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Pure rotational spectra of 25MgH (X2Σ+) and 67ZnH (X2Σ+) were recorded using direct absorption techniques. These free radicals were synthesized by the reaction of metal vapor, generated in a Broida-type oven, with H2in a dc discharge. The N = 0 → 1 and N = 1 → 2 transitions were recorded for both species in the frequency range of 342 – 789 GHz. Spin-rotation and metal and proton hyperfine interactions were resolved in the spectra. From these data, rotational, fine structure, and hyperfine constants were determined, including the Fermi contact, dipolar, and electric quadrupole parameters of the 25Mg and 67Zn nuclei. Comparison of the metal and proton hyperfine terms suggests that the unpaired electron resides in a σ molecular orbital that has significant s contributions from both the metal and the hydrogen atoms. The dipolar terms for both metals are relatively large, in contrast to those of the proton, and indicate spσ and possibly sdσ (zinc only) orbital hybridization. The quadrupole constants arise principally from the p/d orbital character of the unpaired electron, although there is a non-negligible polarization contribution. These results suggest significant covalent character in both MgH and ZnH, in contrast to their fluoride analogs.

Brücken schlagen: Durch Kombination von hochauflösender Molekülspektroskopie im cm‐ und mm‐Wellenbereich mit quantenchemischen Coupled‐Cluster‐Rechnungen auf hohem Theorieniveau wurde die Struktur von HPSi ermittelt. Seine verbrückte Struktur (siehe Bild; berechnete Werte in Klammern) unterscheidet sich in bemerkenswerter Weise von den linearen Strukturen der C‐ und N‐Analoga wie HCN/HNC und HNSi.

The detection of the fullerenes C60and C70in the interstellar medium (ISM) has transformed our understanding of chemical complexity in space. These discoveries also raise the possibility for the presence of even larger molecules in astrophysical environments. Here we report in situ heating of analog silicon carbide (SiC) presolar grains using transmission electron microscopy (TEM). These heating experiments are designed to simulate the temperature conditions occurring in post-AGB stellar envelopes. Our experimental findings reveal that heating the analog SiC grains to the point of decomposition initially yields hemispherical C60-sized nanostructures, with five- and six-membered rings, which transform into multiwalled carbon nanotubes (MWCNTs) if held isothermally >2 min. These MWCNTs are certainly larger than any of the currently observed interstellar fullerene species, both in overall size and number of C atoms. These experimental simulations suggest that such MWCNTs are likely to form in post-AGB circumstellar material, where the structures, along with the smaller fullerenes, are subsequently injected into the ISM.

Previous mid-infrared, far-infrared, and new submillimeter data relating solely to 52CrH in its X 6S+ state have been reanalyzed by a least-squares fit using a Hund's case (b) Hamiltonian to determine the best obtainable set of parameters for the molecule. In particular, the fine structure and hyperfine constants have been improved. From these parameters, transition frequencies are determined that are more reliable than those published previously; the latter show systematic errors of up to 15 MHz. Such frequencies will facilitate the identification of CrH in the interstellar medium.

Rotational lines of 13C14N have been identified in high-resolution (l/Dl ~ 60,000) echelle spectra of the CN B2S+-X2S+ (0-0) band in three comets. The 12C/13C abundance ratios determined using a full fluorescence excitation model for comets Levy (C/1990 K1), Austin (C/1989 X1), and Okazaki-Levy-Rudenko (C/1989 XIX) are 90 +- 10, 85 +- 20, and 93 +- 20, respectively, consistent with the solar system ratio, 90. A lower limit for the nitrogen isotope ratio, 12C14N/12C15N [?] 200, found for comet Levy is consistent with previous comet measurements and the solar system value, 272. The mean CN carbon isotope ratio in the five comets measured to date is 12C14N/12C15N = 90 +- 10, and the mean for the three molecular species C2, HCN, and CN measured in nine comets is 101 +- 15. Consistency of the cometary carbon isotope ratios with the bulk solar system value indicates (1) chemical homogeneity in the outer protosolar nebula, (2) minimal isotopic fractionation in the protosolar precursor molecular cloud, and (3) that comets formed coevally with the solar system. The 14% difference between the solar system (90) and the present solar neighborhood interstellar 12C/13C ratio (77 +- 7) may be indicative of significant Galactic 13C enrichment over the past 4.6 Gyr. However, even though models can match to within a factor of 2 the solar system abundances, including the carbon isotope ratio, other evidence suggests that simple homogeneous Galactic evolution models may not be adequate to explain detailed stellar and interstellar abundances in the Galaxy.

The simplest alkali metal acetylides MCCH were made in the gas phase for the first time (see drawing), and their bond lengths were determined by millimeter/submillimeter spectroscopy of their isotopomers. The M−C bond lengths rCM are the shortest known for organoalkali metal compounds. In the case of LiCCH, the experimentally determined Li−C distance of 1.888 Å has an estimated accuracy of ±0.0005 Å, which should allow a rigorous test of theoretical methods.

Die einfachsten Alkalimetallacetylide MCCH wurden erstmals in der Gasphase hergestellt (siehe Formel), und ihre Bindungslängen wurden durch Millimeter-/Submillimeterwellenspektroskopie ihrer Isotopomere ermittelt: Die M-C-Bindungslängen rCM sind die kürzesten aller bekannten alkalimetallorganischen Verbindungen. Im Fall von LiCCH wurde der Li-C-Abstand von 1.888 Å mit einer geschätzten Genauigkeit von ±0.0005 Å bestimmt – dieser Wert sollte weitere Verbesserungen bei theoretischen Methoden ermöglichen.