Infrared photodissociation spectroscopy of [Aniline-(Water)n]+ (n = 1-8): Structural change from branched and cyclic to proton-transferred forms

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, and 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.This is a preprint of an article published by American Chemical Society in Journal of Physical Chemistry A, 2003, available online: http://pubs.acs.org/doi/abs/10.1021/jp022552