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Electron Lone Pairs (electron + lone_pair)
Selected AbstractsThallium Halides , New Aspects of the Stereochemical Activity of Electron Lone Pairs of Heavier Main-Group ElementsCHEMINFORM, Issue 16 2007Anja-Verena Mudring Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] NMR, solvation and theoretical investigations of conformational isomerism in 2-X-cyclohexanones (X=NMe2, OMe, SMe and SeMe)JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 11 2003Matheus P. Freitas Abstract The conformational equilibria of 2- N,N -dimethylamino- (1), 2-methoxy- (2), 2-methylthio- (3) and 2-methylselenocyclohexanone (4) were determined in various solvents by measurement of the 3JH-2,H-3 couplings. The observed couplings were analyzed using theoretical and solvation calculations to give both the conformer energies in the solvents studied plus the vapor-phase energies and the coupling constants for the distinct conformers. These gave the conformer energies and couplings of 2,4. The intrinsic couplings for the 2- N,N -dimethylamino compound were determined by the molecular mechanics PCMODEL program. The axial conformation in 1 is the most polar and also more stable in DMSO solution (Eeq,Eax=0.05,kcal,mol,1) and the pure liquid, while the equatorial conformer predominates in the remaining solvents studied (except in CCl4, where self-association is observed). In the methoxy ketone (2) the equatorial conformation is more stable in the vapor (Eeq,Eax=,0.30,kcal,mol,1) and in all solvents. The opposite behavior is shown by 3 and 4, where the axial conformation is the more stable one in the vapor phase (Eeq,Eax=1.60 and 2.95,kcal,mol,1 for 3 and 4, respectively) and is still the prevailing conformer in solution. The axial predominance for 3 and 4 is attributed to hyperconjugation between the electron lone pair of the hetero-substituent and the ,*CO orbital. This interaction is stronger for 3 and 4 than in the case of 1 and 2, where the ,gauche effect' in the equatorial conformation should be more effective in stabilizing this conformation. Copyright © 2003 John Wiley & Sons, Ltd. [source] A Density Functional Theory Study on the Electrocyclization of 1,2,4,6-Heptatetraene Analogues: Converting a Pericyclic to a Pseudopericyclic ReactionCHEMISTRY - A EUROPEAN JOURNAL, Issue 20 2005Enrique M. Cabaleiro-Lago Prof. Dr. Abstract A comprehensive B3LYP/6-31+G* study on the electrocyclization of 1,2,4,6-heptatetraene analogues was conducted. Starting from the cyclization of (2Z)-2,4,5-hexatrienal, a pericyclic disrotatory process favored by the assistance of a electron lone pair, we incorporated small modifications in its molecular structure to obtain a truly pseudopericyclic process. To this purpose electronegative atoms (fluorine and nitrogen) were added to give a more electrophilic character on the carbon atom which is attacked by the electron lone pair of the oxygen atom. The complete pathway for each reaction was determined, and changes in magnetic properties were monitored with a view to estimating the aromatization associated with each process. This information, together with the energetic and structural results, allowed us to classify the reactions as pseudopericyclic or pericyclic. Among all studied reactions only one was a truly pseudopericyclic process and another was a borderline case. The features of this unequivocally pseudopericyclic case were analyzed in depth. Se ha llevado a cabo un exhaustivo estudio B3LYP/6-31+G* de la electrociclación de análogos del 1,2,4,6-heptatetraeno. Partiendo de la ciclación del (2Z)-2,4,5-hexatrienal, un proceso pericíclico disrotatorio favorecido por la ayuda de un par electrónico solitario, hemos incorporado pequeñas modificaciones en su estructura molecular con el fin de obtener un proceso verdaderamente pseudopericíclico. Con este propósito se añadieron átomos electronegativos (flúor y oxígeno) para conseguir un mayor carácter electrofílico en el átomo de carbono que es atacado por el par electrónico solitario del átomo de oxígeno. Se han determinado los caminos de reacción completos y se ha observado la variación de las propiedades magnéticas con el fin de estimar la aromatización asociada a cada proceso. Esta información, junto con los resultados energéticos y estructurales, nos han permitido clasificar las reacciones como pseudopericíclicas o pericíclicas. De todas las reacciones estudiadas sólo una resultó ser realmente pseudopericíclica y otra resultó ser un caso límite. Las características de este caso inequívocamente pseudopericíclico han sido analizadas en profundidad. [source] Polymorphism of CaTeO3 and solid solutions CaxSr1,,,xTeO3ACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2009Berthold Stöger Single crystals and microcrystalline samples of the calcium tellurate(IV) phases ,-, ,-, ,,- and ,-CaTeO3 as well as of two solid solutions CaxSr1,,,xTeO3 (x = 0.55 and 0.77) have been synthesized and characterized by X-ray diffraction and thermal analysis. A comparative description of the structures and the relations between the polymorphs is given. The main building units of the hitherto unknown structures are isolated [TeIVO3]2, units and [(Ca,Sr)Ox] (x = 6,8) polyhedra. All structures exhibit channels in which the TeIV electron lone pairs protrude. The low-temperature phase ,-CaTeO3 is stable up to 1168,K. It exhibits nearly cylindrical channels (diameter ,,4,Å) and differs structurally from the other phases, whereas the metastable high-temperature phases are closely related to each other. They feature oval channels (shortest and longest diameter ,,2 and 8,Å). ,-CaTeO3 can be described as an order,disorder (OD) structure of two non-polar layers with layer groups p121/m1 and p1211. The ,-CaTeO3 crystal under investigation consists of two polytypes with a maximum degree of order. The two phases CaxSr1,,,xTeO3 (x = 0.55 and 0.77) are isostructural to the MDO1 polytype of ,-CaTeO3. ,-CaTeO3 shows a distinct reversible phase transition at ,,293,K. The low-temperature modification ,,-CaTeO3 as well as its high-temperature modification ,-CaTeO3 can be considered as threefold superstructures along [100] based on the MDO1 polytype of ,-CaTeO3. [source] Tl2CO3 at 3.56 GPaACTA CRYSTALLOGRAPHICA SECTION C, Issue 8 2008A. Grzechnik The crystal structure of thallium carbonate, Tl2CO3 (C2/m, Z = 4), is stable at least up to 3.56,GPa, as demonstrated by hydrostatic single-crystal X-ray diffraction measurements in a diamond anvil cell at room temperature. Our results contradict earlier observations from the literature, which found a structural phase transition for this compound at about 2,GPa. Under atmospheric conditions, all atoms except for one O atom reside on the mirror plane in the high-pressure structure. The compression mainly affects the part of the structure where the nonbonded electron lone pairs on the Tl+ cations are located. [source] Cyanomethylidyne: A Reactive Carbyne RadicalCHEMPHYSCHEM, Issue 3 2006Jian Wang Abstract The cyanomethylidyne radical (CCN) has been a long-standing subject of extensive structural and spectroscopic studies. However, its chemical reactivity has received rather little attention. Recently, we studied the reaction of CCN with the simplest alkane, CH4, which follows a mechanism of carbyne insertion,dissociation rather than that of direct H abstraction proposed by a recent experimental study. However, we are aware that alkanes like CH4 bear no electron lone pairs and thus are not ideal diagnostic molecules for distinguishing between the carbyne-insertion and H-abstraction mechanisms. Hence, we chose a series of ,-bonded molecules HX (X=OH, NH2, and F) which bear electron lone pairs and are better diagnostics for carbyne-insertion behavior. The new results at the CCSD(T)/6-311+G(2df,p)//B3LYP/6-311G(d,p)+ZPVE, CCSD(T)/aug-cc-pVTZ//B3LYP/6-311G(d,p)+ZPVE, G2M(CC1), and MC-QCISD//B3LYP/6-31G(d)+ZPVE levels definitively confirm the carbyne-insertion behavior of the CCN radical towards HX. In addition, we make the first attempt to understand the reactivity of the CCN radical toward ,-bonded molecules, using the CCN+C2H2 model reaction. This reaction involves carbenoid addition to the CC bond without a potential-energy barrier to form a C3 three-membered cyclic intermediate followed by H extrusion. Therefore, the reactions of CCN with both ,- and ,-bonded molecules conclusively show that CCN is a reactive carbyne radical and may be more reactive than the well-known CN radical. Future experimental studies, especially on product characterization, are strongly desired to test our proposed carbyne mechanism. The studied reactions of CCN with CH4,, NH3,, H2O, and C2H2 could be of interest to combustion science and astrophysics, and they could provide efficient routes to form novel cyano-containing molecules in interstellar space. [source] | ||||||||||