Metal-catalyzed aziridination of alkenes by organic azides: a mechanistic DFT investigation

Structural Chemistry - The DFT B3LYP/6–31G(d,p) approach is used to study alkene aziridination by azides through catalyzed routes involving a metal nitrenoid intermediate. The catalysts...     

Algebraic expressions for effective potential characteristic parameters in heavy ion scattering

In this paper, we obtain reliable expressions to calculate the barrier and pocket positions of the real part of the effective phenomenological optical potential having Woods-Saxon form factor, for different partial waves. The comparison of the results obtained from these formulae, when compared with the numerical results obtained using Newton-Raphson iterative procedure are found to be quite accurate, with error less than 1%. We also obtain algebraic expressions for estimatingl _poc, the angular momentum at which the potential pocket vanishes, the accuracy of which is tested with the exact calculations, using self-consistent iterative procedures. These and other expressions deduced in this paper provide simple and useful methods for calculating critical parameters of heavy ion effective potentials like barrier and pocket positions, curvatures at the barrier and pocket positions,l _poc and the grazing angular momentuml _g to carry out the analysis of heavy ion scattering.     

Ozonolysis of methyl, amino and nitro substituted ethenes: A semi-empirical molecular orbital study

The three-step ozonolysis reaction is studied for a number of methyl, amino and nitro substituted ethenes (classified as symmetrically, asymmetrically and cis/trans substituted) using the PM3 SCF-MO method. Substituent effects are predicted to generally yield the order NH₂ > Me > NO₂ for the ability to enhance facility of ozonolysis, in line with the electrophilic nature of ozone. Geometry and conformation allow for a variety of different pathways, and the lowest energy pathway is predicted for each case, with consequences for identity of the intermediates preferentially involved. Greater stability of the trans isomer of the carbonyl oxide intermediate is the main factor for its preferred involvement in the second step of the reaction. For the cis/trans substituted ethenes, the major product (the secondary ozonide) is predicted as being the cis ozonide and the trans ozonide for the cis and the trans substituted ethenes, respectively.     

Lantern-Type Divanadium Complexes with Bridging Ligands: Short Metal−Metal Bonds with High Multiple Bond Orders

Vanadium forms binuclear complexes with a variety of ligands often containing V≡V triple bonds. Many tetragonal divanadium paddlewheel complexes with bridging bidentate ligands have been experimentally characterized. This research exhaustively treats model tetragonal, trigonal, and digonal paddlewheel-type divanadium complexes V₂L_x (L=formamidinate, guanidinate, and carboxylate; x=2, 3, 4), each in the three lowest-energy spin states. The V−V formal bond orders are obtained from metal−metal MO diagrams for representative structures. A number of short V−V multiple bonds of order 3, 3.5, and 4 are found in these model complexes. The short V≡V triple bonds and singlet ground state predicted here for the model tetragonal complexes correspond well with the limited experimental results for the series of known tetragonal paddlewheels. Digonal divanadium lanterns with very short V−V quadruple bonds are predicted as interesting synthetic targets. The V−V bond distances are categorized into distinct ranges according to the formal bond order values from 0.5 to 4. These bond length ranges are compared with the ranges compiled for other divanadium complexes including carbonyl complexes.     

Ring expansions in substituted phenylnitrenes: an AM1 SCF-MO study

The semi-empirical AM1 SCF-MO method is used to study the ring expansion of ortho-substituted phenylnitrenes in the singlet state. This two-step rearrangement involves nitrene atom insertion into the phenyl ring to give a bicyclic azepine intermediate, which undergoes electrocyclic ring opening to yield the monocyclic keteneimine product. The first step of nitrene insertion in ortho-substituted phenylnitrenes may occur either towards the substituent side or away from it, while the second step is simply sequential to the first. Nine ortho-substituted phenylnitrene systems X–C₆H₄–N (X=H, CH₃, CN, NH₂, NO₂, OH, F, SH and Cl) are considered for study here.

Comparison of activation energy barriers for both steps predicts that the first step of azepine formation would be the rate-determining step. Electron-withdrawing substituents promote this step, and vice versa for electron-donating ones. The azepine intermediate is unstable compared to the keteneimine product due to strain. The transition state for the second step is predicted to have aromatic character, unlike the azepine and ketenimine. In general, the ring expansion is predicted to be favoured towards the unsubstituted side of the ring rather than towards the substituted side, where steric factors play the major role. The two successive steps of the ring expansion proceed via transition states which are more or less similar to each other, the second one having aromatic character. Competition between ring expansion and decay of the singlet phenylnitrene to the triplet state, as estimated by calculated enthalpy terms, is predicted to favour decay to the triplet state.     

Lantern-type dinickel complexes: An exploration of possibilities for nickel–nickel bonding with bridging bidentate ligands

Many binuclear nickel complexes have Ni Ni distances suggesting Ni Ni covalent bonds, including lantern-type complexes with bridging bidentate ligands. This DFT study treats tetragonal, trigonal, and digonal lantern-type complexes with the formamidinate, guanidinate, and formate ligands, besides some others. Formal bond orders (ranging from zero to two) are assigned to all the Ni Ni bonds on the basis of MO occupancy considerations. A VB-based electron counting approach assigns plausible resonance structures to the dinickel cores. Model tetragonal complexes with the dimethylformamidinate and the dithioformate ligands have singlet ground states whose non-covalently bonded Ni Ni distances are close to those in their experimentally known counterparts. Trigonal dinickel complexes are unknown, but are predicted to have quartet ground states with Ni Ni bonds of order 0.5. The model digonal complexes are predicted to have triplet ground states, but the predicted Ni Ni bond lengths are longer than those found in their experimentally known counterparts. This could owe to inadequate treatment of electron correlation by DFT in these short Ni Ni bonds with their multiconfigurational character. All the Ni Ni bond distances here are categorized into ranges according to the Ni Ni bond orders of 0, 0.5, 1, 1.5, and 2, no Ni Ni bonds of order higher than two being identified. The Ni Ni bonds of given order in these lantern-type complexes are consistently shorter than the corresponding Ni Ni bonds in dinickel complexes having carbonyl ligands, attributable to the metal metal bond lengthening effect of CO ligands.     

Generation, structure and reactivity of arynes: A theoretical study

The semiempirical AM1 SCF-MO method is used to study the benzyne mechanism for aromatic nucleophilic substitution of various m-substituted chlorobenzenes and 3-chloropyridine. The calculations predict that most of the fixed substituents studied here would induce the formation of 2,3-arynes through their electron-withdrawing resonance or inductive effects. The geometry and electronic structure of the 2,3- and 3,4-arynes investigated here, confirm the generally acceptedo-benzyne structure postulated for arynes. The sites of nucleophilic addition to arynes as predicted here are in fair agreement with expectation and experimental findings.     

Synthesis and antimicrobial studies of half-sandwich arene platinum group complexes containing pyridylpyrazolyl ligands

The reaction of [(arene)MCl₂]₂ with pyridylpyrazolyl ligands (L1 and L2) in the presence of ammonium hexafluorophosphate leads to formation of cationic complexes having the general formula [(arene)M(L)Cl]PF₆ {M?=?Ru, arene = p-cymene (1, 4); Cp*, M?=?Rh (2, 5); Cp*, M?=?Ir (3, 6); L?=?2-(1H-pyrazol-1-yl)pyridine (L1), 2-(3,5-dimethyl-1H-pyrazol-1-yl)pyridine (L2)}. Similarly the reaction of [CpRu(PPh₃)₂Cl] and [(ind)Ru(PPh₃)₂Cl] (ind?=?η⁵-C₉H₇) with L1 and L2 yielded cationic complexes which have been formulated as [(Cp/ind)Ru(L)PPh₃]PF₆ (7–10). All these complexes were characterized by analytical and spectroscopic techniques. The pyridylpyrazolyl ligands coordinated metal through pyridyl and pyrazolyl nitrogens forming a six-membered metallacycle. The ligands as well as the complexes were evaluated for their in vitro antibacterial activity by agar well diffusion method against two Gram negative bacteria (Escherichia coli and Pseudomonas aeruginosa) and two Gram positive bacteria (Staphylococcus aureus and Bacillus thuriengiensis). Results show that the ligands and the complexes have significant antibacterial activity against Gram negative bacteria.     

Correlation between zeroes and poles ofS matrix for complex potentials

Using several illustrative examples, the nature of resonance poles and the corresponding zeroes of the s-waveS matrix is examined for several potentials having an absorptive pocket followed by a barrier. It is shown that even though the presence of absorption practically suppresses the manifestation of resonance in the elastic scattering cross section, the effect of the resonances generated by the absorptive pocket is more clearly manifested in the absorption cross section provided the barrier width is not too large. We further find that the signature of barrier top resonances are also more clearly manifested in the absorption cross section rather than in the elastic scattering cross section. These results have been interpreted in terms of complex resonance poles and corresponding zeroes of theS matrix. This implies that in complex potential scattering like heavy ion collisions, the reaction channel cross section peak is a more reliable signature of resonance phenomenon than the variation of the elastic channel cross section with energy.