A theoretical study on the molecular and electronic structures and the enthalpies of formation of several polychlorinated dibenzo-p-dioxins, PCDDs, has been carried out using DFT at the B3LYP/6-31G(d) and B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) levels of theory. The calculated molecular structures are planar in all of the studied PCDDs, in agreement with the available X-ray diffraction data. NBO charges, and HOMO and LUMO energies have also been obtained and discussed. The enthalpies of formation have been calculated through isodesmic reactions, and the values obtained have been compared with the few available experimental values and with values estimated by other authors using semiempirical methods or group additivity approaches.

ABSTRACT:The gas-phase thermal decomposition of 3-hydroxypropionitrile, 3-hydroxybutyronitrile, and 3-hydroxy-3-methylbutyronitrile has been studied at the MP2/6-31G(d) level of theory at 683.15 K and 0.06 atm. Results based both in energy and structure data seem to indicate a favorable route of decomposition via a six-membered cyclic transition state (similar to those suggested for thermal decomposition of other related compounds, such as ␤-hydroxyketones, ␤-hydroxyalkenes, and ␤-hydroxyalkynes) rather than a four-membered cyclic transition state or even a quasiheterolytic pathway. Wiley Periodicals, Inc. Int J Quantum Chem 91: 618 -625, 2003

No abstract

Theoretical studies of the thermolysis of two a-amino acid ethyl esters in the gas phase were carried out using ab initio theoretical methods, at the HF/6-31G(d) and the MP2/6-311 G(2d,p)//MP2/6-31G(d) levels of theory. The reactions studied have two steps: the first one corresponds to the formation of ethylene and a neutral amino acid intermediate via a six-membered cyclic transition state, and the second is the rapid decarboxylation of this intermediate via a five-membered cyclic transition state. The progress of the first step of the reactions was followed by means of the Wiberg bond indices. The results indicate that the transition states have an intermediate character between reactants and products, and the calculated synchronicities show that the reactions are concerted and slightly asynchronous. The bond-breaking processes are more advanced than the bond-forming processes, indicating a bond deficiency in the transition states. The kinetic parameters calculated for both reactions agree very well with the available experimental results.