Large-amplitude vibrations of an N–H⋯π hydrogen bonded cis-amide–benzene complex

Abstract

The ground-state N–H⋯π interaction of 2-pyridone·benzene (2PY·Bz) has been studied by infrared-UV depletion spectroscopy of the supersonic-jet cooled complex [P. Ottiger et al., J. Phys. Chem. B (2009) 113, 2937]. Here, we investigate the large-amplitude vibrations of 2PY·Bz and its d₁-2PY and benzene-d₆ isotopologues in the S₁ state, using two-color resonant two-photon ionization and UV-holeburning spectroscopies, complemented by RI-CC2 and SCS-RI-CC2 calculations of the S₁ state. The latter predict a tilted T-shaped structure with an N–H⋯π hydrogen bond to the benzene ring, similar to the S₀ state. The binding energy is predicted to increase by 1.5 kJ mol⁻¹ upon S₁ ← S₀ excitation, in close agreement with the experimental value of 1.2 kJ mol⁻¹. The vibronic band structure up to 60 cm⁻¹ above the 0⁰₀ band is dominated by large-amplitude δ tilting excitations, reflecting a change in the tilt angle of the T-shaped complex. The S₀ and S₁ state δ potentials were fitted to experiment, yielding a single minimum in the S₀ state and a double-minimum S₁ potential with δ_min = ±13 degrees. The second large-amplitude vibration is the θ twisting or benzene internal-rotation mode. Due to the C₆ symmetry of the benzene moiety the S₀ and S₁ state θ potentials are sixfold symmetric. Analysis of the θ band structure reveals that the S₀ and S₁θ potentials are mutually aligned and that the internal rotation barriers are V₆(S₀) < 0.2 kJ mol⁻¹ and V₆(S₁) = 0.10(1) kJ mol⁻¹, in close agreement with the calculations. Weaker excitations of the totally symmetric intermolecular vibrations χ (shear), ω (bend) and σ (stretch) vibrations are also observed. The 2PY intramolecular ν₁ overtone, corresponding to an 2PY amide out-of-plane twist distortion, lies ∼30% higher than in bare 2PY, reflecting the hindrance of this motion by the strong N–H⋯π interaction.