Dipolar Species Research Articles

Dynamics of polarization relaxation in a dipolar mixture at a solid–liquid interface

Dynamics of polarization relaxation in a dipolar mixture at solid–liquid interface is investigated theoretically by using time dependent density functional theory. The time dependence of the building up of polarization of different dipolar species next to a newly charged solid surface is calculated by solving both Markovian and non-Markovian equations of motion for density and momentum variables. The inhomogeneity of the interface is taken into account in the dynamical theory. The theory also includes the various intra- and interspecies molecular correlations and the surface–molecule interactions. Both the molecular size and the dipole moment of various species in the dipolar mixture can be unequal. The initial inhomogeneity of the interface is calculated by using a weighted density functional theory. Explicit numerical results are obtained for the polarization relaxation in a binary dipolar liquid at varying distances from the solid surface and the dynamics of interfacial relaxation of the two species are compared with that in the bulk phase. A slowing down of the rate of relaxation is found for interfacial molecules of both the species. However, the extent of slowing down is found to be different for different species. Also, the rate of relaxation is found to change nonmonotonically with distance from the surface due to the layering of solvent molecules at the interface. © 2000 American Institute of Physics.

Novel heterocyclic constructionviadipolar cycloadditions to 1,2-dicarbonyl compounds

The reactivity of o-quinones and other 1,2-diones towards a variety of dipolar species viz-, nitrile oxides, carbonyl ylides, betaines and mesoionic compounds has been investigated. In most cases, these reactions occur with the participation of C=O group as the dipolarophile leading to the synthesis of novel heterocyclic compounds.

Dehydrogenative Route to Metallo Dithiolenes: The Reaction of Zinc Polysulfides and Alkenes

Complexes of the type ZnS6(TMEDA), ZnS4(PMDETA), and ZnS4(Me3TACN) react with electrophilic alkenes to give dithiolene complexes ZnS2C2R2(amine)n. The Me3TACN complex is the most reactive, while the more conveniently prepared PMDETA complex also undergoes this reaction at useful rates. The following alkenes were successfully tested: C2H2(CO2Me)2 (cis and trans isomers), C2H3(CO2Me), C2H3(CN), 1,2-C2H2Me(CN), C2H3(CHO), and 1,2-C2H2(CN)(Ph). Crystallographic analysis shows that the highly reactive complex ZnS4(Me3TACN) is structurally similar to ZnS4(PMDETA), including the presence of an elongated Zn−Nax bond. Model studies indicate that the reaction of alkenes with LnZnSx proceeds via the reversible formation of a dipolar intermediate, as indicated by the ability of the polysulfido complexes to catalyze the isomerization of cis-C2H2(CO2Me)2. It is proposed that such dipolar species undergoes ring closure to give alkanedithiolato intermediates, e.g., Zn[S2C2H2(CO2Me)2](PMDETA). The dithiolato complexes Zn...

ChemInform Abstract: Reactions of Group 6 Metal Carbene Complexes with Ylides and Related Dipolar Species

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Time-resolved fluorescence of O-acetylserine sulfhydrylase

Static and time-resolved fluorescence of the internal aldimine of the pyridoxal 5′-phosphate (PLP)-dependent enzyme O-acetylserine sulfhydrylase (OASS) and those of free PLP, and the PLP- l-valine Schiff base have been measured to gain insight into the photophysics of PLP bound to OASS. Exciting at 330 nm, free coenzyme exhibits a band at 415 nm, whereas PLP-valine and OASS (also when excited at their absorbance maxima) exhibit a structured emission with a peak at 420 nm and shoulders at 490 and 530 nm. The emission bands at 420 and 490 nm are attributed to the enolimine and ketoenamine tautomers of the internal aldimine, respectively, while the 530 nm emission might arise from a dipolar species formed upon proton dissociation in the excited state. Time-resolved fluorescence of OASS (PLP-valine), excited at 412 nm (415 nm) and collected at λ>470 nm, indicates the presence of two components characterized by lifetimes ( τ) of 0.6 (0.08) and 3.8 (1.55) ns with equal fractional intensity ( f). In the presence of acetate the slow component dominates OASS emission with f of 0.98. Excitation at 350 nm as a function of emission wavelengths (400–560 nm) shows at least three components. The f of the slow component increases from 400 to 440 nm, then decreases, whereas the f of the intermediate and fast components behave in the opposite way. Results indicate that: (i) the fast component is associated with the emission at 530 nm; (ii) the slow component is associated with the emission at 420 nm; (iii) a fast additive component, characterized by a very short lifetime, is present on the blue side of the emission spectrum; (iv) the intermediate component results from overlapping contributions, including the emission of the band at 490 nm, that could not be resolved; (v) the increased emission at 490 nm, caused by acetate binding, is likely due to the stabilization of the ketoenamine tautomer induced by an increase in polarity of the active site microenvironment and/or a decrease in proton dissociation in the excited state; (vi) excitation at 330 nm, where the enolimine tautomer absorbs, leads to emission decays typical of the ketoenamine.

Volume Change Associated with Large Photoinduced Dipole Formation in a Rigid Donor−Acceptor Compound: New Approach to Optoacoustic Volume Determination

Time-resolved laser-induced optoacoustic spectroscopy was used to study the charge separation process taking place in the excited state of a donor−bridge−acceptor (D-br-A) compound in alkane solvents. This molecule contains a D−A pair separated by a rigid saturated hydrocarbon bridge 6 sigma bonds long. Excitation at 308 nm results in very efficient long-range charge separation, leading to formation of a charge-transfer (CT) state with a dipole moment of 38 D. By monitoring the pressure waves generated during the decay of the excited species, we could discern three consecutive relaxation processes. To separate the contributions of enthalpy and structural volume changes (ΔVstr), experiments were carried out with a series of n-alkanes having different photothermal properties. However, the usual separation method clearly failed for this system, and a new approach was developed to ensure that ΔVstr remained constant across the alkane series. This consisted of selecting conditions where the solvent compressibility could be considered constant over the experimental range. In this way, ΔVstr values between −110 and −200 mL/mol were obtained, depending on alkane length and temperature. These are remarkably large contractions, which can be attributed only to electrostriction of the alkane solvent around the dipolar CT species. Interestingly, the contractions are as large as predicted by classical electrostatic theory. The quantum yield of triplet formation, ΦT, determined from the optoacoustic data, revealed a strong dependence on the oxygen content of the solution. A value of ΦT = 0.04 was obtained for the intrinsic quantum yield in the absence of oxygen.

Reactions of Group 6 Metal Carbene Complexes with Ylides and Related Dipolar Species

Abstract: Among the amazing diversity of reactions discovered on group 6 metal carbene complexes, including now fully synthetically useful reactions, such as D6tz's benzannulation and Hegedus' photochemical ketene generation, the behavior of these complexes towards ylides has been much less studied. In this article, following a brief résumé on the common preparation methods and general reactivity of group 6 carbene complexes, their reactions with ylides and ylide-like derivatives will be reviewed. Three major reaction modes of metal carbenoids with ylides will be discussed: reactions producing new carbene complexes, reactions leading to ketenimine complexes and, finally, those reactions in which the metallic moiety is not present in the final products. The first type is exemplified by the reaction of 1,3-dipoles and a,p-unsaturated complexes. The second class is exclusive of the reactivity of these complexes and isocyanides. The remaining group of reactions is more diverse, and their outcome is determined by the nature of the ylide. So far, SO, NO, NS, NC, and PC ylides have been reported to react with Fischer carbenes. Organic products such as esters, imidates, vinyl ethers, cyclopropanes, and captodative allenes have been isolated in these processes, and their formation, a_s consequence of different reactivity patterns, will be commented through the paper.

Time-resolved fluorescence of O-acetylserine sulfhydrylase catalytic intermediates.

The reaction of the substrate O-acetyl-L-serine (OAS) with the pyridoxal 5'-phosphate (PLP)-dependent enzyme O-acetylserine sulfhydrylase-A (OASS-A) proceeds via the transient formation of an external aldimine absorbing at 420 nm and a stable alpha-aminoacrylate intermediate absorbing at 330 and 465 nm. Stable external aldimine species are obtained by reaction of the enzyme with either the reaction product L-cysteine or the product analog L-serine. Static and time-resolved fluorescence emission properties of the coenzyme in the above catalytic intermediates have been used to directly probe the active site conformation at different stages of the catalytic pathway. Upon excitation at either 420 or 330 nm, the external aldimines with L-cysteine and L-serine exhibit a structured emission centered at 490 nm with a shoulder at 530 nm. Fluorescence decays upon excitation at 420 nm are best fitted using two components with lifetimes of 1.1 and 3.8 ns, with the fractional intensity of the slow component being 0.92 with L-cysteine and 0.75 with L-serine, respectively. The fast component, emitting at 530 nm, is attributed to a dipolar species formed in the excited state by proton dissociation, and the slow component, emitting at 490 nm, is attributed to a ketoenamine tautomer of the external aldimine. The slow component for external aldimine fluorescence decay is characterized by the same lifetime value as that of the internal aldimine with an increased fractional intensity, indicating that the distribution between the ketoenamine and the dipolar species is shifted toward the ketoenamine tautomer in the external aldimine, compared to the internal aldimine. Differences in equilibrium distribution of ketoenamine and enolimine tautomers can also account for differences in the emission properties of the external aldimines of L-cysteine and L-serine. The alpha-aminoacrylate species is characterized by a relatively weak emission. Upon excitation at 330 nm, the emission exhibits two bands centered at 420 and 540 nm, whereas upon excitation at 420 nm the emission bands are centered at 500 and 540 nm, and upon excitation at 465 nm, the main absorbance peak of the alpha-aminoacrylate species, the emission spectrum shows a band at 540 nm. The fluorescence decays, upon excitation at 330 nm, are best fitted using three components with lifetime values similar to those found for the internal aldimine, with the slow component predominating. Species-associated spectra, collected between 400 and 520 nm upon excitation at 350 nm, indicate the presence of a fast component overlapping the slow component on the blue side of the emission spectrum, as detected for the internal aldimine. When the excitation wavelength is 420 nm, there are only two components with the fast one predominating. A further increase in the fractional intensity of the fast component is observed upon excitation at 465 nm. The weak emission and the short lifetime of the emission excited at 465 nm indicate that this alpha-aminoacrylate tautomer interacts significantly with neighboring groups of the protein matrix and may be endowed with a higher mobility than the external aldimine.

Dielectric and mechanical spectroscopies for the study of thermal and radiochemical ageing of polymers

Thermal and/or radiochemical ageing of polymer has been extensively studied by thermal, FTIR, UV or static mechanical analysis. The aim of this work was to make a parallel study of dynamic mechanical and dielectrical properties of EPDM elastomers, and to present the possibility of using such methods for studying ageing. These analyses reflect not only the molecular dynamic, but also through dielectric spectroscopy the chemical evolution due to modification of dipolar species. Changes in the main relaxation peak, both in mechanical and dielectrical spectra, allow the degradation to be quantified.

Reaction of CCl2 with CH2NH and the formation of dipolar and biradical ylide structures

The potential energy surface for the reaction between CH2NH and CCl2 has been investigated using ab initio methods. We have performed geometry optimizations at the MP2/6-31G* level of theory and single point calculations at the MP4(SDQ)/6-311++G** level. The reaction step for ylide formation has a free energy of activation predicted to be 5.0 kcal mol–1. The parallel 1,2-cycloaddition reaction has a calculated free energy barrier of 16.5 kcal mol–1, indicating that this second pathway is not competitive with ylide formation. The structure of the azomethine ylide formed in the first reaction step is similar to that found for the ylide resulting from the reaction of methylene with ammonia and corresponds to a dipolar species. This is highly unstable and rearranges to its more stable isomer, the biradical azomethine ylide, which has a structure similar to the corresponding carbonyl ylide. This species has a free energy barrier to ring closure calculated to be 21.2 kcal mol–1, so it has reasonable kinetic stability. The resulting aziridine has a free energy of 24.1 kcal mol–1 lower than the biradical azomethine ylide, and the activation free energy of ring opening is calculated to be 45.3 kcal mol–1.

Ground and Excited State Dipole Moments ofN ,N ′-Bis(4-methoxycarbonylphenyl)piperazine and its Implications†

G. Saroja, A. Samanta and N. P. Rath, J. Chem. Res. (S), 1997, 332 DOI: 10.1039/A700638A

2 + 3] Cycloaddition reactions of dimethylamino-bis(trifluoromethyl)borane, (CF 3) 2BNMe 2, with nitrile oxides, nitrones and Me 2CSO 2. Crystal and molecular structure of (CF 3) 2[formula omitted] and (CF 3) 2[formula omitted

Dimethylamino-bis(trifluoromethyl)borane, (CF 3) 2BNMe 2 (A), undergoes [2 + 3] cycloaddition reactions with various 1,3 dipolar species to yield five-membrane heterocycles. While nitrile oxides RCNO give (CF 3) 2 BNMe 2RCNO ; R = Mes (I), Ph (II) pCIC 6H 4 (III), tBu (IV), iPr (V), nitrones R 1HCN(R 2)O furnish the thermally less stable and moisture sensitive heterocycles (CF 3) 2 BNMe 2-R 1HCNR 2O R 2 = Me, R 1 = Ph ( VI), pCIC 6H 4 ( VIII, tBu ( IX), iPr ( X); R 2 = R 1 = Ph ( XI) Analogously, Me 2CSO 2 yields the five-membrane ring (CF 3) 2 BNMe 2CMe 2S(O)O ( XII). The constitution of the novel boron compounds has been deduced from multinuclear NMR, IR and mass spectra. The structures of IV and XII have been investigated by single-crystal X-ray diffraction.

Dipolar lattice model of disorder in random media analytical evaluation of the gaussian disorder model

Abstract One contribution to the distribution of site energies (DOS) in a random medium with localized charge-transport sites is the electrostatic potential of randomly placed and randomly oriented dipolar species nearby. This contribution has recently been modelled using a lattice on which a certain fraction (f) of the points is occupied by dipoles with random orientations. Simulations indicated that the DOS is close to a Gaussian distribution as long as f is not too small. This fact was taken to justify the application of the Gaussian disorder model (GDM) of charge transport in situations where dipolar contributions to the disorder are important. Analytical results on this model are presented here, including an expression for the r.m.s. width of the DOS that differs considerably from the previous result. While the central portion of the DOS is well approximated by a Gaussian distribution in many cases, the low- and high-energy tails generally deviate from this distribution. A simple principle is introdu...

Criticality of charged systems. I. The restricted primitive model

A new method, which improves upon the mean spherical approximation (MSA), is developed by including the ionic-pairing contribution using a recent theory of association. The association constant of the new approximation is obtained through the second ionic virial coefficient. In the simplest version of our theory, which we call the pairing MSA 1 (PMSA1), we neglect the activity coefficient of the fully associated ionic-pairs, which are regarded as a separate dipolar species, and obtain the critical point (ρc*, Tc*) at (0.025, 0.075). In the second PMSA ( or PMSA2), we include the activity coefficient of these dipolar particles at the MSA level. The new critical point is located at (0.023, 0.073). In the third PMSA ( or PMSA3), we further include the effect of the presence of the dipolar-particle cores. The final critical point is located at (0.0245, 0.0745). These critical points are considerably closer than the MSA result (0.014, 0.079) to the most recent Monte Carlo estimates of ρc* from 0.025 to 0.04 and Tc* from 0.053 to somewhat over 0.057. Both PMSA2 and PMSA3 appear to improve the critical values of pressure and the degree of association significantly over PMSA1. All expressions for the thermodynamic properties in the PMSA1, PMSA2, and PMSA3 are of simple analytic form. The equation of state in the PMSA3 reduces to the very accurate Carnahan-Starling equation of state for hard spheres if the charges are turned off, and it reduces to an accurate equation of state for a mixture of hard spheres and hard dumbbells if the charges of the associated pairs are turned off. A comparison is made between our theory and that of a recent approach of Fisher and Levin, which is in good agreement with the simulation results if the hard-core contribution to the thermodynamics is neglected, but which falls out of agreement when an accurate core contribution is included. A discussion of the importance of an accurate core term in the treatment of the restrictive primitive model is given. Finally, the most likely reasons that the Tc* predicted by the PMSA is somewhat too high are briefly noted.

Effects of impurity concentration on dielectric loss in Zn-doped InP at microwave X-band frequencies

Microwave dielectric measurements have been performed on various Zn-doped InP crystals using a vector network analyzer. There are two kinds of dielectric response in Zn-doped InP crystals depending on their Zn concentration. The general dielectric response in InP in the microwave frequency region is dielectric relaxation, which is related to the dipolar species formed from the ionized substitutional ZnIn−. The other dielectric response of InP crystals doped with a higher Zn concentration is dielectric loss. The crystal doped with Zn to a concentration of 2.14×1018 cm−3 shows a strong dielectric loss at 11 GHz, but no dielectric loss peaks are found in crystals doped with a lower Zn concentration of 4.36×1017 cm−3. The dipolar species, which gives rise to the dielectric loss in Zn-doped InP crystals, is believed to be a result of vacancy complex defects of neutral substitutional ZnIn and two P vacancies.

Unexpected [3 + 2] cycloaddition between ethyl α-( N,N-dimethylamino)pentadienoate and acrylonitrile

The reaction of ethyl 2-( N,N-dimethylamino)penta-2,4-dienoate 4 with acrylonitrile 2 gave the methyl N-methyl-2-(2-propenyl)-3-cyano-pyrrolidine-2-carboxylate 5a and 5b as a [3 + 2]-atoms “cycloadducts”. The putative [1,3] dipolar intermediate species azomethine ylide 4c , results from [1,6] hydrogen shift of one N(CH 3) proton.

Transient Displacement Currents Generated by Excited-State Dipole Moments in Liquids

We consider the general form of the transient signals which arise in a dc conductivity cell when dipoles are photoinduced in a liquid solution by laser excitation. The formalism is based on the solution of Debye's equation for rotating molecules but includes a time-variable concentration of dipoles. Onsager's treatment of the internal field acting on the dipoles is used. It is shown that, given adequate time resolution, this method allows measurement of the rotational time, the lifetime, and the dipole moment of various excited states. When the ground state is dipolar, the signal is shown to depend on the polarization of the light with respect to the applied electric field. Variation of the light polarization in such an experiment increases the accuracy of derived parameters and even allows the ground state dipole moment to be measured. Results depend substantially on the shape of the dipolar species. An extension of Onsager's treatment to the case of an ellipsoidal cavity is applied to the problem. Approximations to the general result for an ellipsoidal cavity are also given. 12 refs., 8 figs.

The chemistry of pseudomonic acid. Part 12. Preparation of diazole and triazole derivatives

The preparations of examples of normonyl †; imidazoles, pyrazoles and triazoles are described. Most derivatives were formed by treatment of monic acid amides with triphenylphosphine based dehydrating agents, resulting in cyclisation or cycloaddition of the intermediate dipolar species. One example is the novel one pot reduction–cyclisation of an α-acylamino nitrite to an imidazole.

Dielectric behavior at microwave frequencies of an epoxy resin during crosslinking

AbstractThe dielectric behavior during the crosslinking process of a low molecular weight liquid diglycidyl ether of bisphenol‐A (DGEBA) and ethylenediamine, as the curing agent, has been investigated at microwave frequencies (0.1–10 GHz) by means of the time domain reflectometry. It was found that both the real and the imaginary part of the dielectric constant, which regularly decreased as the cure reaction proceeded, were mainly affected by the disappearance of specific dipolar species, whose relaxation time did not change significantly. A simple model based on disappearance of dipoles agrees with the experimental data and explains the coincidence between kinetic data obtained by dielectrometry and calorimetry. The microwave dielectric constant can thus be used to directly measure in real time the conversion of the reaction. © 1993 John Wiley & Sons, Inc.

Poly(ammonium sulfopropylbetaine)s: 5. Interactions in dilute aqueous solution with low molecular weight salts or zwitterions and with poly(electrolyte)s

Interactions in dilute aqueous solution at 25°C between atactic poly[diethyl-(2-methacryloyloxyethoxy-2-ethyl)-1-(3-sulfopropyl)ammonium betaine] (lateral group N +(CH 2) 3SO − 3 with dipole moment μ = 22.7 D) and a series of low and high molecular weight dipolar or ionic species were studied by viscometry and fluorescence. For constant additive concentrations (<1.5 M), the salt CH 3SO − 3, N +(C 2H 5) 4 is more efficient than the homologous zwitterions such as (C 2H 5) 3N +(CH 2) 3SO − 3 ( μ=22.7 D) or (C 2H 5) 3N +(CH 2) 2N(COCH 3)(CH 2) 3SO − 3 ( μ=28.9 D) for promoting the poly(zwitterion) chain expansion. Progressive increase of the concentration of the salt C 2H 5NH + 3, NO − 3 results first in a moderate poly(zwitterion) chain expansion followed by an asymptotic behaviour over a very broad concentration range from 0.75 up to 11M (pure liquid salt). At low concentration (<0.04 M) where the ammonium salts (CH 3) 4N +, X − (X − = Cl −, Br −) behave as inert additives, the corresponding poly(methacryloyloxyethyltrimethylammonium) halides interact with the poly(zwitterion) chain and lead to water-soluble interpolymer complexes of no definite stoichiometry and of very high viscosity: this binding is more efficient with Br − than with Cl −. The lack of any significant modification of the fluorescence emission wavelength and quantum yield (ΔФ ∼ 10%) in the same dansyl labelled poly(electrolyte) (X −= Cl −) shows that stacking of the two polymers over rather long blocks is very unlikely. However, poly(zwitterion)-poly(sodium acrylate) interactions lead to a hypsochromic shift of the emission wavelength of ∼ 15 nm.