Car–Parrinello and path integral molecular dynamics study of the intramolecular hydrogen bond in the novel class of anionic H-chelates: 6-Nitro-2,3-dipyrrol-2-ylquinoxaline anion

Theoretical studies of the structure and proton motion in the intramolecular N–H⋯N hydrogen bond in 6-nitro-2,3-dipyrrol-2-ylquinoxaline anion were carried out at the DFT, MP2 and molecular dynamics levels. Geometry optimization at the 6-311++G(2d,2p) level demonstrate existence of two tautomers on the potential energy surface. The difference in energy between both tautomers is equals 1.62 (1.42) kcal/mol. Dynamics of proton motion in the N–H⋯N hydrogen bond was investigated in vacuum at 233 K using Car–Parrinello and path integral molecular dynamics. Very large delocalization of bridging proton is noted especially in path integral simulation. DFT calculated the coupling constant across the hydrogen bond equals 16.5 Hz is in good agreement with experimental value.