Conformational analysis of 1,3-dioxane in nanotubes

Russian Journal of Organic Chemistry - Computer simulation of conformational transformations of 1,3-dioxane molecule within model single-walled carbon nanotubes (hybrid DFT-method PBE/3ζ)...     

Conformational analysis of 5,5-dinitro-1,3-dioxane

Study of conformational transformations of 5,5-dinitro-1,3-dioxane by the method HF/6-31G(d), and also hybrid DFT PBE/3z and RI-MP2/λ2 indicated the only inversion route chair-chair through an intermediate minimum corresponding to the 2,5-twist-form. The potential barrier to this process was estimated and also the features of the intramolecular rotation of the axial and equatorial nitro groups in the chair conformer were investigated.     

Conformational Analysis of 5-Ethyl-5-hydroxymethyl-2,2-dimethyl-1,3-dioxane

Russian Journal of Organic Chemistry - Computer simulation of conformational transformations of 5-ethyl-5-hydroxymefhyl-2,2-dimefhyl-1,3-dioxane in the gas phase, as well as in mixtures with...     

Development of a solid-phase microextraction gas chromatography with microelectron-capture detection method for the determination of 5-bromo-5-nitro-1,3-dioxane in rinse-off cosmetics

5-Bromo-5-nitro-1,3-dioxane (bronidox) is a bromine-containing preservative often used in rinse-off cosmetics but also subjected to several restrictions according to the European Cosmetic Products Regulation. Thus, as a part of a quality control procedure, analytical methods for the determination of this compound in different types of cosmetics are required. In the present work, a solvent-free and simple methodology based on solid-phase microextraction (SPME) followed by gas chromatography with microelectron capture detection (GC-μECD) has been developed and validated for the determination of bronidox in cosmetic samples such as shampoos, body cleansers or facial exfoliants. As far as we know, this is the first application of SPME to this preservative. Negative matrix effects due to the complexity of the studied samples were reduced by dilution with ultrapure water. The influence of several factors on the SPME procedure such as fiber coating, extraction temperature, salt addition (NaCl) and sampling mode has been assessed by performing a 2⁴-factorial design. After optimization, the recommended procedure was established as follows: direct solid-phase microextraction (DSPME), using a PDMS/DVB coating, of 10 mL of diluted cosmetic with 20% NaCl, at room temperature, under stirring for 30 min. Using these suggested extraction conditions, linear calibration could be achieved, with limits of detection (LOD) and quantification (LOQ) well below the maximum authorized concentration (0.1%) established by the European legislation. Relative standard deviations (RSD) lower than 10% were obtained for both within a day and among days precision. The method was applied to diverse types of formulations spiked with bronidox at different concentration levels (0.008–0.10%); these samples were quantified by external calibration and satisfactory recoveries (≥70%) were obtained in all cases. Finally, the SPME–GC-μECD methodology was applied to the analysis of several cosmetics labeled or not as containing bronidox. The presence of this preservative in some of these samples was confirmed by GC–MS.     

Structure and Conformational Analysis of 5-Methyl-2,2-diphenyl-1,3-dioxane

Russian Journal of Organic Chemistry - The structure of 5-methyl-2,2-diphenyl-1,3-dioxane has been studied by1H and 13C NMR spectroscopy and X-ray analysis. Its molecules in crystal, as well as in...     

Conformational isomerization of 4-methyl-1,3-dioxane

The whole possible course of conformational isomerization of 4-methyl-1,3-dioxane has been established using empirical (MM+) and nonempirical [STO-3G, 3-21G, 6-31G(d) and 6-31G(d,p)] approximations within the limits of the Hartree-Fock method. It was shown that the potential energy surface of this compound contains a principal (equatorial chair conformer) and local minima corresponding to the axial chair conformer and series flexible forms.     

Conformational analysis of 2-methyl-5-nitro-1,3,2-dioxaborinane

Quantum-chemical methods HF/6-31G(d), HF/6-31+G(d), MP2/6-31G(d)//HF/6-31G(d), and MP2/6-31+G(d)//HF/6-31+G(d) were used to investigate the conformational isomerization of 2-methyl-5-nitro-1,3,2-dioxaborinane. It has been shown that a potential energy surface of this compound includes two minima: an axial form of semi-chair and equatorial sofa together with a transition state belonging to the conformation of 2,5-twist-form. A comparison between experimental NMR ¹H and theoretical vicinal coupling constants was used to determine the quantitative conformational composition of cyclic boric acid ester and a value of ΔG ⁰ for nitro group at the ring carbon atom C⁵ in CCl₄ and C₆D₅NO₂ solutions.     

Conformational transformations and autooxidation of 5-bromo-2-(2-methylpropyl)-5-nitro-1,3,2-dioxaborinane

Conformational study of 5-bromo-2-(2-methylpropyl)-5-nitro-1,3,2-dioxaborinane at the DFT PBE/3ξ level of theory revealed the only sofa–sofa interconversion pathway through a transition state corresponding to 2,5-twist conformer. The barrier to internal rotation of the axial nitro group is several times higher than that for the equatorial nitro group. According to the ¹H, ¹³C, and ¹¹B NMR, IR, and X-ray diffraction data, the main autooxidation products of 5-bromo-2-(2-methylpropyl)-5-nitro-1,3,2-dioxaborinane are 2-bromo-2-nitropropane-1,3-diol and boric acid.     

Conformational analysis of 5-substituted 1,3-dioxanes

Quantum-chemical study on the potential energy surface of 5-alkyl- and 5-phenyl-1,3-dioxanes at the RHF/6-31G(d) level of theory revealed two pathways for conformational isomerizations of the equatorial and axial chair conformers. Potential barriers to this process were estimated. The Gibbs conformational energies ΔG° of substituents at C⁵ in the 1,3-dioxane ring were determined on the basis of experimental (¹H NMR) and theoretical vicinal coupling constants, which turned out to be consistent with published data.     

Improved Studies on the Synthesis,Characterization and Crystal Structure of 2,2-Dimethyl-5-nitro-5-nitroso-1,3-dioxane

The synthetic process for 2,2-dimethyl-5-nitro-5-nitroso-1,3-dioxane(DMNNDO) was improved by using tri(hydroxymethyl)nitromethane and acetone as starting materials through a "one-pot" method combined with alkaline hydrolysis and nitrosation reactions. The yield of DMNNDO was increased from 37% to 45%. The structure of DMNNDO was characterized by IR,~1H NMR,~(13) C NMR,and elemental analysis. Also the thermal decomposition of DMNNDO was studied by using DSC and TG-DTG to find that there are primarily two exothermic decomposition processes between 90 and 300 oC. The crystal structure of DMNNDO was studied by X-ray single-crystal diffraction for the first time. The molecular structure exists as a dimeric form due to the presence of nitroso group. The crystal belongs to monoclinic system,space group C2/c with a = 14.515(3),b = 9.955(2),c = 11.897(2) ?,β = 98.500(3)°,V = 1.6998(2) nm~3,Z = 8,D_c = 1.486 g×cm~(-3),μ = 0.128 mm~(-1),F(000) = 800,S = 1.055,R = 0.0358 and wR = 0.0917. In particular,DMNNDO could be easily hydrolyzed in hydrous systems and the hydrolysis mechanism in CDCl_3 was revealed by means of NMR monitoring.     

Electrochemical reduction of 5-halo-5-nitro-1,3-dioxanes and 2-halo-2-nitro-1,3-propanediols

The character of the first stage of reduction of 5-X-5-nitro-1,3-dioxanes 1—10 and 2-X-2-nitro-1,3-propanediols 11 and 12 is independent of the nature of halogen (X = Br, Cl) and substituents in position 2 of the dioxane cycle. The transfer of two electrons to a molecule of compound 1—12 is accompanied by the anionoid elimination of halogen and formation of the anion of nitronic acid. The high mobility of halogen is mainly due to the acceptor nitro group capable of further transformations in the -position to halogen. The direction of further reduction involving the electron transfer to electrochemically active groups in the aromatic fragment of the molecule is determined by the nature of these groups. Chloro-, bromo-, and iodophenyl-substituted derivatives 4, 5, and 8—10 are reduced as typical halobenzenes. In the case of nitrophenyl-substituted compounds 3 and 7, the dioxane cycle opens to form dianions of p- and m-nitrobenzaldehydes along with the reduction of the nitroso group through the stages of formation of the radical anion and radical anion of the nitroso group. The radical anions of the nitro and nitroso derivatives were identified by ESR.     

Comparative conformational analysis of 1,3-dioxane and 1,3-dithiane

It was shown by ab initio quantum-chemical approximations HF/6-31G(d) and MP2/6-31G(d)//HF/6-31G(d) that the conformational isomerization of 1,3-dioxane and 1,3-dithiane proceeded along common routes. The potential energy surface of both compounds contains six minima including the chair invertomers and enantiomeric flexible forms. They are separated by several potential barriers. It was established by molecular dynamics method that the flexible conformers at heating and keeping at 295–300 K transformed into each other and in the chair conformer.     

Conformational analysis of 5-methyl- and 2,2,5-trimethyl-1,3-dithianes

Conformational isomerization of 5-methyl- and 2,2,5-trimethyl-1,3-dithianes was studied using quantum-chemical HF/6-31G(d), HF/pVDZ, and PBE/3z approximations. The potential energy surface of molecules of both compounds is shown to contain the main minimum corresponding to the equatorial chair conformer C-5e. The calculated potential barriers of the conformational isomerization were found. Based on the experimental (¹H NMR) and theoretical vicinal coupling constants the ??G ⁰ values were determined of the methyl group at the ring C⁵ atom. The reasons for the distinctions between the values of this parameter from the ??G ⁰, obtained by energy minimizations of the equatorial and axial chair conformers are discussed.     

Conformational analysis of 1,3-oxathiane

Computer simulation of pathways of conformational isomerization of 1,3-oxathiane molecule carried out with the help of HF/6-31G(d), MP2.6-31G(d)/HF/6-31G(d), and PNE/3z quantum-chemical approximations showed that interconversion between the degenerate in energy chair conformers proceeds through seven independent pathways: directly and via six flexible forms. Potential energy surface contains eight minimum points including chair conformers and enantiomeric pairs of twist forms, and also five transition states, among them different modification of semi-chair, symmetric and unmmetrical boat. Molecular dynamics methods show that flexible forms at room temperature convert into one another and into the chair conformers.