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

Keywords: 4 , 6 - dimethyl - 1 , 3 - Thioxane , conformational preference , NMR properties , NBO analysis

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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

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