摘要:

AbstractAb initio MO methods have been used to study the structures and energetics of dicyandiamide, [(NH2)2CNCN], its isomers, protonated species, radical anions, transition structures for internal conformational change and transition structures for isomerization. Structures were optimized at the HF/STO‐3G, HF/3–21G and HF/6–31G* levels; selected barrier heights for smaller analogues were also computed at the MP4SDTQ/6–31G* level. The most stable isomer of dicyandiamide has the cyano group on the imine nitrogen [1, (NH2)2CNCN]; the other isomer [2, HNC(NH2)NHCN] lies 12.8 kcal mol−1higher. Inversion at the imino nitrogen proceeds by a linear, in plane process with a barrier of 32.5 kcal mol−1. The amino rotation barriers are 19 kcal mol−1(single NH2) and 40 kcal mol−1(both NH2in a conrotaory or a disrotatory fashion; if the NH2groups are allowed to pyramidalize the disrotatory barrier drops to 20 kcal mol)−1. Protonation occurs preferentially on the imine nitrogen (PA = 219.7 kcal mol−1for 1); the proton affinities PA of the amino nitrogens are 25–30 kcal mol−1lower. Isomerization between 2 and 1 would go via a 1,3‐sigmatropic hydrogen shift, but the barrier is high (48.3 kcal mol−1); protonation reduces the hydrogen shift barrier by ca 15 kcal mol−1. However, the most likely mechanism for isomerization involves protonation of the imine nitrogen in 2 followed by deprotonation of the cyano‐substituted nitrogen to form 1, circumventing the energetically costly 1,3‐sigmatropic hydrogen shift. When an electron is transferred to dicyandiamide, a sizeable fraction of the resona

ISSN:0894-3230    

DOI:10.1002/poc.610040302

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe curve crossing model was applied to a series of hydrogen abstraction reactions from a family of alkanes, RH (R = methyl, ethyl, isopropyl, tert‐butyl) by alkyl, hydrogen and chlorine radicals. The analysis was based on quantitative data obtained from an ab initio MO study. Schematic reaction profiles for the reaction of RH with alkyl and hydrogen radicals are built up from just two configurations: reactant, DA, and product D3*A. For the Cl atom reaction, however, a significant contribution of D+A−, a charge‐transfer configuration, is also shown to be present. A simple explanation for differences in the intrinsic barrier for the identity radical abstraction reaction based on the initial gap size between DA and D3*A configurations is provided. The influence of the D+A−configuration on the nature of the transition state of the Cl atom reaction and its intrinsic barrier is described. It is the D+A−configuration that is responsible for the polar character often observed in radical abstraction and addition

ISSN:0894-3230    

DOI:10.1002/poc.610040303

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe influence of solvation on the Z—E isomerization process of three representative molecules of simple push‐pull ethylenes [H2N(H)C1=C2(H)R=NO2,COHandCN] derived from aminoethylene was investigated by means of RHF‐SCF ab initio calculations at the 3–21 + G level. Solute‐solvent interactions were modelled by a cavity model. The shape of the cavity is based on electronic isodensity surfaces. By using an ellipsoidal cavity very close to the isodensity surface, the perturbation due to the solvent takes an analytical form which is incorporated into the Hartree‐Fock equations and leads to efficient quantum chemical computations. The polarization of the solutes under the influence of the solvent is noticeable and was analysed in detail. Similarly, the barriers to internal rotations are substantially modified by the solvent: the barrier around the CC double bond is appreciably decreased in the thermal mechanism whereas its lowering is less important in the anionic mechanism; in contrast, the barrier around the C‐1N bond is slightly increased. The variation of the barriers with the nature of the acceptor group is fairly well reproduced by the computations. The electronic structure of the push‐pull ethylene molecules and the modifications of this structure under the influence of the solvent are

ISSN:0894-3230    

DOI:10.1002/poc.610040304

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe kinetics of the nucleophilic substitution between n‐hexyl bromide and solid potassium chloride (KCl) in toluene were studied in the presence of catalytic amounts of tetra‐n‐butylammonium bromide at 84°C. This reaction is characterized by an induction period that disappears on activation of the solid salt. The substitution with the activated salt constitutes a reversible pseudo‐first‐order reaction with variable orders in the substrate and the catalyst equal to 0 ≤ n ≤ 1. A quantitative increase in the solid KCl at first accelerates the substitution reaction, but when the level reaches KCl/RBr ≥ 7 the rate constant is no longer influenced by the amount of salt. Comparing data obtained with kinetic equations for several possible substitution schemes, the mechanism of the reaction can be derived, including the formation of a ternary adsorption complex (TC) from the substrate, catalyst and solid salt. The data obtained allow the evaluation of the equilibrium constant of TC formation (KΣ= 20.48 l2mol−2) and the rate constant of its disintegration into substitution products (k+= 0.75 × 10−2s−1). Through kinetic analysis the adsorption sequence on the surface of the solid salt was determined, including primary formation of the binary complex KCl.QX and subsequent TC formation. The concentration of active KCl molecules, estimated on the basis of the primary kinetic data is 10−2M and is commensurate with the catalyst concentration. This concentration of active KCl molecules is achieved owing to the formation of subcolloidal‐sized particles in the process

ISSN:0894-3230    

DOI:10.1002/poc.610040305

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractAlthough carbon (normal)‐ and oxygen (abnormal)‐attacked products are usually afforded in the Grignard reaction of α‐diketones, the relative ratio of the two are strongly dependent on the structure of the Grignard reagents. The product‐determining factors are discussed on the basis of the intermediate radicals formed from α‐diketones and Grigna

ISSN:0894-3230    

DOI:10.1002/poc.610040306

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe classification of aromaticity criteria is studied. New aromaticity criteria are developed and discussed. A statistical analysis of such criteria is presented and compared with a similar analysis of the recent literature. It is shown that aromaticity is at least a two‐dimensional phenomenon. The classification of the compounds depends on the type of criteria included in the analysi

ISSN:0894-3230    

DOI:10.1002/poc.610040307

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe patterns of the NH stretching vibrations in the solid‐state IR spectra of a series of 5‐pyridylmethylenehydantoins revealed the presence of various modes of hydrogen bonding: intermolecular NH&4nldr;&4nldr;OC and intermolecular or intramolecular NH&4nldr; ‥‥N(py). These variations are related to the orientation of the pyridyl nitrogen and to stereochemistry. A distinction between intramolecularly and intermolecularly hydrogen‐bonded compounds was also provided by comparison of the1H NMR spectra in (CD3)2SO and

ISSN:0894-3230    

DOI:10.1002/poc.610040308

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractHartree—Fock calculations were performed to investigate the structure and relative stabilities of complexes between halogen cations and first‐ and second‐row bases. It is shown by means of both a qualitative perturbation molecular orbital treatment and a topological analysis of the electronic charge density that second‐row bases present enhanced halogen cation basicities compared with first‐row bases. In this respect the results predict that although the fluorine cation basicity of water is smaller than its proton affinity, the fluorine cation basicities of SH2, thioether, phosphine and trimethylphosphine are considerably higher than their proton basicities. Similarly, phosphine and trimethylphosphine should have chlorine cation basicities greater than their proton affinities, in contrast with ammonia and trimethylamine. The results also show that fluorine and chlorine cation basicities are more sensitive to methyl substitution than proton a

ISSN:0894-3230    

DOI:10.1002/poc.610040309

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

ISSN:0894-3230    

DOI:10.1002/poc.610040301

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY