Strain energies experimental data

For a Hookian material, the concept of minimum strain energy states that a material fails, for example cell wall disruption occurs, when the total strain energy per unit volume attains a critical value. Such an approach has been used in the past to describe a number of experimental observations on the breakage of filamentous micro-organisms [78,79]. Unfortunately, little direct experimental data are available on the Young s modulus of elasticity, E, or shear modulus of elasticity G representing the wall properties of biomaterial. Few (natural) materials behave in an ideal Hookian manner and in the absence of any other information, it is not unreasonable to assume that the mechanical properties of the external walls of biomaterials will be anisotropic and anelastic. 

The equilibrium interconversion between an ethylene phosphite and a bicyclic spirophosphorane is shown to proceed by the insertion of the phosphite into the labile O-H bond of the hydroxyethyl ester. The mechanism is similar to the insertion of carbenes or nitrenes. Energy relationships of reaction intermediates were studied by MO RHF, MP2(full), MP4SDTQ, and DFT calculations. In most cases, they predicted that hydroxyethyl ethylene phosphates were more stable than the strained spirophosphoranes, which is not supported by the experimental evidence. The best correspondence to experimental data was obtained by DFT calculations with Perdew-Wang correlation functions <2003JST35>. 

SCHEME 14.5 Different synthetic routes for the preparation of metallacyclocumulene (5). Note The perception of strain energy in the cyclocumulene is so high that initial attempts at publishing the experimental single crystal data of the metallacyclocumulene (5) met with strong resistance from referees (personal comments from Prof. U. Rosenthal). Characterization of the structure computationally helped in gaining acceptance of the structure. 

All these methods have found applications in theoretical considerations of numerous problems more or less directly related to solvent extraction. The MM calculated structures and strain energies of cobalt(III) amino acid complexes have been related to the experimental distribution of isomers, their thermodynamic stability, and some kinetic data connected with transition state energies [15]. The influence of steric strain upon chelate stability, the preference of metal ions for ligands forming five- and six-membered chelate rings, the conformational isomerism of macrocyclic ligands, and the size-match selectivity were analyzed [16] as well as the relation between ligand structures, coordination stereochemistry, and the thermodynamic properties of TM complexes [17]. 

Strain energy increment ASE of 8.6 kcal/mol [95], the SE of bicyclopropylidene (1) exceeds the sum of SEs of methylenecyclopropane (2) and cyclopropane by 7.6 kcal/mol (77.4-41.7-28.1), and the SE for methylenespiropentane (6) exceeds the sum of those for cyclopropane and methylenecyclopropane (2) by 4.8 kcal/mol (74.6-28.1-41.7). Assuming that additivity also holds for oligo-spirocyclopropanated bicyclopropylidenes, their SEs can be assessed using a number of basic parameters such as 28.1 (SE of cyclopropane), 41.7 (SE of methylenecyclopropane 2), 8.6, 7.6, and 4.8 kcal/mol (excess increments ASE for spiropentane, bicyclopropylidene, and methylenespiropentane, respectively), but the lack of experimental data does not allow us to verify this scheme for spiropentane, bicyclopropylidene, and methylenespiropentane linkages. 

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. 

However, in many interesting cases, the requisite experimental data are not available, and the olefinic strain for disubstituted alkenes may be estimated using the calculated energies and the following isodesmic reaction ... 

Some of the potential energy functions used to calculate the total strain energy of a molecule are similar to the functions used in the analysis of vibrational spectra. Because the parameters used to derive the strain energies from these functions are fitted quantities, which are based on experimental data (for example X-ray structures), molecular mechanics may be referred to as empirical force field calculations (more often the simplification force field calculations is used). The quality of such calculations is strongly dependent on the reliability of potential energy functions and the corresponding parameters (the force field). Thus, the selection of experimental data to fit the force field is one of the most important steps in a molecular mechanics study. An empirical force field calculation is in essence a method where the structure and the strain energy of an unknown molecule are interpolated from a series of similar molecules with known structures and properties. 

The parameterization can be based on any type of experimental data that are directly related to the results available from molecular mechanics calculations, i. e., nuclear coordinates, nuclear vibrations, or strain energies. Most of the force fields available, and this is certainly true for force fields used in coordination chemistry are at least partially based on structural data. The Consistent Force Field (CFF)[43,49,601 is an... 

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