We present a new quantum accurate Spectral Neighbor Analysis Potential (SNAP) machine-learning potential for simulating carbon under extreme conditions of dynamic compression (pressures up to 1 TPa and temperatures up to 10,000 K). The development of SNAP potential involves (1) the generation of the training database comprised of the consistent and meaningful set of first-principles DFT (Density Functional Theory) data for carbon materials at high pressure and temperature; (2) the robust and physically guided training of the SNAP parameters on first-principles data involving statistical data analysis; and (3) the validation of the SNAP potential in MD simulations of carbon at high PT conditions. The excellent performance of quadratic SNAP potential is demonstrated by simulating the radial distribution functions at high pressure-temperature conditions and melt curve of diamond, which were found in good …
Investigations of the low-temperature radiation-induced transformations in the C2H2–H2O system are relevant to the chemistry occurring in interstellar and cometary ices. In this work, we applied a matrix isolation technique to study the radiation-driven evolution of this system at molecular level in order to get new mechanistic insight. The 1:1 C2H2⋅ ⋅ ⋅H2O complexes were prepared in various solid noble-gas matrices (Ar, Kr, Xe) and these icy matrices were subjected to X-ray irradiation at 5 K. Decomposition of initial complex and formation of products were monitored by Fourier-transform infrared (FTIR) spectroscopy. It was found that complexation with H2O resulted in significant enhancement of the radiolytic decay of C2H2 molecules and provided new channels for its radiation-induced transformations. Ketene (both isolated H2CCO and in the form of H2CCO–H2 pair), ketenyl radical (HCCO), carbon monoxide (CO), and methane (CH4) were observed as main products of the C2H2⋅ ⋅ ⋅H2O radiolysis. In addition, vinyl alcohol (CH2CHOH) was detected in an Ar matrix. The mechanistic interpretation (reaction routes leading to formation of these products) is discussed on the basis of consideration of kinetic dependences and matrix effects. Conversion of C2H2⋅ ⋅ ⋅H2O to CH4 is a prominent example of water-mediated cleavage of the C≡C triple bond, which may occur under prolonged irradiation in rigid media. Possible astrochemical implications of the obtained results are highlighted.
Using molecular dynamics simulation and evolutionary metadynamic calculations, a series of structures were revealed that possessed enthalpies and Gibbs energies lower than those of aragonite but higher than those of calcite. The structures are polytypes of calcite, differing in the stacking sequence of close-packed (cp) Ca layers. The two- and six-layered polytypes have hexagonal symmetry P6322 and were named hexarag and hexite, respectively. Hexarag is similar to aragonite, but with all the triangles placed on the middle distance between the cp layers. On the basis of the structures found, a two-step mechanism for the transformation of aragonite to calcite is suggested. In the first step, CO3 triangles migrate to halfway between the Ca layers with the formation of hexarag. In the second step, the two-layered cp (hcp) hexarag structure transforms into three-layered cp (fcc) calcite through a series of many …
We use a traditional surface science approach to create and study an atomically thin NiI2 film (a promising two-dimensional ferromagnetic material) formed on nickel substrate as a result of molecular iodine adsorption. The I/Ni(100) system was examined with scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and density functional theory calculations. We found out that the iodine adsorption on Ni(100) at 300 K leads to the formation of non-equilibrium phases, whereas the adsorption at elevated temperature (≥390 K) gives rise to the thermodynamically stable phases. In both cases, a simple p(2 × 2) structure is formed at 0.25 ML. As more iodine is adsorbed at 300 K, the p(2 × 2) phase is replaced by the small coexisting domains of c(3 × 2) and c(6 × 2) phases both corresponding to the coverage of 0.33 ML, while adsorption at elevated temperature results in the formation of only one c(3 × 2) phase. At further iodine adsorption the c(3 × 2) phase transforms into the c(5 × 2) one, while the c(6 × 2) phase – into the (√10×√10)R18°(√10×√10)R18° one both corresponding to the coverage of 0.40 ML. In addition to simple chemisorbed phases, a new shifted-row reconstruction of Ni(100) induced by iodine adsorption was discovered. At coverages exceeding 0.40 ML, we observed complex LEED patterns and superstructures in STM and assigned them to specific surface reconstructions. We also found that prolonged iodine dosing leads to the nucleation of nickel iodide islands and the growth of a 2D atomically thin iodide film partially exfoliated from the substrate.
The photophysical and photochemial properties of dipyrido[3,2-a:2′,3′-c]phenazine (dppz) in acetonitrile have been investigated by laser flash photolysis and ultrafast pump-probe spectroscopy. The excitation of dppz to the first singlet excited state was followed by vibrational cooling with solvent relaxation for ∼3 ps and then intersystem crossing for ∼50 ps. Spectral and kinetic characteristics of the triplet excited state have been determined.
Early processes in photochemistry of 2,3-bis(2,5-dimethylthiophen-3-yl)cyclopent-2-enone were explored using the ultrafast pump-probe spectroscopy. The photocyclization occurs from the antiparallel conformation of an open form on a time scale of 7 ps, while the parallel conformation exhibits an intersystem crossing within 130 ps. The mechanism and timescales were revealed as typical of other thiophene-containing diarylethenes.
The time of ultrafast intersystem crossing for the PtIVBr62– complex in aqueous solution was measured by a fluorescence up-conversion technique. The corresponding time constant (140 ± 40 fs) is in agreement with that estimated from previous ultrafast transient absorption measurements and with the reported results of theoretical calculations.
It is known that trans,cis,cis-[RuCl2(DMSO)2(H2O)2] (1a) complexes, which are formed upon dissolution of trans-[RuCl2(DMSO)4] in water, demonstrate light-induced cytotoxicity. The mechanistic study of 1a photochemistry has been performed using ultrafast pump–probe spectroscopy, laser flash photolysis and stationary photolysis. The first stage of 1a photochemistry is the photoexchange of a DMSO ligand to a water molecule; its quantum yield is wavelength-dependent (estimating by values 0.3 and 0.04 upon irradiation at 308 and 430 nm, respectively). The mechanism of photoexchange is complicated involving at least four Ru(II) intermediates. Two tentative mechanisms of the process are proposed.
The photochemistry of the OsIVCl62− complex in ethanol was studied by means of stationary photolysis, nanosecond laser flash photolysis, ultrafast pump–probe spectroscopy and quantum chemistry. The direction of the photochemical process was found to be wavelength-dependent. Irradiation in the region of the d–d and LMCT bands results in the photosolvation (with the wavelength-dependent quantum yield) and photoreduction of Os(IV) to Os(III), correspondingly. The characteristic time of photosolvation is ca. 40 ps. Photoreduction occurs in the micro- and millisecond time domains via several Os(III) intermediates. The nature of intermediates and the possible mechanisms of photoreduction are discussed. We believe that the lability of the photochemically produced Os(IV) and Os(III) intermediates determines the synthetic potential of OsIVCl62− photochemistry.