Conformational Preferences of 3-(Dimethylazinoyl)propanoic Acid as a Function of pH and Solvent; Intermolecular versus Intramolecular Hydrogen Bonding

The conformational equilibrium of 3-(dimethylazinoyl)propanoic acid (DMAPA, azinoyl = N^+(O^−) has a weak pH-dependence in D_2O, with a slight preference for trans in alkaline solutions. The acid ionization constants of the protonated amine oxide and carboxylic functional groups as determined by NMR spectroscopy were 7.9 × 10^(−4) and 6.3 × 10^(−6), respectively. The corresponding value of K_1/K_2 of 1.3 × 10^2 is not deemed large enough to provide experimental NMR evidence for a significant degree of intramolecular hydrogen bonding in D_2O. Conformational preferences of DMAPA are mostly close to statistical (gauche/trans = 2/1) in other protic solvents, e.g., alcohols. However, the un-ionized form of DMAPA appears to be strongly intramolecularly hydrogen-bonded and gauche in aprotic solvents

Electrochemistry of Dihalogenated Nicotonic Acids in Aqueous and Aprotic Media

The electrochemical reduction of several 2,5- and 5,6- dihalonicotinic acids have been studied in dimethyl sulfoxide as well as in aqueous buffers of different pH. The polarographic half-wave potentials for the reduction of these compounds in both media are reported here. The compounds appear to reduce at the carboxyl group. The presence of halogen atoms on the pyridine ring facilitates reduction

Swelling of acetylated wood in organic liquids

To investigate the affinity of acetylated wood for organic liquids, Yezo spruce wood specimens were acetylated with acetic anhydride, and their swelling in various liquids were compared to those of untreated specimens. The acetylated wood was rapidly and remarkably swollen in aprotic organic liquids such as benzene and toluene in which the untreated wood was swollen only slightly and/or very slowly. On the other hand, the swelling of wood in water, ethylene glycol and alcohols remained unchanged or decreased by the acetylation. Consequently the maximum volume of wood swollen in organic liquids was always larger than that in water. The effect of acetylation on the maximum swollen volume of wood was greater in liquids having smaller solubility parameters. The easier penetration of aprotic organic liquids into the acetylated wood was considered to be due to the scission of hydrogen bonds among the amorphous wood constituents by the substitution of hydroxyl groups with hydrophobic acetyl groups.Comment: to be published in J Wood Science (Japanese wood research society

Processing for maximizing the level of crystallinity in linear aromatic polyimides

The process of the present invention includes first treating a polyamide acid (such as LARC-TPI polyamide acid) in an amide-containing solvent (such as N-methyl pyrrolidone) with an aprotic organic base (such as triethylamine), followed by dehydrating with an organic dehydrating agent (such as acetic anhydride). The level of crystallinity in the linear aromatic polyimide so produced is maximized without any degradation in the molecular weight thereof

Boron-containing organosilane polymers and ceramic materials thereof

The present invention relates to a polyorgano borosilane ceramic precursor polymer comprising a plurality of repeating units of the formula: (R(sup 1) single bond B)(sub p) being linked together at B by second units of the formula: single bond (R sup 2) single bond (Si single bond R sup 3) single bond (sub q), where R(sup 1) is a lower alkyl, cycloalkyl, phenyl, or (R(sup 2)R(sup 3) single bond Si single bond B single bond)(sub n) and R(sup 2) and R(sup 3) are each independently selected from hydrogen, lower alkyl, vinyl, cycloalkyl, or aryl, n is an integer between 1 and 100; p is an integer between 1 and 100; and q is an integer between 1 and 100. These materials are prepared by combining an organo borohalide of the formula R(sup 4) single bond B single bond (X sup 1) (sub 2) where R(sup 4) is selected from halogen, lower alkyl, cycloalkyl, or aryl, and an organo halosilane of the formula: R(sup 2)(R sup 3)Si(X sup 2)(sub 2) where R(sup 2) and R (sup 3) are each independently selected from lower alkyl, cycloalkyl, or aryl, and X(sup 1) and X(sup 2) are each independently selected from halogen, in an anhydrous aprotic solvent having a boiling point at ambient pressure of not greater than 160 C with in excess of four equivalents of an alkali metal, heating the reaction mixture and recovering the polyorgano borosilane. These silicon boron polymers are useful to generate high-temperature ceramic materials, such as SiC, SiB4, and B4C, upon thermal degradation above 600 C

A new insight into the oxidative mechanism of caffeine and related methylxanthines in aprotic medium: May caffeine be really considered as an antioxidant?

Background: Antioxidant properties have been recently suggested for caffeine that seems showing protective effects against damages caused by oxidative stress. In particular, a HO% scavenging activity has been ascribed to caffeine. Even if the oxidation of caffeine has been widely studied, the antioxidant mechanism is still far to be understood. Methods: The electrochemical behavior of caffeine, theobromine and theophylline was studied in aprotic medium by cyclic voltammetry and electrolysis in UV–vis cell; a computational analysis of the molecular structures based on the Density Functional Theory was performed; the reactivity of all substrates towards lead dioxide, superoxide and galvinoxyl radical was followed by UV–vis spectrophotometry. Results: Results supported the mono-electronic oxidation of the C4]C5 bond for all substrates at high oxidation potentials, the electron-transfer process leading to a radical cation or a neutral radical according to the starting methylxanthine N7-substituted (caffeine and theobromine) or N7-unsubstituted (theophylline), respectively. A different following chemical fate might be predicted for the radical cation or the neutral radical. No interaction was evidenced towards the tested reactive oxygen species. Conclusions: No reactivity via H-atom transfer was evidenced for all studied compounds, suggesting that an antiradical activity should be excluded. Some reactivity only with strong oxidants could be predicted via electron- transfer. The acclaimed HO% scavenging activity should be interpreted in these terms. The study suggested that CAF might be hardly considered an antioxidant. General significance: Beyond the experimental methods used, the discussion of the present results might provide food for thought to the wide audience working on antioxidants

Ultrafast Intramolecular Charge Transfer of Formyl Perylene Observed Using Femtosecond Transient Absorption Spectroscopy

The excited-state photophysics of formylperylene (FPe) have been investigated in a series of nonpolar, polar aprotic, and polar protic solvents. A variety of experimental and theoretical methods were employed including femtosecond transient absorption (fs-TA) spectroscopy with 130 fs temporal resolution. We report that the ultrafast intramolecular charge transfer from the perylene unit to the formyl (CHO) group can be facilitated drastically by hydrogen-bonding interactions between the carbonyl group oxygen of FPe and hydrogen-donating solvents in the electronically excited state. The excited-state absorption of FPe in methanol (MeOH) is close to the reported perylene radical cation produced by bimolecular quenching by an electron acceptor. This is a strong indication for a substantial charge transfer in the S1 state in protic solvents. The larger increase of the dipole moment change in the protic solvents than that in aprotic ones strongly supports this observation. Relaxation mechanisms including vibrational cooling and solvation coupled to the charge-transfer state are also discussed

Solvatochromic probes for detecting hydrogen-bond-donating solvents

Hydrogen bonding heavily influences conformations, rate of reactions, and chemical equilibria. The development of a method to monitor hydrogen bonding interactions independent of polarity is challenging as both are linked. We have developed two solvatochromic dyes that detect hydrogen-bond-donating solvents. The unique solvatochromism of the triazine architecture has allowed the development of probes that monitor hydrogen-bond-donating species including water

Sub-2 cm/s passivation of silicon surfaces by aprotic solutions

Minimizing recombination at semiconductor surfaces is required for the accurate determination of the bulk carrier lifetime. Proton donors, such as hydrofluoric acid and superacids, are well known to provide highly effective short-term surface passivation. We demonstrate here that aprotic solutions based on bis(trifluoromethanesulfonyl)methane (TFSM) in hexane or pentane can also result in excellent passivation of (100)-orientation silicon surfaces. We show that the optimized TFSM-pentane passivation scheme can measure effective lifetimes up to 20 ms, with a surface recombination velocity of 1.7 cm s1 at an excess carrier density of 1015 cm3 . Fitting injection-dependent lifetime curves requires chemical passivation and field effect passivation from a negatively charged layer with a charge density of 1010–1011 q cm2 . The slightly higher recombination velocity of 2.3 cm s1 measured with TFSM-hexane can be explained by a lower charge density in the passivating layer, suggesting that the steric hindrance associated with the solvent size could play a role in the passivation mechanism. Finally, phosphorus nuclear magnetic resonance experiments confirm that TFSM-based solutions have Lewis acidity without being superacids, which opens up opportunities for them to be used in materials systems sensitive to superacidic environments