The beryllium dimer is a deceptively simple molecule that, in spite of having only eight electrons, poses difficult challenges for ab initio quantum chemical methods. More than 100 theoretical investigations of the beryllium dimer have been published, reporting a wide range of bond lengths and dissociation energies. In contrast, there have been only a handful of experimental studies that provide data against which these models could be tested. Ultimately, the uncertain extrapolation behavior associated with the available data has prevented quantitative comparisons with theory. In our experiment, we resolve this issue by recording and analyzing spectra that sample all the bound vibrational levels of the beryllium dimer molecule's electronic ground state. After more than 70 years of research on this problem, the experimental data and theoretical models for the dimer are finally reconciled.
Beryllium clusters provide an ideal series for exploring the evolution from discrete molecules to the metallic state. The beryllium dimer has a formal bond order of zero, but the molecule is weakly bound. In contrast, bulk-phase beryllium is a hard metal with a high melting point. Theoretical calculations indicate that the bond energies increase dramatically for Be(n) clusters in the range n=2-6. A triplet ground state is found for n=6, indicating an early emergence of metallic properties. There is an extensive body of theoretical work on smaller Be(n) clusters, in part because this light element can be treated using high-level methods. However, the apparent simplicity of beryllium is deceptive, and the calculations have proved to be challenging owing to strong electron correlation and configuration interaction effects. Consequently, these clusters have become benchmark systems for the evaluation of a wide spectrum of quantum chemistry methods.
The perfect separation
with optimal productivity, yield, and purity
is very difficult to achieve. Despite its high selectivity, in crystallization
unwanted impurities routinely contaminate a crystallization product.
Awareness of the mechanism by which the impurity incorporates is key
to understanding how to achieve crystals of higher purity. Here, we
present a general workflow which can rapidly identify the mechanism
of impurity incorporation responsible for poor impurity rejection
during a crystallization. A series of four general experiments using
standard laboratory instrumentation is required for successful discrimination
between incorporation mechanisms. The workflow is demonstrated using
four examples of active pharmaceutical ingredients contaminated with
structurally related organic impurities. Application of this workflow
allows a targeted problem-solving approach to the management of impurities
during industrial crystallization development, while also decreasing
resources expended on process development.
Rotationally resolved infrared spectra are reported for halogen atom-HF free radical complexes formed in helium nanodroplets. An effusive pyrolysis source is used to dope helium droplets with Cl, Br and I atoms, formed by thermal dissociation of Cl2, Br2 and I2. A single hydrogen fluoride molecule is then added to the droplets, resulting in the formation of the X-HF complexes of interest. Analysis of the resulting spectra confirms that the observed species have 2Pi3/2 ground electronic states, consistent with the linear hydrogen bound structures predicted from theory. Stark spectra are also reported for these species, from which the permanent electric dipole moments are determined.
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