A theoretical study of the chemical reaction between ethylene and N2H2

The interaction between the molecules ethylene and cis-N₂H₂ has been studied using a gaussian basis in a series of ab initio SCF calculations. The results obtained indicate that the synchronous hydrogen transfer reaction is a one-step reaction having an activation energy of around 60 kcal/mol. Our results do not lend support to the hypothesis that the rate of the overall reaction between C₂H₄ and N₂H₂ is controlled by the rate of isomerization of trans-diimide to the cis form.     

A quantum chemical investigation of pyrolysis reactions of glyoxylic acid ethylester

Two possible reaction paths for the pyrolysis of the ethylester of glyoxylic acid have been studied by ab initio molecular orbital calculations. The basis sets 3-21G and 6-31G * have been used, and electron correlation has been included by Møller–Plesset calculations up to fourth order. Our calculations indicate that the reaction leading to acid and ethylene through a 6-membered ring transition state is favored relative to a process involving a formyl hydrogen transfer via a 5-membered ring to the alkyl unit leading to ethane, CO, and CO₂. The predicted activation energies for these two reactions obtained at the highest level of calculation, MP 4(SDTQ )/6–31G *, are 50.4 and 71.7 kcal/mol, respectively. The transition states have RHF wave functions that are stable relative to UHF solutions using the 3–21G basis. The geometry of the transition states and IRC following indicate that both reactions are strongly asynchronous: The C? O bond rupture is virtually completed before hydrogen transfer occurs. For comparative purposes, analogous calculations have been performed for the ethylester of formic acid, where it is confirmed that a 6-membered ring transition state is preferred relative to a 4-membered one by around 42 kcal/mol at the highest level of calculation.     

Spin squeezing and quantum correlations

We discuss the notion of spin squeezing considering two mutually exclusive classes of spin-s states, namely, oriented and non-oriented states. Our analysis shows that the oriented states are not squeezed while non-oriented states exhibit squeezing. We also present a new scheme for construction of spin-s states using 2s spinors oriented along different axes. Taking the case of s=1, we show that the ‘non-oriented’ nature and hence squeezing arise from the intrinsic quantum correlations that exist among the spinors in the coupled state.     

On the interplay between theory and experiment in molecular structure problems: Some case studies

Some brief, general comments on the concept of molecular structure will be given. Some important points connected to the use of models in the interpretation of molecular structures will also be mentioned. The main theme in one part of the presentation will be the cooperative efforts made by experimentalists (mainly spectroscopists) and theoreticians (computational chemists) in order to measure, predict, and analyze the perturbations on the cyclopropane ring by different substituents. The aim is to demonstrate that ab initio calculations of a certain quality can constitute an important support for experimental studies in that they are able to discriminate between different models that otherwise are equally probable. The second part of the presentation will be concerned with a class of molecules that gives both experimental structural chemists and computational chemists a great challenge, namely, the metallocenes. A discussion of some of the grave discrepancies between theory and experiment regarding their geometry will be given.     

A theoretical study of the strcture of (NO)2

Ab initio SCF calculations on the cis form of nitric oxide dimer, (NO)₂, indicate that the NN bond distance is 1.74 Å, the NO bond distance is 1.16 Å, and the angle NNO is 107°.     

A force field study of the chemical reaction between ethylene and cis-N2H2

The fully optimized geometry of the activated complex which occurs as an intermediate in the concerted H-transfer reaction between C₂H₄ and cis-N₂H₂ has been determined using the ab initio FORCE method of Pulay. The activation energy for the synchronous transfer of two hydrogen atoms from cis-N₂H₂ to ethylene is found to be 18.8kcal/mol, i.e. substantially lower than the previously estimated energy barrier of around 60 kcal/mol. The same method applied to trans-N₂H₂ and semilinear N₂H₂ gave an isomerization energy of 49.7 kcal/mol indicating that the isomerization of trans-N₂H₂ to the cis form might be the overall rate-controlling step.     

A molecular orbital study of the cis-trans energy difference in glyoxal

The barrier to internal rotation around the C-C bond in glyoxal, and the energy difference between the cis and the trans form of the molecule have been estimated using (7,$\text{3}\text{4}$) and (9,$\text{5}\text{4}$) Gaussian basis sets including polarization functions. The predicted energy of the cis form relative the ground state trans form was found to be substantially higher (4.9–6.3 kcal mol^?1) than the experimental value of 3.2 kcal mol^?1.     

Evaluation of strain in heterosiliranes: Systematics,surprises, and problems

A series of different isodesmic and homodesmic reactions defining strain energies in 3-membered rings are described and discussed. One isodesmic and one homodesmic reaction are applied numerically with the purpose of estimating strain in heterosiliranes including second-row and third-row heteroatoms. All molecules involved are optimized using SCF /6-31G **, and energies are calculated using MP 2/6-31G **//SCF /6-31G ** calculations. The results are discussed with reference to limitations of the chosen models. © 1996 John Wiley & Sons, Inc.     

Structural aspects of a possible transannular interaction in silatranes and azasilatranes: An ab initio study

Complete geometry optimizations using ab initio SCF/3-21G* calculations were performed on silatrane and azasilatrane, on their fluoro derivatives, and on methylsilatrane. By comparison with optimized geometries obtained for a series of model systems, the predicted transannular internuclear distances were interpreted in terms of the combined effects of electronegative substituents on Si and the anomeric interaction in the silatranes. The predicted geometries indicate that there is a weak transannular interaction in silatrane and a more significant one in azasilatrane, the predicted equilibrium distances being 2.66 and 2.15 Å, respectively. © 1996 John Wiley & Sons, Inc.