Coordination compounds of 2-mercapto-1-methylimidazole with platinum(II), palladium(II), rhodium(III) and ruthenium(III)

Summary Complexes of 2-mercapto-1-methylimidazole (TMZ) with Pt^II, Pd^II, Rh^III and Ru^III of the general formulae Pt(TMZ)₂Cl₂, Pd(TMZ)₄Cl₂. Rh(TMZ)Cl₃ and Ru(TMZ)Cl₃ have been obtained. The thermal stabilities of the compounds were estimated by derivatographic measurements and the electron-donating atom of the measurements and the electron-donating atom of the ligand was identified from the i.r. absorbtion spectra. Lattice constants for the Pt^II and Pd^II complexes were estimated from their x-ray powder diffraction patterns.     

Photophysics of aromatic molecules with low-lying pi sigma* states: excitation-energy dependence of fluorescence in jet-cooled aromatic nitriles

Excitation-energy dependence of fluorescence intensity and fluorescence lifetime has been measured for 4-dimethylaminobenzonitrile (DMABN), 4-aminobenzonitrile (ABN), 4-diisopropylaminobenzonitrile (DIABN), and 1-naphthonitrile (NN) in a supersonic free jet. In all cases, the fluorescence yield decreases rather dramatically, whereas the fluorescence lifetime decreases only moderately for S1 (pi pi*, L(b)) excess vibrational energy exceeding about 1000 cm(-1). This is confirmed by comparison of the normalized fluorescence excitation spectrum with the absorption spectrum of the compound in the vapor phase. The result indicates that the strong decrease in the relative fluorescence yield at higher energies is due mostly to a decrease in the radiative decay rate of the emitting state. Comparison of the experimental results with the TDDFT potential energy curves for excited states strongly suggests that the decrease in the radiative decay rate of the aminobenzonitriles at higher energies is due to the crossing of the pi pi* singlet state by the lower-lying pi sigma*(C[triple bond]N) singlet state of very small radiative decay rate. The threshold energy for the fluorescence "break-off" is in good agreement with the computed energy barrier for the pi pi*/pi sigma* crossing. For NN, on the other hand, the observed decrease is in fluorescence yield at higher excitation energies can best be attributed to the crossing of the pi pi* singlet state by the pi sigma* triplet state.     

Square-Planar Nickel(II) O,O′-Dialkyldithiophosphato Complexes with Triphenylphosphine of the Type [NiX(S2P{OR}2)(PPh3)] (X = Cl,Br, I and NCS)

A series of new [NiX(S₂P{O-c-Hex}₂)(PPh₃)](X = Cl^–, Br^–, I^– and NCS^–)(1)–(4) and [Ni(NCS)(S₂P{OR}₂)(PPh₃)][R =n-Pr (5), i-Pr (6)] complexes has been synthesized and characterized by elemental analyses, f.i.r., i.r., u.v.–vis., ¹H-, ¹³C{¹H}- and ³¹P{¹H}-n.m.r. spectra, magnetochemical and conductivity measurements. A single crystal X-ray analysis of [Ni(NCS)(S₂P{O-n-Pr}₂)(PPh₃)](5) reveals the molecular structure of the complex and confirms a square-planar geometry around the central atom of nickel with the NCS anion coordinated via the nitrogen atom.     

Synthesis, Crystal Structure, and IR Spectroscopic Characterization of 1,6-Hexanediammonium Dihydrogendecavanadate

Summary.  Anhydrous 1,6-hexanediammonium dihydrogendecavanadate ((HdaH₂)₂H₂V₁₀O₂₈, 1) was prepared by reaction of V₂O₅ with 1,6-hexanediamine in aqueous solution. The crystal structure of 1 was determined, and the proton positions in the H₂V₁₀O₂₈ ⁴⁻ anion were calculated by the bond length/bond number method. The protons are bound to the centrosymmetrically oriented μ–OV₃ groups of the decavanadate anion. Based on the analysis of IR spectra of 1 prepared from H₂O and D₂O, the absorption band at 871 cm⁻¹ can be attributed to δ(V–O_b–H) vibrations. Received August 3, 2001. Accepted (revised) October 8, 2001     

Solvatochromic equilibria of cobalt(II) chloride in binary mixtures of dipolar aprotic solvents with water

The visible absorption spectra of CoCl₂ in binary mixtures of acetone, dmf, dma and dmso with H₂O show the existence of configuration equilibria between tetrahedral (T) and octahedral (O) species. Linear correlations between the values of log K (K = [O]/[T]) and the mole fraction of H₂O, _w, have been observed in all the systems under study; their slopes decrease linearly with an increase of the donor number, DN, of the organic component of the mixture. The formation of octahedral species is enthalpy favoured but entropy disfavoured. It was found that both H and S values change linearly with _w and the slopes of these relationships depend clearly on the DN values.     

The electronic absorption study of imide anion radicals in terms of time dependent density functional theory

The absorption spectra of the N-(2,5-di-tert-butylphenyl) phthalimide (1-), N-(2,5-di-tert-butylphenyl)-1,8-naphthalimide (2-) and N-(2,5-di-tert-butylphenyl)-perylene-3,4-dicarboximide (3-) anion radicals are studied in terms of time dependent density functional theory (TDDFT). For these anion radicals a large number electronic states (from 30 to 60) was found in the visible and near-IR regions (5000-45,000 cm(-1)). In these regions the TD/B3LYP treatment at the 6-1+G* level is shown to reproduce satisfactorily the empirical absorption spectra of all three anion radicals studied. The most apparent discrepancies between purely electronic theory and the experiment could be found in the excitation region corresponding to D0-->D1 transitions in the 2- and 3- molecules. For these species we argue that the structures seen in the lowest energy part of the absorptions of the 2- and 3- species are very likely due to Franck-Condon (FC) activity of the totally symmetric vibrations not studied in this Letter.     

Coordination compounds of nickel with trithiocyanuric acid. Part IV. Structure of [Ni(pmdien)(ttcH)] (pmdien = N,N,N′,N′,N′′-pentamethyldiethylenetriamine,ttcH3 = trithiocyanuric acid)

Ni^II mixed-ligand complexes of compositions [Ni(pmdien)(ttcH)] (1), [Ni(baphen)₂(ttcH)] · 4H₂O (2), [Ni-(dpa)(ttcH)(H₂O)] (3), [Ni(cyclam)(ttcH)] · 2H₂O (4), [Ni(hexaa)](ttcH) (5) and [Ni(hexab)(ttcH)] · 2H₂O (6), (baphen = 4,7-diphenyl-1,10-phenanthroline, dpa = 2,2-dipyridylamine, cyclam = 1,4,8,11-tetraazacyclotetradecane, hexaa = 1,3,6,9,11,14-hexaazatricyclo[12.2.1.1^6,9]-octadecane, hexab = 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane) have been prepared and characterized by means of i.r., u.v.–vis. spectroscopies and magnetochemical measurements. The redox properties of the complexes were studied by cyclic voltammetry. The crystal and molecular structure of [Ni(pmdien)(ttcH)] was determined. The nickel atom is penta-coordinated by three N atoms of pmdien, and by S and N atoms of trithiocyanurate(2–) anion.     

Decoding the intricate network of molecular interactions of a hyperstable engineered biocatalyst

Computational design of protein catalysts with enhanced stabilities for use in research and enzyme technologies is a challenging task. Using force-field calculations and phylogenetic analysis, we previously designed the haloalkane dehalogenase DhaA115 which contains 11 mutations that confer upon it outstanding thermostability (T_m = 73.5 °C; ΔT_m > 23 °C). An understanding of the structural basis of this hyperstabilization is required in order to develop computer algorithms and predictive tools. Here, we report X-ray structures of DhaA115 at 1.55 Å and 1.6 Å resolutions and their molecular dynamics trajectories, which unravel the intricate network of interactions that reinforce the αβα-sandwich architecture. Unexpectedly, mutations toward bulky aromatic amino acids at the protein surface triggered long-distance (∼27 Å) backbone changes due to cooperative effects. These cooperative interactions produced an unprecedented double-lock system that: (i) induced backbone changes, (ii) closed the molecular gates to the active site, (iii) reduced the volumes of the main and slot access tunnels, and (iv) occluded the active site. Despite these spatial restrictions, experimental tracing of the access tunnels using krypton derivative crystals demonstrates that transport of ligands is still effective. Our findings highlight key thermostabilization effects and provide a structural basis for designing new thermostable protein catalysts.

Illustration of cooperative thermostabilization effects of the double-lock system that: (i) induced backbone changes, (ii) closed the molecular gates, (iii) reduced the volumes of the main and slot access tunnels, and (iv) occluded the active site.