We present a high-level theoretical study on the gas-phase reaction between formaldehyde and hydroperoxyl radical carried out using the DFT-B3LYP, QCISD, and CCSD(T) theoretical approaches in connection with the 6-311+G(d,p), 6-311+G(2df,2p), and aug-cc-pVTZ basis sets. The most favorable reaction path begins with the formation of a pre-reactive complex and produces the peroxy radical CH(2)(OO)OH in a process that is computed to be exothermic by 16.8 kcal/mol. This reaction involves a process in which the oxygen terminal of the HO(2) moiety adds to the carbon of formaldehyde, and, simultaneously, the hydrogen of the hydroperoxyl group is transferred to the oxygen of the carbonyl in a proton-coupled electron-transfer mechanism. Our calculations show that this transition state lies below the sum of the energy of the reactants, and we computed a rate constant at 300 K of 9.29 x 10(-14) cm(3) molecule(-1) s(-1), which is in good agreement with the experimental results. Also of interest in combustion chemistry, we studied the hydrogen abstraction process by HO(2), the result of which is the formation of HCO + H(2)O(2). We found two reaction paths with activation enthalpies close to 12 kcal/mol. For this process, we computed a rate constant of 1.48 x 10(-16) cm(3) molecule(-1) s(-1) at 700 K, which also agrees quite well with experimental results.

Conformational preferences of 2,2′-bithiophene, 2-(2-thienyl)pyrrole, and N-methyl-2-(2-thienyl)pyrrole have been investigated by means of computational methods. Calculations were performed at the ab initio HF/6-31G(d) and MP2/6-31G(d) levels and, additionally, with the density functional B3-LYP/6-31G(d). The results indicate that 2-(2-thienyl)pyrrole behaves similarly to the 2,2′bithiophene. Thus, two minimum energy conformations were found for each compound, which correspond to anti-gauche and syn-gauche. Such minima are separated by barriers of about 1.7 and 1.3 kcal/mol at the HF and MP2 levels. On the contrary, the preferences found for N-methyl-2-(2-thienyl)pyrrole were different, giving an almost negligible energy barrier between the two minimum energy conformations. Furthermore, at the MP2 level the anti-gauche and syn-gauche minimum energy conformations present an inter-ring dihedral of 135°and 68°, respectively, displaying deviations greater than those found in 2,2′-bithiophene and 2-(2-thienyl)pyrrole. The conformational analysis was extended to the tricyclic compound N-methyl-2,5-di(2-thienyl)pyrrole. For this molecule, a contour map of the conformational energy as a function of the inter-ring dihedral angles was computed at the HF/6-31G(d) level. Minimum energy conformations were subsequently computed at the same level of theory. Results were in agreement with those obtained for the bicyclic compound N-methyl-2-(2-thienyl)pyrrole. Thus, the inter-ring dihedral angles of the minimum energy conformations present a large distortion with respect to the planarity, and such minimum energy conformations are separated by almost negligible energy barriers. Finally, N-hexyl-2,5-di-(2-thienyl)pyrrole radical cation was observed by EPR spectroscopy. This is the first time that a mixed trimer of thiophene and pyrrole is oxidized to its radical cation and detected by means of EPR spectroscopy. The spectrum seems to be symmetric, which is accounted for by a fast interconversion between conformers. This supports the small energy barrier calculated between minima for the bicyclic and tricyclic compounds. Overall, the results presented in this work indicate that N-methylpyrrole rings are able to induce large rotational deffects in [...-(pyrrole) n -(thiophene) n -...] block copolymers.

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