Large-amplitude vibrations of an N-H center dot center dot center dot pi hydrogen bonded cis-amide-benzene complex

The ground-state N-H center dot center dot center dot pi interaction of 2-pyridone.benzene (2PY center dot 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 center dot Bz and its d(1)-2PY and benzene-d(6) isotopologues in the S-1 state, using two-color resonant two-photon ionization and UV-holeburning spectroscopies, complemented by RI-CC2 and SCS-RI-CC2 calculations of the S-1 state. The latter predict a tilted T-shaped structure with an N-H center dot center dot center dot pi hydrogen bond to the benzene ring, similar to the S-0 state. The binding energy is predicted to increase by 1.5 kJ mol(-1) upon S-1 <- S-0 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 0(0)(0) band is dominated by large-amplitude delta tilting excitations, reflecting a change in the tilt angle of the T-shaped complex. The S-0 and S-1 state delta potentials were fitted to experiment, yielding a single minimum in the S-0 state and a double-minimum S-1 potential with delta(min) = +/- 13 degrees. The second large-amplitude vibration is the theta twisting or benzene internal-rotation mode. Due to the C-6 symmetry of the benzene moiety the S-0 and S-1 state theta potentials are sixfold symmetric. Analysis of the theta band structure reveals that the S-0 and S-1 theta potentials are mutually aligned and that the internal rotation barriers are V-6(S-0) < 0.2 kJ mol(-1) and V-6(S-1) = 0.10(1) kJ mol(-1), in close agreement with the calculations. Weaker excitations of the totally symmetric intermolecular vibrations chi (shear), omega (bend) and sigma (stretch) vibrations are also observed. The 2PY intramolecular nu(1) overtone, corresponding to an 2PY amide out-of-plane twist distortion, lies similar to 30% higher than in bare 2PY, reflecting the hindrance of this motion by the strong N-H center dot center dot center dot pi interaction.