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Issue 29, 2010
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Large-amplitude vibrations of an N–H⋯π hydrogen bonded cis-amide–benzene complex

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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 S1S0 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 δmin = ±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.

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

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Publication details

The article was received on 29 Jan 2010, accepted on 16 Mar 2010 and first published on 18 May 2010


Article type: Paper
DOI: 10.1039/C002056G
Citation: Phys. Chem. Chem. Phys., 2010,12, 8208-8218

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

    C. Pfaffen, H. Frey, P. Ottiger, S. Leutwyler, R. A. Bachorz and W. Klopper, Phys. Chem. Chem. Phys., 2010, 12, 8208
    DOI: 10.1039/C002056G

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