Properties and isomerization mechanism of the singlet state imidazole-imidazolium system

The singlet state complexes of imidazole and imidazolium are optimized using the B3LYP method in conjunction with the 6-311+G* basis set. Totally, four stable complexes are found on the global potential energy surface, and they are classified as: N-H⋯N mode, C-H⋯N mode, and C-N mode. The harmonic vibrational frequency and the Mulliken population analyses are carried out. The stability of the isomers are compared, and the most stable complexes are those with N-H⋯N type H-bonds. Another important finding is that the HOMO electrons of the N-H⋯N type coupling mode complexes are more difficult to be excited or to be removed for their low HOMO energies as compared with the other isomers, while these complexes are easier to be reduced by the electron attachment for their high LUMO energies. The low barriers for the self-isomerization pathways of C(N)-H⋯N mode complexes demonstrate that their N-H⋯N or C-H⋯N type H-bonds belong to the low barrier hydrogen bond, implying their flexibility for isomerization. The C-N mode complex should be a metastable isomer that has finite lifetime, and upon interconversion, it should transform to C-H⋯N mode complex and release a lot of energies.