First local minimum of the formic acid dimer exhibits simultaneously red-shifted O-H center dot center dot center dot O and improper blue-shifted C-H center dot center dot center dot O hydrogen bonds

The first local minimum of the formic acid dimer exhibits simultaneously red-shifted O-H...O and blue-shifted C-H...O hydrogen bonds. The improper, blue-shifted hydrogen bond was investigated by means of the NBO analysis, rehybridization model and optimization in the electric field. It was shown that the electrostatic model cannot describe the nature of the blue-shifted H-bond. From the NBO analysis it becomes evident that the formation of the O-H...O hydrogen bond and C-H...O improper hydrogen bond can be explained on the basis of an increase of electron density in the sigma* antibonding O-H orbital and a decrease of electron density in the sigma* antibonding C-H orbital. While the former effect is easily explained on the basis of hyperconjugation, the latter requires the existence of a new mesomeric structure characterized by delocalization of electron density from the C-H sigma* antibonding orbital to the remaining part of the complex. The rehybridization model explains properly the formation of both hydrogen bonds but fails to interpret the changes of the other bonds.