Spectral Analysis for Systems of Atoms and Molecules Coupled to the Quantized Radiation Field

We consider systems of static nuclei and electrons – atoms and molecules – coupled to the quantized radiation field. The interactions between electrons and the soft modes of the quantized electromagnetic field are described by minimal coupling, p→p−e A (x), where A(x) is the electromagnetic vector potential with an ultraviolet cutoff. If the interactions between the electrons and the quantized radiation field are turned off, the atom or molecule is assumed to have at least one bound state. We prove that, for sufficiently small values of the fine structure constant α, the interacting system has a ground state corresponding to the bottom of its energy spectrum. For an atom, we prove that its excited states above the ground state turn into metastable states whose life-times we estimate. Furthermore the energy spectrum is absolutely continuous, except, perhaps, in a small interval above the ground state energy and around the threshold energies of the atom or molecule. Received: 3 September 1998 / Accepted: 17 March 1999     

Supramolecular Assembly of Heterogeneous Multiporphyrin Arrays—Structures of [{ZnII(tpp)}2(tpyp)] and the Coordination Polymer [{[MnIII(tpp)]2(tpyp)(ClO4)2}∞]

Channels make up 46% of the crystal volume: A two-dimensional coordination polymer of exceptional structural regularity (see schematic drawing; tpp=tetraphenylporphyrin, tpyp=tetrapyridylporphyrin) was assembled from [Mn^III(tpp)]⋅ClO₄ and tpyp. The crystal packing of these polymers yields an open bulk structure with networks of perpendicular channels filled with nitrobenzene solvent molecules. Only oligomeric entities are formed by the reaction of [Zn^II(tpp)] with tpyp.     

Reactions along the astrophysical s-process path and prospects for neutron radiotherapy with the Liquid-Lithium Target (LiLiT) at the Soreq Applied Research Accelerator Facility (SARAF)

The European Physical Journal A - An empirical formula is proposed for the two-proton decay half-lives. This study is an extension of the empirical formula reported recently by us for calculating...     

Tulearins A, B, and C; structures and absolute configurations

The relative configuration of tulearin A (1) is determined by X-ray diffraction analysis of a cyclic carbonate derivative 2 and the absolute configuration (2R,3R,5S,8S,9S,15R,17S) from the 9-MTPA-esters 1R and 1S is determined using the modified Mosher’s method. A mechanism for the unexpected formation of carbonate 2 is suggested. Two N-phenyltriazolinedione derivatives 3 and 4 are also prepared. Two additional tulearins, B and C (5 and 6) are isolated in very small amounts and their structures are elucidated by spectroscopic means.     

A novel Ce<Superscript>III</Superscript>-cyclam type complex and its redox chemistry in aqueous solutions

A new Ce^III complex was synthesized by mixing the ligand “dioxocyclam” with Ce^III ions at pH 8.0 and its redox properties were investigated.

The Interplay between Steric and Electronic Effects in SN2 Reactions

Myths of steric hindrance : In contrast with current opinion, energy decomposition analysis shows that the presence of bulky substituents at carbon leads to the release of steric repulsion in the transition state shown in the graphic. It is rather the weakening of the electrostatic attraction, and in particular the loss of attractive orbital interactions, that are responsible for the activation barrier.

     

Effect of the metal fragment in the thermal cycloaddition between alkynyl metal(0) Fischer carbene complexes and nitrones

The thermal cycloaddition between alkynyl metal(0) Fischer carbenes and nitrones has been studied computationally within the Density Functional Theory framework. It is found that the [3 + 2] cycloaddition takes place via transition structures that are more asynchronous and less aromatic than their nonorganometallic analogues. These reactions are also found to be completely regioselective in favor of the cycloadduct possessing the Fischer carbene moiety and the oxygen atom of the nitrone in a 1,3-relative disposition. These results are consistent with the role of the Fischer carbene moiety as an electron withdrawing group that enhances the electrophilic character of the alkyne group acting as a Michael acceptor as a dipolarophile. In terms of the isolobal analogy model, it can be concluded that alkynylalkoxy metal(0) carbene complexes act in this reaction as organometallic analogues of organic alkyl-propiolates with enhanced electrophilic character.     

Correlation between Hammett substituent constants and directly calculated pi-conjugation strength

The results of an energy decomposition analysis of ortho-, meta-, and para-substituted benzylic cations and para-substituted benzylic anions H2C-C6H4Rq (R = H, F, CN, Me, OH, NH2, NO2, CHO, CO2H; q = +, -) are presented and discussed. The calculated values for the pi bonding between CH2(q) and C6H4R show for substituents which have pi orbitals a linear correlation with the Hammett sigma(p), sigma(+)(p), and sigma(m) constants.