DRAMIŃSKI - Nir spectroscopy

Many so-called inorganic compounds are in reality largely organic, and for these we look for the same functional group bands in the IR as we do for purely organic compounds. However, the infrared spectra of relatively simple, purely inorganic compounds containing only a few atoms–specifically, inorganic salts containing polyatomic (complex) ions–are quite distinctive and can be used to rapidly identify the ions. Consider a simple inorganic salt, such as KNO2. On the basis of the empirical formula, we might naively expect there to be a total of 3(4)-6 = 6 normal modes of vibration associated with this material. However, this assumes that KNO2 is covalent. In fact, KNO2 consists of an ionic lattice of K+ and NO2- ions arranged in an infinite and very regular array. The crystal consists of essentially isolated K+ ions and NO2- ions. Thus we are able to consider the vibrational modes of the cation and anion independently of one another. In this case, since the potassium ions are monatomic, they have no vibrations (3(1)-3 = 0), so we need only consider the nitrite anions. The VSEPR (Valence Shell Electron Pair Repulsion) Theory predicts a bent structure for the nitrite ion. We thus anticipate three normal vibrational modes for NO2-, corresponding to the diagrams drawn earlier for H2O, and they should all be infrared active. Indeed, three bands are observed in the IR spectrum of KNO2: the symmetric stretch at 1335 cm-1, the asymmetric stretch at 1250 cm-1, and the bending vibration at 830 cm-1 (bending vibrations occur in general at lower frequencies than stretching vibrations). The frequencies of these vibrations are about the same regardless of counter ion, substantiating the independence of the anion and cation in the crystal. (This independence is only an approximation, but we will not worry about the complicating factors now.) We can, therefore, diagnose the presence of nitrite ion in a salt from the infrared spectrum of the material. This diagnostic application can be implemented only when the spectrum of the material is relatively uncomplicated, but despite this restriction it is an enormously useful application.

Let us turn now to the somewhat more complex case of NaNO3. Here we anticipate 3(4)-6 = 6 normal vibrational modes. The infrared spectrum, however, exhibits only three fundamental bands, at 831, 1405, and 692 cm-1. Why? There is no doubt that there are 6 normal vibrational modes. The formula is always valid. In the case of NO3-, however, the symmetric stretch is not IR active because it does not cause a change in the dipole moment of the ion, and so cannot give rise to absorption of IR radiation. This eliminates one of the anticipated bands from the IR spectrum. Among the remaining 5, there are two sets of doubly degenerate vibrations–i.e., two instances in which 2 vibrations occur with exactly the same frequency. Thus although 5 vibrations absorb IR radiation, they are manifested in only three spectral bands. However, these absorptions can be used diagnostically just as for nitrite. In similar fashion, other relatively simple anionic (and cationic) species can be identified via their IR spectra.

Organic functional groups (atom groups bonded in particular ways) differ both in the strengths of the bond(s) and in the masses of the atoms involved. For instance, the O-H and C=O functional groups each contain atoms of different masses connected by bonds of different strengths. According to equation (1), we therefore expect the O-H and C=O groups to absorb IR radiation at different positions in the spectrum. The presence of a strong, broad band between 3200 and 3400 cm-1 indicates the presence of an O-H group in the molecule, while the presence of a strong band around 1700 cm-1 confirms the presence of a C=O group.

For organic molecules, the infrared spectrum can be divided into three regions. Absorptions between 4000 and 1300 cm-1 are primarily due to specific functional groups and bond types. Those between 1300 and 909 cm-1, the fingerprint region, are primarily due to more complex vibrational motions; and those between 909 and 650 cm-1 are usually associated with the presence of benzene rings in the molecule.

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.