Introduction to Infrared Spectroscopy
When a beam of electromagnetic radiation of intensity Io is passed through a substance, it can be either absorbed or transmitted, depending upon its frequency, n, and the structure of the molecule it encounters. Electromagnetic radiation is energy; thus when a molecule absorbs radiation it gains energy as it undergoes a quantum transition from one energy state (Einitial) to another (Efinal). The frequency of the absorbed radiation is related to the energy of the transition by Planck’s law: Efinal - Einitial = DE = hn = hc/l. If a transition exists that is related to the frequency of the incident radiation by Planck’s constant, the radiation can be absorbed. If the frequency does not satisfy the Planck expression, then the radiation will be transmitted. A plot of the frequency of the incident radiation vs. some measure of the percent radiation absorbed by the sample is the absorption spectrum of the compound.
The type of absorption spectroscopy depends upon the frequency range of the electromagnetic radiation absorbed. Microwave spectroscopy involves a transition from one molecular rotational energy level to another. Rotational energy level spacings correspond to radiation from the microwave portion of the electromagnetic spectrum. Vibrational spectroscopy (or infrared spectroscopy) measures transitions from one molecular vibrational energy level to another, and requires radiation from the infrared portion of the electromagnetic spectrum. Ultraviolet-visible spectroscopy (also called electronic absorption spectroscopy) involves transitions among electron energy levels in the molecule, which require radiation from the UV-visible portion of the electromagnetic spectrum. Such transitions alter the configuration of the valence electrons in the molecule.
Molecules may undergo several types of motion. First, the entire molecule may move through space in some direction and with a particular velocity. This is called translational motion and with it we associate the translational kinetic energy of the molecule, 1/2mv2 (v = velocity of the center of mass of the molecule). The velocity of translation may be resolved into components along the three axes of a Cartesian coordinate system, so that we may write 1/2mv2 = 1/2mvx2 + 1/2mvy2 + 1/2mvz2, where vx is the x-component of velocity, etc., and m is the mass of the molecule. This equation tells us that the total translational KE of the molecule consists of three parts, each of which represents the kinetic energy along one of the reference directions. Since any translation of the molecule may be viewed as the vector sum of its motions along the three axes, the kinetic energy may always be broken up into the sum of three contributions, one arising from motion along each axis. We say that the molecule has 3 translational degrees of freedom, one corresponding to each Cartesian axis.