Structures, aromaticity, AIM, and NBO analyses of hydroxy and mercapto azaazulene: a DFT study†

This work contributes to an overview of the structure of tautomers and rotamers of 2- and 8-hydroxy azaazulene and their mercapto analogues. This can be accomplished by optimizing the structures at the B3LYP functional with the 6-311+G(d,p) basis sets. Single-point energy calculations were performed at the M06-2X/6-311++G(2d,2p) level. The G3MP2 composite method was employed to obtain more accurate energies. The variations in the local aromaticity of aromatic rings were discussed by using the nucleus-independent chemical shift (NICS), the harmonic oscillator model of aromaticity index (HOMA), the multicenter index (MCI), and the aromatic fluctuation index (FLU and FLU-π) as probes of aromaticity. Additionally, the intramolecular proton transfer and rotation barriers of tautomerization and rotamerization have been extensively discussed. The discussion also encompassed natural bond orbital (NBO) charges and analysis, as well as atoms in molecules (AIM) analysis. The results reveal the presence of intramolecular hydrogen bonds in the 8OH-AZ, 8S-AZ, and 8SH-AZ structures.