AM1 computational study of cycloaddition reaction of vinyl sulfene generated from thiete 1,1-dioxide

Theoretical studies of the cycloaddition reaction between vinyl sulfene, norbornene and related systems are presented. Since the parameterized AMI semiempirical method was used, the generated data are only of qualitative value. The vinyl sulfene reactivity was assessed by using frontier molecular orbital (FMO) energy gaps, bond orders, and the charge distribution of the reactants. To obtain more information, as far as the reaction outcome and its feasibility, the transition state structures and activation barriers for competitive pathways are computed.     

An ab initio study of heterodienophiles addition to 2,3-diaza-1,3-butadiene: An example of endo-lone-pair effect on the reaction energy barrier

Transition states for the Diels-Alder reactions of 2,3-diaza-1,3-butadiene with ethylene, formaldehyde, formaldimine, cis- and trans- diazene, and nitrosyl hydride were located by ab initio molecular orbital calculations. The bond orders of the new forming bonds have been used to determine the asynchronicity of the reactions. Ab initio calculations show that the energy barrier for the hetero-Diels-Alder reactions is relatively high. The highest energy barrier of 34.76 kcal/mol calculated at the MP4/6-31G*//MP2/6-31G* level was found for the exo-cis-diazene addition to 2,3-diaza-1,3-butadiene. In all cases, when two diastereomeric transition structures are possible, the one with the endo hydrogen, exo lone pair was predicted to have a lower activation barrier. This behavior can be explained by the n-π and n-n loan pair repulsion interaction between the dienophile and diene heteroatoms in the corresponding transition state. The barrier is higher for those reactions which in the transition state have more lone electron pairs. Also, the barrier is higher when the lone pairs are endo oriented than when they are exo oriented in the transition state. © 1996 by John Wiley & Sons, Inc.     

Density functional calculations of difluorodiazete structures with Gaussian-orbital-type approach

The comparison of local nonlocal and hybrid DFT methods with RHF , MP 2, CCSD , and CPF ab initio methods in generating geometries and relative energies of cis- and trans-difluorodiazete, SVWN , BLYP , BP 86, BECKE 3LYP , and BECKE 3P 86 DFT methods with 6-311 + g(2d) and 6-311 + + g(3df) basis sets. The geometries generated with RHF ab initio models are quite different from experimental values and energy evaluation prefers the wrong isomer. The hybrid methods give superior geometries while energies evaluated with nonlocal DFT methods are better than the one obtained with MP 2 or CCDS ab initio methods. The results suggest DFT as the method of choice of studying similar systems. © 1996 John Wiley & Sons, Inc.     

Highlevel ab initio and hybrid density functional theory study of the energy profile for the CO+CO reaction

Two complete basis set and three hybrid density functional computational studies were applied in the exploration of the ¹CO+²CO⁺ reaction potential energy surface. One molecular carbon monoxide–carbon monoxide cation molecular associate was elucidated as the structure with the lowest energy on the potential energy surface. Ionization energies, bond dissociation energies, and enthalpies of formation for every di and tri-atomic molecule on the potential energy surface were estimated with the two complete basis sets and the three hybrid density functional theory methods. Six different endothermic channels for the ¹CO+²CO⁺ reaction were evaluated with ab initio and DFT methods. The computed energies and structural parameters are compared with experimental values where available. Some new energies for this reaction system were suggested.     

High level of ab initio and density functional theory evaluation of the C?O bond dissociation energies in the dimethyl ether anion

The computational study of four possible first steps for the Wittig rearrangement of the dimethyl ether anion was investigated with a highly accurate complete basis set ab initio and density functional theory method. The initial step in all of these pathways is the C O bond breaking. The energies for these paths were computed and compared with the discussion of the mechanism of the Wittig [1, 2]‐rearrangement. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 73: 299–306, 1999     

Complete basis set ab initio and hybrid density functional theory exploration of the potential energy surface in the reaction between an amino radical and nitrogen oxide

The potential energy surface for the reaction involving NH₂ plus NO was explored with a quadratic complete basis set ab initio approach and three hybrid density functional theory methods, the target being to accurately estimate activation barriers and the relative stability of the nitrogen–oxygen isomers. The computational results were compared with previously performed ab initio calculations and new, more accredited values for the NH₂NO rearrangement to HNNOH and for the HNNOH decomposition reaction were suggested. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 409–414, 1998     

Density functional theory and quadratic complete basis set ab initio studies of the HNSi to HSiN isomerization and hydrogen insertion reactions with further isomerizations of the insertion products

Ab initio and density functional theory (DFT) computational studies were performed in order to determine the potential energy profiles for the inter-conversions of the small silicon–nitrogen built chemical systems: HSiN, HNSi, HSiNH₂, and HNSiH₂. Three hybrid DFT methods demonstrated high reliability in computing structures and potential energy profiles for these chemical transformations. The stability of some of these molecular systems and their interconversions was discussed and computational results were compared with experimental observations when available.     

Transition metal free reductive dimerization of nitrogen containing barbituric acid benzylidenes

Through the NMR monitoring of the reactive intermediates in the condensation of barbituric acid with nitrogen containing heterocyclic 2‐carboxaldehydes, a synthetic procedure was developed for the preparation of each of the intermediates. The simple high yield procedure for the preparation of the reductive dimer from corresponding barbituric acid benzylidenes was developed, although we were not able to elucidate the source of the reduction. The structure of the dimer was confirmed by X‐ray analysis.