Quantum chemical approaches conformation

In molecular-mechanics calculations, the atoms are considered to move in a force field defined by an energy function based on classical (rather than quantum) mechanics. Thus, the energy of a given molecular conformation is not calculated in an iterative SCF procedure, as in quantum-chemical approaches, but rather uses an analytical formula based on effective potentials. 

Joshi, R.K., Meister, Th., Scapozza, L. and Ha, T.-K. (1994). A New Quantum Chemical Approach in QSAR-Analysis. Parametrisation of Conformational Energies into Molecular Descriptors Jmn (Steric) and Jsn (Electronic). Arzneim.Forsch., 44,779-790. 

A new quantum chemical approach in QSAR-analysis. Parametrisation of conformational energies into molecular descriptors Jmn (steric) and Jsn (electronic). Arzneim. Forsch. (German), 44, 779-790. 

Despite the extremely wide use of multi-dimensional NMR techniques for conformational studies of biological macromolecules, at present, only a small part of the information contained in the NMR spectrum can be used for structure determination [118,119]. The reason is that there are no well-established correlations between NMR chemical shifts and the structural parameters [118] and only a few useful correlations besides Karplus relations [120,121] for nuclear spin-spin coupling constants. As a consequence, a great deal of important information about the system is not available without turning to quantum chemical approaches for the theoretical interpretation. 

Hpp describes the primary system by a quantum-chemical method. The choice is dictated by the system size and the purpose of the calculation. Two approaches of using a finite computer budget are found If an expensive ab-initio or density functional method is used the number of configurations that can be afforded is limited. Hence, the computationally intensive Hamiltonians are mostly used in geometry optimization (molecular mechanics) problems (see, e. g., [66]). The second approach is to use cheaper and less accurate semi-empirical methods. This is the only choice when many conformations are to be evaluated, i. e., when molecular dynamics or Monte Carlo calculations with meaningful statistical sampling are to be performed. The drawback of semi-empirical methods is that they may be inaccurate to the extent that they produce qualitatively incorrect results, so that their applicability to a given problem has to be established first [67]. 

The currently available quantum chemical computational methods and computer programs have not been utilized to their potential in elucidating the electronic origin of zeolite properties. As more and more physico-chemical methods are used successfully for the description and characterization of zeolites, (e.g. (42-45)), more questions will also arise where computational quantum chemistry may have a useful contribution towards the answer, e.g. in connection with combined approaches where zeolites and metal-metal bonded systems (e.g. (46,47)) are used in combination. The spectacular recent and projected future improvements in computer technology are bound to enlarge the scope of quantum chemical studies on zeolites. Detailed studies on optimum intercavity locations for a variety of molecules, and calculations on conformation analysis and reaction mechanism in zeolite cavities are among the promises what an extrapolation of current developments in computational quantum chemistry and computer technology holds out for zeolite chemistry. 

The quantitative prediction of the stereochemistry of a chemical reaction by strain energies requires the knowledge of the reaction mechanism, i.e., the selective intermediates and/or transition states involved, and an accurate force field for the transient species. As discussed above, these are two demanding problems and, with a small number of exceptions involving combined quantum-mechanical/molecular-mechan-ical methods there are no reports of studies in this area that have used molecular mechanics for quantitative predictions at the same level of accuracy as for conformational analyses. Quantum-mechanical approaches are more appropriate in this area. 

The explicit modeling approach surrounds a solute molecule with solvent molecules and then examines each molecule in that solvated environment. Quantum chemical methods, both semiempiricaP and ab initio" have been used to do this however, molecular dynamics and Monte Carlo simulations using force fields are used most often.Calculations on ensembles of molecules are more complex than those on individual molecules. Dykstra et al. discuss calculations on ensembles of molecules in a chapter in this book series. Because of the many conformations accessible to both solute and solvent molecules, in addition to the great number of possible solute molecule-solvent molecule orientations, such direct QM calculations are very computer intensive. However, the information resulting from this type of calculation is comprehensive because it provides molecular structures of the solute and solvent, and takes into account the effect of the solvent on the solute. This is the method of choice for assessing specific bonding information. 

In microscopic approaches the solvent molecules are described as true discrete entities but in some simplified form, generally based on fotee-field methods (Allinger, 1977). These theories may be of the semicontinuum type if the distant bulk solvent is accounted for, or of the fully discrete type if the solvent description includes a large number of molecules. As an example, the spectrum of formaldehyde in water has been examined using a combination of classical molecular dynamics and ab initio quantum chemical methods and sampling the calculated spectrum at different classical conformations (Blair et al., 1989 Levy et al., 1990). These calculations predict most of the solvent shift as well as the line broadening. 

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