Molecular mechanics angle bending

As the most notable contribution of ab initio studies, it was revealed that the different modes of molecular deformation (i.e. bond stretching, valence angle bending and internal rotation) are excited simultaneously and not sequentially at different levels of stress. Intuitive arguments, implied by molecular mechanics and other semi-empirical procedures, lead to the erroneous assumption that the relative extent of deformation under stress of covalent bonds, valence angles and internal rotation angles (Ar A0 AO) should be inversely proportional to the relative stiffness of the deformation modes which, for a typical polyolefin, are 100 10 1 [15]. A completly different picture emerged from the Hartree-Fock calculations where the determined values of Ar A0 AO actually vary in the ratio of 1 2.4 9 [91]. 

Molecular mechanics calculations use an empirically devised set of equations for the potential energy of molecules. These include terms for vibrational bond stretching, bond angle bending, and other interactions between atoms in a molecule. All of these are summed up as follows ... 

The molecular mechanics energy of a molecule is described in terms of a sum of contributions arising from distortions from ideal bond distances ( stretch contributions ), bond angles ( bend contributions ) and torsion angles ( torsion contributions ), together with contributions due to non-bonded (van der Waals and Coulombic) interactions. It is commonly referred to as a strain energy , meaning that it reflects the strain inherent to a real molecule relative to some idealized form. 

Empirical force fields used in molecular mechanics/molecular dynamics calculations all share common components, among them components which describe bond-stretching, angle-bending and torsional motions, as well as components which account for non-bonded steric and electrostatic interactions. While much of the information needed to parameterize force fields can be obtained from experiment, quite frequently critical data are missing. Information about torsional potentials, in particular, is often very difRcult to obtain from experiment, and here calculations can prove of great value. 

Molecular Mechanics Models. Methods for structure, conformation and strain energy calculation based on bond stretching, angle bending and torsional distortions, together with Non-Bonded Interactions, and parameterized to fit experimental data. 

A molecule-independent, generalized force field for predictive calculations can be obtained by the inclusion of additional terms such as van der Waals and torsional angle interactions. This adds an additional anharmonic part to the potential (see below) but, more importantly, also leads to changes in the whole force field thus the force constants used in molecular mechanics force fields are not directly related to parameters obtained and used in spectroscopy. It is easy to understand this dissimilarity since in spectroscopy the bonding and angle bending potentials describe relatively small vibrations around an equilibrium geometry that, at least... 

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