Influence of substitution of oxygen by sulfur on maltol properties

Replacing oxygen by sulfur in a maltol molecule generates a family of new very interesting ligands: thiohydroxypyrones. In this work, theoretical calculations have been performed for all possible maltol derivatives created by consecutive substitutions of oxygen atoms by sulfur. The study is focused on molecular properties of thioligands, which are important for the formation of their metal complexes and potentially useful in medicinal and environmental chemistry. Energetic, tautomeric, aromatic, and charge distribution data are reported and the results are compared with maltol properties. It is shown that, similar to maltol, the most stable tautomer for all thio derivatives, is the one with the keto‐enol group. The protonation in cations occurs always on the heteroatom of the (thio)ketone group. The study has been carried out with the aid of some aromaticity indices, such as HOMA, NICS, and ASE. Aromaticity is studied in the heterocyclic pyran ring and in the XCCX part (where X – oxygen or sulfur). All calculations were performed at the B1LYP/6‐311++G(d,p) level of theory. We conclude that the aromaticity order determined previously for maltol (cation > neutral molecule > anion) is also preserved for thiohydroxopyrones. The results of the population analysis indicate that upon protonation, a large portion of additional positive charge delocalizes on the entire molecule, whereas upon deprotonation, negative charge accumulates mainly on the heteroatoms of the XCCX group. Copyright © 2009 John Wiley & Sons, Ltd.     

Theoretical studies on aromaticity of selected hydroxypyrones. Part 3#. Chelatoaromaticity phenomenon in metalcomplexes of hydroxypyrones

The aim of this project is to study the aromatic properties of various forms (neutral, cationic, and anionic) of selected hydroxypyrones (pyromeconic acid, maltol, and ethylmaltol) and their metalcomplexes with aluminum, gallium, and indium ions. Aromaticity of hydroxypyrone metalcomplexes is important because it can influence the stability of such complexes, which is crucial for their applications in medicinal and environmental chemistry. Results from ten different indices of aromaticity (HOMA, NICS(0), NICS(1), NICS_scan, ASE_iso, PDI, FLU, I_ring, MCI, and KMCI) show that aromaticity in hydroxypyrones decreases in the order cations > neutral molecules > anions. Performed calculations situate the aromaticities of ligands in metalcomplexes close to their respective cations. This means that complexation causes a significant increase of the aromaticity of ligands, which stabilizes formed chelatocomplexes. On the other hand, we clearly show that rings that are involved in binding metal ions are not aromatic. Copyright © 2010 John Wiley & Sons, Ltd.     

Substituent effect on local aromaticity in mono and di‐substituted heterocyclic analogs of naphthalene

A quantitative study on local aromaticity has been performed on a series of mono‐ and di‐substituted biheterocycles (quinoline, isoquinoline, quinoxaline, quinazoline). Three electronically based indices (PDI, ATI, and FLU) have been employed to investigate the substituent effect on the π‐electron delocalization in both heterocycle and benzenoid rings. Three typical substituents (Cl, OCH₃, and CN) with different inductive and resonance power have been selected. Generally, substituent causes a reduction in aromaticity irrespective of whether it is electron attracting or electron donating. It is shown that the maximum aromaticity exhibits a similar trend of Cl > CN > OCH₃ for all the studied rings. Moreover, it is found that the substituent situation with respect to the heteroatom has a significant influence on the aromaticity. It results from our study that in di‐substituted derivatives, irrespective of whether the two substituents form a meta or para isomer, they preferably choose the position which leads to the maximum aromaticity character. Copyright © 2009 John Wiley & Sons, Ltd.     

Theoretical study on the molecular tautomerism of the 3-hydroxy-pyridin-4-one system

3-hydroxy-pyridin-4-one is a parent molecule for the family of hydroxypyridinones that are known in coordination chemistry as efficient metal ions chelators. In this work, relative stabilities of some possible tautomers were investigated using several quantum chemical methods: CBS (complete basis set methods), Gn, DFT (density functional theory), Hartree–Fock and MP2. Performed calculations show that the system under consideration exists as a mixture of two tautomers with comparable energies. Among them, the hydroxypyridinone structure of the studied molecular system seems to be a bit more stable than the o-dihydroxypyridine one, by a few kJ/mol only. Aromaticity and intra-molecular hydrogen bonding are the main effects influencing the stability of the studied tautomeric structures. Consequently, aromatic effects were calculated using several indices of aromaticity: HOMA (harmonic oscillator model of aromaticity), NICS (nucleus independent chemical shift), H, PDI (para delocalisation index), MCI (multi-centre index) and ASE (aromatic stabilisation energy). The strength of possible intra-molecular hydrogen bonds (H-bonds) was determined by means of the AIM (atoms-in-molecules) method and by calculating enthalpies for theoretical reactions that do or do not involve H-bonds. The AIM method was employed to understand how variations in atomic energies influence the stability of different tautomeric structures.     

Aromaticities of azines relative to benzene; a theoretical approach through the dimethyldihydropyrene probe

The aromaticities of azines relative to benzene have been estimated by fusion with 15,16‐dimethyldihydropyrene. Chemical shift data for the azine‐fused dihydropyrenes (calculated at GIAO HF/6‐31G*//B3LYP/6‐31 + G*) were used to estimate the reduction in the dihydropyrene nucleus aromaticity. Choice of the saturated reference model was quite crucial in reliable estimation of aromaticity. Reference models with partial unsaturation at azine (21,23,25–32) gave better estimate of aromaticity than the parent dimethyldihydropyrene. Aromaticities of azines through chemical shift data and geometric parameter analysis were found to be 90–100% to that of benzene, highly consistent with the aromaticity estimation by nucleus independent chemical shift_(0)πzz calculations. Copyright © 2014 John Wiley & Sons, Ltd.     

Theoretical prediction of size‐expansion effect on the C8‐site activity in the modified guanine‐cytosine analogs

Homo/hetero ring‐expanded DNA analogs have been shown to be rationally modified DNA motifs with improved physical or biological properties. In this work, using density functional theory, the stability of these artificial DNA base pairs was examined with regard to three aspects associated with DNA damage, namely deprotonation, H‐abstraction, and H‐radical addition. The effect of size expansion on C8 activity was investigated because C8‐oxidative guanine (G) is one of the most important products of DNA damage. Computational results indicate that the insertion of an aromatic spacer ring in G considerably decreases the electron density over the C8 site, leading to easier deprotonation or H‐abstraction from the C8 site and more difficult H ^. ‐radical attack on the C8 site. However, the opposite phenomenon is observed if the spacer ring is antiaromatic, because of the increased electron density over the C8 site. Moreover, these effects are more prominent the larger the aromaticity or antiaromaticity of the spacer ring. Further analyses, using natural bond orbitals (NBOs) and the nucleus‐independent chemical shift (NICS) index of aromaticity, indicate that the changes of the electron distribution over the C8 site arise because the aromatic spacer ring, involved in the conjugation structure, increases the electron delocalization from the electron‐rich imidazole ring to the diatropic six‐membered rings, while the antiaromatic spacer ring acts as an electron‐donating group, not only inhibiting the above electron delocalization, but also slightly increasing the electron density over the C8 site. The improved stability of these size‐expanded base pairs in different DNA‐damaged environments may encourage their use in practical applications. Copyright © 2009 John Wiley & Sons, Ltd.     

Tuning the absorption spectra and nonlinear optical properties of D‐π‐A azobenzene derivatives by changing the dipole moment and conjugation length: a theoretical study

The optical properties of several azobenzene derivatives were modulated by varying the dipole moments and conjugation lengths of the D‐π‐A systems. The relationship between the structure and absorption spectrum and polarizability was studied in the gas phase, THF and MeOH solutions, respectively, by using the density functional theory. The calculated absorption spectra and second‐order polarizabilities are in good agreement with the available experimental observations. In comparison with the D‐π‐A monomer, the H‐shaped D‐π‐A dimer almost doubles the dipole moments and hence increases the second‐order polarizabilities, without a significant shift in the maximum absorption bands. The addition of another azobenzol group between electron‐donating and ‐accepting groups increases the second‐order polarizabilities by 4–6 times, but leads to an evident red‐shift of about 65–80 nm in spectra. The relative second‐order polarizability of the halogen‐substituted derivatives is in the sequence of ? CF₃ > ? F > ? Cl > ? Br, without obvious substituent effects on the optical transparency. The D‐π‐A chromophores with the strong electron‐donating (amino) and ‐accepting (acetyl) substituent present the larger second‐order polarizabilities, at the cost of about 20 nm red‐shift of the maximum absorption lengths relative to the halogen‐substituted species. It is also demonstrated that both the linear and nonlinear optical properties augment with the increase in solvent polarity, accompanied by a red‐shift in the wavelengths of maximum absorption by about 18 and 23 nm, respectively, in THF and MeOH solutions. The changes in optical properties upon the structural modifications are further rationalized by the electronic structures of various H‐shaped dimers. Copyright © 2010 John Wiley & Sons, Ltd.     

Theoretical studies on aromaticity of selected hydroxypyrones and their cations and anions. Part 2. Electron delocalisation in the OCCO group

Electron delocalisation in the OCCO part of hydroxypyrones with two exocyclic oxygens linked by two carbon atoms was studied using well‐known aromaticity indices like: HOMA, EN, GEO, NICS(0) and NICS(1). This part of hydroxypyrones is very important due to the fact that the two exocyclic oxygens of deprotonated hydroxypyrone units are responsible for metal ion binding. Values of the aromaticity indices were obtained for geometries calculated by several theoretical methods (HF, SVWN, B3LYP and B1LYP) with 6‐311 ++ G(d,p) basis set. Electron delocalisation in the corresponding cations and anions was also considered. The data obtained in this work revealed that the relative delocalisation order in the OCCO group is the same as the aromaticity order determined earlier for the heterocyclic rings of hydroxypyrones (anions < neutral molecule < cation). The problem of quasiaromaticity in neutral hydroxypyrones and their cations is also considered and discussed. Copyright © 2008 John Wiley & Sons, Ltd.     

Assessment of the nature of interactions of cations with cycloheptatriene derivatives using change in the aromaticity: Comparison with electron density and NBO results

ABSTRACT

Interactions of cycloheptatriene derivatives, C₇H₆X, (X?=?NH, PH, AsH, O, S, Se) with the cations H⁺, CH₃⁺, Cu⁺, Al⁺, Li⁺, Na⁺, and K⁺ are studied using B3LYP functional and 6-311++G(d,p) basis set. The calculated gas-phase cation affinities (CA) and cation basicities (CB) for all molecules decrease as H⁺ > CH₃⁺ > Cu⁺ > Al⁺ > Li⁺ > Na⁺ > K⁺. We used the induced aromaticity in the 7-membered ring of C₇H₆X upon interaction with the cations, M⁺, as a measure of C₇H₆X/M⁺ interaction. Nucleus-independent chemical shift (NICS) and harmonic oscillator model of aromaticity (HOMA) were used as two indices of aromaticity. The highest and lowest induced aromaticities were observed for interactions of H⁺ and K⁺, respectively. Also, the aromaticity induced by interaction with a cation in C₇H₆AsH and C₇H₆PH was larger than that in C₇H₆NH and C₇H₆O. Hence, the aromaticity was considered as a measure of covalency for the C₇H₆X/M⁺ interactions showing a rational dependence on both the molecule and cation. The nature of the interactions was also assessed using electron density, charge distribution analysis and NBO calculations. The results of the aromaticity indices, NICS and HOMA, were compared with the electron density and NBO results.     

CTED: the new aromaticity index based on corrected total electron density at bond critical points

In this work, the corrected total electron density based on ellipticity (ε) at C–C bond critical points in a given ring and bond length alternations was introduced to estimate π‐electron density distributions in the ring. Then, to evaluate aromaticity of rings with any number of members, the was normalized relative to a system assumed as a full aromatic, which is named as the corrected total electron density (CTED) aromaticity index. For a wide range of aromatic, nonaromatic and antiaromatic compounds, we have compared CTED index with the other commonly used aromatic indices, such as HOMA, PDI, FLU, NICS and recently introduced EL. CTED index was seen to be in agreement with the defined indices, and with general expectations. Hence, as similar to the other indices except from PDI, we have proposed that CTED index could be applied to study the aromaticity of rings without any restriction in the number of members of rings and used to analyze both the local and global aromatic character of rings as a new aromaticity index. Copyright © 2014 John Wiley & Sons, Ltd.     

Ability of DFT to evaluate the torsional barriers in neutral and CO* protonated aromatic carbonyl compounds

Before the recent development of new functionals, the Density Functional Theory (DFT) was considered as a failing quantum chemical method in accurately computing the rotational barrier height of the gaseous benzaldehyde. Since the 2004 polemical Speakman's paper about the accuracy of microwave value of this quantity [L. D. Speakman, B. N. Papas, H. L. Woodcock, H. F. Schaefer, J. Chem. Phys. 2004, 120, 4247], the question is still relevant. This paper aims to display the ability of the DFT to evaluate the torsional barriers of a series of para‐substituted benzaldehydes in solution. The method is also tested in computing barriers of other solvated aromatic carbonyl compounds (in both neutral and carbonyl protonated forms) for which accurate experimental data are available. Computations have been carried out at two DFT methods using the popular hybrid generalized‐gradient‐approximation (GGA) density functional B3LYP and the global hybrid meta‐GGA Minnesota functional M06‐2X with a 6‐311++g (2d,2p) basis set. Solvent effects investigations were undertaken within the framework of the polarisable continuum model solvation approach. It has been concluded that the used computational methods are very satisfactory in predicting barriers of para‐substituted benzaldehydes in the liquid phase and less satisfactory for their CO*‐protonated forms. M06‐2X functional particularly leads to full agreement. For para‐substituted acetophenones, good agreement with experiment is observed only with M06‐2X applied to the CO*‐protonated forms. The present investigations have allowed to demonstrate that the Density Functional Theory does not constantly fail in accurately computing the rotational barrier heights of aromatic carbonyl compounds. Copyright © 2014 John Wiley & Sons, Ltd.     

Structural and aromatic aspects of tautomeric equilibrium in hydroxy aryl Schiff bases

The synthesis and X‐ray measurements of four Schiff bases were carried out at 100 K. The HOMA and HOSE aromaticity indices were estimated on the basis of the experimental data. The aromaticity of the phenyl ring and the chelate chain was analysed. A comparison of the aromaticity of naphthalene and phenyl derivatives of hydroxy aryl Schiff bases is presented. The balance between the aromaticity of adjacent rings of the naphthalene fragment and its effect on proton transfer is defined. Research on the interrelations between aromaticity and the intramolecular proton transfer in hydroxy aryl Schiff bases is shown. Copyright © 2008 John Wiley & Sons, Ltd.     

Aromaticity of Heterofullerenes C18BxNy (x+y=2) and Their Molecular Ions

采用拓扑共振能(TRE)和百分拓扑共振能(%TRE)方法研究了从富勒烯C20(Ih)产生的异质富勒烯C18N2、C18B2和C18BN的所有可能的异构体和分子离子的芳香性．讨论了C18BxNy异构体的芳香性和杂原子在C20(Ih)中取代位置间的关系．结果表明,中性状态和阳离子状态时各异构体的TRE都为负值,具有反芳香性．但它们高价阴离子的TRE都为正值,具有芳香性．C18N2、C18B2 和C18BN中都是杂原子在1,11-取代的异构体最稳定．异质富勒烯都比C20的芳香性高．在中性状态下的稳定性顺序为C18N2>C18BN>C18B2>C20．在形成闭壳层结构时的稳定性顺序为C18B28->C206->C18BN6->C18N24-．理论上预测C22BxNy的高价阴离子具有很高的芳香性．     

Effects of pnictogen and chalcogen bonds on the aromaticities of carbazole‐like and dibenzofuran‐like molecular skeletons: Cambridge Crystallographic Data Centre (CCDC) Study

Parameterization scheme used in geometry‐based aromaticity index Harmonic Oscillator Model for Heterocycles with π‐electrons and n‐electron delocalization was extended to cover certain pnictogen (group 15) and chalcogen (group 16) elements, for example, phosphorus, arsenic, selenium, and tellurium. Thus, assessment of aromaticities of dibenzofuran‐like and carbazole‐like molecular skeletons including the aforementioned pnictogens and chalcogens besides nitrogen, oxygen, and sulfur was made possible. Our results have shown that aromaticity of five‐membered and six‐membered rings in the considered skeletons can be independently treated from each other. Arsenic, phosphorus, and sulfur are the most suitable heteroatoms in the considered molecular skeletons to be used in designing π‐conjugated functional materials, because they have potentially smaller band gap due to the presence of the less aromatic flanking benzenes. Differences in aromaticities of two flanking benzenes of the molecular skeletons including pnictogens are more pronounced than those of molecular skeletons including chalcogens. Copyright © 2015 John Wiley & Sons, Ltd.     

Prototropy and π‐electron delocalization for purine and its radical ions – DFT studies

Complete tautomeric equilibria and π‐electron delocalization were studied at the B3LYP/6‐311+G** level for neutral purine ( P ) and its charged radicals ( P ^+? and P ^??). All possible nine tautomers (four NH and five CH forms) and all possible 36 tautomeric equilibria (six NⁱH → N^kH, twenty NH → CH, and ten CⁱH → C^kH conversions) were considered. The greatest variations of the tautomeric equilibrium constants (as pK_T) were observed for the NH → CH conversions when proceeding from neutral to reduced purine ( P + e → P ^??). These variations completely change the tautomeric preferences. One‐electron oxidation ( P ? e → P ^+?) has considerably smaller effect on the pK_T values and does not change the tautomeric preferences. π‐Electron delocalization depends on the position of the moving proton and on the type of the electron transfer. For individual tautomers, some linear relations between the relative stabilities and the HOMA (harmonic oscillator model of aromaticity) indices occur for neutral and oxidized purine. For reduced purine, a scatter plot is found. Copyright © 2010 John Wiley & Sons, Ltd.     

An Investigation of 13C-NMR Spectra of Some Intramolecular Hydrogen Bonded and Non-Bonded Azo Dyes

The ¹³C-NMR spectra of the azo dyes which were synthesized by reacting substituted benzenediazonium chloride with derivatives of some phenol have been measured. The spectral data of these compounds were described considering intramolecular hydrogen bond and not. The chemical shift assignments were made regarding substituent effects.     

A natural abundance Se solid-state NMR study of inorganic compounds

Various inorganic selenium-based compounds were analysed by ⁷⁷Se solid-state NMR, and a distinct difference in chemical shift ranges for compounds where selenium is present as selenide (Se²⁻) ionically and covalently bonded systems was observed. The selenides exhibit a shift range of approximately −700 to −100 ppm, as opposed to 700 to 1600 ppm for the compounds where there tends to be more direct covalent bonding to the selenium. The anisotropic hyperfine shift observed in NbSe₂ is shown to be axially symmetric, where the H₁₁ component is found to be normal to the Se3-trigonal plane.     

Critical evaluation of HOMA and MBL as local aromaticity indices

The harmonic oscillator model of aromaticity (HOMA) index is an excellent structural indicator of aromaticity. We found that HOMA values for internal benzene rings in large pericondensed polycyclic aromatic hydrocarbons are often overestimated because of the local aromaticity of adjacent benzene rings. The occurrence of this phenomenon was confirmed by comparing the HOMA with the corresponding SSE(LA) values; SSE(LA) is a graph‐theoretical index of local aromaticity not disturbed by the aromaticity of the adjacent benzene rings. Mean bond length values were likewise assessed by comparing them with the corresponding HOMA and SSE(LA) values.     

2D—NMR研究5—氟脲嘧啶衍生物的结构

2D—NMR是近十年来出现的新技术,在有机分子结构的研究中起着重要作用.本文是用同核、异核化学位移相关二维谱、异核中继根干传递二维谱和异核远程偶合相关二维谱,对四个新的5—氟脲嘧啶衍生物进行了全面的研究,确定了它们的结构及~1H、~(13)C谱线归属.