Elusiveness of bishomoaromaticity in anionic systems: the bicyclo[3.2.1]octa-3,6-dien-2-yl anion

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Stabilization of the alleged bishomoromatic bicyclo[3.2.1]octa-2,6-dienyl anion by counterion interactions and by hyperconjugation

Hyperconjugation and inductive effects, rather than homoaromaticity, are responsible for the stabilization of the title anion in the gas phase; interaction of the double bond with the Li+ gegenion in the endo geometry contributes additionally in solution

The interplay of angle strain and aromaticity: molecular and electronic structures of [0<SUB>n</SUB>]paracyclophanes

The belt-like polyphenylenes, [0n]paracyclophanes, (n = 5 and 6), have been investigated using semi-empirical, ab initio and DFT methods. The molecular structure, rotational barrier on twisting a single phenyl ring and the aromatic character within each ring as well as in the whole molecule have been evaluated. [05]Paracyclophane is predicted to have a quinonoid structure. In contrast, the equatorial pentaphenyl fragment found in C70 as well as the hexagons of the less strained [06]paracyclophane have benzenoid character. Approximate band structures have been derived for larger cycles of [0n] paracyclophanes

Nucleophilic additions to 4-substituted snoutanones: getting a measure of long range electrostatic and orbital control of π-face selectivity

Pentacyclic ketones 10a-e (snoutan-9-ones) undergo nucleophilic additions with the same facial preference as the corresponding norsnoutanones 9a-e, but with markedly reduced selectivity, revealing the involvement of electrostatic effects in the former and implying the importance of hyperconjugative orbital interactions in determining π-face selectivity in the latter systems

4-Substituted norsnoutanones: a new probe system forevaluatingelectronic effects in π -facial selectivity in nucleophilicadditions

Remotely substituted norsnoutanone derivatives show significant and consistent π-face selectivity in nucleophilic additions, revealing the interplay of orbital and electrostatic effects

Managing the natural disasters from space technology inputs

Natural disasters, whether of meteorological origin such as Cyclones, Floods, Tornadoes and Droughts or of having geological nature such as earthquakes and volcanoes, are well known for their devastating impacts on human life, economy and environment. With tropical climate and unstable land forms, coupled with high population density, poverty, illiteracy and lack of infrastructure development, developing countries are more vulnerable to suffer from the damaging potential of such disasters. Though it is almost impossible to completely neutralise the damage due to these disasters, it is, however possible to (i) minimise the potential risks by developing disaster early warning strategies (ii) prepare developmental plans to provide resilience to such disasters, (iii) mobilize resources including communication and telemedicinal services and (iv) to help in rehabilitation and post-disaster reconstruction. Space borne platforms have demonstrated their capability in efficient disaster management. While communication satellites help in disaster warning, relief mobilisation and telemedicinal support, Earth observation satellites provide the basic support in pre-disaster preparedness programmes, in-disaster response and monitoring activities, and post-disaster reconstruction. The paper examines the information requirements for disaster risk management, assess developing country capabilities for building the necessary decision support systems, and evaluate the role of satellite remote sensing. It describes several examples of initiatives from developing countries in their attempt to evolve a suitable strategy for disaster preparedness and operational framework for the disaster management Using remote sensing data in conjunction with other collateral information. It concludes with suggestions and recommendations to establish a worldwide network of necessary space and ground segments towards strengthening the technological capabilities for disaster management and mitigation

PDDG/PM3 and PDDG/MNDO: improved semiempirical methods

Two new semiempirical methods employing a Pairwise Distance Directed Gaussian modification have been developed: PDDG/PM3 and PDDG/MNDO; they are easily implemented in existing software, and yield heats of formation for compounds containing C, H, N, and O atoms with significantly improved accuracy over the standard NDDO schemes, PM5, PM3, AM1, and MNDO. The PDDG/PM3 results for heats of formation also show substantial improvement over density functional theory with large basis sets. The PDDG modifications consist of a single function, which is added to the existing pairwise core repulsion functions within PM3 and MNDO, a reparameterized semiempirical parameter set, and modified computation of the energy of formation of a gaseous atom. The PDDG addition introduces functional group information via pairwise atomic interactions using only atom-based parameters. For 622 diverse molecules containing C, H, N, and O atoms, mean absolute errors in calculated heats of formation are reduced from 4.4 to 3.2 kcal/mol and from 8.4 to 5.2 kcal/mol using the PDDG modified versions of PM3 and MNDO over the standard versions, respectively. Several specific problems are overcome, including the relative stability of hydrocarbon isomers, and energetics of small rings and molecules containing multiple heteroatoms. The internal consistency of PDDG energies is also significantly improved, enabling more reliable analysis of isomerization energies and trends across series of molecules; PDDG isomerization energies show significant improvement over B3LYP/6-31G&#8727; results. Comparison of heats of formation, ionization potentials, dipole moments, isomer, and conformer energetics, intermolecular interaction energies, activation energies, and molecular geometries from the PDDG techniques is made to experimental data and values from other semiempirical and ab initio methods

Analysis of geometric and strain effects in homo-Diels-Alder reactions

Molecular mechanics calculations have been carried out to quantify the key geometric and strain effects which are likely to control the homo-Diels-Alder reactivity of 1,4-dienes. The criteria considered include C1..C5 and C2..C4 distances in the diene, twist angle of the two &#960; units, and the magnitude of strain increase as a result of cycloaddition. By first considering these factors in a number of nonconjugated dienes with known reactivity, the ranges of values within which the reaction is favoured are proposed. Calculations are also reported on several substrates which have not been investigated so far. Promising systems for experimental study are suggested which, in addition to being intrinsically interesting, would place the present proposals on a firm basis

Factors contributing to phantom bonds in inorganic molecules: interpretations and predictions

The validity of various qualitative proposals for interpreting and predicting the existence of short contacts between formally non-bonded atoms, as in cyclodisiloxane and related inorganic ring systems, is critically evaluated. The models range from simple considerations of geometric constraints, lone pair repulsions and &#960;-complex formation to proposals such as the unsupported &#960;-bond model and the s-bridged-&#960; bond concept. It is pointed out that a unified description based on a combination of closed and open 3-centre 2-electron bonds is possible. The role of hybridisation is emphasized in the short phantom bond computed in an earlier model system. These insights are used to predict structures with exceptionally short Si..Si and B..B phantom bonds. The proposals are confirmed byab initio calculations

Structure of the triplet excited state of bromanil from time-resolved resonance Raman spectra and simulation

Time-resolved resonance Raman (TR3) spectroscopy has been used to study the structure of the triplet excited state of bromanil. These experimental results were then simulated using parameters from density functional theoretical (DFT) calculations and wave packet dynamics, in order to understand the structure and mode-specific displacements of the resonant excited state. The transition dipole moments and the energy separation of the T1 and Tn states were obtained from time-dependent DFT calculations. We have demonstrated application of the technique to tetrabromo-p-benzoquinone. From our calculations, the observed T1->Tn absorption spectrum has been assigned to the 3Bg&#8594;3Bu transition. The geometry has been optimized for the resonant higher triplet state, Tn, and is found to be in good agreement with the predictions of the wave packet dynamical simulations. Mode-specific displacements of the triplet state geometry have been obtained from simulations and these have been rationalized with respect to the molecular orbital involved. Thus, we have demonstrated that from the simulations of the experimental TR3 spectral data, valuable additional information can be derived on the structure of the transient states that may then be used for elucidation of structure-reactivity correlation in the future