RADICAL PROCESSES IN LIPIDS. SELECTIVITY OF HYDROGEN ABSTRACTION FROM LIPIDS BY BENZOPHENONE TRIPLET

Laser photolysis techniques have been used to measure the reactivity of benzophenone triplet (3BZP) toward various fatty acids and two glycerides in benzene solution. Eight compounds varying both in number and in the configuration of olefinic bonds have been examined. It has been found that the rate constant for hydrogen abstraction from these compounds by 3BZP may be related to the number of secondary, allylic and doubly allylic hydrogens in each molecule by the equation: kH = [0.023[H sec] + 0.112[H allylic] + 1.78[H doubly allylic]] x 10(7) M-1s-1     

Adiabatic-diabatic transformations for molecular systems: a model study of two interacting conical intersections

A model is presented for studying the interaction between two conical intersections (e.g. a dimer of two bound molecules each characterized by a conical intersection). The model is an extension of a previous model for a single conical intersection formed by an electron housed in a vibrating molecule (Baer, M. and Englman, R., 1992, Molec. Phys., 75, 293). We distinguish between two situations: when the coupling is weak (for instance when it takes place in the asymptotic region) and when it is strong. The study is accomplished by calculating the adiabatic—diabatic transformation (ADT) matrix. Whereas the features of the ADT matrix for weak coupling seem reasonable (and to a certain extent expected), we find some unexpected features in the case of strong coupling. In particular, the two characteristic ADT angles of the uncoupled systems namely (?_1/2) and (?_2/2) are replaced by two new ADT angles, namely, (?₁ + ?₂)/2 and (?₁—?₂)/2. This implies that the corresponding nuclear wavefunctions, which originally were multi-valued, become single-valued in cases of strong interaction.     

Rotational spectrum,structure and internal motions of the ethylene-OCS weakly bound dimer

The rotational spectra of five isotopomers of the ethylene-OCS dimer have been observed by Fourier transform microwave spectroscopy and its structure was determined. The dipole moment components and rotational constants for this complex are consistent with a stacked geometry in which the OCS lies above the ethylene molecular plane, approximately parallel to the C=C bond. Two internal motions of the monomer subunits split each rotational transition into four components. The larger tunneling splittings have been analysed to give a twofold barrier for the internal motion of the ethylene subunit about its c inertial axis of 16(3) cm^?1. The results are compared with calculations with a semi-empirical model employing electrostatic, dispersion and repulsion interactions.     

Electrostatic interactions between molecules from relaxed one-electron density matrices of the coupled cluster singles and doubles model

The influence of electron correlation on the electrostatic interaction between closed shell molecules is studied using the relaxed electron densities of the coupled cluster singles and doubles (CCSD) model. The corresponding CCSD one-electron density matrices are efficiently computed without full four-index transformation by employing the generalized exchange and Coulomb operator technique. Using several representative van der Waals and hydrogen bonded complexes it was found that in most cases the convergence of the M?ller-Plesset expansion of the electrostatic energy, restricted to single, double and quadruple excitations, is satisfactory and the fourth-order triple excitation term is more important than the sum of the fifth- and higher-order contributions from CCSD theory. The importance of the CCSD correlation correction to the electrostatic energy was gauged by comparison of the total interaction energy computed by symmetry-adapted perturbation theory (SAPT) and by the super-molecular CCSD(T) approach (coupled cluster singles and doubles model with a non-iterative inclusion of triple excitations). Except for the CO and N₂ dimers, very good agreement between the two sets of results is observed. For the difficult case of the CO dimer the difference between the SAPT and CCSD(T) results can be explained by the truncation of the SAPT expansion for the dispersion energy at second order in the intramonomer correlation operator.     

A Nystrm method for the numerical solution of Laplace's equation with non-linear boundary conditions on a polygon

A non-linear boundary value problem for Laplace's equation ona polygonal domain is solved with a Nystrm method applied toa boundary integral equation reformulation. The method usesmeshes which are graded toward the corners. Numerical examplesare included.     

The Generalized Matrix Sector Function and the Separation of Matrix Eigenvalues

The matrix sector function of A is introduced and generalizedto the matrix sector function of g(A), where the complex matrixA may have a real or complex characteristic polynomial and g(A)is a matrix function of a conformal mapping. The generalizedmatrix sector function of A is employed to separate the matrixeigenvalues relative to a sector, a circle, and a sector ofa circle in the complex plane without actually seeking the characteristicpolynomial and the matrix eigenvalues relative to a sector,a circle, and a sector of a circle in the complex plane withoutactually seeking the characteristic polynomial and the matrixeigenvalues themselves. Also, the generalized matrix sectorfunction of A is utilized to carry out the block-diagonalizationand block-triangularization of a system matrix, which are usefulin developing applications to mathematical science and control-systemproblems.     

Democritus – scientific wizard of the 5th century bc

Roughly 2400 years ago, during an era largely characterized by unscientific thought, a school of natural philosophers led by Democritus of Abdera developed a remarkably accurate understanding of our physical world. How could this small group have discovered so much at a time when technology and mathematics were at such a rudimentary level? What if their methods and ideas had caught on immediately, instead of being virtually ignored for 2000 years?     

SOLVENT INFLUENCES ON THE SINGLET QUENCHING OF CHLOROPHYLL a BY 2, 5-DIMETHYL-p-BENZOQUINONE

Abstract— The quenching of chlorophyll a excited singlet states by 2.5-dimethyl- p -benzoquinone has been investigated in solvents of varying viscosity and polarity. The observed singlet lifetimes showed little variation in several hydrocarbon solvents. Stern-Volmer constant K depends on the viscosity of the solvent, although cyclic and straight-chain hydrocarbons behave somewhat differently. The decrease of the K values with increase of viscosity suggests that the quenching mechanism is at least partly dynamic, although there is evidence for static quenching as well. The influence of solvent polarity on the K values was found to be insignificant, which is consistent with a very short-lived ion pair intermediate formed by electron-transfer quenching.