Molecular interpretation of Trouton’s and Hildebrand’s rules for the entropy of vaporization of a liquid

The entropy of vaporization at a liquid’s boiling point is well approximated by Trouton’s rule and even more accurately by Hildebrand’s rule. A cell method is used here to calculate the entropy of vaporization for a range of liquids by subtracting the entropy of the gas from that of the liquid. The liquid’s entropy is calculated from the force magnitudes measured in a molecular dynamics simulation based on the harmonic approximation. The change in rotational entropy is not accounted for except in the case of liquid water. The predicted entropies of vaporization agree well with experiment and Trouton’s and Hildebrand’s rules for most liquids and for water except other liquids with hydrogen bonds. This supports the idea that molecular rotation is close to ideal at a liquid’s boiling point if hydrogen bonds are absent; if they are present, then the rotational entropy gain must be included. The method provides a molecular interpretation of those rules by providing an equation in terms of a molecule’s free volume in a liquid which depends on the force magnitudes. Free volumes at each liquid’s boiling point are calculated to be ～1 Å³ for liquids lacking hydrogen bonds, lower at ～0.3 Å³ for those with hydrogen bonds, and they decrease weakly with increasing molecular size.     

Correction to: First-principles study on the Jahn-Teller distortion in trigonal bipyramidal coordinated LiFe1−xMxBO3 (M=Mn,Co, and Ni) compounds

The development of new cathode materials with high capacity, good stability, and high safety is important for the future improvement of Li batteries. LiFeBO₃ is considered to be a type of promising electrode materials for Li-ion batteries due to its low cost, high theoretical capacity of 220 mAh/g (about 30% larger than that of LiFePO₄), low toxicity, and small volume change of 2% during the Li⁺ reversible extraction/insertion process. However, its electronic conductivity and rate performance still need further improvement. To optimize the performance of the LiFeBO₃, Mn, Cr, and Ni doping at Fe site have been studied experimentally, while the effect of minor addition of 3d transition metals on the electronic structure of LiFeBO₃ is rarely investigated. Thus, density functional theory calculations corrected by on-site Coulomb interactions have been conducted to study the crystal structure and electronic property of the LiFe_1−xM_xBO₃ (M = Mn, Co, and Ni) electrode systems. The results indicate that the coordination geometry about Fe in LiFeBO₃ is a distorted trigonal bipyramid with a distortion which can be attributed to a Jahn-Teller effect. The band gap energy of LiFeBO₃ is calculated to be 3.40 eV, which is in reasonable agreement with the previously computed values. The doping at Fe site with Mn cannot reduce the distortion of Jahn-Teller effect, whereas Co doping intensifies Jahn-Teller distortion of the FeO₅ trigonal bipyramid in LiFeBO₃. Ni substitution is predicted to be able to introduce impurity levels, and the Jahn-Teller distortion degree of the trigonal bipyramid decreased from 11.9 of the FeO₅ to 8.7% of the NiO₅. Thus, Ni doping is expected to increase stability and the electronic conductivity of the LiFeBO₃ structure.

     

Radiation-chemical processes leading to origination and accumulation of oxygen in the Earth’s atmosphere

The role of natural radioactivity in the formation of the oxygen atmosphere of the Earth is discussed. The origination of oxygen in the free state in the hydrosphere and atmosphere is associated with the radiolysis of the water of the World ocean under the irradiation of radio active isotopes ⁴⁰K, ²³⁵U, ²³⁸U, and ²³²Th. The calculations showed that within the last 3.8 billion years the total weight of oxygen, which could be formed due to this process, is about 7.4?10¹⁷ kg, i.е., the value of the same order as the oxygen content in the modern atmosphere of the Earth. The Ocean was an intermediate collector and provided the nucleation of new forms of biological life with oxygen respiration. The consecutive transformation of the Earth’s atmosphere with a gradual increase in the oxygen content became the result of photosynthesis in algae and green plants and thus opened a way to the origination of complicated forms of the life.     

Zinc(II) and copper(II) complexes of edampda2−: octahedral,trigonal bipyramidal and in between [edampda2−=N,N′-bis-(pyridylmethyl)ethylenediamine-N,N′-diacetate]

ZnII and CuII complexes of the ligand edampda2– [N,N-bis(pyridylmethyl)-ethylenediamine-N,N-diacetate] have been studied in solution by 1H n.m.r. and e.p.r. spectroscopies, respectively. [ZnII(edampda)] exists in solution in a major octahedral isomer (ca. 83%) in which the two carboxylate donors and two pyridylmethyl donors remain stereochemically rigid up to 333K at pD=6.0. The major octahedral complex has equivalent glycinato and pyridyl donors as shown by equivalent AB quartets for each type of chelate. By contrast, the [ZnII(edta)]2– analogue complex is known to have processes which rapidly equilibrate the coordinated carboxylates leading to coalesced, broad singlets instead of AB quartets down to 273K (freezing point of the sample). The minor [ZnII(edampda)] species has one pendant pyridylmethyl arm. The complex does not increase in abundance up to 333K via dissociation of the major species, suggesting that it possesses a different five-coordinate geometry (approximate trigonal bipyramid). The [CuII(edampda)] complex exhibits an e.p.r. spectrum that is intermediate between rhombic or tetragonal CuII complexes (near D4h) and the reversed-e.p.r. type of trigonal bipyramidal CuII complexes (ca. D3h). The single g value of 2.079 for gg>2.03 identifies the [CuII(edampda)] complex as distorted toward trigonal bipyramidal whereas its [CuII(edtaH2)(H2O)] analogue is known to be distorted toward square pyramidal. A binuclear CuI complex of edampda2– is formed only as a transient, and it rapidly disproportionates into [CuII(edampda)] and Cu metal. A mononuclear [CuI(edampda)]– complex persists for up to 8 h, but is oxidized within 3 min by O2 to the CuII complex. [CuII(edampda)] oxidizes to CuIII with a highly irreversible wave on glassy-carbon at +1.09V compared to the [NiII/III(edampda)] wave at +1.32V.     

Chemistry of HCNH+: mechanisms, structures, and relevance to Titan’s atmosphere

The chemistry of HCNH⁺ in Titan’s atmosphere is not completely understood despite previous experimental and theoretical studies. In response to recent suggestions in the literature, we have searched for specific products of the reactions of HCNH⁺ with H₂, CH₄, C₂H₂, and C₂H₄ using the flowing afterglow-selected ion flow tube technique. We have probed for an association mechanism for reaction with H₂, and associative-H₂ loss for the reactions involving CH₄, C₂H₂, and C₂H₄. In all cases, these reaction mechanisms were found to be inefficient pathways for the depletion of HCNH⁺. Our ab initio computational studies characterize the structures and energies for these mechanisms and indicate that the proposed pathways are endothermic or possess reaction barriers. We compare our studies to previous experimental and computational work, and we suggest other ion-neutral reactions with HCNH⁺ that have not been included in previous models of Titan’s ionosphere.     

Spectroscopic characterization of Brooker’s merocyanine in Silicalite-1 and ZSM-5 leading to second harmonic generation

The use of porous materials has become increasingly significant in the pursuit of novel composite materials with nonlinear optical properties. The microporous channels in Silicalite-1 and ZSM-5, which have identical crystal frameworks but different chemical composition, were used to study the role of zeolite composition on dye adsorption and organization, leading to second-harmonic generation (SHG). The dye molecule, known as Brooker’s merocyanine (4-[(1-methyl-4(1H)-pyridinylidene)ethylidene]-2,5-cyclohexadien-1-one), exhibits one of the largest known molecular hyperpolarizabilities for small dye molecules, but it crystallizes centrosymmetrically. When Brooker’s merocyanine was adsorbed to Silicalite-1 powder, no SHG was detected, but when adsorbed to ZSM-5, the powder was SHG-active. Spectroscopic characterization of the new materials was used to determine the dye loading in each zeolite. The hydrophilic ZSM-5 channels proved to be essential for dye adsorption and alignment within the channels as compared to the hydrophobic Silicalite-1 which did not appreciably adsorb the dye. The effect of dye protonation and isomerization on adsorption to each zeolite was also studied, with neither having an effect on the amount of dye adsorbed.     

Anion-/cation-directed reaction routes to polymorphic forms of a pyrazole-type ligand and its coordination compounds with zinc. Key structural differences between polymorphs’

Synthetic paths toward the two polymorphs of a monohydrate, one anhydrous polymorph of 1-carboxamidino-5-hydroxy-3-methylpyrazole (hcmp) and two polymorphs of zinc complexes containing hcmp ligand are presented. By choosing ions which are not part of the final product, it is possible to direct the synthesis toward the particular polymorph. In all three modifications of hcmp, the same hydrogen bonding motif appears, leading to formation of similar molecular chains. Differences arise due to different modes of chain aggregation and the presence of solvent water. Analysis of the crystal packing and the energetic features of hcmp polymorphs is made using the PIXEL model. The thermal decomposition processes are examined using differential scanning calorimetry and thermogravimetry. Analysis of crystal packing in the two polymorphs of zinc complex suggests the key role of the hydrogen bonding capacity of the aqua ligand for the appearance of the two polymorphic forms. In both polymorphs of zinc complex, stacking interactions have an important role. However, the enhanced hydrogen bonding capacity of the aqua ligand influences the formation of multistacking arrangement.     

Copper catalysed 1,4-addition of organozinc reagents to α,β-unsaturated carbonyl compounds: a mechanistic investigation

A mechanistic study of the 1,4-addition of diethylzinc to 2-cyclohexenone catalysed by copper complexes of the Schiff base ligand H₂L1 was performed. The kinetic law of this system was determined and the nature of the different copper complexes involved in the reaction was investigated. The experimental results indicate a first-order dependence of the reaction rate on 2-cyclohexenone concentration, a zero-order on diethylzinc concentration, and a first-order dependence with respect to the concentration of a 1:1.2 mixture of Cu(OTf)₂ and H₂L1. A sharp change in the kinetics of the reaction was observed at catalyst concentrations higher than 9 mM, indicating the possible formation of catalytically inactive species. An aggregate copper complex, with the molecular formula [(CuL1)₂ · Cu(OTf)₂](TfOH)_1/3 (1), is formed upon mixing Cu(OTf)₂ and ligand H₂L1 in toluene. Complex 1 is reduced in situ to a catalytically active copper(I) species by addition of 12 equivalents of Et₂Zn. This species is able to perform the conjugate addition to 2-cyclohexenone under stoichiometric conditions and resumes its catalytic activity in the presence of 2-cyclohexenone and Et₂Zn.     

Static mass spectrographs of a new type using Euler’s homogeneous electric and magnetic fields. II: Conditions of high-order double focusing for two-cascade schemes

Static mass spectrometers are masked by more modern and more sophisticated mass spectrometric instruments, but their potential nowhere is almost exhausted. One of the principle advantages of static mass spectrometers is that they are the only type of mass spectrometric instruments capable of operating in a spectrographic mode by measuring the whole range of masses per a single run. However, systems with good spectrographic properties significantly differ from the optical point of view from systems with good spectrometric properties, and the development of such devices leads to completely new optical problems and, accordingly, to novel methods of their solution. In this paper, we consider possibilities related to the application of electrostatic and magnetostatic fields, homogeneous in Euler’s terms, to the development of two-cascade mass spectrographs with double focusing.     

Phase behaviour of fluorocarbon and hydrocarbon mixtures: interpretation of type II and type III behaviour in terms of a ‘hard-sphere + attractive force’ equation of state

Christou, G., Morrow, T., Sadus, R.J. and Young, C.L., 1986. Phase behaviour of fluoro-carbon and hydrocarbon mixtures: interpretation of type II and type III behaviour in terms of a ‘hard-sphere + attractive force’ equation of state. Fluid Phase Equilibria, 25: 263–272.Experimental measurements of the upper critical solution temperature of octadecafluorooctane + n-alkanes are reported. These measurements and those discussed by Schneider (1983) on the phase behaviour of tetrafluoromethane + alkanes are compared with results obtained from the one-fluid model using a ‘hard sphere + attractive term’ equation of state.Interaction parameters, ξ, obtained from the comparison together with values from the literature on other fluorocarbon + hydrocarbon mixtures show trends which are briefly discussed.     

Grob fragmentation of norbornyl α-diketones: a route to α-ketoenols and aromatic compounds

An efficient acid-catalyzed Grob fragmentation of symmetrical and asymmetrically substituted norbornyl α-diketones to the corresponding six-membered α-ketoenols is reported. The regio- and stereochemical outcome of the Grob fragmentation of C2 mono- and disubstituted α-diketones was investigated. A single regioisomer resulting from a favorable half-chair intermediate was normally observed. A departure from the normal course was noticed for C2 disubstituted α-diketones possessing an exo-methyl and an endo-methoxycarbonyl derivative, giving the opposite regioisomers due to initial formation of the hemiketal. The bromo analogues of the C2 disubstituted α-diketones furnished an unusual byproduct, which appears to have been formed through highly reactive fused four-membered bicyclo[2.2.0]hexane intermediates. A plausible mechanistic proposal involving the gem-dihalo intermediate, which in one case was actually isolated as its BF(2)-complex, is outlined. The fragmentation protocol was applied to various norbornyl substrates including bis-α-diketone derivatives. The methodology was successfully utilized for the synthesis of substituted aromatic compounds.     

XI International Symposium on Electrochemistry,dedicated to the 125th anniversary of A. N. Frumkin’s birth

Russian Chemical Bulletin -     

ChromiumVI oxide −70% tert.butyl hydroperoxide,a simple catalytic system for oxidation of alcohols to carbonyl compounds

Oxidation of alcohols to carbonyl compounds was carried out by using catalytic amounts of CrO₃ and excess of aqueous t.BuOOH. This system is highly efficient in the case of secondary benzylic alcohols. A catalytic cycle which retains the initial oxidation state of chromium is proposed.     

Front Cover: In Situ Generated DBU⋅HF Acts as a Fluorinating Agent in a Hexafluoroisobutylation Tandem Reaction: An Effective Route to 5,5,5,5’,5’,5’-Hexafluoroleucine (Eur. J. Org. Chem. 10/2023)

We report the direct incorporation of the hexafluoroisobutyl group on a chiral glycine Schiff base complex mediated by 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). The fluoroalkylation involves 2-(bromomethyl)-1,1,1,3,3,3-hexafluoropropane reagent, which generates in situ hexafluoroisobutylene (HFIB), and reacts then with the enolate through a tandem allylic shift/hydrofluorination process. We showed that the use of neutral organic base DBU generates in situ an original DBU⋅HF salt, which preserves the fluoride nucleophilicity and acts as a fluorinating agent. This fluoride salt promotes the hydrofluorination of the pentafluorinated alkene overcoming the usual fluoride β-elimination observed with α-CF₃-vinyl reagents. With alkali metal bases, by contrast, the hydrofluorination is disfavored and the pentafluorinated alkene intermediate is obtained predominantly. This study highlights the critical role of the fluoride counter ion to preserve its nucleophilicity. The protocol is amenable to multidecagram scale synthesis of enantiopure (S)- and (R)-5,5,5,5’,5’,5’-hexafluoroleucine and their N-Fmoc or N-Boc derivatives in good overall yield.     

Photochemical transformation of β,β′-dithienyl substituted o-divinylbenzenes leading to 1,2-dihydronaphthalenes or fused pentacyclic compounds: first evidence of electrocyclization process via 2,3-dihydronaphthalene intermediates

The photochemical behaviour of 2,2′-(o-phenylenedivinylene)dithiophenes (7a–c), 3,3′-(o-phenylenedivinylene)dithiophene (8a) and 3,3′-(o-phenylenedivinylene)dibenzothiophene (8b) was studied under the low concentrations. An intramolecular reaction via the 2,3-dihydronaphthalene intermediate has been observed in all studied examples accompanied by dimerization and polymerization. The 1,2-dihydro-2,3-dithienylnaphthalenes (9a–c) were isolated (7–9%) from the 2-thiophene derivatives while the 3-thiophene derivatives gave polycyclic structures (13–44% yield).