Interaction of adenine with dichloro-[1-alkyl-2-(naphthylazo)-imidazole]palladium(II) complexes: kinetic and mechanistic studies

Interaction of adenine (A) with dichloro-[1-alkyl-2-(α-naphthylazo)imidazole] palladium(II) [Pd(α-NaiR)Cl₂], 1 and dichloro-[1-alkyl-2-(β-naphthylazo)imidazole] palladium(II) [Pd(β-NaiR)Cl₂], 2 {where R=Me (a), Et (b) or Bz (c)} in MeCN-water (50% v/v) medium to yield [{1-alkyl-2-(α-naphthylazo)imidazole}(adenine)]palladium(II) perchlorates (3a, 3b, 3c) and [{1-alkyl-2-(β-naphthylazo)imidazole}(adenine)]palladium(II) perchlorates (4a, 4b, 4c) was studied. The products were characterized by physico-chemical and spectroscopic methods. The reaction kinetics were second order overall, being first order in both the Pd(II) complex and adenine. The effect of adding chloride was consistent with rate-limiting dissociation of chloride from the complex. Thermodynamic parameters were determined from temperature variation experiments. The second-order rate constant k ₂ corroborates with the experimental Δ^‡H° values, while the negative values of Δ^‡S° indicate that the reaction proceeds through an associative inner sphere mechanism.     

Composite MR image reconstruction and unaliasing for general trajectories using neural networks

In rapid parallel magnetic resonance imaging, the problem of image reconstruction is challenging. Here, a novel image reconstruction technique for data acquired along any general trajectory in neural network framework, called “Composite Reconstruction And Unaliasing using Neural Networks” (CRAUNN), is proposed. CRAUNN is based on the observation that the nature of aliasing remains unchanged whether the undersampled acquisition contains only low frequencies or includes high frequencies too. Here, the transformation needed to reconstruct the alias-free image from the aliased coil images is learnt, using acquisitions consisting of densely sampled low frequencies. Neural networks are made use of as machine learning tools to learn the transformation, in order to obtain the desired alias-free image for actual acquisitions containing sparsely sampled low as well as high frequencies. CRAUNN operates in the image domain and does not require explicit coil sensitivity estimation. It is also independent of the sampling trajectory used, and could be applied to arbitrary trajectories as well. As a pilot trial, the technique is first applied to Cartesian trajectory-sampled data. Experiments performed using radial and spiral trajectories on real and synthetic data, illustrate the performance of the method. The reconstruction errors depend on the acceleration factor as well as the sampling trajectory. It is found that higher acceleration factors can be obtained when radial trajectories are used. Comparisons against existing techniques are presented. CRAUNN has been found to perform on par with the state-of-the-art techniques. Acceleration factors of up to 4, 6 and 4 are achieved in Cartesian, radial and spiral cases, respectively.     

Theoretical study of the electronic nonadiabatic transitions in the photoelectron spectroscopy of F2O

The photoelectron spectrum of F2O pertaining to ionizations to the ground (X2B1) and low-lying excited electronic states (A2B2, B2A1, and C2A2) of F2O+ is investigated theoretically. The near equilibrium potential energy surfaces of the ground electronic state (X2B1) of F2O and the mentioned ground and excited electronic states of F2O+ reported by Wang et al. ( J. Chem. Phys. 2001, 114, 10682) for the C2v configuration are extended for the Cs geometry assuming a harmonic vibration along the asymmetric stretching mode. The vibronic interactions between the A2B2 and B2A1 electronic states of F2O+ are treated within a linear coupling approach, and the strength of the vibronic coupling parameter is calculated by an ab initio method. The nuclear dynamics is simulated by both time-independent quantum mechanical and time-dependent wave packet approaches. Although the first photoelectron band exhibits resolved vibrational progression along the symmetric stretching mode, the second one is highly overlapping. The latter is attributed to the nonadiabatic interactions among the energetically close A2B2, B2A1, and C2A2 electronic states of F2O+. The theoretical findings are in good accord with the available experimental results.     

Model pharmaceutical co-crystallization: Guest-directed assembly of caffeine and aromatic tri-hydroxy and dicarboxylic acids into different heteromolecular hydrogen bonding networks in solid state

Three model pharmaceutical caffeine-containing co-crystals of 1,3,5-trihydroxybenzene (phloroglucinol), isophthalic acid and 5-hydroxyisophthalic acid were synthesized and characterized via single-crystal X-ray diffraction. The three crystalline forms reported are an anhydrous co-crystal and other two are co-crystal hydrates. Also their binding properties were studied by UV-vis analysis. In each of these structures, an organised intermolecular hydrogen bonding motif was observed. A comparison of hydrogen bonding motifs in the crystal sheets was presented.     

An efficient chemical method for separation of corsolic acid from its isomeric maslinic acid

Based on the difference in steric bulk around C12-C13 double bond, the two isomeric dihydroxy pentacyclic triterpenic acids viz: corsolic acid and maslinic acid have been quantitatively separated via their methyl esters by reacting with the bulky m-chloroperbenzoic acid. Corsolic acid methyl ester was obtained in pure form, whereas maslinic acid methyl ester was separated as 12-oxo derivative formed via its epoxide. Alkaline hydrolysis of corsolic acid methyl ester afforded the desired acid. This method was also found to work well with the isomeric amyrin mixture (alpha- and beta-), but not highly selective. The high selectivity of this method with corsolic maslinic acid system can be rationalized in terms of 2alpha-hydroxy functionality, which provides additional crowding around the double bond and completely prevented corsolic acid from its reaction with perbenzoic acid.     

Asymmetric synthesis of hydroxy-skipped bishomo-inositols as potential glycosidase inhibitors

Four isomeric hydroxy-skipped bishomo-inositol analogs have been synthesized from both enantiomers of 5-hydroxymethyl-2-cyclohexenone. Sharpless asymmetric dihydroxylation and substrate-directed anionic hydroxymethylation are the key reactions which have been employed successfully for the synthesis of new cyclitols. The synthesized cyclitols have been screened for their inhibitory effect on α- and β-glycosidases.     

Substitution of ligands in dichloro-2-(arylazo)heterocyclepalladium(II) by 8-quinolinol: kinetics and mechanistic studies

Nucleophilic substitutions of Pd(N,N)Cl₂[(N,N = 1-methyl-2-(arylazo)imidazole (RaaiMe), p-RC₆H₄N=NC₃H₂NN-1-Me; 2-(arylazo)pyridine (Raap), p-RC₆H₄N=NC₅H₄N; 2-(arylazo)pyrimidine (Raapm), p-RC₆H₄N=NC₄H₃N₂ where R = H (a), Me (b), Cl (c)] with 8-quinolinol (HQ) have been examined by spectrophotometry at 298 K in MeCN solution. The product, Pd(Q)₂, has also been confirmed by independent synthesis from Na₂[PdCl₄] and HQ in EtOH. The kinetics of the reaction have been studied under pseudo-first-order conditions and the analyses support a nucleophilic association path. A single phase reaction has been observed and follows the rate law, rate = a + k [Pd(N,N)Cl₂] [HQ]². Thus, the reaction is first order in [Pd(N,N)Cl₂] and second order in [HQ]. External addition of Cl^–(LiCl) suppresses the rate. The rate increases as follows: Pd(RaaiMe)Cl₂ < Pd(Raap)Cl₂ < Pd(Raapm)Cl₂.     

p-benzoquinone-benzene clusters as potential nanomechanical devices: a theoretical study

The equilibrium structures and binding energies of the benzene complexes of p-benzoquinones (PBQ) and its negatively charged anionic species (PBQ- and PBQ2-) have been investigated theoretically using second-order M?ller-Plesset calculations. While neutral p-benzoquinone-benzene clusters (PBQ-Bz) prefer to have a parallel displaced geometry (P-c), CH...pi interactions (T-shaped geometries) prevail in the di-anionic PBQ-benzene (PBQ2- -Bz) complexes (T-e2-). Studies on dianionic p-benzoquinone-benzene clusters showed that two nonbonded intermolecular interactions compete in the most stable conformation. One is H-bonding interaction (C-H...O type) between carbonyl oxygen of p-benzoquinone and one of the hydrogen atoms of benzene, and the other is a pi-H interaction between pi-electron cloud of PBQ2- and another hydrogen atom of benzene. Blueshifted H-bonds were observed in T-shaped clusters. The changes in the geometrical preference of PBQ-Bz complex upon addition of electrons would be useful in designing optimized molecular mechanical devices based on the edge-to-face and face-to-face aromatic interactions.     

Photochemistry of pyrrole: time-dependent quantum wave-packet description of the dynamics at the 1pi sigma*-S0 conical intersections

The photoinduced hydrogen-elimination reaction in pyrrole via the conical intersections of the two (1)pi sigma(*) excited states with the electronic ground states [(1)B(1)(pi sigma(*))-S(0) and (1)A(2)(pi sigma(*))-S(0)] have been investigated by time-dependent quantum wave-packet calculations. Model potential-energy surfaces of reduced dimensionality have been constructed on the basis of accurate multireference ab initio electronic-structure calculations. For the (1)B(1)-S(0) conical intersection, the model includes the NH stretching coordinate as the tuning mode and the hydrogen out-of-plane bending coordinate as the coupling mode. For the (1)A(2)-S(0) conical intersection, the NH stretching coordinate and the screwing coordinate of the ring hydrogens are taken into account. The latter is the dominant coupling mode of this conical intersection. The electronic population-transfer processes at the conical intersections, the branching ratio between the dissociation channels, and their dependence on the initial preparation of the system have been investigated for pyrrole and deuterated pyrrole. It is shown that the excitation of the NH stretching mode strongly enhances the reaction rate, while the excitation of the coupling mode influences the branching ratio of different dissociation channels. The results suggest that laser control of the photodissociation of pyrrole via mode-specific vibrational excitation should be possible. The calculations provide insight into the microscopic details of ultrafast internal-conversion processes in pyrrole via hydrogen-detachment processes, which are aborted at the (1)pi sigma(*)-S(0) conical intersections. These mechanisms are of relevance for the photostability of the building blocks of life (e.g., the DNA bases).