. The average ionization energy, j(r), is introduced and is demonstrated to be useful as a guide to chemical reactivity in aromatic systems. I(r) is rigorously defined within the framework of self-consistent-field molecular orbital (SCF-MO) theory and can be interpreted as the average energy needed to ionize an electron at any point in the space of a molecu_le. An a b initio SCF-MO approach has been used to calculate I(r) at the 6-31G* level, using STO-3G optimized geometries. I(r) has been computed on molecular surfaces defined by the contour of constant electronic density equal to 0.002 electrons/bohr3, for a series of aromatic systems. This surface I(r) provides site specific predictions for preferred positions of electrophilic aromatic substitution. Relative reactivity toward electrophiles increases as the magnitudes of the smallest I(r) values (Imin) for these systems decr5ase. An excellent relationship, with a correlation coefficient of 0.99, has been found between the Hamrnett constants and Imin; this allowed us to predict the values of these constants for the substituents NHF and NF2, for which they were previously not known.Key words: average local ionizations energy, chemical reactivity, electrophilic aromatic substitution, molecular surfaces, Hammett constants.PER SJOBERG, JANE S. MURRAY, TORE BRINCK et PETER POLITZER. Can. J. Chem. 68, 1440 (1990). On introduit le concept d'tnergie rnoyenne d'ionisation locale, j(r), et on dtmontre qu'il est un guide utile pour prtdire la rtactivitt chimique dans les systernes aromatiques. On dtfinitI(r) d'une f a~o n rigoureuse dans le cadre de la thtorie des orbitales moltculaires en champ auto-cohtrent (OM-CAC) et on peut l'interprtter c o m e l'tnergie moyenne ntcessaire pour ioniser un tlectron dans un point quelconque de l'espace d'une moltcule. On a utilist une approche ab initio d'OM-CAC pour calculeri(r) au nivequ 6-3 IG*, en utilisant des gtomttries optimistes au niveau STO-3G. Pour une strie de systemes aromatiques, on a aussi calcul< I(r) sur les surfaces moltculaires dtfinies par le contour d'une densitt Clectronique constante de 0,002 tlectrons/bohr3. Cette I(r) de surface fournit des prtdictions sptcifiques au site pour les positions prtftrtes par les substitutions arornatiques Clectrophiles, La rtactivitt relative vis-i-vis des tlectrophiles augmente avec une diminution des amplitudes des valeurs I(r) les plus faibles Imin de ces systemes. On a ob_tenu une excellente corrtlation, avec un coefficient de corrtlation de 0,99, entre les constantes de Hammett et les valeurs de Imin; cette corrtlation nous permet de prtdire les valeurs de ces constantes pour les substituants NHF et NF2 pour lesquels ils n'ttaient pas encore connus.Mots clis : tnergie moyenne d'ionisation locale, rtactivitt chimique, substitution arornatique tlectrophile, surfaces rnoltculaires, constantes de Harnmett.[Traduit par la revue] Introduction derivatives, I-IX, on three-dimensional surfaces encompassing The interpretation and prediction of molecular reactive the molecules. These r...

No abstract

The natural aquatic environment contains an enormous pool of dissolved reduced carbon, present as ultracomplex mixtures that are constituted by an unknown number of compounds at vanishingly small concentrations. We attempted to separate individual structural isomers from several samples using online reversedphase chromatography with selected ion monitoring/tandem mass spectrometry, but found that isomeric complexity still presented a boundary to investigation even after chromatographic simplification of the samples. However, it was possible to determine that the structural complexity differed among samples. Our results also suggest that extreme structural complexity was a ubiquitous feature of dissolved organic matter (DOM) in all aquatic systems, meaning that this diversity may play similar roles for recalcitrance and degradation of DOM in all tested environments.Dissolved organic matter (DOM) is the dominant form of organic carbon in most aquatic environments. Upon mineralization, it is an important precursor of outgassing of CO 2 from inland waters (Tranvik et al. 2009), it carries substantial amounts of nutrients and energy from land to sea (Medeiros et al. 2016), and it persists in the deep ocean for millennia (Dittmar and Stubbins 2014). It is an ultra-complex mixture of compounds that provides the ultimate test of the capabilities of analytical chemistry (Rodgers et al. 2005;Dittmar and Stubbins 2014), and investigations into its nature have been confounded by its extreme molecular complexity. High-resolution mass spectrometry (HRMS) is able to resolve many thousands of molecular masses from complex natural mixtures of organic compounds (Marshall et al. 1998;Riedel and Dittmar 2014;Hendrickson et al. 2015), but is unable to differentiate between structural isomers of a molecular formula.Most recent research has utilized advanced visualization or multivariate statistical approaches to interpret HRMS data (Wu et al. 2004;Sleighter et al. 2010;Kellerman et al. 2015), but when the HRMS data are not combined with more This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Scientific Significance StatementMolecular complexity is an inherent feature of dissolved organic matter (DOM), confounding investigations into its nature. Recent research has suggested that this complexity may explain the persistence of DOM due to the implied low abundance of individual compounds. Here, we use chromatographic separation and collision induced dissociation of deprotonated molecules to demonstrate the extreme isomeric complexity of individual molecular formulas in DOM and show that isomeric complexity occurs across diverse aquatic environments. 21Limnology and Oceanography Letters 3, 2018, 21-30

Onion waste is a renewable raw material, rich in different molecular species of the antioxidant quercetin. To utilize this resource, an environmentally sustainable procedure has been developed, using pressurized hot water to extract the quercetin species, followed by biocatalytic conversion of the quercetin glycosides to quercetin and carbohydrates. Two different recombinantly expressed thermostable b-glucosidases, Thermotoga neapolitana b-glucosidase A and B, were utilized as catalysts. These enzymes maintain activity at temperatures around 90 uC, and are therefore ideal to use in combination with hot water extraction. Our results, based on experimental design, showed that they converted quercetin glycosides to active quercetin in less than 10 min reaction time in water at 90 uC, pH 5.0. Experimental design showed that the optimal extraction conditions included three 5 min extraction cycles with water at 120 uC and 50 bars, giving a total extraction time of 15 min. Several different types of quercetin and isorhamnetin glycosides as well as kaempferol were detected in onion waste using LC-MS/MS analysis. After converting the different glycosidic compounds to their respective aglycones, the quercetin content was 10 to 50 mg g 21 dry weight of onion waste (RSD 8%). In summary, our research demonstrates that subcritical water extraction followed by b-glucosidase-catalyzed hydrolysis is a rapid method to determine the content of quercetin and isorhamnetin in onion samples, and is environmentally sustainable as it only uses water as solvent and enzymes as catalysts.

855of T -N. The difference of the activation energy of this reaction was estimated to be 1.3 kcal mol-', which is consistent with that of the zero-point energy of the initial state of T?1.22 On the other hand, the isotope effect on the proton-transfer reactions in both the ground and excited states may be interpreted in terms of the nonradiative process as a vibrational coupling between the double minimum potentials of T and N (or N * and T*). This type of nonradiative process is known as an isoenergetic transition between two upper vibrational states of T and N with an activation barrier. Acknowledgment.We are indebted to Drs. M. Sumitani and K. Hashimoto, Institute for Molecular Science, Okazaki, for supplying an amplifier circuit of the pin photodiode.Registry No. 7-Hydroxyquinoline, 580-20-1; methanol, 67-56-1; deuterium, 7782-39-0; methanol-d,, 1455-13-6. (22) Kishi, T.; Tanaka, J.; Kouyama, T. Chem. Phys. Let?. 1976,41,497.of the T* decay time suggests that the N-H(D) vibration seems to take an important role in the nonradiative process of T*. The isotope effect of the activation energy of the excited-state proton transfer was observed to be unusually large, though the determination df activation energy from the fluorescence rise time of T* has less accuracy. The difference of the activation energy of 1.4 kcal mol-' is fortunately close to the difference of the zero-point energy between 0-H and 0-D vibratiom2' Furthermore, the decay constant of T and the activation energy of T to N exhibit also unusually large deuterium isotope effect. Siiice any photochemical reactions and any other decay processes than that to the ground-state normal form N do not seem to be involved in the decay of T, the observed deuterium isotope effect on the decay of T may be really reflected by that of the reaction rate constant (21) OFerrall, R. Abstract:We have calculated the electrostatic potentials of benzene, aniline, nitrobenzene, and the three isomeric nitroanilines in order to study the effects of -NH2 and -NO2 substituents in activating or deactivating the aromatic ring toward electrophilic attack. The potentials were computed with use of SCF STO-5G wave functions, after the molecular geometries were first optimized at the STO-3G level. (Resonance considerations are notably effective in rationalizing the changes in geometry that accompany the introduction of -NO2 into aniline.) Benzene itself has extensive negative potentials (attractive toward electrophiles) above and below the aromatic ring, in the T regions. These are significantly stronger in aniline, indicating a considerable degree of activation, but are entirely eliminated in nitrobenzene. When both -NH2 and -NO2 are present, the deactivating influence of the latter dominates, although there remains evidence of the directing properties of -NH2. An interesting feature observed in nitroaromatic potentials is a positive buildup that occurs over the C-N02 bond regions. This may indicate a possible pathway for nucleophilic attack.The effect of an amino substituent upon an...