Компьютерная томография и магнитно-резонансная томография в диагностике опухолей средостения

Резюме. Проанализированы результаты обследования 95 больных с опухолями средостения (ОС) с использованием рентгенологических методов: компьютерной томографии (КТ), магнитно-резонансной томографии (МРТ). Приведена КТ- и МРТ-семиотика ОС, оценена информативность результатов каждого метода в зависимости от локализации опухоли. На основании полученных данных обоснованы показания к применению и последовательность использования неинвазивных лучевых методов в целях диагностики ОС. В частности, МРТ наиболее показана при локализации опухоли в заднем и передневерхнем отделах средостения, тогда как КТ целесообразнее проводить, если опухоль располагается в переднесреднем, передненижнем или среднем отделах средостения. Ключевые слова: опухоли средостения, рентгенологическое исследование, компьютерная томография, магнитно-резонансная томография, диагностическая эффективность.Summary. The paper reports findings of examination of 95 patients with mediastinum tumors (MS) using the following roentgenologic methods: computed tomography (CT) and magnetic resonance tomography (MRT). CR and MRT semeiology of MS is described and the informative value of each method is assessed in dependence on tumor’s localization. Based on these findings, indications for application of non-invasive radiation methods in examination of mediastinum tumors are developed as well as the sequence of their using. In particular, MRT is especially promising in tumors localized in posterior and antero-superior portions of the mediastinum, while CT is more appropriate in tumors localized in antero-medial, antero-lower or medial portions of the mediastinum. Key Words: mediastinum tumors, roentgenologic examination, computed tomography, magnetic resonance tomography, diagnostic efficiency

Delocalization in valence bond-hyperconjugation

We consider delocalization in small molecules. The valence bond technology allows an arbitrary division of the 1-electron space in strictly separate, but nonorthogonal, spaces. Allowing or obstructing orbital mixing between subspaces during the SCF procedure is associated with incorporating or eliminating the hyperconjugation effect. We show an example for tert-butyl and trimethylsilyl cations and radicals. For H2, for which really extensive basis sets are feasible, we extend the basis so far that the delocalization or hyperconjugation effect has nearly disappeare

Spin coupling and resonance

The resonating block localize wave function (RBLW) method is introduced, a resonating modification of the block localized wave functions introduced by Mo et al. [Mo, Y.; Peyerimhoff, S. D. J. Chem. Phys. 1998, 109, 1687].This approach allows the evaluation of resonance energies following Pauling’s recipe. The method is tested on two model molecules, hexagonal H6 and benzene. Calculations have been done with (local) and without local restrictions (delocal). Resonance energies for both molecules have been obtained for each type of calculation, in agreement with Pauling’s concept. From a comparison of the resonance energies obtained from RBLW and standard valence bond calculations, the resonating block localize wave functions prove to yield resonance energies close to standard valence bond delocal calculations

In Memory of Jaap G. Snijder (1951-2003)

In Memory of Jaap G. Snijders (1951-2003

Generation of Kekule valence structures and the corresponding valence bond wave function

A new scheme, called “list of nonredundant bonds”, is presented to record the number of bonds and their positions for the atoms involved in Kekulé valence structures of (poly)cyclic conjugated systems. Based on this scheme, a recursive algorithm for generating Kekulé valence structures has been developed and implemented. The method is general and applicable for all kinds of (poly)cyclic conjugated systems including fullerenes. The application of the algorithm in generating Valence Bond (VB) wave functions, in terms of Kekulé valence structures, is discussed and illustrated in actual VB calculations. Two types of VBSCF calculations, one involving Kekulé valence structures only and the second one involving all covalent VB structures, were performed for benzene, pentalene, benzocyclobutadiene, and naphthalene. Both strictly local and delocalised p-orbitals were used in these calculations. Our results show that, when the orbitals are restricted to their own atoms, other VB structures (Dewar structures) also have a significant contribution in the VB wave function. When removing this restriction, the other VB structures (Dewar and also the ionic structures) are accommodated in the Kekulé valence structures, automatically. Therefore, at VBSCF delocal level, the ground states of these systems can be described almost quantitatively by considering Kekulé valence structures only at a considerable saving of time

Note on the Calculation of Analytical Hessians in the Zeroth-Order Regular Approximation (ZORA)

The previously proposed atomic zeroth-order regular approximation (ZORA) approch, which was shown to eliminate the gauge dependent effect on gradients and to be remarkably accurate for geometry optimization, is tested for the calculation of analytical second derivatives. It is shown that the resulting analytic second derivatives are indeed exact within this approximation. The method proves to yield frequencies that are remarkably close to the experimental frequency for uranium hexafluoride but less satisfactory for the gold dimer

Ab initio calculation of the NH(3Sigma-)-NH(3Sigma-) interaction potentials in the quintet, triplet, and singlet states

Contains fulltext : 33311.pdf (publisher's version ) (Open Access

Atoms in Valence Bond – AiVB : synopsis and test results

The Atoms in Valence Bond (AiVB) method, a new approach in the Valence Bond, is introduced. This approach combines the ideas behind the early Atoms in Molecules (AIM) developments, e.g. by Moffit [21], to understand a molecular wave function in terms of proper atomic wave functions, with the available framework of the VBSCF [17,18] as implemented in TURTLE [19]. The fundamental theoretical tools, to explain the AiVB concept, are shown and the initial test results are presented