Über die Kronenetherkomplexe [K(15-Krone-5)2]3[Sb3I12], [TeCl3(15-Krone-5)][TeCl5] und [TeCl3(15-Krone-5)]2[TeCl6]

On the Crown Ether Complexes [K(15-Crown-5)₂]₃[Sb₃I₁₂], [TeCl₃(15-Crown-5)][TeCl₅], and [TeCl₃(15-Crown-5)]₂[TeCl₆] Orange-coloured crystals of [K(15-crown-5)₂]₃[Sb₃I₁₂] are formed in the reaction of potassium iodide with antimony triiodide and 15-crown-5 in acetonitrile solution. An X-ray structure determination reveals severe disorder of the crown ether molecules, which coordinate to the potassium atoms in a sandwich array; so only the [Sb₃I₁₂]^3? ion and the potassium positions were ascertained. The anion is a centrosymmetric trimer (symmetry C_2h), which can be understood as central SbI₆^3? ion, coordinated by two SbI₃ molecules. (Space group C2/m), Z = 2, 3263 observed, independent reflections, R = 0.06, lattice dimensions at 20°C: a = 2541.1 pm, b = 1441.7 pm, c = 1588.4 pm, β = 113.33°. The tellurium complexes [TeCl₃(15-crown-5)] [TeCl₅] and [TeCl₃(15-crown-5)]₂[TeCl₆] are prepared by reaction of TeCl₄ with 15-crown-5 in acetonitrile solution, forming yellow-green crystals sensitive to moisture. They are characterized by their i.r. spectra.     

Solution Conformations,Photophysics, and Photochemistry of Bile Pigments; Bilirubin and Biliverdin,Dimethyl Esters and Related Linear Tetrapyrroles

Bilirubin and biliverdin dimethyl esters (BRE and BVE, respectively) and related linear tetrapyrroles have been studied using a combination of photochemical and spectroscopic techniques, the latter including absorption, fluorescence fluorescence excitation, medium-induced circular dichroism, and proton magnetic resonance. Both types of tetrapyrroles form mixtures of different topological isomers in very dilute solutions. In the case of the bilirubins the heterogeneity of the solutions is caused by two coexisting conformers with different orientations of the A/B and C/D pyrromethenone moieties with repect to each other. The spectral properties of one conformer resemble the isolated parent pyrromethenone, whereas those of the other result from electronic coupling of the two subchromophores presumably held in a “ridge tile” -like orientation. C? C rotations at the C-5 and C-15 bridges substantially compete in both components with the photochemical channels (E→Z isomerization and lumirubin formation) for the radiationless deactivation of the excited singlet state. The more rigid “ridge tile” component additionally undergoes hydrogen bond-mediated deactivation, and it photoisomerizes more efficiently. The situation is markedly more complex with the biliverdins. In order to obtain a more detailed insight into the mechanisms of the radiationless excited-state processes, time-resolved optoacoustic spectroscopy and ultrafast absorption (pump-probe) and fluorescence detection (single-photon-timing) techniques were used to supplement the stationary methods. The solution mixtures are composed of a (family of) helically coiled all-Z, all-syn species, and of species differing from the former by stretched arrangements of the rings B and C around the central C-10 bridge (E-anti, E-syn, and Z-anti). Two excited singlet states with picosecond lifetimes are attributed to either one or two coiled ground-state forms, and two remarkably long-lived nanosecond excited states arise each from a stretched ground state. The radiationless deactivation of the shorter-lived of the picosecond states is brought about by ultrafast intramolecular proton transfer between the B/C nitrogen atoms, in addition to the C? C rotational modes operative in both. Z→E photoisomerization is also an appreciable deactivation channel of excited biliverdin dimethyl ester. It is confined to the central C-10 double bond and selectively affords a stretched isomer (10E-anti), which thermally reforms the coiled starting meterial at room temperature via a sequence of tautomerization and C? C rotation. Heating or ultrasonic treatment can reverse this sequence and drive it farther to populate another stretched isomer (10E-syn) which is thermally stable at room temperature. This stretched form aggregates (presumably to dimers) already at concentrations at which the coiled species still appears to be fully monomeric.     

Quantitative molecular thermochemistry based on path integrals

The calculation of thermochemical data requires accurate molecular energies and heat capacities. Traditional methods rely upon the standard harmonic normal-mode analysis to calculate the vibrational and rotational contributions. We utilize path-integral Monte Carlo for going beyond the harmonic analysis and to calculate the vibrational and rotational contributions to ab initio energies. This is an application and an extension of a method previously developed in our group [J. Chem. Phys. 118, 1596 (2003)].     

Internal degrees of freedom,structural motifs,and conformational energetics of the 5'-deoxyadenosyl radical: implications for function in adenosylcobalamin-dependent enzymes. A computational study

The potential energy surface of the free 5'-deoxyadenosyl radical in the gas phase is explored using density functional and second-order M?ller-Plesset perturbation theories with 6-31G(d) and 6-31++G(d,p) basis sets and interpreted in terms of attractive and repulsive interactions. The 5',8-cyclization is found to be exothermic by approximately 20 kcal/mol but kinetically unfavorable; the lowest cyclization transition state (TS) lies about 7 kcal/mol higher than the highest TS for conversion between most of the open isomers. In open isomers, the two energetically most important attractive interactions are the hydrogen bonds (a) between the 2'-OH group and the N3 adenine center and (b) between the 2'-OH and 3'-OH groups. The relative ribose-adenine rotation about the C1'-N9 glycosyl bond in a certain range changes the energy by as much as 10-15 kcal/mol, the origin being (i) the repulsive 2'-H.H-C8 and O1'.N3 and (ii) the attractive 2'-OH.N3 ribose-adenine interactions. The hypothetical synergy between the glycosyl rotation and the Co-C bond scission may contribute to the experimentally established labilization of the Co-C bond in enzyme-bound adenosylcobalamin. The computational results are not inconsistent with the rotation about the C1'-N9 glycosyl bond being the principal coordinate for long-range radical migration in coenzyme B(12)-dependent enzymes. The effect of the protein environment on the model system results reported here remains an open question.     

Kristall- und Molekülstruktur von 6-Ethoxy-12,12-dimethyldibenzo[d,g]-1,3,2,6-dioxaphosphasilocin-6-oxid

Synthesis and Crystal Structure of the Bis(amidinatochelate) Complex ClSb[Ph? C(NSiMe₃)₂]₂ The antimony(III) amidinato complex ClSb[Ph? C(NSiMe₃)₂]₂ was obtained by the reaction of antimony trichloride and N,N,N′-tris(trimethylsilyl)benzamidine in dichloromethane in form of pale-yellow, moisture sensitive crystals. X-ray crystal structure determinations were performed at 20°C and at ?93°C. Crystal data at 20°C: space group P1 , Z = 2, a = 1160.3(2), b = 1305.4(2), c = 1336.5(2) pm, α = 68.32(1), β = 79.79(1), γ = 71.47(1)°; at ?93°C the lattice vectors are 1.20 to 0.85% shorter. In the molecule two Ph? C(NSiMe₃)₂ groups are attached with their N atoms in a chelate manner to the Sb atom. Together with the Cl atom they form an irregular coordination polyhedron about the Sb atom with a stereochemically strongly effective lone electron pair. The SiMe₃ groups show considerable twisting vibrations about the N? Si axes even at ?93°C.     

Phosphanimin- und Phosphaniminato-Komplexe von Magnesium. Die Kristallstrukturen von [MgBr1,25I0,75(Me3SiNPMe3)(OEt2)], [MgI2(Me3SiNPMe3)2], [Mg2I2(Me3SiNPMe2CH2)Me3SiNPMe2CH2CH(Me)O)(OEt2)] und [MgBr(NPMe3)]4·C7H8

Phosphaneimine and Phosphoraneiminato Complexes of Magnesium. The Crystal Structures of [MgBr_1,25I_0,75(Me₃SiNPMe₃)(OEt₂)], [MgI₂(Me₃SiNPMe₃)₂], [Mg₂I₂(Me₃SiNPMe₂CH₂)(Me₃SiNPMe₂CH₂CH(Me)O)(OEt₂)], and [MgBr(NPMe₃)]₄ · C₇H₈ By reactions of the silylated phosphaneimine Me₃SiNPMe₃ with the Grignard reagents EtMgBr and MeMgI, respectively, the carbanionic phosphoraneiminato derivatives [XMg(CH₂PMe₂NSiMe₃)]_n (X ? Br, I) can be isolated as main products. The by-products of these reactions, [MgBr_1.25I_0.75(Me₃SiNPMe₃)(OEt₂)], [MgI₂(Me₃SiNPMe₃)₂] and [Mg₂I₂(CH₂PMe₂NSiMe₃)(O(Me)CHCH₂PMe₂NSiMe₃)(OEt₂)] were identified by crystal structure determinations. The phosphoraneiminato complex [MgBr(NPMe₃)]₄ · C₇H₈ with hetero cubane structure is formed by a metathesis reaction of [ZnBr(NPMe₃)]₄ with RMgBr (R ? Ph. Mes).     

Synthese und Kristallstruktur von SMe3 ⊕[MoBr4(SMe2)2]−

Synthesis and Crystal Structure of SMe₃^⊕ [MoBr₄(SMe₂)₂]^? . The title compound is obtained from MoBr₄ and excess dimethyl sulfide, forming red crystals which are only slightly sensitive to moisture. Whereas the intermediately formed adduct of MoBr₄ and SMe₂ is unstable, the stable adduct [Mo(NO)₂Br₂(SMe₂)₂] can be prepared from Mo(NO)₂Br₂ and SMe₂. According to the structural analysis by X-ray methods, SMe₃ ^⊕[MoBr₄(SMe₂)₂]^? crystallizes orthorhombic in the space group Imma with eight formula units per unit cell, the cell dimensions being a = 1578, b = 2820, c = 856 pm (1303 observed, independent reflexions). The compound consists of S(CH₃)₃^⊕ cations with S? C bond lengths of 180 pm and C? S? C bond angles of 102° and 103° resp., and anions [MoBr₄(SMe₂)₂]^?. The molybdenum atom is coordinated octahedral by four bromine atoms in equatorial positions and the two S atomes of the SMe₂ donor molecules in axial sites with Mo? S bond lengths of 254 pm.