Vibrational spectroscopy fundamentals

W. H. Weinberg. In Methods of Experimental Physics. 22,23, 1985. Fundamentals of HREELS and comparisons to other vibrational spectroscopies. 

In cases where information about atomic arrangements cannot be obtained by X-ray crystallography owing to the insolubility or instability of a compound, vibrational spectroscopy may provide valuable insights. For example, the explosive and insoluble black solid SesNaCla was shown to contain the five-membered cyclic cation [SesNaCl]" by comparing the calculated fundamental vibrations with the experimental IR spectrum. ... 

Vibrational spectroscopy and in particular Raman spectroscopy is by far the most useful spectroscopic technique to qualitatively characterize polysulfide samples. The fundamental vibrations of the polysulfide dianions with between 4 and 8 atoms have been calculated by Steudel and Schuster [96] using force constants derived partly from the vibrational spectra of NayS4 and (NH4)2Ss and partly from cydo-Sg. It turned out that not only species of differing molecular size but also rotational isomers like Ss of either Cy or Cs symmetry can be recognized from pronounced differences in their spectra. The latter two anions are present, for instance, in NaySg (Cs) and KySg (Cy), respectively (see Table 2). 

The use of computer simulations to study internal motions and thermodynamic properties is receiving increased attention. One important use of the method is to provide a more fundamental understanding of the molecular information contained in various kinds of experiments on these complex systems. In the first part of this paper we review recent work in our laboratory concerned with the use of computer simulations for the interpretation of experimental probes of molecular structure and dynamics of proteins and nucleic acids. The interplay between computer simulations and three experimental techniques is emphasized (1) nuclear magnetic resonance relaxation spectroscopy, (2) refinement of macro-molecular x-ray structures, and (3) vibrational spectroscopy. The treatment of solvent effects in biopolymer simulations is a difficult problem. It is not possible to study systematically the effect of solvent conditions, e.g. added salt concentration, on biopolymer properties by means of simulations alone. In the last part of the paper we review a more analytical approach we have developed to study polyelectrolyte properties of solvated biopolymers. The results are compared with computer simulations. 

Vibrational spectroscopy can help us escape from this predicament due to the exquisite sensitivity of vibrational frequencies, particularly of the OH stretch, to local molecular environments. Thus, very roughly, one can think of the infrared or Raman spectrum of liquid water as reflecting the distribution of vibrational frequencies sampled by the ensemble of molecules, which reflects the distribution of local molecular environments. This picture is oversimplified, in part as a result of the phenomenon of motional narrowing The vibrational frequencies fluctuate in time (as local molecular environments rearrange), which causes the line shape to be narrower than the distribution of frequencies [3]. Thus in principle, in addition to information about liquid structure, one can obtain information about molecular dynamics from vibrational line shapes. In practice, however, it is often hard to extract this information. Recent and important advances in ultrafast vibrational spectroscopy provide much more useful methods for probing dynamic frequency fluctuations, a process often referred to as spectral diffusion. Ultrafast vibrational spectroscopy of water has also been used to probe molecular rotation and vibrational energy relaxation. The latter process, while fundamental and important, will not be discussed in this chapter, but instead will be covered in a separate review [4],... 

When an electron is injected into a polar solvent such as water or alcohols, the electron is solvated and forms so-called the solvated electron. This solvated electron is considered the most basic anionic species in solutions and it has been extensively studied by variety of experimental and theoretical methods. Especially, the solvated electron in water (the hydrated electron) has been attracting much interest in wide fields because of its fundamental importance. It is well-known that the solvated electron in water exhibits a very broad absorption band peaked around 720 nm. This broad absorption is mainly attributed to the s- p transition of the electron in a solvent cavity. Recently, we measured picosecond time-resolved Raman scattering from water under the resonance condition with the s- p transition of the solvated electron, and found that strong transient Raman bands appeared in accordance with the generation of the solvated electron [1]. It was concluded that the observed transient Raman scattering was due to the water molecules that directly interact with the electron in the first solvation shell. Similar results were also obtained by a nanosecond Raman study [2]. This finding implies that we are now able to study the solvated electron by using vibrational spectroscopy. In this paper, we describe new information about the ultrafast dynamics of the solvated electron in water, which are obtained by time-resolved resonance Raman spectroscopy. 

This result is tremendously useful, it not only leads to selection rules for vibrational spectroscopy but also, as was the case with electronic wavefunctions (see 8-2), allows us to predict from inspection of the character table the degeneracies and symmetries which are allowed for the fundamental vibrational wavefunctions of any particular molecule. 

Vibrational spectroscopy is based on two fundamental processes excitation and detection. As we shall see later in this chapter, they are not equivalent, and indeed both have to be treated to understand the origin of active modes in the spectra. The excitation is based on inelastic scattering processes, thus connecting initial and final states with different energy. The detection relies on the effect of the new inelastic channel on experimentally observable magnitudes, i.e. the junction conductance. 

The topic discussed here pertains to classical mechanics, namely to the theory of small oscillations. The fundamental sources are [6] and [7]. More chemistry (vibration spectroscopy) oriented is [8], A concise and clear (not the same thing ) description of classical mechanics is presented in [9]. 

There are some difficulties we should be aware of just the same. The maximum that is supposed to appear at co = 0 shows up in the INM calculations as a full-blown divergence (43,44). Indeed this infinity is just one instance of the fundamental problems with INMs at zero frequency. It probably should not be a surprise that a theory that pretends that basic liquid structure does not change with time is going to be ill-suited to studying behavior at the lowest frequencies. The same level of theory predicts liquid diffusion constants to be identically zero, for example. Fortunately, realistic molecular vibrational frequencies tend to be well outside this low-frequency regime, so the effects on predicted Tis are likely to be minimal. Still, as we shall note in Section VI, not every aspect of vibrational spectroscopy will be quite so insulated from this basic issue. 

Vibrational spectroscopy has proven to be a powerful method of studying biological molecules. Continued technical improvement (FT spectroscopy, time resolved spectroscopy, etc.) open up new domains of investigation which help solve fundamental problems of structure-function correlation at the molecular level. Many domains are beginning to be explored, and results are expected in the fields of compatible biomaterials, intelligent drug development, and in vivo spectroscopic measurement. 

After a short outline of the early history of infrared and Raman spectroscopy (Section 1), a general survey is given of different aspects of vibrational spectroscopy (Section 2). This survey is sufficient for readers who intend to get an impression of the fundamentals of vibrational spectroscopy. It serves as a common basis for subsequent chapters, which de.scribe special experimental features, the theory, and applicational details Section 3, Tools for infrared and Raman Spectroscopy Section 4, Vibrational spectroscopy of different classes and states of compounds Section 5, Evaluation procedures, and Section 6, Special techniques and applications. 

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