Understanding the crystalline structure of racemic carvedilol phosphate hemihydrate presents several challenges that were overcome using a combination of single-crystal X-ray diffraction, solid-state NMR (SSNMR), and other analytical techniques. Initial attempts to obtain a crystal structure were hampered by difficulties with twinning and problematic disorder in the final refinements. Multinuclear SSNMR analysis localized the disorder to portions of the molecule near the chiral center. As a result, single-enantiomer carvedilol phosphate was prepared and was found to crystallize in a phase that was isomorphous with the racemate, while SSNMR spectra of the single enantiomers did not contain the disorder observed in the racemate. The singlecrystal X-ray structure of the (R)-enantiomer was solved and used as a starting point to successfully progress the solution of the disordered racemic crystal structure. Thermal analysis and construction of a phase diagram, along with crystallographic and spectroscopic analysis, found the crystal structure of the racemate to be a solid solution of (R)-and (S)-enantiomers, with the conformation of the molecule adjusting to fit. The crystal structures show the stoichiometry of the both the racemate and (R)-enantiomer to be a hemihydrate. The phase isomorphically dehydrates below relative humidity values of 1% and above temperatures of 125 °C as assessed by water vapor sorption studies, powder X-ray diffraction, and SSNMR. Single-crystal diffraction detected significant changes in the unit cell dimensions as the phase dehydrated, which was related to the visual appearance of opacity in a single crystal of the (R)-enantiomer. The mechanism of water incorporation was further probed spectroscopically via exchange with deuterium, 17 O-, and 18 O-labeled water; the results suggest that dehydration and rehydration likely proceed via narrow tunnels in the crystal structure, combined with the formation of fissures in the crystal. 2 H SSNMR experiments showed that the water does not engage in solid-state jump motion even at higher temperatures.
The disubstituted boron cations CH3OBOCH3 + and CH3BCH3 + readily cleave CO and C−C bonds in gaseous long-chain aldehydes and ketones in a dual-cell Fourier transform ion cyclotron resonance mass spectrometer. Abstraction of OH by the borocations yields a hydrocarbon product ion that contains the entire carbon skeleton of the aldehyde or ketone. A competing abstraction of part of the carbonyl compound as a small aldehyde results in a borocation product that is indicative of the location of the carbonyl group in the neutral substrate. The mechanisms of these two reactions likely involve common intermediates formed via 1,2-hydride shifts in an initially formed B−OC adduct. Both reactions are highly exothermic. The OH abstraction reaction is the thermodynamically favored pathway while aldehyde abstraction is kinetically favored by the smaller carbonyl compounds. The overall enthalpy change associated with the latter reaction is likely to be relatively insensitive to the size of the carbonyl compound. In contrast, the OH abstraction reaction becomes more exothermic as the size of the substrate increases. This results in a predominant hydrocarbon ion product for the larger aldehydes and ketones.
We report here preliminary results on a mediumperformance FT-ICR mass spectrometer, based on a 0.4-T permanent magnet, that could easily be configured into bench-top proportions. Among other properties, we have examined the mass range, mass resolution, and mass accuracy of this mass spectrometer, as well as the stability of the magnetic field over time. Further, the use of this device in studies of gas-phase ion/molecule reactions has been briefly explored. The data presented here demonstrate that this simple mass spectrometer can perform a variety of useful experiments, including the measurement of EIMS, MS/MS, and MS/MS/MS spectra, the determination of reaction rate constants for bimolecular ion/molecule reactions, and the measurement of the exact mass of ions.
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