Computational chemistry spectroscopic parameters

Based on surface science and methods such as TPD, most of the kinetic parameters of the elementary steps that constitute a catalytic process can be obtained. However, short-lived intermediates cannot be studied spectroscopically, and then one has to rely on either computational chemistry or estimated parameters. Alternatively, one can try to derive kinetic parameters by fitting kinetic models to overall rates, as demonstrated below. 

The interaction between computational chemistry and experimental magnetic resonance spectroscopies is increasing at a fast pace in recent years. This parallels a more general trend of successful synergic interactions between experimentalists and computational chemists based on the capability of quantum mechanical methods to provide reliable estimates for a large number of spectroscopic parameters. 

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. 

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. 

Electron transfer reactions have been characterized with much more rigor in inorganic chemistry than with organic molecules. Marcus has provided the principal description relating the kinetics and thermodynamics of electron transfer between metal complexes (1). The Marcus theory, a computationally simple approach with good predictive power, is an empirical treatment which uses thermodynamic parameters and spectroscopic measurements to calculate kinetic data. It assumes that bimolecular electron transfer reactions occur in three stages as shown in Scheme 1 (1) formation of the precursor complex, (2) electron transfer, and (3) solvation of the redox pair. 

Whaf has primarily hampered MRS and MRSI from realizing their potential in cancer diagnostics is the dependence on total shape spectra without reliable quantification. There is a vital need to reconstruct component spectra and the spectral parameters from which metabolite concentrations are computed. The critical role here is played by mathematics by which the measured time signals are transformed info spectra. Spectroscopic methods are the key strategy for investigating the structure and content of matter. Physics and chemistry developed their full pofenfial by means of fhese methods. Medical diagnostics need to take full advanfage of such developments. 

The time-dependent theory of spectroscopy bridges this gap. This approach has received less attention than the traditional time-independent view of spectroscopy, but since 1980, it has been very successfully applied to the field of coordination chemistry.The intrinsic time dependence of external perturbations, for example oscillating laser fields used in electronic spectroscopy, is also expKdtly treated by modern computational methods such as time-dependent density functional theory, a promising approach to the efficient calculation of electronic spectra and exdted-state structures not based on adjustable parameters, as described in Chapter 2.40. In contrast, the time-dependent theory of spectroscopy outlined in the following often relies on parameters obtained by adjusting a calculated spectrum to the experimental data. It provides a unified approach for several spectroscopic techniques and leads to intuitive physical pictures often qualitatively related to classical dynamics. The concepts at its core, time-dependent wave functions (wave packets) and autocorrelation functions, can be measured with femtosecond (fs) techniques, which often illustrate concepts very similar to those presented in the following for the analysis of steady-state spectra. The time-dependent approach therefore unifies spectroscopic... 

Setting the right chemical model is essential to the computational study of chemical behaviour. This applies to problems that are associated with aU aspects of chemistry, i.e. structure, properties and reactivity patterns. In this chapter, we focus on structural and reactivity issues. Physical properties, for instance, the calculation of thermodynamic parameters (formation energies, pK, etc.) and spectroscopic properties (vibrational, electronic, NMR, etc.), present different challenges that will not be documented here. 

---