Abstract
Problem statement: Hydrogen bonding is one of the most important concepts in chemistry because it is crucial to understand many different interactions both in gas phase and in condensed media. A particular subset is presented by the intramolecular hydrogen bonds where two ends of the same molecule interact, resulting in a ringlike structure. Approach: The study involved theoretical investigation of proton transfer and transition states in β-thioxoketones. Both structures of the enol and enethiol tautomers of β-thioxoketones were geometrically optimized then the transition states were obtained. The calculations were done at the B3LYP hybrid and 6-31G(d,p), 6-31+G(d,p), 6- 31++G(d,p), 6-311G(d,p), 6-311+G(d,p) and 6-311++G(d,p) basis sets. Results: The calculated energies of the enethiol tautomer are higher than those of the enol tautomer for all the thioxoketones studied in this work. The S...O distances are in the order: enethiol > enol > transition state in addition the hydrogen bonding in the enol tautomer is stronger than that in the enethiol tautomer. Conclusion: The interconversion within the tautomeric mixture in these compounds is preferable in the enethiol → enol direction.
Highlights
The presence of a strong intramolecular hydrogen bond that adjoins hydroxylic and ketonic oxygen and sulpher centers markedly stabilizes the enolic tautomers causing them to dominate over their diketo counterparts under ambient, isolated-molecule conditions (Nguyen et al, 2006; Andersen et al, 2000; Posokhov et al, 2001) as is the case of their structurally related compounds, βdiketones (Chatterjee et al, 2010; Yamabe et al, 2004)
In this study we investigate the transition states of the equilibrium reaction enol-enethiol and the effect of intrahydrogen bonding in β-thioxoketones
No imaginary frequencies were found in the calculated vibrational spectra of the stable enol and enethiol structures, while a single imaginary stretching frequency was found for each Transition State (TS)
Summary
The presence of a strong intramolecular hydrogen bond that adjoins hydroxylic (proton-donating) and ketonic (proton-accepting) oxygen and sulpher centers markedly stabilizes the enolic tautomers causing them to dominate over their diketo counterparts under ambient, isolated-molecule conditions (Nguyen et al, 2006; Andersen et al, 2000; Posokhov et al, 2001) as is the case of their structurally related compounds, βdiketones (Chatterjee et al, 2010; Yamabe et al, 2004). Jorgensen et al (1981) on the basis of photoelectron spectroscopic and theoretical study for thioacetylacetone and related compounds concluded that simple β-Thioxoketones exist as equilibrium mixtures of rapidly interconverting intramolecularly hydrogen bonded enol and enethiol tautomeric forms. An ab initio study using the HF/6-31G(d,p) level for the thioketo-thioenol tautomerism of dithiomolandialdehyde and dithioacetylacetone reveled that all of the enethiol conformers of dithiomolandialdehyde were found to be more stable than the keto conformers, eventhough the chelate structures are expected to be less stable than the most of the unchelated isomers. The hydrogen bonded dithioacetylacetone was predicted to be energetically favoured with respect to the other enethiol isomers, but a non-planar thioketo structure is found to be the most stable conformation (Buemi, 1992). In this study we investigate the transition states of the equilibrium reaction enol-enethiol and the effect of intrahydrogen bonding in β-thioxoketones
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