Structures and properties of the hydrogen-bond complexes: Theoretical studies for the coupling modes of the pyrazole-imidazole system

The calculations at the B3LYP/6-311+G* level have been performed for Pyrazole-Imidazole (Py-Im) system. Eight Py-Im complexes are found and their calculated geometric structures, relative energies, IR spectra and stabilization energies are presented. Those Py-Im complexes can be classified into single H-bond mode and double H-bond mode. The single H-bond complexes can be subdivided into N-H⋯N mode and C-H⋯N mode. The four single H-bond complexes are defined as SA, SB, SC, and SD and the four double H-bond complexes are defined as DA, DB, DC, and DD. SA and SB belong to the N-H⋯N coupling mode while SC and SD belong to the C-H⋯N coupling mode, respectively. The double H-bond coupling mode complexes can be further divided into two types. DA and DB have a N-H⋯N type H-bond and a C-H⋯N type H-bond and DC and DD have two C-H⋯N type H-bonds. For the single H-bond modes and double H-bond modes, the complexes with N-H⋯N type H-bond have stronger interactions than those with C-H⋯N type H-bond. DB is the most stable isomer among the eight complexes. With corrections for the zero-point vibrational energies (ZPE) and basis set superposition errors (BSSE), the stabilization energy of DB is 7.60 kcal/mol at the B3LYP/6-311+G* level. The stability order of the eight Py-Im complexes is DB>DA>SB>SA>SC=SD>DD>DC. Moreover, this result is in close agreement with that of the natural bond orbital (NBO) analysis.