Thermally induced incomplete high-spin (5T2) ⇌ low-spin (1A1) transition in solid bis[2-(2-pyridylamino)-4-(2-pyridyl) thiazolato]iron (II)

The ⁵⁷Fe Mössbauer effect in two samples (A and B) of [Fe(papt)₂] and in its solvates with CHCl₃ and C₆H₆ has been studied between 4.2 and 343 K and clearly indicates a temperature induced high-spin (⁵T₂) ? low-spin (¹A₁) transition in these compounds [paptH = 2-(2-pyridylamino)-4-(2-pyridyl) thiazole]. At 343 K, sample B shows a doublet with ΔE_Q = 2.03 mm s^?1 and δIS = +0.87 mm s^?1, characteristic of a ⁵T₂ ground state. At 257 K, a second doublet, typical for a ¹A₁ ground state, is observed and its intensity increases as the transition progresses but levels off below ～ 100 K. At 4.2 K, 83% of the intensity is due to the ¹A₁ state, and ΔE_Q(¹A₁) = 1.56 mm s^?1 and δ^IS(¹A₁ = +0.32 mm s^?1. In an applied magnetic field, V_zz(¹A₁) < 0 and η ≈ 0.7 have been determined, whereas for the ^sT₂ ground state, V_zz(^sT₂) > 0, η ≈ 0.75, and an internal hyperfine field H_n ≈ ?13 kG have been observed. Similar results have been obtained with the other samples.Debye-Waller factors f5_T2 and f1_A1 were determined from the saturation corrected areas in the Mössbauer spectra, assuming Curie-Weiss dependence of the magnetic susceptibility for the ⁵T₂ and constant υ_cff for the ¹A₁ ground state. The temperature dependence of ?In f1_A1 closely follows the Debye model with Θ1_A1 = 165 K, whereas the same applies to ?ln f5_T2 only above ～ 210 K and Θ5_T2 = 134 K. The nature of the observed transition is discussed and the data presented are shown to be incompatible with a model based on a Boltzmann distribution between the two states.     

Valency-bond studies of some conjugated hydrocarbons

A valency bond study of naphthalene has been made in which, assuming the molecular symmetry to be D _2h, successively more canonical structures up to the total of 42 have been included. All four chemically differentiable bonds have been independently varied in length to find the minimum molar energy and thus the equilibrium ground state molecular structure. The result is remarkably close to the observed structure and also agrees well with those deduced by other types of calculation.

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Zusammenfassung Im Rahmen der Methode der Valenzstrukturen wurde eine Untersuchung des Naphthalins durchgeführt, wobei als Symmetriegruppe des Moleküls D _2hangenommen wurde. Die Anzahl der einbezogenen Strukturen wurde schrittweise auf die Gesamtzahl von 42 erhöht. Bei der Minimisierung der molekularen Energie zur Bestimmung der Gleichgewichtsstruktur des Grundzustandes wurden auch die Längen aller vier chemisch unterscheidbaren Bindungen variiert. Diese Gleichgewichtsabstände stimmen sowohl mit den Meßwerten als auch mit den Ergebnissen anderer Rechenverfahren gut überein.

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Résumé Une étude du naphtalène a été faite par la méthode de la mésomérie. On a supposé que la symétrie soit D _2het on a compris successivement de plus en plus formules canoniques jusqu'au total de 42. On a fait varier indépendamment les longueurs de toutes les quatre liaisons qu'on peut distinguer chimiquement afin qu'on peut estimer l'énergie moléculaire minimale et ainsi la structure stable de la molécule en état fondamental. Le résultat s'approche remarquablement de la structure observée et s'accorde bien à celles qu'on a déterminées par des autres types de calculs.

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The author gratefully acknowledges helpful discussions with Professor C. A. Coulson (Oxford), Professor D. H. W. Den Boer and Mevrouw P. C. Den Boer (Utrecht) and Dr. E. Clar (Glasgow).     

A note on upper bounds for forecast-value-added relative to naïve forecasts

In forecast value added analysis, the accuracy of relatively sophisticated forecasting methods is compared to that of naïve 1 forecasts to see whether the extra costs and effort of implementing them are justified. In this note, we derive a ratio that indicates the upper bound of a forecasting method’s accuracy relative to naïve 1 forecasts when the mean squared error is used to measure one-period-ahead accuracy. The ratio is applicable when a series is stationary or when its first differences are stationary. Formulae for the ratio are presented for several exemplar time series processes.     

A high-throughput, high-resolution strategy for the study of site-selective DNA binding agents: analysis of a "highly twisted" benzimidazole-diamidine

A general strategy for the rapid structural analysis of DNA binding ligands is described as it was applied to the study of RT29, a benzimidazole-diamidine compound containing a highly twisted diphenyl ether linkage. By combining the existing high-throughput fluorescent intercalator displacement (HT-FID) assay developed by Boger et al. and a high-resolution (HR) host-guest crystallographic technique, a system was produced that was capable of determining detailed structural information pertaining to RT29-DNA interactions within approximately 3 days. Our application of the HT/HR strategy immediately revealed that RT29 has a preference for 4-base pair (bp), A.T-rich sites (AATT) and a similar tolerance and affinity for three A-T-bp sites (such as ATTC) containing a G.C bp. On the basis of these selectivities, oligonucleotides were designed and the host-guest crystallographic method was used to generate diffraction quality crystals. Analysis of the resulting crystal structures revealed that the diphenyl ether moiety of RT29 undergoes conformational changes that allow it to adopt a crescent shape that now complements the minor groove structure. The presence of a G.C bp in the RT29 binding site of ATTC did not overly perturb its interaction with DNA-the compound adjusted to the nucleobases that were available through water-mediated interactions. Our analyses suggest that the HT/HR strategy may be used to expedite the screening of novel minor groove binding compounds leading to a direct, HR structural determination.     

The photochemistry of intramolecular excimers: the role of intersystem crossing and product formation

The quantum yields of triplet formation by some α,ω-dinaphthylalkanes and related compounds which exhibit intramolecular excimer fluorescence and/or intramolecular fluorescence quenching have been determined. Although most compounds have quite high quantum yields (~ 0.4) others are much lower and no single reason could be found to explain this variance.     

Shear field modification of strongly flocculated suspensions — Aggregate morphology

Rheological and microscopical studies have been made to elucidate the effects of shear fields on the morphology of concentrated, aggregated model colloids. The models employed are well-characterised, predominantly chargestabilised polymer latices, coagulated by the addition of excess electrolyte. Continuous shear rheological and viscoelastic measurements indicate a very significant decrease in shear yield stress, apparent viscosity and shear modulus following prolonged shearing.Electron microscopy reveals the source of these changes. Freshly coagulated suspensions form networks that are porous, strong and qualitatively similar to simulated structures for diffusion limited aggregation. Following protracted shearing, the network structure is rearranged to yield discrete, tightly packed aggregates with a characteristic size, which is principally a function of the primary particle size.