s-Tetrazine in Aqueous Solution: A Density Functional Study of Hydrogen Bonding and Electronic Excitations

Using density functional theory based methods we studied vertical and adiabatic excitations of the s-tetrazine molecule, small clusters with water molecules and a single s-tetrazine molecule within 60 water molecules using periodic boundary conditions. We therefore achieve a consistent description of s-tetrazine from the isolated molecule to full solvation in water. The explicit treatment of solvent molecules allows for an accurate treatment of solute−solvent interactions. For the isolated s-tetrazine molecule a comparison with earlier high level ab initio calculations and other density functional calculations is made. In accordance with experiment the most favorable two−water-adduct displays a homodromic feature, i.e., a chain of hydrogen bonding from the nitrogen of the s-tetrazine to its methenyl (CH) group. Radial distribution functions calculated from a Car−Parrinello molecular dynamics simulation of the aqueous solution clearly show an unexpected preference of water for hydrogen bonding to the C−H group over the nitrogen lone pairs. Only infrequent and short-lived hydrogen bonds from water molecules to the nitrogen atoms are found. Calculations of vertical excitations using time-dependent density functional theory showed that the solvent shifts can be explained from the polarization of the Kohn−Sham orbitals of the solute. Hydrogen bonding has only a minor effect on the solvent shifts of low lying states of s-tetrazine

s-Tetrazine in Aqueous Solution: A Density Functional Study of Hydrogen Bonding and Electronic Excitations

Using density functional theory based methods we studied vertical and adiabatic excitations of the s-tetrazine molecule, small clusters with water molecules and a single s-tetrazine molecule within 60 water molecules using periodic boundary conditions. We therefore achieve a consistent description of s-tetrazine from the isolated molecule to full solvation in water. The explicit treatment of solvent molecules allows for an accurate treatment of solute−solvent interactions. For the isolated s-tetrazine molecule a comparison with earlier high level ab initio calculations and other density functional calculations is made. In accordance with experiment the most favorable two−water-adduct displays a homodromic feature, i.e., a chain of hydrogen bonding from the nitrogen of the s-tetrazine to its methenyl (CH) group. Radial distribution functions calculated from a Car−Parrinello molecular dynamics simulation of the aqueous solution clearly show an unexpected preference of water for hydrogen bonding to the C−H group over the nitrogen lone pairs. Only infrequent and short-lived hydrogen bonds from water molecules to the nitrogen atoms are found. Calculations of vertical excitations using time-dependent density functional theory showed that the solvent shifts can be explained from the polarization of the Kohn−Sham orbitals of the solute. Hydrogen bonding has only a minor effect on the solvent shifts of low lying states of s-tetrazine

Simulations of the Aqueous “Brown-Ring” Complex Reveal Fluctuations in Electronic Character

Ab initio molecular dynamics (AIMD) simulations of the aqueous [Fe(H2O)5(NO)]2+ “brown-ring” complex in different spin states, in combination with multiconfigurational quantum chemical calculations, show a structural dependence on the electronic character of the complex. Sampling in the quartet and sextet ground states show that the multiplicity is correlated with the Fe–N distance. This provides a motivation for a rigid Fe–N scan in the isolated “brown-ring” complex to investigate how the multiconfigurational wave function and the electron density change around the FeNO moiety. Our results show that subtle changes in the Fe–N distance produce a large response in the electronic configurations underlying the quartet wave function. However, while changes in spin density and potential energy are pronounced, variations in charge are negligible. These trends within the FeNO moiety are preserved in structural sampling of the AIMD simulations, despite distortions present in other degrees of freedom in the bulk solution

Interdependent Electronic Structure, Protonation, and Solvatization of Aqueous 2‑Thiopyridone

2-Thiopyridone (2-TP), a common model system for excited-state proton transfer, has been simulated in aqueous solution with ab initio molecular dynamics. The interplay of electronic structure, protonation, and solvatization is investigated by comparison of three differently protonated molecular forms and between the lowest singlet and triplet electronic states. An interdependence clearly manifests in the mixed-character T1 state for the 2-TP form, systematic structural distortions of the 2-mercaptopyridine (2-MP) form, and photobase protolysis of the 2-TP– form, in the aqueous phase. In comparison, simplified continuum models for the solvatization are found to be significantly inaccurate for several of the species. To facilitate future computational studies, we therefore present a minimal representative solvatization complex for each stable form and electronic state. Our findings demonstrate the importance of explicit solvatization of the compound and sets the stage for including it also in future studies

Composition Related Tunability of “Green” Core/Shell Quantum Dots for Photovoltaic Applications from First Principles

Quantum dots (QDs) with core/shell (c/s) type configurations are promising candidates for photovoltaic (PV) applications, as they are known to enhance the QD stability, and are also expected to reduce charge carrier recombination both by reducing the trap states and increasing charge carrier separation. Hence, here we report detailed first-principles studies of different compositions of c/s QDs made from nontoxic materials, namely, CuInSe2/ZnS, CuInSe2/ZnSe, and CuInSe2/CuInS2 and their inverts, namely, ZnS/CuInSe2, ZnSe/CuInSe2, and CuInS2/CuInSe2. The geometric and electronic properties are studied using first-principles density functional theory (DFT). The optimized structures of all the QDs were found to have a defect-free c/s interface, which would reduce charge-carrier recombination rates arising due to charge trapping. The projected density of states (PDOS) of the QDs shows that the highest occupied molecular orbital (HOMO) is mainly composed of either S or Se states, whereas Zn or In states constitute the lowest unoccupied molecular orbital (LUMO). Time-dependent DFT (TDDFT) calculations of the optical transitions show that these systems have a strong absorption in the visible region of the spectrum. Interestingly, the c/s configuration enables the tailoring of the electronic and optical properties compared to the bulk as well as QD systems; as in the c/s QDs, the relative thickness as well as material composition of the core and shell is also a tunable parameter, in addition to the QD size. A natural transition orbital (NTO) analysis of the charge transfer upon light absorption shows, surprisingly, that charge separation occurs only in certain c/s configurations, such as the CuInSe2/ZnS QD, via fast direct electron transfer from core to shell, and CuInSe2/ZnSe QD, via indirect electron transfer, which may be slower. Hence, these compositions are expected to exhibit better efficiencies for PV applications. Thus, our study also highlights the importance of the NTO analysis in giving a detailed insight into local excitations and charge transfer excitations in these promising systems

Elucidating the Mechanism of Zn²⁺ Sensing by a Bipyridine Probe Based on Two-Photon Absorption

In this work, we examine, by means of computational methods, the mechanism of Zn2+ sensing by a bipyridine-centered, D-π-A-π-D-type ratiometric molecular probe. According to recently published experimental data [Divya, K. P.; Sreejith, S.; Ashokkumar, P.; Yuzhan, K.; Peng, Q.; Maji, S. K.; Tong, Y.; Yu, H.; Zhao, Y.; Ramamurthy, P.; Ajayaghosh, A. A ratiometric fluorescent molecular probe with enhanced two-photon response upon Zn2+ binding for in vitro and in vivo bioimaging. Chem. Sci. 2014, 5, 3469–3474], after coordination to zinc ions the probe exhibits a large enhancement of the two-photon absorption cross section. The goal of our investigation was to elucidate the mechanism behind this phenomenon. For this purpose, linear and nonlinear optical properties of the unbound (cation-free) and bound probe were calculated, including the influence of solute–solvent interactions, implicitly using a polarizable continuum model and explicitely employing the QM/MM approach. Because the results of the calculations indicate that many conformers of the probe are energetically accessible at room temperature in solution and hence contribute to the signal, structure–property relationships were also taken into account. Results of our simulations demonstrate that the one-photon absorption bands for both the unbound and bound forms correspond to the bright π → π* transition to the first excited state, which, on the other hand, exhibits negligible two-photon activity. On the basis of the results of the quadratic response calculations, we put forward a notion that it is the second excited state that gives the strong signal in the experimental nonlinear spectrum. To explain the differences in the two-photon absorption activity for the two lowest-lying excited states and nonlinear response enhancement upon binding, we employed the generalized few-state model including the ground, first, and second excited states. The analysis of the optical channel suggests that the large two-photon response is due to the coordination-induced increase of the transition moment from the first to the second excited state

Nitrogen <i>K</i>‑Edge X‑ray Absorption Spectra of Ammonium and Ammonia in Water Solution: Assessing the Performance of Polarizable Embedding Coupled Cluster Methods

The recent development of liquid jet and liquid leaf sample delivery systems allows for accurate measurements of soft X-ray absorption spectra in transmission mode of solutes in a liquid environment. As this type of measurement becomes increasingly accessible, there is a strong need for reliable theoretical methods for assisting in the interpretation of the experimental data. Coupled cluster methods have been extensively developed over the past decade to simulate X-ray absorption in the gas phase. Their performance for solvated species, on the contrary, remains largely unexplored. Here, we investigate the current state of the art of coupled cluster modeling of nitrogen K-edge X-ray absorption of aqueous ammonia and ammonium based on quantum mechanics/molecular mechanics, where both the level of coupled cluster calculations and polarizable embedding are scrutinized. The results are compared to existing experimental data as well as simulations based on transition potential density functional theory

Cations Strongly Reduce Electron-Hopping Rates in Aqueous Solutions

We study how the ultrafast intermolecular hopping of electrons excited from the water O1s core level into unoccupied orbitals depends on the local molecular environment in liquid water. Our probe is the resonant Auger decay of the water O1s core hole (lifetime ∼3.6 fs), by which we show that the electron-hopping rate can be significantly reduced when a first-shell water molecule is replaced by an atomic ion. Decays resulting from excitations at the O1s post-edge feature (∼540 eV) of 6 m LiBr and 3 m MgBr2 aqueous solutions reveal electron-hopping times of ∼1.5 and 1.9 fs, respectively; the latter represents a 4-fold increase compared to the corresponding value in neat water. The slower electron-hopping in electrolytes, which shows a strong dependence on the charge of the cations, can be explained by ion-induced reduction of water–water orbital mixing. Density functional theory electronic structure calculations of solvation geometries obtained from molecular dynamics simulations reveal that this phenomenon largely arises from electrostatic perturbations of the solvating water molecules by the solvated ions. Our results demonstrate that it is possible to deliberately manipulate the rate of charge transfer via electron-hopping in aqueous media

Fingerprint of Dipole Moment Orientation of Water Molecules in Cu²⁺ Aqueous Solution Probed by X‑ray Photoelectron Spectroscopy

The electronic structure and geometrical organization of aqueous Cu2+ have been investigated by using X-ray photoelectron spectroscopy (XPS) at the Cu L-edge combined with state-of-the-art ab initio molecular dynamics and a quantum molecular approach designed to simulate the Cu 2p X-ray photoelectron spectrum. The calculations offer a comprehensive insight into the origin of the main peak and satellite features. It is illustrated how the energy drop of the Cu 3d levels (≈7 eV) following the creation of the Cu 2p core hole switches the nature of the highest singly occupied molecular orbitals (MOs) from the dominant metal to the dominant MO nature of water. It is particularly revealed how the repositioning of the Cu 3d levels induces the formation of new bonding (B) and antibonding (AB) orbitals, from which shakeup mechanisms toward the relaxed H-SOMO operate. As highlighted in this study, the appearance of the shoulder near the main peak corresponds to the characteristic signature of shakeup intraligand (1a1 → H-SOMO(1b1)) excitations in water, providing insights into the average dipole moment distribution (≈36°) of the first-shell water molecules surrounding the metal ion and its direct impact on the broadening of the satellite. It is also revealed that the main satellite at 8 eV from the main peak corresponds to (metal/1b2 → H-SOMO(1b1) of water) excitations due to a bonding/antibonding (B/AB) interaction of Cu 3d levels with the deepest valence O2p/H1s 1b2 orbitals of water. This finding underscores the sensitivity of XPS to the electronic structure and orientation of the nearest water molecules around the central ion

Coupling Methylammonium and Formamidinium Cations with Halide Anions: Hybrid Orbitals, Hydrogen Bonding, and the Role of Dynamics

The electronic structures of four precursors for organic–inorganic hybrid perovskites, namely, methylammonium chloride and iodide, as well as formamidinium bromide and iodide, are investigated by X-ray emission (XE) spectroscopy at the carbon and nitrogen K-edges. The XE spectra are analyzed based on density functional theory calculations. We simulate the XE spectra at the Kohn–Sham level for ground-state geometries and carry out detailed analyses of the molecular orbitals and the electronic density of states to give a thorough understanding of the spectra. Major parts of the spectra can be described by the model of the corresponding isolated organic cation, whereas high-emission energy peaks in the nitrogen K-edge XE spectra arise from electronic transitions involving hybrids of the molecular and atomic orbitals of the cations and halides, respectively. We find that the interaction of the methylammonium cation is stronger with the chlorine than with the iodine anion. Furthermore, our detailed theoretical analysis highlights the strong influence of ultrafast proton dynamics in the core-excited states, which is an intrinsic effect of the XE process. The inclusion of this effect is necessary for an accurate description of the experimental nitrogen K-edge X-ray emission spectra and gives information on the hydrogen-bonding strengths in the different precursor materials