ABSTRACT
Benzene, toluene, ethylbenzene, and xylenes are volatile hydrocarbons known as BTEX, which present concerns about environmental problems. Density functional theory (DFT) functionals were used for the BTEX gas phase adsorption on TiO2 (110) of rutile and (101) of anatase surfaces. Dispersion terms have shown the importance to treat weak interactions and were used to study these adsorptions using plane wave DFT calculations. All BTEX molecules have the same trend for the adsorption on rutile and anatase surfaces. The inclusion of dispersion terms has a significant contribution for the interaction energy. Density of states results suggest the hybridization between the d state of pentacoordinated titanium atoms (Ti5C) and carbon p states of benzene. The adsorption energy values indicate an effective interaction between the BTEX and surfaces, mainly due to the aromatic π interaction, which is present in all adsorbates. However, for p-xylene the methyl hydrogen directs the second major influence. Graphical abstract Charge difference showing the system with the smallest interaction and the one with the largest interaction.
ABSTRACT
We performed a combined DFT and Monte Carlo (13)C NMR chemical-shift study of azadirachtin A, a triterpenoid that acts as a natural insect antifeedant. A conformational search using a Monte Carlo technique based on the RM1 semiempirical method was carried out in order to establish its preferred structure. The B3LYP/6-311++G(d,p), wB97XD/6-311++G(d,p), M06/6-311++G(d,p), M06-2X/6-311++G(d,p), and CAM-B3LYP/6-311++G(d,p) levels of theory were used to predict NMR chemical shifts. A Monte Carlo population-weighted average spectrum was produced based on the predicted Boltzmann contributions. In general, good agreement between experimental and theoretical data was obtained using both methods, and the (13)C NMR chemical shifts were predicted highly accurately. The geometry was optimized at the semiempirical level and used to calculate the NMR chemical shifts at the DFT level, and these shifts showed only minor deviations from those obtained following structural optimization at the DFT level, and incurred a much lower computational cost. The theoretical ultraviolet spectrum showed a maximum absorption peak that was mainly contributed by the tiglate group.
