CONFORMATIONAL PREFERENCE IN 4,6-DIMETHYL-l,3-THIOXANE

Abstract

Trans-4,6-dimethyl-1,3-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 ∆G o 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 1 H-NMR spectra. The experimental NMR properties (δ and 1 J C-H) have not been reported since I and II separation could not be achieved. The results obtained for two long range 4 J 2ax,6eq and 6 J 2eq,5eq 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 δHeq < δHax implying that some anomalous effects are operating, though the δ 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 1 J C-H are predicted for C2-H and C5-H are operating, respectively. The σC5-Heq → σ * C-S interaction would be the responsible for the very small elongation of the C5-Heq over the C5-Hax bond distances.

Modeling the transition state structure to probe a reaction mechanism on the oxidation of quinoline by quinoline 2-oxidoreductase

“…The transition state structure was located by the presence of one imaginary negative frequency [16]. The geometries from single point energy calculations were used for AOMix molecular analysis using AOMix 2011/2012 (reversion 6.6) software programs [17, 18].…”

Adsorción y oxidación de monóxido de carbono en una lamina de ß-grafino: Estudio teórico

“…Se calcularon también frecuencias vibracionales para verificar la naturaleza de los puntos estacionarios como mínimos o estados de transición de primer orden al presentar solo una frecuencia imaginaria. Asimismo para el estado de transición se realizaron cálculos de "Coordenada Intrínseca de Reacción" (CIR) para verificar que dichos estados de transición localizados conectaban con los correspondientes puntos mínimos estacionarios asociados a reactantes y productos [17][18].…”

Modeling the Transition State Structures of the Reductive-Half Reaction Active Site of Xanthine Oxidase Bound to Guanine Analogues: A Density Functional Theory Approach

“…In this work, the effect of chalcogen replacement and methyl group at the transition is expected to be addressed. Although the transition state structure is characterized by one imaginary negative frequency [Ayala, and Schlegel, (1997) and Contreras, Lorena and Gerli, (2008)], the imaginary negative frequencies and linear motions of the H RH , at the transition state, is also proposed to vary depending on the affinity of the substrates (orientation of substrates and substrate-binding sites) [Bayse, (2009)]. The frequencies and intensities are expected to depend on the conformation and binding-sites of the substrates.…”