Estudo teorico de equilibrios tautomericos contendo C e Si como atomos centrais averiguando-se efeitos de substituintes na afinidade por protons atraves de caculos teoricos ab initio e funcional de densidade

AUTOR(ES)
DATA DE PUBLICAÇÃO

2000

RESUMO

Theoretical study (ab initio and density functional) of the effects to different substituents, in tautomeric equilibrium:CH3(Y=O)CH2-X - CH3(Y-OH)=CH-X, with X = H, CH3, Cl, F, NH2 or OH and Y = C or Si. The analysis was carried out throught calculation of electronic properties: energy, molecular geometry and proton affinity. The basis sets used in the description of the valence electrons, and correction of the diffuse character , were obtained with Generator Coordinate Method (GCM). The core electrons were represented by pseudopotential (ECP), resulting lower the computer processing cost. Theoretical calculation method was employed to obtaine the precise energy: In DFT we used the potential B3PW91 and basis sets with the addition of polarization functions. To obtain an ab initio calculation at high level, like QCISD(T)/ /(ECP /GCM+ ), we use simpler calculations (MP2 and QCISD(T)) with basis set without extra functions (ECP /GCM) and MP2 with extra functions (ECP /GCM+ ). The resulting errors of ab initio calculations for the proton affinity for X = H, CH3 and F, were 2, 4 and 9 kJ .mol-1 respectively, and the experimental values accepted are 10 kJ.mol-1. The proton affinity dependents on several factors: electronegativity, p donor effect, charge distribution and structural disposition of the molecules. These factors can provide ( or not) an additional stabilization to the present structures in the considered tautomeric equilibriuns. Any alteration in the differences between the energy of the protonate and deprotonate forms can affect the values of the proton affinity. The discussion of these effects results in the explanation of the sequence obtained to proton affinity in tautomeric equilibriuns. For example,in ketones, the largest proton affinity was obtained with X = H. It is due to the inability of the hydrogen in dispersing the negative charge in the deprotonate form, because of the small inductive and steric (volume) effect. The smallest proton affinity was obtained when X = Cl. This atom belongs to the second period. It possesses a large atomic volume and there is no large competition between p donor and the s aceptor ( electronegativity ) effect.

ASSUNTO(S)

metodo pseudopotencial

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