Photoelectron Spectroscopy of the Water Dimer Reveals Unpredicted Vibrational Structure

Abstract

Hydrogen bonds and proton transfer reactions can be considered as being at the very heart of aqueous chemistry and of utmost importance for many processes of biological relevance. Nevertheless, these processes are not yet well understood, even in seemingly simple model systems like small water clusters. We present a study of the photoelectron spectrum of the water dimer, revealing previously unresolved vibrational structure with 10–30 meV (80–242 cm–1) typical splitting, in disagreement with a previous theoretical photoionization study predicting an apparent main vibrational progression with an ∼130 meV spacing [ Kamarchik Kamarchik J. Chem. Phys.2010132194311]. The observed vibrational structure and its deviation from the theoretical prediction is discussed in terms of known difficulties with calculations of strongly coupled anharmonic systems involving large amplitude motions. Potential contributions of the nonzero vibrational energy of the neutral water dimer at a finite experimental internal temperature are addressed. The internal temperature is estimated from the breakdown diagram associated with the dissociative ionization of the water dimer to be around to 130 K. This analysis also provides two additional, independently measured values for the 0 K appearance energy of the hydronium ion (H3O+) from dissociative ionization of the water dimer.

A plethora of isomerization processes and hydrogen scrambling in the fragmentation of the methanol dimer cation: a PEPICO study

“…Alternatively, this largeamplitude methyl motion can also lead to the methyl-loss fragment ion OHCH2OH2 + , [9], without a reverse barrier and at the same, 9.84 eV threshold. Another methyl roaming transition state [10] ‡ is found 0.17 eV above the minimum [8]. The reaction path bifurcates following the next intermediate minimum [11] at 9.46 eV.…”

“…In addition to hydroxymethyl loss, H2O and CH3 loss was also observed experimentally, and we explored the reaction coordinates leading to them, too. In the stable intermediate [6] at 9.04 eV, the methyl group can roam around the cation core and explore the potential energy well of intermediate [8] at 9.39 eV through transition state [7] ‡ at 9.84 eV. Alternatively, this largeamplitude methyl motion can also lead to the methyl-loss fragment ion OHCH2OH2 + , [9], without a reverse barrier and at the same, 9.84 eV threshold.…”

“…Much remains unknown even about the simplest systems, such as the water dimer. Its photoelectron spectrum, 8 exhibiting major discrepancies compared with Franck-Condon simulations 9 as well as the appearance energy of H3O + have only recently been determined. 10 Vibrational studies of both small 11 and large 12 methanol and protonated methanol cation clusters found evidence for delocalized protons between hydroxyl groups, i.e., conventional hydrogen bonding.…”

Photoionization Dynamics and Proton Transfer within the Adenine-Thymine Nucleobase Pair

“…We cannot completely rule out the occurrence of such reactions due to lower experimental sensitivity in the nonprotonated mass channels. Nevertheless, we note that the exclusive observation of protonated fragments is common in dissociative ionization experiments of hydrogen-bonded systems. ,, In addition, we performed high-level ab initio calculations of different hydrogen-bonded conformers of the AT pair and conclude that our experimental observations can be attributed to a single most stable conformer. The offset between calculated and measured dissociation thresholds allows the determination of an upper limit of the reaction barrier occurring in the proton transfer step prior to the dissociation.…”

Vacuum ultraviolet photochemistry of sulfuric acid vapor: a combined experimental and theoretical study

“…24 A mixture of helium carrier gas (1.5 bars), water vapor (from the vaporization of liquid water at room temperature) and H 2 SO 4 vapor, generated from the vaporization of liquid H 2 SO 4 (95% purity) in an in-vacuum oven heated at 180 1C, was expanded through a 70 mm diameter nozzle to form a continuous molecular beam which was skimmed twice before reaching the center of the spectrometer. 25,26 The multiplex coincidence scheme between the ion and electron yields mass-selected photoelectron spectra (PES) as a function of photon energy, which are then reduced to SPES, as previously detailed. 27,28 Meanwhile, the coincidence scheme can provide spectral fingerprints to identify and separate isomers, and accurately measure thermochemical data such as fragment appearance energies and bond energies.…”