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Inorganic Chemistry has a long-standing tradition in using advanced spectroscopic methods for the characterization of compounds that are central to our field. Among those, the compounds of the d and f elements probably take center stage. These molecules typically have partially filled d or f shells, with the molecular orbitals of the respective ions frequently split by low-symmetry ligand fields. However, given that these orbitals are rather well-shielded, the splittings are not overly large. As a consequence, there is a complex manifold of low-lying electronic states that give rise to rich optical spectra. In addition, the various multiplets that arise from distributing the d or f electrons among the available orbitals lead to an array of states with nonzero total spin. Consequently, these states can be probed by magnetic spectroscopies. Last, the d or f elements often “stand out” in these complexes in the sense that only a few of them are surrounded by ligands built from main-group elements. This feature opens another rich venue of element-specific experimental techniques that probe the deep-lying core electrons (like in various forms of X-ray spectroscopy) or even the nuclear states themselves (as in Mößbauer spectroscopy). Obviously, the physics underlying all of these experimental techniques quickly becomes fairly complicated such that a fruitful interplay between theory and experiment is almost mandatory to develop the full information content of the experimental data. Theory also acts as an indispensable tool in cross-connecting the results of different spectroscopic measurements and provides a link to the reactive properties of the studied systems.
Because the electronic and magnetic states of molecules determine their reactivities and materials properties, we have witnessed rapid progress in both the experimental and theoretical application of spectroscopy to inorganic compounds. We thus feel that it would be a good time to share the excitement and assemble an Inorganic Chemistry Virtual Issue that highlights the multiple ways in which spectroscopy can be used in our discipline. In doing so, we have focused on articles published between January 2018 and July 2020 in Inorganic Chemistry. In addition, we have selected some publications from the Journal of the American Chemical Society that also impressively demonstrate the power of spectroscopy to probe inorganic systems. Our task was a complicated one indeed because there are far more excellent articles published than we can reasonably cover in this Virtual Issue. We emphasize that the many articles that we could not highlight here are of no lesser scientific value than the excellent articles that are included in this issue.
In choosing the articles, we have given preference to younger investigators because they will lead the way into the future while building on the impressive legacy of the field of inorganic spectroscopy. We have largely excluded articles in the fields of inorganic photochemistry, molecular magnetism, and inorganic solid-state chemistry. These are large and prosperous fields that, we feel, deserve their own and independent special issues. For good reason, a recent Inorganic Chemistry Forum focused on innovative work in the field of magnetism (DOI: 10.1021/acs.inorgchem.9b02488), a recent Virtual Issue touched on solid-state chemistry (https://pubs.acs.org/page/inocaj/vi/emerging-solid-state-chemistry), and a Forum on light-controlled reactivity of metal complexes will appear soon. Hence, this Virtual Issue focuses on molecular compounds from all areas of inorganic chemistry. To that end, we have selected articles where, we feel, spectroscopy is front and center of the study instead of serving as a useful routine tool for the characterization of new and exciting compounds.
As will become quickly evident from glancing over the contents of this Virtual Issue, we include articles from the field of optical spectroscopy featuring uses of absorption, circular dichroism, and magnetic circular dichroism spectroscopy. We also highlight the powers of vibrational spectroscopy in the form of resonance Raman, infrared, or nuclear resonance vibrational spectroscopy. A large field of investigation with many uses is offered by the wide array of magnetic resonance, including all forms of electron paramagnetic or nuclear magnetic resonance. These techniques are widely used in the characterization of new compounds, in the construction of sensors, or as spin probes for distance measurements, to name only a few applications. Synchrotron-based X-ray spectroscopies have witnessed a very impressive development in the past years, and these advances are also reflected in the numerous contributions that focus on this technique. Various one- or two-dimensional absorption and emission techniques are being widely used to probe metal centers in molecules and materials in an element-specific and even time-resolved manner. An application of spectroscopy that deserves special mention revolves around the study of reactive intermediates, where many available spectroscopic tools are used, often in combination with time-resolved or quenching techniques. Quite impressive insights have been achieved on the basis of these multimethod approaches. Last but not least, there also are an increasing number of articles dealing with theoretical inorganic spectroscopy. Here, the focus is on understanding structure–spectra correlations and on identifying or highlighting spectroscopic fingerprints that can be used for the identification of structural motifs in unknown compounds or materials.
We strongly believe that these are exciting times for inorganic spectroscopy. The progress is rapid, and the fields of application are both broad and deep. Inorganic spectroscopy is certainly a field that is fairly demanding from an experimental as well as a theoretical perspective. However, we foresee a richly flourishing future for this field, and we hope that the highlights that we have assembled in this Inorganic Chemistry Virtual Issue will help motivate inorganic chemists, in particular those of the younger generation, to engage in spectroscopic studies.
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS.
