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

Abstract

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.