s-p Separation Model Indo-Mo Method and the Theoretical Study of 1H Hyperfine Coupling Constant

We have recently developed a modification of the traditional semiempirical INDO-MO method especially useful for open shell problems: INDO with s-p separation (J. Chin. Chem. Soc., 32, 385(1985)). In order to show the reliability of our new method fifty seven free radicals and radical ions were selected as examples for our MO calculation. We are particularly interested in the unpaired electron distribution of hydrogen atoms. A linear correlation analysis between the spin density and the isotropic hyperfine coupling constant (a_H) for ¹H has been carried out for these radicals and ions. Most of our calculated a_H's are better than the results reported using the conventional INDO method.     

Measurements of Long-Range Homonuclear Coupling Constants

Coupling constants in proton systems provide access to useful structural information. Several methods have been proposed to measure these constants in high-resolution spectra, but many of them are not well suited when the coupling constants are comparable to the spectral linewidth. In such a case the measurement of the apparent splitting, obtained from conventional NMR or from two-dimensional correlation spectroscopy (COSY), can cause a miscalculation of the true coupling constant value. In this work, data processing for extracting small coupling constants is described. Signals are obtained from spin-echo experiments and analysed in the time domain in such a way that couplings are apparently multiplied by n+1, where n is positive. Small coupling constants in 4-methyl-1,3-dioxane were obtained by this method. © 1997 John Wiley & Sons, Ltd.     

HCS自由基超精细结构的密度泛函理论计算

１９９８年,李远哲等［１］对ＨＣＳ自由基进行了傅里叶变换毫米波光谱实验研究,测定了质子的超精细耦合常数Ａ（Ｈ）为１２７．４２７ＭＨｚ．他们注意到此值远小于ＨＣＯ自由基的Ａ（Ｈ）值,并认为这与ＨＣＳ的键角比ＨＣＯ的键角大有关．我们用密度泛函理论中的（Ｕ）Ｂ３ＬＹＰ［２,３］方法对ＨＣＳ及有关的ＨＣＯ、ＨＳＩＳ和ＨＳＩＯ自由基进行了计算研究．本研究结果表明：Ｂ３ＬＹＰ方法计算的ＨＣＳ的Ａ（Ｈ）与实验值非常一致;键角的大小不足以解释ＨＣＳ的Ａ（Ｈ）值远小于ＨＣＯ这一事实;用自旋密度却可简明地加以解释．…     

A theoretical study on the hyperfine coupling constant of the radical cations of aliphatic ethers

Summary The hyperfine coupling constants of the radical cations of dimethylether, oxetane (oxacyclobutane), and tetrahydrofuran (oxacyclopentane) are studied byab-initio molecular orbital theories. The extraordinarily large hyperfine coupling constants of the protons of the ethers that have been found experimentally are analyzed to conclude that an important mechanism of the hole delocalization is the spin polarization in the H-C-O-C-H bond. It is also found that for the ethereal systems conventional molecular orbital calculations give glaringly small spin densities but the SAC-CI calculation gives remarkably improved values.     

J(Si,H) Coupling Constants in Nonclassical Transition‐Metal Silane Complexes

We will outline that the sign and magnitude of J(Si,H) coupling constants provide a highly sensitive tool to measure the extent of Si?H bond activation in nonclassical silane complexes. Up to now, this structure–property relationship was obscured by erroneous J(Si,H) sign determinations in the literature. These new findings also help to identify the salient control parameters of the Si?H bond activation process in nonclassical silane complexes.     

Precise Measurement of Long‐Range Heteronuclear Coupling Constants by a Novel Broadband Proton–Proton‐Decoupled CPMG‐HSQMBC Method

A broadband proton–proton‐decoupled CPMG‐HSQMBC method for the precise and direct measurement of long‐range heteronuclear coupling constants is presented. The Zangger–Sterk‐based homodecoupling scheme reported herein efficiently removes unwanted proton–proton splittings from the heteronuclear multiplets, so that the desired heteronuclear couplings can be determined simply by measuring frequency differences between singlet maxima in the resulting spectra. The proposed pseudo‐1D/2D pulse sequences were tested on nucleotides, a metal complex incorporating P heterocycles, and diglycosyl (di)selenides, as well as on other carbohydrate derivatives, for the extraction of ⁿJ(¹H,³¹P), ⁿJ(¹H,⁷⁷Se), and ⁿJ(¹H,¹³C) values, respectively.     

Sensitivity—enhanced Experiments for the Measurement of J and Dipolar Coupling Constants

A sensitivity-enhanced IPAP NMR experiment was described in this paper,which separates the ^1H-^15N doublets into two different spectra to alleviate the problem of resonance overlaps and achieve the accurate measurement of J and residual dipolar coupling constants in proteins.This experiment offered 20%-60% sensitivity enhancement over the original IPAP experiment,and therefore produced more measurable resonances.Pulsed field gradient was used for coherence selection.Water-flip-back approach was used for water suppression.The sensitivity-enhanced IPAP experiment was employed in the measurement of ^1JNH and ^1DNH constants of the protein UBC9.     

一种确定具有相似晶体结构材料介电常数的简单方法(英文)

作者将前人推导出来的理论推广到计算和预测材料的介电常数上面来 ,提出了一个很实用的指导性方法。此法仅仅从其相应组成原子的平均原子序数与其介电常数间的定量关系出发 ,使人们可以轻松地处理材料的介电常数问题。对广大的材料学工作者来说 ,此方法是很有帮助的。     

Evaluation of Acidity Constants of Amphoteric Substances by the Corresponding Solubilities Method

Abstract

A simple method for the evaluation of acidity constants of amphoteric substances (HR) is reported in this paper. The method is based on the use of pH values which have corresponding solubilities and has been checked against the p-diethylaminobenzylidenrhodanine system. In order to apply the method it is not necessary to know the intrinsic solubility of HR.     

Can One Assess the π Character of a C–C Bond with the Help of the NMR Spin–Spin Coupling Constants?

Measured one‐bond spin–spin coupling constants (SSCC) ¹J(CC) can be used to describe the nature of the C–C bond, provided one is able to separate the various coupling mechanisms leading to ¹J(CC). The Fermi‐contact (FC) term probes the first‐order density at the positions of the coupling nuclei, whereas the noncontact terms (the paramagnetic spin orbit (PSO) and the spin–dipole (SD) terms) probe the π character of the C–C bond (the diamagnetic spin orbit (DSO) term can mostly be neglected). A model is tested, in which the value of the FC(CC) term is estimated with the help of measured SSCCs ¹J(CH). The difference between the measured J(CC) and the estimated FC(CC) values, Δ(CC)=PSO(CC)+SD(CC)+DSO(CC), provides a semiquantitative measure of the π character of a C–C multiple bond. The applicability and limitations of this approach are discussed by partitioning the four Ramsey terms of the SSCC ¹J(CC) into one‐ and two‐orbital contributions. The FC, PSO, and SD terms of ¹J(CC) are explained and analyzed with regard to their relationship to other C–C bond properties. It is shown that empirical relationships between measured SSCCs and the s character of a bond need reconsideration.     

Visible-Light Photocatalysis of the Ketyl Radical Coupling Reaction

Ketyl radical coupling reactions for the construction of diversely functionalized alcohols have been continuously developed for many decades. Based on the recently widespread application of photocatalysis, ketyl radical coupling reactions have also witnessed rapid development. This Minireview aims to briefly and concisely summarize the methods to construct ketyl radical intermediates through visible-light photocatalysis over the past 10 years. The ketyl-radical-generation activators are grouped as Lewis acids, Brønsted acids, in situ generated Brønsted acids, and others.     

Estimation of Free Radical Polymerization Rate Coefficients Using Computational Chemistry

This study explores the application of computational chemistry to estimate free radical polymerization rate coefficients. The Evans-Polanyi relationship is combined with computed heats of polymerization to estimate copolymerization reactivity ratios for many vinyl monomer pairs, focusing on acrylates, methacrylates and styrene, with accuracy assessed by comparison to experimental values. The effect of different optimization approaches on the values of thermodynamic properties is explored, and it is concluded that a combination of conventional optimization and relaxed potential energy scans was most effective at identifying global minima. The difference between thermodynamic properties calculated using the harmonic oscillator treatment and a hindered rotor model is evaluated for methyl methacrylate polymerization.     

Coupling Interactions between Sulfurous Acid and the Hydroperoxyl Radical

Radical–molecule complexes associated with the hydroperoxyl radical (HOO) play an important role in atmospheric chemistry. Herein, the nature of the coupling interactions between sulfurous acid (H₂SO₃) and the HOO radical is systematically investigated at the B3LYP/6‐311++G(3df,3pd) level of theory in combination with the atoms in molecules (AIM) theory, the natural bond orbital (NBO) method, and energy decomposition analyses (EDA). Eight stable stationary points possessing double H‐bonding features were located on the H₂SO₃???HOO potential energy surface. The largest binding energies of ?12.27 and ?11.72 kcal mol^?1 are observed for the two most stable complexes, where both of them possess strong double intermolecular H‐bonds of partially covalence. Moreover, the characteristics of the IR spectra for the two most stable complexes are discussed to provide some help for their possible experimental identification.     

已烯自由基阳离子的密度泛函理论研究

使用密度泛函理论B3LYP方法和6-31G(d,p),6-31+G(d,p),6-311G(d,p)及6- 311+G(d,p)基组,分别对1-C_6H_(12)~+,2-C_6H_(12)~+和3-C_6H_(12)~+的各种构 象进行了几何构型优化,并在B3LYP/6-311G(d,p)水平上进行了频率分析计算,在 各优化构型上,使用B3LYP和MP2(full)方法进行了超精细结构的计算。计算的3- C_6H_(12)~+的超精细偶合常数比以往的计算结果更好;1-C_6H_(12)~+和2-C_6H_ (12)~+的超精细偶合常数目前尚无实验数据报道,本计算预言了它们的超精细偶合 常数和最稳定构型。     

A Three Equation Method for the Numerical Evaluation of Acidity Constants of Single Equilibria from Spectrophotometric Data

Abstract

In this paper we devise a numerical method for the evaluation of acidity constants of single equilibria suitable for use when the limit absorbance of the pure species are unavailable since these species cannot be obtained pure in solutions. The method has been applied to the evaluation of the first acidity constant of methylglyoxal bis (4-pheny1-3-thiosemicarbazone) (MGBPT).     

Characterization of Doubly Ionic Hydrogen Bonds in Protic Ionic Liquids by NMR Deuteron Quadrupole Coupling Constants: Differences to H‐bonds in Amides,Peptides, and Proteins

We present the first deuteron quadrupole coupling constants (DQCCs) for selected protic ionic liquids (PILs) measured by solid‐state NMR spectroscopy. The experimental data are supported by dispersion‐corrected density functional theory (DFT‐D3) calculations and molecular dynamics (MD) simulations. The DQCCs of the N−D bond in the triethylammonium cations are the lowest reported for deuterons in PILs, indicating strong hydrogen bonds between ions. The NMR coupling parameters are compared to those in amides, peptides, and proteins. The DQCCs show characteristic behavior with increasing interaction strength of the counterion and variation of the H‐bond motifs. We report the similar presence of the quadrupolar splitting pattern and the narrow liquid line in the NMR spectra over large temperature ranges, indicating the heterogeneous nature of PILs.     

Comprehensive Study of Free Radical Copolymerization Using a Monte Carlo Simulation Method, 1

Summary: In order to investigate the influence of reactivity ratios and initial feed composition on the microstructure of macromolecules in free radical copolymerization, a comprehensive study was carried out using a Monte Carlo simulation method. As a result, a new procedure was introduced to modify the works of others on the initiation step. The variation of the copolymer composition and the fashion of the arrangement of monomers in simulated chains were evaluated as a function of copolymerization parameters. The model was capable of monitoring any change in azeotropy as well as the magnitude and direction of composition drift from the azeotrope point. The maximum reachable conversion (MRC) was predicted for different combinations of initial feed compositions and reactivity ratios. According to the simulation results, a critical conversion where the macromolecules produced inherited the maximum allowed alternation was obtained for the reactivity ratios given.

Change of sequence distribution of simulated copolymer chains with conversion for various initial feed compositions on a triangular graph (r_A = 0.5, r_B = 0.9).     

电位滴定法测定弱酸电离常数与混合酸组分浓度的一种通用处理方法

本文导出了由电位滴定数据求算弱酸电离常数与混合酸组分浓度的通用算式,按Rosenbrock算法设计程序并处理了某些实验数据,得到良好的结果。与处理同类问题的其它方法相比,本法是方便并可靠的。