Gyusung Chung

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.

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.

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.

Molecular structures of gauche and anti conformers for oxalyl bromide: ab initio and DFT calculations

Abstract The molecular structure and conformational nature of oxalyl bromide are investigated by the ab initio and DFT methods. Both the MP2 and B3LYP optimized structures are better consistent with the experimental result in the gas phase. The CC bond torsional angle of the gauche conformation is quite sensitive to the choice of the calculational level. The B3LYP potential energy surface around the anti conformation is so flat that the existence of the anti conformation is not certain. It is suggested that the geometry calculations at the MP2 level can explain the nature of conformation in oxalyl bromide better than those at the B3LYP level.

Ab Initio Study of Cyclenphosphorane: The Molecular Structures of Phosphorus(III) and Phosphorus(V) Tautomers

Ab initio calculations using the GAUSSIAN 92 program have been performed to investigate the molecular structure of cyclenphosphorane and the tautomerism between the P(V) tautomer ((cyclen)PH) and the P(III) tautomer (H(cyclen)P). The optimized geometry around the P atom in (cyclen)PH is a distorted trigonal bipyramid at the RHF/6-31G*, RHF/6-311G*, and MP2/6-31G* levels, while the most stable conformer among four conformers of H(cyclen)P is found to have a distorted pyramidal geometry around the P atom at the RHF/6-31G* and RHF/6-311G* levels. The results show that (cyclen)PH is more stable than H(cyclen)P at higher levels of theory.