Ab initio Study and NBO Analysis of Conformational Properties of 2-Substituted Cyclohexane-1,3-diones and its Analogues Containing S and Se Atoms

NBO analysis, hybrid density functional theory (B3LYP/6-311+G**) and ab initio molecular orbital (HF/6-311+G**) based methods were used to study the anomeric effects (AE), electrostatic interactions, dipole-dipole interactions and steric repulsion effects on the conformational properties of 2-methoxy- (1), 2-methylthio- (2), 2-methylseleno- (3), 2-fluoro- (4), 2-chloro- (5) and 2-bromocyclohexane-1,3-dione (6). The B3LYP/6-311+G** and HF/6-311+G** results indicates the axial preference in these compounds. The methods used show that these compounds exist predominantly in the axial chair conformation and the axial conformation stability and calculated Gibbs free energy difference (ΔGeq-ax) values between the axial and equatorial conformations increase from 1 to its analogous 3 and also from 4 to its analogues 6. The NBO analysis of donor-acceptor interactions show that the GAE (Generalized Anomeric Effect) increases from compound 1 to compound 3 and also from compound 4 to compound 6. GE (Gauche Effect) does not have significant impact on the conformational behaviors of compounds that have been studied and GAE succeeds in accounting qualitatively for the increase of the axial preferences. On the other hand, the calculated differences between the dipole moment values of the axial and equatorial conformations, Δ(μeq-μax), are not in the same trend observed for the corresponding GAE and ΔG values. These findings led to the proposal that the calculated GAE values due to donor→acceptor hyperconjugation effects are more significant for the explanation of the conformational preferences of compounds that have been studied than the electrostatic interactions. Also similar results are obtained for their analogous containing S and Se atoms. The correlations between the GAE, GE, dipole-dipole interactions, donor and acceptor orbital energies and occupancies, bond orders, structural parameters and conformational behavior of compounds (1-6) and their analogous containing S and Se atoms have been investigated.