1hJFH coupling in 2-fluorophenol revisited: is intramolecular hydrogen bond responsible for this long-range coupling?

The present study shows that a hydrogen bond between the OH group and the fluorine atom is not involved in the (1h)J(FH) spin-spin coupling transmission either for 4-bromo-2-fluorophenol or 2-fluorophenol. In fact, according to a quantum theory of atoms in molecules analysis, no bond critical point is found between O-H and F moieties. The nature of the transmission mechanism of the Fermi contact term of the (1h)J(FH) spin-spin coupling is studied by analyzing canonical molecular orbitals (see J. Phys. Chem. A 2010, 114, 1044), and it is observed that virtual orbitals play only a quite minor role in its transmission. This is typical of a Fermi contact term transmitted mainly through exchange interactions owing to the overlap of proximate electronic clouds; therefore, it is suggested to identify them as (nTS)J(FH) coupling where n stands for the number of formal bonds separating the coupling nuclei. In the cases studied in this work is n = 4. Results presented in this work could provide an interesting rationalization for different experimental signs known in the current literature for proximate J(FH) couplings.

Conformational analysis of small molecules: NMR and quantum mechanics calculations

This review deals with conformational analysis in small organic molecules, and describes the stereoelectronic interactions responsible for conformational stability. Conformational analysis is usually performed using NMR spectroscopy through measurement of coupling constants at room or low temperature in different solvents to determine the populations of conformers in solution. Quantum mechanical calculations are used to address the interactions responsible for conformer stability. The conformational analysis of a large number of small molecules is described, using coupling constant measurements in different solvents and at low temperature, as well as recent applications of through-space and through-hydrogen bond coupling constants JFH as tools for the conformational analysis of fluorinated molecules. Besides NMR parameters, stereoelectronic interactions such as conjugative, hyperconjugative, steric and intramolecular hydrogen bond interactions involved in conformational preferences are discussed.

Achieving regio- and stereo-control in the fluorination of aziridines under acidic conditions

We developed an efficient fluorination protocol that converts easily accessible aziridines into β-fluoroamines, which are important motifs in biologically active molecules. In contrast with traditional fluorination approaches, DMPU-HF has shown both higher reactivity and regioselectivity and good functional group tolerance; thus, a wide scope of β-fluoroamines can now be accessed conveniently. The stereochemical behavior of the ring opening depends on the substitution pattern of the aziridine substrate.

The unexpected roles of σ and π orbitals in electron donor and acceptor group effects on the 13C NMR chemical shifts in substituted benzenes

NH₂ and NO₂ group effects on ¹³C NMR chemical shifts in substituted benzenes are explained by σ- instead of π-orbitals.

Effects of electron-donating (R = NH₂) and electron-withdrawing (R = NO₂) groups on ¹³C NMR chemical shifts in R-substituted benzene are investigated by molecular orbital analyses. The ¹³C shift substituent effect in ortho, meta, and para position is determined by the σ bonding orbitals in the aryl ring. The π orbitals do not explain the substituent effects in the NMR spectrum as conventionally suggested in textbooks. The familiar electron donating and withdrawing effects on the π system by NH₂ and NO₂ substituents induce changes in the σ orbital framework, and the ¹³C chemical shifts follow the trends induced in the σ orbitals. There is an implicit dependence of the σ orbital NMR shift contributions on the π framework, via unoccupied π* orbitals, due to the fact that the nuclear shielding is a response property.

Conformational preferences for some 5-substituted 2-acetylthiophenes through infrared spectroscopy and theoretical calculations

The s-cis-trans isomerisms of some derivatives of thiophene (2-acetyl, AT; 2-acetyl-5-bromo, ABT and 2-acetyl-5-chloro, ACT) were analyzed, using data from deconvolution of their carbonyl absorption bands in two solvents (CCl4 and CHCl3). These infrared data showed that the O,S-cis conformer largely predominates in the studied solvents and that the same occurs in the gas phase, as observed from theoretical calculations. The latter results were obtained using B3LYP/6-311++G(3df,3p) and MP2/6-311++G(3df,3p) levels of theory, with zero-point energy correction. Moreover, the use of the IEFPCM (Integral Equation Formalism Polarizable Continuum Model) to take into account the solvent effects, using the same levels of theory, confirmed the results observed from infrared data. Low temperature 13C NMR spectra in CS2/CD2Cl2 (-90 °C) and in acetone-d6 (-80°C) did not show pairs of signals for each carbon, due to the known low energy barrier (∼8 kcal mol(-1)) for the cis-trans interconversion. Data from NBO calculations show that the nO(2)→σS-C5* and nO(2)→σC2-C3* interactions occur only in the O,S-cis isomer and can explain its conformational preference.

Trends of intramolecular hydrogen bonding in substituted alcohols: a deeper investigation

NBO analyses were used to perform a deeper investigation on the effects that drive IAHB strength in substituted acyclic alcohols.

The halogen effect on the 13C NMR chemical shift in substituted benzenes

X (F, Cl, Br, I) and R (NH₂, NO₂) group effects on ¹³C NMR chemical shifts are explained by π and σ orbitals, respectively.

Dealing with supramolecular structure for ionic liquids: a DOSY NMR approach

Diffusion-ordered spectroscopy (DOSY) is a powerful method for the NMR analysis of ionic liquids. Thus the dynamic-structural behaviour of imidazolium ionic liquids has been investigated by measurements of direct ¹H diffusion coefficients in different solvents.

Computational evidence for intramolecular hydrogen bonding and nonbonding X···O interactions in 2'-haloflavonols

The conformational isomerism and stereoelectronic interactions present in 2'-haloflavonols were computationally analyzed. On the basis of the quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analysis, the conformer stabilities of 2'-haloflavonols were found to be dictated mainly by a C=O···H-O intramolecular hydrogen bond, but an unusual C-F···H-O hydrogen-bond and intramolecular C-X···O nonbonding interactions are also present in such compounds.

Monoamine oxidase inhibitory activities of indolylalkaloid toxins from the venom of the colonial spider Parawixia bistriata: functional characterization of PwTX-I

Colonial spiders evolved a differential prey-capture behaviour in concert with their venom chemistry, which may be a source of novel drugs. Some highly active tetrahydro-beta-carboline (THbetaC) toxins were recently isolated from the venom of the colonial spider Parawixia bistriata; the spiders use these toxins as part of their chemical arsenal to kill and/or paralyze preys. The major THbetaC compound isolated from this venom was identified as 6-hydroxytrypargine, also known as PwTX-I. Most natural compounds of animal origin occur in low abundance, and the natural abundance of PwTX-I is insufficient for complete functional characterization. Thus, PwTx-I was synthesized using a Pictet-Spengler condensation strategy, and the stereoisomers of the synthetic toxin were separated by chiral chromatography. The fraction of venom containing a mixture of three natural THbetaC toxins and enantiomers of PwTx-I were analyzed for inhibition of monoamine oxidase (MAO)-A and -B and for toxicity to insects. We reveal that the mixture of the natural THbetaC toxins, as well as the enantiomers of PwTx-I, were non-competitive inhibitors of MAO-A and MAO-B and caused potent paralysis of honeybees. The (-)-PwTX-I enantiomer is 2-fold more potent than the (+)-PwTX-I enantiomer in the assays performed.

The Venomous Secrets of the Web Droplets from the Viscid Spiral of the Orb-Weaver SpiderNephila clavipes (Araneae, Tetragnatidae)

The capture web of N. clavipes presents viscous droplets, which play important roles in web mechanics and prey capture. By using scanning and transmission electron microscopy, it was demonstrated that the web droplets are constituted of different chemical environments, provided by the existence both of an aqueous and a lipid layer, which, in turn, present a suspension of tenths of vesicles containing polypeptides and/or lipids. GC/EI-MS Analysis of the contents of these vesicles led to the identification of some saturated fatty acids, such as decanoic acid, undecanoic acid, dodecanoic acid, tetradecanoic acid, octadecanoic acid, and icosanoic acid, while other components were unsaturated fatty acids, such as (Z)-tetradec-9-enoic acid, (Z)-octadec-9-enoic acid, and (Z)-icosa-11-enoic acid; and polyunsaturated fatty acids like (9Z,12Z)-octadeca-9,12-dienoic acid, (9Z,12Z,15Z)-octadeca-9,12,15-trienoic acid, and (11Z,14Z)-icosa-11,14-dienoic acid. Toxic proteins such as calcium-activated proteinase and metalloproteinase jararhagin-like precursor were also identified by using a proteomic approach, indicating the possible involvement of these enzymes in the pre-digestion of spiders' preys web-captured. Apparently, the mixture of fatty acids are relatively toxic to insects by topical application (LD50 64.3+/-7.6 ng mg(-1) honeybee), while the proteins alone present no topical effect; however, when injected into the prey-insects, these proteins presented a moderate toxicity (LD50 40.3+/-4.8 ng mg(-1) honeybee); the mixture of fatty acids and proteins is very toxic to the preys captured by the web droplets of the viscid spiral of Nephila clavipes when topically applied on them (LD50 14.3+/-1.8 ng mg(-1) honeybee).