Electronic properties of the low‐lying spin states of dimethylnitrosamine coordinated to Fe(III) heme models: An ab initio study

The unusual O‐coordination mode of nitrosamines to Fe(III) heme models has been observed in the bis(dimethylnitrosamine)(meso‐tetraphenylporphyrinate)iron(III) cation. For the first time, this latter as well as the simpler bis(dimethylnitrosamine)(porphinate)iron(III) heme model cations have been studied through ab initio methods. The sextet, quartet, and doublet spin states of both cations have been studied through single‐point calculations based on the experimental (X‐ray) geometry. Their energies, charges, and spin densities have been analyzed. The obtained results (at the UHF/cc‐pVDZ and ROHF/cc‐pVDZ levels) indicate that the peripheral benzene rings are of secondary importance for the coordination of dimethylnitrosamine to the Fe(III) porphyrin core. The obtained energy ordering is sextet < quartet < doublet, at all computational levels. The UHF, ROHF, and UMP2 results indicate an excess of alpha spin density around the Fe atom, a low covalency for the Fe? O bond and a substantial charge transfer to the Fe atom. Our best estimates [obtained at ROMP2 level with the mixed cc‐pVDZ/cc‐pVTZ‐DK(Fe) basis set] for the energy differences (in eV) between the three spin states considered are 0.929 for the sextet‐quartet gap and 0.812 for the quartet‐doublet gap, which indicate that the spin crossover (at room temperature) is very unlikely. These results represent the substantial decrease in the uncorrelated values. The implications of spin contaminations at the UHF and UMP2 levels for subsequent geometry optimizations to be performed in the smaller cation have also been discussed. © 2013 Wiley Periodicals, Inc.     

The deformable universe

The concept of smooth deformations of Riemannian manifolds, recently evidenced by the solution of the Poincaré conjecture, is applied to Einstein’s gravitational theory and in particular to the standard FLRW cosmology. We present a brief review of the deformation of Riemannian geometry, showing how such deformations can be derived from the Einstein-Hilbert dynamical principle. We show that such deformations of space-times of general relativity produce observable effects that can be measured by four-dimensional observers. In the case of the FLRW cosmology, one such observable effect is shown to be consistent with the accelerated expansion of the universe.     

Terrestrial gamma-ray flashes as powerful particle accelerators

Strong electric discharges associated with thunderstorms can produce terrestrial gamma-ray flashes (TGFs), i.e., intense bursts of x?rays and γ?rays lasting a few milliseconds or less. We present in this Letter new TGF timing and spectral data based on the observations of the Italian Space Agency AGILE satellite. We determine that the TGF emission above 10 MeV has a significant power-law spectral component reaching energies up to 100 MeV. These results challenge TGF theoretical models based on runaway electron acceleration. The TGF discharge electric field accelerates particles over the large distances for which maximal voltages of hundreds of megavolts can be established. The combination of huge potentials and large electric fields in TGFs can efficiently accelerate particles in large numbers, and we reconsider here the photon spectrum and the neutron production by photonuclear reactions in the atmosphere.     

Dynamic Effects Dictate the Mechanism and Selectivity of Dehydration–Rearrangement Reactions of Protonated Alcohols [Me2(R)CCH(OH2)Me]+ (R=Me,Et, iPr) in the Gas Phase

The gas‐phase dehydration–rearrangement (DR) reactions of protonated alcohols [Me₂(R)CCH(OH₂)Me]⁺ [R=Me ( ME ), Et ( ET ), and iPr ( I‐PR )] were studied by using static approaches (intrinsic reaction coordinate (IRC), Rice–Ramsperger–Kassel–Marcus theory) and dynamics (quasiclassical trajectory) simulations at the B3LYP/6‐31G(d) level of theory. The concerted mechanism involves simultaneous water dissociation and alkyl migration, whereas in the stepwise reaction pathway the dehydration step leads to a secondary carbocation intermediate followed by alkyl migration. Internal rotation (IR) can change the relative position of the migrating alkyl group and the leaving group (water), so distinct products may be obtained: [Me(R)CCH(Me)Me ??? OH₂]⁺ and [Me(Me)CCH(R)Me ??? OH₂]⁺. The static approach predicts that these reactions are concerted, with the selectivity towards these different products determined by the proportion of the conformers of the initial protonated alcohols. These selectivities are explained by the DR processes being much faster than IR. These results are in direct contradiction with the dynamics simulations, which indicate a predominantly stepwise mechanism and selectivities that depend on the alkyl groups and dynamics effects. Indeed, despite the lifetimes of the secondary carbocations being short (<0.5 ps), IR can take place and thus provide a rich selectivity. These different selectivities, particularly for ET and I‐PR , are amenable to experimental observation and provide evidence for the minor role played by potential‐energy surface and the relevance of the dynamics effects (non‐IRC pathways, IR) in determining the reaction mechanisms and product distribution (selectivity).     

Controlling Proton Delivery through Catalyst Structural Dynamics

The fastest synthetic molecular catalysts for H₂ production and oxidation emulate components of the active site of hydrogenases. The critical role of controlled structural dynamics is recognized for many enzymes, including hydrogenases, but is largely neglected in designing synthetic catalysts. Our results demonstrate the impact of controlling structural dynamics on H₂ production rates for [Ni(P^Ph₂N^C6H4R₂)₂]²⁺ catalysts (R=n‐hexyl, n‐decyl, n‐tetradecyl, n‐octadecyl, phenyl, or cyclohexyl). The turnover frequencies correlate inversely with the rates of chair–boat ring inversion of the ligand, since this dynamic process governs protonation at either catalytically productive or non‐productive sites. These results demonstrate that the dynamic processes involved in proton delivery can be controlled through modification of the outer coordination sphere, in a manner similar to the role of the protein architecture in many enzymes. As a design parameter, controlling structural dynamics can increase H₂ production rates by three orders of magnitude with a minimal increase in overpotential.     

Low-lying singlet and triplet electronic states of RhB

The low-lying XSigma+, a3Delta, A1Delta, b3Sigma+, B1Pi, c3Pi, C1Phi, D1Sigma+, E1Pi, d3Phi, and e3Pi electronic states of RhB have been investigated at the ab initio level, using the multistate multiconfigurational second-order perturbation (MS-CASPT2) theory, with extended atomic basis sets and inclusion of scalar relativistic effects. Among the eleven electronic states included in this work, only three (the X1Sigma+, D1Sigma+, and E1Pi states) have been investigated experimentally. Potential energy curves, spectroscopic constants, dipole moments, binding energies, and chemical bonding aspects are presented for all electronic states.     

Stereo- and regiospecific cu-catalyzed cross-coupling reaction of vinyl iodides and thiols: a very mild and general route for the synthesis of vinyl sulfides

A mild and efficient method for the copper-catalyzed formation of vinylic carbon-sulfur bonds has been developed. The desired vinyl sulfides are obtained in good to excellent yields, with full retention of stereochemistry. This method is particularly noteworthy given its mild reaction conditions, simplicity, and generality, as well as low cost of the catalyst system.     

Resolving Ultrafast Photoinitiated Dynamics of the Hachimoji 5-aza-7-deazaguanine Nucleobase: Impact of Synthetically Expanding the Genetic Alphabet†

The guanine derivative, 5-aza-7-deazaguanine (^5N7CG) has recently been proposed as one of four unnatural bases, termed Hachimoji (8-letter) to expand the genetic code. We apply steady-state and time-resolved spectroscopy to investigate its electronic relaxation mechanism and probe the effect of atom substitution on the relaxation mechanism in polar protic and polar aprotic solvents. Mapping of the excited state potential energy surfaces is performed, from which the critical points are optimized by using the state-of-art extended multi-state complete active space second-order perturbation theory. It is demonstrated that excitation to the lowest energy ¹ππ* state of ^5N7CG results in complex dynamics leading to ca. 10- to 30-fold slower relaxation (depending on solvent) compared with guanine. A significant conformational change occurs at the S₁ minimum, resulting in a 10-fold greater fluorescence quantum yield compared with guanine. The fluorescence quantum yield and S₁ decay lifetime increase going from water to acetonitrile to propanol. The solvent-dependent results are supported by the quantum chemical calculations showing an increase in the energy barrier between the S₁ minimum and the S₁/S₀ conical intersection going from water to propanol. The longer lifetimes might make ^5N7CG more photochemically active to adjacent nucleobases than guanine or other nucleobases within DNA.