Hydrogen bond stabilization in Diels–Alder transition states: The cycloaddition of hydroxy-ortho-quinodimethane with fumaric acid and dimethylfumarate

DFT investigations on the mechanism of Diels–Alder reactions of a hydroxy-ortho-quinodimethane with fumaric acid derivatives were performed to understand the origin of the syn or anti configuration of the adducts. The diene hydroxyl group and the dieneophile carboxyl group show hydrogen bonding in the transition state, significantly favouring the syn product. This reaction is poorly diastereoselective for R = CO₂Me (ratio syn/anti = 57:43) and significantly improved for R = CO₂H (ratio syn/anti = 71:29). The stereoselectivities are properly predicted from transition structures calculated at the B3LYP/6-31G(d) level

Electrostatic control on endo/exo selectivity in ionic cycloaddition

DFT method has been used in combination with various basis sets to model the ionic cycloaddition of cationic heteroaromatic diene, 2,3-dimethylisoquinoliniuim ion with cyclopentadiene with a view to understand the factors that influence the stereochemical outcome of the reaction. Calculations show that this reaction is an inverse electron demand type reaction and it passes through highly asynchronous transition states and mainly electrostatic repulsion govern the endo/exo selectivity of the reaction. Endo TS is more destabilized than exo TS due to repulsion between positive charges delocalized over the two reacting partners in the transition state and hence, exo adduct is more preferred than endo adduct in total agreement with experiment

Possible ring structures of armchair single-walled carbon nanotubes

Energetics and the electronic structure of various types of single-walled carbon nanotubes have been investigated by using Density Functional Theory. Armchair [n,n], zigzag [n,0] and chiral [n,m] C₄₀H₂₀ nanotubes have been considered. Calculations show that the armchair isomer is the most stable among the three types and they further reveal the factors that stabilize this isomer. Nucleus-independent chemical shift calculations indicate the aromaticity of the individual hexagonal rings in the carbon nanotubes and explain the extent of electron delocalization in them

A DFT based ligand field study of the EPR spectra of Co(II) and Cu(II) porphyrins

Using a DFT based Ligand Field treatment (LFDFT) of the electronic structure of Co(II) and Cu(II) porphyrins (CoP and CuP) we analyse the origin of their EPR spectra. From a comparison between theoretical result on Co model clusters (CoP and CoP–ZnP dimer) we conclude that the g-tensor values are very sensitive to the axial coordination which stabilizes a ²A₁ ground state in good agreement with experimental data. In contrast, DFT overestimates Cu–ligand covalency, leading to large discrepancy with experiments, and hence the orbital contribution to the computed g-values is too small. Using a numerical adjustment of nuclear charge for Cu, a good agreement between the computed and the experimental g-tensor values is observed. The influence of the DFT functional on the calculated g-tensor is also discussed