Migration of an Excess Proton upon Asymmetric Hydration:  H⁺[(CH₃)₂O](H₂O)_n as a Model System

Abstract

An excess proton can migrate from a solute to solvent molecules upon asymmetric solvation. The migration depends sensitively on solvation number, solvation structure, and proton affinity differences between solute and solvent molecules. The present study demonstrates this intriguing solvation-induced effect using protonated dimethyl ether-water clusters as the benchmark system. An integrated examination of H + [(CH 3 ) 2 O]-(H 2 O) n by vibrational predissociation spectroscopy and ab initio calculations indicates that the excess proton is (1) localized on (CH 3 ) 2 O at n ) 1, (2) equally shared by (CH 3 ) 2 O and (H 2 O) 2 at n ) 2, and (3) completely transferred to (H 2 O) n at n g 3. The dynamics of proton transfer is revealed by the characteristic free-and hydrogen-bonded-OH stretching vibrations of the water molecules in direct contact with the excess proton. Both hydrogen bond cooperativity and zero-point vibrations have crucial influences on the final position of the proton in the clusters. Further insight into this remarkable phenomenon of intracluster proton transfer is provided by a comparison between H + [(CH 3 ) 2 O](H 2 O) n and its structural analogues, H + (H 2 O) n+1 and H + [(C 2 H 5 ) 2 O](H 2 O) n .

Ground State Proton Transfer in Phenol–(NH3)n (n ≤ 11) Clusters Studied by Mid-IR Spectroscopy in 3–10 μm Range

“…These predictions agree with the spectral appearance around the C−H bending vibration (colored in blue) that does not change for n ≥ 9. Enhancement of ν 12 and ν 9a modes in the PT(I) and PT(II) forms is consistent with the observed bands.…”

“…94 The IRMPD spectrum gives the ν 9a mode of PhO − at ∼1150 cm −1 with an intensity similar to those of CC and C−O stretching vibrations, but the spectrum did not cover the frequency range below the 1000 cm −1 . These IR and Raman spectra imply a solvation induced enhancement of ν 12 and ν 9a modes of PhO − . The observation of sharp features for n ≥ 7 agrees well with these reported features for PhO − .…”

“…It is different from n = 6 inferred from the size dependence of the clusters IP. 29 For n = 6−8, the non-PT structures are suggested by the presence of the C−O−H bending vibration; however, the signatures of the PhO − vibrations (ν 12 and ν 9a of PhO − ) are also observed at n = 7 with almost comparable intensities to those in n = 8. For n = 6, ν 12 can be seen weakly, while ν 9a is missing.…”

Intermolecular proton-transfer in acetic acid clusters induced by vacuum-ultraviolet photoionization

“…It has been known for the clusters composed of more than two molecules that proton affinities of bare molecules are not necessarily good indicators for possibilities of proton-transfer because of cooperative effects of intermolecular interactions among molecules. [17][18][19]62 The acetic acid-H 2 O ͑1:1͒ cluster cation prefers the non-proton-transferred structure. Since the proton affinity of water becomes larger with increasing the number of water molecules, the proton of acetic acid moiety would be transferred to the water cluster moiety in largesized acetic acid-͑H 2 O͒ n cations.…”

“…relative proton affinities or gas phase basicities between proton donors and acceptors. 4,17,20,[60][61][62] Correlations of the proton-transfer reaction with the proton affinity in the microsolvated phenol and aniline cations have been illustrated by electronic and vibrational spectroscopies. [17][18][19][20] Generation of protonated species with the matrix-assisted laser desorption/ ionization technique has also been examined with relationship between proton affinities of samples and acidities of matrix molecules.…”