. Unlike formaldehyde, a molecule such as CO2, with all its atoms in a line, will have only 3n - 5 degrees of vibrational freedom.
Stretching refers to shortening and stretching of the bonds between two atoms. This stretching can be symmetric or asymmetric. For example, the two carbon-hydrogen bonds of formaldehyde can symmetrically shorten and lengthen in the same direction at the same time, or one carbon-hydrogen bond can asymmetrically shorten while the other lengthens and then subsequently lengthen while the other shortens. Symmetric CH2 stretching absorbs infrared energy at 2785cm^-1 in the infrared spectrum; asymmetric CH2 stretching absorbs infrared energy at 2850cm^-1 in the spectrum. Symmetric stretching of the carbon-oxygen double bond produces a strong absorption signature at 1750cm^-1. The higher energy level of the carbon-oxygen absorption derives from the greater strength of the carbon-oxygen double bond and from changes in the dipole moment, or concentration of electrical charge, between the two atoms. Triple-bonded atoms such as the nitriles (CN), in turn, produce higher infrared energy absorption than double-bonded atoms.
Rocking and Wagging
Rocking and wagging represent simultaneous planar vibrations of two bonded molecules without changes in the bond lengths of those molecules. The CH2 of formaldehyde can undergo these types of vibration as well. Stand up and hold your arms apart at a 90-degree angle. Your body is the carbon atom and each of yours hands is a hydrogen atom. Now rotate your torso back and forth while keeping your arms straight. Your hands, the hydrogens, will swing back and forth in space while maintaining the same spatial separation. This is rocking. Now hold your arms at the same angle but move your arms up and down at the same time. This is the vibrational movement scientists call wagging. Formaldehyde shows CH2 rocking infrared absorption at 1250cm^-1; it shows a lower-energy CH2 wagging at 1165cm^-1. Wagging and rocking vibrations represent lower energy levels than stretching vibrations because it takes more energy to compress or stretch a bond than to bend it.
The last type of vibrational movement demonstrated by bonded atoms is scissoring. Like a pair of scissors, this planar movement describes two bonded atoms moving toward each other and then away without changing the length of their bonds just like the tips of a pair of scissors as you open and close the scissors. The scissoring virbrational movement of CH2 in formaldehyde is higher in energy than wagging and rocking but lower in energy than stretching. It produces a relatively low level of absorption at 1485cm^-1.
Compounds in which a single hydrogen is bonded to an oxygen, nitrogen or carbon absorb energy at higher infrared frequencies, producing absorption signatures on the left of the spectrum. These bonds are high in energy because the lightweight hydrogen stretches, or vibrates, very rapidly in its bond, similar to the way a paddle ball on a rubber string reverberates rapidly against the paddle if you hold the paddle correctly. Next in order of energy and close to the centre of the spectrum, are double- and triple-bonded atoms of carbon, oxygen and nitrogen. Lower in energy and closer to the right-hand side of the spectrum, in what is called the fingerprint region, are single-bonded carbon-carbon, carbon-oxygen and carbon-nitrogen atoms.
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