Quantum chemical studies on some potentially tautomeric oxazolidin-4-one derivatives and their thio and azo anologs

The basicities and nucleophilicities along with prototautomerism of biologically active oxazolidin-4-one and its thio and azo analogs were investigated by semi-empirical methods. The oxo and thion protonation were found to be easier than that of azo protonation for 4-oxo and 4-thion derivatives whereas amino protonation was found to be easier than imino and azo protonation in 4-imino derivative. The preferred tautomeric form for 4-oxo and 4-thion derivatives were found to be the keto and thion forms, respectively, whereas the amino form was found to be preferred in 4-imino derivatives. An acceptable correlation between gas phase proton affinities and aqueous phase acidity constants as well as the correlation between nucleophilicity and acidity constants was observed.     

Aspects of tautomerism. Part 16. Influence of the y-keto function on the reactions of sulphonic acids

Reaction of sodium 2-formylbenzenesulphonate (1) with thionyl chloride or phosphorous pentachloride gives a mixture of pseudo (2) and normal (3) sulphonyl chlorides. Whereas ammonium 2-carboxybenzenesulphonate (6) gives only the normal sulphonyl chloride (7) on reaction with thionyl chloride, a mixture of normal (7) and pseudo (8) isomers are formed on reaction with phosphorous pentachloride. Sodium 2-benzoylbenzenesulphonate (15), on the other hand, gives the corresponding normal sulphonyl chloride (16) on reaction with both of the reagents mentioned above. Based on these observations it is concluded that γ-keto sulphonic acids are amenable to the influence of γ-carbonyl group as in the case of γ-keto carboxylic acids but to a lesser extent.     

Quantum chemical studies on tautomerism of barbituric acid in gas phase and in solution

Ab initoand density functional theory (DFT) methods were used to study the tautomers of barbituric acid in the gas phase and in a polar medium. In the gas phase, the tautomers were optimized at the HF/6-31G^*, MP2/6-31G^*and B3LYP/6-31G^*, B3PW91/6-31G^*levels of theory. The self-consistent reaction field theory (SCRF) at the HF/6-31G^*level of theory has been used to optimize the tautomers in a polar medium. The relative stability of the tautomers was compared in the gaseous and polar mediums. The ability of maximum hardness principle to predict the stable tautomer has been studied. The ¹³C-NMR chemical shift for carbon atoms in the tautomers was calculated and the results are discussed.     

丙烯醛光致脱羰机理的量子化学研究

用量子化学方法研究了丙烯醛基态和激发态的反应途径, 通过比较不同反应途径的反应势垒和中间产物构型的稳定性, 从理论上得出该反应的反应机理。     

Quantum chemical studies on tautomerism of 2-, 3- or 4-hydroxyquinoline derivatives along with their thio and azo analogs

The geometries, relative stabilities and proton affinities for the different tautomers of 2-, 3- and 4-hydroxyquinoline derivatives and their thio and azo analogs along with their fixed forms (i.e. model molecules in which the proton migration is eliminated) were calculated with full geometry optimization using AM1, PM3 and MNDO methods. The predominance of oxo forms over hydroxy forms were confirmed with all three methods both in gas and liquid phases, as cited in the literature, with the exception of 3-hydroxyquinoline for which the AM1 and MNDO methods both in gas and liquid phases suggest the predominance of the hydroxy form. For the thio analogs the predominance of thione forms over mercapto forms and for the amino analogs the predominance of the amino forms over imino forms were confirmed both in gas and liquid phases with all three methods as indicated in the literature.     

Quantum chemical studies on antimalarial of artemisinin (qinghaosu) derivatives

In this paper theoretical studies were performed on artemisinin (qinghaosu) derivatives with semiempirical quantum chemical methods AMI and PM3. The antimalarial activity -logC has an obvious correlation with the net charge of C(16) and bond orders of bonds O(1)-C(10), O(2)-C(6), O(1)-O(2) and O(5)-C(16). According to the calculation results, we derived structure-activity relationship, presented the probable pharmacophore of qinghaosu derivatives and the interaction fashion between the drugs and the plasmodium receptor.     

Substituent effects on the ring-chain tautomerism of some 1,3-oxazolidine derivatives

The 14 and 70 eV electron ionization mass spectra of five sets (R1 = Me, Et, i-Pr, t-Bu and Ph) of seven 2-aryl-4-R1-substituted (Ar = C6H4X; X = p-NO2, m-Br, p-Cl, H, p-Me, p-OMe and p-NMe2) (1-5) and of seven 2-aryl-5-phenyl-substituted 1,3-oxazolidines (6; for Ar, see above) were recorded to study their ring-chain equilibria in the gas phase. These equilibria were also studied by 1H NMR spectroscopy in CDCl3 for compounds 5 and 6. A few 2,4- and 2,5-dimethyl-2-aryl derivatives (7, 8: Ar = C6H4X; X = m-Br, H and p-OMe) were studied both in CDCl3 and in the gas phase. The main characteristics of the ring-chain equilibria expressed by the variable SigmaRA% of the ring and of the chain form proved to be a strong dependence on the nature of the substituents on C-2 and C-4. The results in the gas phase are compared with those in CDCl3.     

Quantum chemical calculations of the molecular structure of diphenylguanidine and experimental studies of its tautomerism in solution

MNDO-PM3 and AM1 quantum chemical calculations of the molecular structure of N,N′-diphenyl-guanidine (DPG) prove that the molecule preferably exists as an asymmetric tautomer with a basic center C = N−. IR and UV spectral data, dipole moments, and average molecular weights of DPG solutions in nonpolar and low-polar solvents confirm the results of calculations, indicating formation of hydrogen-bonded cyclic self-associates of DPG in solution. Translated fromZhumal Strukturnoi Khimii, Vol. 39, No. 4, pp. 618–625, July–August, 1998.     

Quantum chemical studies on the structures, properties and decomposition of the azido derivatives of trinitrobenzenes

The geometries,heats of formation and electronic structures of 15 azido-derivatives of 1,2,3-TNB (Ⅰ),1,2,4-TNB (Ⅱ) and 1,3,5-TNB (Ⅲ) have been studied using quantum chemical AMI method at HF level.The effect of azido substitution on the structures and properties of TNBs has been discussed and the relative stability of the title compounds has been established.The processes of the decomposition of the title compounds by breaking C-NO2,C-N3 and CN-N2 bonds are investigated at UHF-AM1 level.It is shown that the decomposition of the title compounds may be initiated by the cleavage of both C-NO2 and N-N2 bonds.     

Quantum chemical studies on prototautomerism of 1H‐imidazo[4,5‐c]pyridine

The structural features of the 1H‐imidazo[4,5‐c]pyridine (ICPY) tautomers and homodimers of the most stable tautomers have been studied by quantum chemical methods. FTIR and Raman spectra of the ICPY were recorded in the range of 4000–60 cm^?1 and 3500–5 cm^?1. The predominant tautomer among four possible isomers of ICPY were determined. The optimized geometries and vibrational frequencies of possible ICPY tautomers and dimers were computed by B3LYP/DFT method with 6‐311++G(d,p) and 6‐31G(d) basis sets. All vibrational frequencies assigned in detail with the help of total energy distribution (TED) and isotopic shifts. ICPY dimeric forms were also characterized according to their hydrogen bonding interactions, and it has been found that the most stable ICPY homodimer establishes moderate strong N ? H …N type hydrogen bond. ¹H NMR, ¹³C NMR, and ¹⁵N NMR properties have been calculated for all tautomeric forms using the gauge independent atomic orbital (GIAO) method. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

New synthesis on the basis 2‐allyloxy chalcone and NMR studies its some derivatives

Synthesis and NMR investigations of cyclohexenone, flavanone, isoxazol and indazole derivatives of (2E)‐3‐[2‐(allyloxy)‐5‐bromophenyl]‐1‐(2‐hydroxy‐5‐methylphenyl)‐2‐propen‐1‐one ( I , chalcone) have been carried out. The results confirm the formation of O–H???O type intramolecular hydrogen bond and intramolecular cyclization in the (2E)‐3‐[2‐(allyloxy)‐5‐bromophenyl]‐1‐(2‐hydroxy‐5‐methylphenyl)‐2‐propen‐1‐one ( I ), the presence of conformational and keto–enol tautomeric transitions in the 6‐acetyl‐5‐[2‐(allyloxy)‐5‐bromophenyl]‐3‐(2‐hydroxy‐5‐methylphenyl)‐2‐cyclohexen‐1‐one ( II ), conformational transitions in the 2‐{4‐[2‐(allyloxy)‐5‐bromophenyl]‐3‐methyl‐4.5‐dihydro‐1.2‐benzisoxazol‐6‐yl}‐4‐methylphenol ( III ) and 2‐{4‐[2‐(allyloxy)‐5‐bromophenyl]‐3‐methyl‐4.5‐dihydro‐1H‐indazol‐6‐yl}‐4‐methylphenol ( IV ). The conformational and keto–enol tautomerism in the investigated compounds have been also confirmed by chemical methods. Copyright © 2014 John Wiley & Sons, Ltd.     

Quantum chemical studies on the bonding characteristics of some M3X4 transition-metal halogenide clusters

Ab initio calculations with effective core potentials have been performed on the trinuclear incomplete cubane-type cores in some cluster compounds with halogen as the bridging atoms. By comparison with the bonding and electronic structures of analogous chalcogenides of M₃O _n S _4–n ⁴⁺ and M₃O _n Se _4– ⁴⁺ the metal-metal bonding, metal-ligand bonding and the electronic characteristics of halogenide clusters of Nb₃CI ₄ ³⁺ , M₃OCl ₃ ⁵⁺ (M = Mo, W), Mo₃OBr ₃ ⁵⁺ , M₃Cl ₄ ⁵⁺ (M = Nb, Ta), and Cu₃I ₄ ^– are discussed in terms of delocalized and localized molecular orbitals. One three-centered two-electron bond has been found to exist among the three skeletal metals in the clusters having eight skeletal electrons. The reason why these transition-metal halides do not show the so-called quasi-aromatic (i.e. benzene-like) character is attributed to the lack of strongd – p bonding in the puckeredM ₃(µ–X)₃ six-membered rings.     

Subphase pH effect on the limiting molecular area of amphiphilic β-diketones in Langmuir monolayers

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有机氟化合物量子化学研究 I: 二氟乙炔分子中化学键特性的从头计算

实验发现, F-C≡C-F与H-C≡C-H相比, 其C≡C三重键的离解能要小250.8kJ/mol,而该键的键长却比C~2H~2的短。这与"键越短键就越强"的传统看法不一致。我们通过从头计算研究, 发现主要原因是C~2F~2分子中F原子的孤对电子对C≡C三重键起反键作用, 从而削弱了C≡C三重键的强度; F原子的吸电子性又使C的原子轨道收缩效应增强,而使得C≡C三重键变短。     

Computational Investigation of the Effect of pH on the Color of Firefly Bioluminescence by DFT

In spite of recent advances towards understanding the mechanism of firefly bioluminescence, there is no consensus about which oxyluciferin (OxyLH₂) species are the red and yellow‐green emitters. The crystal structure of Luciola cruciata luciferase (LcLuc) revealed different conformations for the various steps of the bioluminescence reaction, with different degrees of polarity and rigidity of the active‐site microenvironment. In this study, these different conformations of luciferase (Luc) are simulated and their effects on the different chemical equilibria of OxyLH₂ are investigated as a function of pH by means of density functional theory with the PBE0 functional. In particular, the thermodynamic properties and the absorption spectra of each species, as well as their relative stabilities in the ground and excited states, were computed in the different conformations of Luc. From the calculations it is possible to derive the acid dissociation and tautomeric constants, and the corresponding distribution diagrams. It is observed that the anionic keto form of OxyLH₂ is both the red and the yellow‐green emitter. Consequently, the effect of Luc conformations on the structural and electronic properties of the Keto‐(?1) form are studied. Finally, insights into the Luc‐catalyzed light‐emitting reaction are derived from the calculations. The multicolor bioluminescence can be explained by interactions of the emitter with active‐site molecules, the effects of which on light emission are modulated by the internal dielectric constant of the different conformations. These interactions can suffer also from rearrangement due to entry of external solvent and changes in the protonation state of some amino acid residues and adenosine monophosphate (AMP).     

Experimental (13C NMR) and theoretical (ab initio molecular orbital calculations) studies on the prototropic tautomerism of benzotriazole and some derivatives symmetrically substituted on the benzene ring

The prototropic tautomerism in anhydrous DMSO of benzotriazole and six derivatives symmetrically substituted on the benzene ring (5,6-dichloro, tetrachloro, 4,7-dibromo, tetrabromo, 5,6-dimethyl, and tetramethyl), was followed by both experimental (13C NMR and UV spectroscopy) and theoretical methods. In all of the analyzed systems, predominance of the asymmetric form, N(1)/N(3) protonated, was found. The rates of the N(1)-H<-->N(3)-H prototropic equilibrium, estimated by 13C NMR techniques, were in the medium exchange regime of 300-3000 s(-1), and are correlated with the spectroscopically determined pKa values in aqueous medium, and the anionic forms are the putative rate-limiting intermediate states.     

Quantum chemical and experimental studies on the mechanism of alkylation of beta-dicarbonyl compounds. The synthesis of five and six membered heterocyclic spiro derivatives

The alkylation of beta-dicarbonyl compounds in a K2CO3/DMSO system was found to afford O- and C-alkylated derivatives, depending on the type of the beta-dicarbonyl compound involved. The alkyl derivatives obtained were used in the synthesis of some new spiro barbituric acid derivatives. Quantum chemical calculations were carried out to elucidate the reaction mechanisms for some typical synthesis.