Infrared Depletion Spectroscopy of the Hydrogen-Bonded Aniline−Diethylamine (C6H5-NH2···NHC4H10) Complex Produced in Supersonic Jet

The vibrational frequencies of the N−H stretching modes of aniline and diethylamine (DEA), after forming a strong H-bonded complex, are measured with infrared depletion spectroscopy that uses cluster-size-selective REMPI time-of-flight mass spectrometry. Three strong infrared absorption features observed at 3294, 3369, and 3473 cm-1 are assigned to the NH stretching vibration of DEA and H-bonded and free N−H stretching vibrations of aniline, respectively, in the 1:1 aniline−DEA complex. The spectral broadening observed for the free and H-bonded N−H stretching modes of aniline indicates mode-specific vibrational energy dynamics. Although the narrow bandwidth (≈4 cm-1) of the N−H stretch at 3473 cm-1 incorporates all of the common broadening mechanisms including intramolecular vibrational relaxation (IVR), the broader (≈13 cm-1) absorption feature at 3369 cm-1 suggests vibrational predissociation/IVR of the H-bonded complex, with a subpicosecond lifetime. The red-shifts of the N−H stretching vibrations of aniline and DEA agree with the ab initio calculated (MP2/6-31G**) aniline−DEA structure in which one of the N−H bonds of aniline interacts with the nitrogen atom of DEA through a hydrogen bond, giving a binding energy of 15.8 kJ mol-1 with due corrections for BSSE and zero-point energy. The electronic 0−0 band origin for the S1 ← S0 transition is observed at 32 916 cm-1, giving a significant red-shift of 1113 cm-1 from that of the bare aniline. The vibrational progressions associated with the R2PI spectrum are assigned to the intermolecular modes of the complex.