Search

2-Methoxytetrahydropyran (1), -thiopyran (2) and -selenopyran (3) have been chosen as model compounds to investigate the origin of the anomeric effect (AE). The impacts of the hyperconjugation, electrostatic and steric interactions on the conformational preferences of compounds 1–3 have been analysed by means of complete basis set-4, hybrid-density functional theory (B3LYP/6-311+G**) based methods and natural bond orbital (NBO) interpretation. Both levels of theory showed that the axial conformations of compounds 1–3 are more stable than their equatorial conformations. The Gibbs free energy difference (G eq–G ax) values (i.e. ΔG eq–ax) between the axial and equatorial conformations increase from compound 1 to compound 2 but decrease from compound 2 to compound 3. Based on the NBO results obtained, the AE associated with the electron delocalisation [i.e. Σ(endo-AEeq + exo-AEeq) − Σ(endo-AEax + exo-AEax)] increase slightly from compound 1 to compound 2 but decrease from compound 2 to compound 3. Similar trend is also observed for the differences between the calculated total steric exchange energy values [i.e. Δ(TSEE)eq–ax]. On the other hand, the calculated differences between the dipole moment values of the axial and equatorial conformations [i.e. Δ(μeq–μax)] decrease from compound 1 to compound 3. These findings led to the proposal that the AE associated with the electron delocalisation (the hyperconjugation effect) is more significant for the explanation of the conformational preferences of compounds 1–3 than the electrostatic model. The correlations between the AE associated with the electron delocalisation, bond orders, TSEE, ΔG eq–ax, dipole–dipole interactions, structural parameters and conformational behaviours of compounds 1–3 have been investigated. [ABSTRACT FROM PUBLISHER]

Complete basis set CBS-QB3, hybrid-density functional theory (B3LYP/Def2-TZVPP) based methods and NBO interpretation were used to investigate the impacts of the stereoelectronic effects and electrostatic and steric interactions on the conformational properties of halocarbonyl isocyanates (halo = F ( 1), Cl (2), and Br ( 3)), halothiocarbonyl isocyanates (halo = F ( 4), Cl ( 5), and Br ( 6)), and haloselenocarbonyl isocyanates(halo = F ( 7), Cl ( 8), and Br ( 9)). Both methods showed that the Z-conformations of compounds 1, 4, and 7 are more stable than their corresponding E conformations, but the stability of the E conformations, when compared with the corresponding Z conformations, increases from compound 1 to compound 3, compound 4 to compound 6, and also from compound 7 to compound 9. The NBO analysis showed that the generalized anomeric effect (GAE) is in favor of the Z conformations of compounds 1, 4, and 7. The GAE values calculated (i.e., GAEE-GAEZ) increase from compound 1 to compound 3, compound 4 to compound 6, and also from compound 7 to compound 9. On the other hand, there are none of the same trends between the calculated total dipole moment and the Gibbs free energy difference values between the E and Z conformations (i.e., Δ μE-Z and Δ GE-Z) of compounds 1- 3, 4- 6, and 7- 9. Accordingly, the GAE succeeds in accounting for the increase of the E conformation stability from compound 1 to compound 3, compound 4 to compound 6, and also from compound 7 to compound 9. Therefore, the GAE associated with the electron delocalization, not the total dipole moment changes (i.e., Δ μE-Z), is a reasonable indicator of the total energy difference in compounds 1- 3, 4- 6, and 7- 9. There is a direct correlation between the calculated GAE and Δ[ r2-6( E) - r2-6( Z)] parameters. Importantly, there are interesting through-space electron delocalizations (LP2X6→π*C4-O5) that justify the increase of the E conformation stability from compound 1 to compound 3, compound 4 to compound 6, and also from compound 7 to compound 9, when compared with their corresponding Z conformations. The correlations between the GAE, bond orders, total steric exchange energies (TSEE), Δ GZ-E, Δ μE-Z, structural parameters, and conformational behaviors of compounds 1- 9 were investigated. [ABSTRACT FROM AUTHOR]