The ability of quantum-mechanical methods (HF/6-31G and MP2) to estimate additivity increments E XY G (X, Y = CH3, Cl), E XY GT , E XY GG , E XY GG ′ (X = Y = Cl) for conformational energies in chloro-substituted saturated hydrocarbons has been studied. Various basis sets including those extended by different sets of polarization functions both on heavy atoms (1d, 2d, 3d, 2d1f , 3d1f) and on hydrogens (1p, 2p, 3p) have been examined. The obtained results have been compared with available experimental data. It is shown that only the results obtained using the MP2 method and basis sets with polarization functions are in the scope with experimental estimates. The relative arrangement of conformational energies for various conformers in the series of polysubstituted chloroalkanes obtained using the additivity rule as well as direct quantum-mechanical calculations coincide with experimental findings. This fact can be considered as an argument that it is possibile to work out quantum-mechanically based additivity schemes for haloalkanes.
Additivity of conformational energy of saturated molecules has been studied using the perturbation theory for density matrix. Analytical expressions for both total and conformational energies have been obtained and analyzed up to the third-order members. It is shown that the conformational energy of compounds under investigation can be presented as a sum of transferable increments corresponding to the energies of separate conformational segments and their sequences. The role of end groups in determination of energies of conformational segments is revealed. The influence of substitution has been investigated. The dependence of subtle conformational effects on common structural peculiarities of Hamiltonians of saturated molecules has been concluded.
rnThe dependence of the intramolecular charge transfer through the cT-electron systems of substituted hydrocarbons on the spatial arrangement of the X-C, and C,-C, bonds has been studied using the perturbation theory for the one-electron density matrix (DM). Analytical expressions for the populations of the orbitals pertinent to the cis-and trans-bonds with respect to the X-C, bond have been obtained and analyzed. The Hamiltonian matrix elements determining the predominant direction of the above-defined charge transfer (cis or trans) have been revealed. The electron-accepting and electrondonating substituents ( X ) have been considered separately and the dependence of the resulting charge-transfer direction on the properties of substituent has been established.
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