Spectroscopic parameters, quantum chemical calculations

Among van der Waals complexes, the complexes of simple molecules with rare gas atoms has received a great deal of attention because the PES of these species can be reconstructed from spectroscopic data. Studies of isolated complexes in supersonic jets provide a unique opportunity to determine experimentally the parameters of complex multidimensional PES, which may have several minima, with accuracy approaching 0.01-0.001 cm-1 in some cases. This feat seemed to be a formidable challenge only a few years ago On the other hand, in view of their relative simplicity these complexes are reasonable subjects for quantum chemical calculations, so reliable comparisons of theory with experiment are possible. 

The topic of interactions between Lewis acids and bases could benefit from systematic ab initio quantum chemical calculations of gas phase (two molecule) studies, for which there is a substantial body of experimental data available for comparison. Similar computations could be carried out in the presence of a dielectric medium. In addition, assemblages of molecules, for example a test acid in the presence of many solvent molecules, could be carried out with semiempirical quantum mechanics using, for example, a commercial package. This type of neutral molecule interaction study could then be enlarged in scope to determine the effects of ion-molecule interactions by way of quantum mechanical computations in a dielectric medium in solutions of low ionic strength. This approach could bring considerable order and a more convincing picture of Lewis acid base theory than the mixed spectroscopic (molecular) parameters in interactive media and the purely macroscopic (thermodynamic and kinetic) parameters in different and varied media or perturbation theory applied to the semiempirical molecular orbital or valence bond approach [11 and references therein]. 

The Oudar-Chemla equation has been tested for many donor-acceptor TT-conjugated organic molecules. The two-state model works quite well in most cases. However, it should not be left unmentioned that Are values of are usually overestimated in comparison to the more advanced quantum chemical calculations [3, 31, 38—40]. Hence, the two-state model should be treated only as a rather rough approximation to the SOS method. On the other hand, for the most applications, the relative values of the first-order hyperpolarizabilies are of the higher importance. The two-state model allows to establish the structure-NLO properties relationship in terms of relatively simple spectroscopic parameters. 

Quantum chemical calculations on metal clusters in zeolite A [12] and semi-empirical ligand field interpretations of spectroscopic data of transition metal ions [6] have proven to be successful in structural characterizations of molecular sieves and their guest species. The present tendency in catalysis towards a more fundamental approach justifies the expectation that ESR, combined with other spectroscopic techniques, will become important. However, this requires an accurate and unambiguous parameterization of the ESR spectra. The parameter set thus obtained forms a firm basis for a theoretical investigation of the coordination environment of the paramagnetic entity. 

The extent of transmission of substituent effects in selenophene and several condensed derivatives was deduced from CNDO/2 quantum chemical calculations on C-protonated forms or a complexes <82CS75, 82CS208). Quantum chemical calculations (CNDO/S) of spectroscopic parameters of selenophene and several condensed derivatives have been used to determine their ionization potentials <861ZV2251>. 

The solid salt, trimethylammonium chloride (TMAC), has been investigated by Penner et al. using a combination of NMR spectroscopic techniques and quantum chemical calculations. Chemical shift and nuclear quadrupolar interaction parameters have been measured for Cl, H/ h, and These parameters have also been calculated as a function of... 

Force fields contain numerous parameters (here /, bo,f, amino acids in the case of proteins or relevant monomer-analogous molecules in the case of technical polymers. The parameters are adjusted using thermodynamic, spectroscopic, or structural data, and increasingly quantum chemical calculations (most importantly in the case of the torsion potential in the valence part of a force field) available for the training set. 

An overview of the theoretical background and computational requirements needed for the accurate evaluation of the spectroscopic parameters of relevance to rotational spectroscopy is given. The accuracy obtainable from state-of-the-art quantum chemical calculations is mainly discussed by means of signihcant examples, which also allows us to stress the importance of the interplay of theory and experiment in the field of rotational spectroscopy. 

The aim of the present chapter is to provide a resume on the role of quantum chemistry in the field of rotational spectroscopy. Therefore, how the spectroscopic parameters of relevance to rotational spectroscopy can be evaluated by means of quantum chemical calculations will be presented and some emphasis will be given to the computational requirements. It will then be pointed out how quantum chemistry can be used for guiding, supporting, and/or challenging the experimental determinations. As a sort of conclusion the importance of the interplay between theory and experiment in rotational spectroscopy will be demonstrated. By means of significant examples, profits from such an interplay will be shown. 

A prominent example in this context is the recent detection of oxadisulfane (HSOH) via rotational spectroscopy [4]. The successful identification of HSOH among the products of the pyrolysis of (t-Bu)2SO was possible due to accurate predictions of the spectroscopic parameters of HSOH. In fact previous searches for HSOH without such predictions were unsuccessful [4]. As outlined by Winnewisser et al. [4], quantum chemical calculations were used to predict the HSOH rotational-torsional spectrum The equilibrium rotational constants were obtained at the CCSD(T)/cc-pCVQZ level of theory and then augmented by vibrational corrections at the CCSD(T)/cc-pVTZ level. Dipole moment components were also computed in order to predict the type of rotational transitions detectable and their intensity. 

Matrix isolation experimental techniques [1-10] stand out among many other modern chemical research methods with regard to their ability to provide direct comparisons with quantum mechanical calculations. The use of photoexcitation methods to induce reactions [7-9] as well as the applications of multiple spectroscopic techniques to study such photochemical reactions allows for close control of the reaction parameters. Most of the high temperature and entropy effects, otherwise very large in thermochemical reactions, are therefore not present here and thus some of the limitations associated with applications of precise quantum mechanical calculations to kinetic processes disappear. 

Quantum chemical nuclear magnetic resonance (NMR) chemical shift calculations enjoy great popularity since they facilitate interpretation of the spectroscopic technique that is most widely used in chemistry [1-11], The reason that theory is so useful in this area is that there is no clear relationship between the experimentally measured NMR shifts and the structural parameters of interest. NMR chemical shift calculations can provide the missing connection and in this way have proved to be useful in many areas of chemistry. A large number of examples including the interpretation of NMR spectra of carbocations [12], boranes [10, 13], carboranes [10, 13-15], low-valent aluminum compounds [16-18], fullerenes [19-21] as well as the interpretation of solid-state NMR spectra [22-26] can be found in the literature. 

Melander and Saunders (1980) have given a comprehensive description of the development of methods of computer calculations of isotope effects on the kinetics of chemical reactions. Such techniques, originally proposed by Wolfsberg and Stem (1964), Shiner (1975), Buddenbaum and Shiner (1977), and Schowen (1977), marry the methods ofEyring s absolute rate (activated complex) theory with detailed modeling of molecular vibrational properties. Input parameters are a mix of spectroscopically determined or quantum mechanically calculated force constants and/or force constant shifts. The method has resulted in informative and detailed molecular description of the molecular changes that occur as the system proceeds from reactant to product along the reaction coordinate. As a result, kinetic isotope effect studies now constitute one of the most important methods employed in the development of detailed... 

In sharp contrast to conventional spectroscopic methods based on direct mie-photon absorption, IRMPD spectroscopy relies on the sequential absorption of a large number of IR photons. This excitation mechanism leaves an imprint on the observed IR spectrum in the sense that vibrational bands are typically broadened, red-shifted and affected in relative intensity to some extent. While the intramolecular processes underlying these spectral modifications have been addressed and qualitatively modelled in a large number of studies [166-172], it is often hard to predict quantitatively an IRMPD spectrum because the required molecular parameters, in particular the anharmonic couplings between vibrational normal modes at high internal energies, are usually unknown and cannot be calculated accurately using current quantum-chemical methods, fri practice, most experimental IRMPD spectra are therefore analysed oti the basis of computed linear absorption spectra, which usually provide a reasonable approximation to the IRMPD spectrum. 

The parameters have been determined to reproduce energetic, structural and dynamic properties found from ab initio quantum mechanical calculations, spectroscopic measurements, and crystallographic data. Partial charges have been derived once for each atom type and they are independent of the actual chemical environment of atoms. Such an approach produces a transferable set of charges for standard components of the proteins and nucleic acids. Derivation of a charge model for a new system is based on quantum mechanical calculations. 

Quantum chemistry has emerged as an important tool for investigating a wide range of problems in chemistry and molecular physics. With the recent development of computational methods and more powerful computers, it has become possible to solve chemical problems that only a few years ago seemed for ever beyond the reach of a rigorous quantum-mechanical treatment. Today quantum-mechanical methods are routinely applied to problems related to molecular stmcture and reactivity, and spectroscopic parameters calculated quantum-mechanically are often useful in the interpretation of spectroscopic measurements. Wth the development and distribution of sophisticated program packages, advanced computational electronic-stmcture theory has become a practical tool for nonspecialists at universities and in industry. 

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