Introduction

The journal Structural Chemistry covers a wide variety of research fields and includes theoretical and experimental studies with broad impacts on the fields of physical, chemical, biological, and medical research.

It is now nearly 20 years since we have been making the effort to provide an outside perspective (sometimes also from a personal perspective) on the research in this journal by reviewing and commenting on each published article. In these reviews, each article (except for obituaries and errata) is summarized, and a thermochemical comment has been added, sometimes with suggestions for future research. All data for inorganic compounds with otherwise unreferenced enthalpies of formation are assumed from the “Wagman compendium” [1], and corresponding unreferenced enthalpies of formation for organic compounds are assumed from the “Pedley compendium” [2].

The first such study was published solely by JFL in 2003 (for issues in 2000) [3]. Over the last 20 years, different coauthors have worked on the reviews. In addition to JFL, MP (now MPS), who joined in 2008 (for issues in 2007) [4], and DNZ, who joined in 2012 (for issues in 2011) [5], now compose a team. In a recent review, MPS and JFL [6] also summarized some thoughts on the journal, the discipline, bridge building, and our personal and professional practice.

With the newest issue of Structural Chemistry, we provide a review of issues 1 and 2 from the calendar year 2019 and continue our efforts to review and comment on each article published in the journal.

Issue 1

Issue 1 of the 2019 volume of Structural Chemistry opens with an editorial contributed by Hargittai and Hargittai written on the occasion of the 150th anniversary of the first periodic table generated by Mendeleev [7].

The results of a theoretical study on the nature of electronic substituent effects and their transmission through the carbon–carbon triple bond in some β-substituted phenylacetylenes were reported by Campanelli and Domenicano [8]. A comparison with the (E)-β-substituted styrenes showed that in β-substituted phenylacetylenes, the field effect of the substituent plays a more important role, and the resonance effect a less important role, than in (E)-β-substituted styrenes. The calculated results were in good accord with experimental observations. Consider β-substituted phenylacetylenes, species with the generic formula C6H5C ≡ CX. Contemporary enthalpy of formation and hydrogenation data are quite limited: X = H, n-alkyl, phenyl, and phenylethynyl [9]. Studies that are over a century old include those of phenylpropiolic acid (X = COOH) and 4-phenylbut-3-yn-2-one (X = COCH3) [10]. Conceptually bridging both types of studies is the contemporaneous calorimetric investigation of the addition reaction of ethyl phenylpropiolate (X = COOC2H5) with methyl iodide and tetraiodoplatinate [11].

Horton et al. [12] studied the antioxidant activity of OH versus NH groups in simple substituted phenols and anilines to obtain insight into their structure–activity relationship (SAR). The results suggested that the activity ranges overlap, indicating that anilines are comparable radical scavengers to phenols. Experimental activity screening was performed against several radical compounds; among those compounds, 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) was selected for further theoretical investigations. The enthalpy of formation of all three aminophenols (or should we say hydroxyanilines) has been experimentally determined [2, 13,14,15]. These thermochemical data are missing for all of the isomeric aminonaphthols (hydroxynaphthylamines), but some comparisons of their relative hydrogen bonding are available [16].

The mechanism of the reaction between substituted N-benzoylcarbamates and aliphatic/aromatic amines that results in N-benzoyl urea derivatives was theoretically elucidated by Singh [17]. Based on the results of their calculations, the authors suggested that the choice of reaction mechanism depends upon the nature of the leaving group on N-benzoylcarbamate. These carbamates and ureas have the general formulas RC(O)NHC(O)OR and RC(O)NHC(O)NR2, with no constraints as to the R groups. Thermochemical data for the related acyclic amino derivatives, the so-called allophanates and biurets, are surprisingly rare: examples include [18,19,20]; relevant cyclic examples include studies of isatoic anhydride [21] and quinazoline-2,4-dione [22].

Dawoud and Alomari [23] reported the results of a theoretical study of Li+·(N2)n (n = 1–4) complexes. The authors calculated binding energies of 46 and 89 kJ mol−1 for linear mono- and diligated complexes, respectively. Trigonal planar and tetrahedral configurations were predicted for tri- and tetra-ligated complexes, respectively. Consider neutral lithium–nitrogen species, in particular Li3N and LiN3. The enthalpies of formation of these species have been measured in [24,25,26] and [27, 28], respectively. Other lithium–nitrogen species (LiN, LiN2, and LiN5) have been shown to be stable under high-pressure conditions.

In an experimental study, Uchikoshi and Shinoda [29] reported ultraviolet–visible (UV-Vis) and X-ray absorption spectra (XAS) of cupric-chloro complexes in hydrochloric acid solutions at T = 298 K. The thermodynamic parameters of the formation of cupric-chloro complexes were calculated, and the structures of five detected species were suggested. Cupric chloride forms complexes with diverse bases. The enthalpies of formation of such complexes with glycine [30], histidine [31], lysine [30], and p-toluidine [32] have been studied calorimetrically. Can this phenomenon be related to the basicity (both solution- and gas-phase) of the amine? What about related complexes with other cupric halides such as cupric bromide with p-chloroaniline [33]? What about other Cu(II) derivatives such as that from cupric perchlorate and aniline [34]?

Silva and Pliego [35] theoretically elucidated the SNAr reactions of bromobenzene and methoxybromobenzenes (ortho, meta, and para isomers) with different nucleophiles (OH−, CN−, CH3O−, CH3COCH2−, CH3COCHCOCH3−) in the gas phase and in methanol and dimethyl sulfoxide solutions. The results suggested that by decreasing the solvent effect, arylation of enolates with unactivated arenes could become possible. The ambidentate character of CH3COCH2− and CH3COCHCOCH3− ions was discussed. What about that of CN−? The systematics of the enthalpy of formation difference between isomerically related cyano and isocyano species have been systematically studied [36, 37]; the enthalpies of formation of diverse C- and O-alkylated acetylacetones have been determined, but the systematics appear to be unstudied: for example, [38, 39].

A theoretical study on the possible use of black phosphorus monolayer (BPML) to eliminate cadmium, lead, mercury, and arsenic ions from contaminated environments via chemisorption was reported by Ghashghaee and Ghambarian [40]. Although the adatom was four-coordinated to BPML in the case of BP-Pb1, twofold coordination was observed in the cases of BPHg2 and BP-Cd2; metal was three-coordinated to the surface in the others. The calculated adsorption energies varied between − 3020 and − 764 kJ mol−1 in the following order: Pb (II) < Cd (II) < Hg (II) << As (III). These findings were in good accord with the observed charge transfers, which ranged between 0.440 e and 1.236 e. We welcome results for the binding energies of the corresponding metal halides and solvated ions to the phosphorus monolayer.

The sensitivity of pristine, Stone–Wales-defected, and Fe-doped single-walled carbon nanotubes (CNTs) for arsine was studied theoretically by Arasteh and Naseh [41]. Owing to its relatively higher adsorption energy and charge transfer and its shorter interaction distance, the AsH3 molecule was more likely to be absorbed on Fe-doped CNTs than on pristine or Stone–Wales-defected CNTs. The energetics of arsine–iron tricarbonyl derivatives has been compared with those of the related phosphines [42].

The extent to which analogs of the “indenyl effect” might influence the reaction chemistry of the cationic 18-electron tricarbonyl complexes [Mn(CO)3(naphthalene)]+ and [Cr(CO)3(benzotropylium)]+ was theoretically studied by Ahmed [43]. The author reported that in the naphthalene system, the “naphthalene effect” has only very marginal impact on the hydride reduction surface. The emergence of an analog to the indenyl effect for benzotropylium was suggested. The enthalpies of formation of some benzotropones [44] and indenone [45] have been reported.

Structural features and binding mechanisms of a series of novel S-trityl-L-cysteine (STLC) analogs, acting as inhibitors of Eg5 (which plays an essential role in the early stages of mitosis), were elucidated theoretically by Mousavi and Fatemi [46]. Hydrogen bonding and hydrophobic interactions were suggested to play a significant role in the binding of inhibitors to Eg5. A potentiometric study of N-trityl-L-cysteine and other amino acids has been reported [47]. How do S- and N-tritylcysteine compare?

Possible mechanisms and kinetics of the reaction of acetaldehyde with methoxy radical were elucidated theoretically by Zhang et al. [48]. Among the suggested pathways, the one that resulted in CH3CO and CH3OH as products was CH3O abstracting the hydrogen atom of the aldehyde group in CH3CHO. The rate constant is 8.73 × 10−15 cm3 molecule−1 s−1, which is in good accord with the experimental value (k1 = 8.30 × 10−15 cm3 molecule−1 s−1 and 4.23 × 10−15 cm3 molecule−1 s−1). The energetics of the related addition reaction of carbonyl compounds with alcohols to form hemiacetals has been studied theoretically and experimentally [49, 50].

The thermochemistry and thermodynamic favorability of the metallacycle intermediates for ethylene dimerization to 1-butene over NiMCM-41 catalyst were studied by Ghambarian et al. [51]. The enthalpy of dimerization of the most thermochemically favorable intermediate was estimated as − 147 and − 180 kJ mol−1 at the L1 and L2 levels of theory, respectively. The calculated Gibbs free energies of the intermediate complexes ranged from − 42 to 70 kJ mol−1. Dimerization to form 2-butene was unexplored. How do these isomeric dimerization processes compare with the observed dimerization to form 1-butene? (We note that 1-butene is less stable than its 2-butene isomers.)

Lower-energy structures of (H2O)12+ clusters were investigated theoretically by Wang et al. [52]. The authors suggested that the new-found entities adopt 3D cage-like structures in which the radical–ion pair disappears. Comparison with the 12-water cluster (H2O)12 and [(H2O)12H]+ clusters are welcomed.

A theoretical study of the scavenging behavior of the isomeric chromones 4′,5,7 trihydroxy isoflavone dihydrogenistein and 4′,6,7 trihydroxy isoflavone demethyltexasin was performed by Anbazhakan et al. [53]. For 4′,5,7 trihydroxy isoflavone dihydrogenistein, the most stable conformer was identified for the torsion angle around τ = 43°, and for 4′,6,7 trihydroxy isoflavone demethyltexasin, the most stable conformer was obtained around τ = 0°, resulting in high planarity, which confirms the superior structural activity of 4′,6,7 trihydroxy isoflavone demethyltexasin compared with that of 4′,5,7 trihydroxy isoflavone dihydrogenistein. The solvent-dependent dissociation constants of genistein and daidzein have been studied [54]; comparison with isomeric flavones is welcomed.

A paper by Nazifi et al. [55] discusses the structural requirements (charge, linker, vector, cargo) of polyamine conjugates for transportation into cells by means of the polyamine transport system of mammalian cells. Heat capacity [56] and enthalpy of formation [57] measurements have been reported for a series of “polyethyleneamines.”

The C–H···O interactions between cations (1-ethyl-3-methylimidazolium and 1-butyl-3-methylimidazolium) and eight hydrophobic amino acid–derived anions within 64 amino acid–based ionic liquids in the gas phase and in the solvent phase were studied theoretically by Shyama and Lakshmipathi [58]. The authors suggested that cations bind strongly with the investigated hydrophobic anions. The C–H···O hydrogen bonds were predicted to be strong and red-shifted in all complexes. High stability of ionic liquids in the gas phase and moderate stability in solvents were suggested. The enthalpies of formation of ionic liquids are encouragingly well-understood: for example, [59, 60].

The growth of iron clusters, Fen (n ≤ 4), on the surface of B12N12 cages was studied theoretically by Castro and Chigo Anota [61]. Both tetrahedral and parallelogram forms were calculated to be structurally stabilized during the adsorption process, and the lowest energy structure corresponded to the tetrahedron. As a function of n (including and vastly exceeding n = 4), the reaction energy of [Fen]+ with diverse neutrals has been studied experimentally. These neutrals include CO [62], CO2 [63], and CS2 [64]. Related systematic studies for the binding of complexes with CO for neutral Fen or anionic [Fen]− with varying n have also been reported [65].

Structural changes in a hexahydro-1,3,5-trinitro-1,3,5-s-triazine (RDX) lattice induced via ionization were investigated theoretically by Akin and Kiyak [66], who used [(RDX)2]0 complexes as a simple model. [(RDX)2]+ were suggested to exist in the general form RDX+∙RDX. What can be said about the related oligomeric radical anions [67]?

Fragment-based de novo methods were used by Abdullah and Guruprasad [68] to design Wee1 kinase inhibitors. The authors designed new molecules with variable tricyclic core scaffolds [6–6–5] and extended them based on the chemical space available in the active site of Wee1 kinase. The designed molecules were suggested to be comparable with the reference inhibitors in terms of binding location, docking scores, and binding free energies. There are seemingly no species with the [6–6–5] tricyclic core scaffold for which the enthalpy of formation has been determined. We recognize the [6–5–6] scaffold in fluorene and its derivatives. Some calorimetric studies include measurements of the enthalpy of formation of the parent hydrocarbon [69] and of its 9-carbinol and 9-carboxylic acid [70]. The [6–6–6] scaffold as both the isomeric anthracene and phenanthrene has been studied, as have their 9,10-quinones. Two isomeric [6–7–6] scaffolds have been calorimetrically investigated [71, 72], but no such data have been reported for the isomeric [6–6–7] species. Two of the three possible [6–8–6] scaffolds have also been calorimetrically studied [73], but [6–6–8] species remain ignored by thermochemists.

In an experimental study, Ji et al. [74] reported the synthesis and characterization (including X-ray single-crystal structure determination) of [Mn(L)2(H2O)2]n and [Cd2(L)4(H2O)4]n {HL = (benzotriazol-1-yloxy)-acetic acid}. The former compound has a one-dimensional (1D) double-chain structure, and the latter has a 1D infinite-chain structure. O–H···O and O–H···N hydrogen bonds (HBs) present in both structures enable formation of three-dimensional (3D) assemblies. The relative stability of the tautomeric benzotriazole-N-oxide and 1-hydroxybenzotriazole has been shown to vary with the phase and the solvent [75]. How is this manifested in their metal complexes?

Díaz et al. [76] reported possible competitive oxidation reactions between allyl methyl disulfide and the extremely reactive hydroxyl radical in gas and aqueous phases; the authors suggested that the proposed reactions are thermodynamically feasible in both phases. Allyl propyl disulfide is rearranged by strong base to form 1,3-di(propylthio)propene [77], and 3-alkylthiopropene S-sulfides spontaneously rearrange to form alkyl allyl disulfides [78]. What are the enthalpies of these rearrangements?

In the following paper, Nagrimanov et al. [79] reported a reconciliation of reported thermochemical data available for substituted acetanilides that was facilitated by experimental measurements and theoretical methods. Theoretical enthalpies of formation were in good accord with the experimental results. A group-additivity method was suggested for quick assessment of vaporization enthalpies. A comparison of enthalpies of formation and phase transition for comparably substituted derivatives of the isomeric C6H5NHC(O)CH3 and C6H5C(O)NHCH3 is welcomed, in particular using the findings of the present paper [79] and reference [80].

In a theoretical study, Dubey et al. [81] reported the use of theoretical methods to suggest novel pyrazolo-benzimidazole derivatives as spleen tyrosine kinase inhibitors. The results of these authors’ calculations suggested that one hydrogen acceptor, one hydrogen donor, and three aromatic rings are crucial for spleen tyrosine kinase inhibition. Comparison with the isomeric imidazo-benzopyrazoles is welcomed. We note the near thermoneutrality of the benzo transfer reaction

$$ \mathrm{pyrazole}+\mathrm{benzimidazole}\to \mathrm{imidazole}+\mathrm{indazole} $$
(1)

What about the reaction enthalpies for the related pyrazolo-benzimidazole (imidazo-benzopyrazole), benz[1,2-d:3,4-d]bisimidazole (and its 1,2:4,5-isomer) and its bispyrazole counterparts? The enthalpies of formation of these heterocycles remain unknown.

In the next study, Romero [82] computed dipole and quadrupole moments for a series of three-membered rings containing second, third, fourth, and fifth-row elements. The author suggested that the dipole moment increases with the increase in electronegativity and the atomic size of the heteroatom in the ring skeleton; in addition, high relative dipole moment was suggested for three-membered rings containing II, IIIA, VIA, and VIIA elements. We welcome a related study of four-, five-, and six-membered heterocyclic rings. What can be said about the strain energy and indeed the enthalpy of formation of these diverse heterocycles? Sometimes, relative stability can be suggested: for example, cyclopropylborane spontaneously isomerizes (experimentally rearranges) to form boretane [83].

In a theoretical paper, Elguero and Elguero [84] suggested that the use of the Morse potential energy function should be preferred over polynomial regression models. These authors demonstrated use of the Morse potential energy function in previous examples of their work. We note that there are other potential energy functions that have long been suggested for studying the potential energy of diatomic molecules [85, 86]. How do these alternatives compare with the Elgueros’ use of the Morse function?

The effects of stepwise substitution of carbon for silicon on the structure and adiabatic electron affinities of the cyanocarbons tetracyanoethylene and tetracyanoquinodimethane were elucidated theoretically by Maley et al. [87]. Si-substitution was calculated to have a positive increase in the adiabatic electron affinities of both compounds, and it stabilized an open-shell singlet diradical state in the case of tetracyanoquinodimethane. How are electron affinities modified by the presence of silicon in lieu of carbon? Among the simplest cases is that of SiH3 [88, 89] over that of CH3 [90] by ~ 130 kJ mol−1. However, we note the electron affinities of SiN and CN, where the former [91] is lower than the latter [92] by nearly 100 kJ mol−1.

Ilkhani [93] calculated the geometrical parameters of “octa-heterocyclic” dichalcogenin and dipnictogenin systems with “1,2–C6X2” (X = O, N and their congeners) to explain the two-dimensional instability of the molecular structures. The pseudo Jahn–Teller effect was identified as a reason for twisting of the configurations. In addition, intramolecular hydrogen bonding may explain planar instability. Comparison with the related 1,2-C4X2 species is welcomed: we note that experimental enthalpy of formation data are limited to 1,2-C4N2 (pyridazine) and the 3,6-diphenyl derivative of 1,2-C4S2 (1,2-dithiin). We note that although the resonance energy of a substituted (N-t-butyl)-1,2-azaborinine (1,2-C4XY, X = B, Y = N) has been determined through the use of hydrogenation reaction calorimetry [94], no analogous analysis has been reported for any pyridazine or dithiin derivative.

The [3 + 2] cycloaddition reaction between furoxancarbonitrile oxide and electron-deficient 2,2,2-trichloroacetonitrile in the presence of chloroform was investigated theoretically by Hosseini et al. [95]. The sole product of the reaction is a highly substituted 1,2,4-oxadiazole, which is in accord with experimental findings. What is the relative stability of substituted derivatives of the diverse isomeric 1,3,4-oxadiazoles?

In the next paper, Xiang et al. [96] reported a theoretical study in which the authors designed a new family of six novel energetic compounds formed by the introduction of 1,2,3,4-tetrazine-1,3-dioxide into the basic skeleton of 1,1-diamino-2,2-dinitroethene together with amino, nitro, and N-oxide groups. In addition to the detonation properties and impact sensitivities of these compounds, the authors also studied their environmental behavior and potential ecological risks. There are few 1,2,3,4-tetrazines with measured enthalpies of formation [97]; we wonder, what is the enthalpy of formation difference between vicinal bis(diazo)alkanes and the isomeric 1,2,3,4-tetrazine?

Buchanan et al. [98] reported the experimental synthesis and characterization of a series of polymer nanocomposite blends prepared from the engineering thermoplastic polyamide 6 and increasing admixtures (up to 8 wt%) of carbon nanofibers as a reinforcing nanofiller. The most significant improvement in mechanical performance the authors observed was the ~ 500% elongation at break for the 2 wt% carbon nanofibers/polyamide 6 nanocomposite. How different are the properties of polyamide 6 from those of distinct, small, covalently bonded cyclic oligomers of their shared monomer, caprolactam? This dimer has been described as “a very stable compound as indicated by its high m.p. (347 °C)” [99], as opposed to the hydrogen-bonded dimer, which has a dissociation energy of just ~ 25 kJ mol−1 [100]. The covalent dimer of caprolactam may be compared with 1,8-cyclotetradecadiene, the covalent dimer of cycloheptene. Acknowledging the absence of thermochemical data for this last dimeric species, we nevertheless note the evolving value of the enthalpy of formation of its saturated counterpart cyclotetradecane [100,101,102,103]. We also note that the enthalpy of formation of 1,8-cyclotetradecadiyne has been determined, but it is now the unsaturated monomer, cycloheptyne, for which the desired experimentally determined enthalpy data are missing.

Marek et al. [104] reported a theoretical study on selecting the optimal computational level to study stacking interactions in adenine dimers, which stabilize the secondary structure of DNA and RNA. There are many tautomers of adenine and their interconversion that one can consider; perhaps the simplest is the N7–N9 pair. Experiment shows that these two species are of nearly identical energy [105, 106].

The structure and bonding surrounding the boron atoms in the donor–acceptor complexes H3N → BR(OH)2 and H3N → BRH(OH) (R = H, NH2, OH, and F) were studied theoretically by Larkin and Bock [107]. The authors reported that the addition of an ammonia molecule dative bonded to the boron atom for these boronic and borinic acids results in the elongation of bonds to the boron atom. What is found for the corresponding deprotonated species where the proton is from either an N–H bond or from an O–H bond?

The intermolecular interaction energies between the anticancer drug doxorubicin and finite open and hydrogen-terminated single-walled CNTs with different structural parameters were investigated by Contreras et al. [108]. The results of the study suggested that the best interaction occurred when the drug was located inside the cavity of the nanotube. The intermolecular interaction energy of DNA and doxorubicin has been studied [109]. Does that energy depend on the nucleic acid sequence?

Wang et al. [110] reported a theoretical study on N1-aryl-benzimidazole derivatives, which are considered an important class of HIV-1 non-nucleoside reverse transcriptase inhibitors. The results suggested participation of 3,5-dimethylphenyl moieties in forming π–π stacking interactions with amino acid residues (Phe227, Trp229, and Tyr188), the beneficial role of sulfonyl bridges in forming hydrogen bonds with residue Pro236, and the importance of the hydrogen-bond acceptor groups at the C4 position of the arylacetamide moiety. What about the corresponding species with a sulfinyl bridge instead of a sulfonyl bridge? Sulfinyl species have been largely ignored by the biomedical community.

In a theoretical study, Wang et al. [111] investigated the adsorption behavior of Sc on the external and internal surfaces of pristine (CNT) and Stone–Wales (SW) and single-vacancy (SV) defect carbon nanotubes; they also investigated the adsorption ability of CO on Sc@CNT/SW/SV. Based on their analysis, the authors suggested that the SV defect in carbon nanotubes can make the confined Sc atom show good CO adsorption, whereas Sc confined in SW-defect carbon nanotubes has good anti-CO poisoning ability. What is known about scandium carbonyls? Laser ablation matrix isolation has shown the existence of Sc(CO)n, where 1 ≤ n ≤ 4 [112], and mass-selected photoelectron velocity map imaging spectroscopy has shown [ScNi(CO)n′]− where 2 ≤ n′ ≤ 6 [113]. However, none of these homoleptic species has been isolated as bulk species.

In the last entry in this issue, Lawal et al. [114] reported three possible concerted general acid–base pathways for the HIV-1 PR catalysis of its natural substrate. Activation free energies of approximately 63.6 and 69.5 kJ mol−1 were calculated for both concerted acyclic pathways and for the other possible reaction pathway, respectively, in good accord with the experimentally determined value of 65.6 kJ mol−1. Many dicarboxylic acids have been shown to hydrolyze the strained amide benzohomoquinuclidone to form the corresponding carboxylic anhydride and the amide-derived, ring-opened amino acid [115]. Malonic acid does not react in this manner, reflecting the instability of malonic anhydride and its derivatives [116]. Dare we conclude that the strain energy of malonic anhydride is higher than that of benzohomoquinuclidone?

Issue 2

This special issue, which presents a collection of studies in structural chemistry from the Department of Chemistry, Lomonosov Moscow State University, on the occasion of its 90th anniversary, opens with an editorial by Hargittai [117].

In the foreword to the issue, Novakovskaya and Lunin [118] outline important milestones in the development of the Department of Chemistry of Lomonosov Moscow State University.

In an experimental study, Bushmeleva et al. [119] reported the synthesis and characterization (including X-ray single crystal structure determination) of the cyameluric acid salts (NH4)2[C6N7O3H] and (NH4)2(H9O4)[C6N7O3]. The authors state that the packing of anions in both synthesized salts was affected by Coulombic compression. Tautomerism in cyameluric acid and its salts and derivatives has been studied [120]. The enthalpies of formation of the related ammeline, ammelide, melem, and melon have been measured [121]. Despite the one-step, high-yield synthesis of the parent heterocycle, tri-s-triazine (heptazine) [122], there appear to have been no measurements of its enthalpy of formation.

In the next paper, Karpova et al. [123] reported the synthesis and characterization (including X-ray single crystal structure determination) of bimetallic trichloroacetates of the following composition: Cs[M(CCl3COO)3(H2O)3] (M = Co, Ni). The synthesized compounds were suggested to be isostructural. The enthalpies of formation of condensed-phase mono-, di-, and trichloroacetic acids vary widely [124, 125]; the last value is quite close to that of the unhalogenated acetic acid. What would be found for their simple and bimetallic salts?

Shestimerova et al. [126] reported the synthesis and characterization (including X-ray single crystal and electronic structure determination) of the silver-containing metal–inorganic frameworks [Ag3S](NO3) and [Ag4Te](SO4). The authors suggested that both [Ag3S](NO3) and β-[Ag4Te](SO4) may be regarded as salts with polymer cations and isolated anions. These species are also recognizable as inorganic sulfonium and (8-Te-4) telluronium salts. The enthalpies of formation of few organic sulfonium ions have been measured—these include the trimethyl and t-butyldimethyl [127] ions in the gas phase. The solution-phase enthalpy of deprotonation of the organic sulfonium species, benzyldimethyl sulfonium cation, has been reported [128]. We lack corresponding thermochemical information on the analogous tellurium species, (R3Te)+ and (R4Te)2+; indeed, any energetics knowledge of the latter class of ions is, to this point, limited to that from calculational theory [129]. Guidance may be found from the proton affinity measurement for H2Te [130] in that this can be used to derive the enthalpy of formation of (H3Te)+, the simplest but admittedly non-organic, telluronium ion.

The synthesis and characterization (including X-ray single crystal structure determination) of N,N′-dibutyl-N,N,N′,N′-tetramethylethylenediammonium bis(tetrafluoroborate) were reported by Zakharov et al. [131]. The authors suggested the potential use of the synthesized ionic liquid in electrochemical energy storage devices. Isoelectronic to the cation is the hydrocarbon 5,5,8,8-tetramethyldodecane. What can be said about this highly branched species? The enthalpies of vaporization of a variety of highly branched hydrocarbons have been discussed [132], but this species was not among them. Then again, 5,5,8,8-tetramethyldodecane is still unknown.

Marochkin et al. [133] reported the synthesis of 1,2,3-trimethyldiaziridine and the determination of its molecular structure by gas-phase electron diffraction. Gaseous 1,2,3-trimethyldiaziridine was suggested to be present as the conformer with trans H3C–N–N–CH3 moiety orientation, which was confirmed by theoretical methods. What is the difference between the enthalpies of formation of the cis- and trans-isomers? The difference between those of the related hydrocarbons cis- and trans-1,2,3-trimethylcyclopropane is ~ 40 kJ mol−1 [134].

Four novel conjugates of adamantane were synthesized and characterized by Zefirov et al. [135]. The synthesized conjugates were based on experimental and theoretical investigations that suggested they would promote the formation of atypical tubulin assemblies; all of the compounds did so, inhibiting the proliferation of cancer cells (human epithelial lung carcinoma cell line A549). The systematics of the enthalpy of formation of adamantanes have recently been reviewed [136].

Alexeev et al. [137] reported that the reaction of 1-[(1RS,3SR,5SR)-bicyclo[3.3.1]non-6-en-3-yl]-3-t-butylthiourea with bromine under atmospheric moisture conditions did not result in the previously observed product of intramolecular cyclization by sulfur atom but rather in a product of intramolecular cyclization by nitrogen atom of a previously formed urea fragment. The reported compounds represent a novel structural type of tricyclic bridged ureas. The systematics of the enthalpy of formation of substituted ureas have recently been reviewed [138]. No corresponding review seems to have been written for the related thioureas.

The effects of the electrostatic potential created by explicit water molecules in the shells around the active sites of two zinc-dependent enzymes (matrix metalloproteinase-2 and metallo-β-lactamase) were investigated theoretically by Kapusta et al. [139]. Based on their analysis, the authors reported a reasonable convergence of the calculated electrostatic potentials on selected points in the enzyme–substrate complexes with respect to the size of the shell. Metallo-β-lactamase activity has been characterized using isothermal titration calorimetry [140].

Rybakov et al. [141] used the projector augmented-wave (PAW) method to investigate the influence of oxide support on the structure of deposited metal slabs for a series of TiO2 (rutile), the tetragonal form of ZrO2, and some forms of γ-Al2O3 oxides. The results suggested that the properties of Pd slabs depend on the oxide support, but this dependence can be avoided owing to the localization of charges transferred in the reaction to/from the Pd slab. What about palladium oxides? The phase equilibrium of the palladium–water system (and hence of palladium oxides, hydroxides, and their ions) has been studied electrochemically [142].

Next, Golosnaya et al. [143] reported a theoretical study of the structure and reactivity of the [Au4(dmmp)2(С ≡ CСН3)2]2+ (dmmp = dimethylphosphinomethyl) cluster towards O2 and CO. Based on their analysis, the authors proposed that the Au–P fragments of the cluster are the most probable active sites. The binding energy of [(CH3)3PAu]+ with diverse olefins and acetylenes has been studied both experimentally and calculationally [144].

Different theoretical methods were used by Pisarev et al. [145] to study two least-strain conformers for bicyclo[3.3.1]nonane, bicyclo[3.3.1]nonan-9-one, and some heteroanalogs. The authors observed large discrepancies in the results calculated using different methods and discussed possible reasons for these discrepancies, which were mainly attributed to the experimental setup. The energetics of protonation (both N- and O-) of the bridged lactam, 1-azabicyclo[3.3.1]nonan-2-one, have been theoretically and experimentally studied, and these two protonation sites are nearly isoenergetic [146]. Only theoretical results are available for protonation of the isomeric 1-azabicyclo[3.3.1]nonan-9-one [146, 147].

The next paper in this issue presents the results of a theoretical study by Grineva [148] on the influence of fluorine, chlorine, and bromine atoms on the conformational preferences of 35 free non-rigid molecules with the general formula X(CH2)nY (X = Y = CH3, F, Cl, Br, or X = F, Cl, Br, and Y = H3; n = 8, 10, 12, 14 16) as well as on those of n-octane. In accord with previous reports, free n-alkane molecules having a number of carbon atoms (nC) greater than approximately 16–18 prefer to form hairpin conformers rather than fully extended (all-trans) conformers, which are more favorable for compounds with lower nC. How does this finding affect the enthalpies of formation of α,ω-disubstituted alkanes and the alkanes themselves? In our archival enthalpy of formation source, we find measurements of α,ω-disubstituted alkanediols, but only through the C10 species—a bit too short. The word “interamolecular” appears in the chemical physics literature [149]. Can we appropriate the word “interamolecular” [149] for such seemingly long-distance stabilizing effects [150]? This phenomenon has been observed for the stabilization of highly strained hydrocarbons [151].

Godunov et al. [152] analyzed experimentally determined fluorescence spectra of jet-cooled methylpropanal (MPA)-h1 and deuterated (MPA)-d1 to calculate the structure of methylpropanal in the ground (S0) and lowest excited singlet (S1) electronic states. The calculated results were in good accord with their experimental observations. Related information about the related triplet T1 state has been reported [153].

Abramovich [154] studied possible anomalies in the solution system (1–х)C6H5Cl–(х) о-C6H4Cl2 (х = mole fraction) in which molecules of the solvent and solute are close in both size and electro-optical parameters, but the physicochemical properties of the individual liquids are substantially different. The author observed complex non-monotonic behavior of the total light scattering coefficient with changes in concentration; light scattering was also affected by fluctuations of density and of the polarizability anisotropy of the molecules in the system of о-C6H4Cl2–C6H5Cl solutions. As an example of physicochemical properties, we note that the gas-phase acidity of the various chlorobenzenes has been measured [155, 156].

In the next paper, Slovokhotov [157] revisited the main postulates of Aleksandr Kitaigorodskii’s theory of organic crystal structures. With his coworkers, Kitaigorodskii laid the foundation of crystallography of organic compounds consisting of polyatomic, generally low-symmetry molecules. Let it simply be said that Kitaigorodskii’s theory provides a key underpinning of “Structural Chemistry” as defined by the journal and the discipline.

The structure of the CHF3∙∙∙H2O complex was revisited by Sosulin et al. [158] using theoretical methods and IR matrix isolation spectroscopy. A previously unreported C–F bending mode of the complex was observed. The calculated results were in good accord with experimental observations. The solubility of fluoroform in water has been investigated [159, 160].

Korneichuk et al. [161] reported the IR and Raman spectra of 4-piperidinemethanol, which were supported by theoretical investigations. The authors suggested that the gaseous state of 4-piperidinemethanol may be an equilibrium mixture of the two monomers, 4eNe and 4eNa, where “e” and “a” indicate an equatorial or axial position of the hydrogen atom and CH2OH groups connected to the nitrogen atom and opposite to it, C4 carbon, respectively. The enthalpy of formation of 4-piperidinemethanol has been determined, as have those of its 2- and 3-isomers [162]. No such data are available for the 1-isomer, which is known to readily decompose into piperidine and formaldehyde [163].

Shilyaeva and Novakovskaya [164] reported an experimental and computational investigation of possible surface functionalization of graphite sheets upon oxidation, with an emphasis on epoxide and hydroxide groups. Based on their results, the authors noted the important roles of OH groups and hydrating water molecules in functionalization. One should not forget mellitic acid (benzenehexacarboxylic acid) as a product of oxidizing graphite or more precisely for the current discussion, the related anhydride “A new oxide of carbon” [165]. What is its enthalpy of formation, and how exothermic is the reaction

$$ {\mathrm{C}}_{12}{\mathrm{O}}_9\to {3\mathrm{C}}_3{\mathrm{O}}_2+3\mathrm{C}\mathrm{O}? $$
(2)

From literature enthalpy of formation data, dare we extrapolate benzene, phthalic anhydride, 1,2,4,5-benzenetetracarboxylic dianhydride [166, 167], mellitic trianhydride? Is mellitic trianhydride destabilized because of charge/dipole repulsions of the exocyclic oxygens? It is aromatic by the ipsocentric magnetic criterion [168]. Should we also discuss the energetics of 1,2,3,4-benzenetetracarboxylic dianhydride? The literature is rather silent here.

The results of a theoretical study of dynamic behavior modeling of phase states (solid and liquid) of water clusters [(H2O)8, (H2O)10, and (H2O)12] have been reported by Belega [169]. Based on the results of the calculations, the author suggested the dynamic criteria of the phases proposed for the water octamer also work well for water clusters with larger numbers of molecules, particularly for the decamer and the dodecamer. The energetics of hydrocarbons and hydrates have been compared in the context of the relations of the thermodynamic solvate rule and Benson group increments [170]. How shall we interrelate (H2O)8, (H2O)10, and (H2O)12 with cyclooctane, cyclodecane, and cyclododecane?

A new numerical algorithm for calculating scale factors for molecular force fields expressed in Cartesian coordinates was proposed by Kochikov et al. [171], who applied the algorithm to unsubstituted indole and pyrrole molecules. This method was suggested to be effective for calculating the vibrational spectra of large biological molecules, associates, polymers, and nanostructures containing hundreds or even thousands of molecules. Geometrically, indole is quite similar to 1-indenone, but electronically, they are not: indole is generally understood as aromatic [172], and indenone is not [45]. What about vibrationally? The sole studied indenones are all perturbed because they have 3-amino “push-pull” substituents [173].

Gas-phase enthalpies of formation for diamantane and some of its derivatives were calculated by Dorofeeva and Ryzhova [174]. Differences were observed between the theoretically and experimentally determined results, leading to a discussion of available experimental data and inconsistencies in the data. Thermochemical comparison has been made with derivatives of adamantane [136]. What about triamantane and its derivatives? Such species have been synthesized with high isomeric purity [175], but calorimetric data remain absent.

The last paper in this issue is a review by Hargittai [176] of The Posthumous Nobel Prize in Chemistry. Volume 1. Correcting the Errors and Oversights of the Nobel Prize Committee. (edited by E. Thomas Strom and Vera V. Mainz). The book considers 11 cases of omission of those individuals who did not receive the Nobel Prize in recognition of their achievements, although their contributions to science may be comparable to those who did receive the award.