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 (approximate to4 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 (approximate to13 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 S-1 <-- S-0 transition is observed at 32 916 cm(-1), giving a significant red-shift of It 13 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.