Fourier transform microwave spectroscopy of LiCCH, NaCCH, and KCCH: Quadrupole hyperfine interactions in alkali monoacetylides
Graphical abstract
Highlights
► Microwave spectra of LiCCH, NaCCH, KCCH and deuterium isotopologues recorded using FTMW spectroscopy. ► Combined fit of microwave and previous millimeter-wave data. ► Metal and deuterium quadrupole coupling constants determined for the first time. ► Synthesized by ablation of the pure metal and reaction with acetylene using discharge assisted laser ablation.
Introduction
Molecular hyperfine parameters can provide insight into chemical bonding [1], [2], [3]. For example, quadrupole coupling constants have been used to gauge the ionic/covalent character of alkali metal-containing compounds, such as fluorides [4], [5], [6], chlorides [7], [8], [9], hydroxides [10], [11], [12] and borohydrides [13]. Typically, Fourier transform microwave (FTMW) and molecular beam resonance spectroscopic techniques have been employed to measure the hyperfine parameters of such species, where they have been generated in the gas phase by heating or ablating the solid salt. This gas-phase production method, however, limits the type of ligand that can be attached. As a result, hyperfine parameters for a number of alkali metal-containing species such as the amides [14], [15], monomethyls [16] and hydrosulfides [17], [18] have not yet been measured, despite the existence of extensive millimeter-wave measurements.
Another class of compound for which hyperfine parameters have not yet been established is the alkali monoacetylides: LiCCH, NaCCH and KCCH. Their rotational spectra have been recorded at millimeter wavelengths [19], [20], [21], [22], and these studies clearly indicate linear geometries for these molecules. Because these measurements occurred at high frequencies, however, hyperfine structure, which would chiefly arise from quadrupole coupling, was not observed. Information regarding the degree of ionic/covalent bonding character between the metal and the CCH group could only be speculated from the structural parameters.
Recently, the first experimental observation of monomeric copper acetylide, CuCCH, has been reported [23]. The pure rotational spectra of several isotopologues of this molecule were recorded using the FTMW and millimeter-wave spectrometers of the Ziurys group. CuCCH was successfully synthesized in the supersonic nozzle of the FTMW instrument, using the newly developed technique of discharge-assisted laser ablation. Building on the success of this study, we have employed discharge-assisted laser ablation to produce LiCCH, NaCCH, KCCH and their deuterium isotopologues, and to measure their microwave spectra. To conduct this study, specially-designed alkali metal “rods” were fabricated, which, to the best of our knowledge, is a novel approach. Quadrupole hyperfine structure was resolved in the spectra arising from the metals and the deuterium nuclei. In this paper, we present these data, analysis of the spectra, and an interpretation of the quadrupole constants.
Section snippets
Experimental
The FTMW spectrometer of the Ziurys group [24] was used to measure the microwave spectra of LiCCH, NaCCH, KCCH and their deuterium isotopologues in the range 5–37 GHz. The instrument has been described in detail elsewhere. Briefly, the Balle–Flygare type instrument [25] consists of a typical Fabry–Perot cavity that contains two spherical aluminum mirrors in a near confocal arrangement, housed in a cryo-pumped vacuum chamber. Antennas are embedded in opposite mirrors for the injection and
Results
The ground electronic state of the alkali metal acetylides is 1Σ+. Their spectra are therefore quite simple, except for the possible presence of quadrupole coupling and nuclear spin-rotation interactions of the alkali metal nuclei (I = 3/2 for Li, Na and K). Such hyperfine interactions were predicted for the acetylides on the basis of the alkali fluorides. Using these predictions, a 10 MHz frequency region was initially searched, centered on a given rotational transition. Only the J = 1 → 0 transition
Discussion
Table 4 lists the metal quadrupole coupling constants for several alkali-metal containing molecules, namely the fluorides, chlorides, hydroxides and borohydrides. The metal quadrupole coupling constant is small in magnitude for each species, which is consistent with an ionic description of the alkali metal–ligand bond [1], [2], [3]. For the acetylides, the magnitude of the eQq constant for a particular metal is very similar to those listed for the other ligands in Table 4. This comparison
Acknowledgments
This research is supported by NSF-CHE-1057924. P.M.S. and M.K.L.B. would like to thank Canisius College for providing travel funds and the Ziurys group for the use of their spectrometer and their hospitality.
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