Tetrasulfur, S4: rotational spectrum, interchange tunneling, and geometrical structure

The rotational spectrum of S4 has been observed for the first time in an electrical discharge through sulfur vapor. Two techniques have been used: Fourier transform microwave spectroscopy and long-path millimeter-wave absorption spectroscopy. Small, but systematic shifts of the measured transition frequencies of the normal isotopic species indicate that S4 has C2v symmetry but with a low-lying transition state of D2h symmetry, yielding interchange tunneling at 14.1(2) kHz in its ground vibrational state. From the rotational constants of the normal and the single 34S isotopic species, an experimental (r0) structure has been derived: S4 is a singlet planar trapezoid with a terminal bond length of 1.899(7) A, a central bond of 2.173(32) A, and an S-S-S angle of 103.9(8) degrees. Like thiozone (S3), S4 is a candidate for detection in the atmosphere of the Jovian moon Io and in other astronomical sources.     

Vapochromism and its structural basis in a luminescent Pt(II) terpyridine-nicotinamide complex

A novel Pt(II) terpyridine complex that has a nicotinamide moiety linked to the terpyridyl ligand has been synthesized in good yield and studied structurally and spectroscopically. The complex, [Pt(Nttpy)Cl](PF(6))(2) where Nttpy = 4'-(p-nicotinamide-N-methylphenyl)-2,2':6',2' '-terpyridine, is observed to be brightly luminescent in the solid state at room temperature and at 77 K. The complex exhibits reversible vapochromic behavior and crystallographic change in the presence of several volatile organic solvents. Upon exposure to methanol vapors, the complex changes color from red to orange, and a shift to higher energy is observed in the emission maximum with an increase in excited-state lifetime and emission intensity. The crystal and molecular structures of the orange and red forms, determined by single-crystal X-ray diffraction on the same single crystal, were found to be equivalent in the molecular sense and only modestly different in terms of packing. In both forms, the cationic Pt(II) complexes possess distorted square planar geometries. Analysis of the orange form's crystal packing reveals the presence of solvent molecules in lattice voids, Pt...Pt separations averaging 3.75 A and a zigzag arrangement between nearest neighbor Pt atoms, whereas the red form is devoid of solvent within the crystal lattice and contains complexes stacked with a nearly linear arrangement of Pt(II) ions having an average distance of 3.33 A. On the basis of the crystallographic data, it is evident that sorption of methanol vapor induces a change in intermolecular contacts and Pt...Pt interactions in going from red to orange. Disruption of the d(8)-d(8) metallophilic interactions consequently alters the emitting state from (3)[(d)sigma*-pi*(terpyridine)] that is formally a metal-metal-to-ligand charge transfer (MMLCT) state in the red form to one in which the HOMO corresponds to a more localized Pt(d) orbital in the red form ((3)MLCT).     

FLUORESCENCE STUDY OF THE PHYSICO-CHEMICAL PROPERTIES OF A BENZO(A)PYRENE7,8-DIHYDRODIOL9,10-OXIDE DERIVATIVE BOUND COVALENTLY TO DNA

Abstract— Covalent complexes between 7 ,8-dihydrodiol 9.10-oxide benzo(a)pyrene (BPDE) and DNA with a modification of one BPDE molecule per 1000 DNA bases were prepared in vitro . The same stereoselective and chemically homogeneous binding of BPDE to native DNA was observed, as reported earlier for human and bovine bronchial explants. The fluorescence of the pyrene-like aromatic moiety of BPDE bound to DNA in vitro was used as a probe of the microenvironment of the BPDE molecule in order to obtain information about the structure of the BPDE-DNA complex dissolved in aqueous solution. Fluorescence techniques, based on the quenching of the singlet excited states by metal ions such as Ag⁺, by iodide ions, and by molecular oxygen are described, which provide a method for differentiating between external and internal (intercalation) binding of polycyclic aromatic molecules to DNA. Silver ions, which bind specifically to DNA bases, exhibit a strong quenching effect on noncovalently bound, intercalated benzo(a)pyrene; on the other hand, there is no quenching effect on the fluorescence of BPDE in the covalent DNA adduct. Quenchers such as O₂ and iodide ions, which do not specifically bind to DNA and are dissolved in the solution external to the DNA molecule, exhibit a quenching effect on the BPDE chromophore. Furthermore, the fluorescence yield of the BPDE-DNA complex decreases with increasing DNA concentration, an effect which is not observed with non-covalently bound intercalated benzo(a)pyrene-DNA complexes, and which is attributed to intermolecular DNA-DNA interactions. The results of these studies indicate that the pyrene-like chromophore in the covalent BPDE-DNA complex is not intercalated between the base pairs, and that it is located in an accessible region external to the DNA helix. Possible structures are discussed.     

Mono- and dibridged isomers of Si2H3 and Si2H4: the true ground state global minima. Theory and experiment in concert

Highly correlated ab initio coupled-cluster theories (e.g., CCSD(T), CCSDT) were applied on the ground electronic states of Si(2)H(3) and Si(2)H(4), with substantive basis sets. A total of 10 isomers, which include mono- and dibridged structures, were investigated. Scalar relativistic corrections and zero-point vibrational energy corrections were included to predict reliable energetics. For Si(2)H(3), we predict an unanticipated monobridged H(2)Si-H-Si-like structure (C(s), (2)A') to be the lowest energy isomer, in constrast to previous studies which concluded that either H(3)Si-Si (C(s), (2)A') or near-planar H(2)Si-SiH (C(1), (2)A) is the global minimum. Our results confirm that the disilene isomer, H(2)Si-SiH(2), is the lowest energy isomer for Si(2)H(4) and that it has a trans-bent structure (C(2)(h), (1)A(g)). In addition to the much studied silylsilylene, H(3)Si-SiH, we also find that a new monobridged isomer H(2)Si-H-SiH (C(1), (1)A, designated 2c) is a minimum on the potential energy surface and that it has comparable stability; both isomers are predicted to lie about 7 kcal/mol above disilene. By means of Fourier transform microwave spectroscopy of a supersonic molecular beam, the rotational spectrum of this novel Si(2)H(4) isomer has recently been measured in the laboratory, as has that of the planar H(2)Si-SiH radical. Harmonic vibrational frequencies as well as infrared intensities of all 10 isomers were determined at the cc-pVTZ CCSD(T) level.     

Rotational spectrum and equilibrium structure of silanethione, H2Si=S

Unsubstituted silanethione, H(2)Si=S, has been characterized experimentally for the first time by means of rotational spectroscopy; the equilibrium structure of this fundamental molecule has been evaluated through a combination of experimental data from a total of ten isotopologues and results of high-level coupled-cluster calculations.     

The rotational spectrum of CN-

The rotational spectrum of the molecular negative ion CN(-) has been detected in the laboratory at high resolution. The four lowest transitions were observed in a low pressure glow discharge through C(2)N(2) and N(2). Conclusive evidence for the identification was provided by well-resolved nitrogen quadrupole hyperfine structure in the lowest rotational transition, and a measurable Doppler shift owing to ion drift in the positive column of the discharge. Three spectroscopic constants (B, D, and eQq) reproduce the observed spectrum to within one part in 10(7) or better, allowing the entire rotational spectrum to be calculated well into the far IR to within 1 km s(-1) in equivalent radial velocity. CN(-) is an excellent candidate for astronomical detection, because the CN radical is observed in many galactic molecular sources, the electron binding energy of CN(-) is large, and calculations indicate CN(-) should be detectable in IRC+10216-the carbon star where C(6)H(-) has recently been observed. The fairly high concentration of CN(-) in the discharge implies that other molecular anions containing the nitrile group may be within reach.     

Ionic liquid extraction method for upgrading eucalyptus kraft pulp to high purity dissolving pulp

High purity cellulose from wood is an important raw material for many applications such as cellulosic fibers, films or the manufacture of various cellulose acetate products. Hitherto, multi-step refining processes are needed for an efficient hemicellulose removal, most of them suffering from severe cellulose losses. Recently, a novel method for producing high purity cellulose from bleached paper grade birch kraft pulp was presented. In this so called IONCELL process, hemicelluloses are extracted by an ionic liquid–water mixture and both fractions can be recovered without yield losses or polymer degradation. Herein, it is demonstrated that bleached Eucalyptus urograndis kraft pulp can be refined to high purity acetate grade pulp via the IONCELL process. The hemicellulose content could be reduced from initial 16.6 to 2.4 wt% while persevering the cellulose I crystal form by using an optimized 1-ethyl-3-methylimidazolium dimethylphosphate-water mixture as the extraction medium. The degree of polymerization was then reduced by a sulfuric acid treatment for subsequent acetylation of the pulp, resulting in a final hemicellulose content of 2.2 wt%. When pre-treating the pulp enzymatically with endoxylanase, the final hemicellulose content could be reduced even to 1.7 wt%. For comparison, the eucalyptus kraft pulp was also subjected to cold caustic extraction and the same subsequent acid treatment which led to 3.9 wt% of residual hemicelluloses. The performance in acetylation of all produced pulps was tested and compared to commercial acetate grade pulp. The endoxylanase-IONCELL-treated pulp showed superior properties. Thus, an ecologically and economically efficient alternative for the production of highest value cellulose pulp is presented.     

Influence on Pore Structure of Micro/Nanofibrillar Cellulose in Pigmented Coating Formulations

Nano and microfibrillated cellulose (NFC and MFC, respectively, collectively termed MNFC) is known to interact strongly with water, related to its high polarity and surface area. The swelling behaviour acts to form a gel with high water retention properties. The observation that nanocellulose could possibly be used in paper or other coating formulations, as a co-binder, for example, raises a question about the possible effects it could have on coating pore structure. In this study, we analyse the pore structure of pigmented coatings, liquid absorption and permeability, in respect to the influence of partially substituting traditional co-binder carboxymethyl cellulose with MNFC. The contrast between polar water and non-polar liquid, such as alkane, is used to probe the water interactive and extractable in-coating (internal) gel-formation properties of the nanocellulosic materials. These contrasting liquids are important in many processes, such as offset printing, but also in respect to exposure of coatings in general to environmental factors in application. Results show that permeability to liquid water is dramatically reduced when nanocellulosic material is present, though water can permeate by diffusion through the nanocellulose gel network. Long timescale exposure to water during absorption leads to extraction of any soluble salts remaining after the chemical treatment of the fibrillar material during production. Inert alkane, on the other hand, can absorb and permeate freely without interactive hindrance from the nanocellulose, with no extractive effect. Such a construct could in principle be considered for use as an oil-water differential membrane or for slow release concepts in aqueous systems by loading soluble deliverable materials within the nanocellulosic gel.     

The Rotational Spectra of H2CCSi and H2C4Si

The microwave rotational spectra of two singlet chains H₂CCSi and H₂C₄Si, have been observed in a pulsed supersonic molecular beam by Fourier transform microwave spectroscopy. Following detection of the singly substituted rare isotopic species and D₂CCSi, an experimental structure (r₀) has been determined to high accuracy for H₂CCSi by isotopic substitution. In addition, the rotational transitions of the ²⁹Si and the two ¹³C isotopic species of H₂CCSi exhibit nuclear spin-rotation hyperfine structure. The component of the spin-rotation tensor along the a-inertial axis is abnormally large for each of these isotopic species, especially for that with ²⁹Si, where the magnitude of C_aa is in excess of 700 kHz.