Gas-phase vibrational spectroscopy of triphenylamine: the effect of charge on structure and spectra.

The effect of ionization by oxidation and protonation on the structure and IR spectrum of isolated, gas-phase triphenylamine (TPA) has been investigated by infrared multiple photon dissociation (IRMPD) spectroscopy in the fingerprint range from 600 cm to 1800 cm using an infrared free electron laser. IR spectra calculated using density functional theory (DFT) convincingly reproduce the experimental data. Spectral and structural differences are identified among neutral TPA, TPA˙ and protonated TPA and qualitatively related to effects of resonance delocalization. As a consequence of electron delocalization, computed structural parameters for TPA remain virtually unchanged upon removal of an electron. Nonetheless, CC and CN stretching vibrations in the IR spectra of TPA˙ undergo a red shift of up to 52 cm as compared to those in TPA. Since ionization also strongly influences the relative band intensities, a vibrational projection analysis was used to correlate vibrational modes of TPA with those of TPA˙. The experimental IR spectrum of gas-phase protonated TPA indicates that protonation occurs on the nitrogen atom, despite delocalization of the lone electron pair. Upon protonation, the structure changes from the nearly planar geometry to a near-tetrahedral configuration.