Towards an accurate description of the electronic properties of the biphenylthiol/gold interface: the role of exact exchange

We investigate the role of the exact exchange in describing the biphenylthiol/gold interface. The study is performed by simulating the electronic properties of mercaptobiphenylthiol and aminobiphenylthiol molecules adsorbed on a Au(23) cluster, using local, semilocal and hybrid functionals and an effective exact exchange method, namely, the localized Hartree-Fock (LHF). We find that the local/semilocal functionals strongly underestimate the charge transfer and the bond dipole at the interface due to the self-interaction-error (SIE), which alters the correct level alignment. On the other hand the LHF method is SIE free and predicts a larger charge transfer and bond dipole. We also found that LHF results can be reproduced using hybrid functionals and that conventional local/semilocal correlation functionals are unable to improve over the exchange-only description.

Torsional effects on excitation energies of thiophene derivatives induced by β-substituents: Comparison between time-dependent density functional theory and approximated coupled cluster approaches

The influence of methyl or phenyl substitution in beta-position of dioxygenated terthiophene and diphenylthiophene on the optical properties is investigated by first-principles calculations. We compare the approximated singles and doubles coupled cluster (CC2) approach with time-dependent density functional theory methods. CC2 reproduces experimental excitation energies with an accuracy of 0.1 eV. We find that the different substituents modify the inter-ring torsional angle which in turn strongly influences the excitation energies. The steric contribution to the excitation energies have been separated from the total substituent effects.

A comparative quantum chemical study of the ruthenium catalyzed olefin metathesis

The accurate quantum mechanical description of homogeneous catalysis involving transition-metal complexes is a complicated and computationally demanding task. Hence, in this study the performance of different quantum chemical approaches with respect to the ruthenium catalyzed olefin metathesis of ethylene and RuCl(2)(PH(3))(2)CH(2) as a model system is investigated. All intermediates and transition states that are relevant for the rate determining steps of competing reaction mechanisms (associative and two dissociative pathways) are considered. Results from density functional theory calculations employing B-P86, B97-D, B3-LYP, TPSSh, and B2-PLYP functionals, as well as from MP2 and SCS-MP2 perturbation theory are compared to reference values (relative and reaction energies) obtained at the QCISD(T) level of theory. In particular, the applicability of AO basis sets of increasing size ranging from double-zeta to quadruple-zeta quality is evaluated for representative methods. For some reaction steps, large basis set effects on the order of 10 kcal mol(-1) (50% of Delta E) are observed. Double-zeta type basis sets yield very unreliable results while properly polarized triple-zeta sets provide reaction energies quite close to the basis set limit. The performance of recommended methods is B2-PLYP>TPSSh>B-86 approximately B97-D>SCS-MP2. The often used standard approaches B3-LYP and MP2 provide overall the largest errors. The accurate QCISD(T) computations predict in conclusion (and in agreement with a recent other study) that for the model system considered, the dissociative trans pathway is favored over the dissociative cis pathway and also over the associative reaction mechanism.

Systematic quantum chemical study of DNA-base tautomers

The relative energies of the energetically low-lying tautomers of pyridone, cytosine, uracil, thymine, guanine, and iso-cytosine are studied by a variety of different quantum chemical methods. In particular, we employ density functional theory (DFT) using the six functionals HCTH407, PBE, BP86, B-LYP, B3-LYP, and BH-LYP, and the ab initio methods Hartree-Fock (HF), standard second-order Møller-Plesset perturbation theory (MP2), an improved version of it (SCS-MP2), and quadratic configuration interaction including single and double excitations (QCISD) and perturbative triple corrections [QCISD(T)]. A detailed basis set study is performed for the formamide/formamidic acid tautomeric pair. In general, large AO basis sets of at least valence triple-zeta quality including f-functions (TZV) are employed, which are found to be necessary for an accurate energetic description of the various structures. The performance of the more approximate methods is evaluated with QCISD(T)/TZV(2df,2dp) data taken as reference. In general it is found that DFT is not an appropriate method for the problem. For the tautomers of pyridone and cytosine, most density functionals, including the popular B3-LYP hybrid, predict a wrong energetic order, and only for guanine, the correct sequence of tautomers is obtained with all functionals. Out of the density functionals tested, BH-LYP, which includes a rather large fraction of HF exchange, performs best. A consistent description of the nonaromatic versus aromatic tautomers seems to be a general problem especially for pure, nonhybrid functionals. Tentatively, this could be assigned to the exchange potentials used while the functional itself, including the correlation part, seems to be appropriate. Out of the ab initio methods tested, the new SCS-MP2 approach seems to perform best because it effectively reduces some outliers obtained with standard MP2. It outperforms the much more costly QCISD method and seems to be a very good compromise between computational effort and accuracy.

Evolutive behavior towards cardiomyopathy of treated (nifurtimox or benznidazole) and untreated chronic chagasic patients

The aim of this work was to compare the evolution of chronic chagasic untreated patients (UTPs) with that of benznidazole or nifurtimox-treated patients (TPs). A longitudinal study from a low endemic area (Santa Fe city, Argentina) was performed during an average period of 14 years. Serological and parasitological analyses with clinical exams, ECG and X-chest ray were carried out. At the onset, 19/198 infected patients showed chagasic cardiomyopathy (CrChM) while 179 were asymptomatic. In this latter group the frequency of CrChM during the follow-up was lower in TPs compared with UTPs (3.2% vs 7%). Within the CrChM group, 2/5 TPs showed aggravated myopathy whereas this happened in 9/14 UTPs. Comparing the clinical evolution of all patients, 5.9% of TPs and 13% of UTPs had unfavourable evolution, but the difference is not statistically relevant. Serological titers were assessed by IIF. Titers equal to or lower than 1/64 were obtained in 86% of the TPs, but only in 38% of UTPs. The differences were statistically significant (geometric mean: 49.36 vs. 98.2). Antiparasitic assessment of the drugs (xenodiagnosis) proved to be effective. The low sensitivity in chronic chagasic patients must be born in mind. Despite treated patients showed a better clinical evolution and lower antibody levels than untreated ones, it is necessary to carry on doing research in order to improve therapeutic guidelines, according to the risk/benefit equation and based on scientific and ethical principles.

[Diagnostic imaging in non-palpable breast lesions. Targeted++ ultrasonography versus direct radiologic magnification]

The main target of mammography in asymptomatic women is the early diagnosis, or rather the identification, of non-palpable breast cancers. Doubtful or suspicious findings on conventional mammograms with no clinical evidence call for radiologic or other complementary imaging techniques to assess the exact lesion nature. Direct magnification and US are targeted techniques to employ as additional investigations after conventional mammography. Fifty consecutive patients were referred to our department of radiology for the preoperative localization of non-palpable breast lesions previously identified on conventional mammograms. The diagnostic or complementary roles of direct magnification and of US were thus investigated. US was always repeated during the preoperative localization; a 10-MHz immersion sectorial probe was used. Magnification was performed if absent or poor in conventional mammograms. The contribution of each technique to conventional mammography was graded as valuable (A), medium (B), or null (C). The lesions were grouped according to their structure: microcalcifications (a), nodules (b), scars (c), and complex lesions (a+b, a+c, b+c, ecc.). Six cases are included in our series which had been diagnosed as questionable or suspicious on previous mammograms. In our department, they were diagnosed as benign. Two of them were operated on because biopsy was required by the gynecologist and the other underwent stereotaxic FNB: negative cytology was considered the final diagnosis. Forty-six histologic and 4 cytologic examinations diagnosed 25 malignant and 25 benign lesions. Direct magnification was of great value in all cases, whereas US was useless in microcalcifications and useful in nodular or complex lesions, especially those with a nodular component. However, the incidence of US false-negatives was high, even in very suspicious cases on mammography, which suggests that US negativity cannot be considered an adequate sign to rule malignancy out.