Revealing the reaction path of UVC bond rupture in cyclic disulfides with ultrafast x-ray scattering.

Disulfide bonds are ubiquitous molecular motifs that influence the tertiary structure and biological functions of many proteins. Yet, it is well known that the disulfide bond is photolabile when exposed to ultraviolet C (UVC) radiation. The deep-UV-induced S─S bond fragmentation kinetics on very fast timescales are especially pivotal to fully understand the photostability and photodamage repair mechanisms in proteins.

Pattern changes of ecological product trade in countries along the Belt and Road.

The Belt and Road Initiative (BRI) was designed to promote economic and trade cooperation between countries along the Belt and Road (B&R), specifically by building an international trade network. Ecological resources are the basis for human survival. Countries along the B&R transform ecological resources into ecological products by production activities.

Exploring adaptive approaches for social-ecological sustainability in the Belt and Road countries: From the perspective of ecological resource flow.

The countries along the Belt and Road (B&R) are characterized by fragile ecosystems and underdevelopment economy. International trade usually transferred the eco-environmental negative impacts to developing countries. How to avoid the conflict between economic development and eco-environmental protection is the primary concern of building the Green Silk Road.

The external dependence of ecological products: Spatial-temporal features and future predictions.

The instability of international trade can threaten the resource security of resource-importing countries, while international trade helps address spatial mismatches between regional populations, economies, and resources. Ecological products are the basis for human survival and development, in which agri-livestock products are especially sensitive to trade fluctuation and closely related to human well-being. The external dependence is an important indicator to reveal the external supply risk of regional resources.

Ultrafast conformational dynamics of Rydberg-excited -methyl piperidine.

We have observed the ultrafast conformational dynamics of electronically excited -methyl piperidine (NMP) using time-resolved Rydberg fingerprint spectroscopy. Optical excitation at various wavelengths ranging from 212 nm to 229 nm leads to the 3s or 3p Rydberg states and induces coherent oscillatory motions with periods of about 700 fs. These coherent motions survive the internal conversion from 3p to 3s but then dephase on a time scale of a few oscillations.

Transient Symmetry Controls Photo Dynamics near Conical Intersections.

Excited-state chemistry lacks generalized symmetry rules. With many femtochemistry studies focused on individual cases, it is hard to build up the same level of chemical intuition for excited states as that for ground states. Here, we unravel the degrees of freedom involved in ultrafast internal conversion (IC) by mapping the vibrational coherence of the initial wavepacket and the dependence on molecular symmetry in various cyclic tertiary amines.

Risk of Global External Cereals Supply under the Background of the COVID-19 Pandemic: Based on the Perspective of Trade Network.

International food trade is an integral part of the food system, and the COVID-19 pandemic has exposed the fragility of external food supplies. Based on the perspective of cereals trade networks (CTN), the pandemic risk is combined with the trade intensity between countries, and an assessment model of cereals external supply risk is constructed that includes external dependence index (EDI), import concentration, and risk of COVID-19 from import countries index (RICI). The results show that: (1) the global main CTN have typical scale-free characteristics, and seven communities are detected under the influence of the core countries; (2) about 60%, 50%, and 70% of countries face risks of medium and above (high and very high) external dependence, concentration of imports, and COVID-19 in the country of origin, respectively.

Ultrafast X-ray scattering offers a structural view of excited-state charge transfer.

Intramolecular charge transfer and the associated changes in molecular structure in N,N'-dimethylpiperazine are tracked using femtosecond gas-phase X-ray scattering. The molecules are optically excited to the 3p state at 200 nm. Following rapid relaxation to the 3s state, distinct charge-localized and charge-delocalized species related by charge transfer are observed.

Strong-field induced fragmentation and isomerization of toluene probed by ultrafast femtosecond electron diffraction and mass spectrometry.

We investigate the fragmentation and isomerization of toluene molecules induced by strong-field ionization with a femtosecond near-infrared laser pulse. Momentum-resolved coincidence time-of-flight ion mass spectrometry is used to determine the relative yield of different ionic products and fragmentation channels as a function of laser intensity. Ultrafast electron diffraction is used to capture the structure of the ions formed on a picosecond time scale by comparing the diffraction signal with theoretical predictions.

Observation of the molecular response to light upon photoexcitation.

When a molecule interacts with light, its electrons can absorb energy from the electromagnetic field by rapidly rearranging their positions. This constitutes the first step of photochemical and photophysical processes that include primary events in human vision and photosynthesis. Here, we report the direct measurement of the initial redistribution of electron density when the molecule 1,3-cyclohexadiene (CHD) is optically excited.

A deep UV trigger for ground-state ring-opening dynamics of 1,3-cyclohexadiene.

We explore the photo-induced kinetics of 1,3-cyclohexadiene upon excitation at 200 nm to the 3p state by ultrafast time-resolved, gas-phase x-ray scattering using the Linac Coherent Light Source. Analysis of the scattering anisotropy reveals that the excitation leads to the 3px and 3py Rydberg electronic states, which relax to the ground state with a time constant of 208 ± 11 fs. In contrast to the well-studied 266 nm excitation, at 200 nm the majority of the molecules (76 ± 3%) relax to vibrationally hot cyclohexadiene in the ground electronic state.

Scattering off molecules far from equilibrium.

Pump-probe gas phase X-ray scattering experiments, enabled by the development of X-ray free electron lasers, have advanced to reveal scattering patterns of molecules far from their equilibrium geometry. While dynamic displacements reflecting the motion of wavepackets can probe deeply into the reaction dynamics, in many systems, the thermal excitation embedded in the molecules upon optical excitation and energy randomization can create systems that encompass structures far from the ground state geometry. For polyatomic molecular systems, large amplitude vibrational motions are associated with anharmonicity and shifts of interatomic distances, making analytical solutions using traditional harmonic approximations inapplicable.

Ultrafast X-ray scattering reveals vibrational coherence following Rydberg excitation.

The coherence and dephasing of vibrational motions of molecules constitute an integral part of chemical dynamics, influence material properties and underpin schemes to control chemical reactions. Considerable progress has been made in understanding vibrational coherence through spectroscopic measurements, but precise, direct measurement of the structure of a vibrating excited-state polyatomic organic molecule has remained unworkable. Here, we measure the time-evolving molecular structure of optically excited N-methylmorpholine through scattering with ultrashort X-ray pulses.

Simplicity Beneath Complexity: Counting Molecular Electrons Reveals Transients and Kinetics of Photodissociation Reactions.

Time-resolved pump-probe gas-phase X-ray scattering signals, extrapolated to zero momentum transfer, provide a measure of the number of electrons in a system, an effect that arises from the coherent addition of elastic scattering from the electrons. This allows to identify reactive transients and determine the chemical reaction kinetics without the need for extensive scattering simulations or complicated inversion of scattering data. We examine the photodissociation reaction of trimethylamine and identify two reaction paths upon excitation to the 3p state at 200 nm: a fast dissociation path out of the 3p state to the dimethyl amine radical (16.

Determining Orientations of Optical Transition Dipole Moments Using Ultrafast X-ray Scattering.

Identification of the initially prepared, optically active state remains a challenging problem in many studies of ultrafast photoinduced processes. We show that the initially excited electronic state can be determined using the anisotropic component of ultrafast time-resolved X-ray scattering signals. The concept is demonstrated using the time-dependent X-ray scattering of N-methyl morpholine in the gas phase upon excitation by a 200 nm linearly polarized optical pulse.

Assessing the ecological vulnerability of the upper reaches of the Minjiang River.

The upper reaches of the Minjiang River (URMR), located on the eastern edge of the Tibetan Plateau in southwestern China, are an important component of the ecological barrier of the Upper Yangtze River Basin. Climate change and human activities have increased the ecological sensitivity and vulnerability of the region, which may pose a threat to the ecological security of the Yangtze River Basin and have negative impacts on local social and economic development. In this study, we analyzed land use and cover change (LUCC) of the URMR between 2000 and 2010, and found that the total rate of LUCC was less than 0.

[Chemical Constituents from Shoot of Phyllostachys edulis (II)].

Objective: To investigate the chemical constituents from the shoot of Phyllostachys edulis.

Methods: Normal-phase, reversed-phase silica gel, Sephadex LH-20 porous polymer gel column and HPLC chromatography were used for isolation. Spectroscopic methods (13C-NMR, 1H-NMR, DEPT and EI-MS) were used for identification.

Ratio-controlled synthesis of CuNi octahedra and nanocubes with enhanced catalytic activity.

Non-noble bimetallic nanocrystals (NCs) have been widely explored due to not only their low cost and abundant content in the Earth's crust but also their outstanding performance in catalytic reactions. However, controllable synthesis of non-noble alloys remains a significant challenge. Here we report a facile synthesis of CuNi octahedra and nanocubes with controllable shapes and tunable compositions.

[Experimental investigation of shear bond strength on orthodontic bonding on dental fluorosis after air abrading surface preparation technique].

Objective: To find a way to having higher bond strength on mottled enamel.

Methods: Sixty mottled enamel first bicuspid teeth extracted from fifteen patients needing orthodontic force were prepared and divided into four groups. Group A was routine acid etched, group B was air abraded, group C was etched by clearfil liner self-etching primer, group D was air abraded and then etched by clearfil liner self-etching primer.