Duckhwan Lee

Observation of pronounced b•,y cleavages in the electron capture dissociation mass spectrometry of polyamidoamine (PAMAM) dendrimer ions with amide functionalities

We report the electron capture dissociation (ECD) mass spectrometry of the third generation polyamidoamine (PAMAM) dendrimer that contains amide functionalities. The dendrimer was chosen because it offers a unique opportunity to understand the ECD behavior of the amide functionality in a framework other than peptides/proteins. In this study, PAMAM ECD was found to exhibit a fragmentation pattern strikingly different from that of ordinary peptide/protein ECD. Specifically, ECD of multiply protonated PAMAM ions gave rise to significant b•,y cleavages as well as S,E dissociations but, unexpectedly, only minor c,z• fragmentations are observed. In an effort to account for the unexpectedly different fragmentation pattern, a comparative ECD experiment on the poly(propylene imine) dendrimer in which the amide bond moiety is not available and density functional theory calculations (B3LYP/6-311+G(d)) investigations on the model system of a charge-solvated single-repeat unit were carried out. On the basis of these results, we discuss here possible implications of intramolecular charge-solvation, energy barriers in dissociation reactions, and macromolecular properties of the dendritic molecule for understanding the reaction pathway of PAMAM ECD.

Theory of diffusion-influenced fluorescence quenching : dependence of the Stern-Volmer curve on light intensity

Abstract A theory for describing the effects of quencher molecules on the intensity of fluorescence stimulated by steady-state illumination is presented, which is based on a hierarchical system of many-body Smoluchowski equations for the reactant molecule distribution functions. An important aspect of the present theory is that it describes the dependence of fluorescence quenching dynamics on the intensity of external illumination. In the conventional Smoluchowski approach it has been simply assumed that the intensity of illumination should be quite weak. Hence the validity of the present theory may be tested by a fluorescence quenching experiment under intense illumination.

Theoretical study of some bis-verdazyl diradicals: singlet–triplet energy gap

Abstract The bis-verdazyl diradical (BVD) system is closely examined by using the multiconfiguration wavefunctions as well as the density functional theory (DFT). The totally symmetric singlet ground state turns out to have strong multiconfiguration character at all levels of theory. The singlet ground state takes on the planar structure while the most stable triplet state corresponds to the twisted form. The MCSCF+MCQDPT2 calculations are shown to be sufficient to predict the singlet–triplet energy gap which is insensitive to the electronic characters of the ring substituents.

The scaling approach to multicenter molecular integrals with Slater-type orbitals

Abstract A scaling approach to multicenter molecular integrals with Slater-type orbitals (STOs) is presented. The result is significant in that it shows (1) the existence of a simple relationship between multicenter integrals and (2) an implied computational savings. Operation count estimates indicate that the significant savings would occur for a system having large numbers of STOs on each atom.

Tetrasulfur tetranitride and its selenium analogs: ab initio and DFT calculations

Abstract The eight-membered ring systems of Se n S 4− n N 4 ( n =0–4) with weak transannular bondings between chalcogen elements are examined with the DFT and MP2 methods. The previously known cage conformations turn out to be the most stable structure for all molecular systems under investigation. The chair conformations are also found to be stable local minima. It is also concluded that the DFT method is far more superior to the MP2 method in predicting the weak transannular inter-chalcogen bondings. The DFT and MP2 methods get to agree with each other if the transannular inter-chalcogen bondings are weakened enough as in the case of the chair conformations.

Combined Ellipsometric and Reflectance Measurements for Characterization of Films Formed on Electrodes

The conventional form of ellipsometry has difficulty in characterizing a light-absorbing film, because there are only two equations for three unknown properties of the film at a fixed angle of incidence and at fixed wavelength. The three unknown properties are the real and imaginary parts of the dielectric constant and the thickness of the film.(1) In order to overcome this difficulty, supplementary measurements are necessary. However, since these supplementary measurements are often nonoptical or nonsimultaneous with the optical measurements, they are not suitable for in situ monitoring of the formation of thin films on electrode surfaces. The reflectance-ellipsometry technique, or three-parameter ellipsometry as it is often called, in which the ellipsometric parameters and reflectance are simultaneously measured, has proved to be applicable without such difficulties for thin films formed on electrode surfaces.(2–5) With this technique, transient reflectance-ellipsometry measurements become possible and can be used to characterize the films as functions of time during film formation and during the early stage of growth and change of the films.

Evolution of reactions on surfaces exhibiting defect structures

Abstract This paper considers the time and spatial dependent behavior of an active surface exposed to two reactants which may diffuse on the surface, desorb, and also react. The surface is assumed to contain defect structures corresponding to either inherent lattice faults or foreign material on the surface. A number of case studies are examined corresponding to different assumptions about the desorption and kinetic characteristics of the defect sites. The surface concentration profiles are examined to gain physical insight into the competing surface processes. The net effect of surface defects on chemical production rates was examined by integrating the local production rate over a test region on the surface. Of special interest was the study of the poisoning effect of the product species on the diffusion and reaction of the reactants. The reaction-diffusion equations of the models were solved by the alternating direction collocation technique which shows promise for providing an efficient numerical procedure capable of handling practical large scale problems.