Correlation, effects conformational energy

Group 2 complexes are formally electron deficient and conformationally floppy only small energies (often only 1-2 kcal mol-1) are required to alter their geometries by large amounts (e.g., bond angles by 20° or more). In such cases, the inclusion of electron-correlation effects becomes critical to an accurate description of the molecules structures. Both HF/MP2 and density functional theory (DFT) methods have been applied to organoalkaline earth compounds. DFT approaches, which implicitly incorporate electron correlation in a computationally efficient form, are generally the more widely used. Molecular orbital calculations that successfully reproduce bent... 

The conformational characteristics of dimethyl esters of dicarboxylic acids are studies by the NMR and dipole moment method. Conformational energies of the internal CH2-CH2 bonds are determined from the observed 1H-1H vicinal coupling constants. Preferred conformations around the C-C bond are elucidated from the RIS analysis of dipble moments. With the RIS parameters thus established, the orientational correlation between the terminal ester groups is examined. The analysis provides the reason why the odd-even effect in the dipole moment is moderate, and attenuates rapidly with n in the ct.co-diester series. 

Con former A Conformer B Fig. 1. Effect of correlation on the energy of conformers... 

The purpose of the present study is to determine the conformational preference of 2-hydraxy-acetamide using ab initio quantum chemical methods. Intramoleculeur hydrogen-bonding is believed to have a strong influence on the relative stability of the conformers. The study of intramolecular hydrogenbonding by ab initio quantum chemical calculations is made difficult by the approximation techniques which are necessary for a theoretical description of a system such as this. The small energy differences calculated by ab initio quantum mechanical techniques are very sensitive to the choice of basis set used to parametrize the orbitals and to the calculation of electron correlation effects. 

The second-order correlation energies were determined to show whether electron correlation effects might favour one conformation of the 2-hydroxy-acetamide molecule over the other. The effects of the basis set on the calculated correlation energy is also of interest. 

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