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Range separated density functionals capture accurately the dynamics of PdIII–O 2 superoxide formation by revealing the non-bonding (anagostic) and bonding interactions involved in the process. [Display omitted] • Aerobic oxidation of Pd complexes in C−C coupling reactions pass through the formation of PdIII – Superoxo intermediate and the crucial step of O 2 addition to Pd center bearing tetradentate pyridinophane ligands have been investigated with DFT. • Among several functionals tested range separated functionals, particularly M11 describes the dynamics of the process accurately. • Asymmetric substitutions favour O 2 addition to Ni, Pd and Pt centers by decreasing the steric nature (bis anagostic interactions) of N 4 ligand than symmetirc substitutions. Aerobic oxidative C C coupling catalyzed by dimethyl PdII center (L–[Pd]) bearing tetradentate pyridinophane (N 4) ligand L (L = N, N -dimethyl-2,11-diaza[3.3]-(2,6)pyridinophane) has been investigated using density functional theory (DFT) computations. In Pd-catalyzed aerobic oxidation processes, the interaction of dioxygen with PdII center is a crucial step. Computations reveal that the reactant dimethyl PdII complex L–[Pd] has anagostic interactions (4 × Pd···H ∼ 2.60 Å) in total agreement with reported experimental NMR downshift values and these interactions play a significant role in the formation of PdIII – Superoxo intermediate which is less probed. While oxygen approaches the reactant, PdII center becomes PdIII allowing axial amine coordination. This pushes two protons (C sp 3 – H) away from Pd center weakening the two anagostic interactions. The other two anagostic interactions are intact at long ranges and at closer distances they weaken facilitating Pd – O bonding. The formation of the PdIII center in the PdIII – Superoxo were confirmed by reported EPR measurements. To understand this process, this key step has been modelled. Calculations have been performed with fourteen different density functionals (M06L, B3LYP, PBE0, M06, M11 to name a few) and different basis sets both in gas phase and in solvent medium. It is found that the range separated functionals, particularly M11 functional describe the process accurately and this is attributed to their efficiency of handling electron correlation at different ranges by including different percentage of Hartree-Fock exchange. The role of anagostic interactions M⋯H (M = Ni, Pd and Pt) in metal – superoxo species bearing symmetric and asymmetric substituted N 4 ligands have been investigated. These results clearly explain asymmetrically substituted N 4 ligands assist the simultaneous creation of M – O bonds at medium ranges and the weakening of bis anagostic (repulsive) M∙∙∙H interactions at short ranges. These findings suggest that intermediate stabilization in C C coupling processes mediated by transition-metal catalysts containing N 4 ligands depends critically on the strength of anagostic interactions. [ABSTRACT FROM AUTHOR]

Designing new and innovative receptors for the selective binding of radionuclides is central to nuclear waste management processes. Recently, a new multi-topic ion-pair receptor was reported which binds a variety of cesium salts. Due to the large size of the receptor, quantum chemical calculations on the full ion-pair receptors are restricted, thus the binding mechanisms are not well understood at the molecular level. We have assessed the binding strengths of various cesium salts to the recently synthesized multi-topic ion-pair receptor molecule using density functional theory based calculations. Our calculations predict that the binding of cesium salts to the receptor predominantly occurs via the cooperative binding mechanism. Cesium and the anion synergistically assist each other to bind favorably inside the receptor. Energy decomposition analysis on the ion-pair complexes shows that the Cs salts are bound to the receptor mainly through electrostatic interactions with small contribution from covalent interactions for large ionic radius anions. Further, QTAIM analysis characterizes the importance of different inter-molecular interactions between the ions and the receptor inside the ion-pair complexes. The role of the crystallographic solvent molecule contributes significantly by ∼10 kcal mol−1 to the overall binding affinities which is quite significant. Further, unlike the recent molecular mechanics (MM) calculations, our calculated binding affinity trends for various Cs ion-pair complexes (CsF, CsCl and CsNO3) are now in excellent agreement with the experimental binding affinity trends. [ABSTRACT FROM AUTHOR]

A series of 2-quinolinone-based molecular systems with three different acceptor groups and N/ O-alkylated quinolinone compounds have been synthesised in an attempt to understand their optical properties. All the compounds were characterised by 1H NMR, 13C NMR and mass analysis. Absorption measurements revealed that charge-transfer transition was observed by introducing electron-withdrawing acceptor groups. Alkylation of 2-quinolinone at the O-position thwarts the charge-transfer transitions, whereas N-alkylation retains the charge-transfer property. The presence of resonance zwitterionic forms in the unalkylated and N-alkylated quinolinone compounds play an important role in charge-transfer transition. The effects of solvents on the absorption and emission properties of these compounds were probed through Lippert Mataga and ET(30) correlation. The Stokes shifts of O-alkylated compounds were larger than those of the unalkylated and N-alkylated quinolinone compounds. The observed higher quantum yield and Stokes shift for these compounds will make them ideal fluorescent probes. Incorporation of acceptor groups and alkylation in the quinolinone moiety alters their energy levels. Good thermal stability was observed for both unalkylated and alkylated quinolinone compounds. The trends observed in the photophysical and electrochemical properties were supported by theoretical studies. The observed tunable optical properties, which were achieved through simple N- vs. O-alkylation, results in large Stokes shifts and thermal stability, which means that the 2-quinolinone-based molecular systems are expected to emerge as potential candidates for photovoltaic and biological applications. [ABSTRACT FROM AUTHOR]

Azinomycin B-a well-known antitumor drug-forms cross-links with DNA through alkylation of purine bases and blocks tumor cell growth. This reaction has been modeled using the ONIOM (B3LYP/6-31 + g(d):UFF) method to understand the mechanism and sequence selectivity. ONIOM results have been checked for reliability by comparing them with full quantum mechanics calculations for selected paths. Calculations reveal that, among the purine bases, guanine is more reactive and is alkylated by aziridine ring through the C10 position, followed by alkylation of the epoxide ring through the C21 position of Azinomycin B. While the mono alkylation is controlled kinetically, bis-alkylation is controlled thermodynamically. Solvent effects were included using polarized-continuum-model calculations and no significant change from gas phase results was observed. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Imidozirconocene complex is known for its bifunctional reactivity and catalytic ability and this complex mediates ring cleavage of epoxides. Cyclooctene oxide ( 1) Norbornene oxide ( 2) and 2,5-dimethyl cyclohexene oxide ( 3) undergo ring cleavage in the presence of imidozirconocene complex. Epoxide 1 has accessible β-hydrogens (type I) while epoxide 2 and 3 do not have them (type II). Normally type I epoxides undergo elimination while type II epoxides prefer insertion. All the insertion reactions lead to five-membered metallacycle formation and elimination results in thermodynamically stable allyl-alkoxy product. The insertion is a two-step process following either diradical or zwitterionic pathway, while elimination is a one-step concerted reaction. DFT (density functional theory) modelling of these reactions at B3LYP/LANL2DZ level show that epoxide 1 undergoes elimination in agreement with experiment. However, calculations indicate that epoxide ( 2) proceeds through diradical intermediate in contrast to experimental observations. Surprisingly, epoxide ( 3) that has both the β positions blocked by methyl groups undergoes elimination rather than insertion. AIM and EDA analyses offer further insights on the reaction mechanism and bifunctional reactivity of imidozirconozene complex. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

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Biocatalytic azidolysis of 9 unsymmetrical epoxides by halohydrin dehalogenase enzyme (HheC) in gas phase and uncatalysed azidolysis of the same epoxides in gas phase and in aqueous solution have been modelled at DFT level. Aliphatic epoxides (1-6) and aromatic epoxides (9) undergo β cleavage while styrene oxide (7) and p-nitro styrene (8) oxide prefer α cleavage in the gas phase. Inclusion of aqueous solvation effect via Polarizable Continuum Model (PCM) increases the activation barrier and makes the reaction endothermic due to extensive solvation of azide anion and oxido anionic products, but does not alter the regioselectivity. Halohydrin dehalogenase from Agrobacterium radiobactor AD1 catalyses (E1-E9) ring opening of all these epoxides by azide ion with β selectivity and the reversal of selectivity in epoxide 7 and 8 is notable. These reactions follow, in both enzymatic and non-enzymatic environment, S2 mechanism. Calculations while agreeing totally with experimental results offer better insights on the factors determining the regioselectivity and particularly the role of enzyme. Active site model and crystal structure data reveal that the Tyr145 and Ser132 form weak hydrogen bonds with epoxide oxygen lone pair and form reactant enzyme complex (REC). The enzyme complex activates the epoxide ring towards azidolysis. The NBO deletion and second order perturbation analyses clearly bring out the role of catalytic duo Tyr145 and Ser132 and particularly shed light on the dominant contribution of Tyr145 in selectively activating C-O bond. The present results indicate that Arg149 or other residues in the pocket do not seem to have any significant effect on the reaction. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

The antitumor activities of bent metallocenes [Cp-M-Cp] (M = Ti, V, Nb, Mo) and complexes of them with guanine, adenine, thymine and cytosine nucleotides have been probed using electronic structure calculations. DFT/BP86 calculations have revealed that the bent metallocene-nucleotide interaction strongly depends on the stability of the hydrolyzed form of the bent metallocene dichloride [CpM] species, and in turn the stability of the [CpM] species strongly depends on the electronic structure of [CpM]. Detailed electronic structure and Walsh energy analyses have been carried out for the hydrolyzed forms of four [Cp-M-Cp] (M = Ti, V, Nb, Mo) species to find out why the bent structure is unusually stable. Energy changes that occur during the bending process in frontier molecular orbitals as well as the p(π)- d(π) overlap have been invoked to account for the anticipated antitumor activities of these species. The bonding situation and the interactions in bent metallocene-nucleotide adducts were elucidated by fragment analysis. Of the four nucleotides complexed with the four bent metallocenes, adenine and guanine show better binding abilities than the other two nucleotides. Metallocenes of second-row transition metals exhibit better binding with pyrimidine-base nucleotides. In particular, the Lewis acidic bent metallocenes interact strongly with nucleotides. The antitumor activity is directly related to the binding strength of the bent metallocene with nucleotide adducts, and the computed interaction energy values correlate very well with the experimentally observed antitumor activities. [ABSTRACT FROM AUTHOR]

Diels Alder reactivity of corannulene has been probed using density functional theory (DFT) at B3LYP/6-31G* level by employing it both as a diene and a dienophile in cycloaddition with ethylene and 1,3-butadiene as typical partners. Computations reveal that corannulene acts better as a dienophile than as a diene and as a dienophile it undergoes normal electron demand type addition with 1,3-butadiene, and as a diene corannulene undergoes inverse electron demand type addition with ethylene. When employed as a dienophile the addition takes place preferentially in the rim position than in the spoke position due to strong steric and electronic reasons. Further in the rim addition rim exo approach is favored kinetically and thermodynamically. As a diene, corannulene shows regioselectivity for rim–spoke addition over spoke–spoke addition. Concerted type cycloadditions have been studied and the reactions are seen to take place preferentially on the convex face. The effect of substituents in butadiene on the reactivity and the reaction of butadiene–pentaindenocorannulene (an extended corannulene) system has been investigated for the most favorable rim exo positions. Copyright © 2007 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Borirane undergoes ring opening reaction with NOCl and HNF2 yielding the corresponding alkenes. Ab initio and density functional investigations of this reaction with cis- and trans-2,3-dimethylboriranes reveal that these reactions take place in a single step through the formation of a prereactive complex and a transition state giving the alkene with the same stereochemistry. Calculations clearly show that the concerted cleavage of C&bond;B bonds leads to retention of stereochemistry. Further, it shows that HNF2 cleaves boriranes more efficiently than does NOCl. Intrinsic reaction coordinate analyses and bond order analysis describe the nature of the transition state very well and fix the reaction mechanism. Solvent effect calculations through PCM model, with acetonitrile and CCl4 as solvents, do not alter the gas phase results significantly. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [ABSTRACT FROM AUTHOR]

The stereochemical course of the deamination of cis‐2,3‐dimethylaziridine by nitrosyl chloride was investigated at the QCISD/6‐31G(d) level. Calculations reveal that the reaction takes place in two steps. In the first step, the reactants form a pre‐reactive complex, followed by a spiro‐type bicyclic transition state, which on dissociative cycloelimination gives the N‐nitrosoaziridine intermediate. In the second step, this intermediate undergoes cycloreversion through a slightly asynchronous concerted transition state to form an alkene with the same stereochemistry, which is in total agreement with experiment. In the whole reaction, the denitrosation step is found to be rate‐determining. For comparison, geometry optimizations and energies were also obtained at the B3LYP/6‐31G(d) level. It was found that the B3LYP energy results differed significantly from the QCISD ones. To analyze the reason for this difference, B3LYP calculations were repeated by varying the contribution of exact exchange in the Becke functional. With respect to the QCISD results, it has been shown that the functional with 0% exact exchange yields the best activation barriers, whereas the functional with 30% exact exchange is the most suitable one to carry out the complexation and reaction energy calculations. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [ABSTRACT FROM AUTHOR]

Abstract: All the possible carbonyl-nitroso, enol-nitroso and carbonyl-oxime conformations of glyoxalmonoxime and all the rotamers of carbonyl-oxime tautomer of 1-methyl and 2-methylglyoxalmonoximes have been investigated at ab initio HF and correlated level (Möller-Plesset second order) using 6-31G** basis set. Density functional theory calculations have also been done with Becke''s three-parameter hybrid method. The chelated enol-nitroso form is non-existent and converges upon the carbonyl form at both DFT and MP2 calculations. In gas phase, fully linear carbonyl-oxime conformer has been found to be the most favored one over the enol-nitroso and carbonyl-nitroso conformations, unlike the chelated form in the case of malondialdehyde (MDA). The hydrogen bond energy (EHB) of O⋯H–O framework in the closed hexatomic chelate of carbonyl-oxime is affected by the methyl group at the carbonyl carbon atom at all levels (ca. 8kJ/mol HF; ca. 5.2kJ/mol MP2; ca. 5.3kJ/mol B3LYP). A model compound has been chosen for the quantification of EHB. Even though enol-nitroso form is less stable, its hydrogen bond strength is much higher than in MDA. The investigation in aqueous medium was also carried out by means of PCM-SCRF method. Effect of adding one water molecule to the various GM rotamers has also been studied, incorporating the corrections to the basis set super position error. [Copyright &y& Elsevier]