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.     

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).     

Opposing mixed convection and its interaction with radiation inside eccentric horizontal cylindrical annulus

In the present investigation, the coupled phenomenon of opposing mixed convection and radiation within differentially heated eccentric horizontal cylindrical annulus has been numerically simulated. The radiation transfer contributed from the participating medium is obtained by solving the nonlinear integro‐differential radiative transfer equation using discrete ordinate method. The participating gray medium is considered to be emitting, absorbing and isotropically scattering. The walls of the annulus are considered to be opaque, diffuse and gray. In the study it has been observed that the Richardson number ‘Ri’ has a small effect on the total Nusselt number ‘Nu’ in mixed convection heat transfer with or without radiation. From the present investigation it is found that substantial changes occur in isotherms as well as in flow patterns, when the Richardson number is allowed to vary in the range of 0.01–1. The influence of radiative parameters on the interaction phenomenon has been delineated through isotherm and streamline pattern. Copyright © 2008 John Wiley & Sons, Ltd.     

Lamb wave characteristics of thickness-graded piezoelectric IDT

An equivalent single layer model of Lamb wave generation by thickness-graded piezoelectric IDT on host structure is developed. Various additional complexities, such as the coupling between the Lamb wave modes, complicated nature of the electro-mechanical excitation are considered. The model of infinite IDT is extended to deal with the finite IDT with edge discontinuities. The effects of electromechanical coupling and thickness gradation on the wavelength shifts are investigated. The problem of electrically driven instability within the IDT is analyzed. Numerical results are reported by considering Al2O3/PZT IDT as integral part of the host structure, which show that there are significant changes and improvements in the Lamb wave characteristics due to the graded configuration. Most important among these is the reduced dispersiveness of the Lamb wave modes, which is useful in launching a SAW that propagates with narrower pulse width and less attenuation.