A study on the structure and vibrations of diphenylamine by resonance-enhanced multiphoton ionization spectroscopy and ab initio calculations

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Abstract
Laser‐desorption jet‐cooling has been applied in combination with mass‐selective gas‐phase spectroscopic techniques to study the structure and low‐frequency vibrations of diphenylamine (DPA). Two‐color (1+1′) resonance‐enhanced multiphoton ionization has been used to measure the vibrationally resolved excitation spectrum of the S1←S0 transition in the 305–309 nm region. Ion‐dip measurements have been performed to determine the vibrational structure in the electronic ground state. The electronic spectra of DPA are dominated by long progressions in low‐frequency vibrations involving the motion of the phenyl rings as a whole. For the interpretation of the experimental data ab initio calculations have been performed at the Hartree–Fock level for the S0‐state and using single‐excitation configuration interaction for the S1‐state. The DPA molecule is found to change from a pyramidal geometry around the N‐atom with unequal torsional angles of the phenyl groups in the S0‐state to a planar geometry with equal torsional angles in the S1‐state. The two most prominent vibrational motions are the in‐phase wagging and the in‐phase torsion of the phenyl rings. In addition, the resonance‐enhanced multiphoton ionization spectra of the S1←S0 transition in the DPA‐Ar, DPA‐Kr, and DPA‐Xe van der Waals complexes have been measured. From these spectra it is inferred that there is a coupling between the van der Waals modes and the low‐frequency intra‐molecular modes of DPA.