Four centred

This equation can be solved numerically on a grid to determine Vei(ri). The same grid is Iheii used to numerically integrate the four-centre, two-electron integral. Equation (3.60), billows ... 

The neglect of all three- and four-centre two-electron integrals reduces the constructioi of the Fock matrix from a formal order of to M. However, the time required fo... 

If sequences of chiral centres are separated by non-chiral centres, the non-chiral centres are ignored, and the remaining set of chiral centres is assigned the appropriate configurational prefix (for four centres or less) or prefixes (for more than four centres). 

Typical four-centre-type photopolymerization and properties of the resulting polymers 124... 

A perfectly ordered polymer composite by four-centre-type photopolymerization of a molecular complex 166... 

Direct transformation from 2,5-DSP to poly-2,5-DSP through 2,5-DSP oligomer in the crystal is shown in Scheme 1. This new reaction was named a four-centre-type photopolymerization. As well as being the first example of a topochemical reaction in a pure sense, the four-centre-type photopolymerization of 2,5-DSP crystals was the first example of photopolymerization via a step-growth mechanism. 

TYPICAL FOUR-CENTRE-TYPE PHOTOPOLYMERIZATION AND PROPERTIES OF THE RESULTING POLYMERS... 

The polymer = 8.19 dlg in hexafluoro-2-propanol, HFIP, solution) in Figs 1 and 2 is prepared on photoirradiation by a 500 W super-high-pressure Hg lamp for several hours and subjected to the measurements without purification. The nmr peaks in Fig. 1 (5 9.36, 8.66 and 8.63, pyrazyl 7.35 and 7.23, phenylene 5.00, 4.93, 4.83 and 4.42, cyclobutane 4.05 and 1.10, ester) correspond precisely to the polymer structure which is predicted from the crystal structure of the monomer. The outstanding sharpness of all the peaks in this spectrum indicates that the photoproduct has few defects in its chemical structure. The X-ray patterns of the monomer and polymer in Fig. 2 show that they are nearly comparable to each other in crystallinity. These results indicate a strictly crystal-lattice controlled process for the four-centre-type photopolymerization of the [l OEt] crystal. 

However, except for two reports of preliminary results on poly-DSP (Fujishige and Hasegawa, 1969 Kanetsuna et al., 1970), there are few reports so far which describe the film and solution properties of the polymers obtained by four-centre-type photopolymerization. 

Thus, the monomer crystal lattice control of the whole process from the monomer to the polymer is generally established for the four-centre-type photopolymerization of conjugated diolefln compounds. Topotaxies in the four-centre-type photopolymerization of several diolefins are shown in... 

Table Topotaxies in the four-centre-type photopolymerization of conjugated diolefins."...

A PERFECTLY ORDERED POLYMER COMPOSITE BY FOUR-CENTRE-TYPE PHOTOPOLYMERIZATION OF A MOLECULAR COMPLEX... 

Ferrocenyltriphenylphosphonium perchlorate (84) has been synthesized from iodoferrocene, tetrakis(acetonitrile)copper(i), and triphenylphosphine in nitromethane. The authors suggest that iodoferrocene first forms the complex (85) which then breaks down via a four-centred transition state to (84). 

The stereochemistry of the addition of phosphorus pentachloride to isolated acetylenes in non-polar solvents has been shown by n.m.r. to be CIS, as illustrated for the adduct (46) from propyne. This observation has been explained in terms of a four-centre process. Contrary to a previous report, the reaction of triphenylphosphine hydrobromide with phenyl-acetylene carboxylic acid (47) yields both the trans- and the known c/5-adducts. 

Dialkyl dimethyl phosphoramidites (16) react with j8-propiolactone to give the phosphoramidate (17) and the phosphonate (18), A kinetic study suggests a mechanism involving initial attack of phosphorus at saturated carbon to give (17), while a four-centred transition state (19) is invoked to explain the formation of (18). 

A mechanism for this reaction involving nucleophilic attack of the ylide on the cyanide group and formation of the P=N linkage via a four-centred intermediate was formulated. The structure of this phosphazene was confirmed by its synthesis from the vinyl azide, Ph(N3)C=CHPh, and triphenylphosphine. Phosphoranes stabilized by electron-withdrawing... 

The reactions of triphenylphosphazenyl derivatives with many other electrophilic species have been described. With sulphur tetrafluoride, sulphur imines are formed, possibly via the four-centred intermediate (24) ... 

In contrast to the early theoretical work of Rank and coworkers , C-NMR investigations had revealed that the metallated carbon atom in the a-sulphinyl carbanion is nearly planar . A four-centre chelate structure 315 has been proposed for a-lithiosulphoxides, and it is believed to be responsible for the planar configuration of the anionic carbon atom and for the greater stability of o(-sulphinyl carbanions in comparison with a-sulphenyl carbanions This chelation favours one of the two diastereoisomeric carbanions and for this reason a-sulphinyl carbanions react with electrophiles in a highly stereoselective manner (see below). 

A summary of the kinetic parameters is given in Table 11. With the exception of 3,3,3-trifluoropropyltrifluorosilane the decompositions are first-order homogeneous processes. The 2,2-difluoro compounds decompose by a four-centre transition state ... 

The four centre transition state requires the addition of both boron and hydrogen to the same face of the molecule. 

According to the Keele theory the primary reaction between the perchloric acid and the monomer is also (1), but thereafter the growth is by a ring-expansion reaction (4) in which no free end is ever formed this goes by way of a four-centred transition state and gives the cyclic product (IV) ... 

B) Cleavage of ring structure and change of the electrons via a four centre transitional state... 

Kinetic studies of the reaction of Z-phenyl cyclopropanecarboxylates (1) with X-benzylamines (2) in acetonitrile at 55 °C have been carried out. The reaction proceeds by a stepwise mechanism in which the rate-determining step is the breakdown of the zwitterionic tetrahedral intermediate, T, with a hydrogen-bonded four-centre type transition state (3). The results of studies of the aminolysis reactions of ethyl Z-phenyl carbonates (4) with benzylamines (2) in acetonitrile at 25 °C were consistent with a four- (5) and a six-centred transition state (6) for the uncatalysed and catalysed path, respectively. The neutral hydrolysis of p-nitrophenyl trifluoroacetate in acetonitrile solvent has been studied by varying the molarities of water from 1.0 to 5.0 at 25 °C. The reaction was found to be third order in water. The kinetic solvent isotope effect was (A h2o/ D2o) = 2.90 0.12. Proton inventories at each molarity of water studied were consistent with an eight-membered cyclic transition state (7) model. 

A theoretical study of the reaction of water and methanol with HNCO has led to a prediction of a four-centred transition state for both reactions. The interactions of water and of alcohols with alkyl isocyanates have been the subject of both experimental and theoretical study. In the case of hydration, evidence for initial interaction of water and water clusters (n = 1-3) across the N=C bond of the alkyl isocyanate... 

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