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).

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(•).

Polycyclic aromatic hydrocarbon formation mechanism in the "particle phase". A theoretical study.

The synthesis of polycyclic aromatic hydrocarbons (PAHs) and the formation of soot platelets occur both during combustion at relatively low [O(2)], or under pyrolysis conditions. When the PAH size grows beyond the number of three-four condensed cycles, the partitioning of PAHs between the gas and particle phases favours the latter (i.e. adsorption). This study aims to assess which role the soot particle plays during PAH synthesis, in particular if catalytic or template effects of some sort can be exerted by the soot platelet on the adsorbed growing PAH-like radical. Our theoretical calculations indicate that chain elongation by ethyne addition cannot compete with cyclization when both can take place in the growing PAH-like radical, already in the gas phase. When it is adsorbed, cyclization is found to become easier than in the gas phase (more so, in terms of Gibbs free energy barriers, at higher temperatures), hinting at some sort of template effect, while chain elongation by ethyne addition becomes somewhat more difficult. The underlying soot platelet can assist (at lower temperatures) the formation of a larger aromatic hydrocarbon, by a final hydrogen abstraction from that endocyclic saturated carbon the newly formed cycle still bears. As an alternative (at higher temperature), a spontaneous hydrogen atom loss can take place. Finally, at rather low temperatures, the addition of the growing radical to the underlying soot platelet might occur and cause some reticulation, form more disordered structures, i.e. soot precursors instead of PAHs.

The reaction of N2O with phenylium ions C6(H,D)5(+): an integrated experimental and theoretical mechanistic study.

The reaction of N(2)O (known to be an O atom donor under several conditions) with the phenyl cation is studied by experimental and theoretical methods. Phenyl cation (or phenylium), C(6)H(5)(+), and its perdeuterated derivative C(6)D(5)(+) are produced either by electron impact or by atmospheric pressure chemical ionization of adequate neutral precursors, and product mass spectra are measured in a guided ion beam tandem mass spectrometer. The ions C(5)(H,D)(5)(+), C(6)(H,D)(5)O(+), and C(3)(H,D)(3)(+) are experimentally detected as the most relevant reaction products. In addition, the detection of the adduct (C(6)H(5)N(2)O)(+), which is collisionally stabilized in the scattering cell of the mass spectrometer, is reported here for the first time. The reaction pathways, which could bring about the formation of the mentioned ions, are then explored extensively by density functional theory and, for the more promising pathways, by CASPT2/CASSCF calculations. The two reacting species (1) form initially a phenoxydiazonium adduct, C(6)H(5)ON(2)(+) (2a), by involving the empty in-plane hybrid C orbital of phenylium. The alternative attack to the ring pi system to produce an epoxidic adduct 2c is ruled out on the basis of the energetics. Then, 2a loses N(2) quite easily, thus affording the phenoxyl cation 3. This is only the first of several C(6)H(5)O(+) isomers (4-6 and 8-12), which can stem from 3 upon different cleavages and formations of C-C bond and/or H shifts. As regards the formation of C(5)H(5)(+), among several conceivable pathways, a direct CO extrusion from 3 is discarded, while others appear to be viable to different extents, depending on the initial energy of the system. The easiest CO loss is from 4, with formation of the cyclopentadienyl cation 7. Formation of C(3)H(3)(+) is generally hindered and its detection depends again on the availability of some extra initial energy.

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.

PAHs 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 O2 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.

Ozone interaction with polycyclic aromatic hydrocarbons and soot in atmospheric processes: theoretical density functional study by molecular and periodic methodologies.

The ozonization mechanism for polycyclic aromatic hydrocarbons (PAHs) and soot is investigated by quantum mechanical calculations carried out on molecular and periodic systems. PAHs, interesting per se, serve also to model the local features of the graphenic soot platelets, for which another model is provided by a periodic representation of one graphenic layer. A concerted addition leads to a primary ozonide, while a nonconcerted attack produces a trioxyl diradical (in which one of the two unpaired electrons is pi-delocalized). Easy loss of (i) (1)O(2) or (ii) (3)O(2) from either intermediate, with spin conservation, would yield stable (i) singlet or (ii) triplet pi-delocalized species which carry an epoxide group. The trioxyl diradical pathway is estimated to be preferred, in these systems. An intersystem crossing, taking place in the trioxyl diradicals, can be invoked to allow the even easier loss of a ground-state oxygen molecule with the formation of a ground-state epoxide in a more exoergic and less demanding step. We propose that soot ozonization can take place by such a process, with ultimate functionalization of the graphenic platelets by epoxide groups.