Overview vibrational structure

Fig. 5.1 Absorption and emission (fluorescence). a) Overview of electronic and vibro-nic energy levels (schematic). The absorption spectrum displays a vibrational structure which is characteristic of the photoex-cited state (Sn). b) Band spectra with Stokes shift (schematic). The fluorescence spectrum closely resembles the mirror image of the...

A.D. Buckingham, J.E. Del Bene and S.A.C. McDowell (2008) Chem. Phys. Lett., vol. 463, p. 1 - An overview of structural and vibrational spectroscopic properties of hydrogen bonds, and of hydrogen bonding between H2O molecules. 

This Introductory Section was intended to provide the reader with an overview of the structure of quantum mechanics and to illustrate its application to several exactly solvable model problems. The model problems analyzed play especially important roles in chemistry because they form the basis upon which more sophisticated descriptions of the electronic structure and rotational-vibrational motions of molecules are built. The variational method and perturbation theory constitute the tools needed to make use of solutions of... 

Experimentally one can investigate resonances by various spectroscopic schemes, as indicated in Fig. 1 by direct overtone pumping [11] from the ground vibrational state, by vibrationally mediated photodissociation [12] using an excited vibrational level as an intermediate, or by stimulated emission pumping (SEP) [13-15] from an excited electronic state. In all cases it is possible to scan over a resonance and thereby determine its position j4s aHd its width hkU). A schematic illustration of an absorption or emission spectrum is depicted on the left-hand side of Fig. 1 all of the more or less sharp structures at energies above threshold are resonances. Figure 2 shows an overview SEP spectrum measured for DCO [16]. It consists of... 

The identification of species adsorbed on surfaces has preoccupied chemists and physicists for many years. Of all the techniques used to determine the structure of molecules, interpretation of the vibrational spectrum probably occupies first place. This is also true for adsorbed molecules, and identification of the vibrational modes of chemisorbed and physisorbed species has contributed greatly to our understanding of both the underlying surface and the adsorbed molecules. The most common method for determining the vibrational modes of a molecule is by direct observation of adsorptions in the infrared region of the spectrum. Surface spectroscopy is no exception and by far the largest number of publications in the literature refer to the infrared spectroscopy of adsorbed molecules. Up to this time, the main approach has been the use of conventional transmission IR and work in this area up to 1967 has been summarized in three books. The first chapter in this volume, by Hair, presents a necessarily brief overview of this work with emphasis upon some of the developments that have occurred since 1967. 

To get an overview of the relevant potential energy surfaces, several stationary points of each system were calculated with the 6-311+G basis set and the B3LYP functional using the Gaussian package of programs [5]. For the structural identification of the expected species it was also necessary to obtain the calculated vibrational spectra. 

The traditional ways of evaluating the IHB characteristics are to assess the vibrational frequencies or intensities of the OH stretching or torsion in the IR spectra and the chemical shifts of the hydroxyl protons in the NMR spectra which are found to be nicely correlated ". Crystal-structure analysis also provides essential information in this field. Bilton and coworkers carried out a systematic survey of the internally hydrogen-bonded frames in the 200,000 structures of the CSD and gave a general overview of the IHB in the solid state. 

In this account we have attempted to provide a brief overview of the concepts of first-principles methods tailored for the calculation of structures, energetics, and properties of supramolecular assemblies. The presentation of the theory focussed on the most essential building blocks in order to provide a general frame to interrelate the various methods available. Thereafter, we discussed the relation of these methods to experiment and to well-known concepts for the description of typical interaction patterns. Also, new methods tailored for tackling problems specific to supramolecular chemistry have been discussed (like the calculation of local dipole moments in CPMD simulations, the Mode-Tracking protocol for the selective calculation of vibrational frequencies and intensities, or the SEN method for the calculation of hydrogen bond energies). 

Reviews of the methods to determine molecular structures have been published in the past [7-9]. In section II of this article, the basic theory of Are vibration-rotation interactions are summarized and the various methods to determine the structure of molecules are reviewed, begiiuiing with the method. The short introduction to the least squares fitting technique and the tq and pseudo-Kraitchman methods is followed by an overview of newa- developments based on the mass-dependence (rj ) method. Section II concludes with methods employing ab initio derived quantities, methods based on empirical correlations and a short summary on the method. In section HI, complete structure determinations of molecules are listed that have been published from 1980 to 1998. I hope that the information presented in this section is fairly complete for the period, although I know that it is impossible not to miss something. Also in this section, a number of interesting examples of problems and their resolutions are illustrated. 

In this section we describe the required contact transformations. The section is structured in the following manner. It begins with a description of the perturbative transformations as they apply to the (J = 0) vibrational Hamiltonian. Select results are given for C02 and H2CO. The section concludes with a brief overview of how these ideas are extended to include rotations. Here results will be presented for H20, H2CO, and S02. More detailed accounts of these studies can be found elsewhere (43,44,47,49). 

In the development of zeolite science, infrared spectroscopy has been one of the major tools for structure and reactivity characterization. However, the field of zeolite Raman spectroscopy is gaining importance. The Raman effect is an intrinsically weak phenomenon, and Raman spectra of zeolites are often obscured by a broad fluorescence. Just like IR spectroscopy, Raman can detect small. X-ray amorphous zeolite particles. Therefore, Raman spectroscopy has been used to examine zeolite synthesis mixtures with ex-situ methods (with separation of solid and liquid) and in-situ methods. In this work we give an overview of the zeolite framework vibrations, zeolite synthesis, adsorption on zeolites and metal substitution and ion exchange in zeolites. 

The fields of electronic-structure theory and variational nuclear motion computations are diverse and involve a huge number of papers. Consequently, it is impossible to review the advances in these fields. Only efforts in our group related to the computation of complefe rotational-vibrational spectra of small molecules is overviewed and references from other groups are given only when directly relevant to our own efforts. 

Overview Over the last few decades, quanmm chemistry has evolved into a predictive tool for the calculation of energy differences. Of equal importance as a driving force for chemical reactions is the entropy that has received less attention from the computational community. The predictive value of computed rate constants depends not only on the accuracy of the electronic structure calculations but also on the correct description of the molecular entropy, including anharmonic effects in the vibrational modes, which is subject of Section 7.3.1.3. 

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