New chemistry of 1,2-closo-P2B10H10 and 1,2-closo-As2B10H10; in silico and gas electron diffraction structures, and new metalladiphospha- and metalladiarsaboranes

The molecular structures of 1,2-closo-P(2)B(10)H(10) (1) and 1,2-closo-As(2)B(10)H(10) (2) have been determined by gas electron diffraction and the results obtained compared with those from computation at the MP2/6-31G** level of theory. The level of agreement is good for 2 (root-mean-square [rms] misfit for As and B atoms 0.0297 ?) and very good for 1 (rms misfit for P and B atoms 0.0082 ?). In comparing the structures of 1 and 2 with that of 1,2-closo-C(2)B(10)H(12) (I) it is evident that expansion of the polyhedron from I to 1 to 2 is restricted only to the heteroatom vertices and the B(6) face to which these are bound. Following deboronation (at B3) and subsequent metallation, compounds 1 and 2 have been converted into the new metalladiheteroboranes 3-(η-C(9)H(7))-3,1,2-closo-CoAs(2)B(9)H(9) (4), 3-(η-C(10)H(14))-3,1,2-closo-RuAs(2)B(9)H(9) (5), 3-(η-C(5)H(5))-3,1,2-closo-CoP(2)B(9)H(9) (6), 3-(η-C(9)H(7))-3,1,2-closo-CoP(2)B(9)H(9) (7) and 3-(η-C(10)H(14))-3,1,2-closo-RuP(2)B(9)H(9) (8), the last three constituting the first examples of metalladiphosphaboranes. Together with the known compound 3-(η-C(5)H(5))-3,1,2-closo-CoAs(2)B(9)H(9) (3), compounds 4-8 have been analysed by NMR spectroscopy and (except for 8) single-crystal X-ray diffraction. The (11)B NMR spectra of analogous pairs of metalladiphosphaborane and metalladiarsaborane (6 and 3, 7 and 4, 8 and 5) reveal a consistently narrower (9-10 ppm) chemical shift range for the metalladiarsaboranes, the combined result of a deshielding of the lowest frequency resonance (B6) and an increased shielding of the highest frequency resonance (B8) via an antipodal effect. In crystallographic studies, compounds 3 and 5B (one of two crystallographically-independent molecules) suffer As/B disorder, but in both cases it was possible to refine distinct, ordered, components of the disorder, the first time this has been reported for metalladiarsaboranes. Moreover, whilst the Cp compounds 6 and 3 are disordered, their indenyl analogues 7 and 4 are either ordered or significantly less disordered, a consequence of both the reduced symmetry of an indenyl ligand compared to a Cp ligand and the preference of the former for a distinct conformation relative to the cage heteroatoms. Unexpectedly, whilst this conformation in the cobaltadiphosphaborane 7 is cis-staggered (similar to that previously established for the analogous cobaltadicarborane), in the cobaltadiarsaborane 4 the conformation is close to cis-eclipsed.     

Tuning the wall thickness and pore orientation in mesoporous titania films prepared with low-temperature aging

Porous titania thin films with well-ordered mesostructures are prepared by using Pluronic surfactant P123 as the pore template and aging the films in a high-humidity environment at −6 °C. These structures are stable enough to undergo calcination at 400 °C to generate nanocrystalline TiO₂ walls with retention of mesoporosity. Under the aging conditions used, the films have well-ordered mesostructures even with a molar ratio of P123 to titanium (R) as small as 0.006. Because the P123 micelle diameter remains constant across a range of compositions, the pore diameter also remains fixed but the wall thickness of the titania thin films increases as the P123 concentration decreases without decreasing the long-range order of the products. Furthermore, mesoporous titania thin films with hexagonal close-packed channels oriented perpendicular to the substrate can be obtained R values of 0.008–0.012 by sandwiching the as-prepared films between glass slides modified with crosslinked P123. Analysis of the mesophases obtained here indicates that a transition from films containing significant 2D hexagonal channels to 3D hexagonal structure occurs below P123/Ti = 0.008. This does not match the expected volume fraction for this transition based on the mesophases behavior of aqueous P123 at room temperature, suggesting that a more detailed model would be needed to predict mesostructure in titania films aged below the freezing point of water.     

Copolymerization kinetics of a model siloxane system

Using ²⁹Si NMR, we monitor the copolymerization of trimethylethoxysilane and dimethyldiethoxysilane (model compounds for more complex sol-gel copolymer systems) in a batch reactor. Under the chosen conditions, the extents of self- and cross-condensation reactions are readily determined. Using a nonideal polycondensation kinetic model, we show that the copolymerization rate coefficient for a pair of sites of differing functionality is bounded by their homopolymerization rate coefficients, lying closer to the larger one. This reactivity pattern generates a heterogeneous monomer distribution in the copolymerization products. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1293–1302, 1997     

Differences Between Gas-Phase and Solid-State Molecular Structures of the Simplest Phosphonium Ylide,Me3P=CH2

Clearly different from local C ₃ symmetric is the heavy-atom core of Me₃P=CH₂, the simplest phosphonium ylide. The geometry obtained by reanalysis of gas-electron-diffraction data from 1977 is now consistent with theoretical calculations, but different from the molecular structure in the solid state. The picture shows the structure of Me₃P=CH₂ in the gas phase (a) and in the crystal (c) together with the calculated transition state (b) (viewed along the P=C bond).     

Synthesis and 1H NMR spectra of some 1-aryl-2,5-pyrrolidinediones

Three 1-aryl-2,5 pyrrolidinediones, two of which are novel, were prepared by reaction of the requisite primary aromatic amines with succinic anhydride, followed by treatment with acetic anhydride. The ¹H nmr spectra for the derivatives in which aryl is 1-naphthyl and 1-anthracenyl exhibit 32-line multiplets for the four aliphatic hydrogens, indicating that all four are in different environments. Examination of molecular models demonstrates that the pyrrolidinedione and aryl ring systems cannot be coplanar and that rotation about the nitrogen-aryl bond is restricted. Molecular mechanics calculations reveal that a dihedral angle of 50–65° for the two ring systems results in the minimum steric interaction energy.     

Recent applications of density functional theory calculations to biomolecules

The purpose of this overview is to highlight the broad scope and utility of current applications of density functional theory (DFT) methods for the study of the properties and reactions of biomolecules. This is illustrated using examples selected from research carried out within our research group and in collaboration with others. The examples include the hyperfine coupling constants of amino acid radicals, the use of an amino acid as a chiral catalyst for the formation of carbon–carbon bonds in the aldol reaction, hydrogen-bond mediated catalysis of an aminolysis reaction, radiation-induced protein–DNA cross-links, and the mechanism by which an antitumor drug cleaves DNA. We demonstrate that DFT-based methods can be applied successfully to a broad range of problems that remain beyond the scope of conventional electron-correlation methods. Furthermore, we show that contemporary computational quantum chemistry complements experiment in the study of biological systems. Received: 19 December 2001 / Accepted: 8 April 2002 / Published online: 4 July 2002