Strong N-H•••π Hydrogen Bonding in Amide-Benzene Interactions

Among the weak intermolecular interactions found in proteins, the amide N-H center dot center dot center dot pi 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-d(6). Both MP2 and SCS-MP2 geometry optimizations yield a T-shaped structure with a N-H center dot center dot center dot pi hydrogen bond to the benzene ring and the C=O group above, but far from the C-H bonds of benzene. The CCSD(T) calculated binding energy at the optimum geometry is D-e = 25.2 kJ/mol (dissociation energy D-0 = 21.6 kJ/mol), Corresponding to the H-bond strength of the water dimer or of N-H center dot center dot center dot O hydrogen bonds. The T-shaped geometry is supported by the infrared-ultraviolet depletion spectra of 2PY center dot benzene: The N-H stretch vibrational frequency is lowered by 56 cm(-1), and the C=O stretch vibration is lowered by 10 cm(-1), relative to those of bare 2PY, indicating a strong N-H center dot center dot center dot pi interaction and it weak interaction of the C=O group. The benzene C-H infrared stretches exhibit very small shifts (approximate to 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 pi hydrogen bond and a T-shaped structure for 2PY center dot benzene. Symmetry-adapted perturbation theory calculations show that the N-H center dot center dot center dot pi interaction is by far the dominant stabilization factor.