Carbon-13 n.m.r. studies. Part 66—13C spectra of several methyl,hydroxymethyl and carboxylic acid derivatives of bicyclo[2.2.1]heptane and bicyclo[2.2.2]octane

The ¹³C n.m.r. spectra of a series of 52 substituted norbornanes, norbornenes, bicyclo[2.2.2]octanes and -octenes have been recorded to examine the stereochemical effects of closely neighboring substituents on the carbon shieldings of these well-defined molecular frameworks. These moderately rigid skeletons permit detailed examination of a variety of orientations of substituents and, in this study, attention has been focused on a series of vicinally substituted systems. The substituents include methyl, hydroxymethyl and carboxylic acid groups for comparison with earlier data for several bicyclic alcohols and hydrocarbons, from which conclusions regarding the conformational preferences of the hydroxyl group may be drawn. The deviations of the observed shieldings from those predicted by simple additivity follow definite patterns which should be useful for stereochemical assignments in related systems.     

More mutually orthogonal latin squares

Wilson's construction for mutually orthogonal Latin squares is generalized. This generalized construction is used to improve known bounds on the function n_r (the largest order for which there do not exist r MOLS). In particular we find n₇?780, n₈?4738, n₉?5842, n₁₀?7222, n₁₁?7478, n₁₂?9286, n₁₃?9476, n₁₅?10632.     

On the number of unique subgraphs of a graph

A question of Entringer and Erdös concerning the number of unique subgraphs of a graph is answered.     

New antitumour active platinum compounds containing carboxylate ligands in trans geometry: synthesis, crystal structure and biological activity

New asymmetric trans-platinum(II) complexes, composed of an isopropylamine, an azole and two carboxylate leaving groups, are presented. The crystal and molecular structures of one of the complexes has been determined and the cytotoxicity and reactivity with 5'-guanosine monophosphate is reported. The complexes show a reduced reactivity, but no decrease in cytotoxic activity compared to their chloro-counterparts. Furthermore the complexes largely overcome cisplatin resistance, they therefore present an interesting class of antitumour active trans-platinum complexes.     

Ultrafast exciton dynamics in CdSe quantum dots studied from bleaching recovery and fluorescence transients

We have performed ultrafast absorption bleach recovery and fluorescence upconversion measurements ( approximately 100 fs time resolution) for three CdSe samples, with nanoparticle diameters of 2.7, 2.9, and 4.3 nm. The two types of experiments provide complementary information regarding the contributions of the different processes involved in the fast relaxation of electrons and holes in the CdSe quantum dots. Transient absorption and emission experiments were conducted for the 1S [1Se-1S3/2(h)] transition, 1S(e) and 1S3/2(h) representing the lowest electron (e) and hole (h) levels. The bleach recovery of the 1S transition shows a approximately 400-500 fs initial rise, which is followed by a size-dependent approximately 10-90 ps decay and finally a long-lived (approximately ns) decay. The fluorescence upconversion signal for the 1S transition shows quite different temporal behavior: a two times slower rise time (approximately 700-1000 fs) and, when the fluorescence upconversion signal has risen to about 20% of its maximum intensity, the signal displays a slight leveling off (bend), followed by a continued rise until the maximum intensity is reached. This bend is well reproducible and power and concentration independent. Simulations show that the bend in the rise is caused by a very fast decay component with a typical time of about 230-430 fs. Considering that the 1S quantum dot excitation is comprised of five exciton substates (F=+/-2, +/-1L, 0L, +/-1U, and 0U), we attribute the disparity in the rise of the bleaching and emission transients to the results from the dynamics of the different excitons involved in respectively the bleaching and fluorescence experiments. More specifically, in transient absorption, population changes of the F=+/-1U excitons are probed, in emission population effects for the F=+/-2 ("dark") and the F=+/-1L ("bright") exciton states are monitored. It is discussed that the fast (approximately 400-500 fs) rise of the bleach recovery is representative of the feeding of the F=+/-1U exciton (by filling of the 1S(e) electron level) and that the slower (approximately 700-1000 fs) feeding of the emissive +/-2, +/-1L excitons is determined by the relaxation of the hole levels within the 1S3/2 fine structure. Finally, the approximately 230-430 fs component, typical of the bend in the fluorescence transient, is attributed to the thermalization of the close-lying +/-2 ("dark") and +/-1L ("bright") excitons.     

Covering all points except one

In many point-line geometries, to cover all points except one, more lines are needed than to cover all points. Bounds can be given by looking at the dimension of the space of functions induced by polynomials of bounded degree.