Picosecond Proton Transfer Kinetics in Water Revealed with Ultrafast IR Spectroscopy

Abstract

Aqueous proton transport involves the ultrafast interconversion of hydrated proton species that are closely linked to the hydrogen bond dynamics of water, which has been a long-standing challenge to experiments. In this study, we use ultrafast IR spectroscopy to investigate the distinct vibrational transition centered at 1750 cm in strong acid solutions, which arises from bending vibrations of the hydrated proton complex. Broadband ultrafast two-dimensional IR spectroscopy and transient absorption are used to measure vibrational relaxation, spectral diffusion, and orientational relaxation dynamics. The hydrated proton bend displays fast vibrational relaxation and spectral diffusion timescales of 200-300 fs; however, the transient absorption anisotropy decays on a remarkably long 2.5 ps timescale, which matches the timescale for hydrogen bond reorganization in liquid water. These observations are indications that the bending vibration of the aqueous proton complex is relatively localized, with an orientation that is insensitive to fast hydrogen bonding fluctuations and dependent on collective structural relaxation of the liquid to reorient. We conclude that the orientational relaxation is a result of proton transfer between configurations that are well described by a Zundel-like proton shared between two flanking water molecules.

Correlated dynamics in aqueous proton diffusion

“…However, we note that spectroscopic studies aiming to gain insight into aqueous proton dynamics have been done at even higher concentrations than we have studied here, 59 – 63 providing obvious relevance for having a reliable model for the dynamics of protons in more concentrated solutions. The spectroscopy results appear to set upper and lower bounds of 2.5 ps and 480 fs for the proton transfer, and Tokmakoff and co-workers estimate that the transfer occurs in 1–2 ps.…”

“…The spectroscopy results appear to set upper and lower bounds of 2.5 ps and 480 fs for the proton transfer, and Tokmakoff and co-workers estimate that the transfer occurs in 1–2 ps. 63 Unfortunately, none of these studies reported diffusion coefficients for their samples, which complicates interpretation of the results given that the diffusion coefficient is sensitive to concentration. Additionally, a recent theoretical study by Napoli et al concluded that the dynamics measured in the spectroscopy experiments correspond to changes in the proton solvation asymmetry rather than directly relating to the proton transfer.…”

Energy Relaxation and Structural Dynamics of Protons in Water/DMSO Mixtures

“…This observation suggests that the structural lifetime of the proton hydration structures is on the order of picoseconds. 23 …”

Hydrated Excess Protons in Acetonitrile/Water Mixtures: Solvation Species and Ultrafast Proton Motions

“…On the experimental side, femtosecond infrared spectroscopy has probed the fluctuating ensemble by vibrational excitations of water molecules. [30][31][32] Fluctuating forces manifest in transient vibrational lineshapes, most specifically in two-dimensional infrared (2D-IR) spectra. For protons in water, the observed vibrational response is due to both water molecules solvating protons and those in the bulk of the liquid, making an interpretation of the results in terms of structure highly challenging.…”