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The addition of carbonyl oxides to ozone could have an effect on the tropospheric HO• nocturnal formation. Its mechanistic description has provided so far conflicting results. CASPT2 (and CASSCF) g...

Positive ions play a fundamental role in the interstellar chemistry, especially in cold environments where chemistry is believed to be mainly ion-driven. However, in contrast with neutral species, most of the cations present in the astrochemical reaction networks are not fully characterized in the astrochemical literature. To fill up this gap, we have carried out new accurate quantum chemical calculations to identify the structures and energies of 262 cations with up to 14 atoms that are postulated to have a role in the interstellar chemistry. Optimised structures and rotational constants were obtained at M06-2X/cc-pVTZ level, while electric dipoles and total electronic energies were computed with CCSD(T)/aug-cc-pVTZ//M06-2X/cc-pVTZ single point energy calculations. The present work complements the study by Woon & Herbst (2009), who characterised the structure and energies of 200 neutral species involved as well in the interstellar chemistry. Taken together, the two datasets can be used to estimate whether a reaction, postulated in present astrochemical reaction networks, is feasible from a thermochemistry point of view and, consequently, to improve the reliability of the present networks used to simulate the interstellar chemistry. We provide an actual example of the potential use of the cations plus neutral datasets. It shows that two reactions, involving Si-bearing ions and present in the widely used reaction networks KIDA and UMIST, cannot occur in cold ISM because endothermic.

The reactivity of “furan-ynes” in combination with pyridine and quinoline N-oxides in the presence of a Au(I) catalyst, has been studied, enabling the synthesis of three different heterocyclic scaffolds. Selective access to two out of the three possible products, a dihydropyridinone and a furan enone, has been achieved through the fine-tuning of the reaction conditions. The reactions proceed smoothly at room temperature and open-air, and were further extended to a broad substrate scope, thus affording functionalized dihydropyridinones and pyranones.

The reactivity of "furan-ynes" in combination with pyridine and quinoline N-oxides in the presence of a Au(I) catalyst, has been studied, enabling the synthesis of three different heterocyclic scaffolds. Selective access to two out of the three possible products, a dihydropyridinone and a furan enone, has been achieved through the fine-tuning of the reaction conditions. The reactions proceed smoothly at room temperature and open-air, and were further extended to a broad substrate scope, thus affording functionalized dihydropyridinones and pyranones.

We report that the nucleophilic acyl substitution reaction of aliphatic and (hetero)aromatic amides by organolithium reagents proceeds quickly (20 s reaction time), efficiently, and chemoselectively with a broad substrate scope in the environmentally responsible cyclopentyl methyl ether, at ambient temperature and under air, to provide ketones in up to 93 % yield with an effective suppression of the notorious over-addition reaction. Detailed DFT calculations and NMR investigations support the experimental results. The described methodology was proven to be amenable to scale-up and recyclability protocols. Contrasting classical procedures carried out under inert atmospheres, this work lays the foundation for a profound paradigm shift of the reactivity of carboxylic acid amides with organolithiums, with ketones being straightforwardly obtained by simply combining the reagents under aerobic conditions and with no need of using previously modified or pre-activated amides, as recommended.

A small library of 2,6- and 3,5-distyrenyl-substituted carborane-BODIPY dyes was efficiently synthesized by means of a Pd-catalyzed Heck coupling reaction. Styrenyl-carborane derivatives were exploited as molecular tools to insert two carborane clusters into the fluorophore core and to extend the π-conjugation of the final molecule in a single synthetic step. The synthetic approach allows to increase the molecular diversity of this class of fluorescent dyes by the synthesis of symmetric or asymmetric units with enhanced boron content. The structural characterization and the photoluminescence (PL) properties of synthesized dyes were evaluated, and the structure/properties relationships have been investigated by theoretical calculations. The developed compounds exhibit a significant bathochromic shift compared to their parent fluorophore scaffolds, and absorption and emission patterns were practically unaffected by the different substituents (Me or Ph) on the Ccluster atom (Cc) of the carborane cage or the cluster isomer (ortho- or meta-carborane). Remarkably, the presence of carborane units at 2,6-positions of the fluorophore produced a significant increase of the emission fluorescent quantum yields, which could be slightly tuned by changing the Cc-substituent and the carborane isomer, as well as introducing ethylene glycol groups at the meso-position of the BODIPY., We would like to acknowledge Dr. Emanuele Priola (UniTO) and Dr. Francesco Marra (UniTO) for technical support, and Prof. Claudio Medana (UniTO) for HRMS measurements. We acknowledge the italian MIUR, Huvepharma Italia srl, Regione Piemonte and Cassa di Risparmio di Torino for financial support. This research was funded by MINECO ((CTQ2016-75150-R) Agencia Estatal de Investigación AEI from MICINN (PID2019-106832RB-100/AEI/10.13039/501100011033) and Generalitat de Catalunya (2017 SGR1720). The work was also supported by the MICINN through the Severo Ochoa Program for Centers of Excellence FUNFUTURE (CEX2019-000917-S). Sohini Sinha was enrolled in the PhD Program of UAB. S. S. acknowledges financial support from DOC-FAM, European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 754397., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

The gold-catalyzed oxidation of N-tosyl-protected 6-alkynyl-3,4-dihydro-2H-pyridines was studied in detail to obtain divinyl ketones in which one of the double bond is embedded in a heterocyclic framework. The best reaction conditions were then extended to different types of substrates to assess the scope of the reaction. DFT calculations were exploited to gain insight into the regioselectivity and chemoselectivity of the process too. The obtained divinyl ketones were then easily cyclized according to a Nazarov process and the bi- or polycyclic compounds used as scaffolds in the synthesis of analogues of plant hormones Strigolactones

Polycyclic aromatic hydrocarbons and their oxidized derivatives are ubiquitous environmental pollutants that are toxic to different degrees. The NO 3 radical is known to be an important actor in fostering nighttime atmospheric chemistry; hence it could elicit some nocturnal PAH-loss processes. In this study the gas phase pathways open to the initial anthracene and phenanthrene NO 3 π−radical adducts in the presence of O 2 , NO and NO 2 are examined by Density Functional Theory to ascertain the mechanistic features of their NO 3 -initiated oxidative degradation. Unimolecular steps involving the initial adducts (ring closures or fragmentation) present rather high free energy barriers and seem unlikely. Regarding bimolecular reactions, any radical present in the tropospheric environment can give an intrinsically fast radical coupling with the initial adducts, thus producing bifunctional closed shell species. The addition of ground state dioxygen is a relatively fast step that is reversible in some cases, less so in others. It entails subsequent reaction steps, which involve the peroxyl radical intermediate and addition and loss of small species as NO, NO 3 , NO 2 and O 2 . These steps can either trigger a β−fragmentation with formation of a closed shell dialdehyde or formation of a closed shell nitroxy ketone, bifunctional species that appear to be the most likely products when typical tropospheric concentrations are taken into account.

Cyclization of 2-(2-hydroxyethyl)phenol via DMC chemistry in acidic conditions is herein discussed for the first time. Reaction conditions have been investigated and optimized. This substrate is quite appealing as it incorporates a 2-hydroxyethyl moiety in ortho to the aromatic hydroxyl group capable of stabilizing the related phenonium ion. When the reaction mechanism was investigated via theoretical calculations, the results suggest that the most favorable pathway encompasses a DMC-mediated formation of the phenonium ion that is converted into the 2-(2-methoxyethyl)phenol. The related cyclic ether is then formed via intramolecular cyclization of this intermediate. This peculiar cyclization reaction is another example of the versatility of DMC herein used as solvent, methoxycarbonylation agent and leaving group in the intramolecular cyclization leading to the phenonium ion.

We report the results of a DFT study of the electronic properties, intended as highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies, of periodic models of H-passivated armchair graphene nanoribbons (a-GNRs) as that synthetized by bottom-up technique, functionalized by vicinal dialdehydic groups. This material can be obtained by border oxidation in mild and easy to control conditions with 1Δg O2 as we reported in our previous paper (Ghigo et al., ChemPhysChem 2015, 16, 3030). The calculations show that the two models of border oxidized a-GNRs (model A, 0.98 nm and model B, 1.35 nm wide) present LUMO and HOMO energies lowered by an extend roughly linearly dependent on the amount of oxygen chemically bound. The frontier orbital energy variations dependence on the % wt of oxygen bound are, for model A: −0.12 eV for the LUMO and −0.05 eV for the HOMO; for model B: −0.15 eV (HOMO) and −0.06 eV (LUMO). © 2016 Wiley Periodicals, Inc.

Propargyl radical addition to 1,3-butadiene seems to be a promising channel to form 5-, 6-, and 7-membered rings. These are important steps in the growth of polycyclic aromatic hydrocarbons and soot platelets. The reaction mechanism, involving 97 intermediates and 115 transition structures, was studied by CBS-QB3 method (reported here) and density functional theory. All these structures were included in the subsequent RRKM study at different combustion pressures (P=30-0.01 atm) and temperatures (T=1200-2100 K). At P=30 atm, open-chain products dominate in the whole range of temperatures. The importance of 5- and 6-membered rings rises with T, reaching a maximum in the T range 1500-1800 K. A more modest yield in 7-rings is present at T=1500 K. At P =I atm, in the range 1200-1500 K, the yield in 5- and 6-rings dominate. 5- and 6-rings yields are about 41% at 1200 K (CBS-QB3 data). At P =0.1 atm, 6-rings become the main products a 1000 K (35%), and then they decrease to 12% (2100 K) and 5-rings rise up to 44% (1200 K), and then decrease to 14% (2100 K). Open-chains are the main products at T < 1000 K and T > 1500 K. Then, at P=0.01 atm, open-chain products are important below 900 K and above 1500 K,6-rings are main contributors between 900 K and 1500K. 6-Rings reach a maximum yield of 47% (1200 K) and 5-rings 44% (1200K) and 7-rings 17% (900 K). The main products form through to H losses. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The methyl carbocation is ubiquitous in gaseous environments, such as planetary ionospheres, cometary comae, and the interstellar medium, as well as combustion systems and plasma setups for technological applications. Here we report on a joint experimental and theoretical study on the mechanism of the reaction CH

In the plasma dry reforming, CH4 and CO2 are dissociated by electron collisions producing fragments that in turn react to form new molecules. While the production of CO, H2, and small hydrocarbons has been investigated extensively, much less is known about the plasma synthesis of oxygenated molecules, which is the target of the present work. In addition to experimental results, gas phase reactions for the production of acetic, formic and propanoic acid were investigated by DFT calculations.

Combustive formation of a first carbon ring is an important step in the growth of polycyclic aromatic hydrocarbons (PAHs) and soot platelets. Propargyl radical addition to but-1-ene-3-yne (vinylacetylene) can start this process, possibly forming 5-, 6-, and 7-membered rings. A variety of partially intertwined reaction pathways results from density functional theory (DFT), which indicates three C7H7 radicals, benzyl, tropyl, and vinylcyclopentadienyl, as particularly stable. DFT energetics forms a basis for a subsequent Rice-Ramsperger-Kassel-Marcus (RRKM) study at different combustion pressures and temperatures (P = 30-0.01 atm; T = 1200-2400 K). RRKM indicates open-chain structures and 5-rings as the most important products. Open-chain structures, whose main contributors are the initial adducts, are favored by lower T and higher P, while 5-rings are favored by higher T and lower P. The main feature is that the declining yield in open-chain structures with rising T almost mirrors, at all pressures, the growth with T exhibited by 5-rings (main contributor: fulvenallene). Thus, the two yield lines for open chains and 5-rings cross at some T, and their crossing moves toward lower T values as lower P values are considered. Because the T dependence of the yields (slope of the lines) is more pronounced in the T range close to the line crossing, it also becomes less pronounced at the lowest P values considered because the crossing region falls at very low T values. Another constant trait is that 6-rings (mainly benzyl radical) are the third contributor, though they are present at most with a modest maximum yield of 2.4-2.7% in a T range which moves toward lower T as P is reduced.

The mechanisms of the fragmentation and isomerization pathways of o-benzyne were studied at the multi-configurational second-order perturbative level [CAS(12,12)-PT2]. The direct fragmentation of o-benzyne to C2H2 + C4H2 follows two mechanisms: a concerted mechanism and a stepwise mechanism. Although the concerted mechanism is characterized by a single closed-shell transition structure, the stepwise pathway is more complex and structures with a strong diradical character are seen. A third diradicaloid fragmentation pathway of o-benzyne yields C6H2 as the final product. As an alternative to fragmentation, o-benzyne can also undergo rearrangement to its meta and para isomers and to the open chain cis and trans isomers of hexa-3-en-1,6-diyne (HED). These easily fragment to C2H2 + C4H2 or C6H2. Kinetic modelling at several different temperatures between 800 and 3000 K predicted that the thermal decomposition of o-benzyne should yield C2H2, C4H2 and C6H2 as the main products. Small amounts of the HED isomers accumulated at temperatures1200 K, but they rapidly decompose at higher temperatures. Between 1000 and 1400 K, C2H2 + C4H2 are formed exclusively from the decomposition of trans-HED. At temperatures1400 K, C2H2 + C4H2 also form from the direct fragmentation of o-benzyne. The formation of C2H2 + C4H2 prevails up to 1600 K but above this temperature the formation of C6H2 prevails. At temperatures2400 K, the direct fragmentation of o-benzyne again leads to the formation of C2H2 + C4H2. The formation of hydrogen atoms is also explained by our proposed mechanisms.

The solvolysis of two diastereomers may give the same two products, but in different ratios, notwithstanding the fact that the two reaction pathways share an apparently identical intermediate carbocation. This has been dubbed the "memory effect", since the initial carbocation seems to "remember" its origin when undergoing further evolutions through multistep rearrangements. This puzzling result was studied theoretically for the case of the solvolysis of norborn-2-en-7-ylmethyl-X systems by defining the reaction potential energy surface (PES) and then carrying out a dynamical study. The PES shows that upon X(-) loss, multiphase rearrangements concertedly yield the two stablest carbocations, G and L. These carbocations are connected by a transition structure. The carbocation intermediates proposed in the literature do not correspond to any stationary point. The preference for the rearrangement to G or L (the memory effect) is determined by structural and stereoelectronic effects: the competitive interaction between an empty p orbital with a σ orbital or a p/π orbital is guided by geometrical aspects present in the starting carbocations. The dynamical study shows that (1) G and L do not interconvert and (2) the evolving system can switch from one pathway to the other to different extents, thus determining a more or less pronounced memory loss (the leakage).

The acid-catalyzed benzidine rearrangement of hydrazobenzene yields p-benzidine and diphenyline as the main products and some other secondary products such as o-benzidine, o-semidine and p-semidine. Several reaction mechanisms have been proposed for this rearrangement, but none fully describes it. In this work we present the results of an unrestricted DFT(M06-2X) study in which the benzidine rearrangement of the hydrazobenzene is fully described. Some relevant points were also analyzed with the CASSCF and CASPT2 methods. Our theoretical approach properly describes the electronic structure of all species along the reaction pathway. The rearrangement of the hydrazobenzene has been found to take place through a multi-step mechanism in which complexes with dication diradical character similar to those proposed by Dewar play a key role. For the first time, the kinetic isotope effects have been calculated and were found to be in good agreement with the experimental findings. The relative yields of the main products were also correctly estimated. Finally, an explanation of the general acid catalysis observed in some experiments is offered.

PAH-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).

This paper (part 1) and the following one (part 2) aim to assess the viability of some tropospheric oxidation channels for two symmetrical alkynes, ethyne (acetylene) and but-2-yne. Paper 1 defines the features of the DFT(B3LYP)/6-31 1G(3df,2p) energy hypersurface and qualitatively considers the practicability of different pathways through the estimate of free energy barriers. Paper 2 will assess this in more detail by way of master equation simulations. Oxidized in the presence of HO and O 2 (with the possible intervention of NO), ethyne and but-2-yne are known to produce mainly glyoxal or dimethylglyoxal and, to a lesser extent, formic or acetic acid. The initial attack by HO gives an adduct, from which several pathways (la-c, 2a-e) originate. Pathway la passes through the 2-oxoethyl (vinoxyl) radical, or the analogous dimethyl-substituted intermediate, which could in principle undergo O 2 addition (and subsequently, but through a demanding step, give the dialdehydes). However, in paper 2 it is assessed that the vinoxyl, as a nonthermalized intermediate, will preferentially follow unimolecular pathways to ketene or acetyl. Pathway 2a is the most important pathway: a very steep free energy cascade, started by O 2 addition to the initial HO adduct with a concerted barrierless 1,5 H shift, gives a hydroperoxyalkenyloxyl radical intermediate. Peroxy bond cleavage finally produces the dialdehydes and regenerates HO. Pathways 2b and 2c originate from O 2 addition to the initial HO adduct and produce, via different ring closures, either dioxetanyl or alkyl dioxiranyl radicals, respectively. Two subsequent fragmentations occur in both cases and give the carboxylic acids and a carbonyl radical, which can indirectly generate hydroxyl. Two further pathways (1c and 2e) see NO intervention onto the peroxyl radicals formed along pathways 1 and 2. Both could enhance dialdehyde production, while simultaneously depressing the carboxylic acid yield.

The aim of this study is to improve understanding of the tropospheric oxidation of ethyne (acetylene, C2H2) and but-2-yne, which takes place in the presence of HO and O2. The details of the potential energy hypersurface have been discussed in a previous article [Maranzana et al., J. Phys. Chem. A 2008, 112, XXXX]. For both molecules, the initial addition of HO radical to the triple bond is followed by addition of O2 to form peroxyl radicals. In both reaction systems, the peroxyl radicals take two isomeric forms, E1 and E2 for ethyne and e1 and e2 for but-2-yne. Energy transfer parameters (alpha = 250 cm-1) for the ethyne system were obtained by simulating laboratory data for N2 buffer gas, where O2 was not present. In simulations of C2H2 + HO when O2 is present, E1 reacts completely and E2 reacts almost completely, before thermalization. Radical E1 produces formic acid ( approximately 44%) and E2 gives glyoxal ( approximately 53%), in quite good agreement with experiments. For but-2-yne, pressure-dependent laboratory data are too scarce to obtain energy transfer parameters directly, so simulations were carried out for a range of values: alpha = 200-900 cm-1. Excellent agreement with the available experimental yields at atmospheric pressure was obtained with alpha = 900 cm-1. Two reaction channels are responsible for acetic acid formation, but one is clearly dominant. Biacetyl is produced by reactions of e1 and, to a minor extent, e2. The peroxyl radical e2 leads to less than 8% of all products. Vinoxyl radical (which has been reported in experiments involving C2H2 + HO) and products of its reactions are predicted to be negligible under atmospheric conditions.

In $${\rm CH}_{2}{\rm CHCH}_{2}-{\rm GeH}_{3}^{\cdot -}$$ two stable structures have a trigonal bipyramidal arrangement around Ge, with the extra electron in equatorial (tbp eq) or axial (tbp ax) position. In $${\rm CH}_{2} ({\rm CH})_{3}{\rm CH}_{2}-{\rm GeH}_{3}^{\cdot -}$$ only tbp ax is found, while a second structure with a tetrahedral germyl group has the extra electron on the conjugated π system. C−Ge bond cleavage yields allyl/ pentadienyl radicals plus germide. Both dissociation reactions require 4–6 kcal mol−1, less than the analogous C and Si systems (ca. 30 and 14 kcal mol−1, respectively). Fragmentation is dramatically activated with respect to homolysis in the corresponding neutrals. The wavefunction is dominated by one single configuration at all distances, in contrast to homolytic cleavage, in which two configurations are important. C−Ge bond dissociation is at variance also with heterolysis, due to spin recoupling of one of the C−Ge bond electrons with the originally unpaired electron.

Our purpose is to identify a computational level sufficiently dependable and affordable to assess trends in the interaction of a variety of radical or closed shell unsaturated hydro-carbons A adsorbed on soot platelet models B. These systems, of environmental interest, would unavoidably have rather large sizes, thus prompting to explore in this paper the performances of relatively low-level computational methods and compare them with higher-level reference results. To this end, the interaction of three complexes between non-polar species, vinyl radical, ethyne, or ethene (A) with benzene (B) is studied, since these species, involved themselves in growth processes of polycyclic aromatic hydrocarbons (PAHs) and soot particles, are small enough to allow high-level reference calculations of the interaction energy ΔEAB. Counterpoise-corrected interaction energies ΔEAB are used at all stages. (1) Density Functional Theory (DFT) unconstrained optimizations of the A-B complexes are carried out, using the B3LYP-D, ωB97X-D, and M06-2X functionals, with six basis sets: 6-31G(d), 6-311 (2d,p), and 6-311++G(3df,3pd); aug-cc-pVDZ and aug-cc-pVTZ; N07T. (2) Then, unconstrained optimizations by Møller-Plesset second order Perturbation Theory (MP2), with each basis set, allow subsequent single point Coupled Cluster Singles Doubles and perturbative estimate of the Triples energy computations with the same basis sets [CCSD(T)//MP2]. (3) Based on an additivity assumption of (i) the estimated MP2 energy at the complete basis set limit [EMP2/CBS] and (ii) the higher-order correlation energy effects in passing from MP2 to CCSD(T) at the aug-cc-pVTZ basis set, ΔECC-MP, a CCSD(T)/CBS estimate is obtained and taken as a computational energy reference. At DFT, variations in ΔEAB with basis set are not large for the title molecules, and the three functionals perform rather satisfactorily even with rather small basis sets [6-31G(d) and N07T], exhibiting deviation from the computational reference of less than 1 kcal mol(-1). The zero-point vibrational energy corrected estimates Δ(EAB+ZPE), obtained with the three functionals and the 6-31G(d) and N07T basis sets, are compared with experimental D0 measures, when available. In particular, this comparison is finally extended to the naphthalene and coronene dimers and to three π-π associations of different PAHs (R, made by 10, 16, or 24 C atoms) and P (80 C atoms).

The ene reaction of singlet oxygen with α,β-unsaturated carbonyl compounds gives unsaturated hydroperoxides and displays interesting features: Different reactivities of the s-cis and s-trans reactants and a marked regioselectivity, influenced to some extent by solvent polarity. All of these traits are accounted for by a polar diradical mechanism. A perepoxide intermediate is not a critical point on the reaction’s potential energy surface. Also, a trioxene intermediate is located too high in energy to be significantly populated. An alternative pathway leading to dioxetane is not as effective as the ene pathway giving the hydroperoxide. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

A theoretical model for the evaluation of the entropy of activation for organic reactions in the condensed phase is proposed. The required partition functions for solutes are computed taking into account the hindered translation and rotation in the solvent cavity. The activation parameters for the alkylation of 3-bromopyridine by iodomethane in acetonitrile were obtained by regression of experimentally determined rate constants to the Eyring equation with different reference volumes. While the gas-phase reference volume affords an entropy of activation of –40.7 cal mol–1 K–1, the corresponding calculated quantity at the MPW1K/CRENBL·6-311(+)G(d) level of theory is –34.9 cal mol–1 K–1. The condensed-phase reference volume gives ΔS≠ = –24.1 cal mol–1 K–1, in better agreement with the computed value of –22.6 cal mol–1 K–1. Regressions with all reference volumes give an enthalpy of activation of around 13 kcal mol–1, while the computed potential-energy barriers lie between 16.8 and 18.4 kcal mol–1. Transition-state theory, making use of calculated potential-energy barriers and either gas-phase or condensed-phase partition functions, affords second-order rate constants for the reaction of CH3I with 3-bromopyridine at least three orders of magnitude below the experimentally observed value. This discrepancy does not appear to be a consequence of a lack of accuracy of the pre-exponential factor, but might be caused by either overestimated calculated potential-energy barriers or the functional form of the Eyring equation. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Graphite, and particularly defective graphite, is chosen to model soot particles. Quantum mechanical calculations are first carried out on molecular polycyclic-aromatic-hydrocarbon-type systems and then extended to a periodic representation of one graphite layer. The features of the interaction of H, HO, NO, NO2, and NO3 with these model systems are examined, with the aim of defining a suitable representation of the atmospheric or combustion gas−solid interactions by which functionalization reactions can take place. The more interesting interactions with small reactive molecules regard the edge of the graphene sheet and the in-plane carbon vacancies. While these interactions can be well described by sufficiently extended molecular models, periodic models are necessary to describe accurately the equilibrium geometries because they introduce the necessary geometric constraints. The ability of a graphene sheet to easily accommodate unpaired electrons in σ or π orbitals is the basis for its interesting intera...

Alkyl peroxyls form in the atmospheric oxidation of hydrocarbons and in their combustion. When NO concentration is low, they can appreciably react with themselves. This reaction has both propagation and termination channels. Multireference second-order perturbative energy calculations CAS(16,12)-PT2/6-311G(2df,p) have been carried out on the CAS(8,8)-MCSCF/6-311G(d,p) geometries pertaining to the reaction pathways explored. The tetroxide intermediate put forward first by Russell in 1957 is found as a stable energy minimum, but the calculations indicate that, as the system moves from atmospheric to combustion temperatures, its formation becomes problematic. A concerted synchronous transition structure, apt to connect it with the termination products, formaldehyde, methanol, and dioxygen, is not found. The concerted dissociation of the two external O–O bonds in the tetroxide leads to the 3(2CH3O•)2⋯3O2 complex, with overall singlet spin multiplicity. Both termination via H transfer, to give H2CO, CH3OH, and O2, or dissociation to 2 CH3O•+O2 (possible propagation) are feasible. The former could occur in principle with production of either excited 1O2 or excited 3H2CO. However, if a sufficiently easy intersystem crossing (ISC) could take place in the complex, the process would end up with all ground-state molecules. The (possible) propagation channels are favored by higher temperatures, while lower temperatures favor the ISC mediated termination channel. A fairly good qualitative agreement with experimental T dependence of the relevant branching ratio is found. From the tetroxide over again, dissociation of a single external O–O bond leads to CH3O• and CH3O3•, or possibly to a 1(CH3O•⋯CH3O3•) complex, but further transformations along this line are not competitive.

The reactions of (1)O(2) with alkenes can share open-chain diradicals or cyclic peroxiranes as common polar intermediates. The latter in particular has been postulated on the basis of trapping experiments, which exploit the capability of reducing agents (R) to extract an oxygen atom from the putative perepoxide, to generate an epoxide. This theoretical study illustrates that trapping experiments cannot distinguish between a peroxirane and an open-chain intermediate pathway, because an epoxide is the shared outcome of the attack by R.

Butadiyne (diacetylene, HC(4)H) is produced during combustions, and has been quantified in different flames as well as a biomass burning emission. Its reaction with the hydroxyl radical, HO((2)Π(3/2)), under combustion conditions, was investigated in a thorough RRKM study by J. P. Senosiain, S. J. Klippenstein, and J. A.Miller (Proc. Combust. Inst. 2007, 31, 185−192). The present densityfunctional theory (DFT) study focuses on the mechanism of further oxidation by O(2)(3Σ(g)(−)). The DFT(M06-2X)/cc-pVTZ reaction energy hypersurface for the system C(4)H(2)/HO•/O(2) is studied to define a variety of reaction pathways, and the relevant thermochemistry for temperatures ranging from 200 to 2500 K is assessed, thus encompassing combustive, postcombustive, and tropospheric conditions.Energies are then recomputed at the coupled cluster level[CCSD(T)/cc-pVTZ], and combined with the DFT thermochemistry.Finally, the role of the different reaction channels is assessed by RRKM-ME simulations in the same temperature range for P = 1 atm,to comprise the situation of emission in the troposphere and those pertaining to different flames. This shows that, when considering HO addition to the triple bond, dioxygen takes part in C(4)H(2) oxidation with higher efficiency at lower temperatures, whereas, as T rises, the O(2) adducts are inclined to redissociate: for instance, a 50% redissociation is estimated at T = 1800 K. For 200 < T < 1100 K, two polycarbonyl products (CHO.CO.CCH and CHO.CO.CHCO) and two fragmentation products (HCOOH plus OC(•)−CCH) are the main species predicted as products from the addition channel (fragmentation is entropy-favored by higher T values). However, at higher temperatures, aninitial H abstraction by HO can give the but adiynyl radical (HC(4)(•)) as the starting point for subsequent dioxygen intervention.Then, new pathways opened by O(2) addition become accessible and bring about fragmentations mainly to HC(3)(•) + CO(2) and also to HC(3)(•)O + CO.

To assess if reactions with oxygen species can induce a mass increase of polycyclic aromatic hydrocarbons, we exposed naphthalene molecules to an oxidative gas flow containing the radicals H and HO (2Π3/2) and the diradicals O (3P) and O2 (3Σ−g). We observed the formation of 1- and 2-naphtol, 1,4-naphthoquinone, naphthalene-derived cyclic ethers, an ester from ring opening, and ether adducts containing two naphthalene units. We investigated the possible reaction pathways as a function of temperature by density functional calculations. We found that the reactivity is characterized by HO, O and H addition to naphthalene, or by H abstraction from it, with roles depending on temperature. In conclusion, oxygen species can promote, under the experimental conditions, mainly naphthalene oxidation and, to a lesser extent, substantial molecular growth, with an efficiency that the calculations indicate to depend on the system temperature. Future experiments should try to quantify key species to allow defining the relative importance of the various reaction mechanisms uncovered by ab initio calculations.

Polycyclic aromatic hydrocarbons (PAHs) and carbonaceous nanoparticles can be oxidized right from their inception and all through their growth. Oxidation can also promote their degradation. This modelistic density functional theory (DFT) study explores, in a descriptive manner, if oxidation can mediate the earliest stages of nucleation ("functionalized nucleation"), though contrasted by mass declension triggered by oxidation itself. Initial O ((3) P) attack onto pyrene, chosen as a representative of a generic small PAH or nascent soot lamella, forms an oxyl diradical intermediate that can evolve into an open-shell epoxide, phenol, or ketone species or, alternatively, undergo mass depletion from the beginning (without impeding further additions). Open-shell intermediates can add O or O2 ((3) Σg (-) ) and ethyne, in any order, and open the way, through formation of carbon and oxygen bridges, to the addition of a second molecule of pyrene, whereas formation of direct carbon-carbon links between the two PAH-like parts might also occur.

We report the results of a DFT study of the border oxidation by (1) Δg O2 of molecular models of armchair graphene nanoribbons (a-GNRs). The aim of this work is to propose a new method, as an alternative or complementary method to the tuning of the size, to modify the electronic properties of a-GNRs. Here, we investigate modification of the HOMO and LUMO energies, which are some of the most important parameters to be controlled in the design of organic electronic devices. We study the oxidation reaction mechanism of medium-size polycyclic aromatic hydrocarbons, mimicking the stiffness and reactivity of a-GNRs. Thermodynamics and kinetics indicate that the reaction should bring about a decoration of the borders with vicinal dialdehyde groups. We also study the effect of this oxidation on the HOMO and LUMO energies of two series of molecular models of a-GNRs with increasing lengths. The results suggest that the oxidized a-GNRs should present LUMO energies lowered by 0.3-0.5 eV with respect to the original material, whereas the HOMO energies are barely lowered.

The methyl carbocation is ubiquitous in gaseous environments, such as planetary ionospheres, cometary comae, and the interstellar medium, as well as combustion systems and plasma setups for technological applications. Here we report on a joint experimental and theoretical study on the mechanism of the reaction CH3+ + CH3CCCH3 (but-2-yne, also known as dimethylacetylene), by combining guided ion beam mass spectrometry experiments with ab initio calculations of the potential energy hypersurface. Such a reaction is relevant in understanding the chemical evolution of Saturn’s largest satellite, Titan. Two complementary setups have been used: in one case, methyl cations are generated via electron ionization, while in the other case, direct vacuum ultraviolet photoionization with synchrotron radiation of methyl radicals is used to study internal energy effects on the reactivity. Absolute reactive cross sections have been measured as a function of collision energy, and product branching ratios have been derived....

Gas-phase selective reactions have been studied in silane/propene mixtures by ion-trap mass spectrometry and density functional calculations. Products from the third step of reaction of SiH+ ions with SiH4 and C3H6 were selected and reacted in the cell of a quadrupole ion trap. The prior steps have been the subject of a previous study by this research group. While ions Si3H5+, Si4H7+, and SimCnHp+ react selectively with propene, collisions with silane do not lead to any detectable product besides self-condensation of silane. Experimental and collisional rate constants of the main processes have been compared, determining reaction efficiencies. Chain propagation proceeds until the formation of large clusters, such as Si2C5H13+, Si3C2Hn+ (n = 9, 11), and Si4C2H11+, is attained. The critical points on the B3LYP/6-311G(d,p) potential energy surfaces of the Si2H3+/C3H6 and Si3H5+/C3H6 systems have been found, describing the structures of the most stable reaction products. The observed selectivity toward propen...

The kinetics of gas phase reactions of the ions C 5 H 5 M + (M=V, Co, Ni and Ru) with oxygen (Me 2 CO, Me 2 O, MeOH, Me 2 CHOH, H 2 O) and nitrogen (NH 3 , NH 2 Me, NHMe 2 , NMe 3 ) donor ligands have been studied by Ion Trap Mass Spectrometry. The kinetic data confirm the previously found dependence of the log of the rate constants upon the ligand ionization energy. To get some insight on the origin of the existing barrier to the reaction some experiments were performed at various temperatures; it was found that the kinetic constants increased when the temperature decreased. The results are compatible with an entropy effect on the free energy of activation as also shown by some calculations performed for the reaction of the C 5 H 5 Fe + ion with water.

The gas-phase chemistry of gaseous germane/ethene mixtures has been investigated by ab initio theoretical calculations and by experiments to examine the formation and growth of germanium-/carbon-containing species. Ion/molecule reactions in GeH4/C2H4 mixtures have been studied with an ion trap mass spectrometer. Ion abundance variations as a function of reaction time, reaction paths originating from primary ions of both reagents, and reaction rate constants of the main processes have been determined. The highest yield of new Ge−C bonds formed via reactions of Ge-containing ions with ethene molecules was obtained in mixtures carrying similar amounts of germane and ethene. Reactions of GeH2•+ with ethene play a prominent role in this system. High-level theoretical methods were therefore used to determine the geometrical structures and energies of transition structures, reaction intermediates, and final products for several reaction pathways. Formation of the adduct between GeH2•+ and H2CCH2 is the initial s...

Formation of the first ring structure from small aliphatic molecules is often regarded as the rate-determining step (and in any case as a step of paramount importance) in the combustive growth of larger aromatic systems and soot lamellae. Among possible reactions able to start off this process, the radical addition of the propargyl radical to butadiyne (diacetylene) has been recently studied theoretically [G. da Silva, A. Trevitt, Phys. Chem. Chem. Phys. 13 (2011) 8940] and shown to lead preferentially to a 5-membered ring intermediate. The same reaction is here addressed focusing instead on the possible formation of 6-membered rings. The large variety of partially intertwined reaction pathways is the basis for a subsequent RRKM study (carried out at different combustion temperatures). It confirms, on one hand, the favored formation of fulvenallenyl radical indicated by da Silva and Trevitt, but also indicates the possible formation of six-membered cyclic systems as minor products. In particular, at high pressure, six-membered ring system yields are very low, but at lower pressures a more significant yield of six-membered ring product is predicted.

We report on the successful use of a new zinc complex for the selective fluorescent detection of ADP and ATP in water. This is achieved by the complementary coordination of the phosphate groups to the metal centre and hydrogen bonding of the adenosine with the coordinated ligand.

The monoprotonated mechanism of the benzidine acid-catalysed rearrangement of hydrazobenzene (corresponding to a second order kinetics) is studied and compared with the diprotonated mechanism (corresponding to a third order reaction and previously studied). The nature of the two mechanisms is found to be completely different: a concerted closed-shell sigmatropic shift in the monoprotonated, a stepwise radical cation recoupling in the diprotonated. The activation energies, combined with the energetics of the protonation equilibria, are also very different: 35 kcal mol−1 for the former and 26 kcal mol−1 for the latter (in good agreement with the experimental data). These values make the third order diprotonated mechanism favoured with respect to the second order monoprotonated mechanism for the rearrangement of hydrazobenzene, as found at typical experimental acid concentrations.

The reaction of 1,4-diols with dimethyl carbonate in the presence of a base led to selective and high-yielding syntheses of related five-membered cyclic ethers. This synthetic pathway has the potential for a wide range of applications. Distinctive cyclic ethers and industrially relevant compounds were synthesized in quantitative yield. The reaction mechanism for the cyclization was investigated. Notably, the chirality of the starting material was maintained. DFT calculations indicated that the formation of five-membered cyclic ethers was energetically the most favorable pathway. Typically, the selectivity exhibited by these systems could be rationalized on the basis of hard-soft acid-base theory. Such principles were applicable as far as computed energy barriers were concerned, but in practice cyclization reactions were shown to be entropically driven.

Carbon nanoparticles, generated during combustion at relatively low [O2], or under pyrolysis conditions, can be seen both as soot precursors and as primary pollutants in themselves, since they are also directly emitted in the troposphere by vehicles. Soot particle inception (transition of relatively low-mass molecular systems from the gaseous phase to a solid nature) occurs at least in part via polycyclic aromatic hydrocarbon (PAH) coagulation/condensation. Complexes of different PAH systems, bound only by dispersion/multipolar forces, are investigated here by density functional theory, and their structural and energetic features discussed. The energetic features of the complexes allow to define an interplane interaction energy per C atom which compares satisfactorily with published experimental data on graphite exfoliation (i.e., removal of a single layer from the top of its bulk). The temperature dependence of the equilibrium K for these systems is then calculated to estimate the importance of PAH coagu...

Carbon nanoparticles, generated during combustion at relatively low [O2] or under pyrolysis conditions, can be seen both as soot precursors and as primary pollutants in themselves. Soot particle inception, with transition of relatively low-mass molecular systems from the gaseous phase to a solid nature through coagulation/condensation, is believed to take place via chemical reactions as well as van der Waals (vdW) interactions involving polycyclic aromatic hydrocarbons (PAHs) or derivatives. In this Article, radical addition between open and closed shell molecular PAH or PAH-like systems (R• + P → R–P•) is examined by density functional theory, and different σ bond formations are compared to the stacking of the aromatic components (R•:P, previously discussed in J. Phys. Chem. C2011, 115, 1732–1739). Energetic and entropic effects are examined. At higher T, formation of aliphatic bridges (hence reticulation) appears to be of the utmost importance to link PAH-like moieties, with a preference for more extend...