Headspace Solid-Phase Micro-Extraction Versus Hydrodistillation of Volatile Compounds from Leaves of Cultivated Mentha Taxa: Markers of Safe Chemotypes

Various mint taxa are widely cultivated and are used not only for medicinal purposes but also in cosmetic and industrial applications. The development of new varieties or cultivars of mint generates difficulties in their correct identification and safe use. Volatile organic compounds (VOCs) from the leaves of seven different taxa of the genus Mentha obtained by hydrodistillation (HD) and headspace solid-phase microextraction (HS-SPME) were analyzed using gas chromatography–mass spectrometry (GC-MS). Principal component analysis (PCA) was also performed. Comparative GC-MS analysis of the obtained extracts showed similarity in the major compounds. PCA data allowed the separation of two groups of chemotypes among the analyzed mints, characterized by the abundance of piperitenone oxide and carvone. Two out of seven analyzed taxa were not previously examined for VOC profile, one was examined only for patent application purposes, and six out of seven were investigated for the first time using the HS-SPME technique. The presented analysis provides new data on the abundance and qualitative characterization of VOCs in the studied mint plants and on the safety of their use, related to the possibility of the presence of potentially toxic components. HS-SPME is a valuable method to extend the characterization of the VOC profile obtained by hydrodistillation.     

Red clover (<Emphasis Type=Italic>Trifolium pratense</Emphasis> L.) honey: volatiles chemical-profiling and unlocking antioxidant and anticorrosion capacity

Headspace solid-phase micro-extraction (HS-SPME) and ultrasonic solvent extraction (USE) were used for red clover honey volatiles extraction. The extracts were analysed using gas chromatography and mass spectrometry (GC-MS). Lilac aldehyde isomers dominated in the headspace (individual range from 7.6 % to 21.4 %) followed by phenylacetaldehyde (10.1–31.2 %) and benzaldehyde (7.0–15.7 %). Higher aliphatic alcohols and hydrocarbons were the predominant constituents of the honey extracts. The honey and its extracts exhibited rather weak anti-radical activity (DPPH assay) and total antioxidant activity (FRAP assay). On the other hand, the honey’s inhibitive properties towards the corrosion of AA 2017A alloy in NaCl solution (potentiodynamic polarisation and potentiostatic pulse measurements) revealed the honey to be a very good anodic inhibitor (efficiency up to 76 %) while the honey extracts (USE) showed better inhibition efficacy.     

Direct measurement of the twist penetration length in a single smectic A layer of colloidal virus particles

In the 1970s, deGennes discussed the fundamental geometry of smectic liquid crystals and established an analogy between the smectic A phase and superconductors. It follows that smectic layers expel twist deformations in the same way that superconductors expel magnetic field. We make a direct observation of the penetration of twist at the edge of a single isolated smectic A layer composed of chiral fd virus particles subjected to a depletion interaction. Using the LC-PolScope, we make quantitative measurements of the spatial dependence of the birefringence due to molecular tilt near the layer edges. We match data to theory for the molecular tilt penetration profile and determine the twist penetration length for this system.     

Automerization reaction of cyclobutadiene and its barrier height: an ab initio benchmark multireference average-quadratic coupled cluster study

The problem of the double bond flipping interconversion of the two equivalent ground state structures of cyclobutadiene (CBD) is addressed at the multireference average-quadratic coupled cluster level of theory, which is capable of optimizing the structural parameters of the ground, transition, and excited states on an equal footing. The barrier height involving both the electronic and zero-point vibrational energy contributions is 6.3 kcal mol(-1), which is higher than the best earlier theoretical estimate of 4.0 kcal mol(-1). This result is confirmed by including into the reference space the orbitals of the CC sigma bonds beyond the standard pi orbital space. It places the present value into the middle of the range of the measured data (1.6-10 kcal mol(-1)). An adiabatic singlet-triplet energy gap of 7.4 kcal mol(-1) between the transition state (1)B(tg) and the first triplet (3)A(2g) state is obtained. A low barrier height for the CBD automerization and a small DeltaE((3)A(2g),(1)B(1g)) gap bear some relevance on the highly pronounced reactivity of CBD, which is briefly discussed.     

Finite p-groups all of whose cyclic subgroups A, Bwith A ∩ B ≠ {1} generate an abelian group

We characterize the title p-groups for all primes p.     

Copolymers of Bromine-Containing Monomers. 5. Terpolymerization of Acryionitrile,Styrene, and Pentabromophenyl Acrylate

The terpolymerization of acryionitrile, styrene, and pentabromophenyl acrylate in dimethylformamide solution was investigated. Experimental terpolymerization data agreed well with calculations using the Alfrey-Goldfinger equation. The relationship between monomer feed and terpolymer compositions are presented on triangular coordinate graphs, and the lines of unique and the lines of binary azeotropic composition were identified. No point of true ternary azeotropic composition was found but a “pseudoazeotropic” region was identified. The experimental results of the terpolymerization agreed well with the theoretical curves over a wide range of monomer composition up to high conversions. The influence of pentabromophenyl acrylate units on the thermal and flammability characteristics of the terpolymers are described.     

The proton affinity of the superbase 1,8-bis(tetramethylguanidino)naphthalene (TMGN) and some related compounds: a theoretical study

The spatial and electronic structure of the very strong neutral organic bases bis(tetramethylguanidino)naphthalene (TMGN), 4,5-bis(tetramethylguanidino)fluorene (TMGF) and some related compounds are explored by ab initio computational methods. Their affinity towards the proton is scrutinized both in the gas phase and in solution in acetonitrile. The protonation at the most basic center (the imine nitrogen) yields asymmetric and relatively strong intramolecular hydrogen bonds (IHB). It is found that the angular strain effect and steric repulsion practically vanish in TMGN which implies that its high absolute proton affinity (APA) has its origin in the inherent basicity of the guanidine fragment and a relatively strong IHB in [TMGN]H(+). The nonbonded repulsions in TMGF are higher than in TMGN, which in conjunction with a slightly stronger IHB in the corresponding conjugate acid makes it more basic: APA(TMGF)>APA(TMGN). An interesting new phenomenon is observed in both TMGN and TMGF: the proton triggers the resonance stabilization not only in the directly bonded guanidine moiety, but also in the other guanidine fragment which is more distant from the proton, albeit in a less pronounced manner. The latter feature is termed a partial protonation. This supports the hydrogen bonding and contributes to the IHB stabilization. Convincing evidence is presented that the solvent effect in acetonitrile is determined by two antagonistic factors: 1) the intrinsic (gas phase) proton affinity and 2) the size effect which is given by the ratio between the positive charge in molecular cation (conjugate acid) and the magnitude of the molecular surface. The resulting pK(a) values are given by an interplay of these factors.     

In search of ultrastrong Brønsted neutral organic superacids: a DFT study on some cyclopentadiene derivatives

An efficient but reasonably accurate B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) computational procedure showed that pentasubstituted cyclopentadienes such as (CN)5C5H, (NO2)5C5H, and (NC)5C5H containing strongly electron-withdrawing groups are neutral organic superacids of unprecedented strength. The boldface denotes the atom attached to the cyclopentadiene framework. All of them exhibit prototropic tautomerism by forming somewhat more stable structures with C=NH, NO2H, and N=CH exocyclic fragments, respectively. The acidity (DeltaH(acid)) of these is lower, but only to a rather small extent. The DeltaH(acid) enthalpies of these last three tautomers are estimated to be 271, 276, and 282 kcal mol(-1), respectively. Hence, the most stable tautomers of (CN)5C5H and (NC)5C5H represent a legitimate target for synthetic chemists. On the other hand, (NO2)5C5H is less suitable for practical applications, because of its high energy density. The origin of the highly pronounced acidity of these compounds was analyzed by using the recently developed triadic formula. It is found that very high Koopmans' ionization energy (IE)n(Koop) of conjugate bases exerts a decisive influence on acidity. It follows as a corollary that the overwhelming effect leading to very high acidity is due to the properties of the final state. An alternative picture is offered by homodesmotic reactions, wherein the cyclic systems are compared with their linear counterparts. It is found that the acidity of cyclopentadiene (CP) is a consequence of aromatic stabilization in the CP- anion. The extreme acidity of pentacyanocyclopentadiene (CN)5C5H is due to aromatization of the five-membered ring and a strong anionic resonance effect in the resultant conjugate base. The neutral organic superacids predicted by the present calculations may help to bridge the gap between existing very strong acids and bases.