A MASH simulation of the photoexcited dynamics of cyclobutanone.

In response to a community prediction challenge, we simulate the nonadiabatic dynamics of cyclobutanone using the mapping approach to surface hopping (MASH). We consider the first 500 fs of relaxation following photoexcitation to the S2 state and predict the corresponding time-resolved electron-diffraction signal that will be measured by the planned experiment. 397 ab initio trajectories were obtained on the fly with state-averaged complete active space self-consistent field using a (12,11) active space. To obtain an estimate of the potential systematic error, 198 of the trajectories were calculated using an aug-cc-pVDZ basis set and 199 with a 6-31+G* basis set. MASH is a recently proposed independent trajectory method for simulating nonadiabatic dynamics, originally derived for two-state problems. As there are three relevant electronic states in this system, we used a newly developed multi-state generalization of MASH for the simulation: the uncoupled spheres multi-state MASH method (unSMASH). This study, therefore, serves both as an investigation of the photodissociation dynamics of cyclobutanone, and also as a demonstration of the applicability of unSMASH to ab initio simulations. In line with previous experimental studies, we observe that the simulated dynamics is dominated by three sets of dissociation products, C3H6 + CO, C2H4 + C2H2O, and C2H4 + CH2 + CO, and we interpret our predicted electron-diffraction signal in terms of the key features of the associated dissociation pathways.

Using Instanton Theory to Study Quantum Effects in Photosensitization.

Electronic excitation is usually accomplished using light (photoexcitation) and is a key step in a vast number of important physical and biological processes. However, in instances where photoexcitation is not possible, a photosensitizer can excite the target molecule in a process called photosensitization. Unfortunately, full details of its mechanism are still unknown. This perspective gives an overview of the current understanding of photosensitization and describes how instanton theory can be used to fill the gaps, especially with regard tothe importance of quantum tunnelling effects.

Electrostatically tunable interaction of CO2 with MgO surfaces and chemical switching: first-principles theory.

Electric field-assisted CO2 capture using solid adsorbents based on basic oxides can immensely reduce the required energy consumption compared to the conventional processes of temperature or pressure swing adsorption. In this work, we present first-principles density functional theoretical calculations to investigate the effects of an applied external electric field (AEEF) within the range from -1 to 1 V Å-1 on the CO2 adsorption behavior on various high and low-index facets of MgO. When CO2 is strongly adsorbed on MgO surfaces to form carbonate species, the coupling of electric fields with the resulting intrinsic dipole moment induces a 'switch' from a strongly chemisorbed state to a weakly chemisorbed or physisorbed state at a critical value of AEEF. We demonstrate that such 'switching' enables access to different metastable states with variations in the AEEF. On polar MgO(111) surfaces, we find a distinct feature of the adsorptive dissociation of CO2 towards the formation of CO in contrast to that on the non-polar MgO(100) and MgO(110) surfaces. In some cases, we observe broken inversion symmetry because of the AEEF that results in induced polarity at the interaction site of CO2 on MgO surfaces. Our results provide fundamental insights into the possibility of using AEEFs in novel solid adsorbent systems for CO2 capture and reduction.

CO2 Utilization Through its Reduction to Methanol: Design of Catalysts Using Quantum Mechanics and Machine Learning.

Reducing levels of CO2, a greenhouse gas, in the earth's atmosphere is crucial to addressing the problem of climate change. An effective strategy to achieve this without compromising the scale of industrial activity involves use of renewable energy and waste heat in conversion of CO2 to useful products. In this perspective, we present quantum mechanical and machine learning approaches to tackle various aspects of thermocatalytic reduction of CO2 to methanol, using H2 as a reducing agent. Waste heat can be utilized effectively in the thermocatalytic process, and H2 can be generated using solar energy in electrolytic, photocatalytic and photoelectrocatalytic processes. Methanol being a readily usable fuel in automobiles, this technology achieves (a) carbon recycling process, (b) use of renewable energy, and (c) portable storage of H2 for applications in automobiles, alleviating the problem of rising CO2 emissions and levels in atmosphere.

Quantum effects in photosensitization: the case of singlet oxygen generation by thiothymines.

Photosensitization is the indirect electronic excitation of a molecule with the aid of a photosensitizer and is a bimolecular nonradiative energy transfer. In this study, we have attempted to elucidate its mechanism, and we do this by calculating rate constants of photosensitization of oxygen by thiothymines (2-thiothymine, 4-thiothymine and 2,4 dithiothymine). The rate constants have been calculated using two approaches: (a) a classical limit of Fermi's Golden Rule (FGR), and (b) a time-dependent variant of FGR, where the treatment is purely quantum mechanical. The former approach has previously been developed for bimolecular systems and has been applied to the photosensitization reactions studied here. The latter approach, however, has so far only been used for unimolecular reactions, and in this work, we describe how it can be adapted for bimolecular reactions. Experimentally, all three thiothymines are known to have significant singlet oxygen yields, which are indicative of similar rates. Rate constants calculated using the time-dependent variant of FGR are similar across all three thiothymines. While the classical approximation gives reasonable rate constants for 2-thiothymine, it severely underestimates them for 4-thiothymine and 2,4 dithiothymine, by several orders of magnitude. This work indicates the importance of quantum effects in driving photosensitization.

Activation of CO2 and CH4 on MgO surfaces: mechanistic insights from first-principles theory.

One of the most challenging topics in heterogeneous catalysis is conversion of CH4 to higher hydrocarbons. Direct conversion of CH4 to ethylene can be achieved via the oxidative coupling of methane (OCM) reaction. Despite studies which have shown MgO to activate CH4 and initiate the OCM reaction, its large-scale applications face a significant impediment due to formation of a byproduct, CO2, and poisoning of the catalyst due to carbonate formation. In the present work, we address two aspects of the OCM reaction on MgO surfaces: carbonate formation on the surface of the catalyst, and (dissociative) adsorption of CH4. We use first-principles density functional theoretical calculations to determine the energetics and underlying mechanisms of interaction of CO2 and CH4 with various surfaces of MgO: (100), (110), and (111) (both Mg- and O-terminations), and the seldom studied, hydroxylated (111) MgO surface with O-termination. We find that the strength of the interaction of CO2 with MgO surfaces depends on several factors: their surface energies, coordination number of surface O atoms, and ability to donate electrons. However, the O-terminated (111) surface of MgO bucks all aforementioned factors, with only oxygen richness affecting its reactivity towards CO2. The interaction of CH4 with MgO surfaces depends primarily on the coordination number of the surface O atoms and the orientation of the CH4 molecule with respect to the surface. Finally, we provide insights into (a) formation of surface carbonates, which is relevant to CO2 capture and conversion, and (b) C-H bond activation on MgO surfaces, which is crucial for direct conversion of CH4 to value-added chemicals.

The Curious Case of Aqueous Warfarin: Structural Isomers or Distinct Excited States?

Warfarin is a potent anti-coagulant drug and is on the World Health Organization's List of Essential Medicines. Additionally, it displays fluorescence enhancement upon binding to human serum albumin, making warfarin a prototype fluorescent probe in biology. Despite its biological significance, the current structural assignment of warfarin in aqueous solution is based on indirect evidence in organic solvents. Warfarin is known to exist in different isomeric forms-open-chain, hemiketal, and anionic forms-based on the solvent and pH. Moreover, warfarin displays a dual absorption feature in several solvents, which has been employed to study the ring-chain isomerism between its open-chain and hemiketal isomers. In this study, our pH-dependent experiments on warfarin and structurally constrained warfarin derivatives in aqueous solution demonstrate that the structural assignment of warfarin solely on the basis of its absorption spectrum is erroneous. Using a combination of steady-state and time-resolved spectroscopic experiments, along with quantum chemical calculations, we assign the observed dual absorption to two distinct π → π* transitions in the 4-hydroxycoumarin moiety of warfarin. Furthermore, we unambiguously identify the isomeric form of warfarin that binds to human serum albumin in aqueous buffer.

Triplet Decay Dynamics in Sulfur-Substituted Thymine: How Position of Substitution Matters.

Sulfur-substituted analogues of thymine are of three types depending on the position of sulfur substitution: 2-thiothymine (2tThy), 4-thothymine (4tThy), and 2,4-dithiothymine (dtThy). These molecules, on photoexcitation, are known to form in their triplet state with near unity yield. Consequently, they are able to photosensitize ground state molecular oxygen to singlet oxygen, a property which makes them potential drugs for photodynamic therapy (PDT). The singlet oxygen yield is directly correlated with the triplet lifetime of the thiothymine, which in turn is governed by its triplet decay dynamics. In this work, the dependence of the triplet decay dynamics on the position of sulfur substitution is investigated by comparatively studying all three thiothymines. The topology of the triplet potential energy surface and decay mechanism of 2tThy is found to be distinctly different from 4tThy and dtThy. The fundamental reason for this is the different electronic natures of the two C═X (X = O, S) moieties in each molecule, one of which is conjugated with a C═C bond, while the other is not. Further, it is shown that the triplet lifetime of 2tThy can be increased by manipulating the energetic ordering of its molecular orbitals with unobtrusive substitutions, thus making it a better candidate for a PDT drug.

Interplay between Conjugation and Size-Driven Delocalization Leads to Characteristic Properties of Substituted Thymines.

Substituted thymines, where oxygen is replaced by sulfur or selenium, affect a variety of functions in biological systems and are prospective phototherapeutic agents. In this study, we show that an interplay between two types of delocalization, one due to conjugation and the other owing to the varying size of the substituted atom, leads to distinct absorption spectra and electrophilic sites in substituted thymines. This result is supported by ab initio quantum chemical calculations and a simple particle-in-a-box model. The model explains the unexpected variation in the absorption of 2-thiothymine and 4-thiothymine and makes an unanticipated prediction about the nature of the LUMO in 2-selenothymine that is confirmed by quantum chemical calculations. Here, delocalization due to the large size of selenium dominates that due to conjugation; in essence, a 2-center delocalization exerts a greater influence on molecular properties than a 4-center delocalization. The study highlights that the widely used concept of delocalization may be affected not only by the long-established idea of double bond conjugation but also delocalization owing to the size of atoms.