Hyperconjugation versus back bonding in AH3−nXn species (A = Si and Ge; X = F, Cl and Br; and n = 1, 2 and 3)

In this research two competing phenomena, back bonding and hyperconjugation, have been investigated based on Natural Bond Orbital (NBO) and Atoms in Molecules (AIM) analyses for radical AH3−nXn species, where A = Si and Ge, and n = 1, 2 and 3. It is demonstrated in this article that both above phenomena will be occurred significantly, while back bonding is the only event in analogous compounds with carbon and hyperconjugation is rather negligible. It was also found that only one back bonding with the help of keyword $CHOOSE in NBO analysis can be found in this type of compounds with reasonable structure, while it can be sometimes detected in AH3−nXn without using keyword $CHOOSE. It is also shown that there is always an increase in bond length in comparison with reference molecules in mentioned species due to existing hyperconjugation, while if the central atom is carbon, we have always a decrease of bond length due to only having back bonding. Additionally, from AIM point of view, the delocalization indices for α-spin (majority spin) is more than β-spin (minority spin) in radical species for molecules without back bonding, while the situation in our compounds is quite reverse, which can be attributed to the π back bonding in the β-spin electrons.