Behaviors of an excess proton in solute-containing water clusters: A case study of H+(CH3OH)(H2O)1–6

Abstract

Behaviors of an excess proton in solute-containing water clusters were investigated using infrared spectroscopy and ab initio calculations. This investigation characterized the structures of protonated methanol-water clusters, H+(CH3OH)(H2O)n with n=2–6, according to their nonhydrogen-bonded and hydrogen-bonded OH stretches in the frequency range of 2700–3900 cm−1. Ab initio calculations indicated that the excess proton in these clusters can be either localized at a site closer to methanol, forming a methyloxonium ion core (CH3OH2+), or at a site closer to water, forming a hydronium ion core (H3O+). Infrared spectroscopic measurements verified the calculations and provided compelling evidence for the coexistence of two distinct structural isomers, CH3OH2+(H2O)3 and H3O+(CH3OH)(H2O)2, in a supersonic expansion. The spectral signatures of them (either CH3OH2+ or H3O+ centered) are the free-OH stretching absorption band at 3706 cm−1 of a single-acceptor-single-donor H2O, and the band at 3673 cm−1 of a single-acceptor CH3OH. At n=4–6, the clusters adopt structures similar to their pure water analogs with five-membered rings starting to form at n=5. The position of the excess proton in them varies sensitively with the number of solvent water molecules as well as the geometry of the clusters. To further elucidate the behaviors of the excess proton in these clusters, we analyze in detail the potential energy surface along the proton transfer coordinate for two specific isomers of n=2 and 4: MW2II and MW4I. It is found that the proton can be nearly equally shared by methanol and the water dimer subunit in the form of CH3OH–H+–(H2O)2, as substantiated by hydrogen bond cooperativity and zero-point vibrational effects.

A benchmark test suite for proton transfer energies and its use to test electronic structure model chemistries

“…The methyloxonium ion (CH 3 OH 2 + ) plays a significant role in many organic and biological reactions [65]. Experimental [56,66,67] and theoretical [68] studies have been carried out to study the proton transfer in CH 3 OH…H + …OH 2 . Johnson and coworkers [56] suggested that methanol can better accept the excess charge of protonation than can H 2 O.…”

Anharmonic Coupling Revealed by the Vibrational Spectra of Solvated Protonated Methanol: Fermi Resonance, Combination Bands, and Isotope Effect

“…The neutral methanol molecule has one OH bond, and thus it can play as a single proton donor, and/or a single/double proton acceptor in hydrogen bond (H bond) networks through its lone pair electrons on oxygen. − The protonated methanol, by contrast, has two OH bonds and can be a single/double proton donor. As a result, it has been shown to form more various types of H bond networks in the clusters either composed of neat methanol − or a mixture with other species, e.g., water. − Through the cooperation between experimental measurements and computational efforts, a variety of H bond networks such as linear, branched, cyclic, and cage-like forms have been confirmed. − …”

(HCl)2(CH3OH)2(X) (X= NH3 VEYA H2O) KÜMELERİNDE PROTON DİNAMİĞİNİN TEORİK OLARAK İNCELENMESİ

“…Sıvı ve katı fazda bu nedenle 2 boyutlu zig-zag zincir oluşturur ve zincirler van der Waals kuvvetleri ile bir arada tutunurlar [5]. Nötral ve protonlanmış su ve metanol kümeleri bir çok grup tarafından hem deneysel hem de teorik olarak çalışılmıştır [6][7][8][9][10][11][12][13][14][15][16][17].…”