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., [GAE.sub.E]-[GAE.sub.Z]) 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[μ.sub.E-Z] and Δ[G.sub.E- 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[μ.sub.E-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[[r.sub.2-6](E) - [r.sub.2-6](Z)] parameters. Importantly, there are interesting through-space electron delocalizations ([LP.sub.2][X.sub.6]→[π.sup.*.sub.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[G.sub.Z-E], Δ[μ.sub.E-Z], structural parameters, and conformational behaviors of compounds 1-9 were investigated. Key words: halocarbonyl isocyanates, stereoelectronic effect, molecular modeling, ab initio, NBO. On a fait appel a l'ensemble de base complet CBS-QB3, aux methodes basees sur la theorie de la fonctionnelle de la densite hybride (B3LYP/Def2-TZVPP) et a une interpretation par les orbitales de liaison naturelle (OLN) pour etudier les impacts des effets stereoelectroniques et des interactions electrostatiques et steriques sur les proprietes conformationnelles des isocyanates d'halogenocarbonyles (halogeno = F (1), Cl (2)et Br (3)), des isocyanates d'halogenothiocarbonyles (halogeno = F (4), Cl (5)et Br (6)) et des isocyanates d'halogenoselenocarbonyles (halogeno = F (7), Cl (8)et Br (9)). Les deux methodes ont permis de montrer que les conformations Z des composes 1, 4 et 7 sont plus stables que les conformations E, mais que la stabilite des conformations E, comparee celle des conformations Z, augmente du compose 1 au compose 3, du compose 4 au compose 6 ainsi que du compose 7 au compose 9. L'interpretation par les orbitales de liaison naturelle (OLN) montre que l'effet anomere generalise (EAG) favorise les conformations Z des composes 1, 4 et 7. Les valeurs calculees pour l'effet anomere generalise (EAG) (c'est-a-dire [EAG.sub.E]-[EAG.sub.Z]) augmente du compose 1 au compose 3, du compose 4 au compose 6 ainsi que du compose 7 au compose 9. Par ailleurs, il ne semble pas y avoir de tendances semblables pour les valeurs calculees pour les moments dipolaires totaux ou les differences d'energie libre de Gibbs entre les conformations E et Z (c'est-a-dire Δ[μ.sub.E-Z] et Δ[G.sub.E-Z]) pour les composes 1-3, 4-6 et 7-9. En consequence, l'effet anomere generalise (EAG) permet d' expliquer l' augmentation de stabilite de la conformation E du compose 1 au compose 3, du compose 4 au compose 6 ainsi que du compose 7 au compose 9. Il en ressort donc que l'effet anomere generalise (EAG) associe a la delocalisation d'electron, et non pas les changements dans les moments dipolaires totaux (c'est-a-dire Δ[μ.sub.E-Z]), est un indicateur raisonnable de la difference d'energie totale dans les composes 1-3, 4-6 et 7-9. Il existe une correlation directe entre l'effet anomere generalise (EAG) et les parametres Δ[[r.sub.2-6](E) - [r.sub.2-6](Z)]. Il est aussi important de realiser qu'il existe des delocalisations d'electrons interessantes a travers l'espace ([LP.sub.2][X.sub.6]→[π.sup.*.sub.C4-O5]) qui permet de justifier l'augmentation de la stabilite des conformations E du compose 1 au compose 3, du compose 4 au compose 6 ainsi que du compose 7 au compose 9, par comparaison avec les conformations Z correspondantes. On a aussi etudie les correlations entre l'effet anomere generalise (EAG), les ordres de liaison, les energies d'echange sterique totales (EEST), Δ[G.sub.E-Z], Δ[μ.sub.E-Z], parametres structuraux et les comportements conformationnels des composes 1-9. Mots-cles : isocyanates d'halogenocarbonyles, effet stereoelectronique, modelage moleculaire, ab initio, orbitales de liaison naturelle (OLN). [Traduit par la Redaction], Introduction The conformations of organic molecules can be determined by the electron delocalization from donor to acceptor and it is then referred to as stereoelectronic effects. The conformational behavior of [...]