The electron delocalization in isocyanates, amides, and ureas has been studied using ab initio MO and density functional methods. The observed trends in the CN rotational barriers and N-inversion barriers in these systems have been explained in terms of orbital interactions. NBO analysis indicates that there is nN → π*[C - X] electron delocalization in amides, which increases with a decrease in the energy difference (ΔE) between the two interacting orbitals. This phenomenon, rather than electronegativity, is responsible for the observed increase in the CN rotational barrier in amides XC(R)NH2 (R = H, NH2) in the order X = O < S < Se.

Silylenes are known to show ambiphilic character. Ab initio study on (Lewis base)→H2Si coordination shows that the strength of the interaction depends mainly on two factors:  (1) the nucleophilicity of the base and (2) the extent of π delocalization of the lone pair on silylene onto the π frame of the base. The stabilization energies due to the formation of H3N→SiH2, OC→SiH2, and HNC→SiH2 complexes at the G2 level are respectively 23.22, 20.84, and 29.59 kcal/mol. The base coordination triggers the nucleophilicity of silylenes, the strength of which is again dependent on the π interaction between the lone pair on Si and the Lewis base. The energy gains due to the formation of the LB→LA complexes (H3N)H2Si:→BH3, (OC)H2Si:→BH3, and (HNC)H2Si:→BH3 are 42.08, 24.65, and 29.16 kcal/mol, respectively, at the G2 level. The electrophilic nature of base-coordinated silylenes and the nucleophilicity of double-base-coordinated silylenes have been quantitatively estimated. Natural bond orbital (NBO) analysis and charge decomposition analysis (CDA) have been carried out to quantitatively estimate electron distribution.

Ab initio and density functional calculations have been performed on the different possible structures of selenourea (su), urea (u) and thiourea (tu) to understand the extent of delocalisation in selenourea in comparison to urea and thiourea. Selenourea (su-1) with C 2 symmetry has the minima on the potential energy surface at MP2(fu)/6-31+G* level. The C-N rotational barrier in selenourea is 8⋅69 kcal/mol, which is 0⋅29 and 0⋅11 kcal/mol more than that of urea and thiourea respectively at MP2(fu)/6-31+G* level. N-inversion barrier is 0⋅55 kcal/mol at MP2(fu)6-31+G* level. NBO analysis has been carried out to understand the nature of different interactions responsible for the electron delocalisation.

Ab initio and density functional studies show that silylenes can form complexes with BH(3) and the resultant complexes possess 3c-2e bridges. The complexation energy for the formation of the these H-bridged structures is in the range of 18-46 kcal/mol. The characteristics of the electron deficient bridges depend on the substituents attached to the silylenes. With an increase in the pi-donating capacity of the substituents, the exothermicity of complex formation gets reduced but the kinetic stability of the H-bridged structures increase. The natural bond orbital analysis shows that all the H-bridged structures are associated with sigma(B-H)-->ppi(Si) second-order delocalization, which is responsible for the origin of the 3c-2e bonds. The complexation energies of the silylene-BH(3) complexes have been shown to have a correlation to the singlet-triplet energy gaps of silylenes.

The potential energy surfaces of thiohydroxylamine HS-NH 2 , 1, and selenohydroxylamine HSe-NH 2 , 2, have been searched, using ab initio and density functional methods, to study the conformational preferences. There are two minima on the path of rotation around the Se-N bond in 2. High accuracy G2MP2 calculations showed that the Se-N rotational barrier in 2 is 5.41 kcal mol Ϫ1 , which is 1.16 kcal mol Ϫ1 less than the S-N rotational barrier in 1. The inversion around N in 1 and 2 goes through low energy barriers of 1.79 and 2.44 kcal mol Ϫ1 at the same level respectively. Charge analysis using the natural population analysis (NPA) method has been performed to understand the electronic factors responsible for the observed trends in the Se-N interactions. The strength of the negative hyperconjugation in 2 has been estimated using natural bond orbital (NBO) analysis and by studying the substituent effect.