Infrared Photodissociation Spectroscopy of [Aniline−(Water)<i>_n</i>]⁺ (<i>n</i> = 1−8): Structural Change from Branched and Cyclic to Proton-Transferred Forms

Abstract

Infrared photodissociation spectra of [aniline−(H2O)n]+ (n = 1−8) are measured in the 2700−3800 cm-1 region. The spectra are interpreted with the aid of density functional theory calculations. The n = 1 ion has an N−H···O hydrogen bond. The spectrum of the n = 2 ion demonstrates a large perturbation to both of the NH oscillators, indicating the 1−1 structure where each NH bond is bound to a water molecule. For the n = 3 ion, the calculated spectrum of the 2−1 branched structure coincides well with the observed one. For the n = 4 ion, there exist three strong bands at 2960, 3100, and 3430 cm-1, as well as a very weak one at 3550 cm-1. The observed spectrum in the 3600−3800 cm-1 region is decomposed into four bands centered at 3640, 3698, 3710, and 3734 cm-1. The 2−2 branched isomer is responsible for all the features except the 3550 and 3710 cm-1 bands. These two bands are due to another isomer with a five-membered ring. An infrared band characteristic of the n = 5 ion appears at 3684 cm-1, which is not seen in the spectra of the n = 1−4 ions. This band is indicative of a ring structure and assigned to the free OH stretching vibration of the three-coordinated (double acceptor−single donor) H2O. The n = 5 ion has the five-membered ring structure with the fifth water molecule bound to the terminal (double acceptor) H2O. The observed spectra of the n = 6−8 ions show features quite different from those of the n = 1−5 ions; a very strong and broad band emerges around 3400 cm-1, while no prominent bands appear below 3200 cm-1. It is suggested that the n = 6−8 ions have proton-transferred structures.