Charge calculations in molecular mechanics. IX. A general parameterisation of the scheme for saturated halogen,oxygen and nitrogen compounds

Summary The CHARGE2 program for the calculation of partial atomic charges has been amended to include bond parameters for a number of organic functional groups, including halogens, nitrogen and oxygen. These minor amendments to the original scheme produce dipole moments for the fluoro and chloro compounds which are in complete agreement with the observed values.The less complete data sets for the bromo and iodo compounds are also well reproduced, and the dipole moments of a variety of mixed halo compounds are now in better agreement with experiment than previously.The calculated dipole moments of the saturated nitrogen and oxygen compounds are now in much better agreement than in the original scheme, thus the revised parameterisation may be employed with confidence to predict the electrostatic energies of these compounds.Furthermore, the revised scheme now gives a precise proportionality between the charge on the proton in a CH group and the ¹H chemical shift of the corresponding proton, allowing the general prediction, in principle, of ¹H chemical shifts. In addition, attempts to include variable electronegativity in the effect are described for fluoro compounds.For part VIII see Ref. 1.     

Elastic π±-d and π±-4He scattering below 70 MeV

Elastic scattering differential cross section data of ^± d at 65 MeV and ^{± 4}He at 51 MeV are presented and compared respectively to fully relativistic 3-body and optical potential calculations including true pion absorption.Presented at the symposium Mesons and Light Nuclei, Liblice, Czechoslovakia, June 1981.     

Electron spin resonance studies of the radiolysis of bi-π-cyclopentadienylalkyls of molybdenum and tungsten

Exposure of (C₅H₅)₂MO(CH₃)₂ and (C₅H₅)₂W(CH₃)₂, prepared from the corresponding dichlorides, to ²⁰Co γ-rays at 77 K gave H₂C.CML₃ carbene species characterised by their ESR spectra, together with a central feature possibly due to the parent cations. Dilute solutions in CD₃OD gave features assigned to the parent anions which were converted on bleaching with visible light into methyl radicals, and H₂C.ML₃ radicals. From the magnitude of the ¹H and ¹⁸³W hyperfine coupling constants, it is deduced that the SOMO for H₂C.WL₃ radicals is strongly localised on carbon.Dilute solutions in aqueous sulphuric acid also gave species with A(2H) = 20 G, identified as the carbene derivatives, H₂C.Ml₃. These were formed on annealing, as signals assigned to HSO₄ · radicals were lost.     

Experimental and theoretical investigation of the 13C and 15N chemical shift tensors in melanostatin-exploring the chemical shift tensor as a structural probe

The determination of backbone conformations in powdered peptides using 13C and 15N shift tensor information is explored. The 13C and 15N principal shift values in natural abundance 13C and 15N melanostatin (L-Pro-L-Leu-Gly amide) are measured using the FIREMAT technique. Furthermore, the orientation of the C-N bond in the 13C shift principal axis system for the backbone carbons is obtained from the presence of the 13C-14N dipolar coupling. The Ramachandran angles for the title compound are obtained from solid-state NMR data by comparing the experimentally determined shift tensor information to systematic theoretical shielding calculations on N-formyl-L-amino acid-amide models. The effects of geometry optimization and neglect of intermolecular interactions on the theoretical shielding values in the model compounds are investigated. The sets of NMR derived Ramachandran angles are assembled in a set of test structures that are compared to the available single-crystal X-ray structure. Shift tensor calculations on the test structures and the X-ray structure are used to further assess the importance of intermolecular interactions when the shift tensor is used as a structural probe in powdered peptides.     

Structure of 4-[4-(Dimethylamino)-Phenylazo] Benzeneboronic Acid and Its Cyclic Esters with D-Glucose: A Computational Study

We present results from a computational study of 4-[4-(dimethylamino)-phenylazo] benzene boronic acid (DABBA) (the 4'-boronic acid isomer of the aminoazobenzene dye N,N-dimethylaminoazobenzene) and its associated anion, as well as, several cyclic esters formed from these azoborates and various conformers of D-glucose. Azo dyes that also contain one or more boronic acid functional groups are of practical importance in the development of chemical sensors for saccharide recognition because of their ability to induce a visible color change upon binding. The lowest-energy DABBA:D-glucose esters found in this investigation consistently involved at least one of the exocyclic hydroxymethyl groups on the D-glucose moiety rather than vicinal cis or trans diol arrangements of hydroxyl groups on the ring.     

Photochemistry of dicyanopyridines

The photochemistry of a variety of dicyanopyridines (2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-dicyanopyridine) in solution at room temperature was investigated. Pulsed UV (308 nm) laser irradiation in deoxygenated acetonitrile yields the triplet state with lifetimes between 4 and 10 μs and absorption bands in the 400 and 320 nm regions. In the presence of added HCl an air-insensitive transient (τ ≈ 10–12 μs, λ_max ≈ 360–380 nm) was observed, suggesting the formation of a protonated excited state.

Irradiation in the presence of amines resulted in the production of the pyridyl radical anion (τ ≈ 40–80 μs, air sensitive, λ_max ≈ 360–380 nm) formed by electron transfer from the amine to the pyridine triplet excited state. Stern-Volmer analysis gave electron transfer rate constants in the range (1–8) × 10⁻⁸ M⁻¹ s⁻¹.

In methanol solvent, irradiation yielded an air-insensitive transient assigned as the neutral pyridyl radical (τ ≈ 30–200 μs, λ_max ≈ 370–385 nm). The formation of these transients is discussed in the context of previous photochemical electron spin resonance and product studies.     

Addressing lead toxicity: complexation of lead(II) with thiopyrone and hydroxypyridinethione O,S mixed chelators

The lead(II) ion is regarded as a serious environmental contaminant. A considerable need exists to develop selective ligands for remediation of this metal ion. Herein, the coordination chemistry of lead(II) is investigated with three O,S donor ligands: thiomaltol, 3-hydroxy-1-methyl-2(1H)-pyridinethione (3,2-HOPTO), and 3-hydroxy-1,2-dimethyl-4(1H)-pyridinethione (3,4-HOPTO). The X-ray structures of [Pb(thiomaltolato)(2)] and [Pb(3,4-HOPTO)(2)] have been solved, revealing the expected 4-coordinate geometries. Electronic spectra have been obtained for the lead(II) complexes with all three ligands. Preliminary solution studies show that the thiomaltol ligand binds lead(II) preferentially over magnesium(II) and calcium(II); however, [Pb(thiomaltolato)(2)] is not stable in the presence of 1 equiv of EDTA. Tetradentate ligands derived from these O,S chelators are expected to generate higher affinity ligands for lead(II) sequestration.