The effect of substituents on the structure of dioxirane

Ab initio calculations have been performed for methyl- and fluorine-substituted dioxirane to analyze the influence of the substituents on the molecular structure and the peroxy bond. Fluorine substitution increases the O-O bond length and shortens the C-O bond, while methyl substitution does not introduce significant changes. These results are discussed using a molecular orbital interaction diagram. Both substituents affect the basicity of the molecule, and it is concluded that the peroxy bond is stabilized on methyl substitution since the methyl group acts as an electron donor, while fluorine substitution destabilizes the peroxy bond.     

The effect of substituents on the phenyl portion of the imido ligand on the structure and properties of molybdenum(VI) imido complexes

Anilines with alkyl substituents on the phenyl ring (ArNH2 = 2,4,6-trimethylaniline; 2,3-, 2,4-, 2,6-, and 3,4-dimethylaniline; and 2,6-diisopropylaniline) react with MoO(X)2(dtc)2 (X = Cl or Br; dtc = diethyldithiocarbamate) in methanol in the presence of 2 equiv of triethylamine to form ionic imido complexes of the type [MoNAr(dtc)3]2[Mo6O19] or MoNAr(dtc)3]4[Mo8O26]. The same reaction in THF with butyllithium as base yields imido complexes of the type MoNAr(X)2(dtc)2. The structures of three ionic, five chloro, and two bromo complexes have been determined by X-ray crystallography. In all complexes, the molybenum center is a distorted pentagonal bipyramid. While the structures are similar, the angles of the imido linkages differ. The effect of the substituents on the phenyl ring of the imido ligand on the 95Mo NMR chemical shifts was determined. The Mo nucleus becomes more deshielded with the substituents in the following order: 3,4-Me2 < 2,3-Me2 < 2,4-Me2 < 2,6-Me2 < 2,4,6-Me3 < 2,6 isopropyl. Complexes with more deshielded 95Mo centers tend to have angles of the imido linkage that are closer to 180 degrees.     

A simple model for predicting the effect of substituents on the rates of thermal pericyclic reactions

An operationally useful model is proposed for predicting the qualitative effect of substituents on the rates of thermal pericyclic reactions. The method provides a unified interpretation of several previously unrelated experimental observations. It also enables the prediction of substituent effects in pericyclic reactions for which experimental data are not yet available.     

The effect of polar substituents on the heterocyclic benzoxazoles

The synthesis, characterization, and mesomorphic properties of a new type of heterocyclic compounds 1, 2 derived from benzoxazole are reported. In order to understand the relationship between the structure and the mesomorphic behavior, compounds containing a variety of polar substituents (i.e., X=H, F, Cl, Br, CH₃, CF₃, OCH₃, NO₂, CN, OH, NMe₂, COOCH₃) on the terminal end were prepared. The phase behavior of these mesogenic compounds was characterized and studied by differential scanning calorimetry (DSC) and polarization optical microscopy. The formation of mesophases was strongly dependent on the electronic and/or the steric factors of the substituents. In general, a mesophase was better induced by introduction of a polar substituent. Compounds (X=H) formed a crystalline phase, however, other compounds, except for X=OH, exhibited nematic or smectic A phases. Interestingly all compounds with electron-donating substituents (X=CH₃, OCH₃, NMe₂) exhibited nematic phases, however, other compounds with electron-withdrawing substituents (X=F, Cl, Br, CF₃, NO₂, CN, COOCH₃) formed smectic A phases. Compounds (X=NO₂, CN, COOCH₃) have higher clearing temperatures than those of other homologues, and the higher T_cl was attributed to an enhanced conjugative interaction. However, no linear correlation between the clearing temperature or the temperature range of mesophases with Hammett σ_p constants was found. The fluorescent properties of the compounds were examined. All λ_max peaks of the absorption and photoluminescence spectra of compounds occurred at ca. 348-381 and 389-478 nm, respectively. Whereas, the quantum yields of some compounds were relatively low.     

Computational and experimental studies of the effect of substituents on the singlet-triplet energy gap in phenyl(carbomethoxy)carbene

The effect of aromatic substitution on the singlet-triplet energy gap in substituted phenyl(carbomethoxy)carbene (X-Ph-C-CO(2)CH(3), PCC) has been explored by time-resolved infrared (TRIR) spectroscopy and gas-phase computational methods. The ground state of para-substituted PCC is calculated to change from the triplet state in p-NO(2)-PCC (Delta G(ST) = 6.1 kcal/mol) to the singlet state in p-NH(2)-PCC (Delta G(ST) = -2.8 kcal/mol). The absence of solvent perturbation in the TRIR spectra of p-N(CH(3))(2)-PCC (which should have electronic properties similar to p-NH(2)-PCC) and parent PCC is consistent with their ground states lying > +/-2 kcal/mol from the next available electronic state, in line with the computational results. The observation of solvent perturbation in the TRIR spectra of p-OCH(3)-PCC and p-CH(3)-PCC implies that their ground states lie < +/-1 kcal/mol from their next available electronic state. This is in agreement with our computational results, which predict a gas-phase Delta G(ST) of -0.8 and 1.6 kcal/mol for p-OCH(3)-PCC and p-CH(3)-PCC as compared to Delta G(ST) values of -3.9 and -1.3 kcal/mol from polarizable continuum model (PCM) calculations with acetonitrile as a solvent. Gas-phase computational results for the meta- and ortho-substituted PCC species are also presented, along with selected linear free energy (LFE) relationships for the para and meta species.     

The effect of aryl substituents on arylcarbene reactivity

Substituted (p-MeO, p-Me, H, p-Cl, p-Br, m-Br, m-MeO, 3,4-Cl₂, p-CO₂Me, m-CN and p-CN) monophenylcarbenes are generated in a binary mixture of substrates (methanol, cis-4-methyl-2-pentene and cyclohexane) and the relative rate of O—H insertion into methanol to stereospecific cyclopropanation of the olefin to C—H insertion into cyclohexane are calculated from the ratios of products and substrates. It is found (i) that the reactivities of the substrates decrease in the order of methanol, olefin and cyclohexane and (ii) that electron-donating substituents generally lead to reaction with the more reactive substrates while the reaction with the less reactive substrates is favoured in the case of electron-withdrawing substituents. These results are interpreted in terms of the change in the electrophilicity of the singlet arylcarbene by the substituents rather than the change in the singlet-triplet equilibrium.     

The effect of electronegative substituents on the electronic spectra of porphyrins

The Hückel approximation MO method is used to calculate -electron levels of energy and forces of dipole transitions, giving rise to a Soret absorption band in the spectra of porphyrins with carbethoxyl substituents. Comparison with experimental data shows that the theory correctly represents changes in electronic absorption spectra with changes in structure.     

Ab initio and experimental study on thermally degradable polycarbonates: the effect of substituents on the reaction rates.

Thermal elimination reactions on polycarbonates are investigated from both theoretical and experimental points of view, to obtain insight into the microscopic aspects that influence the reaction mechanism and rates. In particular, attention is focused on the influence of the type of substituents in the polymer chain on the reaction rates. Ab initio density functional theory calculations are performed on a series of model compound systems for the polycarbonates under study, in particular carbonates differing by the groups attached at the alpha and beta carbon atoms. Reactants, products, and transition states are optimized at the B3LYP/6-311g level of theory. The structures of the activated complex give insight into the mechanistic details of this type of E(i) elimination reactions. The C(alpha)-O bond dissociates before the C(beta)-H bond, developing some carbocation character in the transition state on the C(alpha) atom. The kinematics of the thermal decomposition reactions have been studied by means of transition state theory by construction of the microscopic partition functions. It turns out that the rates of the E(i) elimination reactions are increased by the presence of those substituents on the C(alpha) and C(beta) carbon atoms which are stabilizing the carbocation character in the transition state. In a second part, degradation temperatures have been experimentally measured for some polycarbonates through thermogravimetric analysis. It is investigated whether the relative rates of the model compound carbonate systems are representative of the behavior of the thermal degradation temperatures in polycarbonates. The study as presented here proves that ab initio calculations on small model systems, which are representative for the active area of the degradation process in polycarbonates, can provide insight into the principal ingredients that govern the reaction rates.     

The effect of substituents on the extraction behavior of hydroxamic acids

The extraction behavior of several alkyl-substituted hydroxamic acids in chloroform solution with several metal ions is described. Among the N-phenylacylhydroxamic acids used, those with o-alkyl substituents on either the phenyl or acyl ring generally do not extract metals, apparently because of steric problems at the O=CNO^? chelation site. An improved synthesis of substituted hydroxamic acids is reported.     

The electronic effect of butadienyliron tricarbonyl substituents

The effect of butadienyliron tricarbonyl substituents on the polarographic oxidation potentials of the ferrocenyl nucleus and the ¹⁹F NMR chemical shifts in m- and p-substituted fluorobenzenes is discussed. The data obtained by different physical methods enable us to conclude that all carbon atoms in the coordinated butadiene are sp² hybridized.     

The effect of substituents on the reactivity of aniline with phosphate radical

The reaction between aromatic amines and phosphate radical has been investigated. The reaction is accelerated by electron-releasing substituents and retarded by electron-withdrawing substituents pointing to an electrophilic attack by the PO₄^2xxx radical. σ⁺para values correlate the effect of substituents well. The rho value for PO₄^2xxx is more positive than that for CO₃^xxx indicating higher reactivity of phosphate radical towards aromatic amines than the carbonate radical.     

The effect of side substituents on rotation hindrance in polyheteroarylenes

Hindered rotation was considered in calculations of the conformational parameters of a series of polyheteroarylenes with bulky side substituents by the Monte-Carlo method. Within the range of experimental errors, the results of calculations for several polyarylates coincide with the values of conformational rigidity, determined from hydrodynamic experimental data. The proposed procedure was used to estimate the rigidities of a number of polymers with bulky side substuents for which experimental determination is difficult. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1287–1296, July, 1998.     

The effect of alkyl substituents in nickelocenes on the catalytic dimerization of ethylene

(C₅H₄ i-Pr)₂Ni exhibits the highest catalytic activity in the dimerization of ethylene among the nickelocenes, (C₅H₄R)₂Ni (R = H, Et,n-Pr,i-Pr, ori-Bu) and their analogs (C₅H₄R)Ni(C₃H₅) (R = H,i-Pr). The higher activity is accompanied by lower selectivity with respect to 1-butene and with higher yields of 1-hexene. It is suggested that the introduction of an alkyl substituent in the cyclopentadienyl ring of nickelocene favors the generation of hydride sites involving the nickel atom. These sites participate in the process of ethylene dimerization.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 970–972, May, 1993.     

The effect of arylferrocene ring substituents on the synthesis of multi-walled carbon nanotubes

The synthesis of shaped carbon nanomaterials (SCNMs) such as carbon nanotubes (CNTs), amorphous carbon, carbon fibres (CFs) and carbon spheres (CSs) was achieved using para-substituted arylferrocenes, FcPhX (X = H, OH, Br, COCH₃) or a mixture of ferrocene (FcH) and substituted benzenes (PhX; X = H, OH, Br, COCH₃). The reactions were carried out by an injection chemical vapour deposition (CVD) method using toluene solutions (carrier gas: 5% H₂ in Ar at a flow rate of 100 ml/min) in the temperature range of 800-1000 °C. In most instances multi-walled CNTs (MWCNTs) were produced. Variations in the concentrations of precursor catalysts, the injection rate and temperature affected the type, distribution and dimensions of the SCNMs produced. The overall finding is that the presence of Br and O in these studies significantly reduces CNT growth. A comparative study on the effect of FcPhX versus FcH/PhX mixtures was investigated. The SCNMs were characterized by transmission electron microscopy (TEM), Raman spectroscopy and thermal gravimetric analysis (TGA).