Complete basis set, hybrid-density functional theory study, and natural bond orbital interpretations of the conformational behavior of halocarbonyl, thiocarbonyl, and selenocarbonyl isocyanates.

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]