Investigation of the carbene structure and stability of Imidazopyrimidine isomers by using quantum mechanics and NBO analysis

Quantum mechanical calculations and NBO analysis are performed on carbene of Imidazopyrimidine isomers, based on the zero-point energy (ZPE), the energy of stability, and the first frequency to obtain the most stable structure are calculated. The positivity of the first vibrational frequency in all three structures examined confirms the accuracy of the calculations and structures studied. Accordingly, the most stable structure on these isomers is structure 3, which can be attributed to the relative structure symmetry in the 5-membered imidazole ring. Also, the aromaticity of the ring (NiCS) has been calculated on these structures, and the greater the aromaticity of the six-ring in isomer 3 is to confirm the stability of the structure with the lowest energy level. The analysis for Natural Bonding Orbital is also carried out using the NBO method on an optimal structure, and the results of the bond electron population are consistent with the atomic hybridization and available structures. Also, electron transmissions between the Highest Occupied Molecular Orbitals (HOMO) and the Lowest Unoccupied Molecular Orbitals (LUMO) have been investigated by calculating the difference energy between these orbitals, with the highest transfer rate associated with isomer No. 3 and in full compliance with previous results. The calculations of the atomic charges and the dipole moment of structures are also obtained using the available data in the Gaussian output and confirms the relative stability of the atoms position in the structures.