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Search Results: 1 - 10 of 303012 matches for " J. GUILLERMO; GERLI "
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CONFORMATIONAL PREFERENCE OF 4-ETHYL- 6-METHYL-1,3-DITHIANES
CONTRERAS,J. GUILLERMO; GERLI,LORENA A.;
Journal of the Chilean Chemical Society , 2007, DOI: 10.4067/S0717-97072007000300018
Abstract: interconversion reaction of i ? ii conformers of 4-ethyl-6-methyl-1,3-dithiane (metdit) has been studied by means of ab initio methods on the frame of mo theory. optimized geometries at hf/6-31g** level correlate well with that found for the parent 1,3-dithianefrom an x-ray diffraction study. both conformers possess similar bond distances, but differ in up 12° in some dihedrals. conformer i geometry presents the larger distortion from the regular 1,3-dithiane mainly due to difference between angles κ1 and κ2 (ca. 14°). in conformer ii this difference is just ca. 3°. the energetics and thermodynamics were obtained with basis sets that include diffuse, polarization functions and electronic correlations at the second-order perturbation meller plesset theory. gas phase thermodynamic predicts ii to be in a 98% concentration. low and medium high polarity solvents seem to exert no influence on the conformers concentrations. thus the i ? ii interconversion reaction is largely displaced to ii formation, both in the gas phase and solution. the calculated 1h-nmr data, i.e., chemical shifts and one-bond c-h spin-spin coupling constants are predicted on the basis of the consistent results obtained for the parent 1,3-dithiane. for both conformers reverse perlin effect takes place at c2 and c5. the c2 - hax and c2 - heq bond distances are similar ( ca. 1.092 ?), whereas the c5- heq are larger than the axial ones yielding smaller 1jc-h coupling constants
CONFORMATIONAL PREFERENCE IN 4,6-DIMETHYL-l,3-THIOXANE
CONTRERAS,J. GUILLERMO; GERLI,LORENA A.;
Journal of the Chilean Chemical Society , 2008, DOI: 10.4067/S0717-97072008000100014
Abstract: trans-4,6-dimethyl-l,3-thioxane shows two conformations as a result of the heterocyclic ring inversion. the i and ii conformers interconversion process has been studied by means of ab initio methods. the thermodynamics of this reaction calculated at the mp2/6-311+g**// hf/6-31g" and b3lyp/6-311+g"// b3lyp/6-31g" levels yield ag° values of ca. -1.5 kcal/mol in the gas phase, implying that ii is the most important species in the gas phase and that its relative concentration is ca. 93%. in solution of low and medium polarity solvents, pcm method predicts an 85% concentration for this species, consistent with an 83% estimated from 'h-nmr spectra. the experimental nmr properties ( 8 and 'jc_h ) have not been reported since i and ii separation could not be achieved. the results obtained for two long range "j2ax6e and 6j2e je coupling constants of ca. 0.5 hz are in good agreement with 0.41 hz, calculated in the present work. the calculated chemical shifts for the c2 and c5 protons show that 8heq < 8hax implying that some anomalous effects are operating, though the 8 seems to be moderately insensitive c-h bond distances changes. the spin-spin coupling constants are much more sensitive to changes occurring in the ring. thus, normal and reverse perlin effects for 'j are predicted for c2 -h and c5-h are operating, respectively. the cc5 - heq -> (7*c - s interaction would be the responsible for the very small elongation of the c5 - heq over the c5 - hax bond distances
CONFORMATIONAL PREFERENCE IN 4,6-DIMETHYL-l,3-THIOXANE
J. GUILLERMO CONTRERAS,LORENA A. GERLI
Journal of the Chilean Chemical Society , 2008,
Abstract: Trans-4,6-dimethyl-l,3-thioxane shows two conformations as a result of the heterocyclic ring inversion. The I and II conformers interconversion process has been studied by means of ab initio methods. The thermodynamics of this reaction calculated at the MP2/6-311+G**// HF/6-31G" and B3LYP/6-311+G"// B3LYP/6-31G" levels yield AG° values of ca. -1.5 kcal/mol in the gas phase, implying that II is the most important species in the gas phase and that its relative concentration is ca. 93%. In solution of low and medium polarity solvents, PCM method predicts an 85% concentration for this species, consistent with an 83% estimated from 'H-NMR spectra. The experimental NMR properties ( 8 and 'J C_H ) have not been reported since I and II separation could not be achieved. The results obtained for two long range "J2ax6e and 6J2e Je coupling constants of ca. 0.5 Hz are in good agreement with 0.41 Hz, calculated in the present work. The calculated chemical shifts for the C2 and C5 protons show that 8Heq < 8Hax implying that some anomalous effects are operating, though the 8 seems to be moderately insensitive C-H bond distances changes. The spin-spin coupling constants are much more sensitive to changes occurring in the ring. Thus, normal and reverse Perlin effects for 'J are predicted for C2 -H and C5-H are operating, respectively. The cC5 - Heq -> (7*C - S interaction would be the responsible for the very small elongation of the C5 - Heq over the C5 - Hax bond distances
CONFORMATIONAL PREFERENCE OF 4-ETHYL- 6-METHYL-1,3-DITHIANES
J. GUILLERMO CONTRERAS,LORENA A. GERLI
Journal of the Chilean Chemical Society , 2007,
Abstract: Interconversion reaction of I II conformers of 4-ethyl-6-methyl-1,3-dithiane (METDIT) has been studied by means of ab initio methods on the frame of MO theory. Optimized geometries at HF/6-31G** level correlate well with that found for the parent 1,3-dithianefrom an x-ray diffraction study. Both conformers possess similar bond distances, but differ in up 12° in some dihedrals. Conformer I geometry presents the larger distortion from the regular 1,3-dithiane mainly due to difference between angles κ1 and κ2 (ca. 14°). In conformer II this difference is just ca. 3°. The energetics and thermodynamics were obtained with basis sets that include diffuse, polarization functions and electronic correlations at the second-order perturbation Meller Plesset theory. Gas phase thermodynamic predicts II to be in a 98% concentration. Low and medium high polarity solvents seem to exert no influence on the conformers concentrations. Thus the I II interconversion reaction is largely displaced to II formation, both in the gas phase and solution. The calculated 1H-NMR data, i.e., chemical shifts and one-bond C-H spin-spin coupling constants are predicted on the basis of the consistent results obtained for the parent 1,3-dithiane. For both conformers reverse Perlin effect takes place at C2 and C5. The C2 - Hax and C2 - Heq bond distances are similar ( ca. 1.092 ), whereas the C5- Heq are larger than the axial ones yielding smaller 1J C-H coupling constants
STUDY OF THERMODYNAMIC AND NMR PROPERTIES OF SOME CYCLOHEXANE DERIVATIVES
GERLI C,LORENA A; SALAS,M. PAULINA; CONTRERAS,J. GUILLERMO;
Journal of the Chilean Chemical Society , 2011, DOI: 10.4067/S0717-97072011000400021
Abstract: the preferential conformations of a series of six-membered saturated heterocycles containing oxygen and sulfur atoms, 4-alkyl-6-methyl-1,3-dithiane, with alkyl=methyl, ethyl, propyl, isobuthyl, terbuthyl have been studied by means of ab-initio theoretical methods. the chair conformation is the most stable structure found for all compounds studied here, likewise in its counterparts: cyclohexane, dithiane, and dioxane. the structures show anomalous effects of the chemical shifts, in c2 and c5, due to a σ c-s → σ* c-hec hyperconjugative interaction. they are also affected by normal perlin effect in c2, where c2-hec coupling constants are larger than c2-hax.
STUDY OF THERMODYNAMIC AND NMR PROPERTIES OF SOME CYCLOHEXANE DERIVATIVES
LORENA A GERLI C,M. PAULINA SALAS,J. GUILLERMO CONTRERAS
Journal of the Chilean Chemical Society , 2011,
Abstract: The preferential conformations of a series of six-membered saturated heterocycles containing oxygen and sulfur atoms, 4-alkyl-6-methyl-1,3-dithiane, with alkyl=methyl, ethyl, propyl, isobuthyl, terbuthyl have been studied by means of ab-initio theoretical methods. The chair conformation is the most stable structure found for all compounds studied here, likewise in its counterparts: cyclohexane, dithiane, and dioxane. The structures show anomalous effects of the chemical shifts, in C2 and C5, due to a σ C-S → σ* C-Hec hyperconjugative interaction. They are also affected by normal Perlin effect in C2, where C2-Hec coupling constants are larger than C2-Hax.
MECHANISM OF ISOMERIZATION IN DITHIETANE-1,3-DIOXIDE
CONTRERAS,J.GUILLERMO; GERLI,LORENA A.; HURTADO,S.MARCELA; MADARIAGA,SANDRA T.;
Journal of the Chilean Chemical Society , 2004, DOI: 10.4067/S0717-97072004000400011
Abstract: the cis ¤ trans isomerization reaction of dithietane-1,3-dioxide ( dtdo) has been studied by means of theoretical methods in the framework of molecular orbital theory. the energetics of this reaction was obtained with several basis sets and levels including both polarization and diffuse functions. electronic correlation has been taken into account by applying second-order perturbation mller-plesset theory. the thermodynamics indicates that both isomeric forms are in almost the same concentration in the gas phase, whereas in solution in more polar solvents, the cis form becomes more important. the transition state for the cis ¤ trans interconversion reaction has been fully characterized and the activation energy has been calculated to be 47.8 kcal/mol in the gas phase. in general, the solvent, simulated by the pcm model, lowers ea as its polarity increases, and in dmso (e = 46.7) ea goes down by ca. 20 kcal/mol. the calculated 1h-nmr chemical shifts are consistent with the experimental data. the predicted 17o and 33s-nmr spectra should allow to distinguish between the two conformations
THEORETICAL AND CHEMOMETRIC STUDY OF SUBSTITUTED OXAZINES
CONTRERAS,J. GUILLERMO; FERREIRA,MARCIA M.C; HURTADO,S. MARCELA; GERLI,LORENA; CASTILLO,ROSARIO;
Journal of the Chilean Chemical Society , 2005, DOI: 10.4067/S0717-97072005000400015
Abstract: the application of the principal component of analysis (pca) over the theoretical data of a series of substituted 1,3-oxazines helps to determine the factors that rule the conformational equilibrium. repulsive and intertribal syn-1,3-diaxial interactions have been used as determiners for the axial structures, whereas dipolar interactions are preferred in the equatorial conformation. theoretical variables such as hardness, charges, nuclear repulsion energy and bond order in the different nuclear sectors have been used. pca method has been successfully used in predicting the hyperconjugative interactions that play an important role in the conformational equilibrium between a given pair of conformers of 1,3-oxazines. the studied 1,3-oxazines belong to three different groups of structures having the heterocyclic ring as a common feature. the axial structures are stabilized by two-electron/two-orbital interactions, whereas the equatorial structures are believed to be stabilized by syn-1,3-diaxial and dipole-dipole interactions
THEORETICAL AND CHEMOMETRIC STUDY OF SUBSTITUTED OXAZINES
J. GUILLERMO CONTRERAS,MARCIA M.C FERREIRA,S. MARCELA HURTADO,LORENA GERLI
Journal of the Chilean Chemical Society , 2005,
Abstract: The application of the Principal Component of Analysis (PCA) over the theoretical data of a series of substituted 1,3-oxazines helps to determine the factors that rule the conformational equilibrium. Repulsive and intertribal syn-1,3-diaxial interactions have been used as determiners for the axial structures, whereas dipolar interactions are preferred in the equatorial conformation. Theoretical variables such as hardness, charges, nuclear repulsion energy and bond order in the different nuclear sectors have been used. PCA method has been successfully used in predicting the hyperconjugative interactions that play an important role in the conformational equilibrium between a given pair of conformers of 1,3-oxazines. The studied 1,3-oxazines belong to three different groups of structures having the heterocyclic ring as a common feature. The axial structures are stabilized by two-electron/two-orbital interactions, whereas the equatorial structures are believed to be stabilized by syn-1,3-diaxial and dipole-dipole interactions
MECHANISM OF ISOMERIZATION IN DITHIETANE-1,3-DIOXIDE
J.GUILLERMO CONTRERAS,LORENA A. GERLI,S.MARCELA HURTADO,SANDRA T. MADARIAGA
Journal of the Chilean Chemical Society , 2004,
Abstract: The cis ¤ trans isomerization reaction of dithietane-1,3-dioxide ( DTDO) has been studied by means of theoretical methods in the framework of molecular orbital theory. The energetics of this reaction was obtained with several basis sets and levels including both polarization and diffuse functions. Electronic correlation has been taken into account by applying second-order perturbation Mller-Plesset theory. The thermodynamics indicates that both isomeric forms are in almost the same concentration in the gas phase, whereas in solution in more polar solvents, the cis form becomes more important. The transition state for the cis ¤ trans interconversion reaction has been fully characterized and the activation energy has been calculated to be 47.8 kcal/mol in the gas phase. In general, the solvent, simulated by the PCM model, lowers Ea as its polarity increases, and in DMSO (e = 46.7) Ea goes down by ca. 20 kcal/mol. The calculated 1H-NMR chemical shifts are consistent with the experimental data. The predicted 17O and 33S-NMR spectra should allow to distinguish between the two conformations
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