Lawrence H. Daly

THE VIBRATIONAL ORIGIN OF GROUP FREQUENCIES

Publisher Summary This chapter discusses the vibrational origin of group frequencies with an emphasis on mechanical effects. A characteristic of a group frequency vibration is that the mechanical interaction effects that control the form of the vibration are relatively constant from molecule to molecule, making the frequency readily predictable. Mechanical interaction effects only occur between vibrations belonging to the same symmetry species. In a molecule with a plane of symmetry such as vinyl chloride, in-plane vibrations do not interact with out-of-plane vibration. The chapter focuses on the infrared frequencies that are absorbed by certain chemical functional groups of atoms, such as carbonyls. It further reviews the vibration of a diatomic molecule and a complex module consisting of a number of bonded atoms. X—H stretching frequencies are higher than Y—X stretching frequencies where Y is a nonhydrogen atom directly attached to X. It means that hydrogen stretching vibrations are more or less mechanically independent of the rest of the molecule and tend to make good group frequencies. The chapter presents a list of the approximate X—H stretching frequencies and the force constants for various X—H groups arranged according to the periodic table.

The infrared and Raman spectrum and thermodynamic functions of cyclopropyl cyanide

Abstract Cyclopropane is the only molecule containing the cyclopropane ring which has been thoroughly investigated from a spectroscopic viewpoint. The infrared spectrum of cyclopropyl cyanide in the liquid and vapor phase has been measured from 2 to 25 microns using lithium fluoride, sodium chloride, and cesium bromide prisms. The Raman spectrum of the liquid and qualitative polarization data have been obtained. A normal coordinate analysis was done on the skeletal molecule assuming a Cs point group. Using the calculated values and data on related molecules a complete vibrational assignment of the experimentally determined frequencies has been made. The moments of inertia and the thermodynamic functions have been calculated.

CHAPTER 9 – CARBONYL COMPOUNDS

Publisher Summary Carbonyl compounds give rise to a strong band at 1900–1550 cm−1 caused by the stretching of the C=O bond. This chapter illustrates some carbonyl vibrations and some general spectral features for a selection of carbonyl compounds, which show some bands from the attached groups. It further presents a summarization of the carbonyl spectral regions. The factors that cause shifts in the carbonyl frequencies are also discussed in the chapter. Ketones are characterized by the strong C=O stretching frequency absorption near 1715 cm−1. Ketones with a chlorine on the α-carbon absorb at higher frequencies when the chlorine is rotated near the oxygen than when the chlorine is not near the oxygen due to a field effect.

CHAPTER 8 – AROMATIC AND HETEROAROMATIC RINGS

Publisher Summary This chapter focuses on aromatic and heteroaromatic rings. All the modes of vibration of a monosubstituted benzene ring are illustrated in the chapter. The chapter reviews the general appearance of monosubstituted benzenes and ortho, meta, and para substituted benzenes in the infrared spectrum. It further illustrates the ring modes of benzene1-10 and the modes for mono, ortho, meta, and para substitued benzenes in the 1600 and 1500 cm−1 regions. The chapter presents the characteristic bands for substituted naphthalenes and pyridine compounds. Five-membered ring heteroaromatic compounds with two double bonds in the ring show three ring stretching bands near 1590, 1490, and 1400 cm−1. The chapter reviews the metal complexes of the cyclopentadienyl ring with a large number of different metals and illustrates the bands for the cyclopentadienyl ring.

COMPOUNDS CONTAINING BORON, SILICON, PHOSPHORUS, SULFUR, OR HALOGEN

Publisher Summary This chapter discusses the spectra of various chemical groups such as boron, silicon, phosphorus, sulfur, and halogen under the functional group categories. It presents a list of the spectral for boron compounds. The compounds containing the B—O linkage—such as borates, boronates boronites, boronic anhydrides, borinic acids, and boronic acids—are characterized by intense absorption at 1380–1310 cm−1, involving the stretching of the B—O bond. The high frequency and intensity is due to the fact that the B—O has some polar double bond character. The OH groups in boronic acids and boric acid in the solid state absorb near 3300–3200 cm−1 due to the bonded OH stretch. The compounds having a B—N linkage—such as borazines and amino boranes—have strong absorption at 1465–1330 cm−1 involving the stretching of the B—N bond. This bond has some polar double bond character as the B—O bond. Normal BH and BH2 groups absorb at 2640–2350 cm−1 due to the BH stretch. The chapter further presents a summarization of the more common silicon correlations and a list of correlations for phosphorus groups.

VIBRATIONAL AND ROTATIONAL SPECTRA

Publisher Summary The energy of a molecule consists partly of translational energy, rotational energy, vibrational energy, and electronic energy. Electronic energy transitions give rise to the absorption or emission in the ultraviolet and the visible regions of the electromagnetic spectrum. Pure rotation gives rise to the absorption in the microwave region or the far infrared. Molecular vibrations give rise to the absorption bands throughout most of the infrared region of the spectrum. This chapter focuses on vibrational and rotational spectra and discusses how molecular vibrations and rotations interact with radiation to create the infrared and Raman spectra. The vibrational and the rotational frequencies of molecules can be studied by Raman spectroscopy and by infrared spectroscopy. While they are related to each other, the two types of spectra are not exact duplicates and each has its individual strong points. In Raman spectroscopy, only the wave number is used. Infrared and Raman spectrum both involve vibrational and rotational energy levels; they are not duplicates of each other but rather complement each other. This is because the intensity of the spectral band depends on how effectively the photon energy is transferred to the molecule.

METHYL AND METHYLENE GROUPS

Publisher Summary This chapter discusses the spectra–structure correlations. These correlations are infrared correlations unless they are specifically labeled as Raman correlations. Molecular vibrational frequencies are the same in infrared and Raman spectra and for groups such as O—H, C—H, C≡N, C=0, and C=C frequency, correlations will be the same for both techniques. The chapter focuses on the vibrations of methyl and methylene groups. The vibrations of the CH3 group are illustrated in the chapter. There are three CH bonds in a methyl group; therefore, there are three CH stretching vibrations. In the symmetric, in-phase stretching vibration, the whole CH3 group stretches in-phase. The two out-of-phase stretching vibration is characterized as a “half-methyl” stretch. The chapter further illustrates the vibrations of the CH2 group and presents a list for the CH2 spectral regions.

AMINES, C=N, AND N=O COMPOUNDS

Publisher Summary This chapter presents the general spectral features for amines and discusses other selected nitrogen compounds. The NH2 group gives rise to absorption at 3550–3330 cm−1 (asymmetric stretch) and at 3450–3250 cm−1 (symmetric stretch). Most of the primary amines have an NH2 deformation band at 1650–1590 cm−1. In liquid aliphatic amines, the NH2 wagging and twisting vibrations give rise to broad, strong, and multiple absorption bands at 850–750 cm−1. The NH stretching vibration gives rise to a weak band at 3500–3300 cm−1. In amines—as in alcohols—single bond frequencies are affected by branching at the α-carbon atom. Primary aromatic amines with the nitrogen directly on the ring absorb strongly at 1330–1260 cm−1,3 due to stretching of the phenyl carbon–nitrogen bond. Secondary aromatic amines absorb strongly at 1342–1320 and 1315–1250 cm−1. Dimethyl anilines absorb strongly at 1380–1332 cm−1. For all the tertiary anilines, the region is 1380–1265 cm−1.

MAJOR SPECTRA–STRUCTURE CORRELATIONS BY SPECTRAL REGIONS

Publisher Summary This chapter reviews the group frequencies in terms of the spectral regions in which they occur. The first part of the chapter outlines an orderly procedure for the initial interpretation of an unknown infrared radiation (IR) spectrum by regions. Then, spectra-structure correlations are shown in a chart form where one can look for the groups that absorb in a given region or the regions where a given group absorbs. One of the useful features of infrared spectroscopy is its ability to give information about mixtures. When more than one component is present, the spectra tend to be additive but not completely so because of possible mutual interaction, such as hydrogen bonding for example. If the main component in the spectrum has been identified, a comparison with a reference spectrum may reveal some extra bands in the sample spectrum not in the reference. The chapter also presents some selected Infrared spectra and Raman spectra and illustrates their functional group frequencies.

CHAPTER 7 – OLEFIN GROUPS

Publisher Summary This chapter provides an overview of the vibrations of olefin groups. Olefinic CH stretching vibrations, olefinic C=C stretching vibrations, olefinic CH and CH2 in-plane bending vibrations, and olefinic hydrogen wagging vibrations are discussed in the chapter. Olefinic CH stretching frequencies occur at 3130–2980 cm−1. The C=C stretch vibrations interact to some extent with =CH2 deformation vibration and the attached single bond stretching vibration, because nonhydrogen substituent atoms remain nearly motionless for this mode. The C=C stretching frequency is affected by the changes in these interactions and by mesomeric and inductive effects that alter the strength of the C=C and attached C—X bonds. The chapter presents the general infrared radiation (IR) spectra expected for ethylenes with alkane substituents.