THEORETICAL CALCULATIONS OF THE TRANSITIONS OF PHOTOCOLORED FORMS OF CHROMENES

Abstract— –Theoretical prediction (MO-SCF-CI) of numbers, energies and intensities of transitions are compared with experimental data for the photocolored products of three types of chromenes. Generally good results are obtained. Several conclusions resulted from variation of parameters: (1) When all carbon-carbon bonds are considered equal in length, there is good agreement between experiment and theory regarding band locations and intensities, however, very poor correlation exists between theory and experiment when calculations are done on a model where the carbon-carbon bonds are alternately single and double, (2) theoretical calculations employing the Mataga approximation for the two-center repulsion integrals gives superior correlation with the experimental spectra in comparison with the Pariser-Parr approximation, and (3) variation of certain parameters (as I.P., E.A., β) did produce changes in the results, especially in the oscillator strengths, but there was no clear best choice, and therefore, conventional parameters were used.     

TREATMENT VERSUS PREVENTION OF NITROGEN FERTILIZER POLLUTION: AN INTER‐SECTORAL EXTERNALITY POLICY MODEL

Abstract This article presents an economic, hydrological, dynamic optimization model, which describes the negative external effects of nitrogen fertilizers on groundwater quality. The relative merits of treatment versus prevention of nitrogen pollution were analyzed. A dynamical water and nitrogen flow between land surface, the unsaturated zone, and groundwater was employed. A specific treatment technology, which gives rise to a discontinuous cost function, was also used. Applying the model to the coastal aquifer in Israel, our results showed that in a joint (agricultural and domestic) water source area that supplies a relatively small quantity of drinking water, it is more efficient to combine a policy that imposes restrictions on the use of nitrogen with a drinking‐water treatment process. However, when a relatively large quantity of drinking water is involved, imposing restrictions on the use of nitrogen only is more efficient. The paper, thus, is useful to planners of fast growing urban population centers with regard to regulation and can be used to calculate and evaluate specific policies.     

THE SPECTROSCOPY AND PHOTOCHEMISTRY OF NATURALLY OCCURRING AND SYNTHETIC CHROMENES

Abstract— –All naturally occurring and synthetic chromenes studied exhibit photochromism at — 196°C and in some cases at room temperature. Chromenes containing a coumarin moiety have the least photochemistry and also exhibit the strongest emissions. Chromenes containing a hydroxyl group ortho to an acetyl group exhibit solely a phosphorescence. Lapachenole (a 7,8-benzochromene) shows the most rapid and highest percentage conversion to the colored form and no emission. Photocolored form of evodione can be produced by a triplet energy transfer from benzophenone.     

TWENTY-TWO CARBON HOMOLOGUE OF 11-CIS RETINAL. PHOTOPHYSICAL AND PHOTOCHEMICAL PROPERTIES

Abstract— Results concerning absorption-emission spectra and fluorescence quantum yields at 77 and 298K. triplet absorption spectra, and quantum yields of intersystem crossing and photoisomerization at 298 K, are presented for 11- cis β-apo-14'-carotenal (C₂₂-Ald), the immediate higher homologue of 11- cis retinal. The absorption spectra are characterized by two band-systems with maxima at 390–400 and 270–280 nm, respectively. Upon cooling from 298 to 77 K, the intensities of these two band-systems undergo changes in opposite directions indicating 12-s- cis: 12- s-trans conformational changes. No intermediate band-system analogous to the one located at 270–310 nm in cis retinals is observed for 11- cis C₂₂-Ald. In nonpolar hydrocarbon solvents (e. g. cyclohexane) at room temperature, quantum yields of fluorescence (0.01), intersystem crossing (0.6) and photoisomerization (0.4) are all quite pronounced. The photophysical and photochemical properties of 11- cis C₂₂-Ald are discussed in the light of similarities and dissimilarities with those of all- trans C₂₂-Ald and 11- cis retinal under comparable conditions.