Computational and experimental studies of the effect of substituents on the singlet-triplet energy gap in phenyl(carbomethoxy)carbene

The effect of aromatic substitution on the singlet-triplet energy gap in substituted phenyl(carbomethoxy)carbene (X-Ph-C-CO(2)CH(3), PCC) has been explored by time-resolved infrared (TRIR) spectroscopy and gas-phase computational methods. The ground state of para-substituted PCC is calculated to change from the triplet state in p-NO(2)-PCC (Delta G(ST) = 6.1 kcal/mol) to the singlet state in p-NH(2)-PCC (Delta G(ST) = -2.8 kcal/mol). The absence of solvent perturbation in the TRIR spectra of p-N(CH(3))(2)-PCC (which should have electronic properties similar to p-NH(2)-PCC) and parent PCC is consistent with their ground states lying > +/-2 kcal/mol from the next available electronic state, in line with the computational results. The observation of solvent perturbation in the TRIR spectra of p-OCH(3)-PCC and p-CH(3)-PCC implies that their ground states lie < +/-1 kcal/mol from their next available electronic state. This is in agreement with our computational results, which predict a gas-phase Delta G(ST) of -0.8 and 1.6 kcal/mol for p-OCH(3)-PCC and p-CH(3)-PCC as compared to Delta G(ST) values of -3.9 and -1.3 kcal/mol from polarizable continuum model (PCM) calculations with acetonitrile as a solvent. Gas-phase computational results for the meta- and ortho-substituted PCC species are also presented, along with selected linear free energy (LFE) relationships for the para and meta species.     

Calculation of infrared-divergent Feynman diagrams with zero mass threshold

Two-loop vertex Feynman diagrams with infrared and collinear divergences are investigated by two independent methods. On the one hand, a method of calculating Feynman diagrams from their small momentum expansion [1] extended to diagrams with zero mass thresholds [2] is applied. On the other hand, a numerical method based on a two-fold integral representation is used [3], [4]. The application of the latter method is possible by using lightcone coordinates in the parallel space. The numerical data obtained with the two methods are in impressive agreement. Received: 22 April 1997 / Published online: 20 February 1998     

The closed subalgebras of a topological algebra

Conclusion There are some interesting related problems which we have not solved One is to characterize the family of all closed subalgebras in the case where it is assumed that the algebraic operations are continuous in the topology. If the operations are not determined uniquely by the family of closed subalgebras, is there at least one determination in which they are continuous ? Another problem is to characterize the families of finitely generated closed subalgebras of a topological algebra.Supported by National Science Foundation Research Grant GP-3132.     

The fragmentation pathways of protonated glycine: A computational study

Numerous studies have demonstrated that protonated aliphatic amino acids, [H2NCHRCO2H + H]+, fragment in the gas phase to form iminium ions, H2N=CHR+. Unfortunately none of these studies have probed the structure of the neutral(s) lost as well as the mechanism of fragmentation. Three main mechanisms have been previously proposed: (1) loss of the combined elements of H2O and CO; (2) loss of dihydroxycarbene (HO)2C: and (3) loss of formic acid, HC(=O)OH. Herein, ab initio and density functional theory calculations have been used to calculate the key reactants, transition states, and products of these and several other competing reaction channels in the fragmentation of protonated glycine. The loss of the combined elements of H2O and CO is thermodynamically and kinetically favored over the alternative formic acid or (HO)2C fragmentation processes.     

Separation of 2-naphthol atropisomers on cyclofructan-based chiral stationary phases

A set of fifteen 2-naphthol-derived atropisomers were evaluated on three different cyclofructan-based chiral stationary phases (CSP). The cyclofructan CSPs were a dimethylphenyl-derivatized cyclofructan 7 (CF7-DMP), a isopropyl (CF6-P) and a R-naphthylethyl cyclofructan 6 (CF6-RN) derivative, all bonded to 5-µm spherical fully porous silica particles packed into three 25?cm?×?4.6?mm columns (commercially available as Larihc columns). The 15 atropisomers were analyzed in the normal-phase mode with heptane/alcohol mobile phases. The CF7-DMP column was the most effective partially or fully separating 14 of the 15 compounds (93%). All 15 compounds could be separated by at least one cyclofructan column. Five compounds could be partially or fully separated by all three CSPs. The effect of ethanol, 2-propanol and butanol as 5 and 10% v/v polar modifiers in heptane was studied. A prototype 15?cm?×?4.6?mm column packed with superficially porous 2.7?µm CF6-P bonded particles was tested with the same set of compounds and a standard HPLC system. The increased efficiency and solvent saving were significant.     

Reactions of40Ar with233U,235U,and238U at the barrier

Production cross sections for target-like transfer products in reactions of⁴⁰Ar with^233,235,238U at the barrier were determined using radiochemical techniques. The heaviest products detected are isotopes of californium (Z=98). In addition to the quasi-elastic component of the isotopic distributions observed in the vicinity of uranium, there are also relaxed contributions throughout the entire region. The peak positions of the isotope distributions of this component for fixed atomic number, after the transfer of > 3 charges, approach closely the minimum of the potential energy surfaces (PES). The experimental results thus indicate the cold formation of the reaction products. A search for long-lived heavy actinides produced, by fusion-evaporation, via isotopes of element 110 and their subsequent decay through α-decay chains, remained unsuccessful at a cross section limit of 21 pb.     

Computational investigations of pore forming peptide assemblies in lipid bilayers

This Letter presents the first application of a three-dimensional numerical molecular theory based modeling approach to study the structure and energetics of assemblies of peptides embedded in lipid bilayers. Coarse-grained models were used for both the peptides and lipids. Both barrel-stave and toroidal pore morphologies for the lipids near the peptide assemblies are found, but at different assembly sizes. The free energy of the assembly is found to have a global free energy minimum for a solution with a membrane-spanning toroidal pore. A pairwise approximation to this free energy is found to underpredict the free energy minimum associated with the membrane-spanning pores.     

Fast volatilization of some lanthanide and actinide elements from titanium surfaces

Summary The ad- and desorption of trace quantities of the lanthanide elements lanthanum, praseodymium, neodymium, samarium, europium, gadolinium, dysprosium, and ytterbium and of the actinide elements americium and curium were investigated by thermochromatographic and isothermal measurements with hot titanium columns. The adsorption enthalpies deduced from the experimental data are in reasonable agreement with calculated values which indicate considerable variations in volatility under such conditions. More volatile elements can quickly be separated from less volatile elements by heating the mixture on titanium in high vacuum.

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Schnelle Verflüchtigung einiger Lanthanidenund Aktinidenelemente von Titanoberflächen

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Dedicated to Prof. Dr. G. Tölg on the occasion of his 60th birthday

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Part of a Doctoral Thesis, Mainz 1987