Laser desorption mass spectrometry on thin liquid jets

A recently developed soft desorption method for mass spectrometry is presented, which is called Laser Induced Liquid Beam Ionization/Desorption (LILBID). Analyte ions are desorbed from a thin jet of analyte solution directly into vacuum by means of an IR laser pulse, which has been tuned to a vibrational resonance of the solvent. A comparative experiment with ammonium chloride and aniline hydrochloride shows that ion formation via proton transfer takes place in the solution. Thermally unstable compounds, as well as supra- and biomolecular complexes, can be detected intact and mass analyzed in a reflectron time-of-flight (Re-TOF) mass spectrometer. During the desorption process, noncovalent interactions and some solvation characteristics are preserved. Three examples for the capacity of LILBID are given in this short overview: (a) ion-solvent interactions with the formation of a clathrate structure Cs⁺(H₂O)₂₀, (b) host-guest interactions with the K⁺ selectivity of valinomycin, and (c) noncovalent interactions with the dimerization of gramicidin. Received: 29 July 1997 / Revised: 4 September 1997 / Accepted: 12 September 1997     

Butadienyloxiran-Dihydrooxepin-Isomerisierung. Ringerweiterungsreaktionen von sterisch fixierten und benzoanellierten Epoxyhexadienen durch 1,7-Elektrocyclisierung konjugierter Carbonyl-Ylide

Butadienyloxirane-Dihydrooxepine Isomerisation. Ring Expansion Reactions of Sterically Fixed and Benzoannelated Epoxyhexadienes by 1,7-Electrocyclisation of Conjugated Carbonyl Ylides Thermal ring cleavage of specifically substituted oxiranes results in the formation of conjugated carbonyl ylide intermediates which react by 1,5- and/or 1,7-electrocyclisation to give ring-expanded products. While the spirooxiranes 5t, 6t , and 12t are transformed, as expected, only into 2,3-dihydrofurans ( 21, 22, 50 ), the geometrically fixed compounds give rise to the formation of seven-membered ring isomers either partially ( 6c ), or predominantly ( 5c ), or exclusively ( 7 – 11, 12c ). 1,7-Dipolar cyclisations also take place with participation of one or even two aromatic double bonds ( 7 → 52, 8 → 53, 9 → 54, 12c → 57 , and 10 → 55, 11 → 56 , resp.) affording mono- and dibenzo-dihydrooxepines, respectively. The rearrangements of 5c to 27 and 6c to 28 show for the first time that the 8π-ring closure of 2-oxaheptatrienyl dipoles proceeds in the theoretically predicted conrotatory manner. The exclusive, i.e. periselective formation of dihydrooxepines during the thermolysis of compounds 7 – 11, 12c is explained by assuming a helical geometry of the dipolar intermediates in which the stereoelectronic situation is specially suitable for the – entropically less advantageous – 1,7-cyclisation process.     

Ringerweiterungsreaktionen von (Z)-styryloxiranen unter Phenylring-Beteiligung: bildung von 2,7-dihydro-3,4-benzoxepinen

The thermal ring expansion reaction of (Z)-styryloxiranes ($\text{7}$-$\text{9}$) involves phenylring participation leading to 2,7-dihydro-3,4-benzoxepins ($\text{10}$-$\text{12}$).     

2,6-dicyano-1,5-dimethylsemibullvalene as a probe for homoaromatic molecules of the Dewar-Hoffman type

The 2,6-dicyano-1,5-dimethylsemibullvalene (2d) synthesized via the bicyclo[3.3.0]octanedione 7 exists as a classical ground state which is lower in energy by less than 5 kcal/mole than the homoaromatic transition of the Cope rearrangement of 2d.     

Benchmarking of ruthenium initiators for the ROMP of a norbornenedicarboxylic acid ester

The kinetic study of ring-opening metathesis polymerization (ROMP) of a diester functionalised norbornene derivative, (±)-exo,endo-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid diethyl ester, with a series of ruthenium benzylidene complexes revealed the applicability of these initiators for well defined polymerization reactions. Values for the rate of initiation as well as the rate of propagation of the initiators were determined and correlated to the molecular weight and polydispersity of the isolated polymers. As the only initiator providing an entry to virtually monodisperse polymers the classical “first generation Grubbs-catalyst” was identified, while N-heterocyclic carbene based initiators polymerized with a rate of propagation much higher than the rate of initiation yielding polymers with a broader molecular weight distribution.     

Alkaline Earth Transition Metal Antimonides AT4Sb12 (A = Ca,Sr, Ba; T = Fe,Ru, Os) with LaFe4P12-Structure

The title compounds were prepared by reaction of CaSb₂, SrSb₂, or BaSb₃ with the transition metals and antimony in sealed silica tubes. They crystallize with the cubic LaFe₄P₁₂-structure, which was refined from single-crystal X-ray data of CaFe₄Sb₁₂, SrRu₄Sb₁₂, and BaRu₄Sb₁₂ to residuals of R = 0.014, 0.016, and 0.014, respectively. The thermal parameters of the alkaline earth ions increase with decreasing ionic size. The Sb? Sb distances are greater in the iron compound than they are in the two ruthenium compounds. This is rationalized to be due to a larger portion of electrons in antibonding Sb? Sb states in the iron compound.     

Characterization of Carbohydrates in Paper and Paper Pulps Using Anion Exchange Chromatography and Principal Component Analysis

In the course of this investigation, a method for the characterization and differentiation of paper pulps was developed. After hydrolyzation of the papers and the respective raw pulps with trifluoroacetic acid, the hydrolyzates were analyzed with respect to their carbohydrate composition using ion exchange chromatography. The variations in carbohydrate composition mainly arise from the various hemicelluloses of the papers and pulps. The chromatographic results were then further processed using principal component analysis which allows correlating the various papers to different pulp materials.     

The Role of Binary and Many-centre Molecular Interactions in Spin Crossover in the Solid State. Part I. Equation for Free Energy

Summary. A formalism has been developed that describes spin crossover equilibrium in the solid state by taking into account the effects of n nearest neighbours of a given molecule on its partition function. In this way binary and many-body interactions of the order n + 1 are included into the theoretical model and represented by non-ideality parameters connected with the splitting of free energy levels. Binary interactions are characterised by the main splittings whereas higher order interactions manifest themselves in asymmetries of splittings within multiplets. The contribution of molecular interactions can also be written in terms of formal excess free energies of the second, third, fourth and higher orders. Simple relationships between excess free energies and parameters of multiplets have been found for binary, ternary and quaternary interactions. This formalism is reduced to that of the model of binary interactions when effects of surroundings are additive leading to equidistant free energy multiplets. Higher order interactions may cause an abrupt spin crossover but in a limited range of compositions around the transition point. The regression of experimental transition curves of one-step spin crossover may yield estimates of excess energies up to the fifth order.     

Rotational spectrum, potential energy surface, and bound states of the weakly bound complex He-N2O

Pure rotational transitions of the weakly bound complex He-N(2)O and three minor isotopomers (He-(14)N(15)NO, He-(15)N(14)NO, and He-(15)N(15)NO) were measured in the frequency region from 6 to 20 GHz. Predictions for the microwave transition frequencies were based on the infrared work by Tang and McKellar [J. Chem. Phys. 117, 2586 (2002)]. In the case of (14)N containing isotopomers, nuclear quadrupole hyperfine structure of the rotational transitions was observed and analyzed. The resulting spectroscopic parameters were used to determine geometrical and dynamical information about the complex. An ab initio potential energy surface was calculated at the coupled cluster level of theory with single and double excitations and perturbative inclusion of triple excitations. This surface was constructed using the augmented correlation consistent polarized valence triple zeta basis set for all atoms with the inclusion of bond functions for the van der Waals bond. Bound state calculations were done to determine the energies of low-lying rovibrational levels that are supported by the potential energy surface. The resulting transition energies agree with the experimental values to 1% or better.     

In situ studies of methanol decomposition and oxidation on Pd(111) by PM-IRAS and XPS spectroscopy

Methanol decomposition and oxidation on Pd(111) at millibar pressure were studied by in situ polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS), on-line gas chromatography and pre- and postreaction X-ray photoelectron spectroscopy (XPS). Various dehydrogenation products such as methoxy CH3O, formaldehyde CH2O, formyl CHO, and CO could be spectroscopically identified. Methanol oxidation proceeds via dehydrogenation to formaldehyde CH2O, which either desorbs or is further dehydrogenated to CO, which is subsequently oxidized to CO2. Carbonaceous overlayers that are present during the reaction may favorably affect the selectivity toward CH2O. The reaction takes place on metallic Pd, and no indications of an involvement of Pd surface oxide were observed.