Long-Range Migration of a Water Molecule To Catalyze a Tautomerization in Photoionization of the Hydrated Formamide Cluster

Electron ionization of clusters containing the formamide molecule

Studies on electron interactions with formamide (FA) clusters promote scientific interest as a model system to understand phenomena relevant to astrophysical, prebiotic, and radiobiological processes. In this work, mass spectrometric detection of cationic species for both small bare and microhydrated formamide clusters was performed at an electron ionization of 70 eV. Furthermore, a comparative analysis of the cluster spectra with the literature-reported gas-phase spectra is presented and discussed, revealing different reaction channels affected by the cluster environment. This study is essential in developing our understanding of both low-energy electron phenomena in clusters that can bridge the complexity gap between gas and realistic systems and the effect of hydration on electron-induced processes.

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.

Watching proton transfer in real time: Ultrafast photoionization-induced proton transfer in phenol-ammonia complex cation

In this paper, we give a full account of our previous work [C. C. Shen et al., J. Chem. Phys. 141, 171103 (2014)] on the study of an ultrafast photoionization-induced proton transfer (PT) reaction in the phenol-ammonia (PhOH-NH₃) complex using ultrafast time-resolved ion photofragmentation spectroscopy implemented by the photoionization-photofragmentation pump-probe detection scheme. Neutral PhOH-NH₃ complexes prepared in a free jet are photoionized by femtosecond 1 + 1 resonance-enhanced multiphoton ionization via the S₁ state. The evolving cations are then probed by delayed pulses that result in ion fragmentation, and the ionic dynamics is followed by measuring the parent-ion depletion as a function of the pump-probe delay time. By comparing with systems in which PT is not feasible and the steady-state ion photofragmentation spectra, we concluded that the observed temporal evolutions of the transient ion photofragmentation spectra are consistent with an intracomplex PT reaction after photoionization from the initial non-PT to the final PT structures. Our experiments revealed that PT in [PhOH-NH₃]⁺ cation proceeds in two distinct steps: an initial impulsive wave-packet motion in ∼70 fs followed by a slower relaxation of about 1 ps that stabilizes the system into the final PT configuration. These results indicate that for a barrierless PT system, even though the initial PT motions are impulsive and ultrafast, the time scale to complete the reaction can be much slower and is determined by the rate of energy dissipation into other modes.

Study of Potential Energy Surfaces towards Global Reaction Route Mapping

The potential energy surface (PES) is just a theoretical construct based on the Born-Oppenheimer approximation, but it underlies various phenomena, including molecular vibrations, collisional ionizations, and chemical reactions. This account describes how a new idea for global reaction route mapping (GRRM), which had seemed to be impossible for chemical systems with more than three atoms, was born and has been developed during the course of the study of the PES. GRRM has pioneered new fields of chemistry. Furthermore, techniques for GRRM are still developing, and GRRM is further extending its application to various areas of chemistry and chemical physics.

The mechanism of tautomerisation and geometric isomerisation in thioformic acid and its water complexes: exploring chemical pathways for water migration

A systematic and automated search for chemical pathways of isomerisation between geometric and tautomeric forms of gas-phase thioformic acid (TFA) and its water complexes is performed using a global reaction route mapping (GRRM) method, and an uncovered pathway for cis-trans isomerisation in the thiol form of TFA has been explored through computations performed at CCSD(T)/6-311++G(2d,2p)//B3LYP/6-311++G(2d,2p) level of the coupled cluster and density functional theories. To explore the routes for water migration, a detailed analysis of complexes of TFA with a single-water molecule is presented. Notably, during the isomerisation process in TFA, a positive catalytic effect of water was observed that can arise either by the stabilization of the reactant and/or of the transition state through extensive hydrogen bonding. Interesting behavior of isomeric forms of TFA along the pathways analysed is revealed through the Gibbs free-energy change and its temperature-dependence. The cis form of TFA(thiol) in the complexes of TFA with a single-water molecule is found to be thermodynamically equally feasible as the trans form which though is known to be the most dominating among the isomeric forms of TFA. Besides these, various complexes of TFA with two-water molecules have also been explored to study the hydrogen-bonding interaction through natural bond-orbital (NBO) analysis. The complexes of TFA with two-water molecules have also been characterized using spectral features including vibrational frequency analysis, and the effect of complexation has been observed by noting frequency shift.

Radiation processing of formamide and formamide:water ices on silicate grain analogue

Lyman-α (121.6 nm) photon and 1 keV electron-beam irradiation of pure HCONH2 (FA) ice and H2O:HCONH2 ice mixtures on high-surface-area SiO2 nanoparticles have been investigated with FT-IR spectroscopy and temperature programmed desorption (TPD). Lyman-α photolysis of pure amorphous FA ice grown at 70 K and crystalline FA ice produced by annealing to 165 K gives spectral signatures between 2120 and 2195 cm(-1) that we assign primarily to OCN(-) and CO. The OCN(-) and CO yields are ∼25% less abundant for crystalline FA ice. Photon and electron processing also produces H2 that is released from the ice between ∼90 and 140 K. A decrease in the H2 TPD peak is seen for irradiated crystalline HCONH2 ice. Lyman-α photolysis of H2O:HCONH2 mixed ices increases OCN(-) and CO production, suggesting a catalytic role of H2O. Also, for pure FA, 1 keV electron irradiation slightly increases the yield of OCN(-), while CO decarboxylation is selectively prevented. CO is also not produced in H2O:HCONH2 ices upon electron irradiation. Dissociative ionization, direct dissociative excitation, and dissociative electron attachment (DEA) channels are accessible in the Lyman-α (121.6 nm) photon and 1 keV electron-beam energy range. DEA energetically favors OCN(-) and H(-) formation, with the latter leading to H2 formation. The FA fragment product identities, yields, and branching ratios are considerably different relative to the gas phase and depend upon the radiation type, ice structure, and the presence of SiO2 nanoparticles. The latter may increase ion-electron recombination and radical recombination rates. The main products observed suggest very different condensed-phase dissociation channels from those reported for gas-phase dissociation. Formation of ions/products from FA is not negligible upon Lyman-α photolysis or electron irradiation, both of which could process ices in interstellar regions as well as in Titan's atmosphere.

EVOLUTIONARY DISCOVERY OF TRANSITION STATES IN WATER CLUSTERS

As a basic Aristotle element, water is the most abundant and more importantly crucial substance on earth. Without water, there would not be any form of life as we know. Understanding many phenomena in water such as water evaporation and ice melting and formation requires a deep understanding of hydrogen bond breaking and formation. In particular transition states play a key role in the understanding of such hydrogen bond behavior. Transition states, unlike other metastable states, are energy maxima along the minimum energy path connecting two isomers of molecular clusters. Geometry optimization of transition state structures, however, is a difficult task, and becomes even more arduous, especially when dealing with complex biochemical systems using first-principles calculations. In this paper, a novel molecular memetic algorithm (MOL-MA) composing of specially designed molecular-based water evolutionary operators coupled with a transition-state-local search solver and valley adaptive clearing scheme for the discovery of multiple precise transition states structures is proposed. The transition states of water clusters up to four water molecules uncovered using MOL-MA are reported. MOL-MA is shown not only to reproduce previously found transition states in water clusters, but also established newly discovered transition states for sizes 2–4 water molecules. The search performance of MOL-MA is also shown to outperform its compeers when pitted against those reported in the literature for finding transition states as well as recent advances in niching algorithms in terms of solution precision, computational effort, and number of transition states uncovered.