HYPERICIN DERIVED TRIPLET STATES and TRANSIENTS IN ALCOHOLS and WATER

Abstract— Direct pulse photoexcitation of an antivirally active compound, hypericin sodium salt in ethanol, results in a short-lived transient, attributed to a triplet state. In the presence of reducing agents, a long-lived transient is observed, which indicates a radical anion species. In isopropanol solution, an identical triplet state is formed, accompanied by a long-lived intermediate that consists of a semiquinone-type radical. Laser excitation of hypericin sodium salt aggregates dispersed in water produces a very short-lived transient, also assigned to a triplet state, which decays, leaving an absorption spectrum, indicating a radical anion species. The latter reacts with oxygen with a rate constant of k ∼ 6 × 10⁷ M^-1 s^-1, suggesting the formation of superoxide.     

A modified Weiszfeld algorithm for the Fermat-Weber location problem

This paper gives a new, simple, monotonically convergent, algorithm for the Fermat-Weber location problem, with extensions covering more general cost functions. Received: September 1999 / Accepted: January 2001?Published online April 12, 2001     

Wave scattering from a rough surface: Solution by an iterative method

Two iterative solutions of the Helmholtz equation for a scalar field in above a rough surface that admits the Dirichlet boundary condition are derived. The bases for the two iterative methods are two different boundary integral equations that represent the solution. The first integral equation is classified as a Fredholm integral equation of the first kind. The second is classified as a Fredholm integral equation of the second kind. This classification suggests that it is easier to find stable solution methods to the second equation. In both methods, the boundary integral was separated into a major part which is easy to calculate and a local residual part. The major part is a convolution and thus can be calculated using FFT in complexity O(N log N), where N is the number of surface points in which the surface height and its first derivatives together with the incoming wave and its normal derivative are all known. The residual element of the equations can be approximated efficiently only for surfaces where their amplitude is less than the wavelength of the incoming wave. The iterative schemes were tested numerically against a reference solution in order to examine the applicability range, the error estimation and the stability of the schemes. All tests supported the superiority of the second method. In particular the error estimation and stability tests indicated good performance for surfaces with slope up to 1. Yet, being an equation in the scattered field alone, makes the first method useful as a benchmark solution in its domain of applicability.     

Sinularectin, a new diterpenoid from the soft coral Sinularia erecta

A new norcembrane, designated sinularectin 3, was isolated from the Kenyan soft coral Sinularia erecta. Sinularectin is a chlorinated highly oxygenated norcembrane with an unprecedented functionalisation of the cembrane isopropyl group. The structure of sinularectin was elucidated by interpretation of the HRESMS results as well as 1D and 2D NMR spectra.     

Silicon tetrafluoride visible luminescence induced by high-power CO2 laser irradiation

The visible luminescence induced by a high-power CO₂ laser in SiF₄ occurs within the laser pulse even at pressures as low as 10^?2 Torr. The unirnolecular nature of the process is also supported by pressure-dependence studies of the luminescence intensity and the incubation period. A possible assignment of the transition involving inverse electronic relaxation is discussed.     

Intersystem crossing at singlet conical intersections

The energy of the lowest triplet state of organic molecules is intermediate between the ground state and the first excited singlet. At the S₁/S₀ conical intersection, the two singlet states are degenerate. It is shown that for some molecules (ethylene, benzene, toluene and pyrrole) the T₁ state is also degenerate with the two singlet states. Moreover, the spin orbit coupling matrix element at this structure is necessarily large, so that intersystem crossing can be quite efficient. If the lowest triplet state is repulsive (as in the studied molecules) it may significantly contribute to the dissociation yield under certain experimental conditions.