Infrared Spectrometry. Chapter 6

Chapter 6 Infrared Spectrometry Infrared Spectroscopy is based upon absorption of electromagnetic radiation in the infrared region of the spectrum resulting in changes in the Vibrational energy of molecules. The infrared region wave numbers 12,800 to 10 cm-l or wavelengths 0.78 to 1000 μm. It is convenient to subdivide the spectrum into near, middle, and farinfrared radiation from the standpoint of both application and instrumentation, 1 In order to absorb infrared radiation, a molecule must undergo a net change in dipole moment as a consequence of its vibrational or rotational motion. Only under these circumstances can the alternating electrical field of the radiation interact with the molecule and cause changes in the amplitude of one of its motions. HCl has a significant dipole moment and is can absorb in the infrared. No net change in dipole moment occurs in homonuclear species such as O2 , N2, or Cl2; such compounds cannot absorb in the infrared. The energy required to cause a change in rotational level is minute and corresponds to radiation of 100 μm and greater (<100 cm-1). Vibrational energy levels are also quantized, and range from about 13,000 to 675 cm-1 (0.75 to 15 μm) which correspond to the midinfrared region that correspond to the readily accessible regions of the infrared. 2 Molecular Vibrations The relative positions of atoms in a molecule are not exactly fixed but instead fluctuate continuously as a consequence of a multitude of different types of vibrations. For a simple di-atomic or tri-atomic molecule, it is easy to define the number and nature of such vibrations and relate these to energies of absorption. An analysis of this kind becomes difficult if not impossible for polyatomic molecules, not only because of the large number of vibrating centers, but also because interactions among several centers occur and must be taken into account. Vibrations fall into the basic categories of stretching and bending. A stretching vibration involves a continuous change in the interatomic distance along the axis of the bond between two atoms. Bending vibrations are characterized by a change in the angle between two bonds and are of four types: scissoring, rocking, wagging, and twisting. 3 A non-linear B–A–B type triatomic molecule has three modes, two of which correspond to stretching motions, with the remainder corresponding to a bending motion. 4 Generally, for polyatomic molecules, the complexity of the spectrum increases rapidly with the number of atoms N in the molecule. The number of fundamentals, or normal modes of vibration is given by 3N – 6 and 3N – 5 for bent and linear molecules respectively. Nonlinear molecule: 3N – 6  3 degrees of freedom – i.e., 3 coordinates in space  3 translations and 3 rotations account for 6 motions of molecule Linear molecule: 3N – 5  3 degrees of freedom – i.e., 3 coordinates in space  3 translations and 2 rotations account for 5 motions of molecule because one of the rotations about center bond in linear molecule is indistinguishable There are associated overtones with each fundamental and, in addition, other absorption bands may arise due to interactions between certain of the fundamentals and overtones. These are known as combination and Fermi resonance bands. The most intense bands are fundamentals in which there is a large change of dipole moment during the vibration. However, a band may be weak or absent if the dipole change is small or zero. 5 6.1 Vibrational Frequency Taking first the simple case of diatomic molecules, such molecules have only one vibration that corresponds to the stretching and compression of the bond is possible. This accounts for one degree of vibrational freedom. Two concepts are also used to explain the frequency of vibrational modes. The stiffness of the bond - characterized by a proportionality constant termed the force constant, k (derived from Hooke’s law). The reduced mass, μ, - provides a useful way of simplifying our calculations by combining the individual atomic masses m1m2  m1  m2 1 m  2 k 1   2 k (m1  m2) m1  m2 From Hooke’s law, the equation relating the force constant, the reduced mass and the frequency of absorption is: ΰ= frequency in cm−1, c = velocity of light = 3 × 1010 cm/sec, K = force constant in dynes/cm 6 Calculation of stretching frequencies for different types of bonds C=C bond: K = 10 x 105 dynes/cm C - H bond: K = 5 x 105 dynes/cm 7 Stronger bonds have a larger force constant K and vibrate at higher frequencies than weaker bonds. Bonds between atoms of higher masses (larger reduced mass, m) vibrate at lower frequencies than bonds between lighter atoms. Triple bonds are stronger than double or single bonds between the same two atoms and have higher frequencies of vibration (higher wavenumbers): As the atom bonded to carbon increases in mass, the reduced mass (m) increases, and the frequency of vibration decreases (wavenumbers get smaller): 8 Bending motions occur at lower energy (lower frequency) than the typical stretching motions because of the lower value for the bending force constant K. Hybridization affects the force constant K, also. Bonds are stronger in the order sp > sp2 > sp3, and the observed frequencies of C–H vibration illustrate this nicely. Resonance also affects the strength and length of a bond and hence its force constant K: Resonance lengthens the C=O bond distance and gives it more single-bond character: Resonance has the effect of reducing the force constant K, and the absorption moves to a lower frequency. A normal ketone has its C=O stretching vibration at 1715 cm−1. A ketone that is conjugated with a C=C double bond absorbs at a lower frequency, near 1675 to 1680 cm−1. 9 Hydrogen bonding is a very important effect in infrared spectroscopy. This bonding influences the bond stiffness and so alters the frequency of vibration.  Many solvents are capable of forming hydrogen bonds to solutes.  Three classes of solvent exist which may lead to trouble when used for hydrogen-bonding studies in solution. First, compounds which contain hydrogen donor groups, e.g. halogenated compounds which contain a sufficient number of halogens to activate the hydrogens present, such as chloroform. Secondly, compounds which contain non-bonded electron pairs, such as ethers, aldehydes and tertiary amines. In addition, there are compounds which contain both types of group, e.g. water and alcohols. 10 Dispersive Infrared Spectrometers 11 Fourier-Transform Infrared Spectrometers 12 6.3 Preparation of Samples To determine the infrared spectrum of a compound, one must place the compound in a sample holder, or cell. Cells must be constructed of ionic substances—typically sodium chloride or potassium bromide. Gases: The spectrum of a low-boiling liquid or gas can be obtained by permitting the sample to expand into an evacuated cell. For this purpose, a variety of cells are available with path lengths that range from a few centimeters to several meters. Liquids. A drop of a liquid organic compound is placed between a pair of polished sodium chloride or potassium bromide plates, referred to as salt plates. When the plates are squeezed gently, a thin liquid film forms between them. The pair of plates is inserted into a holder that fits into the spectrometer. 13 Solids: There are at least three common methods for preparing a solid sample The first method involves mixing the finely ground solid sample with powdered potassium bromide and pressing the mixture under high pressure. Under pressure, the potassium bromide melts and seals the compound into a matrix. The result is a KBr pellet that can be inserted into a holder in the spectrometer. The second method, a Nujol mull, involves grinding the compound with mineral oil (Nujol) to create a suspension of the finely ground sample dispersed in the mineral oil. The thick suspension is placed between salt plates and inserted into the spectrometer. The third common method used with solids is to dissolve the organic compound in a solvent, most commonly carbon tetrachloride (CCl4). Again, as was the case with mineral oil, some regions of the spectrum are obscured by bands in the solvent. 14 6.4 Infrared Spectra Early infrared instruments recorded percentage transmittance over a linear wavelength range. It is now unusual to use wavelength for routine samples and the wavenumber scale is commonly used. Most commercial instruments present a spectrum with the wavenumber decreasing from left to right. It is commonplace to have a choice of absorbance or transmittance as a measure of band intensity. Absorbance spectrum of lactic acid. Transmittance spectrum of lactic acid. 15 6.5 Interpretation of Infrared Spectra The mid-infrared spectrum (4000–400 cm−1) can be approximately divided into four regions depending on the nature of a group frequency. the X–H stretching region (4000–2500 cm−1) • O–H (3700–3600 cm−1) • N–H (3400 –3300 cm−1). sharper band than O–H • C–H in aliphatic compounds 3000–2850 cm−1). • C–H adjacent to a double bond or aromatic ring (3100 –3000 cm−1) the triple-bond region (2500–2000 cm−1), • C≡C (2300 –2050 cm−1), small intense band • C≡N (2300 and 2200 cm−1),medium intense band the double-bond region (2000–1500 cm−1) • C=O (1830–1650 cm−1), intense band • C=C stretching1650 cm−1) • C=N stretching also occurs in this region and is usually stronger. the fingerprint region (1500–600 cm−1) • are due to most bending and skeletal vibrations for which small steric or electronic effects in the molecule lead to large shifts. 16 Characteristic infrared bands of aliphatic hydrocarbons: Alkanes Wavenumber (cm−1) 2960 Assignment Methyl sym C–H stretching 2930 Methylene asym C–H stretching 2870 2850 1470 1465 1380 Methyl asym C–H stretching Methylene sym C–H stretching Methyl asym C–H bending Methylene scissoring Methyl sym C–H bending 17 Alkenes 3100–3000 1680–1600 1400 1000–600 =C–H stretching C=C stretching =C–H in-plane bending =C–H out-of-plane bending 18 Alkynes 3300–3250 =C–H stretching 2260–2100 C=C stretching 700–600 =C–H bending 19 Characteristic infrared bands of aromatic compounds Wavenumber (cm−1) Assignment 3100–3000 C–H stretching 2000–1700 Overtone and combination bands 1600–1430 C=C stretching 1275–1000 In-plane C–H bending 900–690 Out-of-plane C–H bending 20 Characteristic infrared bands of oxygen-containing compounds Alcohol and phenols Wavenumber (cm−1) 3600 3550–3500 1300–1000 Assignment Alcohol O–H stretching Phenol O–H stretching C–O stretching 21 Aldehydes and ketones Wavenumber (cm−1) 2900–2700 1740–1720 1720–1680 1730–1700 1700–1680 Assignment Aldehyde C–H stretching Aliphatic aldehyde C=O stretching Aromatic aldehyde C=O stretching Ketone C–H stretching Aliphatic ketone C=O stretching Aromatic ketone C=O stretching 22 Wavenumber (cm−1) Assignment Carboxylic acids 3300–2500 O–H stretching 1700 C=O stretching 1430 C–O–H in-plane bending 1240 C–O stretching 23 6.6 Applications One of the most common applications of infrared spectroscopy is to the identification of organic compounds. As with organic compounds, inorganic compounds can produce an infrared spectrum. Generally, the infrared bands for inorganic materials are broader, fewer in number and appear at lower wavenumbers than those observed for organic materials. 24