Tropospheric Photochemistry of 2-Butenedial: Role of the Triplet States, CO and Acrolein Formation, and the Experimentally Unidentified Carbonyl Compound-Theoretical Study.

Solar irradiation of 2-butenedial in the lower troposphere mainly produces isomeric ketene-enol (a key intermediate product), furanones, and maleic anhydride, the formation pathways of which were investigated in a previous study. The other main products were carbon monoxide and an experimentally unidentified carbonyl compound. This was the subject of the present study. The oxidative reaction mechanisms were studied using DFT calculations. Water intervention is found essential. Its addition and subsequent water-assisted isomerizations (an ene-gem-diol/enol and a carboxylic acid/enol form), followed by cyclization, lead to an interesting cyclic carbonyl compound, but this pathway appears to be rather energy demanding. An alternative implies water cooperation in a ketene-enol + carboxylic acid/enol addition that gives the relevant anhydride. The anhydride is proposed as a candidate for the experimentally unidentified carbonyl product. Regarding CO and acrolein formation, the role of the triplet states, as defined by the probability of intersystem crossing from the excited singlet state S1 to T2 and T1, is discussed. The T1 photolysis pathway connecting butenedial to propenal + CO was then defined.

Mechanism of the Photochemical Isomerization and Oxidation of 2-Butenedial: A Theoretical Study.

Under tropospheric conditions, 2-butenedial is photochemically removed to produce secondary organic aerosol. Upon solar irradiation in the lower troposphere, the main photochemical products are ketene-enol (a key intermediate product), furanones, and maleic anhydride. The oxidative reaction mechanism was studied using the multireference method CASSCF to explore the hypersurface of the two most accessible singlet excited states, and by DFT for the ground state. Photoisomerization of 2-butenedial in the first excited state directly produces ground state ketene-enol upon nonradiative relaxation. From this intermediate, furan-2-ol and successively 3H-furan-2-one and 5H-furan-2-one are formed. The cooperative effect of two water molecules is essential to catalyze the cyclization of ketene-enol to furan-2-ol, followed by hydrogen transfers to furanones. Two water molecules are also necessary to form maleic anhydride from furan-2-ol. For this last reaction, in which one extra oxygen must be acquired, we hypothesize a mechanism with singlet oxygen as the oxidant.

One-Pot, Telescoped Alkenylation of Amides via Stable Tetrahedral Intermediates as Lithium Enolate Precursors.

A mild and efficient telescoped procedure for the stereoselective alkenylation of simple, non-activated amides using LiCH2SiMe3 and carbonyl compounds as surrogates of alkenyllithium reagents is reported. Our methodology relies on the formation of stable tetrahedral intermediates, which, upon collapse into highly reactive lithium enolates in a solvent-dependent fashion, allows for the assembly of α,ß-unsaturated ketones in a single synthetic operation with high stereoselectivity.

Gold(I)-Catalyzed Reactivity of Furan-ynes with N-Oxides: Synthesis of Substituted 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.

A Fast and General Route to Ketones from Amides and Organolithium Compounds under Aerobic Conditions: Synthetic and Mechanistic Aspects.

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.

Multireference Study of the H2COO (Criegee Intermediate) + O3 Addition: A Reaction of Possible Tropospheric Interest.

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) geometry optimizations, focused on the initial addition step, show that the most likely pathway is not a concerted cycloaddition to give directly a c-H2CO5 intermediate, as had been proposed. Concerted cycloaddition is in fact related to a second-order saddle point that puts in turn into relation, obliquely, two distinct diradicaloid transition structures. While O-O addition goes through an energy demanding pathway, C-O addition, potentially producing the open-chain •OOCH2-OOO• diradical intermediate, corresponds to a more viable pathway. CASPT2 and MRCI optimizations, though predicting an addition diradicaloid pathway, describe •OOCH2-OOO• as an unstable structure, because fragmentation to •OOCH2-O• + O2 takes place past the addition TS without any energy barrier: ring closure to c-H2CO5 is thus prevented. The addition E barrier drops as the methods become more demanding, with CASPT3//PT2 at 12.8 kcal mol-1, MRCI at 9.1 kcal mol-1, and MRCC at 4.8 kcal mol-1 (possibly suggesting a significant role of dynamical correlation). Recent experimental data by Chang et al. qualify this addition as a fairly fast reaction.

Comprehensive study on the degradation of ochratoxin A in water by spectroscopic techniques and DFT calculations.

Ochratoxin 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. pK a1,2 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.

Effects of collision energy and vibrational excitation of CH3+ cations on its reactivity with hydrocarbons: But-2-yne CH3CCCH3 as reagent partner.

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. The two most abundant products result from electron and hydride transfer, occurring via direct and barrierless mechanisms, while other channels are initiated by the electrophilic addition of the methyl cation to the triple bond of but-2-yne. Among the minor channels, special relevance is placed on the formation of C5H7+, stemming from H2 loss from the addition complex. This is the only observed condensation product with the formation of new C-C bonds, and it might represent a viable pathway for the synthesis of complex organic species in astronomical environments and laboratory plasmas.

Combustive, Postcombustive, and Tropospheric Butadiyne Oxidation by O2, Following Initial HO Attack. Theoretical Study.

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.

Tuning of the Electronic Properties of Armchair Graphene Nanoribbons through Functionalization: Theoretical Study of (1)Δg O2 Border Addition.

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.

Antagonistic functionalized nucleation and oxidative degradation in combustive formation of pyrene-based clusters mediated by triplet O and O2 : theoretical study.

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.

o-Benzyne fragmentation and isomerization pathways: a CASPT2 study.

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 temperatures <1200 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 temperatures >1400 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 temperatures >2400 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.

First ring formation by radical addition of propargyl to but-1-ene-3-yne in combustion. Theoretical study of the C7H7 radical system.

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.

Memory effects in carbocation rearrangements: structural and dynamic study of the norborn-2-en-7-ylmethyl-X solvolysis case.

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).

Growth of polyphenyls via ion-molecule reactions: an experimental and theoretical mechanistic study.

The reactivity of biphenylium cations C12H9(+) with benzene C6H6 is investigated in a joint experimental and theoretical approach. Experiments are performed by using a triple quadruple mass spectrometer equipped with an atmospheric pressure chemical ion source to generate C12H9(+) via dissociative ionization of various isomers of the neutral precursor hydroxybiphenyl (C12H10O). C-C coupling reactions leading to hydrocarbon growth are observed. The most abundant ionic products are C18H15(+), C18H13(+), C17H12(+), and C8H7(+). The dependence of product ion yields on the kinetic energy of reagent ions, as well as further experiments performed using partial isotopic labelling of reagents, support the idea that the reaction proceeds via a long lived association product, presumably the covalently bound protonated terphenyl C18H15(+). Its formation is found to be exothermic and barrierless and, therefore, might occur under the low pressure and temperature conditions typical of planetary atmospheres and the interstellar medium. Theoretical calculations have focussed on the channel leading to C8H7(+) plus C10H8, identifying, as the most probable fragments, the phenylethen-1-ylium cation and naphthalene, thus suggesting that the pathway leading to them might be of particular interest for the synthesis of polycyclic aromatic hydrocarbons. Both experiments and theory agree in finding this channel exoergic but hampered by small barriers of 2.7 and 3.7 kcal mol(-1) on the singlet potential energy surface.

Density functional theory study of the interaction of vinyl radical, ethyne, and ethene with benzene, aimed to define an affordable computational level to investigate stability trends in large van der Waals complexes.

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).

Selective detection of ATP and ADP in aqueous solution by using a fluorescent zinc receptor.

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.

Synthesis of five-membered cyclic ethers by reaction of 1,4-diols with dimethyl carbonate.

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.

Border reactivity of polycyclic aromatic hydrocarbons and soot platelets toward ozone. A theoretical study.

PAH-based models, with an even or odd number of unsaturated carbon atoms and π 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(•).

Growth of polyaromatic molecules via ion-molecule reactions: an experimental and theoretical mechanistic study.

The reactivity of naphthyl cations with benzene is investigated in a joint experimental and theoretical approach. Experiments are performed by using guided ion beam tandem mass spectrometers equipped with electron impact or atmospheric pressure chemical ion sources to generate C(10)H(7)(+) with different amounts of internal excitation. Under single collision conditions, C-C coupling reactions leading to hydrocarbon growth are observed. The most abundant ionic products are C(16)H(13)(+), C(16)H(n)(+) (with n=10-12), and C(15)H(10)(+). From pressure-dependent measurements, absolute cross sections of 1.0±0.3 and 2±0.6 Å(2) (at a collision energy of about 0.2 eV in the center of mass frame) are derived for channels leading to the formation of C(16)H(12)(+) and C(15)H(10)(+) ions, respectively. From cross section values a phenomenological total rate constant k=(5.8±1.9)×10(-11) cm(3) s(-1) at an average collision energy of about 0.27 eV can be estimated for the process C(10)H(7)(+)+C(6)H(6)→all products. The energy behavior of the reactive cross sections, as well as further experiments performed using partial isotopic labeling of reagents, support the idea that the reaction proceeds via a long lived association product, presumably the covalently bound protonated phenylnaphthalene, from which lighter species are generated by elimination of neutral fragments (H, H(2), CH(3)). A major signal relevant to the fragmentation of the initial adduct C(16)H(13)(+) belongs to C(15)H(10)(+). Since it is not obvious how CH(3) loss from C(16)H(13)(+) can take place to form the C(15)H(10)(+) radical cation, a theoretical investigation focuses on possible unimolecular transformations apt to produce it. Naphthylium can act as an electrophile and add to the π system of benzene, leading to a barrierless formation of the ionic adduct with an exothermicity of about 53 kcal mol(-1). From this structure, an intramolecular electrophilic addition followed by H shifts and ring opening steps leads to an overall exothermic loss (-7.1 kcal mol(-1) with respect to reagents) of the methyl radical from that part of the system which comes from benzene. Methyl loss can take place also from the "naphthyl" part, though via an endoergic route. Experimental and theoretical results show that an ionic route is viable for the growth of polycyclic aromatic species by association of smaller building blocks (naphthyl and phenyl rings) and this may be of particular relevance for understanding the formation of large molecules in ionized gases.