Basic Principles of Vibrational Spectroscopy

Because principles of vibrational spectroscopy have been covered extensively in a number of texts [8,9,28-32], only a few basic points are summarized here. 

To observe particular rotational isomeric states, the method must be much more rapid than the rate of conformational isomerization. Optical methods such as absorption spectroscopy or light-scattering spectroscopy provide a short-time probe of the molecular conformation. If the electronic states of the molecule are strongly coupled to the backbone conformation, the ultraviolet or visible spectrum of the molecule can be used to study the conformational composition. The vibrational states of macromolecules are often coupled to the backbone conformation. The frequencies of molecular vibrations can be determined by infrared absorption spectroscopy and Raman scattering spectroscopy. The basic principles of vibrational spectres-... 

The basic principles of reflectance spectroscopy is firs emphasizing what is particular to each method (vibrational fingerprints... 

In this paper, the basic principles of reflectance spectroscopy will be first discussed, emphasizing what is particular to each method electronic spectra for UV-visible Reflectance Spectroscopy (UVERS), vibrational fingerprints for Electrochemically Modulated Infrared Reflectance Spectroscopy (EMIRS). After a short presentation of the experimental set-up for each technique, various examples, taken mainly from our laboratory, will be given. 

The measurement of vibrational optical activity requires the optimization of signal quality, since the experimental intensities are between three and six orders of magnitude smaller than the parent IR absorption or Raman scattering intensities. To date all successful measurements have employed the principles of modulation spectroscopy so as to overcome short-term instabilities and noise and thereby to measure VOA intensities accurately. In this approach, the polarization of the incident radiation is modulated between left and tight circular states and the difference intensity, averaged over many modulation cycles, is retained. In spite of this common basis, there are major differences in measurement technique and instrumentation between VCD and ROA consequently, the basic experimental methodology of these two techniques will be described separately. 

This book intends to supply the basic information necessary to apply the methods of vibrational spectroscopy, to design experimental procedures, to perform and evaluate experiments. It does not intend to provide a market survey of the instruments which are available at present, because such information would very soon be outdated. However, the general principles of the instruments and their accessories, which remain valid, are discussed. Details concerning sample preparation and the recording of the spectra, which is the subject of introductory courses, are assumed to be known. Special procedures which are described in monographs, such as Fourier transformation or chemometric methods, are also not exhaustively described. This book has been written for graduate students as well as for experienced scientists who intend to update their knowledge. 

Basic Principles of Electronic, Vibrational, and Rotational Spectroscopy (A) P- 278... 

Basic principles of electronic, vibrational and rotational spectroscopy... 

BASIC PRINCIPLES OF ELECTRONIC, VIBRATIONAL AND ROTATIONAL SPECTROSCOPY... 

Infrared (IR) and Raman are both well established as methods of vibrational spectroscopy. Both have been used for decades as tools for the identification and characterization of polymeric materials in fact, the requirement for a method of analysis synthetic polymers was the basis for the original development of analytical infrared instrumentation during World War II. It is assumed that the reader has a general understanding of analytical chemistry, and a basic understanding of the principles of spectroscopy. A general overview of vibrational spectroscopy is provided in Sec. 5 for those unfamiliar with the infrared and Raman techniques. 

Raman scatter, and excitation emission fluorescence spectroscopy (EEFS). They use interaction with radiation from different regions of the electromagnetic spectrum to identify the chemical nature of molecules. For example, absorption of UV and VIS radiation causes valence electron transitions in molecules which can be used to measure species down to parts per million concentrations for fluorophores (i.e., EEFS) determination can even go down to parts per billion levels. Whereas UV, VIS, and EEFS are limited to a smaller, select group of molecules, the NIR, IR, and Raman scatter spectroscopy techniques are probing molecular vibrations present in almost any species their quantification limits are somewhat higher but can still be impressive. The reader is referred to textbooks for further details on basic principles of these spectroscopic techniques [3]. 

As this chapter aims at explaining the basics, operational principles, advantages and pitfalls of vibrational spectroscopic sensors, some topics have been simplified or omitted altogether, especially when involving abstract theoretical or complex mathematical models. The same applies to methods having no direct impact on sensor applications. For a deeper introduction into theory, instrumentation and related experimental methods, comprehensive surveys can be found in any good textbook on vibrational spectroscopy or instrumental analytical chemistry1"4. 

Beyond imaging, CARS microscopy offers the possibility for spatially resolved vibrational spectroscopy [16], providing a wealth of chemical and physical structure information of molecular specimens inside a sub-femtoliter probe volume. As such, multiplex CARS microspectroscopy allows the chemical identification of molecules on the basis of their characteristic Raman spectra and the extraction of their physical properties, e.g., their thermodynamic state. In the time domain, time-resolved CARS microscopy allows recording of ultrafast Raman free induction decays (RFIDs). CARS correlation spectroscopy can probe three-dimensional diffusion dynamics with chemical selectivity. We next discuss the basic principles and exemplifying applications of the different CARS microspectroscopies. 

Chemical symmetry has been noted and investigated for centuries in crystallography which is at the border between chemistry and physics. It was more physics when crystal morphology and other properties of the crystal were described. It was more chemistry when the inner structure of the crystal and the interactions between its building units were considered. Later, descriptions of molecular vibrations and the establishment of selection rules and other basic principles happened in all kinds of spectroscopy. This led to another uniquely important place for the symmetry concept in chemistry with practical implications. 

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