Theoretical calculations were performed to study the nature of the hydrogen bonds in the CH3CHO dimers. The geometric structures and vibrational frequencies of its three stable dimers at the MP2/6-31＋G(d), B3LYP/6-31＋G(d), B3LYP/6-311＋＋G(d,p) and B3LYP/6-311＋＋G(3df,2p) levels were calculated by standard and counterpoise-corrected methods, respectively. The results indicate that in the dimers A and C the C—H bond is strongly contracted and C—H…O blue-shifted hydrogen bonds were observed. From the natural bond orbital analysis it can be seen that the X—H bond length in the X—H…Y hydrogen bond is controlled by a balance of four main factors in the opposite directions: hyperconjugation, electron density redistribution, rehybridization and structural reorganization. Among them the hyperconjugation has the effect of elongating the X—H bond, but the other three factors belong to the bond shortening effects. In the dimers A and C, the shortening effects dominate which lead to the blue shift of the C—H stretching frequencies.

Key words: red-shifted H-bond, blue-shifted H-bond, AIM topological analysis, NBO analysis