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Lone Pair (lone + pair)
Kinds of Lone Pair Selected AbstractsThe Stability and Eventual Lone Pair Distortion of the Hexahalide Complexes and Molecules of the Fifth to Eighth Main-Group Elements with One Lone Pair, as Isolated Entities or in Oligomeric Clusters: A Vibronic Coupling and DFT Study.CHEMINFORM, Issue 40 2005Mihail Atanasov No abstract is available for this article. [source] Thallium 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] Unusual Lone Pairs in Tellurium and Their Relevance for SuperconductivityCHEMINFORM, Issue 14 2006Shuiquan Deng 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] Macrocyclic Receptor Showing Improved PbII/ZnII and PbII/CaII SelectivitiesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 17 2010Raquel Ferreirós-Martínez Abstract Herein we report on the macrocyclic receptor N,N,-bis[(6-carboxy-2-pyridyl)methyl]-1,10-diaza-15-crown-5 (H2bp15c5) and its coordination properties towards ZnII, CdII, PbII, and CaII. The stability constants of these complexes determined by pH-potentiometric titration at 25 °C in 0.1 M KNO3 vary in the following order: PbII > CdII >> ZnII > CaII. As a result, bp15c5 presents very important PbII/ZnII and PbII/CaII selectivities. These results are in contrast to those reported for the related receptor derived from 1,7-diaza-12-crown-4, which provides very similar complex stabilities for ZnII and PbII. The X-ray crystal structure of [Cd(Hbp15c5)]+ shows heptadentate binding of the ligand to the metal ion, with two oxygen atoms of the macrocyclic unit remaining uncoordinated. The 1H NMR spectra of the complexes formed with PbII, ZnII, and CaII (D2O) show very broad peaks in the region 2,5 ppm, indicating an important degree of flexibility of the crownmoiety in these complexes. On the contrary, the 1H and 13C NMR spectra recorded for the CdII complex are well resolved and could be fully assigned. A detailed conformational investigation using theoretical calculations performed at the DFT (B3LYP) level predict a minimum energy conformation for [Cd(bp15c5)] that is very similar to that observed in the solid state. Analogous calculations performed on the [M(bp15c5)] (M = Zn or Pb) systems predict hexadentate binding of the ligand to these metal ions. In the case of the PbII complex our calculations indicate that the 6s lone pair is stereochemically active, which results in a hemidirected coordination geometry around the metal ion. The minimum energy conformations calculated for the ZnII, CdII, and PbII complexes are compatible with the experimental NMR spectra obtained in D2O solution. [source] Fluoride-Free Hiyama and Copper- and Amine-Free Sonogashira Coupling in Air in a Mixed Aqueous Medium by a Series of PEPPSI-Themed Precatalysts,EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 12 2009Chandrakanta Dash Abstract A new series of robust, user-friendly, and highly active PEPPSI-themed (pyridine-enhanced precatalyst preparation, stabilization and initiation) (NHC)PdX2(pyridine)-type (X = Cl, Br) precatalysts of C4,C5 saturated imidazole- (1,4) and triazole-based (5 and 6) N-heterocyclic carbenes for the Hiyama and Sonogashira couplings under amenable conditions are reported. Specifically 1,6 efficiently catalyze the fluoride-free Hiyama coupling of aryl halides with PhSi(OMe)3 and CH2=CHSi(OMe)3 in air in the presence of NaOH as a base in a mixed aqueous medium (dioxane/H2O, 2:1 v/v). Along the same lines, these 1,6 precatalysts also promote the Cu-free and amine-free Sonogashira coupling of aryl bromides and iodides with phenylacetylene in air and in a mixed aqueous medium (DMF/H2O, 3/1 v/v). The complexes 1,6 were synthesized by the direct reaction of the respective imidazolinium and triazolium halide salts with PdCl2 in pyridine in the presence of K2CO3 as a base. DFT studies on the catalytically relevant palladium(0) (NHC)Pd(pyridine) precursors 1a,6a reveal significant donation from the N-heterocyclic carbene lone pair onto the unfilled ,* orbital of the trans Pd,pyridine bond. This weakens the Pd-bound "throwaway" pyridine ligand, and its dissociation marks the initiation of the catalytic cycle.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Hierarchical Structures in Tin(II) OxalatesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 9 2008Padmini Ramaswamy Abstract Six new SnII oxalates exhibiting a hierarchy of structures have been prepared employing hydrothermal methods. The compounds I [C10N2H10][Sn(C2O4)2], II [C10N2H10][Sn2(C2O4)3], and III [C8N4H26][Sn(C2O4)2]2·2H2O possess zero-dimensional molecular structures; IV [C10N2H8]2[Sn(C2O4)]2 and V [C12N2H8][SnC2O4] have one-dimensional chain structures; and compound VI [C5N2H14]2[Sn4(C2O4)6]·7H2O has a two-dimensional layer structure. The SnII ions have 4- and 6-coordination with square-pyramidal or pentagonal-bipyramidal geometry, in which the lone pair of electrons also occupies one of the vertices. Weak intermolecular forces such as hydrogen-bond interactions, ,···, interactions, and lone-pair,, interactions have been observed and appear to lendstructural stability. Theoretical studies indicate that the ,···, interaction energy between the bound 1,10-phenanthroline molecules is of the order of 5,6 kcal,mol,1 in V. Natural bond orbital (NBO) analysis on two model compounds, II and IV, indicates reasonable lone-pair,, interactions. The close structural relationship between all the compounds indicates that a building-up process from the zero-dimensional monomer can be considered. The present structures provide opportunities for evaluating the structure-directing role of the lone pair of electrons of SnII.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Oxidorhenium(V) Complexes of a Family of Bipyridine-Like Ligands Including Pyridyltriazines and Pyrazinyltriazine: Oxygen-Atom Transfer, Metal Redox and CorrelationsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 11 2006Samir Das Abstract The title ligands (general abbreviation L) are bipyridine (bpy), its dimethyl (mbpy) and diphenyl (pbpy) derivatives, phenanthroline (phen), 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine (ppyt) and its dimethyl (mpyt) and pyrazinyl (ppzt) analogues. The concerned oxido complexes are [ReOCl3(L)],[ReOBr3(ppyt)] and [ReOBr3(ppzt)]. The chloro and bromo complexes of ppyt and ppzt were prepared by reacting these ligands with [ReOX3(AsPh3)2] (X = Cl, Br). The X-ray structures of [ReOCl3(ppyt)] and [ReOCl3(ppzt)] reveal that the ReCl3 fragment is meridionally disposed and that the L ligand is N,N -coordinated such that the pyridine/pyrazine nitrogen lies trans to the oxido oxygen atom. The Re,O lengths [1.656(10)/1.625(9) Ĺ] correspond to approximate triple bonding. The rate of oxygen-atom transfer from [ReOX3(L)] to triphenylphosphane in solution follows second-order kinetics and is associated with a large and negative entropy of activation (approx. ,30 cal,K,1,mol,1). The initial attack is believed to involve the phosphane lone pair and Re,O ,*-orbitals. Electron withdrawal from the ReVO moiety by varying X or L facilitates oxygen-atom transfer. Thus, the rates follow the orders Br < Cl; mbpy < bpy < phen < pbpy << mpyt < ppyt < ppzt. The reduction potential of the quasireversible ReVIO/ReVO couple displays similar trends and the logarithmic rate constant of oxygen-atom transfer is found to correlate linearly with the reduction potential. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] X-Ray Structure Analyses of syn/anti -Conformers of N -Furfuroyl-, N -Benzoyl-, and N -Picolinoyl-Substituted (2R)-Bornane-10,2-sultam DerivativesHELVETICA CHIMICA ACTA, Issue 8 2008Karolina Koszewska Abstract The synthesis and the X-ray structure of the three new N -(arylcarbonyl)-substituted derivatives 2a,2c of (2R)-bornane-10,2-sultam are presented and discussed. Direct comparison of the solid-state analyses shows that the dipole-directed SO2/CO anti- /syn- conformations may be very sensitive to weak electronic/electrostatic repulsions of the heteroatom lone pairs. The optimum interactions are reached when the lone pair of the , -positioned heteroatom is oriented in the O(3)C(11)N(1) plane. Such rare syn -conformations may be observed with at least up to 1.8,kcal/mol higher energy as compared to their ground states. Additionally, these anti/syn- conformations are also very sensitive to external influences such as, for example, the crystal-packing forces. [source] A Joint Theoretical and Experimental Insight into the Electronic Structure of Chromophores Derived from 6H,12H -5,11-Methanodibenzo[b,f][1,5]diazocineHELVETICA CHIMICA ACTA, Issue 11 2007Vincent Lemaur Abstract We report on the synthesis and electronic spectra of the chiral, donor-acceptor (push-pull) chromophores (±)- 4 and (±)- 5 with a 6H,12H -5,11-methanodibenzo[b,f][1,5]diazocine scaffold (Scheme,1 and Fig.,2). The electronic structures of these compounds were investigated at a quantum-chemical level (Figs.,2 and 3). The chemical reactivity of 6H,12H -5,11-methanodibenzo[b,f][1,5]diazocine ((±)- 11) towards aromatic electrophilic substitution (Scheme,2 and Table) provided additional information about its electronic structure and confirmed nonnegligible delocalization of the lone pair of the bridge-head N-atoms in this heterocyclic system. [source] Are Oxazolidinones Really Unproductive, Parasitic Species in Proline Catalysis?HELVETICA CHIMICA ACTA, Issue 3 2007Experiments Pointing to an Alternative View, Thoughts Abstract The N,O-acetal and N,O-ketal derivatives (oxazolidinones) formed from proline, and aldehydes or ketones are well-known today, and they are detectable in reaction mixtures involving proline catalysis, where they have been considered ,parasitic dead ends'. We disclose results of experiments performed in the early 1970's, and we describe more recent findings about the isolation, characterization, and reactions of the oxazolidinone derived from proline and cyclohexanone. This oxazolidinone reacts (THF, room temperature) with the electrophiles , -nitrostyrene and chloral (=trichloroacetaldehyde), to give the Michael and aldol adduct, respectively, after aqueous workup (Scheme,5). The reactions occur even at ,75° when catalyzed with bases such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) or EtN(i-Pr)2 (DIPEA) (10%; Table,1). It is shown by NMR (Figs.,1 and 3) and IR analysis (Figs.,2 and 4) that the primarily detectable product (before hydrolysis) of the reaction with the nitro-olefin is again an oxazolidinone. When dissolved in hydroxylic solvents such as MeOH, ,hexafluoroisopropanol' ((CF3)2CHOH; HFIP), AcOH, CF3COOH, or in LiBr-saturated THF, the ring of the oxazolidinone from cyclohexanone and proline opens up to the corresponding iminium ion (Tables,2,4), and when treated with strong bases such as DBU (in (D8)THF) the enamino-carboxylate derived from proline and cyclohexanone is formed (Scheme,8). Thus, the two hitherto putative participants (iminium ion and enamine) of the catalytic cycle (Scheme,9) have been characterized for the first time. The commonly accepted mechanism of the stereoselective C,C- or C,X-bond-forming step (i.e., A,D) of this cycle is discussed and challenged by thoughts about an alternative model with a pivotal role of oxazolidinones in the regio- and diastereoselective formation of the intermediate enamino acid (by elimination) and in the subsequent reaction with an electrophile (by trans -addition with lactonization; Schemes,11,14). The stereochemical bias between endo - and exo -space of the bicyclo[3.3.0]octane-type oxazolidinone structure (Figs.,5 and 6) is considered to possibly be decisive for the stereochemical course of events. Finally, the remarkable consistency, with which the diastereotopic Re -face of the double bond of pyrrolidino-enamines (derived from proline) is attacked by electrophiles (Schemes,1 and 15), and the likewise consistent reversal to the Si -face with bulky (Aryl)2C-substituents on the pyrrolidine ring (Scheme,16) are discussed by invoking stereoelectronic assistance from the lone pair of pyramidalized enamine N-atoms. [source] Aminodiphenylphosphanes: Isotope-induced chemical shifts 1,14/15N(31P), coupling constants 1J(31P,15N), and chemical shifts ,15N and ,31PHETEROATOM CHEMISTRY, Issue 6 2001Rosalinda Contreras A series of aminodiphenylphosphanes 1 [Ph2P-N(H)tBu (a), -NEt2 (b), -NiPr2 (c)], 2 [Ph2P-NHPh (a), -NH-2-pyridine (b), -NH-3-pyridine (c), -NH-4-pyridine (d), NH-pyrimidine (e), NH-2,6-Me2 -C6H3 (f), NH-3-Me-2-pyridine (g)], 3 [Ph2P-N(Me)Ph (a), -NPh2 (b)], and N-pyrrolyldiphenylphosphane 4 (Ph2P-NC4H4) was prepared and studied by NMR (1H, 13C, 31P, 15N NMR) spectroscopy. The isotope-induced chemical shifts 1,14/15N(31P) were determined at natural abundance of 15N by using HEED INEPT experiments. A dependence of 1,14/15N(31P) on the substituents at nitrogen was found (alkyl < H < aryl; increasingly negative values). The magnitude and sign of the coupling constants 1J(31P,15N) (positive sign) are dominated by the presence of the lone pair of electrons at the phosphorus atom. The X-ray structural analysis of 2b is reported, showing the presence of dimers owing to intermolecular hydrogen bridges in the solid state. © 2001 John Wiley & Sons, Inc. Heteroatom Chem 12:542,550, 2001 [source] Comparative studies on the pyrolysis of N -arylideneaminoamides: Kinetic and mechanistic studiesINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 2 2007Nouria A. Al-Awadi Rates of thermal decomposition of title compounds have been measured using a static reaction system. They undergo a unimolecular first-order elimination to give arylnitrile and the corresponding substituted amides. The decomposition parallels that of N -arylidenamino cyclic amide. The relative elimination rates at 600 K were calculated. The kinetic data reveal that the electronic effects of substituents, such as methyl, phenyl, benzyl, and allyl groups, are associated with the opposing directions in which the lone pair of electrons on the nitrogen atom of the arylidene moiety is being delocalized. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 39: 59,66, 2007 [source] Perfect planar tetracoordinate carbon in neutral unsaturated hydrocarbon cages: A new strategy utilizing three-dimensional electron delocalizationJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 13 2009Yang Wang Abstract A new series of unsaturated pure and boron-substituted hydrocarbons containing a perfect planar tetracoordinate carbon (ptC) have been proposed by performing density functional computations. The ptC is effectively stabilized through three-dimensional delocalization of ptC's lone pair into ,-conjugated systems, by utilizing a new strategy opening a brand new way of designing ptC structures. Compared to previously proposed ptC-containing hydrocarbon cage compound, a neutral hydrocarbon designed here might be a more viable target for synthetic attempts. © 2009 Wiley Periodicals, Inc. J Comput Chem 2009 [source] Molecular mechanics (MM4) study of amines,JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 15 2007Kuo-Hsiang Chen Abstract The MM4 force field has been extended to include aliphatic amines. About 20 amines have been examined to obtain a set of useful molecular mechanics parameters for this class. The vibrational spectra of seven amines (172 frequencies) calculated by MM4 have an overall rms error of 27 cm,1, compared with corresponding MM4 value of 24 cm,1 for alkanes. The rms and signed average errors of the moments of inertia of nine simple amines compared with the experimental data were 0.18% and ,0.004%, respectively. The heats of formation of 30 amines were also studied. The MM4 weighted standard deviation is 0.41 kcal/mol, compared with experiment. Electronegativity effects occur in the hydrocarbon portion of an amine from the nitrogen, and are accounted for by including electronegativity induced changes in bond lengths and angles, and induced dipole,dipole interactions in the molecule. Negative hyperconjugation results from the presence of the lone pair of electrons on nitrogen, and leads to the Bohlmann bands in the infrared, and also to strong and unusual geometric changes in the molecules (Bohlmann effect), all of which are fairly well accounted for. The conformational energies in amines appear to be less straightforward than those for most other classes of molecules, apparently because of the Bohlmann effect, and these are probably not yet completely understood. In general, the agreement between the MM4 calculated results and the available data is reasonably good. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [source] Anomeric effect plays a major role in the conformational isomerism of fluorinated pnictogen compoundsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2008Levindo E. Martins Abstract According to our theoretical studies, the anomeric effect, an stereoelectronic interaction between lone pair and a vicinal antibonding orbital, has shown to contribute decisively for the conformational isomerism of 1-fluoro- N,N -dimethylmethanamine (1) and of its corresponding P, As and Sb analogues (2,4). CX bonds in 2,4 are larger than in the parent compound 1, thus providing a LPX/CF* interaction progressively weaker on going from 1 to 4. However, such hyperconjugation contributed by more than 1.3,kcal,mol,1 for the stabilization of anti conformer in 4 (,LPXCF,=,180°), increasing to 24.1,kcal,mol,1 in 1. An isodesmic reaction model supported these findings. Copyright © 2008 John Wiley & Sons, Ltd. [source] Computational study of stereoelectronic effects in fluorinated alkylamines,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 11 2005Joseph J. Urban Abstract A systematic evaluation of the stereoelectronic effects in a series of fluorinated amine compounds has been carried out. A variety of computational methods have been employed, including molecular mechanics, semi-empirical, density functional and ab initio methods. An analysis of the natural bond orbital populations has been carried out and the effect of solvent was considered via the polarized continuum model (PCM). The results indicate a preference for conformations that result in an anti-periplanar arrangement between the nitrogen lone pair and the CX bond (XF or CF3). There are variations in geometry and natural bond orbital population, with conformation that are consistent with a stereoelectronic effect whereby electron density from the nitrogen lone pair is delocalized into the CX ,* orbital. Similar results are obtained for trifluoromethyl methylamines, although the magnitude of the stereoelectronic effect is attenuated resulting in greater expected conformational diversity for these compounds. The PCM results suggest that these preferences should persist in solution. Comparison among the computational methods used reveals that there is good agreement among the ab initio and density functional methods (at the HF/6-31,+,G(d), MP2/6-31,+,G(d), MP2/6-311,+,G(2d,p) and B3LYP/6-31,+,G(d) levels) and these methods agree well with the available experimental data for ethylmethylamine. Published in 2005 by John Wiley & Sons, Ltd. [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] Influence of substituents on the kinetics of epoxy/aromatic diamine resin systemsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 13 2004Heping Liu Abstract Eleven different epoxy/diamine systems, including tetraglycidyl-4,4,-diaminodiphenylmethane (TGDDM), triglycidyl p -aminophenol (TGAP), and diglycidyl ether of bisphenol A (DGEBA) with 4,4,-diaminodiphenylsulfone (DDS), diethyltoluenediamine (DETDA), dimethylthiotoluenediamine (DMTDA), and meta -phenylenediamine (m -PDA), were studied with near-infrared spectroscopy at different temperatures. The reactivities of the epoxies were determined and found to be in the following order when reacted with the same amine: DGEBA > TGAP > TGDDM. When the primary amine was reacted with the same epoxy, the order was DETDA > DDS > DMTDA; for the secondary amine, the order was DETDA > DMTDA > DDS. The relative reaction rates of the secondary amine to the primary amine were compared and discussed in terms of the structural differences and the corresponding substitution effect. It was concluded that the increase in the secondary amine reactivity of DETDA and DMTDA was caused by the deconjugation of the benzene-ring , electrons from the lone pair on the N atom. The overall order of the secondary amine relative reactivity was DMTDA > DETDA > DDS for the same epoxy and TGDDM > TGAP > DGEBA for the same amine. The m -PDA systems had no significant positive or negative substitution effects. Molecular orbital calculations were performed, and the results showed the most significant deconjugation effect in the secondary amine of DETDA. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3143,3156, 2004 [source] Investigations on 1,5-diaminoanthraquinone by laser excitationJOURNAL OF RAMAN SPECTROSCOPY, Issue 1 2003M. Umadevi Abstract Optical absorption and fluorescence emission spectra of 1,5-diaminoanthraquinone (1,5-DAAQ) in different solvents and the surface-enhanced Raman scattering (SERS) spectrum of 1,5-DAAQ in silver sol were investigated. Optical absorption and fluorescence emission studies revealed that the internal and external hydrogen bonds and reorientation of the solvent molecule around the excited state fluorophore are responsible for the observed bathochromic shift and the non-radiative decay process in polar solvents. SERS studies showed that 1,5-DAAQ molecules are chemisorbed through carbonyl groups and the lone pair of electrons on the nitrogen. The orientation of the molecule on the silver surface is found to be stand-on. Copyright © 2002 John Wiley & Sons, Ltd. [source] NMR spin-spin couplings involving nuclei in the neighborhood of a carbonyl group.MAGNETIC RESONANCE IN CHEMISTRY, Issue 3 2008-substituted acetamides, 3JCH couplings in Abstract In this work 3JCH spin-spin coupling constants (SSCCs) for the cis- and trans- conformers for ,-X-acetamides (X = F, Cl, Br and CN) (1,4) were studied in detail since they were found to be notably different for both conformers. These differences are rationalized as originating in the changes of the strong negative hyperconjugative interactions that take place within the carbonyl group. Such changes are found to depend not only on conformation, but also on solvent. For the cis -conformers there is a close proximity between the X-substituent and the in-plane oxygen lone pair of pure p character, which affects notably their respective negative hyperconjugative interactions. Both the efficiency for transmitting the Fermi contact (FC) term through the coupling pathway of 3JCH SSCCs and its potential as a probe to study the stereochemical properties of the XH2C group are discussed. Copyright © 2008 John Wiley & Sons, Ltd. [source] Nonempirical calculations of NMR indirect spin,spin coupling constantsMAGNETIC RESONANCE IN CHEMISTRY, Issue 2 2006-unsaturated aldehydes, : Azomethines of the, Part 1 Abstract Configurational assignment of seven azomethines obtained from the ,-functionally substituted and nonsubstituted ,,,-unsaturated aldehydes has been performed on the basis of experimental measurements and the high-level ab initio calculations of their 1J(C,C) and 1J(C,H), involving the ,-imino carbon that demonstrated the marked stereochemical dependence of both coupling constants upon the orientation of the nitrogen lone pair in the diverse isomers of the title azomethines. Copyright © 2005 John Wiley & Sons, Ltd. [source] Non-empirical calculations of NMR indirect carbon,carbon coupling constants.MAGNETIC RESONANCE IN CHEMISTRY, Issue 6 2005Aliphatic, Part 1, alicyclic oximes Abstract One-bond carbon,carbon coupling constants were calculated in a series of nine aliphatic and alicyclic oximes at the SOPPA (second-order polarization propagator approach) level in good agreement with the available experimental data, and several unknown couplings were predicted with high reliability. The experimental difference between J(C,C) of the corresponding carbon,carbon bonds in cis and trans orientations to the nitrogen lone pair is very well reproduced at the SOPPA level, and this provides an additional tool in the configurational assignment at the CN bond in oximes and related systems. Copyright © 2005 John Wiley & Sons, Ltd. [source] Computation and analysis of 19F substituent chemical shifts of some bridgehead-substituted polycyclic alkyl fluoridesMAGNETIC RESONANCE IN CHEMISTRY, Issue 7 2003William Adcock Abstract The 19F NMR shieldings for several remotely substituted rigid polycyclic alkyl fluorides with common sets of substituents covering a wide range of electronic effects were calculated using the DFT-GIAO theoretical model. The level of theory, B3LYP/6,311+G(2d,p), was chosen based on trial calculations which gave good agreement with experimental values where known. The optimized geometries were used to obtain various molecular parameters (fluorine natural charges, electron occupancies on fluorine of lone pairs and of the CF bond, and hybridization states) by means of natural bond orbital (NBO) analysis which could help in understanding electronic transmission mechanisms underlying 19F substituent chemical shifts (SCS) in these systems. Linear regression analysis was employed to explore the relationship between the calculated 19F SCS and polar substituent constants and also the NBO derived molecular parameters. The 19F SCS are best described by an electronegativity parameter. The most pertinent molecular parameters appear to be the occupation number of the NBO p-type fluorine lone pair and the occupation number of the CF antibonding orbital. This trend suggests that in these types of rigid saturated systems hyperconjugative interactions play a key role in determining the 19F SCS. Electrostatic field effects appear to be relatively unimportant. Copyright © 2003 John Wiley & Sons, Ltd. [source] Orientational ordering, tilting and lone-pair activity in the perovskite methylammonium tin bromide, CH3NH3SnBr3ACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2010Ian Swainson Synchrotron powder diffraction data from methylammonium tin bromide, CH3NH3SnBr3, taken as a function of temperature, reveal the existence of a phase between 230 and 188,K crystallizing in Pmc21, a = 5.8941,(2), b = 8.3862,(2), c = 8.2406,(2),Ĺ. Strong ferroelectric distortions of the octahedra, associated with stereochemical activity of the Sn 5s2 lone pair, are evident. A group analysis and decomposition of the distortion modes of the inorganic framework with respect to the cubic parent is given. The primary order parameters driving this upper transition appear to be an in-phase tilt (rotation) of the octahedra coupled to a ferroelectric mode. The precise nature of the lower-temperature phase remains uncertain, although it appears likely to be triclinic. Density-functional theory calculations on such a triclinic cell suggest that directional bonding of the amine group to the halide cage is coupled to the stereochemical activity of the Sn lone pair via the Br atoms, i.e. that the bonding from the organic component may have a strong effect on the inorganic sublattice (principally via switching the direction of the lone pair with little to no energy cost). [source] Amino and cyano N atoms in competitive situations: which is the best hydrogen-bond acceptor?ACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2001A crystallographic database investigation The relative hydrogen-bond acceptor abilities of amino and cyano N atoms have been investigated using data retrieved from the Cambridge Structural Database and via ab initio molecular orbital calculations. Surveys of the CSD for hydrogen bonds between HX (X = N, O) donors, N,T,C,N (push,pull nitriles) and N,(Csp3)n,C,N molecular fragments show that the hydrogen bonds are more abundant on the nitrile than on the amino nitrogen. In the push,pull family, in which T is a transmitter of resonance effects, the hydrogen-bonding ability of the cyano nitrogen is increased by conjugative interactions between the lone pair of the amino substituent and the C,N group: a clear example of resonance-assisted hydrogen bonding. The strength of the hydrogen-bonds on the cyano nitrogen in this family follows the experimental order of hydrogen-bond basicity, as observed in solution through the pKHB scale. The number of hydrogen bonds established on the amino nitrogen is greater for aliphatic aminonitriles N,(Csp3)n,C,N, but remains low. This behaviour reflects the greater sensitivity of the amino nitrogen to steric hindrance and the electron-withdrawing inductive effect compared with the cyano nitrogen. Ab initio molecular orbital calculations (B3LYP/6-31+G** level) of electrostatic potentials on the molecular surface around each nitrogen confirm the experimental observations. [source] Preferred conformations in the solid state of some ,-(p -phenylsulfinyl)- p -substituted acetophenonesACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2000Paulo R. Olivato Information on the geometrical structures of ,-(p -phenylsulfinyl)- p -substituted acetophenones X,PhC(O)CH2S(O)Ph,Y [X = OMe, Y = H (1); X = NO2, Y = OMe (2); X = OMe, Y = NO2 (3); IUPAC names: (1) 4-methoxyphenyl phenylsulfinylmethyl ketone; (2) 4-nitrophenyl 4-methoxyphenylsulfinylmethyl ketone; (3) 4-methoxyphenyl 4-nitrophenylsulfinylmethyl ketone] have been obtained from X-ray diffraction analyses. A comparison of these results with those previously obtained from X-ray diffraction and ab initio computations of ,-methylsulfinylacetophenone, PhC(O)CH2S(O)Me, indicated that (1) and (2) adopt in the crystal a cis1 conformation and (3) assumes a quasi-gauche geometry. The stabilization of these conformations in the crystal is discussed in terms of the dipole moment coupling, Coulombic and intramolecular charge transfer interactions between the oppositely charged atoms of the C=O and S=O dipoles. The p -substituted benzene ring is quasi -coplanar with the sulfinyl group for (1) and (3), but is quasi -perpendicular for (2). Conjugation and repulsion between the sulfinyl sulfur lone pair and the ,-benzene ring seem to be responsible for the observed geometries. [source] Anomeric effect and hydrogen-bonded supramolecular motif in 5-(3-fluoro-4-methoxyphenyl)-1-[(3-fluoro-4-methoxyphenyl)aminomethyl]-1,3,5-triazinane-2-thioneACTA CRYSTALLOGRAPHICA SECTION C, Issue 11 2009Zhenfeng Zhang In the title compound, C18H20F2N4O2S, the triazinane-2-thione ring adopts an envelope conformation, the ring substituents lie on the same side of the mean plane of the heterocyclic ring and the exo lp,N,C,Ntriaz unit (lp is a lone pair and triaz is the triazinane ring) exhibits an antiperiplanar orientation, which is shown to be governed by strong anomeric effects. Molecules are linked into a complex three-dimensional framework by a combination of two N,H...S hydrogen bonds, three C,H...F hydrogen bonds and a ,,, stacking interaction. [source] GeTe2O6, a germanium tellurate(IV) with an open frameworkACTA CRYSTALLOGRAPHICA SECTION C, Issue 5 2009Nefla Jennene Boukharrata The structure of an already evidenced but still uncharacterized GeTe2O6 phase consists of isolated GeO6 octahedra connected via isolated TeO3 units. The germanium cations occupy a site with symmetry. The Te and O atoms are in general positions of the P21/n space group. This structure corresponds to a new type of tetravalent tellurate and is different from other AB2X6 structures in which the B cation presents a stereochemically active electronic lone pair. It derives from the pseudo-hexagonal MI2O6 (M = Mg, Mn, Co and Fe) type by a strong monoclinic distortion caused by the much smaller size of Ge4+ compared with the divalent M cations. [source] Sodium iodine(V) oxyfluoride, NaIO2F2ACTA CRYSTALLOGRAPHICA SECTION C, Issue 7 2008Jean-Paul Laval As an extension of a general structural study concerning fluorides and oxyfluorides of cations presenting a stereochemically active electronic lone pair, until now limited to tellurium(IV) phases, the previously unknown structure of NaIO2F2 corresponds to a new structure type based on isolated IO2F2, polyhedra forming sheets separated by Na+ layers. The sodium ion is octahedrally coordinated with 2/m site symmetry, while the IV atom has m2m symmetry with a stereochemically active lone electron pair. The O and F atoms (both with m symmetry) are bonded to the IV atoms in a fully ordered manner. A comparison with the structure of ferroelastic KIO2F2 and with structures based on hexagonal close packing of anions, mainly rutile-type and FeTeO3F-type, reveals differences that are attributed to the smaller ionic radius of Na+ and the ordering of the Na and I cations. [source] New oxyfluorotellurates(IV): MTeO3F (M = FeIII, GaIII and CrIII)ACTA CRYSTALLOGRAPHICA SECTION C, Issue 2 2008Jean Paul Laval The crystal structures of the new isomorphous compounds iron(III) oxyfluorotellurate(IV), FeTeO3F, gallium(III) oxyfluorotellurate(IV), GaTeO3F, and chromium(III) oxyfluorotellurate(IV), CrTeO3F, consist of zigzag chains of MO4F2 distorted octahedra alternately sharing O,O and F,F edges and connected via TeO3 trigonal pyramids. A full O/F anionic ordering is observed and the electronic lone pair of the TeIV cation is stereochemically active. [source] | |||||||||||||||||||||||||||||||