No abstract

No abstract

ABSTRACT:For substituted benzenes such as (p-F-C 6 H 4 -Z), Linderberg et al. [1] demonstrated the validity of an equation similar to: ⌬CEBE Ϸ , where ⌬CEBE is the difference in core-electron binding energies (CEBEs) of the fluorinated carbon in p-F-C 6 H 4 -Z and that in FC 6 H 5 , the parameter is a function of the type of reaction, and is the Hammett substituent () constant. In this work, CEBEs of ring carbon atoms for a series of para disubstituted molecules p-F-C 6 H 4 -Z were first calculated using Density Functional Theory (DFT) with the scheme ⌬E KS (PW86-PW91)/TZPϩC rel //HF/6-31G*. An average absolute deviation of 0.13 eV from experiment was obtained for the CEBEs. Then we performed a linear regression analysis in the form of Y ϭ AϩB*X for a plot of Hammett p constants against calculated shifts ⌬CEBEs (in eV) for the fluorinated carbon. The results were: A ϭ -0.08 and B ϭ 1.01, with correlation coefficient R ϭ 0.973, standard deviation ϭ 0.12, and P Ͻ 0.0001. The intercept A of the fitted line, close to zero, shows that the Hammett p constant is proportional to the calculated ⌬CEBEs. On the other hand, the slope B of the straight line gives an estimate of the parameter . Similar statistical correlations were obtained for the carbon atoms ortho and meta to the substituent Z.

Recently a new methodology based on local density of state (LDOS) calculations using topological and semiempirical methods was proposed to identify the carcinogenic activity of polycyclic aromatic hydrocarbons (PAHs). In this work we perform a comparative study of this methodology with principal component analysis (PCA) and neural networks (NN). The PCA and NN results show that LDOS quantum chemical descriptors are relevant descriptors to identify the carcinogenic activity of methylated and non-methylated PAHs. Also, we show that the combination of these distinct methodologies can be an efficient and powerful tool in the structure-activity studies of PAHs compounds. We have studied 81 methylated and non-methylated PAHs, and our study shows that with the use of these methods it is possible to correctly predict the carcinogenic activity of PAHs with accuracy higher than 80%.

ABSTRACT:We calculated the accurate core-electron binding energies (CEBEs) of pyrimidine and purine bases in their isolated forms in the gas phase, i.e., uracil (U), thymine (T), cytosine (C), adenine (A), and guanine (G), using density functional theory (DFT) with the scheme ⌬E (PW86-PW91)/TZP//HF/6-31G*. The relative magnitude of calculated CEBEs of the same type of atom in the gas phase pyrimidine and purine bases reflect its chemical environment. Comparison between the calculated CEBEs of the bases in the gas phase and observed CEBEs of the same molecules in the solid state permitted estimation of the approximate work functions (WD). Using the approximate WD, it was possible to calculate approximate CEBEs of the DNA bases in the solid state. The average absolute deviation from experiment was 0.37 eV.