Correlations between Quantum Calculations and the Contribution of Factors Affecting the Structural Stability and Electronic Properties of Vinyl Azide and Vinyl Isocyanate

The conformational features of vinyl isocyanate (1) and vinyl azide (2) have been analyzed using G4MP2, CCSD(T)/6-311+G**, B3LYP/6-311+G** and LC-ωPBE/6-311+G** theory levels. Published experimental data indicate that the stability of the cis-isomer of vinyl azide (2) is more than the trans-isomer. This is consistent with the findings attained using G4MP2 and LC-ωPBE/6- 311+G** levels, whereas B3LYP/6-311+G** and CCSD (T)/6-311+G** levels provide false energetic outcomes. Natural Bond Orbital (NBO) interpretation is used to interpret the reason for the stability of the cis stereoisomer of compound 2 and the structural features of compound 1. Dipole-dipole interactions, steric effects, and resonance energies on the structural behaviors of compounds 1 and 2 are also investigated using NBO analysis. The results showed that the stability of the cis conformer of compound 2 compared to its trans conformer could be explained using the steric effect (Pauli Exchange Type Repulsion or PETR). Dipole moments of the cis conformer of compound 2 are evaluated experimentally (from Rotational Spectroscopy) and theoretically. The experimental (from Rotational Spectroscopy) and theoretically evaluated dipole moments of trans-conformation of compound 2 are slightly greater than that in cis-conformation, which is in favor of the cis-conformation, while dipoledipole interactions are in favor of the cis-conformation of compound 1. Accordingly, the electrostatic interactions associated with dipole-dipole interactions do not explain the conformational behaviors of compound 1. The stabilization energies related to the delocalization of electrons are in favor of the cis-conformation of compound 1, whereas it has no considerable effect on compound 2’s conformational behavior.