Theoretical studies substitutions

With appropriately substituted oxetanes, aluminum-based initiators (321) impose a degree of microstmctural control on the substituted polyoxetane stmcture that is not obtainable with a pure cationic system. A polymer having largely the stmcture of poly(3-hydroxyoxetane) has been obtained from an anionic rearrangement polymerisation of glycidol or its trimethylsilyl ether, both oxirane monomers (322). Polymerisation-induced epitaxy can produce ultrathin films of highly oriented POX molecules on, for instance, graphite (323). Theoretical studies on the cationic polymerisation mechanism of oxetanes have been made (324—326). 

This equation is a reasonable model of electrokinetic behavior, although for theoretical studies many possible corrections must be considered. Correction must always be made for electrokinetic effects at the wall of the cell, since this wall also carries a double layer. There are corrections for the motion of solvated ions through the medium, surface and bulk conductivity of the particles, nonspherical shape of the particles, etc. The parameter zeta, determined by measuring the particle velocity and substituting in the above equation, is a measure of the potential at the so-called surface of shear, ie, the surface dividing the moving particle and its adherent layer of solution from the stationary bulk of the solution. This surface of shear ties at an indeterrninate distance from the tme particle surface. Thus, the measured zeta potential can be related only semiquantitatively to the curves of Figure 3. 

Although the same theoretical studies indicate very small energy differences between the syn and anti conformers of the 3-carbaldehydes of furan, thiophene and pyrrole with a slight preference for the syn conformer, in chloroform solution the furan- and thiophene-3-carbaldehydes adopt the anti conformers to the extent of 100 and 80% respectively (82X3245). However, A-substituted 3-(trifluoroacetyl)pyrroles exist in solution as mixtures of rotational isomers (80JCR(S)42). 

However, the fundamental contribution to the theoretical study of the structure of substituted pyrazoles has been made by Finar 6SJCS(B)725,70JCS(B)1692). Figure 3 shows... 

Theoretical studies of the relative stabilities of tautomers 14a and 14b were carried out mostly at the semiempirical level. AMI and PM3 calculations [98JST(T)249] of the relative stabilities carried out for a series of 4(5)-substituted imidazoles 14 (R = H, R = H, CH3, OH, F, NO2, Ph) are mostly in accord with the conclusion based on the Charton s equation. From the comparison of the electronic spectra of 4(5)-phenylimidazole 14 (R2 = Ph, R = R3 = H) and 2,4(5)-diphenylimidazole 14 (R = R = Ph, R = H) in ethanol with those calculated by using ir-electron PPP method for each of the tautomeric forms, it follows that calculations for type 14a tautomers match the experimentally observed spectra better (86ZC378). The AMI calculations [92JCS(P1)2779] of enthalpies of formation of 4(5)-aminoimidazole 14 (R = NH2, R = R = H) and 4(5)-nitroimidazole 14 (R = NO2, R = R = H) point to tautomers 14a and 14b respectively as being energetically preferred in the gas phase. Both predictions are in disagreement with expectations based on Charton s equation and the data related to basicity measurements (Table III). These inconsistencies may be... 

The stereochemical outcome of nucleophilic addition reactions to cyclic ketones is the subject of numerous experimental and theoretical studies, with substituted cyclohexanones and cy-clopcntanones having been intensively studied. In addition reactions to substituted cyclohexanones 1 the problem of simple diastereoselectivity is manifested in the predominance of cither axial attack of a nucleophile, leading to the equatorial alcohol 2 A. or equatorial attack of the nucleophile which leads to the axial alcohol 2B. 

Polar factors can play an extremely important role in determining the overall reactivity and specificity of hotnolytic substitution.97 Theoretical studies on atom abstraction reactions support this view by showing that the transition state has a degree of charge separation.101 10 ... 

Very recently there has been an experimental and theoretical study of electronic substituent effects in 4-aminoaryl (4-substituted aryl) sulfones146. PMR, 13C NMR and infrared measurements were involved and semi-empirical all-valence CNDO/2 calculations, with and without sulfur d orbitals, were carried out. Various correlations between spectral results and substituent constants are presented. There is good agreement between experimental and theoretical data, which does not depend on the inclusion or exclusion of the sulfur d orbitals from the calculations. 

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

A theoretical study of model apically-substituted and equatorial ly-substituted acyclic phosphoranes, H PX, revealed a ligand apicophilicity in the order, Ct > CN > F > CeCH > H > CHg > OH >... 

The recent interest in substituted silane polymers has resulted in a number of theoretical (15-19) and spectroscopic (19-21) studies. Most of the theoretical studies have assumed an all-trans planar zig-zag backbone conformation for computational simplicity. However, early PES studies of a number of short chain silicon catenates strongly suggested that the electronic properties may also depend on the conformation of the silicon backbone (22). This was recently confirmed by spectroscopic studies of poly(di-n-hexylsilane) in the solid state (23-26). Complementary studies in solution have suggested that conformational changes in the polysilane backbone may also be responsible for the unusual thermochromic behavior of many derivatives (27,28). In order to avoid the additional complexities associated with this thermochromism and possible aggregation effects at low temperatures, we have limited this report to polymer solutions at room temperature. 

An ab initio theoretical study was conducted on 1,2,5-oxadiazole and 3-phenyl-l,2,5-oxadiazole to determine the molecular structures of these heterocyclic compounds. The rotational energy barrier between Ph ring and diazole nucleus was also evaluated. No considerable change of bond lengths inside the diazole nucleus was observed in the Ph-substituted heterocyclic compounds as compared to the oxadiazole and thiadiazole alone <2001MI215>. 

It is well known that the electrical activity of many deep-level defects disappears when the crystal is exposed to atomic hydrogen (see Pearton et al., 1987 and Chapter 5 of this Volume). This has been attributed to complex formation with the hydrogen, and it is very common for transition-metal impurities. Unfortunately, very little theoretical work has been reported for these systems. The deactivation of second- and third-period deep-level impurities is better understood theoretically. The substitutional oxygen defect in silicon ( A center Watkins and Corbett, 1961 Corbett et al., 1961) can be deactivated by exposure to hydrogen. Recently, a theoretical study of the deactivation of substitutional sulfur through the formation of a hydrogen-sulfur pair has been reported (Yapsir etal., 1988). 

Formation of nitrones can be achieved in the first stage of a Krohnke type reaction in which p-n trosodi methy 1 an dine reacts with 2-oo-bromoacetylphenoxathiin in alkaline medium (336). The synthesis of a series of cyclic nitrones of structure (182) has been achieved by regioselective, and by an unusual [3 + 2] cycloaddition of a-nitrosostyrenes (181) to 1,3-diazabuta-l,3-dienes (180) (Scheme 2.64) (337a). Theoretical studies of the substitution effect at the imine nitrogen on the competitive [3 + 2] and [4 + 2] mechanisms of cycloaddition of simple acyclic imines with nitrosoalkenes have been reported (337b). 

There are only a few new theoretical studies of new 5-5 bicyclic ring systems. In one study azido-tetrazole isomerization was investigated (Scheme 1) <1998JOC2354, 1998JA4723>. The model systems chosen were fully conjugated (10-7T-electrons) thiazolo[3,2-<7]tetrazole in its unsubstituted form as well as substituted derivatives. 

Theoretical studies on the experimentally observed selectivity of alkylation of tetrazolo[l,5-tf]pyridine 1 and its benzologues to alkyltetrazolo[l,5- ]pyridinium salts revealed that the site of the alkylation can be fairly well predicted by the help of molecular electronic potential (MEP) maps <1998JMT191>. Earlier it was shown that transformation of 1 to the three possible alkyl-substituted salts 8-10 (Scheme 3) results in a mixture containing high... 

M. Fahlman, M. Logdlund, S. Stafstrom, W.R. Salaneck, R.H. Friend, P.L. Burn, A.B. Holmes, K. Kaeriyama, Y. Sonnoda, O. Lhost, F. Meyers, and J.L. Bredas, Experimental and theoretical studies of the electronic structure of poly(p-phenylene vinylene) and some ring-substituted derivatives, Macromolecules, 28 1959-1965, 1995. 

Mendez, F., Romero, M. d. L., De Proft, F., and Geerlings, P. 1998. The basicity of p-substituted phenolates and the elimination-substitution ratio in p-nitrophenethyl bromide A HSAB theoretical study. J. Org. Chem. 63 5774-5778. 

A theoretical study (144) and several experimental studies using selectively deuterated mono-substituted benzenes (145), chlorobenzene (146), and warfarin (147) (Fig. 4.79) provided strong evidence for the stepwise mechanism. Most recently, a theoretical study using density functional calculations reported the same basic conclusion (148). The reaction proceeds by a stepwise mechanism involving initial attack of FeO3"1" on the jt system to form a tetrahedral intermediate (pathway 2). The tetrahedral intermediate then goes... 

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