Presence Of Substituent Groups Research Articles

Structural and Substituent Group Effects on Multielectron Standard Reduction Potentials of Aromatic N-Heterocycles.

Aromatic N-heterocycles have been used in electrochemical CO2 reduction, but their precise role is not yet fully understood. We used first-principles quantum chemistry to determine how the molecular sizes and substituent groups of these molecules affect their standard redox potentials involving various proton and electron transfers. We then use that data to generate molecular Pourbaix diagrams to find the electrochemical conditions at which the aromatic N-heterocycle molecules could participate in multiproton and electron shuttling in accordance with the Sabatier principle. While one-electron standard redox potentials for aromatic N-heterocycles can vary significantly with molecule size and the presence of substituent groups, the two-electron and two-proton standard redox potentials depend much less on structural modifications and substituent groups. This indicates that a wide variety of aromatic N-heterocycles can participate in proton, electron, and/or hydride shuttling under suitable electrochemical conditions.

Understanding the Role of Cholinium Carboxylate Ionic Liquids in PEG-Based Aqueous Biphasic Systems

This work aims at exploring new sustainable separation processes based on ionic liquids. Aqueous biphasic systems (ABS) based on poly(ethylene glycol) (PEG) with low molecular weight (600 and 4000 g mol–1) and cholinium-based ionic liquids and salts containing anions derived from carboxylic acids (oxalate, malonate, succinate, l-malate, fumarate, glutarate and citrate), available in natural compounds, are here presented. Contrary to common ionic liquids, the cholinium-based ionic liquids used in this work are biodegradable, nontoxic, cheap, and simple to prepare, and PEG is also a cheap and nontoxic phase promoter agent. The data reported in this work allows novel insights into the phase splitting mechanism of these ABSs regarding the influence of alkyl chain length of the anion and the presence of substituent groups in the anion. The effect of PEG molecular weight in the ABS was also addressed. Furthermore, the possible application of these systems for the extraction/separation of antioxidants, namely, tert-butylhydroquinone (TBHQ), was evaluated.

The Structure and Function of Quinones in Biological Solar Energy Transduction: A Cyclic Voltammetry, EPR, and Hyperfine Sub-Level Correlation (HYSCORE) Spectroscopy Study of Model Naphthoquinones

Quinones function as electron transport cofactors in photosynthesis and cellular respiration. The versatility and functional diversity of quinones is primarily due to the diverse midpoint potentials that are tuned by the substituent effects and interactions with surrounding amino acid residues in the binding site in the protein. In the present study, a library of substituted 1,4-naphthoquinones are analyzed by cyclic voltammetry in both protic and aprotic solvents to determine effects of substituent groups and hydrogen bonds on the midpoint potential. We use continuous-wave electron paramagnetic resonance (EPR) spectroscopy to determine the influence of substituent groups on the electronic properties of the 1,4-naphthoquinone models in an aprotic solvent. The results establish a correlation between the presence of substituent group(s) and the modification of electronic properties and a corresponding shift in the midpoint potential of the naphthoquinone models. Further, we use pulsed EPR spectroscopy to determine the effect of substituent groups on the strength and planarity of the hydrogen bonds of naphthoquinone models in a protic solvent. This study provides support for the tuning of the electronic properties of quinone cofactors by the influence of substituent groups and hydrogen bonding interactions.

Role of fluorine in distortion from planarity of Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are a large class of compounds that pose a cancer risk to humans. Previously, it has been shown that increased molecular distortion decreases the amount of PAH precursors that are transformed into carcinogenic metabolites through cytochrome P450‐mediated oxidation of Benzo[c]phenanthrene (BcPh) derivates to DNA‐alkylating diol epoxides. The presence of substituent groups in and around the bay or fjord regions has been shown to influence out‐of‐plane distortion. In our studies, we analyzed the effect of the introduction of Fluorine atoms into the angular ring of chrysene and BcPh. Chrysene has a less hindered bay region and is very planar, while BcPh is the smallest PAH with a highly hindered fjord region and has been shown to be distorted 26.7° between the outermost rings. The 3‐D structures of 5‐FluoroBcPh, 6‐FluoroBcPh, 5‐Fluorochrysene, Benzo[g]chrysene and 10‐Fluorobenzo[g]chrysene were determined by single‐crystal X‐ray diffraction with Mo radiation using the Bruker Smart APEX II diffractometer. Results reveal a decrease in distortion from planarity upon the introduction of Fluorine atoms. The angle between the outermost rings was found to be decreased from 26.7° in BcPh to 24.3° and 23.6° in the 6‐Fluoro‐ and 5‐FluoroBcPh respectively. The angle between the outermost rings in 10‐Fluorobenzo[g]chrysene was found to be 33.33°, which is a decrease from 36.04° in its non‐fluorinated precursor. Future studies on these and other similar compounds will clarify the role of substituent groups on the out‐of‐plane distortion and measure the relationship between distortion and DNA alkylating ability. Research is supported by a grant from CNR for Short Term Mobility Funding (FC) and an NSF grant for the Bruker X‐ray diffractometer (CHE 0521237).

Characteristics of acetylated and enzyme-modified potato and sweet potato flours

The properties of modified potato and sweet potato flours have been determined by incorporating acetyl groups (acetylation) and by treating with glucoamylase (enzymatic modification). Fractionation studies on Sepharose CL-2B showed that the content of high molecular weight fraction decreased, with a proportionate increase in the lower molecular weight carbohydrate fraction, whereas FT-IR indicated changes in crystallinity of the modified starches. The data showed that the degradation of starch is dependent on the type of modification which, in turn, determined its crystallinity and digestibility. The swelling power and solubility patterns of modified flours indicated a greater degree of associative forces in the starch granules. Scanning electron microscopy revealed indentation on acetylated starch granules, and the granules appeared as bunches/clusters, whereas surface erosion was observed in the enzyme-treated samples. The presence of substituent groups in acetylated flour influenced digestibility inversely, whereas improved digestibility was observed in enzyme-modified samples.

Photoelectron spectroscopy of free radicals: 4-Methyl-sulfonyloxy-TEMPO

HeI and HeII photoelectron spectra of persistent free radical: 2,2,6,6-tetramethyl-4-(methylsulfonyloxy)piperidinyl-1-oxide (CH 3SO 3-TEMPO) have been measured. The analysis of the electronic structure is based on comparison with the spectra of related radicals and with DFT calculations. The electronic structure of the nitroxide group in the title radical seems unaffected by the presence of substituent group. This is consistent with results observed for other nitroxide radicals and with the electrochemical oxidation potentials.

Effect of dopants on electron localization length in polyaniline

The electronic behavior of chemically synthesized polyaniline depends on the amount of disorder—whether homogeneous or inhomogeneous—created during synthesis and doping. Its physical properties are significantly affected by a number of parameters such as catalyst type, solution pH, oxidant/monomer ratio, solvent type, secondary doping, dopant type, dopant size and synthesis temperature. With the view of studying the dependence of electron localization behavior on the dopant type, all other factors remaining the same, we report in this paper a systematic study of NMR, EPR, magnetic susceptibility, and dc conductivity measurements for seven doping acids. The EPR lineshape in all these cases is close to a Lorentzian, but the linewidth is sensitively dependent on the type of the dopant. In spite of differences in the magnitude and the temperature dependences of dc conductivity and magnetic susceptibility data, the experiments reveal a localization length of approximately 30 Å for all the samples. It is concluded that, unlike drastically different charge transport behaviors caused by the presence of substituent groups on phenyl rings of the polymer chains, dopant molecules belonging to a given family have virtually no effect on the electron localization length.

Cyclopolymerization

AbstractDiallylamines readily undergo free‐radical polymerization to form polymers which contain cyclic structures. Five‐membered rings predominate, and this conclusion is supported by ESR spectroscopy of the radicals formed in a flow system, by chemical characterization of the compounds formed under nonpropagating conditions, and by NMR spectroscopy of the polymers. The presence of substituent groups in the diallylamine molecule has an important bearing on the cyclization reaction. For example, methyl groups in the β‐position, (dimethallylamines) form a mixture of 5‐ and 6‐membered rings, and the presence of bulkier groups, as in methyl bis‐2‐tert‐butyl allylamine, can prevent reaction with certain free‐radical initiators. Similarly, increasing temperature results in an increase in the amount of 6‐, relative to 5‐, membered ring structures with N‐methyldimethallylamine. These results are generally similar to those obtained with the carbon analogues.From a theoretical viewpoint, the formation of the 5‐membered ring structure indicates that the generally accepted less stable primary radical is formed in preference to the 6‐membered structure which would contain a secondary radical. This apparent conflict is explained by the kinetics of the reaction and particularly the stereo‐electronic effects which operate in the transition state. Thus, the reaction can be under kinetic control and lead predominantly to 5‐membered ring structures, but when thermodynamic control starts to operate, e.g., at higher temperatures, the more stable 6‐membered ring structures may become significant. Changing the polymer structure from a predominantly 5‐membered one to one containing 6‐membered rings can be expected to alter the properties of the polyelectrolyte. Thus, differences in the titration curves have been noted, and the possibility exists of relating these differences to the polymerization conditions and, hence, to polymer structure.