Transition from vehicle to Grotthuss proton transfer in a nanosized flask: cryogenic ion spectroscopy of protonated p-aminobenzoic acid solvated with D2O

Proton transfer (PT) is one of the most ubiquitous reactions in chemistry and life science. The unique nature of PT has been rationalized not by the transport of a solvated proton (vehicle mechanism) but by the Grotthuss mechanism in which a proton is transported to the nearest proton acceptor along a hydrogen-bonded network. However, clear experimental evidence of the Grotthuss mechanism has not been reported yet. Herein we show by infrared spectroscopy that a vehicle-type PT occurs in the penta- and hexahydrated clusters of protonated p-aminobenzoic acid, while Grotthuss-type PT is observed in heptahydrated clusters, indicating a change in the PT mechanism depending on the degree of hydration. These findings emphasize the importance of the usually ignored vehicle mechanism as well as the degree of hydration. It highlights the possibility of controlling the PT mechanism by the number of water molecules in chemical and biological environments.

Hydration-induced protomer switching in p-aminobenzoic acid studied by cold double ion trap infrared spectroscopy

Para-Aminobenzoic acid (PABA) is a benchmark molecule to study solvent-induced proton site switching. Protonation of the carboxy and amino groups of PABA generates O- and N-protomers of PABAH⁺, respectively. Ion mobility mass spectrometry (IMS) and infrared photodissociation (IRPD) studies have claimed that the O-protomer most stable in the gas phase is converted to the N-protomer most stable in solution upon hydration with six water molecules in the gas-phase cluster. However, the threshold size has remained ambiguous because the arrival time distributions in the IMS experiments exhibit multiple peaks. On the other hand, IRPD spectroscopy could not detect the N-protomer for smaller hydrated clusters because of broad background due to annealing required to reduce kinetic trapping. Herein, we report the threshold size for O → N protomer switching without ambiguity using IR spectroscopy in a double ion trap spectrometer from 1300 to 1800 cm^-1. The pure O-protomer is prepared by electrospray, and size-specific hydrated clusters are formed in a reaction ion trap. The resulting clusters are transferred into a second cryogenic ion trap and the distribution of O- and N-protomers is determined by mid-IR spectroscopy without broadening. The threshold to promote O → N protomer switching is indeed five water molecules. It is smaller than the value reported previously, and as a result, its pentahydrated structure does not support the Grotthuss mechanism proposed previously. The extent of O → N proton transfer is evaluated by collision-assisted stripping IR spectroscopy, and the N-protomer population increases with the number of water molecules. This result is consistent with the dominant population of the N-protomer in aqueous solution.

Migrations and Catalytic Action of Water Molecules in the Ionized Formamide-(H2O)2 Cluster

Isomerization dynamics involving the migrations, proton transfer reaction, and catalytic actions of water molecules upon vertical ionization of the formamide (FA)-(H₂O)₂ cluster is investigated by the infrared spectroscopy and theoretical reaction path search calculation. The infrared spectroscopic result indicates the [FA-(H₂O)₂]⁺ cation has the hydrogen-bonded structure of the enol isomer cation of formamide and the water dimer. This structure is formed by proton transfer from the CH bond to the carbonyl group through the catalytic action of the water molecules. The isomerization paths involving this enolization in ionized FA-(H₂O)₂ are explored by using the anharmonic downward distortion following method. We found multiple enolization paths which accompany proton exchanges among the formamide moiety and water molecules through the catalytic actions of the water molecules.

Mechanistic insight on water and substrate catalyzed the synthesis of 3-(1H-indol-3-yl)-2-(4-methoxybenzyl)isoindolin-1-one: Driving by noncovalent interactions

The mechanisms of the synthesis of 2-substituted-3-(1H-indol-3-yl)-isoindolin-1-one derivatives have been investigated theoretically under unassisted, self-assisted, and water-assisted conditions. Being different from previously proposed catalyst-free by Hu et al., our results show that the title mechanism can be altered and accelerated by solvent and substrate 2. Two types of mechanisms have been developed by DFT calculations differ in the reaction sequence of substrates 1 with 3 (M1) or 2 (M2) followed by 2 (M1) or 3 (M2), and water-assisted M1 is the most favored one. It was found that the nucleophilicity of substrate 3 is stronger than that of 2. Our calculations suggest that the water-assisted pathway in M1 is the most favorable case, which undergoes nucleophilic addition and H-shift, C-N bond formation and water elimination, and intramolecular cyclization and water elimination. The rate-determining step is the nucleophilic attack process. Moreover, we also explored the effect of nucleophilic attack of the nitrogen of (4-methoxyphenyl)methanamine on hydroxyl or carbonyl group carbon of phthalaldehydic acid on the activation energy. More importantly, we found that water molecules play a critical role in the whole reaction, not only act as solvent but also as an efficient catalyst, proton shuttle, and stabilizer to stabilize the structures of transition states and intermediates via π···H-O, O···H-N, O···H-C, and O···H-O interactions. The origin of the different reactivity of M1 and M2 is ascribed to the pivotal noncovalent interactions exist between catalyst (water and substrate 2) and reactants. © 2018 Wiley Periodicals, Inc.

An integrated experimental and theoretical reaction path search: analyses of the multistage reaction of an ionized diethylether dimer involving isomerization, proton transfer, and dissociation

A multistage reaction involving isomerization, proton transfer, and dissociation of an ionized diethylether dimer is studied by combination of infrared spectroscopy, tandem mass spectrometry, and a theoretical reaction path search.

Hydrogen bonding and dominant conformations of hydrated sugar analogue complexes using tetrahydrofurfuryl alcohol as the model sugar molecule

Water molecules, which serve as both hydrogen bond donors and acceptors, have been found to influence the conformational landscape of gas-phase phenyl-β-d-glucopyranoside.

Mechanistic insight on (E)-methyl 3-(2-aminophenyl)acrylate cyclization reaction by multicatalysis of solvent and substrate

The reaction mechanism of (E)-methyl 3-(2-aminophenyl)acrylate (A) with phenylisothiocyanate (B) as well as the vital roles of substrate A and solvent water were investigated under unassisted, water-assisted, substrate A-assisted, and water-A-assisted conditions. The reaction proceeds with four processes via nucleophilic addition, deprotonation and protonation, intramolecular cyclization with hydrogen transfer, and keto-enol tautomerization. According to the different H-shift mode, two possible types of H-shift P1 and P2 are carefully investigated to identify the most preferred pathway, differing in the NH2 group deprotonation and CH group of A protonation processes. It is found that substrate A and water not only act as reactant and solvent, but also as catalyst, proton shuttle, and stabilizer in effectively lowering the energy barrier. Therefore, the results demonstrate that the strong donating and accepting ability of NH2 group on A and the presence of bulk water are the keys to the title reaction proceed. © 2016 Wiley Periodicals, Inc.

Mass-selected IR-VUV (118 nm) spectroscopic studies of radicals, aliphatic molecules, and their clusters

Mass-selected IR plus UV/VUV spectroscopy and mass spectrometry have been coupled into a powerful technique to investigate chemical, physical, structural, and electronic properties of radicals, molecules, and clusters. Advantages of the use of vacuum ultraviolet (VUV) radiation to create ions for mass spectrometry are its application to nearly all compounds with ionization potentials below the energy of a single VUV photon, its circumventing the requirement of UV chromophore group, its inability to ionize background gases, and its greatly reduced fragmenting capabilities. In this review, mass-selected IR plus VUV (118 nm) spectroscopy is introduced first in a general manner. Selected application examples of this spectroscopy are presented, which include the detections and structural analysis of radicals, molecules, and molecular clusters in a supersonic jet.

A combined Raman- and infrared jet study of mixed methanol-water and ethanol-water clusters

The vibrational dynamics of vacuum-isolated hydrogen-bonded complexes between water and the two simplest alcohols is characterized at low temperatures by Raman and FTIR spectroscopy. Conformational preferences during adaptive aggregation, relative donor/acceptor strengths, weak secondary hydrogen bonding, tunneling processes in acceptor lone pair switching, and thermodynamic anomalies are elucidated. The ground state tunneling splitting of the methanol-water dimer is predicted to be larger than 2.5 cm(-1). Two types of alcohol-water trimers are identified from the spectra. It is shown that methanol and ethanol are better hydrogen bond donors than water, but even more so better hydrogen bond acceptors. As a consequence, hydrogen bond induced red shifts of OH modes behave non-linearly as a function of composition and the resulting cluster excess quantities correspond nicely to bulk excess enthalpies at room temperature. The effects of weak C-H···O hydrogen bonds are quantified in the case of mixed ethanol-water dimers.