Strong N-H. . .π hydrogen bonding in amide-benzene interactions

Abstract

Among the weak intermolecular interactions found in proteins, the amide N-Hπ interaction has been widely observed but remains poorly characterized as an individual interaction. We have investigated the isolated supersonic-jet-cooled dimer of the cis-amide and nucleobase analogue 2-pyridone (2PY) with benzene and benzene-d6. Both MP2 and SCS-MP2 geometry optimizations yield a T-shaped structure with a N-Hπ hydrogen bond to the benzene ring and the CdO group above, but far from the C-H bonds of benzene. The CCSD(T) calculated binding energy at the optimum geometry is De ) 25.2 kJ/mol (dissociation energy D0 ) 21.6 kJ/mol), corresponding to the H-bond strength of the water dimer or of N-HO hydrogen bonds. The T-shaped geometry is supported by the infrared-ultraviolet depletion spectra of 2PY. benzene: The N-H stretch vibrational frequency is lowered by 56 cm-1, and the CdO stretch vibration is lowered by 10 cm-1, relative to those of bare 2PY, indicating a strong N-Hπ interaction and a weak interaction of the CdO group. The benzene C-H infrared stretches exhibit very small shifts (≈2 cm-1) relative to benzene, signaling the absence of interactions with the benzene C-H groups. The infrared spectral shifts are consistent with a strong nonconventional π hydrogen bond and a T-shaped structure for 2PY benzene. Symmetry-adapted perturbation theory calculations show that the N-Hπ interaction is by far the dominant stabilization factor. © 2009 American Chemical Society.