428 research outputs found
Border Reactivity of Polycyclic Aromatic Hydrocarbons and Soot Platelets Toward Ozone. A Theoretical Study
No full textPAH-based models, with an even or odd number of unsaturated carbon atoms and pi electrons (even and odd PAHs for short), are selected to investigate, by molecular and periodic methods, their electron distribution and border reactivity toward ozone, and also to represent local features and edge reactivity of even or odd soot platelets. These results will contrast those previously collected for the internal positions of similar even (J. Phys. Chem. A 2005, 109, 10929.) or odd systems (J. Phys. Chem. A 2008, 112, 973.). Topologically different peripheral positions, representative of armchair and zigzag borders, exhibit different reactivity right from the beginning. Ozone attacks start off either to give primary ozonides by concerted addition or, nonconcertedly, to first produce trioxyl intermediates. Then, a variety of pathways are described, whose viability depends on both model and position. They can open the way to the possible formation of epoxide, aldehyde, and phenol groups (all entailing O(2) production) or ether (+CO(2)), lactone (+H(2)CO), and ketone functionalities. To sum up, functionalization, regardless of how achieved, can give a number of groups, most of which actually observed in PAH ozonization experimental studies. This picture can be matched up to the results on internal sites of our preceding papers, for which epoxidation was the only outcome. Most interestingly, formation of a ketone group may turn an even system into an odd one (and conversely) while involving production of HOO(center dot)
Soot platelets and PAHs with an odd number of unsaturated carbon atoms and pi electrons: Theoretical study of their spin properties and interaction with ozone
No full textPAHs made from an odd number of unsaturated carbon atoms and pi electrons (odd PAHs) have been detected in flames and flank the more familiar even PAHs, having approximately the same quantitative importance, particularly for PAHs containing more than 25 carbon atoms. Similarly, soot platelets containing an odd number of carbon atoms can be reasonably assumed to form during combustion. PAHs are intended here as small models for the investigation of some, of their local features. To this end, quantum mechanical calculations were also carried out on periodic models. The spin density patterns were found to be highly dependent on the PAH size and shape. PAHs and soot, once released in the environment, can undergo several oxidation processes. Ozone is then taken as a probe of the reactivity properties of some internal exposed portions of a platelet. A primary ozonide (PO) corresponds to an energy minimum, but the relevant concerted addition pathway does not exist, because a PO-like saddle point is second-order. The reaction begins with a nonconcerted attack that produces a trioxyl radical (TR). Subsequent O-2 loss from the TR leaves either an epoxide with a pi-delocalized electron or a pi-delocalized oxepine, by cleavage of the ring carbon-carbon bond. The initial doublet spin multiplicity thus provides a description of the reaction surface unlike that for the internal reactivity of the closed-shell even systems investigated in a previous work, even though the final functionalization is the same
Aromatic nitration under tropospheric and combustion conditions. A theoretical mechanistic study
No full textA quantitative analysis of the factors controlling the inversion barriers in AH3 molecules
No full textCan a photochemical reaction be concerted? A theoretical study of the photochemical sigmatropic rearrangement of but-1-ene
No full textMC-SCF computations at the 4-31G level using a complete active space (CAS) of four orbitals demonstrate the existence of a concerted photochemical pathway for [1,2] and [1,3] alkyl sigmatropic shifts. The central feature of this concerted path is a conical intersection (e.g., a genuine crossing) between ground and excited state from which a fully efficient return to the ground state is possible. Thus the excited-state surface has no minimum with zero gradient (i.e., a critical point) but only a singularity which corresponds to the lowest energy point of a conical intersection between ground and excited states. Thus there is no bottleneck corresponding to a short-lived intermediate that would correspond to the minimum on the excited-state surface at an avoided crossing. Intrinsic reaction coordinate computations have been performed on the excited-state surface that demonstrate the existence of two "channels" on the excited-state surface that simply continue on the ground-state surface. One of these channels leads to a [1,2] sigmatropic shift, the other to a [ 1,3] sigmatropic shift. The proposed mechanism is consistent with experimental observations where both [1,2]- and [1,3]-shift products are observed, and where the migrating group moves according to a supra process with retention of configuration of the migrating group
Comprehensive study on the degradation of ochratoxin A in water by spectroscopic techniques and DFT calculations
No full textOchratoxin A (OTA) is one of the most important dietary risk factors and is classified as a possible carcinogen to humans. Assessing the conditions to remove it from foodstuffs in a simple and effective way is of the utmost importance. OTA behaviour in water in the pH range 1.0–12.5 was elucidated to investigate the conditions for irreversible toxicity inactivation of OTA. The results indicate that four forms, from neutral
to trianionic, intervene depending on the pH. pKa1,2 were rigorously established by independent spectroscopic techniques to overcome the scarcity of literature. Then, Density Functional Theory (DFT) calculations were used to determine the most probable degradation mechanism and this was confirmed by fluorescence spectroscopy. At pH 12.5, hydrolyzation of the lactone ring starts in less than one hour, but only after two hours does the degradation process lead to fragmentation. After one week this process is not yet completed. The reaction products occurring upon re-acidification were also investigated. OTA degradation is still reversible if acidic conditions are promptly restored, yielding again a hazardous molecule. However, degradation becomes irreversible after fragmentation. This finding suggests proceeding with due caution if a base is exploited to remove the toxin
The oxidized soot surface: Theoretical study of desorption mechanisms involving oxygenated functionalities and comparison with temperature programed desorption experiments
No full textThe desorption mechanism for oxygenated functionalities on soot is investigated by quantum
mechanical calculations on functionalized polycyclic aromatic hydrocarbon PAH models and
compared with recently published temperature programed desorption-mass spectrometry results.
Substituents on PAHs of increasing size up to 46 carbon atoms in the parent PAH are chosen to
reproduce the local features of an oxidized graphenic soot platelet. Initially, the study is carried out
on unimolecular fragmentation extrusion, in some cases processes producing HO, CO, or CO2, in
model ketones, carboxylic acids, lactones, anhydrides, in one aldehyde, one peroxyacid, one
hydroperoxide, one secondary alcohol, and one phenol. Then, a bimolecular process is considered
for one of the carboxylic acids. Furthermore, some cooperative effect which can take place by
involving two vicinal carboxylic groups derived from anhydride hydrolysis is investigated for
other four bifunctionalized models. The comparison between the computed fragmentation
desorption barriers for the assessed mechanisms and the temperature at which maxima occur in
TPD spectra for HO, CO, or CO2 desorption offers a suggestion for the assignment of these
maxima to specific functional groups, i.e., a key to the description of the oxidized surface. Notably,
the computations suggest that 1 the desorption mode from a portion of a graphenic platelet
functionalized by a carboxylic or lactone groups is significantly dependent from the chemical and
geometric local environment. Consequently, we propose that 2 not all carboxylic groups go lost at
the relatively low temperatures generally stated, and 3 lactone groups can be identified as
producing not only CO2 but also CO. © 2006 American Institute of Physics
Diabatic surface methods for the study of the reactivity of organic molecules. 1. Cycloaddition of two ethylenes
No full textIn this paper we discuss the quantitative diabatic surface analysis of the surface associated with the thermal cycloaddition of two ethylene molecules. We show that the main features of a saddle point, such as the index (i.e., the number of negative eigenvalues of the Hessian) and the origin, can be understood by analyzing the behavior of the reactant and product diabatic surfaces computed in a three-dimensional subspace involving two relevant geometrical variables at a time. We show also that the behavior of the constituent diabatic surfaces can be easily rationalized with the simple energy expressions of qualitative MO theory. © 1986, American Chemical Society. All rights reserved
Ozone interaction with polycyclic aromatic hydrocarbons and soot in atmospheric processes: Theoretical density functional study by molecular and periodic methodologies
No full textReaction of dialkyl carbonates with alcohols:Defining a scale of the best leaving and entering groups
No full textA series of dialkyl and methyl alkyl carbonates has been synthesized and their reactivity investigated. The behavior of preferential leaving and entering groups for the newly synthesized carbonates has been accurately investigated. Both experimental and computational studies agreed that the scale of leaving groups follows the trend: PhCH2O–, MeO– ≥ EtO–, CH3(CH2)2O–, CH3(CH2)7O– > (CH3)2CHO– > (CH3)3CO–. Accordingly, the scale of the entering group has the same trend, with t-butoxide being the worst entering group. A preliminary attempt to rationalize the nucleofugality trends, limited to the (CH3)3CO– and CH3O– groups, has indicated that a likely origin of the observed trends lies in the different entropic contributions and solvation effects
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