Blue-shifted and red-shifted hydrogen bonds: Theoretical study of the CH3CHO⋯NH3 complexes

Two stable CH3CHO⋯NH3 complexes with two H-bonds were found by ab initio methods at MP2/6-31G(d), MP2/6-311+G(d,p) and MP2/6-311++G(2d,2p) levels, respectively. The complex A exhibits simultaneously red-shifted N-H⋯O and blue-shifted C-H⋯N H-bonds. The complex B possesses simultaneously two red-shifted H-bonds: N-H⋯O and C-H⋯N. From the NBO analysis, it becomes evident that the three red-shifted H-bonds can be explained on the basis of the two opposite effects: hyperconjugation and rehybridization. The blue-shifted H-bond is a result of conjunct C-H bond strengthening effects of hyperconjugation and rehybridization due to existence of the significant electron density redistribution effect.