Structural characterization of organic molecules by negative ions in laser microprobe mass spectrometry. Part 1—Neutral compounds

Laser microprobe mass spectrometry (LMMS) has been used to systematically study polyfunctional molecules, covering a wide range of structure and polarity. The knowledge about the mechanisms actually involved for desorption and ionization (DI) of organics by laser microbeam irradiation of solid samples at high-power density is rather limited. Therefore we have elaborated a set of tentative hypotheses about DI in LMMS, permitting consistent rationalization of detected signals. The technique apparently combines desorption under mild conditions, shown by the release of intact thermolabiles, with extensive fragmentation. Structural data are typically distributed between cations and anions. Interpretation of negative-ion detection mode mass spectra often represents intricate problems, partly due to the lack of sustaining background information from conventional mass spectrometry. Selected examples are presented to illustrate the occurrence of electron capture ionization, the role of heteroatoms in the formation of negative ions and the tendency to undergo complex skeletal rearrangements. Although LMMS was originally aimed at microprobe applications, it has been found to be a valuable tool in organic mass spectrometry.     

New manganese pyrophosphates: The syntheses, crystallographic characterizations and magnetic properties of BaMnP2O7 and CaMnP2O7

The new compound BaMnP₂O₇ was obtained in two crystallographic modifications, (monoclinic-1) and (triclinic-2), by heating mixtures of BaCO₃, P₂O₅ and MnO₂ to 1100°C and to 1000°C for 72 h, respectively. CaMnP₂O₇ (3) was obtained by heating a mixture of CaCO₃, P₂O₅ and MnO₂ to 1050°C for 48 h. Both crystallographic forms of BaMnP₂O₇ and CaMnP₂O₇ (3) were investigated by single-crystal X-ray diffraction analysis. The high-temperature monoclinic form of BaMnP₂O₇ could not be obtained free of the low-temperature triclinic form in the bulk form. In monoclinic-1 the manganese ions exist in a distorted MnO₆ octahedron surrounded by five closely and one remotely positioned oxygen atoms. In triclinic-2 and -3 forms the manganese ions are associated in pairs by the formation of Mn₂O₁₀ units that share one edge of two adjacent octahedra. The magnetic properties of the triclinic-2 and -3 forms were also investigated. The effective magnetic moments, μ_eff, are 5.7 B.M. and 5.8 B.M./Mn atom for triclinic-2 and -3, respectively, and are consistent with a high-spin Mn²⁺ ion in an octahedral environment with five unpaired electrons. The temperature-dependent magnetic measurements of 2 and 3 have revealed a combination of short-range antiferromagnetic coupling, J, between the two Mn ions within the Mn₂O₁₀ units and a longer range weaker antiferromagnetic coupling, J′, between the neighbouring Mn₂O₁₀ units, |J′/J| = 0.18 and 0.074 for 2 and 3, respectively.     

A new synthetic route to boron-10 enriched pentaborane(9) from boric acid and its conversion to anti-(10)B(18)H(22)

Boron-10 enriched boric acid, H310BO3, was converted to the corresponding sodium borohydride, Na10BH4, in essentially quantitative yields, by using slightly modified literature methods involving the formation of butyl borate, (n-OBu)310B, first and then reacting it with NaH in mineral oil. The oxidation reaction of Na10BH4 with I2 in diglyme and subsequent addition/purification in dioxane gave Na[10B3H8]0.3(C4H8O2) that reacted further with NiCl2 in either anhydrous benzene or heavy mineral oil at 110 degrees C to produce the corresponding 10B5H9 as the first isolated 10B-enriched liquid boron hydride in a laboratory environment. Treatment of this 10B5H9 with NaH or t-BuLi in 2:1 molar ratio underwent a cage expansion reaction to produce the [M]10B9H14 that undergoes a redox reaction in situ with anhydrous NiCl2 in n-hexane to yield the corresponding fused cage anti-10B18H22 as the only solid borane product in 42% yield, thus establishing new synthetic routes for the preparation of 10B-enriched polyhedral boranes.     

Base hydrolysis of halolactones in the bicyclo(2.2.1)heptyl and bicyclo(2.2.2)octyl systems

The mechanism of ketocarboxylic acid formation in the base treatment of bicyclo[2.2.1]heptyl halolactones has been determined. On the basis of stoichiometry, kinetics, effect of leaving group, and isotopic labeling experiments, a dehydrohalogenation mechanism is proposed in which cleavage of the carbon-halogen bond precedes that of the carbon-hydrogen bond in the transition state. The relationship between El-like and syn-E2 transition states in these systems is discussed.In the bicyclo[2.2.2]octyl series base treatment of the halolactones yields oxidocarboxylic acids. The difference in behavior between the two systems is rationalized on the basis of molecular geometry.     

Sequencing of bacitracin A and related minor components by liquid chromatography/electrospray ionization ion trap tandem mass spectrometry

A selective reversed-phase liquid chromatography/mass spectrometry (LC/MS(n)) method is described for the characterization of related compounds in commercial bacitracin samples. Mass spectral data for these polypeptide antibiotics were acquired on a LCQ ion trap mass spectrometer equipped with an electrospray ionization probe operated in the positive and negative ion mode. The LCQ ion trap is ideally suited for the sequencing of those linear side-chain cyclized peptides because it provides on-line LC/MS(n) capability. Using this method bacitracin A, 1-epibacitracin A, bacitracins B(1), B(2), B(3) and bacitracin F were sequenced and previous sequencing was confirmed. Bacitracins C(1), C(2), C(3), D, H(2) and H(3) were resolved chromatographically and their ring portion was sequenced for the first time. Four components not described in the literature (1-epibacitracin B(1), 1-epibacitracin B(2), 1-epibacitracin C(1) and H(4)) were sequenced completely for the first time. The main advantage of this hyphenated LC/MS(n) technique is the characterization of the related substances without time-consuming isolation and purification procedures.