Israel Journal of Chemistry

Các hằng số tốc độ dập tắt huỳnh quang, k_q, nằm trong khoảng từ 10⁶ đến 2 × 10¹⁰ M⁻¹ giây⁻¹, của hơn 60 hệ thống cho-nhận electron điển hình đã được đo trong acetonitrile tách oxy và cho thấy có mối quan hệ với sự thay đổi thế năng tự do, ΔG₂₃, liên quan đến quá trình chuyển electron thực tế

Hình ảnh phóng to

trong phức hợp gặp gỡ và thay đổi trong khoảng từ +5 đến −60 kcal/mol. Mối quan hệ này dựa trên cơ chế chuyển electron ngoại-đường dãn cơ adiabatic yêu cầu ΔG^≠₂₃, thế năng tự do kích hoạt của quá trình này, là một hàm đơn điệu của ΔG₂₃ và cho phép tính toán các hằng số tốc độ dập tắt chuyển electron từ dữ liệu quang phổ và điện hóa.

Một nghiên cứu chi tiết về một số hệ thống mà các hằng số dập tắt tính toán khác biệt với các hằng số thí nghiệm đến vài bậc quy mô đã tiết lộ rằng cơ chế dập tắt hoạt động trong những trường hợp này là chuyển nguyên tử hydro thay vì chuyển electron.

Các điều kiện mà các cơ chế khác nhau này áp dụng và hậu quả của chúng được thảo luận.

A linear synchronous transit or quadratic synchronous transit approach is used to get closer to the quadratic region of the transition state and then quasi‐newton or eigenvector following methods are used to complete the optimization. With an empirical estimate of the hessian, these methods converge efficiently for a variety of transition states from a range of starting structures.

New scaling factors have been determined for obtaining fundamental vibrational frequencies and zero‐point vibrational energies from harmonic frequencies calculated at the HF/6–31G* and MP2/6–31G* levels. The scaling factors for the fundamental frequencies have been derived from a comparison of a total of 1066 calculated frequencies for 122 molecules with corresponding experimental values, while the zero‐point energy scaling factors were determined from a comparison of the computed values with the experimental zero‐point energies for a set of 24 molecules. The scaling factors recommended are, respectively, 0.8929 and 0.9427 for HF/6–31G* and MP2/6–31G* fundamental frequencies, and 0.9135 and 0.9646 for HF/6–31G* and MP2/6–31G* zero‐point energies. RMS errors were determined to be around 50 cm⁻¹ for the HF and MP2 fundamental frequencies, and around 0.4 kJ mol⁻¹ for the HF and MP2 zero‐point energies.

Many mutations disappear from the population because they impair protein function and/or stability. Thus, amino acid positions that are essential for proper function evolve more slowly than others, or in other words, the slow evolutionary rate of a position reflects its importance. ConSurf (http://consurf.tau.ac.il), reviewed in this manuscript, exploits this to reveal key amino acid positions that are important for maintaining the native conformation(s) of the protein and its function, be it binding, catalysis, transport, etc. Given the sequence or 3D structure of the query protein as input, a search for similar sequences is conducted and the sequences are aligned. The multiple sequence alignment is subsequently used to calculate the evolutionary rates of each amino acid site, using Bayesian or maximum‐likelihood algorithms. Both algorithms take into account the evolutionary relationships between the sequences, reflected in phylogenetic trees, to alleviate problems due to uneven (biased) sampling in sequence space. This is particularly important when the number of sequences is low. The ConSurf‐DB, a new release of which is presented here, provides precalculated ConSurf conservation analysis of nearly all available structures in the Protein DataBank (PDB). The usefulness of ConSurf for the study of individual proteins and mutations, as well as a range of large‐scale, genome‐wide applications, is reviewed.

With rising levels of CO₂ in our atmosphere, technologies capable of converting CO₂ into useful products have become more valuable. The field of electrochemical CO₂ reduction is reviewed here, with sections on mechanism, formate (formic acid) production, carbon monoxide production, reduction to higher products (methanol, methane, etc.), use of flow cells, high pressure approaches, molecular catalysts, non‐aqueous electrolytes, and solid oxide electrolysis cells. These diverse approaches to electrochemical CO₂ reduction are compared and contrasted, emphasizing potential processes that would be feasible for large‐scale use. Although the focus is on recent reports, highlights of older reports are also included due to their important contributions to the field, particularly for high‐rate electrolysis.

The terms “chiral” and “achiral” are sharply defined when applied to geometric figures or models. The same terms are also commonly used to refer to the real systems to which these models have been adjoined, e.g., molecules, solvents, or reagents. Here, the terms are not sharply defined but depend upon conditions of measurement. The contrast between the geometric and operational usages is discussed in detail.

This paper gives an overview of the author's activities in the research of extremely small metal oxide particles in recent years. In particular, the synthesis of transparent colloidal solutions of extremely small zinc oxide, titanium dioxide, hematite, and titanium/iron mixed oxide particles (2 nm < d 20 nm) in water, ethanol, and 2‐propanol is described. Quantum (Q)‐size effects are observed during particle growth and at me final stages of synthesis. A simple molecular orbital (MO) picture is presented for the qualitative interpretation of these effects, while quantitative calculations have been carried out using a quantum mechanical model developed by Brus. The photophysical properties of the particles have also been investigated extensively. Fluorescence spectra of the ZnO sols suggest that adsorbed electron relays are necessary to shuttle electrons from the conduction band to lower‐lying traps. Excess negative charge on the particles, resulting from either deprotonated surface hydroxyl groups or from photogenerated or externally injected charge carriers, causes a blue‐shift in the electronic absorption spectrum, which is explained by electrostatic and MO models. The zero point of charge (pH_zpc) of the aqueous colloidal suspensions has been determined by several independent methods. While zinc oxide, titanium dioxide, and titanium/iron mixed oxide particles exhibit considerable photocatalytic activity (as illustrated for the reduction of molecular oxygen and the oxidation of various halogenated carboxyl acids), hematite particles are only found to oxidize S(IV) under bandgap illumination to a reasonable extent (ϕ < 0.3). A mechanism involving surface‐bound molecules and free radical intermediates is presented to explain these differences in reactivity.

The energy transfer from an oxacyanine to a thiacyanine dye was investigated in monolayer assemblies. No temperature‐dependence of the efficiency was observed between 300 K and 20 K in arrangements where the donor molecules were isolated. However, in arrangements where the donor was organized into a large aggregate and the acceptor was highly diluted in the adjacent monolayer, the efficiency of the energy transfer was proportional to the absolute temperature; the spectral distributions of the fluorescence of the donor and the absorption of the acceptor were equally well matched at low and high temperatures. A simple model is discussed based on the idea that the energetic match needed for energy and electron transfer occurs occasionally due to thermal fluctuations and that this match will not be achieved below a certain temperature. A mechanism based on exciton hopping in the aggregate followed by energy transfer from a molecule in the vicinity of the acceptor to the acceptor does not explain the observed temperature‐dependence. However, the present experimental data can be rationalized by assuming that the excitation extends over a certain domain (consisting of about 10 donor dye molecules at room temperature and about 150 molecules at 20 K) which moves over the aggregate, occasionally reaching the vicinity of an acceptor molecule.

Over the past few decades, crystalline silicon solar cells have been extensively studied due to their high efficiency, high reliability, and low cost. In addition, these types of cells lead the industry and account for more than half of the market. For the foreseeable future, Si will still be a critical material for photovoltaic devices in the solar cell industry. In this paper, we discuss key issues, cell concepts, and the status of recent high‐efficiency crystalline silicon solar cells.

Boundary conditions and quantum mechanical numerical methods for evaluating transition probabilities of photodissociation and half‐collisional processes are presented. Direct and indirect dissociation modes are discussed. In each case we write a set of equations tying the particular boundary behaviour of each dissociation mode to the standard scattering boundary conditions. Numerical schemes based on these equations are presented. The case of competing dissociation reactions and exchange is also considered for the indirect case. When interference between these modes is expected to play a significant role, we show how the quantum jump due to photon and electron impact can be incorporated into the set of equations describing the dissociation. Finally we indicate how a numerical construction of potential surfaces from photofragment spectra and absorption spectra in the continuous and predissociation range can be obtained using these numerical schemes.