Ultrafast Dynamics of Nitro−Nitrite Rearrangement and Dissociation in Nitromethane Cation

We report new insights into the ultrafast rearrange- ment and dissociation dynamics of nitromethane cation (NM+) using pump−probe measurements, electronic structure calculations, and ab initio molecular dynamics simulations. The “roaming” nitro−nitrite rearrangement (NNR) pathway involving large- amplitude atomic motion, which has been previously described for neutral nitromethane, is demonstrated for NM+. Excess energy resulting from initial population of the electronically excited D2 state of NM+ upon strong-field ionization provides the necessary energy to initiate NNR and subsequent dissociation into NO+. Both pump−probe measurements and molecular dynamics simulations are consistent with the completion of NNR within 500 fs of ionization with dissociation into NO+ and OCH3 occurring ∼30 fs later. Pump−probe measurements indicate that NO+ formation is in competition with the direct dissociation of NM+ to CH3+ and NO2. Electronic structure calculations indicate that a strong D0 → D1 transition can be excited at 650 nm when the C−N bond is stretched from its equilibrium value (1.48 Å) to 1.88 Å. On the other hand, relaxation of the NM+ cation after ionization into D0 occurs in less than 50 fs and results in observation of intact NM+. Direct dissociation of the equilibrium NM+ to produce NO2+ and CH3 can be induced with 650 nm excitation via a weakly allowed D0 → D2 transition

Redox Chemistry of the Subphases of α-CsPbI₂Br and β-CsPbI₂Br: Theory Reveals New Potential for Photostability

The logic in the design of a halide-mixed APb(I1−xBrx)3 perovskite is quite straightforward: to combine the superior photovoltaic qualities of iodine-based perovskites with the increased stability of bromine-based perovskites. However, even small amounts of Br doped into the iodine-based materials leads to some instability. In the present report, using first-principles computations, we analyzed a wide variety of α-CsPbI2Br and β-CsPbI2Br phases, compared their mixing enthalpies, explored their oxidative properties, and calculated their hole-coupled and hole-free charged Frenkel defect (CFD) formations by considering all possible channels of oxidation. Nanoinclusions of bromine-rich phases in α-CsPbI2Br were shown to destabilize the material by inducing lattice strain, making it more susceptible to oxidation. The uniformly mixed phase of α-CsPbI2Br was shown to be highly susceptible towards a phase transformation into β-CsPbI2Br when halide interstitial or halide vacancy defects were introduced into the lattice. The rotation of PbI4Br2 octahedra in α-CsPbI2Br allows it either to transform into a highly unstable apical β-CsPbI2Br, which may phase-segregate and is susceptible to CFD, or to phase-transform into equatorial β-CsPbI2Br, which is resilient against the deleterious effects of hole oxidation (energies of oxidation >0 eV) and demixing (energy of mixing 2Br offers an opportunity to obtain a mixed perovskite material with enhanced photostability and an intermediate bandgap between its constituent perovskites

Pathways of Growth of CdSe Nanocrystals from Nucleant (CdSe)₃₄ Clusters

The initial steps in the growth of quantum platelets from the wurtzite-type (CdSe)₃₄ clusters are simulated using density functional theory with the generalized gradient approximation. The nucleant (CdSe)₃₄ cluster has been chosen for simulations because it has experimentally been found to be a magic-size nucleant for the low-temperature growth of CdSe quantum platelets. According to the results of our calculations, the growth is anisotropic and favors the (0001) direction, which is consistent with the experimental findings. We found that growth in other directions lowers the symmetry of the resulting clusters and that the asymmetrical positioning of rhombic defects causes the growing platelet to bend due to the surface strain, which appears to be the limiting factor of growth. An alternative pathway to quantum platelet growth could proceed via the decomposition of (CdSe)₃₄ to (CdSe)₁₃ in electron-donating media, which was found to be thermodynamically favorable. Side product (CdSe)₂₁ generated in this process is capable of growing via hexagonal stacking as well as propagating as a nanotube

Joint Studies of Spin Frustration Induced by Doping Small ZnSe Nanoparticles with Fe Atoms

Abstract As a 1.8 nm ZnSe nanocrystal is progressively doped with 1%, 5%, and 10% Fe, it shows a progressive change in its magnetic properties from a superparamagnetic FM‐dominated exchange type to an onset of AFM exchange with evidence of spin frustration. Magnetization measurements allow to obtain exchange coupling constants that are compared to the results of a Broken‐Symmetry Density Functional Theory (BS‐DFT) model of a doped (ZnSe)34 cluster. DFT shows a capability to reproduce the experimental pattern of the increasing influence of AFM exchange as doping concentration increases. The material phase segregates at the edges where strained rhombic surface sites are the preferred doping sites of iron. Large concentrations of iron leads to the formation of Fe clusters and complex exchange patterns that result in spin frustration in some iron trimers but none in the others. The spin frustration of these complex systems by assuming mirror symmetry of the sites when fitting by using BS‐DFT formalism is classified and analyzed. While some individual J constants obtained have significant errors, the averaged exchange constants are generally in good agreement with our experimental data

Structure and magnetic properties of Fe

The electronic, geometrical, and magnetic structures of iron clusters Fen substituted with a single Gd atom are studied using density functional theory with generalized gradient approximation for n = 12 − 19. An all electron basis set of a triple-ζ quality is chosen for the iron atoms whereas an effective core potential and the basis set of a triple-ζ quality are used for the Gd atom in optimizations of FenGd clusters. The lowest total energy state of a FenGd cluster was found to possess a geometrical structure where the Gd atom substitutes for a surface Fe atom of the Fen+1 cluster at given n. The total spin of a substituted cluster is larger than the total spin of the lowest total energy state of a unary iron cluster with the same number of atoms. The binding energy per atom in a substituted Fen−1Gd cluster is somewhat smaller than the binding energy per atom in a non-substituted Fen cluster. That is, the Gd substitution increases the total spin magnetic moment but destabilizes substituted iron clusters

Dissociation of Singly and Multiply Charged Nitromethane Cations: Femtosecond Laser Mass Spectrometry and Theoretical Modeling

Dissociation pathways of singly- and multiply charged gas-phase nitromethane cations were investigated with strong-field laser photoionization mass spectrometry and density functional theory computations. There are multiple isomers of the singly charged nitromethane radical cation, several of which can be accessed by rearrangement of the parent CH3–NO2 structure with low energy barriers. While direct cleavage of the C–N bond from the parent nitromethane cation produces NO2+ and CH3+, rearrangement prior to dissociation accounts for fragmentation products including NO+, CH2OH+, and CH2NO+. Extensive Coulomb explosion in fragment ions observed at high laser intensity indicates that rapid dissociation of multiply charged nitromethane cations produces additional species such as CH2+, H+, and NO22+.  On the basis of analysis of Coulomb explosion in the mass spectral signals and pathway calculations, sufficiently intense laser fields can remove four or more electrons from nitromethane