Effect of substituents on the curing of liquid crystalline epoxy resin

The synthesis of an aromatic ester based liquid crystalline epoxy resin (LCE) with a substituent in the mesogenic central group is described. Chlorine and methyl groups were introduced as substituents. The curing behaviors of three epoxy resins were investigated using diaminodiphenyl ester as the curing agent. The curing rate and heat of curing of LCE were measured with dynamic and isothermal DSC. The chlorine substituent accelerated the curing of LCE, while the methyl substituent decelerated the curing of LCE. The heat of curing of substituted LCE was diminished compared to LCE with no substituent. Glass transition temperature and elastic modulus of LCE decreased with increasing the size of the substituent. Three liquid crystalline epoxy resins based on aromatic ester mesogenic groups formed a liquid crystalline phase after curing, and the liquid crystalline phase was stable up to the decomposition temperature. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 911–917, 1998     

Substituent Effect on the C–NO2 and N–NO2 Bond Dissociation Energies of Nitroaromatic Molecules

Six density function theory methods (B3LYP, B3P86, MPWB1K1, MPWPW91, PBEPBE, TPSS1KCIS3) were used to calculate bond dissociation enthalpies of nitro compounds, where the B3P86 method was found to give the most accurate predictions. Using the B3P86 method meta‐ and para‐substituted nitroaromatics were systematically studied for the first time. The remote substituent effects, Hammett relationships, and the origin of the substituent effects were discussed on the basis of the calculated results. Both meta‐ and para‐substituted nitromethyl‐benzenes showed significant substituent effects and a fair correlation against substituent constants σ_p⁺ The ground state effects were found to play the major role in determining the overall substituent effects. Meanwhile, nitroamino‐ benzenes showed irregular substituent effects and a poorer Hammett correlation, where both ground and radical state effects contributed to the overall substituent effects.     

Ineffective OH Pinning of the Flipping Dynamics of a Spherical Guest within a Tight-Fitting Tube

A supramolecular/synthetic method has been devised to affix a sterically hindered substituent onto a fullerene guest encapsulated in a tubular host. A two-wheeled complex of (C₅₉N)-(C₅₉N) with a tubular host was oxidatively bisected to afford a C₅₉N⁺ cation captured in the tube. The C₅₉N⁺ cation in the tube was then trapped by ethanol or water, which led to an oxy substituent pinned on the guest. The guest motions within the tube were modulated by the pinned substituent, and up-and-down flipping motions were halted by an ethoxy substituent. A hydroxy substituent, however, was ineffective in halting the flipping motions, despite the tight-fitting relationship between the tubular host and the spherical guest. Theoretical calculations of the dynamics revealed that the flipping motions were assisted by OH-π hydrogen bonds between the guest and the carbon-rich wall and that sliding motions of the OH group were also facilitated by deformations of the tube.     

Ineffective OH Pinning of the Flipping Dynamics of a Spherical Guest within a Tight‐Fitting Tube

A supramolecular/synthetic method has been devised to affix a sterically hindered substituent onto a fullerene guest encapsulated in a tubular host. A two‐wheeled complex of (C₅₉N)‐(C₅₉N) with a tubular host was oxidatively bisected to afford a C₅₉N⁺ cation captured in the tube. The C₅₉N⁺ cation in the tube was then trapped by ethanol or water, which led to an oxy substituent pinned on the guest. The guest motions within the tube were modulated by the pinned substituent, and up‐and‐down flipping motions were halted by an ethoxy substituent. A hydroxy substituent, however, was ineffective in halting the flipping motions, despite the tight‐fitting relationship between the tubular host and the spherical guest. Theoretical calculations of the dynamics revealed that the flipping motions were assisted by OH‐π hydrogen bonds between the guest and the carbon‐rich wall and that sliding motions of the OH group were also facilitated by deformations of the tube.     

Properties of 2-, 3-, and 4-acetylpyridine substituted ruthenium(II) bis(bipyridine) complexes: substituent effect on the electronic structure,spectra, and photochemistry of the complex

Bis(2,2′-bipyridine) complexes of ruthenium(II) with 2-, 3-, and 4-acetylpyridine derivatives were synthesized and structurally characterized. The effect of changing the location of the pyridine's acetyl substituent was studied experimentally and theoretically to clarify the effect of substituent position on the chemical behavior and photochemical properties of the complex. The substituent position on the heterocyclic-pyridine was found to strongly affect the chemical and photochemical properties of the complex. Variation of the position of the substituent, and thus ligand modification brought by as a consequence of this variation, offers possibilities to design complexes of desired structural and photochemical properties.     

Application of the indo molecular orbital method to ortho effects in mass spectra. Cleavage of cis-β-methoxystyrene

The principal fragmentation of the cis-β-methoxystyrene molecular ion is loss of the methyl group. Although this compound has a π system similar to that of phenyl acetate, the substituent effect of a strongly electronegative ortho substituent is insignificant for this fragmentation; but for the same substitution in phenyl acetate, it had been shown to be important. The INDO method was used to show that bond orders confirm the lack of the substituent effect in the cis-β-methoxystyrene system.     

Effect of Substituents on the Reaction of Aromatic Primary Amines with Fluorescamine

Abstract

The effects of substituents on the reactivity of the monosubstituted anilines with fluorescamine (FLA) and the fluorescent properties of the reaction products, FI (Ph), were investigated. Generally, the substituent at o-position markedly inhibited the reactivity of the amino group (ortho-effect). While the electron-donating substituent seemed to be favorable for the formation of FI (Ph), the electron-attracting one seemed to lower reactivity of the amino group. However, the presence of the latter substituent led to the enhanced fluorescence of FI (Ph). The highly significant correlation was observed between the wavelengths (nm) of the emission maxima and the Hammett's substituent constants.     

A kinetic study of the mechanism of esterification of 1-arylethanols in trifluoroacetic acid

Aryl substituent effects upon the rate constants for the esterification of a series of 1-arylethanols in trifluoroacetic acid are in accordance with a reverse A_AL1 mechanism, for which the Hammett reaction constant ? = ?3.69 has been determined by correlation with σ⁺ substituent constants. The rates of reaction are ca. 50-fold faster than those for corresponding benzyl alcohols which bear an electron-donating aryl substituent and which are also believed to undergo esterification in trifluoroacetic acid by the reverse A_AL1 mechanism; the reverse A_AC2 mechanism applies to benzyl alcohols which bear an electron- withdrawing aryl substituent.     

Nucleophilic substitution in the furazan series. Reactions of nitrofurazans with ammonia

The reactions of 3-nitro-4-R-furazans with ammonia were studied. The effect of the substituent R on the specific features of the nucleophilic substitution reaction observed was considered. The nitro group attached to the furazan ring can act as both the leaving group and the activating group facilitating the displacement of the second substituent (for example, OR" or N(NO₂)R").     

Structure-activity relationship in the case of intramolecular ortho-cyclization of aromatic nitroso oxides: Inverted steric effect of substituent in the 2-R-C6H4NOO transformation

A systematic theoretical study at the M06L/6-311+G(d, p) level of theory was carried out to calculate the activation barriers ΔH^≠ for the intramolecular ortho-cyclization of aromatic nitroso oxides 2-R-C₆H₄NOO and to reveal the effect of substituent nature and position in the benzene ring on the nitroso oxides reactivity. A set of 24 substituents with widely differing spatial and electronic properties (inductive, resonant, steric effects of R) was studied. The para-substituent was shown to have little effect on the ΔH^≠ value. The full set of effects of the R substituent contributes to the reactivity of ArNOO for 3-substituted aromatic nitroso oxides. In the case of 5-substituted ArNOO the Hammett-type relationship was obtain to describe inductive and resonant effects of R on the ortho-cyclization reactivity. The ortho-cyclization for 2-substituted nitroso oxides is a nontrivial example of the existence of an “inverted” steric effect, when an increase in substituent size accelerates intramolecular transformation. The substituent in position 6 also exhibits an “inverted” steric effect, but it is noticeably weaker than that for 2-R-C₆H₄NOO.     

Systematic study on the thermal cycloreversion reactivity of diarylethenes with alkoxy and alkyl groups at the reactive carbons

The relationship between the thermal cycloreversion reactivity of diarylethenes and the bulkiness of the substituents at the reactive carbons was systematically investigated. Two photochromic diarylethenes, 1,2-bis(2-isobutoxy-5-phenyl-3-thienyl)perfluorocyclopentene (1a) and 1,2-bis(2-neopentoxy-5-phenyl-3-thienyl)perfluorocyclopentene (2a), were newly synthesized and their optical properties and thermal cycloreversion reactivity were examined, because there is insufficient data for diarylethenes with alkoxy groups at the reactive carbons. The steric substituent constant was employed to correlate the relationship between the thermal cycloreversion reactivity of diarylethenes with alkyl and alkoxy groups at the reactive carbons and the bulkiness of the substituent. A good correlation was obtained for the substituent constant using CH₂ instead of oxygen in the alkoxy groups. The results indicate that this is a very useful strategy for the design of novel diarylethenes with desired thermal cycloreversion reactivity.     

Multinuclear NMR and vibrational spectroscopy studies of the substituent effects in benzensulphonamide inhibitors of the enzyme carbonic anhydrase

Substituent effects on the electronic structure of sixteen biologically active benzensulphonamide derivatives were investigated by means of ¹⁵N, ¹³C, ¹H NMR, and IR spectroscopy, as well as by quantum chemical calculations. Good correlations were found between the spectroscopic data and both substituent constant and computed electronic charges. On this basis the substituent effects are interpreted in terms of electronic charge perturbation, which is linearly transmitted from the substituent to the biofunctional −SO₂NH₂ group. The resonance nature of the substituent seems most important in determining the ¹⁵N chemical shifts, which follow a “reverse” trend; i.e., electron-donor substituents induce downfield ¹⁵N shifts.     

Charge migration in the molecular ions of isoxazole derivatives

The effect of charge migration in the molecular ion on the dissociative ionization of isoxazole derivatives was examined. It is shown that charge redistribution between the substituent in the 3 position and the oxygen atom of the heteroring precedes isomerization of the molecular ion and its disintegration. The intensity of the peaks of the RCO⁺ ions in the mass spectra of 5-R-3-arylisoxazoles increases as the acceptor effect of the aryl substituent becomes stronger. It is shown that the average internal energy with which the nitrobenzoyl ions are formed decreases as the probability of their formation increases, i.e., as the intensity of charge migration from the aryl substituent to the oxygen atom increases.     

A carbon-13 NMR study of some alkyl-substituted indenes. An experimental study of the baker-nathan order of hyperconjugation in the neutral ground state

Carbon-13 NMR spectra of several alkyl-substituted indenes have been assigned and analysed. For structures having an alkyl substituent in the C-2 position an increased shielding was observed for the remote conjugated positions C-4 and C-6, an alternating trend similar to that found for the 2-cycloaminoindenes. The hyperconjugative behaviour was slightly more pronounced for the methyl group compared with an isopropyl or tert-butyl substituent, a fact which supports a model where C? H σ–π interactions are more efficient than those involving C? C bonds.     

A DNMR study on the ring inversion of oxazolidines

Ring inversion of eight substituted 2-phenyl-4, 4-dimethyloxazolidines in o-dichlorobenzene-d₄ was studied by ¹H DNMR. Very high inversion barriers (ΔG_o^≠ = 86.7–93.4 kJ·mol^?1) were found for these oxazolidine rings. The magnitude of the inversion barrier is dependent on the polarity of the substituent on the phenyl group. A linear correlation is present between the ΔG_o^≠ of the inversion and the substituent constant, σ+, of the substituent.     

1，4-二取代苯紫外吸收光谱中的取代基效应

构建激发态取代基参数与1,4-二取代苯的紫外吸收波数之间的模型,成功地关联80个1,4-二取代苯的紫外吸收波数,其方程的相关系数为0．9805,标准偏差仅为672．27cm^-1．结果表明激发态取代基参数适用于1,4-二取代苯紫外吸收能量的研究．同时提供了研究芳香化合物的紫外吸收光谱的新方法,并有利于深入理解多取代共轭化合物的激发态物理化学性质中的取代基效应．     

Substituent Effect on the Electronic Structure of Sulfines: Molecular Orbital Treatment

The electronic structure and ground state properties of the gaseous sulfine H₂CSO and some of its derivatives were studied at the RHF, MP2, and B3LYP levels. The calculations showed that the Z-isomer is more stable than the E-one and their difference in energy depends on their level of calculation, basis set, and substituent. The factors affecting the isomer stability are the electrostatic interactions, the steric factor, and π-electrons delocalization. The substituent has little effect on the geometry of the CSO moiety but greatly affects its charge distribution and polarizability. The ΔE value of the E- to Z-isomerization process via rotation of the S═O bond was calculated. The substituent has an appreciable effect on both the geometry and energy barrier of isomerization depending on its electronegativity, electrostatic attraction, and effect on π-delocalization over the molecule.     

Kinetics and mechanism of the acid cyclization of N-substituted N-nitroso-α-aminoacetonitriles to sydnoneimines

The cyclization of N-nitrosoaminonitriles was studied by a kinetic method with spectrophotometric recording. It is shown that the reaction rate increases as the electron-donor properties of the substituent increase and as the size of the substituent increases. The rate-determining step is detachment of a proton from the C₄ atom in the 4H-1,2,3-oxodiazole-5-imine dication.This study was carried out with a Shimazdu MPS-50L septrophotometer.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 39–46, January, 1977.     

Structure-reactivity correlations on the kinetics of substituted 2-phenylethyl m-nitrobenzenesulfonates with substituted pyridines

The rates of the substituted 2-phenylethyl m-nitrobenzenesulfonate(2-PNS) with pyridines were determined in acetonitrile. The reaction was accelerated by an electron - donating substituent on both substrate and nucleophlie. Substitutent effects 2-PNS and pyridine are correlated by Brønsted and Hammett equations, respectively. The sensitivity parameters, β and ϱ, obtained from the free-energy relationships, are inter-related and are themselves sensitive to the reactivity of the system. Thus, β varies from 0.246 for p-MeO 2-PNS to 0.284 for p-NO₂PNS, and are linearly related to the α values for 2-PNS substituents. Likewise the ϱ_z (Z is a substituent of substrate) values are linearly related to pKa of the pyridines and ϱ_y (Y is a substituent of pyridine) values are also correlated to the β values. These data show that electron-withdrawing substituents in 2-PNS increse bond formation between C and N atoms and such subsituents in the pyridines also lead to increased bond formation relatively to bond breaking in the transition state. The More O'Ferral and Swain, Thornton, and Harris approaches were applied for the prediction of substituent effects on above interpretation.