Spectrochemical Methods - III (Contd.)

This paragraph introduces the topic of infrared (IR) spectroscopy, emphasizing the study of how analytes absorb in the IR region. It explains the connection between the energy levels observed in invisible spectroscopy and the absorption patterns in the UV-visible range. The discussion then shifts to vibrational quantum levels and how they contribute to the absorption in the IR spectrum. The paragraph highlights the ability of small molecules, such as acetone, to exhibit numerous closely spaced IR absorption peaks due to the various types of bonds and vibrational modes present. The unique stretching frequencies of different bond types (CC, CH, CO) in acetone are discussed, as well as the significance of these frequencies in identifying the molecular structure and the number of bonds.

This paragraph delves into the analysis of IR spectra, focusing on the complexity of the spectra and the bands observed. It references a textbook by Skoog, which provides a comprehensive coverage of these bands and their characteristics. The paragraph explains the standard range of wave numbers plotted in an IR spectrum and the use of transmittance instead of absorbance, as seen in invisible spectra. It also discusses the identification of specific functional groups, such as carbonyl (CO) and aldehyde (CHO), through their characteristic stretching frequencies. The differences in stretching frequencies for these groups when they are part of different molecular structures, such as ketones and aldehydes, are highlighted. Additionally, the paragraph touches on how changes in functional groups, such as the conversion of aldehyde to imine during condensation reactions, alter the stretching frequencies and how these changes can be monitored using IR spectroscopy.

This paragraph explores how the nature of bonding between carbon and oxygen, as well as carbon and carbon, can be deduced from the stretching frequencies observed in an IR spectrum. It discusses the differences in stretching frequencies for coordinate bonds compared to covalent bonds. The paragraph also examines how the formation of new bonds, such as in the formation of metal ion complexes, affects the vibrational modes and the corresponding stretching frequencies. The use of IR spectroscopy to identify the presence of specific functional groups and the changes in their bonding environment is emphasized, providing insight into the molecular structure and potential reactivity of the analyte.

This paragraph discusses the various types of instruments used for infrared spectroscopy, including dispersive spectrometers, Fourier transform spectrometers, and filter photometers. It explains the different applications of these instruments, with dispersive and Fourier transform spectrometers being useful for qualitative identification of spectra and filter photometers designed for quantitative analysis. The paragraph also describes the basic configuration of these instruments, which typically include a source, monochromator, and detector. The use of specific optical materials, such as halide salts, is highlighted due to their transparency in the IR range. Additionally, the paragraph touches on the sources of IR radiation, including heated inert solids like Nernst Glower, Nichrome wire, and Globar, which are essential for the operation of these instruments.

This paragraph focuses on the sources and detectors used in infrared spectroscopy. It explains that heated solids, such as Nernst Glower, Nichrome wire, and Globar, serve as sources of IR radiation due to their ability to glow when heated to high temperatures. The paragraph also discusses the importance of using appropriate optical materials that are transparent to IR radiation, such as KBr and other halide salts. Furthermore, the paragraph describes the role of detectors, which respond to the thermal energy or heat from the sample. It introduces the Fourier transform IR spectrometer, which has been a significant advancement in the field since the 1970s, allowing for the measurement of all wavelengths simultaneously and the conversion of time-domain spectra to frequency-domain spectra using Fourier transformation. The Michaelson interferometer is mentioned as a key component in this process, modulating the source signal and producing an interferogram that is then transformed into a spectrum.