Atom-molecule conversion with particle losses

Based on the mean-field approximation and the phase space analysis, we study the dynamics of an atom-molecule conversion system subject to particle loss. Starting from the many-body dynamics described by a master equation, an effective nonlinear Schr\"odinger equation is introduced. The classical phase space is then specified and classified by fixed points. The boundary, which separate different dynamical regimes have been calculated and discussed. The effect of particle loss on the conversion efficiency and the self-trapping is explored.Comment: 6 pages, 5 figure

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Mechanism for the Nonadiabatic Photooxidation of Benzene to Phenol: Orientation-Dependent Proton-Coupled Electron Transfer

An efficient catalytic one-step conversion of benzene to phenol was achieved recently by selective photooxidation under mild conditions with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) as the photocatalyst. Herein, high-level electronic structure calculations in the gas phase and in acetonitrile solution are reported to explore the underlying mechanism. The initially populated 1ππ* state of DDQ can relax efficiently through a nearby dark 1nπ* doorway state to the 3ππ* state of DDQ, which is found to be the precursor state involved in the initial intermolecular electron transfer from benzene to DDQ. The subsequent triplet-state reaction between DDQ radical anions, benzene radical cations, and water is computed to be facile. The formed DDQH and benzene-OH radicals can undergo T1→S0 intersystem crossing and concomitant proton-coupled electron transfer (PCET) to generate the products DDQH2 and phenol. Two of the four considered nonadiabatic pathways involve an orientation-dependent triplet PCET process, followed by intersystem crossing to the ground state (S0). The other two first undergo a nonadiabatic T1→S0 transition to produce a zwitterionic S0 complex, followed by a barrierless proton transfer. The present theoretical study identifies novel types of nonadiabatic PCET processes and provides detailed mechanistic insight into DDQ-catalyzed photooxidation

Study on utilization of carboxyl group decorated carbon nanotubes and carbonation reaction for improving strengths and microstructures of cement paste

Carbon nanotubes (CNTs) have excellent mechanical properties and can be used to reinforce cement-based materials. On the other hand, the reaction product of carbonation with hydroxides in hydrated cement paste can reduce the porosity of cement-based materials. In this study, a novel method to improve the strength of cement paste was developed through a synergy of carbon nanotubes decorated with carboxyl group and carbonation reactions. The experimental results showed that the carboxyl group (–COOH) of decorated carbon nanotubes and the surfactant can control the morphology of the calcium carbonate crystal of carbonation products in hydrated cement paste. The spindle-like calcium carbonate crystals showed great morphological differences from those observed in the conventional carbonation of cement paste. The spindle-like calcium carbonate crystals can serve as fiber-like reinforcements to reinforce the cement paste. By the synergy of the carbon nanotubes and carbonation reactions, the compressive and flexural strengths of cement paste were significantly improved and increased by 14% and 55%, respectively, when compared to those of plain cement paste

Excited-State Proton-Transfer-Induced Trapping Enhances the Fluorescence Emission of a Locked GFP Chromophore

The chemical locking of the central single bond in core chromophores of green fluorescent proteins (GFPs) influences their excited-state behavior in a distinct manner. Experimentally, it significantly enhances the fluorescence quantum yield of GFP chromophores with an ortho-hydroxyl group, while it has almost no effect on the photophysics of GFP chromophores with a para-hydroxyl group. To unravel the underlying physical reasons for this different behavior, we report static electronic structure calculations and nonadiabatic dynamics simulations on excited-state intramolecular proton transfer, cis–trans isomerization, and excited-state deactivation in a locked ortho-substituted GFP model chromophore (o-LHBI). On the basis of our previous and present results, we find that the S1 keto species is responsible for the fluorescence emission of the unlocked o-HBI and the locked o-LHBI species. Chemical locking does not change the parts of the S1 and S0 potential energy surfaces relevant to enol–keto tautomerization; hence, in both chromophores, there is an ultrafast excited-state intramolecular proton transfer that takes only 35 fs on average. However, the locking effectively hinders the S1 keto species from approaching the keto S1/S0 conical intersections so that most of trajectories are trapped in the S1 keto region for the entire 2 ps simulation time. Therefore, the fluorescence quantum yield of o-LHBI is enhanced compared with that of unlocked o-HBI, in which the S1 excited-state decay is efficient and ultrafast. In the case of the para-substituted GFP model chromophores p-HBI and p-LHBI, chemical locking hardly affects their efficient excited-state deactivation via cis–trans isomerization; thus, the fluorescence quantum yields in these chromophores remain very low. The insights gained from the present work may help to guide the design of new GFP chromophores with improved fluorescence emission and brightness

Concerted Asynchronous Hula-Twist Photoisomerization in the S65T/H148D Mutant of Green Fluorescent Protein

Fluorescence emission of wild-type green fluorescent protein (GFP) is lost in the S65T mutant, but partly recovered in the S65T/H148D double mutant. These experimental findings are rationalized by a combined quantum mechanics/molecular mechanics (QM/MM) study at the QM(CASPT2// CASSCF)/AMBER level. A barrierless excited-state proton transfer, which is exclusively driven by the Asp148 residue introduced in the double mutant, is responsible for the ultrafast formation of the anionic fluorescent state, which can be deactivated through a concerted asynchronous hula-twist photoisomerization. This causes the lower fluorescence quantum yield in S65T/H148D compared to wild-type GFP. Hydrogen out-of-plane motion plays an important role in the deactivation of the S65T/H148D fluorescent state