Blue‐shifted and red‐shifted hydrogen bonds: Theoretical study of the CH 3 CHO· · ·HNO complexes

The blue‐shifted and red‐shifted H‐bonds have been studied in complexes CH 3 CHO…HNO. At the MP2/6‐31G( d ), MP2/6‐31+G( d,p ) MP2/6‐311++G( d,p ), B3LYP/6‐31G( d ), B3LYP/6‐31+G( d,p ) and B3LYP/6‐311++G( d,p ) levels, the geometric structures and vibrational frequencies of complexes CH 3 CHO…HNO are calculated by both standard and CP‐corrected methods, respectively. Complex A exhibits simultaneously red‐shifted CH…O and blue‐shifted NH…O H‐bonds. Complex B possesses simultaneously two blue‐shifted H‐bonds: CH…O and NH…O. From NBO analysis, it becomes evident that the red‐shifted CH…O H‐bond can be explained on the basis of the two opposite effects: hyperconjugation and rehybridization. The blue‐shifted CH…O H‐bond is a result of conjunct CH bond strengthening effects of the hyperconjugation and the rehybridization due to existence of the significant electron density redistribution effect. For the blue‐shifted NH…O H‐bonds, the hyperconjugation is inhibited due to existence of the electron density redistribution effect. The large blue shift of the NH stretching frequency is observed because the rehybridization dominates the hyperconjugation. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006