Mechanisms for the reaction of thiophene and methylthiophene with singlet and triplet molecular oxygen.

Mechanisms for the reaction of thiophene and 2-methylthiophene with molecular oxygen on both the triplet and singlet potential energy surfaces (PESs) have been investigated using ab initio methods. Geometries of various stationary points involved in the complex reaction routes are optimized at the MP2/6-311++G(d, p) level. The barriers and energies of reaction for all product channels were refined using single-point calculations at the G4MP2 level of theory. For thiophene, CCSD(T) single point energies were also determined for comparison with the G4MP2 energies. Thiophene and 2-methylthiophene were shown to react with O(2) via two types of mechanisms, namely, direct hydrogen abstraction and addition/elimination. The barriers for reaction with triplet oxygen are all significantly large (i.e., >30 kcal mol(-1)), indicating that the direct oxidation of thiophene by ground state oxygen might be important only in high temperature processes. Reaction of thiophene with singlet oxygen via a 2 + 4 cycloaddition leading to endoperoxides is the most favorable channel. Moreover, it was found that alkylation of the thiophene ring (i.e., methyl-substituted thiophene) is capable of lowering the barrier height for the addition pathway. The implication of the current theoretical results may shed new light on the initiation mechanisms for combustion of asphaltenes.

(rac)-1,1'-binaphthyl-based simple receptors designed for fluorometric discrimination of maleic and fumaric acids.

(rac)-1,1'-Binaphthyl-based simple receptors 1 and 2 have been designed, synthesized and studied theoretically. The receptors utilize naphthyridine as the binding motifs for complexation of dicarboxylic acids in CHCl(3). The emission of the BINOL moiety was monitored experimentally to ascertain the selectivity and sensitivity of the receptors. Receptor 1 distinguishes maleic acid from isomeric fumaric acid by exhibiting different fluorescence behavior and demonstrates stronger binding in the excited state. Modulation of the binding sites of 1 leads to a new receptor structure 2, which was found to be less efficient in distinguishing maleic from fumaric acid, fluorometrically. Both 1 and 2 also recognize other hydroxy di- and tricarboxylic acids. The binding interactions were monitored by (1)H NMR, fluorescence and UV-vis spectroscopic methods. Structures of apo-hosts, guests and host-guest complexes were determined using force-field based conformational searching. Low energy ensembles were grouped into geometrically similar families, and low energy structures from each family were verified using B3LYP/6-31G*/PB-SCRF(CHCl(3)) calculations. The atomistic calculations provide insight into the differential dicarboxylic acid binding behavior of receptors 1 and 2.