Structure and bonding differences in C3N4 and Si3N4 isomers - A comparative study of [Si3,N4] and [C3,N4] potential energy surfaces using DFT and MP2 methodologies

Potential energy surface (PES) of the singlet ground states of [Si3,N4] and [C3,N4] in the gas phase have been mapped using high level DFT and ab initio calculations. The energy surface, consisting of many stationary points with several atom connectivities like chain (linear and non linear), planar, rings, tetrahedral (Td) and three-dimensional geometries, has been analyzed in search of the minima. Comparing only the most stable isomers in the two PES it is observed that there are no common structures. It has also been observed that on the Si-PES there is only one very low lying minimum, a Td like structure and all other isomers are more than 30 kcal/mol higher in relative energy. Unlike this in the C-series there are around three low-lying minima with very little energy difference among them. These are the chain isomer (C2) and the isomers showing higher symmetry like C2v and D3h. The Td structure is of very high energy in this series. Natural bond orbital analysis (NBO) has been carried out for the most stable isomers in both series to understand the bonding nature. Here both the localization and delocalization energy has been estimated. The most stable structure in the Si-series is the one with the largest localization energy unlike in the case of carbon series where the stability of the most stable isomer compared to the Td structure is due to the delocalization energy. This understanding of these two potential energy surfaces and the bonding in these isomers would be helpful in the synthesis of C3N4 films.