Water network-mediated, electron-induced proton transfer in [C5H5N ⋅ (H2O)n](-) clusters

Disentangling Acid-Base Chemistry through Blue Shifting Hydrogen Bond Contributions

The blue shifting of vibrational frequencies in hydrogen bonded molecules, as observed in aqueous environments, has been attributed to local partial charge transfer from solvation. Here, we extrapolate the blue shift model to the stronger ionic interactions between hydrogen bond acceptors associated with protonation through augmented pH levels and competitive interactions with counter ion pairing. The chemical model we utilize in this work is the aqueous pyridine-pyridinium equilibrium to characterize the blue shifts observed in the pyridinium chloride ionic system. The observed agreement between observed experimental and calculated spectral shifts shows that the blue shifting model can be extrapolated to stronger interactions and accurately describe the nature of the hydrogen bond.

Water Network Shape-Dependence of Local Interactions with the Microhydrated -NO2- and -CO2- Anionic Head Groups by Cold Ion Vibrational Spectroscopy

We report the structural evolutions of water networks and solvatochromic response of the CH₃NO₂^- radical anion in the OH and CH stretching regions by analysis of the vibrational spectra displayed by cryogenically cooled CH₃NO₂^-·(H₂O)_n=1-6 clusters. The OH stretching bands evolve with a surprisingly large discontinuity at n = 6, which features the emergence of an intense, strongly red-shifted band along with a weaker feature that appears in the region assigned to a free OH fundamental. Very similar behavior is displayed by the perdeuterated carboxylate clusters, RCO₂^-·(H₂O)_n=5-7 (R = CD₃CD₂), indicating that this behavior is a general feature in the microhydration of the triatomic anionic domain and not associated with CH oscillators. Electronic structure calculations trace this behavior to the formation of a "book" isomer of the water hexamer that adopts a configuration in which one of the water molecules resides in an acceptor-acceptor-donor (AAD) (A = acceptor, D = donor) H-bonding site. Excitation of the bound OH in the AAD site explores the local network topology best suited to stabilize an incipient -XO₂H-OH^-(H₂O)₂ intracluster proton-transfer reaction. These systems thus provide particularly clear examples where the network shape controls the potential energy landscape that governs water network-mediated, intracluster proton transfer. The CH stretching bands of the CH₃NO₂^-·(H₂O)_n=1-6 clusters also exhibit strong solvatochromic shifts, but in this case, they smoothly blue-shift with increasing hydration with no discontinuity at n = 6. This behavior is analyzed in the context of the solute-ion polarizability response and partial charge transfer to the water networks.

Sprayed water microdroplets containing dissolved pyridine spontaneously generate pyridyl anions

Water microdroplets can accelerate chemical reactions by orders of magnitude compared to the same reactions in bulk water and/or trigger spontaneous reactions that do not occur in bulk solution. Among the properties of water microdroplets, the unique redox ability resulting from the spontaneous dissociation of OH⁻ into a released electron and •OH at the air–water interfaces is especially intriguing. At the air–water interface, OH⁻ exhibits a strong reducing potential, and the resulting •OH is highly oxidative, making water microdroplets a unity of opposites. We report the reduction of pyridine into pyridyl anions (C₅H₅N⁻) and the oxidation of pyridine into hydroxypyridine, which extends what we know about the redox power of water microdroplets.

The anion of pyridine, C₅H₅N⁻, has been thought to be short lived in the gas phase and was only previously observed indirectly. In the condensed phase, C₅H₅N⁻ is known to be stabilized by solvation with other molecules. We provide in this study striking results for the formation of isolated C₅H₅N⁻ from microdroplets of water containing dissolved pyridine observed in the negative ion mass spectrum. The gas-phase lifetime of C₅H₅N⁻ is estimated to be at least 50 ms, which is much longer than previously thought. The generated C₅H₅N⁻ captured CO₂ molecules to form a stable (Py-CO₂)⁻ complex, further confirming the existence of C₅H₅N⁻. We propose that the high electric field at the air–water interface of a microdroplet helps OH⁻ to transfer an electron to pyridine to form C₅H₅N⁻ and the hydroxyl radical •OH. Oxidation products of the Py reacting with •OH are also observed in the mass spectrum recorded in positive mode, which further supports this mechanism. The present study pushes the limits of the reducing and oxidizing power of water microdroplets to a new level, emphasizing how different the behavior of microdroplets can be from bulk water. We also note that the easy formation of C₅H₅N⁻ in water microdroplets presents a green chemistry way to synthesize value-added chemicals.

Theoretical insight into 7,8-dihydrogen-8-oxoguanine radical cation deprotonation

The pK_a values of reactive protons in 8-oxoG˙⁺ and potential energy profiles for 8-oxoG radical cation deprotonation reaction (N1–H and N7–H) were firstly calculated.

Deprotonation of Guanine Radical Cation G•+ Mediated by the Protonated Water Cluster

Proton transfer is regarded as a fundamental process in chemical reactions of DNA molecules and continues to be an active research theme due to the connection with charge transport and oxidation damage of DNA. For the guanine radical cation (G^•+) derived from one-electron oxidation, experiments suggest a facile proton transfer within the G^•+:C base pair, and a rapid deprotonation from N1 in free base or single-strand DNA. To address the deprotonation mechanism, we perform a thorough investigation on deprotonation of G^•+ in free G base by combining density functional theory (DFT) and laser flash photolysis spectroscopy. Experimentally, kinetics of deprotonation is monitored at temperatures varying from 280 to 298 K, from which the activation energy of 15.1 ± 1.5 kJ/mol is determined for the first time. Theoretically, four solvation models incorporating explicit waters and the polarized continuum model (PCM), i.e., 3H₂O-PCM, 4H₂O-PCM, 5H₂O-PCM, and 7H₂O-PCM models are used to calculate deprotonation potential energy profile, and the barriers of 5.5, 13.4, 14.4, and 13.7 kJ/mol are obtained, respectively. It is shown that at least four explicit waters are required for properly simulating the deprotonation reaction, where the participation of protonated water cluster plays key roles in facilitating the proton release from G^•+.

A Raman Spectroscopic and Computational Study of New Aromatic Pyrimidine-Based Halogen Bond Acceptors

Two new aromatic pyrimidine-based derivatives designed specifically for halogen bond directed self-assembly are investigated through a combination of high-resolution Raman spectroscopy, X-ray crystallography, and computational quantum chemistry. The vibrational frequencies of these new molecular building blocks, pyrimidine capped with furan (PrmF) and thiophene (PrmT), are compared to those previously assigned for pyrimidine (Prm). The modifications affect only a select few of the normal modes of Prm, most noticeably its signature ring breathing mode, ν1. Structural analyses afforded by X-ray crystallography, and computed interaction energies from density functional theory computations indicate that, although weak hydrogen bonding (C–H···O or C–H···N interactions) is present in these pyrimidine-based solid-state co-crystals, halogen bonding and π-stacking interactions play more dominant roles in driving their molecular-assembly.

Photoinduced water splitting in pyridine water clusters

Ab initio calculations predict that pyridine (Py) can act as a photo-catalyst to split water by the absorption of a UV photon following the reaction Py-H₂O + hν → PyH˙ + OH˙. To test this prediction, we performed two types of experiments: in the first, we characterize the electronic spectroscopy of the PyH˙ radical in the gas phase. In the second, we evidence the reaction through the UV excitation of molecular Py-(H₂O)_n clusters obtained in a supersonic expansion and monitoring the PyH˙ reaction product. The results show unambiguously that PyH˙ is produced, and thus that water is split using pyridine as a photo-catalyst.

Water network-mediated, electron-induced proton transfer in [C5H5N ⋅ (H2O)n](-) clusters

The role of proton-assisted charge accommodation in electron capture by a heterocyclic electron scavenger is investigated through theoretical analysis of the vibrational spectra of cold, gas phase [Py ⋅ (H2O)n=3-5](-) clusters. These radical anions are formed when an excess electron is attached to water clusters containing a single pyridine (Py) molecule in a supersonic jet ion source. Under these conditions, the cluster ion distribution starts promptly at n = 3, and the photoelectron spectra, combined with vibrational predissociation spectra of the Ar-tagged anions, establish that for n > 3, these species are best described as hydrated hydroxide ions with the neutral pyridinium radical, PyH((0)), occupying one of the primary solvation sites of the OH(-). The n = 3 cluster appears to be a special case where charge localization on Py and hydroxide is nearly isoenergetic, and the nature of this species is explored with ab initio molecular dynamics calculations of the trajectories that start from metastable arrangements of the anion based on a diffuse, essentially dipole-bound electron. These calculations indicate that the reaction proceeds via a relatively slow rearrangement of the water network to create a favorable hydration configuration around the water molecule that eventually donates a proton to the Py nitrogen atom to yield the product hydroxide ion. The correlation between the degree of excess charge localization and the evolving shape of the water network revealed by this approach thus provides a microscopic picture of the "solvent coordinate" at the heart of a prototypical proton-coupled electron transfer reaction.