The formation mechanism of polycyclic aromatic hydrocarbons (PAH) with three fused aromatic rings starting from naphthalene has been studied using accurate ab initio G3(MP2,CC)//B3LYP/6-311G** calculations followed by the kinetic analysis of various reaction pathways and computations of relative product yields. The results reveal new insights into the classical hydrogen abstraction-C2H2 addition (HACA) scheme of PAH growth. The HACA mechanism has been shown to produce mostly cyclopentafused PAHs instead of PAHs with six-member rings only, in contrast to the generally accepted view on this mechanism. Considering naphthalene as the initial reactant, the HACA-type synthesis of higher PAHs with all six-member rings, anthracene and phenanthrene, accounts only for 3-6% of the total product yield at temperatures relevant to combustion (1000-2000 K), whereas cyclopentafused PAHs, including acenaphthalene (41-48%), 4-ethynylacenaphthalene (∼14%), 3-ethynylacenaphthalene (∼7.5%), 1-methylene-1H-cyclopenta[b]naphthalene (∼6%), and 3-methylene-3H-cyclopenta[a]naphthalene (∼5%), account for another ∼75%. It has been shown that acetylene addition to the radical site adjacent to the bay region in naphthalene (as in 1-naphthyl radical) or other similar PAH with a bay region is highly unlikely to be followed by the addition of a second acetylene molecule; alternatively, the bay region closure with a buildup of a new five-member ring occurs. Acetylene addition to a nonbay carbon atom (as in 2-naphthyl radical) can be followed by the second acetylene addition only at T < 1000 K, producing anthracene and phenanthrene. However, at temperatures relevant to combustion, such pathways give negligible contributions to the total product yield, whereas the dominant reaction product, 2-ethynylnaphthalene, is formed by simple hydrogen atom elimination from the attached ethenyl group. An additional six-member ring buildup may occur only after intermolecular hydrogen abstraction from ethynyl-substituted PAH (2-ethynylnaphthalene), in particular, from the carbon atoms adjacent to the existing ethynyl (C2H) fragment, followed by C2H2 addition producing adducts with two ethynyl C2H and ethenyl C2H2 groups next to each other, which then undergo a fast six-member ring closure. Nevertheless, this process has been shown to be relatively minor (∼25%), whereas the major process is a five-member ring closure involving the same C2H and C2H2 groups and leading to a cyclopentafused PAH molecule. Although the computed product yields show a good agreement with experimentally observed concentrations of acenaphthalene and anthracene in various aliphatic and aromatic flames, the yield of phenanthrene, which exhibits an order of magnitude higher concentration than anthracene both in combustion flames and environmental mixtures, via the considered pathways is significantly underpredicted. This result points at the possible existence of another mechanism responsible for the formation of phenanthrene and other all-six-member-ring PAHs. The overall kinetic scheme for...
Kinetics of N-acylation of glycine, L-a-alanine, L-valine, L-leucine, and DL-isoleucine with 4-nitrophenyl benzoate in a water32-propanol solvent at various temperatures were studied. The activation energy, enthalpy, and entropy of the process were determined. Correlations of the N-acylation rate constants of the a-amino acids with the composition of the binary solvent at various temperatures were established, and the N-acylation rate constants of the a-amino acids in water were determined.Kinetics of ammonolysis of esters in nonaqueous media have been studied in [1,2]. There have been a number of works devoted to the effect of the structure of the RCO radical in ROX on the reactions of phenyl esters and derivatives of aliphatic carboxylic acids with various amines in dioxane [335]. Over the past years we have studied the kinetics of N-acylation of a-amino acids with benzoyl chloride in water3dioxane[638] and with carboxylic acid esters in water3 2-propanol, water32-methyl-2-propanol, and water3 acetonitrile [9, 10].The present work deals with the kinetics of N-acylation of glycine (Gly), L-a-alanine (L-Ala), L-valine (L-Val), L-leucine (L-Leu), and DL-isoleucine (DL-ILe) with 4-nitrophenyl benzoate (I) in water3 2-propanol.The N-acylation occurs at pH 839.5 by scheme (1).The reaction was performed with a considerable excess of an a-amino acid (~10 !2 M) with respect to ester I (~10 !5 M). The concentration of the anionic form of the a-amino acid was created by adding a specified amount of NaOH into the reaction mixture. We found that at a 1 : 431 : 10 ratio of the anionic (c a ) and zwitter ionic forms (c zi ) of the a-amino acid the rate of hydrolysis of ester I in water32-propanol can be neglected. The rate of reaction (1) follows the firstorder kinetic law (apparent rate constant k ap ). The second-order rate constant k ac of reaction (1) was calculated by Eq. (2). k a = k ap /c a .(2) Table 1 lists the acylation rate constants k ac of the amino acids studied with ester I in water32-propanol. By the ability for acylation, the a-amino acids can be arranged in the following order: Gly > L-Ala > L-Leu > DL-Ile > L-Val. As seen from Table 1, the N-acylation rate increases with increasing water fraction in the binary solvent, which is probably explained by formation of more reactive complexes II and III of the a-amino acids and ester I with the OH groups of solvent components R`OH (R`= H, i-Pr) via hydrogen bonding. The formation of H-complexes II and III should increase the negative charge of the a-amino acid nitrogen and the positive charge on the ester carbonyl carbon, which favors N3C bond formation and accelerates the reaction.
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