Browsing by Author "Monte, Silmar Andrade do"

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    An ab initio study of the C2H2 HF, C2H(CH3) HF and C2(CH3)2 HF hydrogen-bonded complexes.
    (2006) Ramos, Mozart Neves; Lopes, Kelson Carvalho; Silva, Washington Luís Vieira da; Figueirêdo, Alessandra Marcone Tavares Alves de; Castrini, Fátima Aparecida; Monte, Silmar Andrade do; Ventura, Elizete; Araújo, Regiane de Cássia Maritan Ugulino de
    MP2/6-31++G** and B3LYP/6-31++G** ab initio molecular orbital calculations have been performed in order to obtain molecular geometries, binding energies and vibrational properties of the C2H2 HF, C2H(CH3) HF and C2(CH3)2 HF H-bonded complexes. As expected, the more pronounced effects on the structural properties of the isolated molecules due to complexation was verified for the C C and H F bond lengths, which are directly involved in the H-bond formation. These bond distances increased after complexation. BSSE uncorrected B3LYP binding energies are always lower than the corresponding MP2 values. However, the opposite trend has been verified after BSSE correction by the counterpoise method since it is much lower at B3LYP than at MP2 level. The binding energies for these complexes as well as for the HF acid submolecule modes (the HF stretching and vibrational frequency modes) showed an increasing hydrogen-bonding strength with increasing methyl substitution. The splitting in the HF in-plane and out-of-plane bending modes reflects the anisotropy in the hydrogen-bonding interaction with the _ system of the C C bond. The H F stretching frequency is shifted downward after complexation and it increases with the methyl substitution. The IR intensities of the HF acid submolecule fundamentals are adequately interpreted through the atomic polar tensor of the hydrogen atom using the charge–charge flux-overlap model. The skeletal stretching modes of the Alkyne submolecule are decreased in the complex. The new vibrational modes arising from complexation show several interesting features.
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    A theoretical study of hydrogen complexes of the X H- type between propyne and HF, HCL or HCN.
    (2006) Figueirêdo, Alessandra Marcone Tavares Alves de; Silva, Washington Luís Vieira da; Lopes, Kelson Carvalho; Ventura, Elizete; Araújo, Regiane de Cássia Maritan Ugulino de; Monte, Silmar Andrade do; Silva, João Bosco Paraíso da; Ramos, Mozart Neves
    The present manuscript reports a systematic investigation of the basis set dependence of some properties of hydrogen-bonded (_ type) complexes formed by propyne and a HX molecule, where X= F, Cl and CN. The calculations have been performed at Hartree–Fock, MP2 and B3LYP levels. Geometries, H-bond energies and vibrational have been considered. The more pronounced effects on the structural parameters of the isolated molecules, as a result of complexation, are verified on RC C and HX bond lengths. As compared to double-_ (6-31G**), triple-_ (6-311G**) basis set leads to an increase of RC C bond distance, at all three computational levels. In the case where diffuse functions are added to both hydrogen and ‘heavy’ atoms, the effect is more pronounced. The propyne–HX structural parameters are quite similar to the corresponding parameters of acetylene–HX complexes, at all levels. The largest difference is obtained for hydrogen bond distance, RH, with a smaller value for propyne–HX complex, indicating a stronger bond. Concerning the electronic properties, the results yield the following ordering for H-bond energies, _E: propyne• • •HF > propyne• • •HCl > propyne• • •HCN. It is also important to point out that the inclusion of BSSE and zero-point energies (ZPE) corrections cause significant changes on_E. The smaller effect of ZPE is obtained for propyne• • •HCN at HF/6-311++G** level, while the greatest difference is obtained at MP2/6-31G** level for propyne• • •HF system. Concerning the IR vibrational it was obtained that larger shift can be associated with stronger hydrogen bonds. The more pronounced effect on the normal modes of the isolated molecule after the complexation is obtained for H X stretching frequency, which is shifted downward.