The physical nature of the lone pair

Despite the large number of experimental and theoretical studies on the size, shape, and orientation of lone pairs and their resulting stereochemical character, lone pairs still remain poorly defined in terms of quantitative observable properties of a molecule. Using the conformation of saturated molecules and barriers to internal rotation, experimental chemists have arrived at conflicting sizes and orientations for lone pairs. Most theoretical attempts to define lone pair properties have centered on such non-observables as localized molecular orbitals or have been based on studies on isolated molecules.The use of observable properties to construct a consistent set of physical models to analyze the physical nature of lone pairs is discussed. Much as one probes an electric field with a test charge, probes such as H⁺, H^–, He and H could be used to probe regions of molecules such as NH₃ and H₂O where lone pairs are often postulated to exist.Ab initio quantum mechanical studies can be analyzed using electron density (and resulting changes during interaction), total pair density of electrons, the electrostatic potential about the molecule and bond energy analysis to study lone pair properties. A simple study of NH₃ using an H⁺ probe is presented to clarify the approach.     

A prediction of the three-dimensional structure of maize NADP+-dependent malate dehydrogenase which explains aspects of light-dependent regulation unique to plant enzymes

Summary A model has been built for the plant NADP-malate dehydrogenase from Zea mays, a key enzyme in photosynthesis, which undergoes light-dependent regulation. The model was based on sequence and presumed structural homology to the known three-dimensional structure of mammalian porcine cytosolic NAD-malate dehydrogenase. A cystine-loop present in an extended C-terminal region of plant NADP-malate dehydrogenases was modelled using molecular mechanics and computer graphical methods, based on the assumption that a disulphide bridge exists in the inactive form of the enzyme between Cys³⁵¹ and Cys³⁶³. The predicted conformation of the intact C-terminal cystine-loop suggests that the extended polypeptide will bind in the active centre and inhibit enzyme activity. Another ionizable cysteine residue in the active site is predicted to control the charge of the catalytic His²¹⁵ and might be responsible for the uniquely tight binding of the positively charged nicotinamide ring of NADP⁺ in this and other C4 and C3 plant NADP-malate dehydrogenases.     

Basis set dependence of localized orbitals

The effects of Gaussian basis set contraction and addition of polarization functions on H₂O localized orbitals have been studied at the experimental geometry. It is shown that the electric moments and moment features of localized orbitals are not influenced very much by basis set quality variations, as going from medium size to enlarged basis sets. The difference between bond pair and lone pair charge densities was found to be larger on approaching the Hartree-Fock limit. A minimal basis set, however, does not suitably characterize the localized charge distributions.     

Synthesis ofN-alkylamides of 3-fluoroalkylaziridine-2-carboxylic acids

The N-alkylamides of 3-fluoroalkylaziridine-2-carboxylic acids have been prepared for the first time by the interaction of ethyl 0-fltioroalkyl-,-dibromocarboxylates with primary amines.Prepared by a known procedure;² characteristics of previously unknown ester1b are given in the Experimental section.     

Synthesis and Dehydrohalogenation of 3 β-Chloro-5 α,7 α-dibromo-6-ketosteroids of the Stigmastane and Cholestane Series

3-Chloro-5,7-dibromo-6-ketosteroids 5a and 5b are synthesized from -sitosterol (1a) and cholesterol (1b). Dehydrohalogenation of these forms 7-bromo-2,4-dien-6-ones (6a-b), 2,4-dien-6-ones (7a-b), and 14-hydroperoxy-2,4,7-trien-6-ones (8a-b). Woodward hydroxylation of dienone 6a produces 2-iodo-7-bromo-3-acetoxy-⁴-6-ketone 9 and 7-bromo-2,3-diacetoxy-⁴-6-ketone 10. 2-Iodo-3-acetoxy-^4,7,14-trien-6-one 11 is prepared analogously from trienone 8a.     

1H,13C and19FNMR studies on the structure of the intramolecularly hydrogen bondedcis-enols of 2-trifluoroacetylcycloalkanones

Summary The¹H,¹³C and¹⁹F NMR spectra of some 2-trifluoroacetylcycloalkanones comprising five-, six-, seven-, and eight-membered ring systems have been studied. These systems have been shown by¹⁹F NMR spectroscopy to be over 90% enolized. The effects of ring size on the¹H,¹³C and¹⁹F chemical shifts and¹³C-¹⁹F coupling constants are discussed. The direction of enolization in these systems was investigated by¹H,¹³C and¹⁹F NMR spectroscopy and evidences were presented in favour of a dominant exocyclic enol form in the five- and an endocyclic enol form in the six-, seven- and eight-membered ring systems.

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¹H-,¹³C- und¹⁹F-NMR-Untersuchungen zur Struktur der intramolekularen Wasserstoffbindung in dencis-Enolen von 2-Trifluoracetylcycloalkanonen

Zusammenfassung Die¹H-,¹³C- und¹⁹F-NMR-Spektren einiger 2-Trifluoracetylcycloalkanone mit fünf-, sechs-, sieben- und achtgliedrigen Ringsystemen wurden untersucht. Mittels¹⁹F-NMR konnte über 90% Enolisierung nachgewiesen werden. Der Effekt der Ringgröße auf die chemischen Verschiebungen in den¹H-,¹³C- und¹⁹F-NMR-Spektren und auf die¹³C-¹⁹F-Kopplungskonstanten werden diskutiert. Die Enolisierungsrichtung dieser Systeme wurde unter Einsatz der NMR-Spektren untersucht: dabei ergab sich eine dominierende exocyclische Enolform für die Fünfringe und eine endocyclische Enolform für die sechs-, sieben- und achtgliedrigen Ringe.

     

Nucleophilic addition of methanol and methanethiol to acetylene in the superbasic system KOH-DMSO: a quantum chemical model

Cluster-continuum models (KOH·nDMSO, n = 1, 5) were used to model the superbasic system “alkali metal hydroxide-dimethyl sulfoxide” within the framework of MP2/6-311++G**/ and B3LYP/6-31G* methods. The KOH molecule surrounded by five DMSO molecules exists as “solvate-loosened” ion pair with elongated K-O distance. It is proposed to consider the “solvate-loosened” ion pair of potassium cation with hydroxide anion in the surroundings of five solvent molecules as the catalytic coordination sphere of the superbasic system KOH-DMSO. Methanol and methanethiol molecules can be incorporated with ease into the first coordination sphere of potassium cation to form methoxide and methanethiolate ions. The possibility of nucleophilic attack of methoxide and methanethiolate ions on acetylene molecule in the first coordination sphere of potassium cation was studied. The model reaction system C₂H₂-CH₃OK-H₂O with one DMSO molecule included explicitly to maintain the “solvate-loosened” [CH₃O]^?...K⁺ ion pair and additional inclusion of solvent effects within the framework of the IEFPCM continuum model is the most preferable for serial calculations.     

Non-Enzymatic Activation of Molecular Nitrogen

Several transition metal complexes that can absorb nitrogen from the gas phase are now known. Some of the N₂-metal complexes are stable enough to be isolated and their structure elucidated, the N₂ molecule remaining chemically inert. In other cases reduction to N^3? is possible, but the structure of the reactive intermediate N₂-metal complex can be approached only by mechanistic studies. In the stable complexes the nitrogen is bound via a lone pair of electrons in the direction of the molecular axis (“end-on”), in the reducible complexes possibly “edge-on”.     

Ab initio calculations including electron correlation for the minimum energy path of the (1A1) CH2+(1Σ)H2→ (1A1) CH4 insertion reaction

Ab initio calculations on the SCF level and with the inclusion of valence shell electron correlation in the IEPA–PNO (independent electron pair approximation with pair natural orbitals), the PNO–CI (pair-natural-orbital configuration interaction) and the CEPA–PNO (coupled electron pair approximation with pair natural orbitals) schemes with Gaussian lobe functions of “double zeta quality” have been performed for the minimum energy path of the insertion of singlet (¹A₁) methylene to the (¹Σ)H₂ molecule to yield methane. The energy was minimized on the SCF level to all geometrical parameters for various values of the “approximate” reaction coordinate. The energy along the reaction path decreases monotonically without a barrier and the curves representing the total energy of the system as a function of approximate reaction coordinates obtained at different levels of approximations have the same shape. From the physical point of view three phases of the reaction can be distinguished (chemically two steps) with different geometrical arrangements and different internal geometries of the partners.     

Non-additivity in water-ion-water interactions

The three-body system Li⁺(H₂O)₂ was analyzed to study that non-additive part of the interaction potential which can be obtained by the Hartree-Fock approximation.For long and intermediate distances the three-body correction was found to be well represented by the induction energy, where bond dipoles are induced on each water molecule by point charges located on the (unpolarizable) lithium ion and on the other molecule respectively: for shorter distances this approximation was corrected by means of an exponential repulsive term. Such a potential model for non-additive interactions was extended to the more general situation Li⁺(H₂O)_n, and Monte-Carlo calculations were carried out on clusters containing up to six water molecules; comparison with other simulation results and with available data showed a significantly improved agreement with experiment. Tentative values for H are presented for n =7, 8,..., 20, where experimental data are not available.     

Optimization of a sensitive method for the “switch-on” determination of mercury(II) in waters using Rhodamine B capped gold nanoparticles as a fluorescence sensor

Monodisperse and “naked” gold nanoparticles (GNPs) were modified with thioglycolic acid (TGA). The fluorescence of rhodamine B (RB) is quenched completely by the gold NPs surface with negative charge mainly as a result of fluorescence resonance energy transition (FRET) and collision. The quenching mechanism can be described by a Langmuir isotherm, which was systematically investigated by steady-state fluorescence spectrometry and absorption spectrometry. Hg(II) ion disrupts the GNPs–RB pair, producing a large “switch-on” fluorescence. A low background, highly sensitive and reproducible fluorescence assay for Hg(II) is presented. Under the optimum conditions, the restoration fluorescence intensity is proportional to the concentration of Hg(II). The calibration graphs are linear over the range of 1.0?×?10^?9 to 3.1?×?10^?8 mol L^?1 with a detection limit of 4.0?×?10^?10 mol L^?1. The relative standard deviation was 1.2% for a 5.0?×?10^?9 mol L^?1 Hg(II) solution (N?=?6). This method was applied to the analysis of Hg(II) in environmental water samples, and the results were consistent with those of atomic absorption spectroscopy (AAS).     

Chemie der Pleuromutiline, 11. Mitt.: Konfigurationsumkehr der Vinylgruppe am Kohlenstoff 12 durch reversible Retro-En-Spaltung

The reaction of pleuromutilin-22-0-tosylat(2) withEtZnI afforded a mixture of the 12-epimeric compounds2 and3. A reversible retro-en-reaction of the homoallylic system is discussed as a possible mechanistic pathway. The structure assignment is based on¹H- and¹³C-NMR spectroscopic data.

Für Verbindungen dieser Substanzklasse, die auf ihre antibakterielle Aktivität geprüft werden, ist erfahrungsgemäß eine basisch substituierte Acylgruppierung in Position 14 erforderlich⁴. Heterocyclische Acylgruppierungen erwiesen sich dabei als besonders wirkungsvoll.     

Kinetic method for the study of xylan hydrolysis by xylan hydrolases

Summary The Somogyi-Nelson colorimetric method was used in a new manner more suitable for evaluating the kinetics of the enzyme hydrolysis of xylan catalyzed by xylan hydrolases. The values of the Michaelis parameters (Km=5.56 g l^–1 andV=2.94 · 10^–5 M s^–1) were determined.

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Eine kinetische Methode zur Untersuchung der Hydrolyse von Xylan durch Xylan-Hydrolase

Zusammenfassung Die kolorimetrische Methode nach Somogyi-Nelson wurde nach einem neuen Verfahren zur Verfolgung der Kinetik der hydrolytischen Spaltung von Xylan, katalysiert durch Xylan-Hydrolasen vonAspergillus oryzae, angewandt. Es wurden die Michaelis-Parameter (Km=5.56 g l^–1 undV=2.94 · 10^–5 M s^–1) bestimmt.

     

The isomeric structures of SiH2LiF

The structures of four isomeric forms of the model “silylenoid” SiH₂LiF have been investigated by ab initio molecular orbital theory. The two most stable forms are suggested to be the SiH₂Li⁺F^? ion pair and the H₂Si : FLi complex, analogous to the similar structures previously found for carbenoids. Two further species, a H₂Si : LiF complex and the “classical” (tetrahedral) SiH₂LiF, are also local minima on the potential energy surface, but are higher in energy.     

Crystal Structure of Ammonium(2.2.2‐cryptand) Thiocyanate Monohydrate

Crystals of the ammonium (2.2.2cryptand) thiocyanate monohydrate complex, [NH₄(2.2.2Crypt)]⁺ × SCN^- · H₂O, were studied by Xray structural analysis: space group P2/c, a = 11.303(2)*angst;, b = 8.313(1), c = 14.392(3), = 110.39(2)°, Z = 2, 3680 independent measured reflections, R = 0.064. This complex is of the guest–host type: the cryptand ligand cavity contains the NH₄ ⁺ cation with statistically disordered H atoms forming H bonds with each of its O and N heteroatoms. The SCN^- anions and H₂O molecules are linked by H bonds to form chains that are infinite along the z axis.     

Some opinions of an innocent bystander regarding the Hofmeister series

Hofmeister's “water absorbing effect” of ions is quantified. A clear definition is provided for the term “hydration number” as the average number of water molecules bound to the solute more strongly (by at least 13.3 kcal mol^− 1) than are bound to other waters. Such hydration numbers are reported for various ions of electrolytes along with the extent of ion pair formation of each salt. Experimental measurements of various colligative properties demonstrate that the hydration numbers and extents of ion pair formation remain constant over large ranges of concentration and that solutions behave “ideally” up to the point where about 50% of the total water is strongly bound to the solute and is not available to act as solvent for additional solute.     

On the affinity of cytosine towards electrophiles

Ab initio SCF computations on the intrinsic preferences of the H⁺, CH ₃ ⁺ and C₂H ₅ ⁺ cations towards the two principal sites of protonation or alkylation on cytosine, N₃ or O₂, show that this preference undergoes a continuous modification with the increase in size and complexity of the cation. N₃ is the preferred site of fixation of H⁺, O₂ the preferred site of C₂H ₅ ⁺ , while CH ₃ ⁺ has no marked preference. The exchange repulsion term of the binding energy appears responsible for the preference of C₂H ₅ ⁺ for O₂.This work was supported by the Ligue Francaise contre le Cancer and the National Foundation for Cancer Research (USA)