|
Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
| ABSTRACT - Internet Electron. J. Mol. Des. July 2006, Volume 5, Number 7, 387-402 |
Enolization of Cyclopropanone and Cyclopropanethione: A Theoretical Study
Chittaranjan Rout, Sapan K. Jain, Neera Sharma, and Ramesh C. Rastogi
Internet Electron. J. Mol. Des. 2006, 5, 387-402
|
Abstract:
Cyclopropanones and cyclopropanethiones are highly reactive organic
systems containing a number of labile sites on a small carbon skeleton.
These are small unsaturated cyclic molecules with scarcely available
experimental data making theoretical studies more relevant. The
enolization reactions of cyclopropanone and its sulphur analog have
been investigated by ab initio and DFT methods. Geometry
optimizations for the structures were performed by means of analytical
gradients using the standard 6-31G(d), 6-31+G(d), 6-31G(d,p), 6-31++G(d)
and 6-31++G(d,p) basis sets with HF, MP2 and DFT with
B3LYP hybrid functional SCF methods. The analysis of electron
density distributions was carried out at MP2 and B3LYP levels using
the AIM facility in the Gaussian 03W software package. Both the
reactions are predicted to be endothermic. Inclusion of electron
correlation through B3LYP hybrid functional in DFT and MP2 energy
correction leads to significant decrease in activation enthalpies as
compared to RHF results. Bader's topological analysis predicts all the
bonds to be covalent except C=X (X = O, S). Our calculations also
predict the syn-conformer of the enol/enethiol to be more stable than
its anti form. The rotational barrier for the conversion of syn- to
anti-conformer along the C-X bond lies in the range 3.3 to 5.0 kcal mol-1.
The lower enolization and activation enthalpies for the thiocarbonyl
compounds are attributed to the thiocarbonyl bond being weaker than
the carbonyl bond. It has also been shown that the diffusability and
polarizability of sulphur play an important role in the determination of
electronic distribution in small strained rings. NPA clearly predicts
that enolization proceeds through Hδ+ (proton) transfer for
cyclopropanone, while Hδ- (hydride) like transfer is indicated for
cyclopropanethione.
|