Modelamento mecanico-quantico de tensores polares

AUTOR(ES)

Harley Paiva Martins Filho

DATA DE PUBLICAÇÃO

1994

RESUMO

Experimental polar tensors for the CH2Cl2, CF2Cl2, CF3Cl, CFCl3, CH3F, CH3Cl, CH3I, Cl2CO, F2CO, Cl2CS and F2CS molecules were determined with the aim of testing previously established electronegativity models for tensor invariants estimation. The method of determination uses comparison of the several possible sets of polar tensors for a molecule (corresponding to the several posssible sign choices for the dipole moment derivatives) with theoretical estimates or sets of polar tensors for an isotopically related molecule. Comparisons are made in two-dimensional graphs by deriving principal components from the elements of the tensors. For the choice of a convenient quantum-mechanical method of theoretical estimation a systematic analysis of previously published results was done in which the sum of intensities for each molecule was used as basic parameter for comparison. The accuracy of the Hartree-Fock calculations with the 6-31G* basis set and correlation treatment at MP2 leveI was considered to suffice for our goaIs. The methyl halide molecules fit a previous model of mean dipole moment estimation to a good accuracy. Estimation in this model is done through correlation with parcial charges obtained from the principie of eletronegativity equalization. Calculations of CCFO (Charge - Charge Flux - Overlap) contributions to the tensors were done for quantum-mechanical interpretation of the model and it was observed that the mean dipole moment of the sum of the charge flux and overlap contributions can be estimated as the sum of specific constant quantities for each substituent atom in the molecule. Another previous electronegativity model allows intensity sum estimates through effective charge estimation. Effective charges are estimated through correlation with Mulliken electronegativities. From the methyl halides studied here, only the CF3CI and CFCl3 molecules do not fit that model, mainly due to failure in estimating the chlorine and fluorine atoms effective charges. X2,CY molecules do not fit the models, at least in the original parameterization. Nevertheless, their polar tensors show some tendency to obey the correlations upon which the models are based.

ASSUNTO(S)

calculo tensorial

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