Infrared Depletion Spectroscopy Suggests Mode-Specific Vibrational Dynamics in the Hydrogen-Bonded Aniline−Diethyl Ether (C6H5−NH2···OC4H10) Complex

Abstract

The vibrational frequencies of the N−H stretching modes of aniline, after forming a strong H-bonded complex with diethyl ether (DEE), are measured with infrared depletion spectroscopy that uses cluster-size-selective REMPI time-of-flight mass spectrometry. Two strong absorption features observed at 3372 and 3478 cm-1 are assigned to the H-bonded and free N−H stretching vibrations of the 1:1 aniline−DEE complex. The spectral broadening observed for the free and H-bonded N−H stretching modes indicates mode-specific vibrational energy dynamics. While the narrow bandwidth (≈3 cm-1) of the N−H stretch at 3478 cm-1 incorporates all the common broadening mechanisms including IVR, the broader (≈10 cm-1) absorption feature at 3372 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 agree with the ab initio calculated (MP2/6-31G**) aniline−DEE structure in which one of the N−H bonds of aniline interacts with the oxygen atom of DEE through a hydrogen bond, giving a binding energy of 13 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 33292 cm-1, giving a significant red shift of 737 cm-1 from that of the bare aniline. The vibrational bands associated with the R2PI spectrum are assigned to the intermolecular modes of the complex.