Study of influence of anomeric effect on conformational preferences using hybrid density functional theory (DFT) and natural bond orbital (NBO) interpretation

For the relationship between thermodynamic parameters, global hardness, global electronegativity, anomeric effect, structural parameters, and structural properties of dodecahedro-spiro [isochromene-3,2'-pyran] (A), dodecahedro-spiro [isothiocromene-3,2'- thiopyran) (B), dodecahedro-spiro [isochroman-3,2'-thiopyran] (C) and dodecahedro-spiro [isothiochrome-3,2'-pyran] (D) computational methods (B3LYP / 6-311 ++ G **) were used. Results show that the calculated Gibbs free energy and enthalpy differences between the axial- and equatorial- stereoisomers [i.e. ΔG = Geq – Gax, ΔH = Heq- Hax] decrease from compound A to compound B and from compound C to compound D. NBO results show that the anomeric effect associated with the electron delocalization is in benefit of the axial-stereoisomers and with the decrease of the anomeric effect, the values of global hardness (η), and global electronegativity (χ), from compound A to compound B and from compound C to Composition D is also decrease. The correlations between the global hardness, global electronegativity, anomeric effect, structural parameters, and thermodynamic parameters [ΔH, ΔG, and ΔS] of compounds A-D have been investigated. Also, it can be concluded that the stability energy obtained from the numerical values of the anomeric effect and the values calculated theoretically (B3LYP / 6-311 ++ G **) confirmed each other.