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Hydrogen bonds (HBs) are crucial non-covalent interactions in chemistry. Recently, the occurrence of an HB in (H2S)2 has been reported (Arunan et al., Angew. Chem., Int. Ed., 2018, 57, 15199), challenging the textbook view of H2S dimers as mere van der Waals clusters. We herein try to shed light on the nature of the intermolecular interactions in the H2O, H2S, and H2Se dimers via correlated electronic structure calculations, Symmetry Adapted Perturbation Theory (SAPT) and Quantum Chemical Topology (QCT). Although (H2S)2 and (H2Se)2 meet some of the criteria for the occurrence of an HB, potential energy curves as well as SAPT and QCT analyses indicate that the nature of the interaction in (H2O)2 is substantially different (e.g. more anisotropic) from that in (H2S)2 and (H2Se)2. QCT reveals that the HB in (H2O)2 includes substantial covalent, dispersion and electrostatic contributions, while the last-mentioned component plays only a minor role in (H2S)2 and (H2Se)2. The major contributions to the interactions of the dimers of H2S and H2Se are covalency and dispersion as revealed by the exchange-correlation components of QCT energy partitions. The picture yielded by SAPT is somewhat different but compatible with that offered by QCT. Overall, our results indicate that neither (H2S)2 nor (H2Se)2 are hydrogen-bonded systems, showing how the nature of intermolecular contacts involving hydrogen atoms evolves in a group down the periodic table.
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Invited for the cover of this issue is Alberto Fernández-Alarcón and co-workers at The Institute of Chemistry of the National Autonomous University of Mexico and The School of Chemistry of the University of Oviedo. The image depicts the real space analysis of the excitation energies in the double blue and red shift of the water dimer. Read the full text of the article at 10.1002/chem.202002854.
RESUMO
The development of chemical intuition in photochemistry faces several difficulties that result from the inadequacy of the one-particle picture, the Born-Oppenheimer approximation, and other basic ideas used to build models. It is shown herein how real-space approaches can be efficiently used to gain valuable insights in photochemistry through a simple example of red and blue shift effects: the double hypso- and bathochromic shifts in the low-lying valence excited states of (H2 O)2 . It is demonstrated that 1)â the use of these techniques allows the perturbative language used in the theory of intermolecular interactions, even in the strongly interacting short-range regime, to be maintained; 2)â one and only one molecule is photoexcited in each of the addressed excited states and 3)â the electrostatic interaction between the in-the-cluster molecular dipoles provides a fairly intuitive rationalisation of the observed batho- and hypsochromism. The methods exploited and illustrated herein are able to maintain the individuality and properties of the interacting entities in a molecular aggregate, and thereby they allow chemical intuition in general states, at any geometry and using a broad variety of electronic structure methods to be kept and built.
RESUMO
Different developments in chemistry and emerging technologies have generated a renewed interest in the properties of molecular excited states. We present herein the partition of black-box, size-consistent equation-of-motion coupled cluster singles and doubles (EOM-CCSD) excitation energies within the framework of the interacting quantum atoms (IQA) formalism. We denote this method as IQA/EOM-CCSD. We illustrate this approach by considering small molecules used often in the study of excited states. This investigation shows how the combination of IQA and EOM-CCSD may provide valuable insights into the molecular changes induced by electron excitation via the real space distribution of the energy of an absorbed photon in a molecular system. Our results reveal (i) the most energetically deformed atomic basins and (ii) the most affected covalent and non-covalent interactions within a molecule due to a given photoexcitation. In other words, this kind of analysis provides insights into the spatial energetic redistribution accompanying an electronic excitation, with interesting foreseeable applications in the rational design of photoexcitations with tailored chemical effects. Altogether, we expect that the IQA/EOM-CCSD excitation energy partition will prove useful in the understanding of systems and processes of interest in photophysics and photochemistry.