Strong N-H...pi hydrogen bonding in amide-benzene interactions.
Among the weak intermolecular interactions found in proteins, the amide N--H...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-d6. Both MP2 and SCS-MP2 geometry optimizations yield a T-shaped structure with a N--H...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 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--H...O hydrogen bonds. The T-shaped geometry is supported by the infrared-ultraviolet depletion spectra of 2PY x 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...pi interaction and a weak interaction of the C=O group. The benzene C--H infrared stretches exhibit very small shifts (approximately 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 x benzene. Symmetry-adapted perturbation theory calculations show that the N--H...pi interaction is by far the dominant stabilization factor.
Ottiger
Philipp
P
Departement für Chemie and Biochemie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
Pfaffen
Chantal
C
Leist
Roman
R
Leutwyler
Samuel
S
Bachorz
Rafał A
RA
Klopper
Wim
W
D000577:Amides / Q000737:chemistry*; D001554:Benzene / Q000737:chemistry*; D002244:Carbon / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry*; D006860:Hydrogen Bonding; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D009584:Nitrogen / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry; D011728:Pyridones / Q000737:chemistry; D013055:Spectrophotometry, Infrared / Q000379:methods; D013056:Spectrophotometry, Ultraviolet / Q000379:methods
D000577:Amides; D011728:Pyridones; D002244:Carbon; D006859:Hydrogen; D001554:Benzene; D009584:Nitrogen; D010100:Oxygen