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Carbyne, an infinite-length straight chain of carbon atoms, is supposed to undergo a second order phase transition from the metallic bond-symmetric cumulene (═C═C═) ∞ toward the distorted insulating polyyne chain (-C≡C-) ∞ displaying bond-length alternation. However, recent synthesis of ultra long carbon chains (∼6000 atoms, [Nat. Mater., 2016 , 15 , 634]) did not show any phase transition and detected only the polyyne phase, in agreement with previous experiments on capped finite carbon chains. Here, by performing first-principles calculations, we show that quantum-anharmonicity reduces the energy gain of the polyyne phase with respect to the cumulene one by 71%. The magnitude of the bond-length alternation increases by increasing temperature, in stark contrast with a second order phase transition, confining the cumulene-to-polyyne transition to extremely high and unphysical temperatures. Finally, we predict that a high temperature insulator-to-metal transition occurs in the polyyne phase confined in insulating nanotubes with sufficiently large dielectric constant due to a giant quantum-anharmonic bandgap renormalization.

Low-dimensional systems with a vanishing band gap and a large electron-hole interaction have been proposed to be unstable toward exciton formation. As the exciton binding energy increases in low dimension, conventional wisdom suggests that excitonic insulators should be more stable in 2D than in 3D. Here we study the effects of the electron-hole interaction and anharmonicity in single-layer TiSe 2 . We find that, contrary to the bulk case and to the generally accepted picture, in single-layer TiSe 2 , the electron-hole exchange interaction is much smaller in 2D than in 3D and it has weak effects on phonon spectra. By calculating anharmonic phonon spectra within the stochastic self-consistent harmonic approximation, we obtain T CDW ≈ 440 K for an isolated and undoped single layer and T CDW ≈ 364 K for an electron-doping n = 4.6 × 10 13 cm -2 , close to the experimental result of 200-280 K on supported samples. Our work demonstrates that anharmonicity and doping melt the charge density wave in single-layer TiSe 2 .

At ambient pressure, bulk 2H-NbS 2 displays no charge density wave instability, which is at odds with the isostructural and isoelectronic compounds 2H-NbSe 2 , 2H-TaS 2 , and 2H-TaSe 2 , and in disagreement with harmonic calculations. Contradictory experimental results have been reported in supported single layers, as 1H-NbS 2 on Au(111) does not display a charge density wave, whereas 1H-NbS 2 on 6H-SiC(0001) endures a 3 × 3 reconstruction. Here, by carrying out quantum anharmonic calculations from first-principles, we evaluate the temperature dependence of phonon spectra in NbS 2 bulk and single layer as a function of pressure/strain. For bulk 2H-NbS 2 , we find excellent agreement with inelastic X-ray spectra and demonstrate the removal of charge ordering due to anharmonicity. In the two-dimensional limit, we find an enhanced tendency toward charge density wave order. Freestanding 1H-NbS 2 undergoes a 3 × 3 reconstruction, in agreement with data on 6H-SiC(0001) supported samples. Moreover, as strains smaller than 0.5% in the lattice parameter are enough to completely remove the 3 × 3 superstructure, deposition of 1H-NbS 2 on flexible substrates or a small charge transfer via field-effect could lead to devices with dynamical switching on/off of charge order.

The oxidation of water to O2 by the oxidized species [L2(O)Ru(V)ORu(V)(O)L2]4+ of the Ru blue dimer catalyst (L = bpy, bipyridine) is examined using density functional theory with model ligands and explicit solvent approaches. Following our earlier study of the initial O–O formation by addition of water (step I) ( J. Phys. Chem. A 2011, 115, 8003), we report calculations on the subsequent, penultimate step in the superoxide production (denoted step II), involving proton transfer from the reactant [L2(HOO)Ru(IV)ORu(IV)(OH)L2]4+ to form [L(O2(–))Ru(IV)ORu(III)(H2O)L2]4+. The reaction profile of step II commences with a rearrangement of the HOO and OH groups and associated solvent relaxation in the complex, accompanied by a barrier of ~9 kcal/mol and a free-energy change of +3 kcal/mol. Subsequently, a water molecule connecting these two groups mediates a double proton transfer in a proton relay chain that proceeds spontaneously with a free-energy decrease of 8 kcal/mol to form step II’s product. Comparison with other calculations is made, and the implications for the overall water oxidation to O2 are discussed.

The first key step in the oxidation of water to O(2) by the oxidized species [(bpy)(2)(O)Ru(V)ORu(V)(O)(bpy)(2)](4+) of the Ru blue dimer is studied using density functional theory (DFT) and an explicit solvent treatment. In the model reaction system [L(2)(O)Ru(V)ORu(V)(O)L(2)](4+)·(H(2)O)(4)·W(76), the surrounding water solvent molecules W are described classically while the inner core reaction system is described quantum mechanically using smaller model ligands (L). The reaction path found for the O--O single bond formation involves a proton relay chain: direct participation of two water molecules in two proton transfers to yield the product [L(2)(HOO)Ru(IV)ORu(IV)(OH)L(2)](4+)·(H(2)O)(3)·W(76). The calculated ∼3 kcal/mol reaction free energy and ∼15 kcal/mol activation free energy barrier at 298 K are consistent with experiment. Structural changes and charge flow along the intrinsic reaction coordinate, the solvent's role in the reaction barrier, and their significance for water oxidation catalysis are examined in detail.

The acid dissociation of a nitric acid HNO(3) molecule located at various depths in a water slab is investigated via Car-Parrinello molecular dynamics simulations. HNO(3) is found to remain molecular when it is adsorbed on top of the surface with two hydrogen-bonds, and to dissociate--although not always--by transferring a proton to a water molecule within a few picoseconds when embedded at various depths within the aqueous layer. The acid dissociation events are analyzed and discussed in terms of the proton donor-acceptor O-O hydrogen bonding distance and the configurations of the nearest-neighbor solvent waters of an HNO(3).H(2)O pair. Four key structural features for the HNO(3) acid dissociation are identified and employed to analyze the trajectory results. Key solvent motions for the dissociation include the decrease of the proton donor-acceptor O-O hydrogen bonding distance and a 4 to 3 coordination number change for the proton-accepting water. The Eigen cation (H(3)O(+)), rather than the Zundel cation (H(5)O(2)(+)), is found to be predominant next to the NO(3)(-) ion in contact ion pairs in all cases.

The infrared signatures of nitric acid HNO3 and its conjugate anion NO3(-) at the surface of an aqueous layer are derived from electronic structure calculations at the HF/SBK+* level of theory on the HNO3 x (H2O)3 --> NO3(-) x H3O(+) x (H2O)2 model reaction system embedded in clusters comprising 33, 40, 45, and 50 classical, polarizable waters, mimicking various degrees of solvation [Bianco, R.; Wang, S.; Hynes, J. T. J. Phys. Chem. A 2007, 111, 11033]. The molecular level character of the various bands is discussed, and the solvation patterns are described in terms of hydrogen bonding and resulting polarization of the species' intramolecular bonds. Connection is made with assorted experimental results, including surface-sensitive Sum Frequency Generation spectroscopy of aqueous nitric acid solutions, infrared spectroscopy of amorphous thin films of nitric acid monohydrate (NAM) and dihydrate (NAD), and infrared and Raman spectroscopic results for bulk aqueous solutions of nitric acid and nitrate salts.

The issue of acid dissociation of nitric acid at an aqueous surface is relevant in various portions of the atmosphere in connection with ozone depletion. This proton-transfer reaction is studied here via electronic structure calculations at the HF/SBK+(d) level of theory on the HNO(3) x (H(2)O)(3) model reaction system embedded in clusters comprising 33, 40, 45, and 50 classical, polarizable waters with an increasing degree of solvation of the nitrate group. Free energy estimates for all the cases examined favor undissociated, molecular nitric acid over the 0-300 K temperature range, including that relevant for the upper troposphere, where it is connected to the issue of the mechanism of nitric acid uptake by water ice aerosols. The presence of molecular HNO(3) at 300 K at the surface is further supported by vibrational band assignments in good agreement with a very recent surface-sensitive vibrational spectroscopy study of diluted HNO(3)/H(2)O solutions.

Theoretical studies of the mechanisms of several heterogeneous reactions involving ClONO(2), H(2)O, HCl, HBr, and H(2)SO(4) important in atmospheric ozone depletion are described, focused primarily on reactions on aqueous aerosol surfaces. Among the insights obtained is the active chemical participation of the surface water molecules in several of these reactions. The general methodology adopted allows reduction of these complex chemical problems to meaningful model systems amenable to quantum chemical calculations.

Electronic structure calculations on the H2SO4.(H2O)(4,6) model system embedded at the surface of an aqueous layer have been performed to examine the feasibility of the first acid dissociation of H2SO4 to an H2SO4-.H3O+ contact ion pair over a wide temperature range, with a special focus on the 190-250 K range relevant for atmospheric sulfate aerosols. The results indicate that the acid dissociation can be either thermodynamically favored or disfavored depending on the degree of solvation of the acid and the produced ions, as well as on the temperature.